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Fedora 18

System Administrator's Guide

Deployment, Configuration, and Administration of Fedora 18

Edition 1

Jaromír Hradílek

Red Hat, Inc. Engineering Content Services

Douglas Silas

Red Hat, Inc. Engineering Content Services

Martin Prpič

Red Hat, Inc. Engineering Content Services

Stephen Wadeley

Red Hat, Inc. Engineering Content Services

Eliška Slobodová

Red Hat, Inc. Engineering Content Services

Tomáš Čapek

Red Hat, Inc. Engineering Content Services

Petr Kovář

Red Hat, Inc. Engineering Content Services

Miroslav Svoboda

Red Hat, Inc. Engineering Content Services

John Ha

Red Hat, Inc. Engineering Content Services

David O'Brien

Red Hat, Inc. Engineering Content Services

Michael Hideo

Red Hat, Inc. Engineering Content Services

Don Domingo

Red Hat, Inc. Engineering Content Services

Legal Notice

Copyright © 2013 Red Hat, Inc. and others.
The text of and illustrations in this document are licensed by Red Hat under a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). An explanation of CC-BY-SA is available at http://creativecommons.org/licenses/by-sa/3.0/. The original authors of this document, and Red Hat, designate the Fedora Project as the "Attribution Party" for purposes of CC-BY-SA. In accordance with CC-BY-SA, if you distribute this document or an adaptation of it, you must provide the URL for the original version.
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Abstract
The System Administrator's Guide documents relevant information regarding the deployment, configuration, and administration of Fedora 18. It is oriented towards system administrators with a basic understanding of the system.

Preface
1. Target Audience
2. How to Read this Book
3. Document Conventions
3.1. Typographic Conventions
3.2. Pull-quote Conventions
3.3. Notes and Warnings
4. We Need Feedback!
5. Acknowledgments
I. Basic System Configuration
1. Configuring the Language and Keyboard
1.1. Changing the Language
1.2. Changing the Date, Time, and Numeric Format
1.3. Changing the Keyboard Layout
1.4. Viewing the Current Configuration
2. Configuring the Date and Time
2.1. Using the Date and Time Configuration Tool
2.2. Using the Command Line Tools
2.2.1. Changing the Date
2.3. Additional Resources
2.3.1. Installed Documentation
3. Configuring NTP Using the chrony Suite
3.1. Introduction to the chrony Suite
3.1.1. Differences Between ntpd and chronyd
3.1.2. Choosing Between NTP Daemons
3.2. Understanding chrony and Its Configuration
3.2.1. Understanding chronyd
3.2.2. Understanding chronyc
3.2.3. Understanding the chrony Configuration Commands
3.2.4. Security with chronyc
3.3. Using chrony
3.3.1. Checking if chrony is Installed
3.3.2. Installing chrony
3.3.3. Checking the Status of chronyd
3.3.4. Starting chronyd
3.3.5. Stopping chronyd
3.3.6. Checking if chrony is Synchronized
3.3.7. Manualy Adjusting the System Clock
3.4. Setting Up chrony for Different Environments
3.4.1. Setting Up chrony for a System Which is Infrequently Connected
3.4.2. Setting Up chrony for a System in an Isolated Network
3.5. Using chronyc
3.5.1. Using chronyc to Control cronyd
3.5.2. Using chronyc for Remote Administration
3.6. Additional Resources
3.6.1. Installed Documentation
3.6.2. Useful Websites
4. Managing Users and Groups
4.1. Introduction to Users and Groups
4.1.1. User Private Groups
4.1.2. Shadow Passwords
4.2. Using the User Accounts Tool
4.2.1. Configuring an Account
4.2.2. Adding a New User
4.2.3. Removing a User
4.3. Using the User Manager Tool
4.3.1. Viewing Users and Groups
4.3.2. Adding a New User
4.3.3. Adding a New Group
4.3.4. Modifying User Properties
4.3.5. Modifying Group Properties
4.4. Using Command Line Tools
4.4.1. Adding a New User
4.4.2. Adding a New Group
4.4.3. Enabling Password Aging
4.4.4. Enabling Automatic Logouts
4.4.5. Creating Group Directories
4.5. Additional Resources
4.5.1. Installed Documentation
II. Package Management
5. Yum
5.1. Checking For and Updating Packages
5.1.1. Checking For Updates
5.1.2. Updating Packages
5.1.3. Preserving Configuration File Changes
5.2. Packages and Package Groups
5.2.1. Searching Packages
5.2.2. Listing Packages
5.2.3. Displaying Package Information
5.2.4. Installing Packages
5.2.5. Removing Packages
5.2.6. Working with Transaction History
5.3. Configuring Yum and Yum Repositories
5.3.1. Setting [main] Options
5.3.2. Setting [repository] Options
5.3.3. Using Yum Variables
5.3.4. Viewing the Current Configuration
5.3.5. Adding, Enabling, and Disabling a Yum Repository
5.3.6. Creating a Yum Repository
5.4. Yum Plug-ins
5.4.1. Enabling, Configuring, and Disabling Yum Plug-ins
5.4.2. Installing Additional Yum Plug-ins
5.4.3. Plug-in Descriptions
5.5. Additional Resources
6. PackageKit
6.1. Updating Packages with Software Update
6.1.1. Setting the Update-Checking Interval
6.1.2. Setting the Software Sources
6.2. Using Add/Remove Software
6.2.1. Refreshing Software Sources (Yum Repositories)
6.2.2. Finding Packages with Filters
6.2.3. Installing and Removing Packages (and Dependencies)
6.2.4. Installing and Removing Package Groups
6.2.5. Viewing the Transaction Log
6.3. PackageKit Architecture
6.4. Additional Resources
III. Networking
7. NetworkManager
7.1. The NetworkManager Daemon
7.2. Interacting with NetworkManager
7.2.1. Connecting to a Network
7.2.2. Configuring New and Editing Existing Connections
7.2.3. Connecting to a Network Automatically
7.2.4. User and System Connections
7.3. Establishing Connections
7.3.1. Establishing a Wired (Ethernet) Connection
7.3.2. Establishing a Wireless Connection
7.3.3. Establishing a Mobile Broadband Connection
7.3.4. Establishing a VPN Connection
7.3.5. Establishing a DSL Connection
7.4. Configuring Connection Settings
7.4.1. Configuring 802.1x Security
7.4.2. Configuring Wireless Security
7.4.3. Configuring PPP (Point-to-Point) Settings
7.4.4. Configuring IPv4 Settings
7.4.5. Configuring IPv6 Settings
7.4.6. Configuring Routes
7.5. NetworkManager Architecture
8. Network Interfaces
8.1. Network Configuration Files
8.2. Interface Configuration Files
8.2.1. Ethernet Interfaces
8.2.2. Ethtool
8.2.3. Channel Bonding Interfaces
8.2.4. Network Bridge
8.2.5. Setting Up 802.1q VLAN Tagging
8.2.6. Alias and Clone Files
8.2.7. Dialup Interfaces
8.2.8. Other Interfaces
8.3. Interface Control Scripts
8.4. Using Hostnamectl
8.4.1. View all the Hostnames
8.4.2. Set All The Hostnames
8.4.3. Set A Particular Hostname
8.4.4. Clear A Particular Hostname
8.4.5. Changing Host Names Remotely
8.4.6. Additional Resources
8.5. Static Routes and the Default Gateway
8.6. Network Function Files
8.7. Additional Resources
8.7.1. Installed Documentation
8.7.2. Useful Websites
IV. Infrastructure Services
9. Services and Daemons
9.1. Configuring Services
9.1.1. Enabling the Service
9.1.2. Disabling the Service
9.2. Running Services
9.2.1. Checking the Service Status
9.2.2. Running the Service
9.2.3. Stopping the Service
9.2.4. Restarting the Service
9.3. Additional Resources
9.3.1. Installed Documentation
9.3.2. Related Books
10. Configuring Authentication
10.1. Configuring System Authentication
10.1.1. Launching the Authentication Configuration Tool UI
10.1.2. Selecting the Identity Store for Authentication
10.1.3. Configuring Alternative Authentication Features
10.1.4. Configuring Authentication from the Command Line
10.1.5. Using Custom Home Directories
10.2. Using and Caching Credentials with SSSD
10.2.1. About the sssd.conf File
10.2.2. Starting and Stopping SSSD
10.2.3. Configuring SSSD to Work with System Services
10.2.4. Creating Domains
10.2.5. Configuring Access Control for SSSD Domains
10.2.6. Configuring Domain Failover
10.2.7. Managing the SSSD Cache
10.2.8. Configuring OpenSSH to Check SSSD for Cached Keys (TECH PREVIEW)
10.2.9. Using NSCD with SSSD
10.2.10. Troubleshooting SSSD
11. OpenSSH
11.1. The SSH Protocol
11.1.1. Why Use SSH?
11.1.2. Main Features
11.1.3. Protocol Versions
11.1.4. Event Sequence of an SSH Connection
11.2. An OpenSSH Configuration
11.2.1. Configuration Files
11.2.2. Starting an OpenSSH Server
11.2.3. Requiring SSH for Remote Connections
11.2.4. Using a Key-Based Authentication
11.3. OpenSSH Clients
11.3.1. Using the ssh Utility
11.3.2. Using the scp Utility
11.3.3. Using the sftp Utility
11.4. More Than a Secure Shell
11.4.1. X11 Forwarding
11.4.2. Port Forwarding
11.5. Additional Resources
11.5.1. Installed Documentation
11.5.2. Useful Websites
V. Servers
12. DHCP Servers
12.1. Why Use DHCP?
12.2. Configuring a DHCP Server
12.2.1. Configuration File
12.2.2. Lease Database
12.2.3. Starting and Stopping the Server
12.2.4. DHCP Relay Agent
12.3. Configuring a DHCP Client
12.4. Configuring a Multihomed DHCP Server
12.4.1. Host Configuration
12.5. DHCP for IPv6 (DHCPv6)
12.6. Additional Resources
12.6.1. Installed Documentation
13. DNS Servers
13.1. Introduction to DNS
13.1.1. Nameserver Zones
13.1.2. Nameserver Types
13.1.3. BIND as a Nameserver
13.2. BIND
13.2.1. Configuring the named Service
13.2.2. Editing Zone Files
13.2.3. Using the rndc Utility
13.2.4. Using the dig Utility
13.2.5. Advanced Features of BIND
13.2.6. Common Mistakes to Avoid
13.2.7. Additional Resources
14. Web Servers
14.1. The Apache HTTP Server
14.1.1. New Features
14.1.2. Notable Changes
14.1.3. Updating the Configuration
14.1.4. Running the httpd Service
14.1.5. Editing the Configuration Files
14.1.6. Working with Modules
14.1.7. Setting Up Virtual Hosts
14.1.8. Setting Up an SSL Server
14.1.9. Additional Resources
15. Mail Servers
15.1. Email Protocols
15.1.1. Mail Transport Protocols
15.1.2. Mail Access Protocols
15.2. Email Program Classifications
15.2.1. Mail Transport Agent
15.2.2. Mail Delivery Agent
15.2.3. Mail User Agent
15.3. Mail Transport Agents
15.3.1. Postfix
15.3.2. Sendmail
15.3.3. Fetchmail
15.3.4. Mail Transport Agent (MTA) Configuration
15.4. Mail Delivery Agents
15.4.1. Procmail Configuration
15.4.2. Procmail Recipes
15.5. Mail User Agents
15.5.1. Securing Communication
15.6. Additional Resources
15.6.1. Installed Documentation
15.6.2. Useful Websites
15.6.3. Related Books
16. Directory Servers
16.1. OpenLDAP
16.1.1. Introduction to LDAP
16.1.2. Installing the OpenLDAP Suite
16.1.3. Configuring an OpenLDAP Server
16.1.4. Running an OpenLDAP Server
16.1.5. Configuring a System to Authenticate Using OpenLDAP
16.1.6. Additional Resources
17. File and Print Servers
17.1. Samba
17.1.1. Introduction to Samba
17.1.2. Samba Daemons and Related Services
17.1.3. Connecting to a Samba Share
17.1.4. Configuring a Samba Server
17.1.5. Starting and Stopping Samba
17.1.6. Samba Server Types and the smb.conf File
17.1.7. Samba Security Modes
17.1.8. Samba Account Information Databases
17.1.9. Samba Network Browsing
17.1.10. Samba with CUPS Printing Support
17.1.11. Samba Distribution Programs
17.1.12. Additional Resources
17.2. FTP
17.2.1. The File Transfer Protocol
17.2.2. FTP Servers
17.2.3. Files Installed with vsftpd
17.2.4. Starting and Stopping vsftpd
17.2.5. vsftpd Configuration Options
17.2.6. Additional Resources
17.3. Printer Configuration
17.3.1. Starting the Printer Configuration Tool
17.3.2. Starting Printer Setup
17.3.3. Adding a Local Printer
17.3.4. Adding an AppSocket/HP JetDirect printer
17.3.5. Adding an IPP Printer
17.3.6. Adding an LPD/LPR Host or Printer
17.3.7. Adding a Samba (SMB) printer
17.3.8. Selecting the Printer Model and Finishing
17.3.9. Printing a test page
17.3.10. Modifying Existing Printers
17.3.11. Additional Resources
VI. Monitoring and Automation
18. System Monitoring Tools
18.1. Viewing System Processes
18.1.1. Using the ps Command
18.1.2. Using the top Command
18.1.3. Using the System Monitor Tool
18.2. Viewing Memory Usage
18.2.1. Using the free Command
18.2.2. Using the System Monitor Tool
18.3. Viewing CPU Usage
18.3.1. Using the System Monitor Tool
18.4. Viewing Block Devices and File Systems
18.4.1. Using the lsblk Command
18.4.2. Using the blkid Command
18.4.3. Using the partx Command
18.4.4. Using the findmnt Command
18.4.5. Using the df Command
18.4.6. Using the du Command
18.4.7. Using the System Monitor Tool
18.5. Viewing Hardware Information
18.5.1. Using the lspci Command
18.5.2. Using the lsusb Command
18.5.3. Using the lspcmcia Command
18.5.4. Using the lscpu Command
18.6. Monitoring Performance with Net-SNMP
18.6.1. Installing Net-SNMP
18.6.2. Running the Net-SNMP Daemon
18.6.3. Configuring Net-SNMP
18.6.4. Retrieving Performance Data over SNMP
18.6.5. Extending Net-SNMP
18.7. Additional Resources
18.7.1. Installed Documentation
19. Viewing and Managing Log Files
19.1. Configuring rsyslog
19.1.1. Global Directives
19.1.2. Modules
19.1.3. Rules
19.1.4. rsyslog Command Line Configuration
19.2. Locating Log Files
19.2.1. Configuring logrotate
19.3. Viewing Log Files
19.4. Adding a Log File
19.5. Monitoring Log Files
19.6. Additional Resources
19.6.1. Installed Documentation
19.6.2. Useful Websites
20. Automating System Tasks
20.1. Cron and Anacron
20.1.1. Starting and Stopping the Service
20.1.2. Configuring Anacron Jobs
20.1.3. Configuring Cron Jobs
20.1.4. Controlling Access to Cron
20.1.5. Black/White Listing of Cron Jobs
20.2. At and Batch
20.2.1. Configuring At Jobs
20.2.2. Configuring Batch Jobs
20.2.3. Viewing Pending Jobs
20.2.4. Additional Command Line Options
20.2.5. Controlling Access to At and Batch
20.2.6. Starting and Stopping the Service
20.3. Additional Resources
20.3.1. Installed Documentation
21. Automatic Bug Reporting Tool (ABRT)
21.1. Overview
21.2. Installing ABRT and Starting its Services
21.3. Running ABRT
21.3.1. Using the Graphical User Interface
21.3.2. Using the Command Line Interface
21.4. Configuring ABRT
21.4.1. ABRT Events
21.4.2. Standard ABRT Installation Supported Events
21.4.3. Event Configuration in ABRT GUI
21.4.4. ABRT Specific Configuration
21.4.5. Configuring Automatic Reporting
21.4.6. Uploading and reporting using a proxy server
21.5. Configuring Centralized Crash Collection
21.5.1. Configuration Steps Required on a Dedicated System
21.5.2. Configuration Steps Required on a Client System
21.5.3. Saving Package Information
21.5.4. Testing ABRT's Crash Detection
22. OProfile
22.1. Overview of Tools
22.2. Configuring OProfile
22.2.1. Specifying the Kernel
22.2.2. Setting Events to Monitor
22.2.3. Separating Kernel and User-space Profiles
22.3. Starting and Stopping OProfile
22.4. Saving Data
22.5. Analyzing the Data
22.5.1. Using opreport
22.5.2. Using opreport on a Single Executable
22.5.3. Getting more detailed output on the modules
22.5.4. Using opannotate
22.6. Understanding /dev/oprofile/
22.7. Example Usage
22.8. OProfile Support for Java
22.8.1. Profiling Java Code
22.9. Graphical Interface
22.10. OProfile and SystemTap
22.11. Additional Resources
22.11.1. Installed Docs
22.11.2. Useful Websites
VII. Kernel, Module and Driver Configuration
23. Manually Upgrading the Kernel
23.1. Overview of Kernel Packages
23.2. Preparing to Upgrade
23.3. Downloading the Upgraded Kernel
23.4. Performing the Upgrade
23.5. Verifying the Initial RAM Disk Image
23.6. Verifying the Boot Loader
23.6.1. Configuring the GRUB 2 Boot Loader
23.6.2. Configuring the OS/400 Boot Loader
23.6.3. Configuring the YABOOT Boot Loader
24. Working with Kernel Modules
24.1. Listing Currently-Loaded Modules
24.2. Displaying Information About a Module
24.3. Loading a Module
24.4. Unloading a Module
24.5. Setting Module Parameters
24.6. Persistent Module Loading
24.7. Specific Kernel Module Capabilities
24.7.1. Using Multiple Ethernet Cards
24.7.2. Using Channel Bonding
24.8. Additional Resources
24.8.1. Installed Documentation
24.8.2. Useful Websites
25. The kdump Crash Recovery Service
25.1. Installing the kdump Service
25.2. Configuring the kdump Service
25.2.1. Configuring the kdump at First Boot
25.2.2. Using the Kernel Dump Configuration Utility
25.2.3. Configuring kdump on the Command Line
25.2.4. Testing the Configuration
25.3. Analyzing the Core Dump
25.3.1. Running the crash Utility
25.3.2. Displaying the Message Buffer
25.3.3. Displaying a Backtrace
25.3.4. Displaying a Process Status
25.3.5. Displaying Virtual Memory Information
25.3.6. Displaying Open Files
25.3.7. Exiting the Utility
25.4. Additional Resources
25.4.1. Installed Documentation
25.4.2. Useful Websites
A. Consistent Network Device Naming
A.1. System Requirements
A.2. Enabling and Disabling the Feature
A.3. Notes for Administrators
B. RPM
B.1. RPM Design Goals
B.2. Using RPM
B.2.1. Finding RPM Packages
B.2.2. Installing and Upgrading
B.2.3. Configuration File Changes
B.2.4. Uninstalling
B.2.5. Freshening
B.2.6. Querying
B.2.7. Verifying
B.3. Checking a Package's Signature
B.3.1. Importing Keys
B.3.2. Verifying Signature of Packages
B.4. Practical and Common Examples of RPM Usage
B.5. Additional Resources
B.5.1. Installed Documentation
B.5.2. Useful Websites
B.5.3. Related Books
C. The X Window System
C.1. The X Server
C.2. Desktop Environments and Window Managers
C.2.1. Desktop Environments
C.2.2. Window Managers
C.3. X Server Configuration Files
C.3.1. The Structure of the Configuration
C.3.2. The xorg.conf.d Directory
C.3.3. The xorg.conf File
C.4. Fonts
C.4.1. Adding Fonts to Fontconfig
C.5. Runlevels and X
C.5.1. Runlevel 3
C.5.2. Runlevel 5
C.6. Additional Resources
C.6.1. Installed Documentation
C.6.2. Useful Websites
D. The sysconfig Directory
D.1. Files in the /etc/sysconfig/ Directory
D.1.1. /etc/sysconfig/arpwatch
D.1.2. /etc/sysconfig/authconfig
D.1.3. /etc/sysconfig/autofs
D.1.4. /etc/sysconfig/clock
D.1.5. /etc/sysconfig/dhcpd
D.1.6. /etc/sysconfig/firstboot
D.1.7. /etc/sysconfig/i18n
D.1.8. /etc/sysconfig/init
D.1.9. /etc/sysconfig/ip6tables-config
D.1.10. /etc/sysconfig/keyboard
D.1.11. /etc/sysconfig/ldap
D.1.12. /etc/sysconfig/named
D.1.13. /etc/sysconfig/network
D.1.14. /etc/sysconfig/quagga
D.1.15. /etc/sysconfig/radvd
D.1.16. /etc/sysconfig/samba
D.1.17. /etc/sysconfig/selinux
D.1.18. /etc/sysconfig/sendmail
D.1.19. /etc/sysconfig/spamassassin
D.1.20. /etc/sysconfig/squid
D.1.21. /etc/sysconfig/system-config-users
D.1.22. /etc/sysconfig/vncservers
D.1.23. /etc/sysconfig/xinetd
D.2. Directories in the /etc/sysconfig/ Directory
D.3. Additional Resources
D.3.1. Installed Documentation
E. The proc File System
E.1. A Virtual File System
E.1.1. Viewing Virtual Files
E.1.2. Changing Virtual Files
E.2. Top-level Files within the proc File System
E.2.1. /proc/buddyinfo
E.2.2. /proc/cmdline
E.2.3. /proc/cpuinfo
E.2.4. /proc/crypto
E.2.5. /proc/devices
E.2.6. /proc/dma
E.2.7. /proc/execdomains
E.2.8. /proc/fb
E.2.9. /proc/filesystems
E.2.10. /proc/interrupts
E.2.11. /proc/iomem
E.2.12. /proc/ioports
E.2.13. /proc/kcore
E.2.14. /proc/kmsg
E.2.15. /proc/loadavg
E.2.16. /proc/locks
E.2.17. /proc/mdstat
E.2.18. /proc/meminfo
E.2.19. /proc/misc
E.2.20. /proc/modules
E.2.21. /proc/mounts
E.2.22. /proc/mtrr
E.2.23. /proc/partitions
E.2.24. /proc/slabinfo
E.2.25. /proc/stat
E.2.26. /proc/swaps
E.2.27. /proc/sysrq-trigger
E.2.28. /proc/uptime
E.2.29. /proc/version
E.3. Directories within /proc/
E.3.1. Process Directories
E.3.2. /proc/bus/
E.3.3. /proc/bus/pci
E.3.4. /proc/driver/
E.3.5. /proc/fs
E.3.6. /proc/irq/
E.3.7. /proc/net/
E.3.8. /proc/scsi/
E.3.9. /proc/sys/
E.3.10. /proc/sysvipc/
E.3.11. /proc/tty/
E.3.12. /proc/PID/
E.4. Using the sysctl Command
E.5. References
F. Revision History
Index

Preface

The System Administrator's Guide contains information on how to customize the Fedora 18 system to fit your needs. If you are looking for a comprehensive, task-oriented guide for configuring and customizing your system, this is the manual for you.
This manual discusses many intermediate topics such as the following:
  • Installing and managing packages using the graphical PackageKit and command line Yum package managers
  • Setting up a network—from establishing an Ethernet connection using NetworkManager to configuring channel bonding interfaces to increase server bandwidth
  • Configuring DHCP, BIND, Apache HTTP Server, Postfix, Sendmail and other enterprise-class servers and software
  • Gathering information about your system, including obtaining user-space crash data with the Automatic Bug Reporting Tool, and kernel-space crash data with kdump
  • Easily working with kernel modules and upgrading the kernel

1. Target Audience

The Deployment Guide assumes you have a basic understanding of the Fedora operating system. If you need help with the installation of this system, refer to the Fedora 18 Installation Guide.

2. How to Read this Book

This manual is divided into the following main categories:
Part I, “Basic System Configuration”
This part covers basic system administration tasks such as keyboard configuration, date and time configuration, installation and initial setup of an NTP server, and managing users and groups.
Chapter 1, Configuring the Language and Keyboard covers basic language and keyboard setup. Read this chapter if you need to configure the language of your desktop, change the keyboard layout, or add the keyboard layout indicator to the panel.
Chapter 2, Configuring the Date and Time covers the configuration of the system date and time. Read this chapter if you need to set or change the date and time.
Chapter 3, Configuring NTP Using the chrony Suite covers the installation and configuration of the chrony suite, a client and a server for the Network Time Protocol (NTP). Read this chapter if you need to configure the system to synchronize the clock with a remote NTP server, or set up an NTP server on this system.
Chapter 4, Managing Users and Groups covers the management of users and groups in a graphical user interface and on the command line. Read this chapter if you need to manage users and groups on your system, or enable password aging.
Part II, “Package Management”
This part describes how to manage software packages on Fedora using both Yum and the PackageKit suite of graphical package management tools.
Chapter 5, Yum describes the Yum package manager. Read this chapter for information how to search, install, update, and uninstall packages on the command line.
Chapter 6, PackageKit describes the PackageKit suite of graphical package management tools. Read this chapter for information how to search, install, update, and uninstall packages using a graphical user interface.
Part III, “Networking”
This part describes how to configure the network on Fedora.
Chapter 7, NetworkManager focuses on NetworkManager, a dynamic network control and configuration system that attempts to keep network devices and connections up and active when they are available. Read this chapter for information how to run the NetworkManager daemon, and how to interact with it using the corresponding applet for the notification area.
Chapter 8, Network Interfaces explores various interface configuration files, interface control scripts, and network function files located in the /etc/sysconfig/network-scripts/ directory. Read this chapter for information how to use these files to configure network interfaces.
Part IV, “Infrastructure Services”
This part provides information how to configure services and daemons, configure authentication, and enable remote logins.
Chapter 9, Services and Daemons covers the configuration of the services to be run when a system is started, and provides information on how to start, stop, and restart the services on the command line using the systemctl utility.
Chapter 10, Configuring Authentication describes how to configure user information retrieval from Lightweight Directory Access Protocol (LDAP), Network Information Service (NIS), and Winbind user account databases, and provides an introduction to the System Security Services Daemon (SSSD). Read this chapter if you need to configure authentication on your system.
Chapter 11, OpenSSH describes how to enable a remote login via the SSH protocol. It covers the configuration of the sshd service, as well as a basic usage of the ssh, scp, sftp client utilities. Read this chapter if you need a remote access to a machine.
Part V, “Servers”
This part discusses various topics related to servers such as how to set up a Web server or share files and directories over the network.
Chapter 12, DHCP Servers guides you through the installation of a Dynamic Host Configuration Protocol (DHCP) server and client. Read this chapter if you need to configure DHCP on your system.
Chapter 13, DNS Servers introduces you to Domain Name System (DNS), explains how to install, configure, run, and administer the BIND DNS server. Read this chapter if you need to configure a DNS server on your system.
Chapter 14, Web Servers focuses on the Apache HTTP Server 2.2, a robust, full-featured open source web server developed by the Apache Software Foundation. Read this chapter if you need to configure a web server on your system.
Chapter 15, Mail Servers reviews modern email protocols in use today, and some of the programs designed to send and receive email, including Postfix, Sendmail, Fetchmail, and Procmail. Read this chapter if you need to configure a mail server on your system.
Chapter 16, Directory Servers covers the installation and configuration of OpenLDAP 2.4, an open source implementation of the LDAPv2 and LDAPv3 protocols. Read this chapter if you need to configure a directory server on your system.
Chapter 17, File and Print Servers guides you through the installation and configuration of Samba, an open source implementation of the Server Message Block (SMB) protocol, and vsftpd, the primary FTP server shipped with Fedora. Additionally, it explains how to use the Printer Configuration tool to configure printers. Read this chapter if you need to configure a file or print server on your system.
Part VI, “Monitoring and Automation”
This part describes various tools that allow system administrators to monitor system performance, automate system tasks, and report bugs.
Chapter 18, System Monitoring Tools discusses applications and commands that can be used to retrieve important information about the system. Read this chapter to learn how to gather essential system information.
Chapter 19, Viewing and Managing Log Files describes the configuration of the rsyslog daemon, and explains how to locate, view, and monitor log files. Read this chapter to learn how to work with log files.
Chapter 20, Automating System Tasks provides an overview of the cron, at, and batch utilities. Read this chapter to learn how to use these utilities to perform automated tasks.
Chapter 21, Automatic Bug Reporting Tool (ABRT) concentrates on ABRT, a system service and a set of tools to collect crash data and send a report to the relevant issue tracker. Read this chapter to learn how to use ABRT on your system.
Chapter 22, OProfile covers OProfile, a low overhead, system-wide performance monitoring tool. Read this chapter for information how to use OProfile on your system.
Part VII, “Kernel, Module and Driver Configuration”
This part covers various tools that assist administrators with kernel customization.
Chapter 23, Manually Upgrading the Kernel provides important information how to manually update a kernel package using the rpm command instead of yum. Read this chapter if you cannot update a kernel package with the Yum package manager.
Chapter 24, Working with Kernel Modules explains how to display, query, load, and unload kernel modules and their dependencies, and how to set module parameters. Additionally, it covers specific kernel module capabilities such as using multiple Ethernet cards and using channel bonding. Read this chapter if you need to work with kernel modules.
Chapter 25, The kdump Crash Recovery Service explains how to configure, test, and use the kdump service in Fedora, and provides a brief overview of how to analyze the resulting core dump using the crash debugging utility. Read this chapter to learn how to enable kdump on your system.
Appendix A, Consistent Network Device Naming
This appendix covers consistent network device naming for network interfaces, a feature that changes the name of network interfaces on a system in order to make locating and differentiating the interfaces easier. Read this appendix to learn more about this feature and how to enable or disable it.
Appendix B, RPM
This appendix concentrates on the RPM Package Manager (RPM), an open packaging system used by Fedora, and the use of the rpm utility. Read this appendix if you need to use rpm instead of yum.
Appendix C, The X Window System
This appendix covers the configuration of the X Window System, the graphical environment used by Fedora. Read this appendix if you need to adjust the configuration of your X Window System.
Appendix D, The sysconfig Directory
This appendix outlines some of the files and directories located in the /etc/sysconfig/ directory. Read this appendix if you want to learn more about these files and directories, their function, and their contents.
Appendix E, The proc File System
This appendix explains the concept of a virtual file system, and describes some of the top-level files and directories within the proc file system (that is, the /proc/ directory). Read this appendix if you want to learn more about this file system.

3. Document Conventions

This manual uses several conventions to highlight certain words and phrases and draw attention to specific pieces of information.
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts set. The Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not, alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes the Liberation Fonts set by default.

3.1. Typographic Conventions

Four typographic conventions are used to call attention to specific words and phrases. These conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight keycaps and key combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current working directory, enter the cat my_next_bestselling_novel command at the shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a keycap, all presented in mono-spaced bold and all distinguishable thanks to context.
Key combinations can be distinguished from keycaps by the hyphen connecting each part of a key combination. For example:
Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to the first virtual terminal. Press Ctrl+Alt+F1 to return to your X-Windows session.
The first paragraph highlights the particular keycap to press. The second highlights two key combinations (each a set of three keycaps with each set pressed simultaneously).
If source code is discussed, class names, methods, functions, variable names and returned values mentioned within a paragraph will be presented as above, in mono-spaced bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialog box text; labeled buttons; check-box and radio button labels; menu titles and sub-menu titles. For example:
Choose SystemPreferencesMouse from the main menu bar to launch Mouse Preferences. In the Buttons tab, click the Left-handed mouse check box and click Close to switch the primary mouse button from the left to the right (making the mouse suitable for use in the left hand).
To insert a special character into a gedit file, choose ApplicationsAccessoriesCharacter Map from the main menu bar. Next, choose SearchFind… from the Character Map menu bar, type the name of the character in the Search field and click Next. The character you sought will be highlighted in the Character Table. Double-click this highlighted character to place it in the Text to copy field and then click the Copy button. Now switch back to your document and choose EditPaste from the gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific menu names; and buttons and text found within a GUI interface, all presented in proportional bold and all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or variable text. Italics denotes text you do not input literally or displayed text that changes depending on circumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name at a shell prompt. If the remote machine is example.com and your username on that machine is john, type ssh john@example.com.
The mount -o remount file-system command remounts the named file system. For example, to remount the /home file system, the command is mount -o remount /home.
To see the version of a currently installed package, use the rpm -q package command. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version and release. Each word is a placeholder, either for text you enter when issuing a command or for text displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and important term. For example:
Publican is a DocBook publishing system.

3.2. Pull-quote Conventions

Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and presented thus:
books        Desktop   documentation  drafts  mss    photos   stuff  svn
books_tests  Desktop1  downloads      images  notes  scripts  svgs
Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows:
package org.jboss.book.jca.ex1;

import javax.naming.InitialContext;

public class ExClient
{
   public static void main(String args[]) 
       throws Exception
   {
      InitialContext iniCtx = new InitialContext();
      Object         ref    = iniCtx.lookup("EchoBean");
      EchoHome       home   = (EchoHome) ref;
      Echo           echo   = home.create();

      System.out.println("Created Echo");

      System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
   }
}

3.3. Notes and Warnings

Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.

Note

Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should have no negative consequences, but you might miss out on a trick that makes your life easier.

Important

Important boxes detail things that are easily missed: configuration changes that only apply to the current session, or services that need restarting before an update will apply. Ignoring a box labeled 'Important' will not cause data loss but may cause irritation and frustration.

Warning

Warnings should not be ignored. Ignoring warnings will most likely cause data loss.

4. We Need Feedback!

If you find a typographical error in this manual, or if you have thought of a way to make this manual better, we would love to hear from you! Please submit a report in Bugzilla: https://bugzilla.redhat.com/enter_bug.cgi?product=Fedora Documentation&component=system-administrator's-guide
If you have a suggestion for improving the documentation, try to be as specific as possible when describing it. If you have found an error, please include the section number and some of the surrounding text so we can find it easily.

5. Acknowledgments

Certain portions of this text first appeared in the Deployment Guide, copyright © 2007 Red Hat, Inc., available at http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/Deployment_Guide/index.html.
Section 18.6, “Monitoring Performance with Net-SNMP” is based on an article written by Michael Solberg.
The authors of this book would like to thank the following people for their valuable contributions: Adam Tkáč, Andrew Fitzsimon, Andrius Benokraitis, Brian Cleary Edward Bailey, Garrett LeSage, Jeffrey Fearn, Joe Orton, Joshua Wulf, Karsten Wade, Lucy Ringland, Marcela Mašláňová, Mark Johnson, Michael Behm, Miroslav Lichvár, Radek Vokál, Rahul Kavalapara, Rahul Sundaram, Sandra Moore, Zbyšek Mráz, Jan Včelák, Peter Hutterer, T.C. Hollingsworth, and James Antill, among many others.

Part I. Basic System Configuration

This part covers basic system administration tasks such as keyboard configuration, date and time configuration, installation and initial setup of an NTP server, and managing users and groups.

Table of Contents

1. Configuring the Language and Keyboard
1.1. Changing the Language
1.2. Changing the Date, Time, and Numeric Format
1.3. Changing the Keyboard Layout
1.4. Viewing the Current Configuration
2. Configuring the Date and Time
2.1. Using the Date and Time Configuration Tool
2.2. Using the Command Line Tools
2.2.1. Changing the Date
2.3. Additional Resources
2.3.1. Installed Documentation
3. Configuring NTP Using the chrony Suite
3.1. Introduction to the chrony Suite
3.1.1. Differences Between ntpd and chronyd
3.1.2. Choosing Between NTP Daemons
3.2. Understanding chrony and Its Configuration
3.2.1. Understanding chronyd
3.2.2. Understanding chronyc
3.2.3. Understanding the chrony Configuration Commands
3.2.4. Security with chronyc
3.3. Using chrony
3.3.1. Checking if chrony is Installed
3.3.2. Installing chrony
3.3.3. Checking the Status of chronyd
3.3.4. Starting chronyd
3.3.5. Stopping chronyd
3.3.6. Checking if chrony is Synchronized
3.3.7. Manualy Adjusting the System Clock
3.4. Setting Up chrony for Different Environments
3.4.1. Setting Up chrony for a System Which is Infrequently Connected
3.4.2. Setting Up chrony for a System in an Isolated Network
3.5. Using chronyc
3.5.1. Using chronyc to Control cronyd
3.5.2. Using chronyc for Remote Administration
3.6. Additional Resources
3.6.1. Installed Documentation
3.6.2. Useful Websites
4. Managing Users and Groups
4.1. Introduction to Users and Groups
4.1.1. User Private Groups
4.1.2. Shadow Passwords
4.2. Using the User Accounts Tool
4.2.1. Configuring an Account
4.2.2. Adding a New User
4.2.3. Removing a User
4.3. Using the User Manager Tool
4.3.1. Viewing Users and Groups
4.3.2. Adding a New User
4.3.3. Adding a New Group
4.3.4. Modifying User Properties
4.3.5. Modifying Group Properties
4.4. Using Command Line Tools
4.4.1. Adding a New User
4.4.2. Adding a New Group
4.4.3. Enabling Password Aging
4.4.4. Enabling Automatic Logouts
4.4.5. Creating Group Directories
4.5. Additional Resources
4.5.1. Installed Documentation

Chapter 1. Configuring the Language and Keyboard

Fedora 18 is shipped with the Region and Language configuration tool, which allows you to configure keyboard layouts, the language of your desktop environment, and other regional settings. To start the tool, open the System Settings window by selecting ApplicationsSystem ToolsSystem Settings from the Activities menu, and click Region and Language.

1.1. Changing the Language

To configure the language of your desktop, select the Language tab of the Region and Language application. You will be presented with a short list of common languages.
Changing the language
Changing the language
Figure 1.1. Changing the language

By default, this list only contains a few of the available languages. To add another language, click the + (the plus sign) button below the list. A dialog window appears, allowing you to select the desired language. The input field at the bottom part of the dialog window allows you to reduce the number of displayed items by first few letters part of the language name in it (for example, slov for the Slovak language). Once you select a language, click the Select button to confirm your choice.
Adding another language
Adding a language
Figure 1.2. Adding another language

To choose a particular language from the list, click its name to select it. The changes will take effect the next time you log in to the system.

1.2. Changing the Date, Time, and Numeric Format

To change the default date, time, number, and currency format, select the Formats tab of the Region and Language application. You will be presented with a short list of available formats.
Changing the date, time, and numeric format
Changing the date, time, and numeric format
Figure 1.3. Changing the date, time, and numeric format

By default, this list only contains a few of the available formats. To add another format, click the + (the plus sign) button below the list. A dialog window appears, allowing you to select the desired format according to a region. The input field at the bottom part of the dialog window allows you to reduce the number of displayed items by typing first few letters of the region name in it (for example, slov for Slovakia). Once you select a region, click the Select button to confirm your choice.
Adding a format
Adding a format
Figure 1.4. Adding a format

To choose a particular format from the list, click its name to select it. The changes will take effect the next time you log in to the system.

1.3. Changing the Keyboard Layout

Although the installation program allows a system administrator to configure a keyboard layout during the system installation, the default settings may not always suit your current needs. To change the default keyboard layout, select the Layouts tab of the Region and Language application. You will be presented with a list of currently enabled layouts.
Changing the keyboard layout
Changing the keyboard layout
Figure 1.5. Changing the keyboard layout

To add a layout to the list, click the + (the plus sign) button below the list. A dialog window appears, allowing you to select the desired keyboard layout. The input field at the bottom part of the dialog window allows you to reduce the number of displayed items by typing first few letters of the layout name in it (for example, slov for a Slovak layout). Once you select a layout, click the Add button to confirm your choice.
Adding a keyboard layout
Adding a keyboard layout
Figure 1.6. Adding a keyboard layout

The first layout in the list is always considered the default. To move a particular layout up or down in the list, select it and click the (the upwards arrow) or (the downwards arrow) buttons respectively. To remove a layout, click the (that is, the minus sign) button. Additionally, by selecting an option button on the right side of the window, you can choose if you want to use different keyboard layouts for individual windows, or a single layout for all windows.
When more than one layout is enabled, a keyboard indicator appears on the panel in order to allow you to switch between the layouts.
The keyboard layout indicator
The keyboard layout indicator
Figure 1.7. The keyboard layout indicator

1.4. Viewing the Current Configuration

To view the current configuration, select the System tab of the Region and Language application. You will be presented with a comparison of your own configuration and system-wide settings.
Viewing the current configuration
Viewing the current configuration
Figure 1.8. Viewing the current configuration

Chapter 2. Configuring the Date and Time

This chapter covers setting the system date and time in Fedora, both manually and using the Network Time Protocol (NTP), as well as setting the adequate time zone. Two methods are covered: setting the date and time using the Date and Time configuration tool, and doing so on the command line.

2.1. Using the Date and Time Configuration Tool

Fedora 18 is shipped with the Date and Time configuration tool, which allows you to change the date and time of the system, to configure the time zone used by the system, and to set up the Network Time Protocol daemon to synchronize the system clock with a time server. To start the tool, either select ApplicationsSystem ToolsSystem Settings from the Activities menu and click the Date and Time icon, or click the time in the panel and select Date and Time Settings from the drop-down menu.
The Date and Time configuration tool
The Date and Time configuration tool
Figure 2.1. The Date and Time configuration tool

By default, the tool only allows you to review the current settings. This is because only root is allowed to set the system date and time. To unlock the configuration tool for changes, click the Unlock button in the top-right corner of the window, and provide the correct password when prompted.
To change the current time of your system, either configure the system to synchronize it over the network by clicking the Network Time switch, or set it manually by clicking the up and down arrows above and below the numbers. You can also select 24-hour or AM/PM to enable or disable the 24-hour time format.
To change the time zone, either click on the map, or select the region and city from the Region and City drop-down lists.
To change the current date of your system, select a month from the drop-down list below the time, and use the up and down arrows to choose the day and year.
The changes take effect immediately.

2.2. Using the Command Line Tools

Fedora 18 provides command line tools that allow you to configure the date and time both manually and using the NTP protocol.

2.2.1. Changing the Date

To change the system date, type the following at a shell prompt as root:
date +%D -s YYYY-MM-DD
…where YYYY is a four-digit year, MM is a two-digit month, and DD is a two-digit day of the month. For example, to change the date to 2 June 2010, type:
~]# date +%D -s 2010-06-02
You can verify the current settings by running date without any additional argument.

2.3. Additional Resources

For more information about the date and time configuration, refer to the following resources.

2.3.1. Installed Documentation

  • date(1) — The manual page for the date utility.

Chapter 3. Configuring NTP Using the chrony Suite

Accurate time keeping is important for a number of reasons in IT. In networking for example, accurate time stamps in packets and logs are required. In Linux systems, the NTP protocol is implemented by a daemon running in user space.
The user space daemon updates the system clock running in the kernel. The system clock can keep time by using various clock sources. Usually, the Time Stamp Counter (TSC) is used. The TSC is a CPU register which counts the number of cycles since it was last reset. It is very fast, has a high resolution, and there are no interrupts.
There is a choice between the daemons ntpd and chronyd, which are available from the repos in the ntp and chrony packages respectively. This section describes the use of the chrony suite of utilities to update the daemon on systems that do not fit into the conventional permanently networked, always on, dedicated server category.

3.1. Introduction to the chrony Suite

Chrony consists of chronyd, a daemon that runs in user space, and chronyc, a command line program for making adjustments to chronyd. Systems which are not permanently connected, or not permanently powered up, take a relatively long time to adjust their system clocks using the NTP time protocol. This is because many small corrections are made based on observations of the clocks drift and offset. Temperature changes, which may be significant when powering up a system, affect the stability of hardware clocks. Although adjustments begin within a few milliseconds of booting a system, acceptable accuracy may take anything from ten seconds from a warm restart to a number of hours depending on your requirements, operating environment and hardware. chrony is a different implementation of the NTP protocol than ntpd, it can adjust the system clock more rapidly.

3.1.1. Differences Between ntpd and chronyd

One of the main differences between ntpd and chronyd is in the algorithms used to control the computer's clock. Things chronyd can do better than ntpd are:
  • chronyd can work well when external time references are only intermittently accessible whereas ntpd needs regular polling of time reference to work well.
  • chronyd can perform well even when the network is congested for longer periods of time.
  • chronyd can usually synchronise the clock faster and with better accuracy.
  • chronyd quickly adapts to sudden changes in the rate of the clock, for example, due to changes in the temperature of the crystal oscillator, whereas ntpd may need a long time to settle down again.
  • chronyd in the default configuration never steps the time, in order not to upset other running programs. ntpd can be configured to never step the time too, but it has to use a different means of adjusting the clock, which has some disadvantages.
  • chronyd can adjust the rate of the clock on a Linux system in a larger range, which allows it to operate even on machines with a broken or unstable clock. For example, on some virtual machines.
Things chronyd can do that ntpd cannot do:
  • chronyd provides support for isolated networks where the only method of time correction is manual entry. For example, by the administrator looking at a clock. chronyd can look at the errors corrected at different updates to estimate the rate at which the computer gains or loses time, and use this estimate to trim the computer clock subsequently.
  • chronyd provides support to work out the rate of gain or loss of the real-time clock, the hardware clock, that maintains the time when the computer is turned off. It can use this data when the system boots to set the system time using an adjusted value of the time taken from the real-time clock. This is, at time of writing, only available in Linux.
Things ntpd can do that chronyd cannot do:
  • ntpd fully supports NTP version 4 (RFC5905), including broadcast, multicast, manycast clients and servers, and the orphan mode. It also supports extra authentication schemes based on public-key cryptography (RFC5906). chronyd uses NTP version 3 (RFC1305), which is compatible with version 4.
  • ntpd includes drivers for many reference clocks whereas chronyd relies on other programs, for example gpsd, to access the data from the reference clocks.

3.1.2. Choosing Between NTP Daemons

  • Chrony should be considered for all systems which are frequently suspended or otherwise intermittently disconnected and reconnected to a network. Mobile and virtual systems for example.
  • The NTP daemon (ntpd) should be considered for systems which are normally kept permanently on. Systems which are required to use broadcast or multicast IP, or to perform authentication of packets with the Autokey protocol, should consider using ntpd. Chrony only supports symmetric key authentication, such as the use of MD5, SHA1, and SHA256 hash functions, whereas ntpd supports the Autokey authentication protocol which can make use of the PKI system. Autokey is described in RFC5906.

3.2. Understanding chrony and Its Configuration

3.2.1. Understanding chronyd

The chrony daemon, chronyd, running in user space, makes adjustments to the system clock which is running in the kernel. It does this by consulting external time sources, using the NTP protocol, when ever network access allows it to do so. When external references are not available, chronyd will use the last calculated drift stored in the drift file. It can also be commanded manually to make corrections, by chronyc.

3.2.2. Understanding chronyc

The chrony daemon, chronyd, can be controlled by the command line utility chronyc. This utility provides a command prompt which allows entering of a number of commands to make changes to chronyd. The default configuration is for chronyd to only accept commands from a local instance of chronyc, but chronyc can be used to alter the configuration so that chronyd will allow external control. That is to say, chronyc can be run remotely after first configuring chronyd to accept remote connections. The IP addresses allowed to connect to chronyd should be tightly controlled.

3.2.3. Understanding the chrony Configuration Commands

chronyd's default config file is /etc/chrony.conf. The -f option can be used to specify an alternate config file path. Refer to the chronyd man page for further options. We present here a selection of configuration options. For a complete list of the directives that can be used see http://chrony.tuxfamily.org/manual.html#Configuration-file
Comments
Comments should be preceded by #, %, ; or !
allow
Optionally specify a host, subnet, or network from which to allow NTP connections to a machine acting as NTP server. The default is not to allow connections.
Examples:
  1. allow server1.example.com
    Use this form to specify a particular host, by its host name, to be allowed access.
  2. allow 192.0.2.0/24
    Use this form to specify a particular network to be allowed access.
  3. allow 2001:db8::/32
    Use this form to specify an IPv6 address to be allowed access.
cmdallow
This is similar to the allow directive (see section allow), except that it allows control access (rather than NTP client access) to a particular subnet or host. (By control access is meant that chronyc can be run on those hosts and successfully connect to chronyd on this computer.) The syntax is identical. There is also a cmddeny all directive with similar behaviour to the cmdallow all directive.
dumpdir
Path to the directory to save the measurement history across restarts of chronyd (assuming no changes are made to the system clock behaviour whilst it is not running). If this capability is to be used (via the dumponexit command in the configuration file, or the dump command in chronyc), the dumpdir command should be used to define the directory where the measurement histories are saved.
dumponexit
If this command is present, it indicates that chronyd should save the measurement history for each of its time sources recorded whenever the program exits. (See the dumpdir command above)
local
The local keyword is used to allow chronyd to appear synchronized to real time (from the viewpoint of clients polling it), even if it has no current synchronization source. This option is normally used on computers in an isolated network, where several computers are required to synchronize to one other, this being the master which is kept vaguely in line with real time by manual input.
An example of the command is:
local stratum 10
A large value of 10 indicates that the clock is so many hops away from a reference clock that its time is fairly unreliable. Put another way, if the computer ever has access to another computer which is ultimately synchronized to a reference clock, it will almost certainly be at a stratum less than 10. Therefore, the choice of a high value like 10 for the local command prevents the machine’s own time from ever being confused with real time, were it ever to leak out to clients that have visibility of real servers.
log
The log command indicates that certain information is to be logged. It accepts the following options:
measurements
This option logs the raw NTP measurements and related information to a file called measurements.log.
statistics
This option logs information about the regression processing to a file called statistics.log.
tracking
This option logs changes to the estimate of the system’s gain or loss rate, and any slews made, to a file called tracking.log.
rtc
This option logs information about the system’s real-time clock.
refclocks
This option logs the raw and filtered reference clock measurements to a file called refclocks.log.
tempcomp
This option logs the temperature measurements and system rate compensations to a file called tempcomp.log.
refclocks
This option logs the raw and filtered reference clock measurements to a file called refclocks.log.
refclocks
This option logs the raw and filtered reference clock measurements to a file called refclocks.log.
The files are written to the directory specified by the logdir command. An example of the command is:
log measurements statistics tracking
logdir
This directive allows the directory where log files are written to be specified. An example of the use of this directive is:
logdir /var/log/chrony
makestep
Normally chronyd will cause the system to gradually correct any time offset, by slowing down or speeding up the clock as required. In certain situations, the system clock may be so far adrift that this slewing process would take a very long time to correct the system clock. This directive forces chronyd to step system clock if the adjustment is larger than a threshold value, but only if there were no more clock updates since chronyd was started than a specified limit (a negative value can be used to disable the limit). This is particularly useful when using reference clocks, because the initstepslew directive only works with NTP sources.
An example of the use of this directive is:
makestep 1000 10
This would step the system clock if the adjustment is larger than 1000 seconds, but only in the first ten clock updates.
maxchange
This directive sets the maximum allowed offset corrected on a clock update. The check is performed only after the specified number of updates to allow a large initial adjustment of the system clock. When an offset larger than the specified maximum occurs, it will be ignored for the specified number of times and then chronyd will give up and exit (a negative value can be used to never exit). In both cases a message is sent to syslog.
An example of the use of this directive is:
maxchange 1000 1 2
After the first clock update, chronyd will check the offset on every clock update, it will ignore two adjustments larger than 1000 seconds and exit on another one.
maxupdateskew
One of chronyd's tasks is to work out how fast or slow the computer’s clock runs relative to its reference sources. In addition, it computes an estimate of the error bounds around the estimated value. If the range of error is too large, it probably indicates that the measurements have not settled down yet, and that the estimated gain or loss rate is not very reliable. The maxupdateskew parameter allows the threshold for determining whether an estimate may be so unreliable that it should not be used. By default, the threshold is 1000 ppm. The format of the syntax is:
maxupdateskew skew-in-ppm
Typical values for skew-in-ppm might be 100 for a dial-up connection to servers over a telephone line, and 5 or 10 for a computer on a LAN. It should be noted that this is not the only means of protection against using unreliable estimates. At all times, chronyd keeps track of both the estimated gain or loss rate, and the error bound on the estimate. When a new estimate is generated following another measurement from one of the sources, a weighted combination algorithm is used to update the master estimate. So if chronyd has an existing highly-reliable master estimate and a new estimate is generated which has large error bounds, the existing master estimate will dominate in the new master estimate.
noclientlog
This directive, which takes no arguments, specifies that client accesses are not to be logged. Normally they are logged, allowing statistics to be reported using the clients command in chronyc.
reselectdist
When chronyd selects synchronisation source from available sources, it will prefer the one with minimum synchronization distance. However, to avoid frequent reselecting when there are sources with similar distance, a fixed distance is added to the distance for sources that are currently not selected. This can be set with the reselectdist option. By default, the distance is 100 microseconds.
The format of the syntax is:
reselectdist dist-in-seconds
stratumweight
The stratumweight directive sets how much distance should be added per stratum to the synchronisation distance when chronyd selects the synchronisation source from available sources.
The format of the syntax is:
stratumweight dist-in-seconds
By default, dist-in-seconds is 1 second. This usually means that sources with lower stratum will be preferred to sources with higher stratum even when their distance is significantly worse. Setting stratumweight to 0 makes chronyd ignore stratum when selecting the source.
rtcfile
The rtcfile directive defines the name of the file in which chronyd can save parameters associated with tracking the accuracy of the system’s real-time clock (RTC). The format of the syntax is:
rtcfile /etc/chrony.rtc
chronyd saves information in this file when it exits and when the writertc command is issued in chronyc. The information saved is the RTC’s error at some epoch, that epoch (in seconds since January 1 1970), and the rate at which the RTC gains or loses time. Not all real-time clocks are supported as their code system-specific.

3.2.4. Security with chronyc

As access to chronyc allows changing chronyd just as editing the configuration files would, access to chronyc should be limited. Passwords or their hashes can be specified in the key file, to restrict the use of chronyc. One of the entries is used to restrict the use of operational commands and is referred to as the command key. In the default configuration, a random command key is generated automatically on start. It should not be necessary to specify or alter it manually.
Other entries in the key file can be used as NTP keys to authenticate packets received from remote NTP servers or peers. The two sides need to share a key with identical ID, hash type and password in their key file. This requires manually creating the keys and copying them over a secure medium, such as SSH. If the key ID was, for example, 10 then the systems that act as clients must have a line in their configuration files in the following format:
server w.x.y.z key 10
peer w.x.y.z key 10
The location of the key file is specified in the /etc/chrony.conf file. The default entry in the configuration file is:
keyfile /etc/chrony.keys
The command key number is specified in /etc/chrony.conf using the commandkey directive, it is the key chronyd will use for authentication of user commands. The directive in the configuration file takes the following form:
commandkey 1
An example of the format of the default entry in the key file, /etc/chrony.keys, for the command key is:
1 SHA1 HEX:A6CFC50C9C93AB6E5A19754C246242FC5471BCDF
Where 1 is the key ID, SHA1 is the hash function to use, HEX is the format of the key, and A6CFC50C9C93AB6E5A19754C246242FC5471BCDF is the key randomly generated when chronyc was started for the first time. The key can be given in hexidecimal or ASCII format (the default).
A manual entry in the key file, used to authenticate packets from certain NTP servers or peers, can be as simple as the following:
20 foobar
Were 20 is the key ID and foobar is the secret authentication key. The default hash is MD5, and ASCII is the default format for the key.
By default, chronyd is configured to listen for commands only from localhost (127.0.0.1 and ::1). To access chronyd remotely with chronyc, any bindcmdaddress directives in the /etc/chrony.conf file should be removed to enable listening on all interfaces and the cmdallow directive should be used to allow commands from the remote IP address, network, or subnet. Note that the allow directive is for NTP access whereas the cmdallow directive is to enable the receiving of remote commands. It is possible to make these changes temporarily using chronyc running locally. Edit the configuration file to make persistent changes.
The communication between chronyc and chronyd is done over UDP, so it needs to be authorized before issuing operational commands. To authorize, use the authhash and password commands as follows:
chronyc> authhash SHA1
chronyc> password HEX:A6CFC50C9C93AB6E5A19754C246242FC5471BCDF
200 OK
Alternatively, you can use the chrony-helper wrapper, which will do that automatically. For example, to use the reselect command:
~]# /usr/libexec/chrony-helper command reselect
Only the following commands can be used without providing a password: activity , authhash , dns , exit , help , password , quit , rtcdata , sources , sourcestats , tracking , waitsync .

3.3. Using chrony

3.3.1. Checking if chrony is Installed

To check if chrony is installed, run the following command as root:
~]# yum install chrony
The default location for the chrony daemon is /usr/local/sbin/chronyd.

3.3.2. Installing chrony

To install chrony, run the following command as root:
~]# yum install chrony -y
The default installation directory is /usr/local/sbin/

3.3.3. Checking the Status of chronyd

To check the status of chronyd, issue the following command:
~]$ systemctl status chronyd
chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled)
   Active: active (running) since Wed 2013-06-12 22:23:16 CEST; 11h ago

3.3.4. Starting chronyd

To start chronyd, issue the following command as root:
~]# systemctl start chronyd
To ensure chronyd starts automatically at system start, issue the following command as root:
~]# systemctl enable chronyd

3.3.5. Stopping chronyd

To stop chronyd, issue the following command as root:
~]# systemctl stop chronyd
To prevent chronyd from starting automatically at system start, issue the following command as root:
~]# systemctl disable chronyd

3.3.6. Checking if chrony is Synchronized

To check if chrony is synchronized, make use of the tracking, sources, and sourcestats commands.

3.3.6.1. Checking chrony Tracking

To check chrony tracking, issue the following command:
~]$ chronyc tracking
Reference ID    : 1.2.3.4 (a.b.c)
Stratum         : 3
Ref time (UTC)  : Fri Feb  3 15:00:29 2012
System time     : 0.000001501 seconds slow of NTP time
Last offset     : -0.000001632 seconds
RMS offset      : 0.000002360 seconds
Frequency       : 331.898 ppm fast
Residual freq   : 0.004 ppm
Skew            : 0.154 ppm
Root delay      : 0.373169 seconds
Root dispersion : 0.024780 seconds
Update interval : 64.2 seconds
Leap status     : Normal
The fields are as follows:
Reference ID
This is the refid and name (or IP address) if available, of the server to which the computer is currently synchronised. If this is 127.127.1.1 it means the computer is not synchronised to any external source and that you have the ‘local’ mode operating (via the local command in chronyc (see section local), or the local directive in the ‘/etc/chrony.conf’ file (see section local)).
Stratum
The stratum indicates how many hops away from a computer with an attached reference clock we are. Such a computer is a stratum-1 computer, so the computer in the example is two hops away (i.e. a.b.c is a stratum-2 and is synchronised from a stratum-1).
Ref time
This is the time (UTC) at which the last measurement from the reference source was processed.
System time
In normal operation, chronyd never steps the system clock, because any jump in the timescale can have adverse consequences for certain application programs. Instead, any error in the system clock is corrected by slightly speeding up or slowing down the system clock until the error has been removed, and then returning to the system clock’s normal speed. A consequence of this is that there will be a period when the system clock (as read by other programs using the gettimeofday() system call, or by the date command in the shell) will be different from chronyd's estimate of the current true time (which it reports to NTP clients when it is operating in server mode). The value reported on this line is the difference due to this effect.
Last offset
This is the estimated local offset on the last clock update.
RMS offset
This is a long-term average of the offset value.
Frequency
The ‘frequency’ is the rate by which the system’s clock would be would be wrong if chronyd was not correcting it. It is expressed in ppm (parts per million). For example, a value of 1ppm would mean that when the system’s clock thinks it has advanced 1 second, it has actually advanced by 1.000001 seconds relative to true time.
Residual freq
This shows the ‘residual frequency’ for the currently selected reference source. This reflects any difference between what the measurements from the reference source indicate the frequency should be and the frequency currently being used. The reason this is not always zero is that a smoothing procedure is applied to the frequency. Each time a measurement from the reference source is obtained and a new residual frequency computed, the estimated accuracy of this residual is compared with the estimated accuracy (see ‘skew’ next) of the existing frequency value. A weighted average is computed for the new frequency, with weights depending on these accuracies. If the measurements from the reference source follow a consistent trend, the residual will be driven to zero over time.
Skew
This is the estimated error bound on the frequency.
Root delay
This is the total of the network path delays to the stratum-1 computer from which the computer is ultimately synchronised. In certain extreme situations, this value can be negative. (This can arise in a symmetric peer arrangement where the computers’ frequencies are not tracking each other and the network delay is very short relative to the turn-around time at each computer.)
Root dispersion
This is the total dispersion accumulated through all the computers back to the stratum-1 computer from which the computer is ultimately synchronised. Dispersion is due to system clock resolution, statistical measurement variations etc.
Leap status
This is the leap status, which can be Normal, Insert second, Delete second or Not synchronised.

3.3.6.2. Checking chrony Sources

The sources command displays information about the current time sources that chronyd is accessing. The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.
~]$ chronyc sources
	210 Number of sources = 3
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#* GPS0                          0   4   377    11   -479ns[ -621ns] +/-  134ns
^? a.b.c                         2   6   377    23   -923us[ -924us] +/-   43ms
^+ d.e.f                         1   6   377    21  -2629us[-2619us] +/-   86ms

The columns are as follows:
M
This indicates the mode of the source. ^ means a server, = means a peer and # indicates a locally connected reference clock.
S
This column indicates the state of the sources. * indicates the source to which chronyd is current synchronised. + indicates other acceptable sources. ? indicates sources to which connectivity has been lost. x indicates a clock which chronyd thinks is is a falseticker (i.e. its time is inconsistent with a majority of other sources). ~ indicates a source whose time appears to have too much variability. The ~ condition is also shown at start-up, until at least 3 samples have been gathered from it.
Name/IP address
This shows the name or the IP address of the source, or refid for reference clocks.
Stratum
This shows the stratum of the source, as reported in its most recently received sample. Stratum 1 indicates a computer with a locally attached reference clock. A computer that is synchronised to a stratum 1 computer is at stratum 2. A computer that is synchronised to a stratum 2 computer is at stratum 3, and so on.
Poll
This shows the rate at which the source is being polled, as a base-2 logarithm of the interval in seconds. Thus, a value of 6 would indicate that a measurement is being made every 64 seconds. chronyd automatically varies the polling rate in response to prevailing conditions.
Reach
This shows the source’s reachability register printed as octal number. The register has 8 bits and is updated on every received or missed packet from the source. A value of 377 indicates that a valid reply was received for all from the last eight transmissions.
LastRx
This column shows how long ago the last sample was received from the source. This is normally in seconds. The letters m, h, d or y indicate minutes, hours, days or years. A value of 10 years indicates there were no samples received from this source yet.
Last sample
This column shows the offset between the local clock and the source at the last measurement. The number in the square brackets shows the actual measured offset. This may be suffixed by ns (indicating nanoseconds), us (indicating microseconds), ms (indicating milliseconds), or s (indicating seconds). The number to the left of the square brackets shows the original measurement, adjusted to allow for any slews applied to the local clock since. The number following the +/- indicator shows the margin of error in the measurement. Positive offsets indicate that the local clock is fast of the source.

3.3.6.3. Checking chrony Source Statistics

The sourcestats command displays information about the drift rate and offset estimation process for each of the sources currently being examined by chronyd. The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.
~]$ chronyc sourcestats
       	

210 Number of sources = 1
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
===============================================================================
abc.def.ghi
The columns are as follows:
Name/IP address
This is the name or IP address of the NTP server (or peer) or refid of the refclock to which the rest of the line relates.
NP
This is the number of sample points currently being retained for the server. The drift rate and current offset are estimated by performing a linear regression through these points.
NR
This is the number of runs of residuals having the same sign following the last regression. If this number starts to become too small relative to the number of samples, it indicates that a straight line is no longer a good fit to the data. If the number of runs is too low, chronyd discards older samples and re-runs the regression until the number of runs becomes acceptable.
Span
This is the interval between the oldest and newest samples. If no unit is shown the value is in seconds. In the example, the interval is 46 minutes.
Frequency
This is the estimated residual frequency for the server, in parts per million. In this case, the computer’s clock is estimated to be running 1 part in 10**9 slow relative to the server.
Freq Skew
This is the estimated error bounds on Freq (again in parts per million).
Offset
This is the estimated offset of the source.
Std Dev
This is the estimated sample standard deviation.

3.3.7. Manualy Adjusting the System Clock

To update, or step, the system clock immediately, bypassing any adjustments in progress by slewing the clock, issue the following commands as root:
~]# chronyc
      chrony> password
      200 OK
      chrony> makestep
    200 OK
In Fedora, the real-time clock should not be manually adjusted as this would interfere with chrony's need to measure the rate at which the real-time clock drifts if it was adjusted at random intervals. By default, rtcsync will inform the kernel the system clock is synchronized and the kernel will update the real-time clock in 11 minute intervals.

3.4. Setting Up chrony for Different Environments

3.4.1. Setting Up chrony for a System Which is Infrequently Connected

This example is intended for systems which use dial-on-demand connections. The normal configuration should be sufficient for mobile and virtual devices which connect intermittently. First, review and confirm that the default settings in the /etc/chrony.conf are similar to the following:
driftfile /var/lib/chrony/drift
commandkey 1
keyfile /etc/chrony.keys
The command key ID is generated at install time and should correspond with the commandkey value in the key file, /etc/chrony.keys.
  1. Using your editor running as root, add the addresses of four NTP servers as follows:
     
    server 0.pool.ntp.org offline
    server 1.pool.ntp.org offline
    server 2.pool.ntp.org offline
    server 3.pool.ntp.org offline
    
    The offline option can be useful in preventing systems from trying to activate connections. The chrony daemon will wait for chronyc to inform it that the system is connected to the network or Internet.

3.4.1.1. Configuring chrony to Connect and Disconnect

Using your editor running as root, add the following to the /etc/ppp/ip-up file:
/usr/local/bin/chronyc
password commandkey-password
online
Using your editor running as root, add the following to the /etc/ppp/ip-down file:
/usr/local/bin/chronyc
password commandkey-password
offline
The chrony daemon, chronyd, will now only poll the NTP servers when the Point-To-Point link is activated.

3.4.2. Setting Up chrony for a System in an Isolated Network

For a network that is never connected to the Internet, one computer is selected to be the master timeserver. The other computers are either direct clients of the master, or clients of clients. On the master, the drift file must be manually set with the average rate of drift of the system clock. If the master is rebooted it will obtain the time from surrounding systems and take an average to set its system clock. Thereafter it resumes applying adjustments based on the drift file. The drift file will be updated automatically when the settime command is used.
On the system selected to be the master, using a text editor running as root, edit the /etc/chrony.conf as follows:
driftfile /var/lib/chrony/drift
commandkey 1
keyfile /etc/chrony.keys
initstepslew 10 client1 client3 client6
local stratum 8
manual
allow 192.0.2.0
Where 192.0.2.0 is the network or subnet address from which the clients are allowed to connect.
On the systems selected to be direct clients of the master, using a text editor running as root, edit the /etc/chrony.conf as follows:
server master
driftfile /var/lib/chrony/drift
logdir /var/log/chrony
log measurements statistics tracking
keyfile /etc/chrony.keys
commandkey 24
local stratum 10
initstepslew 20 master
allow 192.0.2.123
Where 192.0.2.123 is the address of the master, and master is the host name of the master. These clinet will resynchronize the master if it restarts.
On the client systems which are not to be direct clients of the master, the /etc/chrony.conf file should be the same except that the local and allow directives should be omitted.

3.5. Using chronyc

3.5.1. Using chronyc to Control cronyd

To make changes using the command line utility chronyc in interactive mode, enter the following command as root:
~]# chronyc
chronyc must run as root if some of the restricted commands are to be used.
The chronyc command prompt will be displayed as follows:
/chronyc>
You can type help to list all of the commands.
The utility can also be invoked in non-interactive command mode if called together with an option as follows:
~]# chronyc option

3.5.2. Using chronyc for Remote Administration

To configure chrony to connect to a remote instance of chronyd, issue a command as root in the following format:
~]# chronyc -h hostname
Where hostname is the hostname of a system running chronyd to connect to in order to allow remote administration from that host. The default is to connect to the daemon on the localhost.
To configure chrony to connect to a remote instance of chronyd on a non-default port, issue a command as root in the following format:
~]# chronyc -p port
Where port is the port in use for controlling and monitoring by the instance of chronyd to be connected to.
Note that commands issued at the chrony command prompt are not persistent. Only commands in the configuration file are persistent.
From the remote systems, the system administrator can issue commands after first using the password command at the chronyc command prompt as follows:
chronyc> password secretpasswordwithnospaces
200 OK
Alternatively, the remote system can use the chrony-helper wrapper, which will do that automatically. For example, to use the reselect command:
~]# /usr/libexec/chrony-helper command reselect
The password or hash associated with the command key for a remote system is best obtained by SSH. That is to say, an SSH connection should be established to the remote machine and the ID of the command key from /etc/chrony.conf and the command key in /etc/chrony.keys memorised or stored securly for the duration of the session.

3.6. Additional Resources

The following sources of information provide additional resources regarding chrony.

3.6.1. Installed Documentation

  • chrony(1) man page — Introduces the chrony daemon and the command-line interface tool.
  • chronyc(1) man page — Describes the chronyc command-line interface tool including commands and command options.
  • chronyd(1) man page — Describes the chronyd daemon including commands and command options.
  • chrony.conf(5) man page — Describes the chrony configuration file.
  • chrony.conf(5) man page — User guide for the chrony suite.
  • /usr/share/doc/chrony*/chrony.txt — User guide for the chrony suite.

3.6.2. Useful Websites

http://chrony.tuxfamily.org/manual.html
The on-line user guide for chrony.

Chapter 4. Managing Users and Groups

The control of users and groups is a core element of Fedora system administration. This chapter explains how to add, manage, and delete users and groups in the graphical user interface and on the command line, and covers advanced topics, such as enabling password aging or creating group directories.

4.1. Introduction to Users and Groups

While users can be either people (meaning accounts tied to physical users) or accounts which exist for specific applications to use, groups are logical expressions of organization, tying users together for a common purpose. Users within a group can read, write, or execute files owned by that group.
Each user is associated with a unique numerical identification number called a user ID (UID). Likewise, each group is associated with a group ID (GID). A user who creates a file is also the owner and group owner of that file. The file is assigned separate read, write, and execute permissions for the owner, the group, and everyone else. The file owner can be changed only by root, and access permissions can be changed by both the root user and file owner.
Additionally, Fedora supports access control lists (ACLs) for files and directories which allow permissions for specific users outside of the owner to be set. Refer to For more information about this feature, refer to the Access Control Lists chapter of the Storage Administration Guide.

4.1.1. User Private Groups

Fedora uses a user private group (UPG) scheme, which makes UNIX groups easier to manage. A user private group is created whenever a new user is added to the system. It has the same name as the user for which it was created and that user is the only member of the user private group.
User private groups make it safe to set default permissions for a newly created file or directory, allowing both the user and the group of that user to make modifications to the file or directory.
The setting which determines what permissions are applied to a newly created file or directory is called a umask and is configured in the /etc/bashrc file. Traditionally on UNIX systems, the umask is set to 022, which allows only the user who created the file or directory to make modifications. Under this scheme, all other users, including members of the creator's group, are not allowed to make any modifications. However, under the UPG scheme, this group protection is not necessary since every user has their own private group.

4.1.2. Shadow Passwords

Especially in environments with multiple users, it is very important to use shadow passwords provided by the shadow-utils package to enhance the security of system authentication files. For this reason, the installation program enables shadow passwords by default.
The following is a list of the advantages shadow passwords have over the traditional way of storing passwords on UNIX-based systems:
  • Shadow passwords improve system security by moving encrypted password hashes from the world-readable /etc/passwd file to /etc/shadow, which is readable only by the root user.
  • Shadow passwords store information about password aging.
  • Shadow passwords allow the /etc/login.defs file to enforce security policies.
Most utilities provided by the shadow-utils package work properly whether or not shadow passwords are enabled. However, since password aging information is stored exclusively in the /etc/shadow file, any commands which create or modify password aging information do not work. The following is a list of utilities and commands that do not work without first enabling shadow passwords:
  • The chage utility.
  • The gpasswd utility.
  • The usermod command with the -e or -f option.
  • The useradd command with the -e or -f option.

4.2. Using the User Accounts Tool

The User Accounts configuration tool allows you to view, modify, add, and delete local users. To run the tool, select ApplicationsSystem ToolsSystem Settings from the Activities menu and click the User Accounts icon.
The User Accounts configuration tool
The User Accounts configuration tool
Figure 4.1. The User Accounts configuration tool

By default, the tool only allows you to change certain settings regarding your account. This is because only the root user is allowed to configure users and groups. To unlock the configuration tool for all kinds of changes, click the Unlock button in the top-right corner of the window, and provide the correct password when prompted.

4.2.1. Configuring an Account

To change the image associated with an account, click the icon next to the account name and either select a picture from the pulldown list, or click Browse for more pictures... to use an image from your local drive.
To change the name associated with an account, click the name next to the icon to edit it.
To change the account type, click the text next to the Account type label. Note that this change requires the configuration tool to be unlocked even if you are changing your own account.
To change the default language for an account, click the text next to the Language label and select a language from the list.
To change the password, click the field next to the Password label. A dialog box appears, allowing you to set the new password. Note that the current password must be provided in order to confirm the change. Once done, click the Change button to save the change.
Changing the password
Changing the password
Figure 4.2. Changing the password

Password security advice

It is advisable to use a much longer password, as this makes it more difficult for an intruder to guess it and access the account without permission. It is also recommended that the password not be based on a dictionary term: use a combination of letters, numbers and special characters.
Finally, to set up automatic login for a particular account, enable the Automatic Login switch. The configuration tool must be unlocked to make this change.

4.2.2. Adding a New User

To add a new user, make sure the configuration tool is unlocked, and click the + button (that is, the plus sign) below the account list. A dialog window appears, allowing you to supply user details.
Creating a new account
Creating a new account
Figure 4.3. Creating a new account

Take the following steps to create an account:
  1. Select an account type from the Account type pulldown list. Available account types are Administrator and Standard (the default option).
  2. Fill in the Full name input field to set the name associated with the account. This name will be used by the login manager, and will be displayed on the panel.
  3. Either select a suggested username from the Username pulldown list, or fill in the corresponding input field.
  4. Click the Create button to confirm the settings.
Fedora uses a user private group (UPG) scheme. The UPG scheme does not add or change anything in the standard UNIX way of handling groups; it offers a new convention. Whenever you create a new user, a unique group with the same name as the user is created.
When a new account is created, default configuration files are copied from the /etc/skel/ directory into the new home directory.

4.2.3. Removing a User

To remove a user, make sure the configuration tool is unlocked, select the desired account from the account list, and click the button (that is, the minus sign) below the account list. A dialog window appears, allowing you to confirm or cancel the change.
Removing an account
Removing an account
Figure 4.4. Removing an account

To delete files and directories that belong to the user (that is, the home directory, mail spool, and temporary files), click the Delete Files button. To keep these files intact and only delete the user account, click Keep Files. To abort the deletion, click Cancel.

4.3. Using the User Manager Tool

The User Manager application allows you to view, modify, add, and delete local users and groups in the graphical user interface. To start the application, either select ApplicationsOtherUsers and Groups from the Activities menu, or type system-config-users at a shell prompt. Note that unless you have superuser privileges, the application will prompt you to authenticate as root.

4.3.1. Viewing Users and Groups

The main window of the User Manager is divided into two tabs: The Users tab provides a list of local users along with additional information about their user ID, primary group, home directory, login shell, and full name. The Groups tab provides a list of local groups with information about their group ID and group members.
Viewing users and groups
Viewing users and groups
Figure 4.5. Viewing users and groups

To find a specific user or group, type the first few letters of the name in the Search filter field and either press Enter, or click the Apply filter button. You can also sort the items according to any of the available columns by clicking the column header.
Fedora reserves user and group IDs below 1000 for system users and groups. By default, the User Manager does not display the system users. To view all users and groups, select EditPreferences to open the Preferences dialog box, and clear the Hide system users and groups check box.

4.3.2. Adding a New User

To add a new user, click the Add User button. A window as shown in Figure 4.6, “Adding a new user” appears.
Adding a new user
Adding a new user
Figure 4.6. Adding a new user

The Add New User dialog box allows you to provide information about the newly created user. In order to create a user, enter the username and full name in the appropriate fields and then type the user's password in the Password and Confirm Password fields. The password must be at least six characters long.

Password security advice

It is advisable to use a much longer password, as this makes it more difficult for an intruder to guess it and access the account without permission. It is also recommended that the password not be based on a dictionary term: use a combination of letters, numbers and special characters.
The Login Shell pulldown list allows you to select a login shell for the user. If you are not sure which shell to select, accept the default value of /bin/bash.
By default, the User Manager application creates the home directory for a new user in /home/username/. You can choose not to create the home directory by clearing the Create home directory check box, or change this directory by editing the content of the Home Directory text box. Note that when the home directory is created, default configuration files are copied into it from the /etc/skel/ directory.
Fedora uses a user private group (UPG) scheme. Whenever you create a new user, a unique group with the same name as the user is created by default. If you do not want to create this group, clear the Create a private group for the user check box.
To specify a user ID for the user, select Specify user ID manually. If the option is not selected, the next available user ID above 1000 is assigned to the new user. Because Fedora reserves user IDs below 1000 for system users, it is not advisable to manually assign user IDs 1–999.
Clicking the OK button creates the new user. To configure more advanced user properties, such as password expiration, modify the user's properties after adding the user.

4.3.3. Adding a New Group

To add a new user group, select Add Group from the toolbar. A window similar to Figure 4.7, “New Group” appears. Type the name of the new group. To specify a group ID for the new group, select Specify group ID manually and select the GID. Note that Fedora also reserves group IDs lower than 1000 for system groups.
New Group
Creating a new group
Figure 4.7. New Group

Click OK to create the group. The new group appears in the group list.

4.3.4. Modifying User Properties

To view the properties of an existing user, click on the Users tab, select the user from the user list, and click Properties from the menu (or choose FileProperties from the pulldown menu). A window similar to Figure 4.8, “User Properties” appears.
User Properties
Modifying user properties
Figure 4.8. User Properties

The User Properties window is divided into multiple tabbed pages:
  • User Data — Shows the basic user information configured when you added the user. Use this tab to change the user's full name, password, home directory, or login shell.
  • Account Info — Select Enable account expiration if you want the account to expire on a certain date. Enter the date in the provided fields. Select Local password is locked to lock the user account and prevent the user from logging into the system.
  • Password Info — Displays the date that the user's password last changed. To force the user to change passwords after a certain number of days, select Enable password expiration and enter a desired value in the Days before change required: field. The number of days before the user's password expires, the number of days before the user is warned to change passwords, and days before the account becomes inactive can also be changed.
  • Groups — Allows you to view and configure the Primary Group of the user, as well as other groups that you want the user to be a member of.

4.3.5. Modifying Group Properties

To view the properties of an existing group, select the group from the group list and click Properties from the menu (or choose FileProperties from the pulldown menu). A window similar to Figure 4.9, “Group Properties” appears.
Group Properties
Modifying group properties
Figure 4.9. Group Properties

The Group Users tab displays which users are members of the group. Use this tab to add or remove users from the group. Click OK to save your changes.

4.4. Using Command Line Tools

The easiest say to manage users and groups on Fedora is to use the User Manager application as described in Section 4.3, “Using the User Manager Tool”. However, if you prefer command line tools or do not have the X Window System installed, you can use command line utilities that are listed in Table 4.1, “Command line utilities for managing users and groups”.
Table 4.1. Command line utilities for managing users and groups
Utilities Description
useradd, usermod, userdel Standard utilities for adding, modifying, and deleting user accounts.
groupadd, groupmod, groupdel Standard utilities for adding, modifying, and deleting groups.
gpasswd Standard utility for administering the /etc/group configuration file.
pwck, grpck Utilities that can be used for verification of the password, group, and associated shadow files.
pwconv, pwunconv Utilities that can be used for the conversion of passwords to shadow passwords, or back from shadow passwords to standard passwords.

4.4.1. Adding a New User

To add a new user to the system, typing the following at a shell prompt as root:
useradd [options] username
…where options are command line options as described in Table 4.2, “useradd command line options”.
By default, the useradd command creates a locked user account. To unlock the account, run the following command as root to assign a password:
passwd username
Optionally, you can set password aging policy. Refer to Section 4.4.3, “Enabling Password Aging” for information on how to enable password aging.
Table 4.2. useradd command line options
Option Description
-c 'comment' comment can be replaced with any string. This option is generally used to specify the full name of a user.
-d home_directory Home directory to be used instead of default /home/username/.
-e date Date for the account to be disabled in the format YYYY-MM-DD.
-f days Number of days after the password expires until the account is disabled. If 0 is specified, the account is disabled immediately after the password expires. If -1 is specified, the account is not be disabled after the password expires.
-g group_name Group name or group number for the user's default group. The group must exist prior to being specified here.
-G group_list List of additional (other than default) group names or group numbers, separated by commas, of which the user is a member. The groups must exist prior to being specified here.
-m Create the home directory if it does not exist.
-M Do not create the home directory.
-N Do not create a user private group for the user.
-p password The password encrypted with crypt.
-r Create a system account with a UID less than 1000 and without a home directory.
-s User's login shell, which defaults to /bin/bash.
-u uid User ID for the user, which must be unique and greater than 999.

Explaining the Process

The following steps illustrate what happens if the command useradd juan is issued on a system that has shadow passwords enabled:
  1. A new line for juan is created in /etc/passwd:
    juan:x:501:501::/home/juan:/bin/bash
    The line has the following characteristics:
    • It begins with the username juan.
    • There is an x for the password field indicating that the system is using shadow passwords.
    • A UID greater than 999 is created. Under Fedora, UIDs below 1000 are reserved for system use and should not be assigned to users.
    • A GID greater than 999 is created. Under Fedora, GIDs below 1000 are reserved for system use and should not be assigned to users.
    • The optional GECOS information is left blank.
    • The home directory for juan is set to /home/juan/.
    • The default shell is set to /bin/bash.
  2. A new line for juan is created in /etc/shadow:
    juan:!!:14798:0:99999:7:::
    The line has the following characteristics:
    • It begins with the username juan.
    • Two exclamation marks (!!) appear in the password field of the /etc/shadow file, which locks the account.

      Note

      If an encrypted password is passed using the -p flag, it is placed in the /etc/shadow file on the new line for the user.
    • The password is set to never expire.
  3. A new line for a group named juan is created in /etc/group:
    juan:x:501:
    A group with the same name as a user is called a user private group. For more information on user private groups, refer to Section 4.1.1, “User Private Groups”.
    The line created in /etc/group has the following characteristics:
    • It begins with the group name juan.
    • An x appears in the password field indicating that the system is using shadow group passwords.
    • The GID matches the one listed for user juan in /etc/passwd.
  4. A new line for a group named juan is created in /etc/gshadow:
    juan:!::
    The line has the following characteristics:
    • It begins with the group name juan.
    • An exclamation mark (!) appears in the password field of the /etc/gshadow file, which locks the group.
    • All other fields are blank.
  5. A directory for user juan is created in the /home/ directory:
    ~]# ls -l /home
    total 4
    drwx------. 4 juan juan 4096 Mar  3 18:23 juan
    This directory is owned by user juan and group juan. It has read, write, and execute privileges only for the user juan. All other permissions are denied.
  6. The files within the /etc/skel/ directory (which contain default user settings) are copied into the new /home/juan/ directory:
    ~]# ls -la /home/juan
    total 28
    drwx------. 4 juan juan 4096 Mar  3 18:23 .
    drwxr-xr-x. 5 root root 4096 Mar  3 18:23 ..
    -rw-r--r--. 1 juan juan   18 Jun 22  2010 .bash_logout
    -rw-r--r--. 1 juan juan  176 Jun 22  2010 .bash_profile
    -rw-r--r--. 1 juan juan  124 Jun 22  2010 .bashrc
    drwxr-xr-x. 2 juan juan 4096 Jul 14  2010 .gnome2
    drwxr-xr-x. 4 juan juan 4096 Nov 23 15:09 .mozilla
At this point, a locked account called juan exists on the system. To activate it, the administrator must next assign a password to the account using the passwd command and, optionally, set password aging guidelines.

4.4.2. Adding a New Group

To add a new group to the system, type the following at a shell prompt as root:
groupadd [options] group_name
…where options are command line options as described in Table 4.3, “groupadd command line options”.
Table 4.3. groupadd command line options
Option Description
-f, --force When used with -g gid and gid already exists, groupadd will choose another unique gid for the group.
-g gid Group ID for the group, which must be unique and greater than 999.
-K, --key key=value Override /etc/login.defs defaults.
-o, --non-unique Allow to create groups with duplicate.
-p, --password password Use this encrypted password for the new group.
-r Create a system group with a GID less than 1000.

4.4.3. Enabling Password Aging

For security reasons, it is advisable to require users to change their passwords periodically. This can either be done when adding or editing a user on the Password Info tab of the User Manager application, or by using the chage command.

Shadow passwords must be enabled to use chage

Shadow passwords must be enabled to use the chage command. For more information, see Section 4.1.2, “Shadow Passwords”.
To configure password expiration for a user from a shell prompt, run the following command as root:
chage [options] username
…where options are command line options as described in Table 4.4, “chage command line options”. When the chage command is followed directly by a username (that is, when no command line options are specified), it displays the current password aging values and allows you to change them interactively.
Table 4.4. chage command line options
Option Description
-d days Specifies the number of days since January 1, 1970 the password was changed.
-E date Specifies the date on which the account is locked, in the format YYYY-MM-DD. Instead of the date, the number of days since January 1, 1970 can also be used.
-I days Specifies the number of inactive days after the password expiration before locking the account. If the value is 0, the account is not locked after the password expires.
-l Lists current account aging settings.
-m days Specify the minimum number of days after which the user must change passwords. If the value is 0, the password does not expire.
-M days Specify the maximum number of days for which the password is valid. When the number of days specified by this option plus the number of days specified with the -d option is less than the current day, the user must change passwords before using the account.
-W days Specifies the number of days before the password expiration date to warn the user.

You can configure a password to expire the first time a user logs in. This forces users to change passwords immediately.
  1. Set up an initial password. There are two common approaches to this step: you can either assign a default password, or you can use a null password.
    To assign a default password, type the following at a shell prompt as root:
    passwd username
    To assign a null password instead, use the following command:
    passwd -d username

    Avoid using null passwords whenever possible

    Using a null password, while convenient, is a highly insecure practice, as any third party can log in first and access the system using the insecure username. Always make sure that the user is ready to log in before unlocking an account with a null password.
  2. Force immediate password expiration by running the following command as root:
    chage -d 0 username
    This command sets the value for the date the password was last changed to the epoch (January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place.
Upon the initial log in, the user is now prompted for a new password.

4.4.4. Enabling Automatic Logouts

Especially when the user is logged in as root, an unattended login session may pose a significant security risk. To reduce this risk, you can configure the system to automatically log out idle users after a fixed period of time:
  1. Make sure the screen package is installed. You can do so by running the following command as root:
    yum install screen
    For more information on how to install packages in Fedora, refer to Section 5.2.4, “Installing Packages”.
  2. As root, add the following line at the beginning of the /etc/profile file to make sure the processing of this file cannot be interrupted:
    trap "" 1 2 3 15
  3. Add the following lines at the end of the /etc/profile file to start a screen session each time a user logs in to a virtual console or remotely:
    SCREENEXEC="screen"
    if [ -w $(tty) ]; then
      trap "exec $SCREENEXEC" 1 2 3 15
      echo -n 'Starting session in 10 seconds'
      sleep 10
      exec $SCREENEXEC
    fi
    Note that each time a new session starts, a message will be displayed and the user will have to wait ten seconds. To adjust the time to wait before starting a session, change the value after the sleep command.
  4. Add the following lines to the /etc/screenrc configuration file to close the screen session after a given period of inactivity:
    idle 120 quit
    autodetach off
    This will set the time limit to 120 seconds. To adjust this limit, change the value after the idle directive.
    Alternatively, you can configure the system to only lock the session by using the following lines instead:
    idle 120 lockscreen
    autodetach off
    This way, a password will be required to unlock the session.
The changes take effect the next time a user logs in to the system.

4.4.5. Creating Group Directories

System administrators usually like to create a group for each major project and assign people to the group when they need to access that project's files. With this traditional scheme, file managing is difficult; when someone creates a file, it is associated with the primary group to which they belong. When a single person works on multiple projects, it becomes difficult to associate the right files with the right group. However, with the UPG scheme, groups are automatically assigned to files created within a directory with the setgid bit set. The setgid bit makes managing group projects that share a common directory very simple because any files a user creates within the directory are owned by the group which owns the directory.
For example, a group of people need to work on files in the /opt/myproject/ directory. Some people are trusted to modify the contents of this directory, but not everyone.
  1. As root, create the /opt/myproject/ directory by typing the following at a shell prompt:
    mkdir /opt/myproject
  2. Add the myproject group to the system:
    groupadd myproject
  3. Associate the contents of the /opt/myproject/ directory with the myproject group:
    chown root:myproject /opt/myproject
  4. Allow users to create files within the directory, and set the setgid bit:
    chmod 2775 /opt/myproject
At this point, all members of the myproject group can create and edit files in the /opt/myproject/ directory without the administrator having to change file permissions every time users write new files. To verify that the permissions have been set correctly, run the following command:
~]# ls -l /opt
total 4
drwxrwsr-x. 3 root myproject 4096 Mar  3 18:31 myproject

4.5. Additional Resources

Refer to the following resources for more information about managing users and groups.

4.5.1. Installed Documentation

For information about various utilities for managing users and groups, refer to the following manual pages:
  • chage(1) — A command to modify password aging policies and account expiration.
  • gpasswd(1) — A command to administer the /etc/group file.
  • groupadd(8) — A command to add groups.
  • grpck(8) — A command to verify the /etc/group file.
  • groupdel(8) — A command to remove groups.
  • groupmod(8) — A command to modify group membership.
  • pwck(8) — A command to verify the /etc/passwd and /etc/shadow files.
  • pwconv(8) — A tool to convert standard passwords to shadow passwords.
  • pwunconv(8) — A tool to convert shadow passwords to standard passwords.
  • useradd(8) — A command to add users.
  • userdel(8) — A command to remove users.
  • usermod(8) — A command to modify users.
For information about related configuration files, see:
  • group(5) — The file containing group information for the system.
  • passwd(5) — The file containing user information for the system.
  • shadow(5) — The file containing passwords and account expiration information for the system.

Part II. Package Management

Chapter 5. Yum

Yum is the The Fedora Project package manager that is able to query for information about packages, fetch packages from repositories, install and uninstall packages using automatic dependency resolution, and update an entire system to the latest available packages. Yum performs automatic dependency resolution on packages you are updating, installing or removing, and thus is able to automatically determine, fetch and install all available dependent packages. Yum can be configured with new, additional repositories, or package sources, and also provides many plug-ins which enhance and extend its capabilities. Yum is able to perform many of the same tasks that RPM can; additionally, many of the command line options are similar. Yum enables easy and simple package management on a single machine or on groups of them.

Secure package management with GPG-signed packages

Yum provides secure package management by enabling GPG (Gnu Privacy Guard; also known as GnuPG) signature verification on GPG-signed packages to be turned on for all package repositories (i.e. package sources), or for individual repositories. When signature verification is enabled, Yum will refuse to install any packages not GPG-signed with the correct key for that repository. This means that you can trust that the RPM packages you download and install on your system are from a trusted source, such as The Fedora Project, and were not modified during transfer. Refer to Section 5.3, “Configuring Yum and Yum Repositories” for details on enabling signature-checking with Yum, or Section B.3, “Checking a Package's Signature” for information on working with and verifying GPG-signed RPM packages in general.
Yum also enables you to easily set up your own repositories of RPM packages for download and installation on other machines.
Learning Yum is a worthwhile investment because it is often the fastest way to perform system administration tasks, and it provides capabilities beyond those provided by the PackageKit graphical package management tools. Refer to Chapter 6, PackageKit for details on using PackageKit.

Yum and superuser privileges

You must have superuser privileges in order to use yum to install, update or remove packages on your system. All examples in this chapter assume that you have already obtained superuser privileges by using either the su or sudo command.

5.1. Checking For and Updating Packages

5.1.1. Checking For Updates

To see which installed packages on your system have updates available, use the following command:
yum check-update
For example:
~]# yum check-update
Loaded plugins: langpacks, presto, refresh-packagekit

PackageKit.x86_64                    0.6.14-2.fc15                 fedora
PackageKit-command-not-found.x86_64  0.6.14-2.fc15                 fedora
PackageKit-device-rebind.x86_64      0.6.14-2.fc15                 fedora
PackageKit-glib.x86_64               0.6.14-2.fc15                 fedora
PackageKit-gstreamer-plugin.x86_64   0.6.14-2.fc15                 fedora
PackageKit-gtk-module.x86_64         0.6.14-2.fc15                 fedora
PackageKit-gtk3-module.x86_64        0.6.14-2.fc15                 fedora
PackageKit-yum.x86_64                0.6.14-2.fc15                 fedora
PackageKit-yum-plugin.x86_64         0.6.14-2.fc15                 fedora
gdb.x86_64                           7.2.90.20110429-36.fc15       fedora
kernel.x86_64                        2.6.38.6-26.fc15              fedora
rpm.x86_64                           4.9.0-6.fc15                  fedora
rpm-libs.x86_64                      4.9.0-6.fc15                  fedora
rpm-python.x86_64                    4.9.0-6.fc15                  fedora
yum.noarch                           3.2.29-5.fc15                 fedora
The packages in the above output are listed as having updates available. The first package in the list is PackageKit, the graphical package manager. The line in the example output tells us:
  • PackageKit — the name of the package
  • x86_64 — the CPU architecture the package was built for
  • 0.6.14 — the version of the updated package to be installed
  • fedora — the repository in which the updated package is located
The output also shows us that we can update the kernel (the kernel package), Yum and RPM themselves (the yum and rpm packages), as well as their dependencies (such as the kernel-firmware, rpm-libs, and rpm-python packages), all using yum.

5.1.2. Updating Packages

You can choose to update a single package, multiple packages, or all packages at once. If any dependencies of the package (or packages) you update have updates available themselves, then they are updated too.

Updating a Single Package

To update a single package, run the following command as root:
yum update package_name
For example, to update the udev package, type:
~]# yum update udev
Loaded plugins: langpacks, presto, refresh-packagekit
Updating Red Hat repositories.
INFO:rhsm-app.repolib:repos updated: 0
Setting up Update Process
Resolving Dependencies
--> Running transaction check
---> Package gdb.x86_64 0:7.2.90.20110411-34.fc15 will be updated
---> Package gdb.x86_64 0:7.2.90.20110429-36.fc15 will be an update
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package     Arch         Version                          Repository      Size
================================================================================
Updating:
 gdb         x86_64       7.2.90.20110429-36.fc15          fedora         1.9 M

Transaction Summary
================================================================================
Upgrade       1 Package(s)

Total download size: 1.9 M
Is this ok [y/N]:
This output contains several items of interest:
  1. Loaded plugins:yum always informs you which Yum plug-ins are installed and enabled. Here, yum is using the langpacks, presto, and refresh-packagekit plug-ins. Refer to Section 5.4, “Yum Plug-ins” for general information on Yum plug-ins, or to Section 5.4.3, “Plug-in Descriptions” for descriptions of specific plug-ins.
  2. gdb.x86_64 — you can download and install new gdb package.
  3. yum presents the update information and then prompts you as to whether you want it to perform the update; yum runs interactively by default. If you already know which transactions yum plans to perform, you can use the -y option to automatically answer yes to any questions yum may ask (in which case it runs non-interactively). However, you should always examine which changes yum plans to make to the system so that you can easily troubleshoot any problems that might arise.
    If a transaction does go awry, you can view Yum's transaction history by using the yum history command as described in Section 5.2.6, “Working with Transaction History”.

Updating and installing kernels with Yum

yum always installs a new kernel in the same sense that RPM installs a new kernel when you use the command rpm -i kernel. Therefore, you do not need to worry about the distinction between installing and upgrading a kernel package when you use yum: it will do the right thing, regardless of whether you are using the yum update or yum install command.
When using RPM, on the other hand, it is important to use the rpm -i kernel command (which installs a new kernel) instead of rpm -u kernel (which replaces the current kernel). Refer to Section B.2.2, “Installing and Upgrading” for more information on installing/updating kernels with RPM.

Updating All Packages and Their Dependencies

To update all packages and their dependencies, simply enter yum update (without any arguments):
yum update
Discovering which packages have security updates available and then updating those packages quickly and easily is important. Yum provides the plug-in for this purpose. The security plug-in extends the yum command with a set of highly-useful security-centric commands, subcommands and options. Refer to Section 5.4.3, “Plug-in Descriptions” for specific information.

5.1.3. Preserving Configuration File Changes

You will inevitably make changes to the configuration files installed by packages as you use your Fedora system. RPM, which Yum uses to perform changes to the system, provides a mechanism for ensuring their integrity. Refer to Section B.2.2, “Installing and Upgrading” for details on how to manage changes to configuration files across package upgrades.

5.2. Packages and Package Groups

5.2.1. Searching Packages

You can search all RPM package names, descriptions and summaries by using the following command:
yum search term
This command displays the list of matches for each term. For example, to list all packages that match meld or kompare, type:
~]# yum search meld kompare
Loaded plugins: langpacks, presto, refresh-packagekit
============================== N/S Matched: meld ===============================
meld.noarch : Visual diff and merge tool
python-meld3.x86_64 : HTML/XML templating system for Python

============================= N/S Matched: kompare =============================
komparator.x86_64 : Kompare and merge two folders

  Name and summary matches only, use "search all" for everything.
The yum search command is useful for searching for packages you do not know the name of, but for which you know a related term.

5.2.2. Listing Packages

yum list and related commands provide information about packages, package groups, and repositories.
All of Yum's list commands allow you to filter the results by appending one or more glob expressions as arguments. Glob expressions are normal strings of characters which contain one or more of the wildcard characters * (which expands to match any character multiple times) and ? (which expands to match any one character).

Filtering results with glob expressions

Be careful to escape the glob expressions when passing them as arguments to a yum command, otherwise the Bash shell will interpret these expressions as pathname expansions, and potentially pass all files in the current directory that match the globs to yum. To make sure the glob expressions are passed to yum as intended, either:
  • escape the wildcard characters by preceding them with a backslash character
  • double-quote or single-quote the entire glob expression.
yum list glob_expression
Lists information on installed and available packages matching all glob expressions.
Example 5.1. Listing all ABRT addons and plug-ins using glob expressions
Packages with various ABRT addons and plug-ins either begin with abrt-addon-, or abrt-plugin-. To list these packages, type the following at a shell prompt:
~]# yum list abrt-addon\* abrt-plugin\*
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
abrt-addon-ccpp.x86_64               2.0.2-5.fc15     @fedora
abrt-addon-kerneloops.x86_64         2.0.2-5.fc15     @fedora
abrt-addon-python.x86_64             2.0.2-5.fc15     @fedora
abrt-plugin-bugzilla.x86_64          2.0.2-5.fc15     @fedora
abrt-plugin-logger.x86_64            2.0.2-5.fc15     @fedora
Available Packages
abrt-plugin-mailx.x86_64             2.0.2-5.fc15     updates
abrt-plugin-reportuploader.x86_64    2.0.2-5.fc15     updates
abrt-plugin-rhtsupport.x86_64        2.0.2-5.fc15     updates

yum list all
Lists all installed and available packages.
Example 5.2. Listing all installed and available packages
~]# yum list all
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
ConsoleKit.x86_64                       0.4.4-1.fc15                  @fedora
ConsoleKit-libs.x86_64                  0.4.4-1.fc15                  @fedora
ConsoleKit-x11.x86_64                   0.4.4-1.fc15                  @fedora
GConf2.x86_64                           2.32.3-1.fc15                 @fedora
GConf2-gtk.x86_64                       2.32.3-1.fc15                 @fedora
ModemManager.x86_64                     0.4-7.git20110201.fc15        @fedora
NetworkManager.x86_64                   1:0.8.998-4.git20110427.fc15  @fedora
NetworkManager-glib.x86_64              1:0.8.998-4.git20110427.fc15  @fedora
NetworkManager-gnome.x86_64             1:0.8.998-4.git20110427.fc15  @fedora
NetworkManager-openconnect.x86_64       0.8.1-9.git20110419.fc15      @fedora
[output truncated]

yum list installed
Lists all packages installed on your system. The rightmost column in the output lists the repository from which the package was retrieved.
Example 5.3. Listing installed packages using a double-quoted glob expression
To list all installed packages that begin with krb followed by exactly one character and a hyphen, type:
~]# yum list installed "krb?-*"
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
krb5-libs.x86_64              1.9-7.fc15              @fedora

yum list available
Lists all available packages in all enabled repositories.
Example 5.4. Listing available packages using a single glob expression with escaped wildcard characters
To list all available packages with names that contain gstreamer and then plugin, run the following command:
~]# yum list available gstreamer\*plugin\*
Loaded plugins: langpacks, presto, refresh-packagekit
Available Packages
gstreamer-plugin-crystalhd.x86_64               3.5.1-1.fc14       fedora
gstreamer-plugins-bad-free.x86_64               0.10.22-1.fc15     updates
gstreamer-plugins-bad-free-devel.x86_64         0.10.22-1.fc15     updates
gstreamer-plugins-bad-free-devel-docs.x86_64    0.10.22-1.fc15     updates
gstreamer-plugins-bad-free-extras.x86_64        0.10.22-1.fc15     updates
gstreamer-plugins-base.x86_64                   0.10.33-1.fc15     updates
gstreamer-plugins-base-devel.x86_64             0.10.33-1.fc15     updates
gstreamer-plugins-base-devel-docs.noarch        0.10.33-1.fc15     updates
gstreamer-plugins-base-tools.x86_64             0.10.33-1.fc15     updates
gstreamer-plugins-espeak.x86_64                 0.3.3-3.fc15       fedora
gstreamer-plugins-fc.x86_64                     0.2-2.fc15         fedora
gstreamer-plugins-good.x86_64                   0.10.29-1.fc15     updates
gstreamer-plugins-good-devel-docs.noarch        0.10.29-1.fc15     updates

yum grouplist
Lists all package groups.
Example 5.5. Listing all package groups
~]# yum grouplist
Loaded plugins: langpacks, presto, refresh-packagekit
Setting up Group Process
Installed Groups:
   Administration Tools
   Design Suite
   Dial-up Networking Support
   Fonts
   GNOME Desktop Environment
[output truncated]

yum repolist
Lists the repository ID, name, and number of packages it provides for each enabled repository.
Example 5.6. Listing enabled repositories
~]# yum repolist
Loaded plugins: langpacks, presto, refresh-packagekit
repo id                      repo name                                    status
fedora                       Fedora 15 - i386                             19,365
updates                      Fedora 15 - i386 - Updates                   3,848
repolist: 23,213

5.2.3. Displaying Package Information

To display information about one or more packages (glob expressions are valid here as well), use the following command:
yum info package_name
For example, to display information about the abrt package, type:
~]# yum info abrt
Loaded plugins: langpacks, presto, refresh-packagekit
Installed Packages
Name        : abrt
Arch        : x86_64
Version     : 2.0.1
Release     : 2.fc15
Size        : 806 k
Repo        : installed
From repo   : fedora
Summary     : Automatic bug detection and reporting tool
URL         : https://fedorahosted.org/abrt/
License     : GPLv2+
Description : abrt is a tool to help users to detect defects in applications and
            : to create a bug report with all informations needed by maintainer
            : to fix it. It uses plugin system to extend its functionality.
The yum info package_name command is similar to the rpm -q --info package_name command, but provides as additional information the ID of the Yum repository the RPM package is found in (look for the From repo: line in the output).
You can also query the Yum database for alternative and useful information about a package by using the following command:
yumdb info package_name
This command provides additional information about a package, including the checksum of the package (and algorithm used to produce it, such as SHA-256), the command given on the command line that was invoked to install the package (if any), and the reason that the package is installed on the system (where user indicates it was installed by the user, and dep means it was brought in as a dependency). For example, to display additional information about the yum package, type:
~]# yumdb info yum
Loaded plugins: langpacks, presto, refresh-packagekit
yum-3.2.29-4.fc15.noarch
     checksum_data = 249f21fb43c41381c8c9b0cd98d2ea5fa0aa165e81ed2009cfda74c05af67246
     checksum_type = sha256
     from_repo = fedora
     from_repo_revision = 1304429533
     from_repo_timestamp = 1304442346
     installed_by = 0
     reason = user
     releasever = $releasever
For more information on the yumdb command, refer to the yumdb(8) manual page.

5.2.4. Installing Packages

Yum allows you to install both a single package and multiple packages, as well as a package group of your choice.

Installing Individual Packages

To install a single package and all of its non-installed dependencies, enter a command in the following form:
yum install package_name
You can also install multiple packages simultaneously by appending their names as arguments:
yum install package_name package_name
If you are installing packages on a multilib system, such as an AMD64 or Intel64 machine, you can specify the architecture of the package (as long as it is available in an enabled repository) by appending .arch to the package name. For example, to install the sqlite2 package for i586, type:
~]# yum install sqlite2.i586
You can use glob expressions to quickly install multiple similarly-named packages:
~]# yum install audacious-plugins-\*
In addition to package names and glob expressions, you can also provide file names to yum install. If you know the name of the binary you want to install, but not its package name, you can give yum install the path name:
~]# yum install /usr/sbin/named
yum then searches through its package lists, finds the package which provides /usr/sbin/named, if any, and prompts you as to whether you want to install it.

Finding which package owns a file

If you know you want to install the package that contains the named binary, but you do not know in which bin or sbin directory is the file installed, use the yum provides command with a glob expression:
~]# yum provides "*bin/named"
Loaded plugins: langpacks, presto, refresh-packagekit
32:bind-9.8.0-3.P1.fc15.i686 : The Berkeley Internet Name Domain (BIND) DNS
                             : (Domain Name System) server
Repo        : fedora
Matched from:
Filename    : /usr/sbin/named
yum provides "*/file_name" is a common and useful trick to find the packages that contain file_name.

Installing a Package Group

A package group is similar to a package: it is not useful by itself, but installing one pulls a group of dependent packages that serve a common purpose. A package group has a name and a groupid. The yum grouplist -v command lists the names of all package groups, and, next to each of them, their groupid in parentheses. The groupid is always the term in the last pair of parentheses, such as kde-desktop in the following example:
~]# yum -v grouplist kde\*
Not loading "blacklist" plugin, as it is disabled
Loading "langpacks" plugin
Loading "presto" plugin
Loading "refresh-packagekit" plugin
Not loading "whiteout" plugin, as it is disabled
Adding en_US to language list
Config time: 0.900
Yum Version: 3.2.29
Setting up Group Process
rpmdb time: 0.002
group time: 0.995
Available Groups:
   KDE Software Compilation (kde-desktop)
   KDE Software Development (kde-software-development)
Done
You can install a package group by passing its full group name (without the groupid part) to groupinstall:
yum groupinstall group_name
You can also install by groupid:
yum groupinstall groupid
You can even pass the groupid (or quoted name) to the install command if you prepend it with an @-symbol (which tells yum that you want to perform a groupinstall):
yum install @group
For example, the following are alternative but equivalent ways of installing the KDE Desktop group:
~]# yum groupinstall "KDE Desktop"
~]# yum groupinstall kde-desktop
~]# yum install @kde-desktop

5.2.5. Removing Packages

Similarly to package installation, Yum allows you to uninstall (remove in RPM and Yum terminology) both individual packages and a package group.

Removing Individual Packages

To uninstall a particular package, as well as any packages that depend on it, run the following command as root:
yum remove package_name
As when you install multiple packages, you can remove several at once by adding more package names to the command. For example, to remove totem, rhythmbox, and sound-juicer, type the following at a shell prompt:
~]# yum remove totem rhythmbox sound-juicer
Similar to install, remove can take these arguments:
  • package names
  • glob expressions
  • file lists
  • package provides

Removing a package when other packages depend on it

Yum is not able to remove a package without also removing packages which depend on it. This type of operation can only be performed by RPM, is not advised, and can potentially leave your system in a non-functioning state or cause applications to misbehave and/or crash. For further information, refer to Section B.2.4, “Uninstalling” in the RPM chapter.

Removing a Package Group

You can remove a package group using syntax congruent with the install syntax:
yum groupremove group
yum remove @group
The following are alternative but equivalent ways of removing the KDE Desktop group:
~]# yum groupremove "KDE Desktop"
~]# yum groupremove kde-desktop
~]# yum remove @kde-desktop

Intelligent package group removal

When you tell yum to remove a package group, it will remove every package in that group, even if those packages are members of other package groups or dependencies of other installed packages. However, you can instruct yum to remove only those packages which are not required by any other packages or groups by adding the groupremove_leaf_only=1 directive to the [main] section of the /etc/yum.conf configuration file. For more information on this directive, refer to Section 5.3.1, “Setting [main] Options”.

5.2.6. Working with Transaction History

The yum history command allows users to review information about a timeline of Yum transactions, the dates and times on when they occurred, the number of packages affected, whether transactions succeeded or were aborted, and if the RPM database was changed between transactions. Additionally, this command can be used to undo or redo certain transactions.

Listing Transactions

To display a list of twenty most recent transactions, as root, either run yum history with no additional arguments, or type the following at a shell prompt:
yum history list
To display all transactions, add the all keyword:
yum history list all
To display only transactions in a given range, use the command in the following form:
yum history list start_id..end_id
You can also list only transactions regarding a particular package or packages. To do so, use the command with a package name or a glob expression:
yum history list glob_expression
For example, the list of first five transactions may look as follows:
~]# yum history list 1..5
Loaded plugins: langpacks, presto, refresh-packagekit
ID     | Login user               | Date and time    | Action(s)      | Altered
-------------------------------------------------------------------------------
     5 | Jaromir ... <jhradilek>  | 2011-07-29 15:33 | Install        |    1
     4 | Jaromir ... <jhradilek>  | 2011-07-21 15:10 | Install        |    1
     3 | Jaromir ... <jhradilek>  | 2011-07-16 15:27 | I, U           |   73
     2 | System <unset>           | 2011-07-16 15:19 | Update         |    1
     1 | System <unset>           | 2011-07-16 14:38 | Install        | 1106
history list
All forms of the yum history list command produce tabular output with each row consisting of the following columns:
  • ID — an integer value that identifies a particular transaction.
  • Login user — the name of the user whose login session was used to initiate a transaction. This information is typically presented in the Full Name <username> form. For transactions that were not issued by a user (such as an automatic system update), System <unset> is used instead.
  • Date and time — the date and time when a transaction was issued.
  • Action(s) — a list of actions that were performed during a transaction as described in Table 5.1, “Possible values of the Action(s) field”.
  • Altered — the number of packages that were affected by a transaction, possibly followed by additional information as described in Table 5.2, “Possible values of the Altered field”.
Table 5.1. Possible values of the Action(s) field
Action Abbreviation Description
Downgrade D At least one package has been downgraded to an older version.
Erase E At least one package has been removed.
Install I At least one new package has been installed.
Obsoleting O At least one package has been marked as obsolete.
Reinstall R At least one package has been reinstalled.
Update U At least one package has been updated to a newer version.

Table 5.2. Possible values of the Altered field
Symbol Description
< Before the transaction finished, the rpmdb database was changed outside Yum.
> After the transaction finished, the rpmdb database was changed outside Yum.
* The transaction failed to finish.
# The transaction finished successfully, but yum returned a non-zero exit code.
E The transaction finished successfully, but an error or a warning was displayed.
P The transaction finished successfully, but problems already existed in the rpmdb database.
s The transaction finished successfully, but the --skip-broken command line option was used and certain packages were skipped.

Yum also allows you to display a summary of all past transactions. To do so, run the command in the following form as root:
yum history summary
To display only transactions in a given range, type:
yum history summary start_id..end_id
Similarly to the yum history list command, you can also display a summary of transactions regarding a certain package or packages by supplying a package name or a glob expression:
yum history summary glob_expression
For instance, a summary of the transaction history displayed above would look like the following:
~]# yum history summary 1..5
Loaded plugins: langpacks, presto, refresh-packagekit
Login user                 | Time                | Action(s)        | Altered 
-------------------------------------------------------------------------------
Jaromir ... <jhradilek>    | Last day            | Install          |        1
Jaromir ... <jhradilek>    | Last week           | Install          |        1
Jaromir ... <jhradilek>    | Last 2 weeks        | I, U             |       73
System <unset>             | Last 2 weeks        | I, U             |     1107
history summary
All forms of the yum history summary command produce simplified tabular output similar to the output of yum history list.
As shown above, both yum history list and yum history summary are oriented towards transactions, and although they allow you to display only transactions related to a given package or packages, they lack important details, such as package versions. To list transactions from the perspective of a package, run the following command as root:
yum history package-list glob_expression
For example, to trace the history of subscription-manager and related packages, type the following at a shell prompt:
~]# yum history package-list subscription-manager\*
Loaded plugins: langpacks, presto, refresh-packagekit
ID     | Action(s)      | Package
-------------------------------------------------------------------------------
     3 | Updated        | subscription-manager-0.95.11-1.el6.x86_64
     3 | Update         |                      0.95.17-1.el6_1.x86_64
     3 | Updated        | subscription-manager-firstboot-0.95.11-1.el6.x86_64
     3 | Update         |                                0.95.17-1.el6_1.x86_64
     3 | Updated        | subscription-manager-gnome-0.95.11-1.el6.x86_64
     3 | Update         |                            0.95.17-1.el6_1.x86_64
     1 | Install        | subscription-manager-0.95.11-1.el6.x86_64
     1 | Install        | subscription-manager-firstboot-0.95.11-1.el6.x86_64
     1 | Install        | subscription-manager-gnome-0.95.11-1.el6.x86_64
history package-list
In this example, three packages were installed during the initial system installation: subscription-manager, subscription-manager-firstboot, and subscription-manager-gnome. In the third transaction, all these packages were updated from version 0.95.11 to version 0.95.17.

Examining Transactions

To display the summary of a single transaction, as root, use the yum history summary command in the following form:
yum history summary id
To examine a particular transaction or transactions in more detail, run the following command as root:
yum history info id
The id argument is optional and when you omit it, yum automatically uses the last transaction. Note that when specifying more than one transaction, you can also use a range:
yum history info start_id..end_id
The following is sample output for two transactions, each installing one new package:
~]# yum history info 4..5
Loaded plugins: langpacks, presto, refresh-packagekit
Transaction ID : 4..5
Begin time     : Thu Jul 21 15:10:46 2011
Begin rpmdb    : 1107:0c67c32219c199f92ed8da7572b4c6df64eacd3a
End time       :            15:33:15 2011 (22 minutes)
End rpmdb      : 1109:1171025bd9b6b5f8db30d063598f590f1c1f3242
User           : Jaromir Hradilek <jhradilek>
Return-Code    : Success
Command Line   : install screen
Command Line   : install yum-plugin-fs-snapshot
Transaction performed with:
    Installed     rpm-4.8.0-16.el6.x86_64
    Installed     yum-3.2.29-17.el6.noarch
    Installed     yum-metadata-parser-1.1.2-16.el6.x86_64
Packages Altered:
    Install screen-4.0.3-16.el6.x86_64
    Install yum-plugin-fs-snapshot-1.1.30-6.el6.noarch
history info
You can also view additional information, such as what configuration options were used at the time of the transaction, or from what repository and why were certain packages installed. To determine what additional information is available for a certain transaction, type the following at a shell prompt as root:
yum history addon-info id
Similarly to yum history info, when no id is provided, yum automatically uses the latest transaction. Another way to refer to the latest transaction is to use the last keyword:
yum history addon-info last
For instance, for the first transaction in the previous example, the yum history addon-info command would provide the following output:
~]# yum history addon-info 4
Loaded plugins: langpacks, presto, refresh-packagekit
Transaction ID: 4
Available additional history information:
  config-main
  config-repos
  saved_tx

history addon-info
In this example, three types of information are available:
  • config-main — global Yum options that were in use during the transaction. Refer to Section 5.3.1, “Setting [main] Options” for information on how to change global options.
  • config-repos — options for individual Yum repositories. Refer to Section 5.3.2, “Setting [repository] Options” for information on how to change options for individual repositories.
  • saved_tx — the data that can be used by the yum load-transaction command in order to repeat the transaction on another machine (see below).
To display selected type of additional information, run the following command as root:
yum history addon-info id information

Reverting and Repeating Transactions

Apart from reviewing the transaction history, the yum history command provides means to revert or repeat a selected transaction. To revert a transaction, type the following at a shell prompt as root:
yum history undo id
To repeat a particular transaction, as root, run the following command:
yum history redo id
Both commands also accept the last keyword to undo or repeat the latest transaction.
Note that both yum history undo and yum history redo commands merely revert or repeat the steps that were performed during a transaction: if the transaction installed a new package, the yum history undo command will uninstall it, and vice versa. If possible, this command will also attempt to downgrade all updated packages to their previous version, but these older packages may no longer be available. If you need to be able to restore the system to the state before an update, consider using the fs-snapshot plug-in described in Section 5.4.3, “Plug-in Descriptions”.
When managing several identical systems, Yum also allows you to perform a transaction on one of them, store the transaction details in a file, and after a period of testing, repeat the same transaction on the remaining systems as well. To store the transaction details to a file, type the following at a shell prompt as root:
yum -q history addon-info id saved_tx > file_name
Once you copy this file to the target system, you can repeat the transaction by using the following command as root:
yum load-transaction file_name
Note, however that the rpmdb version stored in the file must by identical to the version on the target system. You can verify the rpmdb version by using the yum version nogroups command.

Starting New Transaction History

Yum stores the transaction history in a single SQLite database file. To start new transaction history, run the following command as root:
yum history new
This will create a new, empty database file in the /var/lib/yum/history/ directory. The old transaction history will be kept, but will not be accessible as long as a newer database file is present in the directory.

5.3. Configuring Yum and Yum Repositories

The configuration file for yum and related utilities is located at /etc/yum.conf. This file contains one mandatory [main] section, which allows you to set Yum options that have global effect, and may also contain one or more [repository] sections, which allow you to set repository-specific options. However, best practice is to define individual repositories in new or existing .repo files in the /etc/yum.repos.d/directory. The values you define in the [main] section of the /etc/yum.conf file may override values set in individual [repository] sections.
This section shows you how to:
  • set global Yum options by editing the [main] section of the /etc/yum.conf configuration file;
  • set options for individual repositories by editing the [repository] sections in /etc/yum.conf and .repo files in the /etc/yum.repos.d/ directory;
  • use Yum variables in /etc/yum.conf and files in the /etc/yum.repos.d/ directory so that dynamic version and architecture values are handled correctly;
  • add, enable, and disable Yum repositories on the command line; and,
  • set up your own custom Yum repository.

5.3.1. Setting [main] Options

The /etc/yum.conf configuration file contains exactly one [main] section, and while some of the key-value pairs in this section affect how yum operates, others affect how Yum treats repositories. You can add many additional options under the [main] section heading in /etc/yum.conf.
A sample /etc/yum.conf configuration file can look like this:
[main]
cachedir=/var/cache/yum/$basearch/$releasever
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=3

[comments abridged]

# PUT YOUR REPOS HERE OR IN separate files named file.repo
# in /etc/yum.repos.d
The following are the most commonly-used options in the [main] section:
assumeyes=value
…where value is one of:
0yum should prompt for confirmation of critical actions it performs. This is the default.
1 — Do not prompt for confirmation of critical yum actions. If assumeyes=1 is set, yum behaves in the same way that the command line option -y does.
cachedir=directory
…where directory is an absolute path to the directory where Yum should store its cache and database files. By default, Yum's cache directory is /var/cache/yum/$basearch/$releasever.
Refer to Section 5.3.3, “Using Yum Variables” for descriptions of the $basearch and $releasever Yum variables.
debuglevel=value
…where value is an integer between 1 and 10. Setting a higher debuglevel value causes yum to display more detailed debugging output. debuglevel=0 disables debugging output, while debuglevel=2 is the default.
exactarch=value
…where value is one of:
0 — Do not take into account the exact architecture when updating packages.
1 — Consider the exact architecture when updating packages. With this setting, yum will not install an i686 package to update an i386 package already installed on the system. This is the default.
exclude=package_name [more_package_names]
This option allows you to exclude packages by keyword during installation/updates. Listing multiple packages for exclusion can be accomplished by quoting a space-delimited list of packages. Shell globs using wildcards (for example, * and ?) are allowed.
gpgcheck=value
…where value is one of:
0 — Disable GPG signature-checking on packages in all repositories, including local package installation.
1 — Enable GPG signature-checking on all packages in all repositories, including local package installation. gpgcheck=1 is the default, and thus all packages' signatures are checked.
If this option is set in the [main] section of the /etc/yum.conf file, it sets the GPG-checking rule for all repositories. However, you can also set gpgcheck=value for individual repositories instead; that is, you can enable GPG-checking on one repository while disabling it on another. Setting gpgcheck=value for an individual repository in its corresponding .repo file overrides the default if it is present in /etc/yum.conf.
For more information on GPG signature-checking, refer to Section B.3, “Checking a Package's Signature”.
groupremove_leaf_only=value
…where value is one of:
0yum should not check the dependencies of each package when removing a package group. With this setting, yum removes all packages in a package group, regardless of whether those packages are required by other packages or groups. groupremove_leaf_only=0 is the default.
1yum should check the dependencies of each package when removing a package group, and remove only those packages which are not not required by any other package or group.
For more information on removing packages, refer to Intelligent package group removal.
installonlypkgs=space separated list of packages
Here you can provide a space-separated list of packages which yum can install, but will never update. Refer to the yum.conf(5) manual page for the list of packages which are install-only by default.
If you add the installonlypkgs directive to /etc/yum.conf, you should ensure that you list all of the packages that should be install-only, including any of those listed under the installonlypkgs section of yum.conf(5). In particular, kernel packages should always be listed in installonlypkgs (as they are by default), and installonly_limit should always be set to a value greater than 2 so that a backup kernel is always available in case the default one fails to boot.
installonly_limit=value
…where value is an integer representing the maximum number of versions that can be installed simultaneously for any single package listed in the installonlypkgs directive.
The defaults for the installonlypkgs directive include several different kernel packages, so be aware that changing the value of installonly_limit will also affect the maximum number of installed versions of any single kernel package. The default value listed in /etc/yum.conf is installonly_limit=3, and it is not recommended to decrease this value, particularly below 2.
keepcache=value
…where value is one of:
0 — Do not retain the cache of headers and packages after a successful installation. This is the default.
1 — Retain the cache after a successful installation.
logfile=file_name
…where file_name is an absolute path to the file in which yum should write its logging output. By default, yum logs to /var/log/yum.log.
multilib_policy=value
…where value is one of:
best — install the best-choice architecture for this system. For example, setting multilib_policy=best on an AMD64 system causes yum to install 64-bit versions of all packages.
all — always install every possible architecture for every package. For example, with multilib_policy set to all on an AMD64 system, yum would install both the i586 and AMD64 versions of a package, if both were available.
obsoletes=value
…where value is one of:
0 — Disable yum's obsoletes processing logic when performing updates.
1 — Enable yum's obsoletes processing logic when performing updates. When one package declares in its spec file that it obsoletes another package, the latter package will be replaced by the former package when the former package is installed. Obsoletes are declared, for example, when a package is renamed. obsoletes=1 the default.
plugins=value
…where value is one of:
0 — Disable all Yum plug-ins globally.

Disabling all plug-ins is not advised

Disabling all plug-ins is not advised because certain plug-ins provide important Yum services. Disabling plug-ins globally is provided as a convenience option, and is generally only recommended when diagnosing a potential problem with Yum.
1 — Enable all Yum plug-ins globally. With plugins=1, you can still disable a specific Yum plug-in by setting enabled=0 in that plug-in's configuration file.
For more information about various Yum plug-ins, refer to Section 5.4, “Yum Plug-ins”. For further information on controlling plug-ins, see Section 5.4.1, “Enabling, Configuring, and Disabling Yum Plug-ins”.
reposdir=directory
…where directory is an absolute path to the directory where .repo files are located. All .repo files contain repository information (similar to the [repository] sections of /etc/yum.conf). yum collects all repository information from .repo files and the [repository] section of the /etc/yum.conf file to create a master list of repositories to use for transactions. If reposdir is not set, yum uses the default directory /etc/yum.repos.d/.
retries=value
…where value is an integer 0 or greater. This value sets the number of times yum should attempt to retrieve a file before returning an error. Setting this to 0 makes yum retry forever. The default value is 10.
For a complete list of available [main] options, refer to the [main] OPTIONS section of the yum.conf(5) manual page.

5.3.2. Setting [repository] Options

The [repository] sections, where repository is a unique repository ID such as my_personal_repo (spaces are not permitted), allow you to define individual Yum repositories.
The following is a bare-minimum example of the form a [repository] section takes:
[repository]
name=repository_name
baseurl=repository_url
Every [repository] section must contain the following directives:
name=repository_name
…where repository_name is a human-readable string describing the repository.
baseurl=repository_url
…where repository_url is a URL to the directory where the repodata directory of a repository is located:
  • If the repository is available over HTTP, use: http://path/to/repo
  • If the repository is available over FTP, use: ftp://path/to/repo
  • If the repository is local to the machine, use: file:///path/to/local/repo
  • If a specific online repository requires basic HTTP authentication, you can specify your username and password by prepending it to the URL as username:password@link. For example, if a repository on http://www.example.com/repo/ requires a username of user and a password of password, then the baseurl link could be specified as http://user:password@www.example.com/repo/.
Usually this URL is an HTTP link, such as:
baseurl=http://path/to/repo/releases/$releasever/server/$basearch/os/
Note that Yum always expands the $releasever, $arch, and $basearch variables in URLs. For more information about Yum variables, refer to Section 5.3.3, “Using Yum Variables”.
Another useful [repository] directive is the following:
enabled=value
…where value is one of:
0 — Do not include this repository as a package source when performing updates and installs. This is an easy way of quickly turning repositories on and off, which is useful when you desire a single package from a repository that you do not want to enable for updates or installs.
1 — Include this repository as a package source.
Turning repositories on and off can also be performed by passing either the --enablerepo=repo_name or --disablerepo=repo_name option to yum, or through the Add/Remove Software window of the PackageKit utility.
Many more [repository] options exist. For a complete list, refer to the [repository] OPTIONS section of the yum.conf(5) manual page.

5.3.3. Using Yum Variables

You can use and reference the following built-in variables in yum commands and in all Yum configuration files (that is, /etc/yum.conf and all .repo files in the /etc/yum.repos.d/ directory):
$releasever
You can use this variable to reference the release version of Fedora. Yum obtains the value of $releasever from the distroverpkg=value line in the /etc/yum.conf configuration file. If there is no such line in /etc/yum.conf, then yum infers the correct value by deriving the version number from the redhat-release package.
$arch
You can use this variable to refer to the system's CPU architecture as returned when calling Python's os.uname() function. Valid values for $arch include: i586, i686 and x86_64.
$basearch
You can use $basearch to reference the base architecture of the system. For example, i686 and i586 machines both have a base architecture of i386, and AMD64 and Intel64 machines have a base architecture of x86_64.
$YUM0-9
These ten variables are each replaced with the value of any shell environment variables with the same name. If one of these variables is referenced (in /etc/yum.conf for example) and a shell environment variable with the same name does not exist, then the configuration file variable is not replaced.
To define a custom variable or to override the value of an existing one, create a file with the same name as the variable (without the $ sign) in the /etc/yum/vars/ directory, and add the desired value on its first line.
For example, repository descriptions often include the operating system name. To define a new variable called $osname, create a new file with Fedora on the first line and save it as /etc/yum/vars/osname:
~]# echo "Fedora" > /etc/yum/vars/osname
Instead of Fedora 18, you can now use the following in the .repo files:
name=$osname $releasever

5.3.4. Viewing the Current Configuration

To display the current values of global Yum options (that is, the options specified in the [main] section of the /etc/yum.conf file), run the yum-config-manager with no command line options:
yum-config-manager
To list the content of a different configuration section or sections, use the command in the following form:
yum-config-manager section
You can also use a glob expression to display the configuration of all matching sections:
yum-config-manager glob_expression
For example, to list all configuration options and their corresponding values, type the following at a shell prompt:
~]$ yum-config-manager main \*
Loaded plugins: langpacks, presto, refresh-packagekit
================================== main ===================================
[main]
alwaysprompt = True
assumeyes = False
bandwith = 0
bugtracker_url = https://bugzilla.redhat.com/enter_bug.cgi?product=Red%20Hat%20Enterprise%20Linux%206&component=yum
cache = 0
[output truncated]

5.3.5. Adding, Enabling, and Disabling a Yum Repository

Section 5.3.2, “Setting [repository] Options” described various options you can use to define a Yum repository. This section explains how to add, enable, and disable a repository by using the yum-config-manager command.

Adding a Yum Repository

To define a new repository, you can either add a [repository] section to the /etc/yum.conf file, or to a .repo file in the /etc/yum.repos.d/ directory. All files with the .repo file extension in this directory are read by yum, and best practice is to define your repositories here instead of in /etc/yum.conf.

Be careful when using untrusted software sources

Obtaining and installing software packages from unverified or untrusted software sources constitutes a potential security risk, and could lead to security, stability, compatibility maintainability issues.
Yum repositories commonly provide their own .repo file. To add such a repository to your system and enable it, run the following command as root:
yum-config-manager --add-repo repository_url
…where repository_url is a link to the .repo file. For example, to add a repository located at http://www.example.com/example.repo, type the following at a shell prompt:
~]# yum-config-manager --add-repo http://www.example.com/example.repo
Loaded plugins: langpacks, presto, refresh-packagekit
adding repo from: http://www.example.com/example.repo
grabbing file http://www.example.com/example.repo to /etc/yum.repos.d/example.repo
example.repo                                             |  413 B     00:00
repo saved to /etc/yum.repos.d/example.repo

Enabling a Yum Repository

To enable a particular repository or repositories, type the following at a shell prompt as root:
yum-config-manager --enable repository
…where repository is the unique repository ID (use yum repolist all to list available repository IDs). Alternatively, you can use a glob expression to enable all matching repositories:
yum-config-manager --enable glob_expression
For example, to disable repositories defined in the [example], [example-debuginfo], and [example-source]sections, type:
~]# yum-config-manager --enable example\*
Loaded plugins: langpacks, presto, refresh-packagekit
============================== repo: example ==============================
[example]
bandwidth = 0
base_persistdir = /var/lib/yum/repos/x86_64/6Server
baseurl = http://www.example.com/repo/6Server/x86_64/
cache = 0
cachedir = /var/cache/yum/x86_64/6Server/example
[output truncated]
When successful, the yum-config-manager --enable command displays the current repository configuration.

Disabling a Yum Repository

To disable a Yum repository, run the following command as root:
yum-config-manager --disable repository
…where repository is the unique repository ID (use yum repolist all to list available repository IDs). Similarly to yum-config-manager --enable, you can use a glob expression to disable all matching repositories at the same time:
yum-config-manager --disable glob_expression
When successful, the yum-config-manager --disable command displays the current configuration.

5.3.6. Creating a Yum Repository

To set up a Yum repository, follow these steps:
  1. Install the createrepo package:
    ~]# yum install createrepo
  2. Copy all of the packages into one directory, such as /mnt/local_repo/.
  3. Run the createrepo --database command on that directory:
    ~]# createrepo --database /mnt/local_repo
This creates the necessary metadata for your Yum repository, as well as the sqlite database for speeding up yum operations.

5.4. Yum Plug-ins

Yum provides plug-ins that extend and enhance its operations. Certain plug-ins are installed by default. Yum always informs you which plug-ins, if any, are loaded and active whenever you call any yum command. For example:
~]# yum info yum
Loaded plugins: langpacks, presto, refresh-packagekit
[output truncated]
Note that the plug-in names which follow Loaded plugins are the names you can provide to the --disableplugins=plugin_name option.

5.4.1. Enabling, Configuring, and Disabling Yum Plug-ins

To enable Yum plug-ins, ensure that a line beginning with plugins= is present in the [main] section of /etc/yum.conf, and that its value is set to 1:
plugins=1
You can disable all plug-ins by changing this line to plugins=0.

Disabling all plug-ins is not advised

Disabling all plug-ins is not advised because certain plug-ins provide important Yum services. Disabling plug-ins globally is provided as a convenience option, and is generally only recommended when diagnosing a potential problem with Yum.
Every installed plug-in has its own configuration file in the /etc/yum/pluginconf.d/ directory. You can set plug-in specific options in these files. For example, here is the refresh-packagekit plug-in's refresh-packagekit.conf configuration file:
[main]
enabled=1
Plug-in configuration files always contain a [main] section (similar to Yum's /etc/yum.conf file) in which there is (or you can place if it is missing) an enabled= option that controls whether the plug-in is enabled when you run yum commands.
If you disable all plug-ins by setting enabled=0 in /etc/yum.conf, then all plug-ins are disabled regardless of whether they are enabled in their individual configuration files.
If you merely want to disable all Yum plug-ins for a single yum command, use the --noplugins option.
If you want to disable one or more Yum plug-ins for a single yum command, add the --disableplugin=plugin_name option to the command. For example, to disable the presto plug-in while updating a system, type:
~]# yum update --disableplugin=presto
The plug-in names you provide to the --disableplugin= option are the same names listed after the Loaded plugins line in the output of any yum command. You can disable multiple plug-ins by separating their names with commas. In addition, you can match multiple plug-in names or shorten long ones by using glob expressions:
~]# yum update --disableplugin=presto,refresh-pack*

5.4.2. Installing Additional Yum Plug-ins

Yum plug-ins usually adhere to the yum-plugin-plugin_name package-naming convention, but not always: the package which provides the presto plug-in is named yum-presto, for example. You can install a Yum plug-in in the same way you install other packages. For instance, to install the security plug-in, type the following at a shell prompt:
~]# yum install yum-plugin-security

5.4.3. Plug-in Descriptions

The following list provides descriptions of a few useful Yum plug-ins:
fs-snapshot (yum-plugin-fs-snapshot)
The fs-snapshot plug-in extends Yum to create a snapshot of a file system before proceeding with a transaction such as a system update or package removal. When a user decides that the changes made by the transaction are unwanted, this mechanism allows the user to roll back to the changes that are stored in a snapshot.
In order for the plug-in to work, the root file system (that is, /) must be on an LVM (Logical Volume Manager) or Btrfs volume. To use the fs-snapshot plug-in on an LVM volume, take the following steps:
  1. Make sure that the volume group with the root file system has enough free extents. The required size is a function of the amount of changes to the original logical volume that is expected during the life of the snapshot. The reasonable default is 50–80 % of the original logical volume size.
    To display detailed information about a particular volume group, run the vgdisplay command in the following form as root:
    vgdisplay volume_group
    The number of free extents is listed on the Free PE / Size line.
  2. If the volume group with the root file system does not have enough free extents, add a new physical volume:
    1. As root, run the pvcreate command in the following form to initialize a physical volume for use with the Logical Volume Manager:
      pvcreate device
    2. Use the vgextend command in the following form as root to add the physical volume to the volume group:
      vgextend volume_group physical_volume
  3. Edit the configuration file located in /etc/yum/pluginconf.d/fs-snapshot.conf, and make the following changes to the [lvm] section:
    1. Change the value of the enabled option to 1:
      enabled = 1
    2. Remove the hash sign (that is, #) from the beginning of the lvcreate_size_args line, and adjust the number of logical extents to be allocated for a snapshot. For example, to allocate 80 % of the size of the original logical volume, use:
      lvcreate_size_args = -l 80%ORIGIN
    Refer to Table 5.3, “Supported fs-snapshot.conf directives” for a complete list of available configuration options.
  4. Run the desired yum command, and make sure fs-snapshot is included in the list of loaded plug-ins (the Loaded plugins line) before you confirm the changes and proceed with the transaction. The fs-snapshot plug-in displays a line in the following form for each affected logical volume:
    fs-snapshot: snapshotting file_system (/dev/volume_group/logical_volume): logical_volume_yum_timestamp
  5. Verify that the system is working as expected:
    • If you decide to keep the changes, remove the snapshot by running the lvremove command as root:
      lvremove /dev/volume_group/logical_volume_yum_timestamp
    • If you decide to revert the changes and restore the file system to a state that is saved in a snapshot, take the following steps:
      1. As root, run the command in the following form to merge a snapshot into its original logical volume:
        lvconvert --merge /dev/volume_group/logical_volume_yum_timestamp
        The lvconvert command will inform you that a restart is required in order for the changes to take effect.
      2. Restart the system as instructed. You can do so by typing the following at a shell prompt as root:
        reboot
To use the fs-snapshot plug-in on a Btrfs file system, take the following steps:
  1. Run the desired yum command, and make sure fs-snapshot is included in the list of loaded plug-ins (the Loaded plugins line) before you confirm the changes and proceed with the transaction. The fs-snapshot plug-in displays a line in the following form for each affected file system:
    fs-snapshot: snapshotting file_system: file_system/yum_timestamp
  2. Verify that the system is working as expected:
    • If you decide to keep the changes, you can optionally remove unwanted snapshots. To remove a Btrfs snapshot, use the command in the following form as root:
      btrfs subvolume delete file_system/yum_timestamp
    • If you decide to revert the changes and restore a file system to a state that is saved in a snapshot, take the following steps:
      1. Determine the identifier of a particular snapshot by using the following command as root:
        btrfs subvolume list file_system
      2. As root, configure the system to mount this snapshot by default:
        btrfs subvolume set-default id file_system
      3. Restart the system. You can do so by typing the following at a shell prompt as root:
        reboot
For more information on logical volume management, Btrfs, and file system snapshots, see the Fedora 18 Storage Administration Guide. For additional information about the plug-in and its configuration, refer to the yum-fs-snapshot(1) and yum-fs-snapshot.conf(5) manual pages.
Table 5.3. Supported fs-snapshot.conf directives
Section Directive Description
[main] enabled=value Allows you to enable or disable the plug-in. The value must be either 1 (enabled), or 0 (disabled). When installed, the plug-in is enabled by default.
exclude=list Allows you to exclude certain file systems. The value must be a space-separated list of mount points you do not want to snapshot (for example, /srv /mnt/backup). This option is not included in the configuration file by default.
[lvm] enabled=value Allows you to enable or disable the use of the plug-in on LVM volumes. The value must be either 1 (enabled), or 0 (disabled). This option is disabled by default.
lvcreate_size_args=value Allows you to specify the size of a logical volume snapshot. The value must be the -l or -L command line option for the lvcreate utility followed by a valid argument (for example, -l 80%ORIGIN).

presto (yum-presto)
The presto plug-in adds support to Yum for downloading delta RPM packages, during updates, from repositories which have presto metadata enabled. Delta RPMs contain only the differences between the version of the package installed on the client requesting the RPM package and the updated version in the repository.
Downloading a delta RPM is much quicker than downloading the entire updated package, and can speed up updates considerably. Once the delta RPMs are downloaded, they must be rebuilt to apply the difference to the currently-installed package and thus create the full, updated package. This process takes CPU time on the installing machine. Using delta RPMs is therefore a tradeoff between time-to-download, which depends on the network connection, and time-to-rebuild, which is CPU-bound. Using the presto plug-in is recommended for fast machines and systems with slower network connections, while slower machines on very fast connections may benefit more from downloading normal RPM packages, that is, by disabling presto.
refresh-packagekit (PackageKit-yum-plugin)
The refresh-packagekit plug-in updates metadata for PackageKit whenever yum is run. The refresh-packagekit plug-in is installed by default.
rhnplugin (yum-rhn-plugin)
The rhnplugin provides support for connecting to RHN Classic. This allows systems registered with RHN Classic to update and install packages from this system.
Refer to the rhnplugin(8) manual page for more information about the plug-in.
security (yum-plugin-security)
Discovering information about and applying security updates easily and often is important to all system administrators. For this reason Yum provides the security plug-in, which extends yum with a set of highly-useful security-related commands, subcommands and options.
You can check for security-related updates as follows:
~]# yum check-update --security
Loaded plugins: langpacks, presto, refresh-packagekit, security
Limiting package lists to security relevant ones
updates-testing/updateinfo                               | 329 kB     00:00
9 package(s) needed for security, out of 270 available

ConsoleKit.x86_64                    0.4.5-1.fc15                  updates
ConsoleKit-libs.x86_64               0.4.5-1.fc15                  updates
ConsoleKit-x11.x86_64                0.4.5-1.fc15                  updates
NetworkManager.x86_64                1:0.8.999-2.git20110509.fc15  updates
NetworkManager-glib.x86_64           1:0.8.999-2.git20110509.fc15  updates
[output truncated]
You can then use either yum update --security or yum update-minimal --security to update those packages which are affected by security advisories. Both of these commands update all packages on the system for which a security advisory has been issued. yum update-minimal --security updates them to the latest packages which were released as part of a security advisory, while yum update --security will update all packages affected by a security advisory to the latest version of that package available.
In other words, if:
  • the kernel-2.6.38.4-20 package is installed on your system;
  • the kernel-2.6.38.6-22 package was released as a security update;
  • then kernel-2.6.38.6-26 was released as a bug fix update,
...then yum update-minimal --security will update you to kernel-2.6.38.6-22, and yum update --security will update you to kernel-2.6.38.6-26. Conservative system administrators may want to use update-minimal to reduce the risk incurred by updating packages as much as possible.
Refer to the yum-security(8) manual page for usage details and further explanation of the enhancements the security plug-in adds to yum.

5.5. Additional Resources

http://yum.baseurl.org/wiki/Guides
The Yum Guides section of the Yum wiki contains more documentation.

Chapter 6. PackageKit

Fedora provides PackageKit for viewing, managing, updating, installing and uninstalling packages compatible with your system. PackageKit consists of several graphical interfaces that can be opened from the GNOME panel menu, or from the Notification Area when PackageKit alerts you that updates are available. For more information on PackageKit's architecture and available front ends, refer to Section 6.3, “PackageKit Architecture”.

6.1. Updating Packages with Software Update

You can open Software Updates by clicking ApplicationsSystem ToolsSoftware Update from the Activities menu, or running the gpk-update-viewer command at the shell prompt. In the Software Updates window, all available updates are listed along with the names of the packages being updated (minus the .rpm suffix, but including the CPU architecture), a short summary of the package, and, usually, short descriptions of the changes the update provides. Any updates you do not wish to install can be de-selected here by unchecking the checkbox corresponding to the update.
Installing updates with Software Update
installing 56 updates with PackageKit's software update window
Figure 6.1. Installing updates with Software Update

The updates presented in the Software Updates window only represent the currently-installed packages on your system for which updates are available; dependencies of those packages, whether they are existing packages on your system or new ones, are not shown until you click Install Updates.
PackageKit utilizes the fine-grained user authentication capabilities provided by the PolicyKit toolkit whenever you request it to make changes to the system. Whenever you instruct PackageKit to update, install or remove packages, you will be prompted to enter the superuser password before changes are made to the system.
If you instruct PackageKit to update the kernel package, then it will prompt you after installation, asking you whether you want to reboot the system and thereby boot into the newly-installed kernel.

6.1.1. Setting the Update-Checking Interval

Selecting ApplicationsOtherSoftware Updates from the Activities menu opens the Software Update Preferences window. The Update Settings tab allows you to define the interval at which PackageKit checks for package updates, as well as whether or not to automatically install all updates or only security updates. Leaving the Check for updates when using mobile broadband box unchecked is handy for avoiding extraneous bandwidth usage when using a wireless connection on which you are charged for the amount of data you download.
Setting PackageKit's update-checking interval
Setting the update-checking interval for PackageKit
Figure 6.2. Setting PackageKit's update-checking interval

6.1.2. Setting the Software Sources

To select which package repositories to use to install software updates, select ApplicationsOtherSoftware Updates from the Activities menu, and click the Software Sources tab of the Software Update Preferences window.
Setting PackageKit's software sources
Setting the software sources for PackageKit
Figure 6.3. Setting PackageKit's software sources

PackageKit refers to Yum repositories as software sources. It obtains all packages from enabled software sources.The Software Sources tab shows the repository name, as written on the name=My Repository Name field of all [repository] sections in the /etc/yum.conf configuration file, and in all repository.repo files in the /etc/yum.repos.d/ directory.
Entries which are checked in the Enabled column indicate that the corresponding repository will be used to locate packages to satisfy all update and installation requests (including dependency resolution). The Enabled column corresponds to the enabled=<1 or 0> field in [repository] sections. Checking an unchecked box enables the Yum repository, and unchecking it disables it. Performing either function causes PolicyKit to prompt for superuser authentication to enable or disable the repository. PackageKit actually inserts the enabled=<1 or 0> line into the correct [repository] section if it does not exist, or changes the value if it does. This means that enabling or disabling a repository through the Software Sources window causes that change to persist after closing the window or rebooting the system. The ability to quickly enable and disable repositories based on our needs is a highly-convenient feature of PackageKit.
Note that it is not possible to add or remove Yum repositories through PackageKit.

Showing source RPM, test, and debuginfo repositories

Checking the box at the bottom of the Software Sources tab causes PackageKit to display source RPM, testing and debuginfo repositories as well. This box is unchecked by default.

6.2. Using Add/Remove Software

PackageKit's Software Update GUI window is a separate application from its Add/Remove Software application, although the two have intuitively similar interfaces. To find and install a new package, select ApplicationsSystem ToolsAdd/Remove Software from the Activities menu, or run the gpk-application command at the shell prompt.
PackageKit's Add/Remove Software window
Viewing PackageKit's Add/Remove Software window
Figure 6.4. PackageKit's Add/Remove Software window

6.2.1. Refreshing Software Sources (Yum Repositories)

To enable or disable a Yum repository, open a dialog box by sclicking SystemSoftware Sources, and select the Software Sources tab. Refer to Section 6.1.2, “Setting the Software Sources” for more information on available configuration options.
After enabling and/or disabling the correct Yum repositories, make sure that you have the latest list of available packages. Click on SystemRefresh Package Lists and PackageKit will obtain the latest lists of packages from all enabled software sources, that is, Yum repositories.

6.2.2. Finding Packages with Filters

You can view the list of all configured and unfiltered (see below) Yum repositories by opening Add/Remove Software and clicking SystemSoftware Sources. Once the software sources have been updated, it is often beneficial to apply some filters so that PackageKit retrieves the results of our Find queries faster. This is especially helpful when performing many package searches. Four of the filters in the Filters drop-down menu are used to split results by matching or not matching a single criterion. By default when PackageKit starts, these filters are all unapplied (No Filter), but once you do filter by one of them, that filter remains set until you either change it or close PackageKit.
Because you are usually searching for available packages that are not installed on the system, click FiltersInstalled and select the Only Available radio button.
Filtering out already-installed packages
filtering out packages which are already installed
Figure 6.5. Filtering out already-installed packages

Also, unless we require development files such as C header files, we can filter for Only End User Files and, in doing so, filter out all of the package_name-devel packages we are not interested in.
Filtering out development packages from the list of Find results
filtering out development packages from our results
Figure 6.6. Filtering out development packages from the list of Find results

The two remaining filters with submenus are:
Graphical
Narrows the search to either applications which provide a GUI interface (Only Graphical) or those that do not. This filter is useful when browsing for GUI applications that perform a specific function.
Free
Search for packages which are considered to be free software Refer to the Fedora Licensing List for details on approved licenses.
The remaining checkbox filters are always either checked or unchecked. They are:
Hide Subpackages
Checking the Hide Subpackages checkbox filters out generally-uninteresting packages that are typically only dependencies of other packages that we want. For example, checking Hide Subpackages and searching for package would cause the following related packages to be filtered out of the Find results (if it exists):
  • package-devel
  • package-libs
  • package-libs-devel
  • package-debuginfo
Only Newest Packages
Checking Only Newest Packages filters out all older versions of the same package from the list of results, which is generally what we want.

Using the Only Newest Packages filter

Checking Only Newest Packages filters out all but the most recent version of any package from the results list. This filter is often combined with the Only Available filter to search for the latest available versions of new (not installed) packages.
Only native packages
Checking the Only Native Packages box on a multilib system causes PackageKit to omit listing results for packages compiled for the architecture that runs in compatibility mode. For example, enabling this filter on a 64-bit system with an AMD64 CPU would cause all packages built for the 32-bit x86 CPU architecture not to be shown in the list of results, even though those packages are able to run on an AMD64 machine. Packages which are architecture-agnostic (i.e. noarch packages such as crontabs-1.10-32.1.el6.noarch.rpm) are never filtered out by checking Only Native Packages. This filter has no affect on non-multilib systems, such as x86 machines.

6.2.3. Installing and Removing Packages (and Dependencies)

With the two filters selected, Only Available and Only End User Files, search for the htop interactive process viewer and highlight the package. You now have access to some very useful information about it, including: a clickable link to the project homepage; the Yum package group it is found in, if any; the license of the package; a pointer to the GNOME menu location from where the application can be opened, if applicable; and the size of the package, which is relevant when we download and install it.
Viewing and installing a package with PackageKit's Add/Remove Software window
Viewing and installing a package with PackageKit's Add/Remove Software window
Figure 6.7. Viewing and installing a package with PackageKit's Add/Remove Software window

When the checkbox next to a package or group is checked, then that item is already installed on the system. Checking an unchecked box causes it to be marked for installation, which only occurs when the Apply button is clicked. In this way, you can search for and select multiple packages or package groups before performing the actual installation transactions. Additionally, you can remove installed packages by unchecking the checked box, and the removal will occur along with any pending installations when Apply is pressed. Dependency resolution , which may add additional packages to be installed or removed, is performed after pressing Apply. PackageKit will then display a window listing those additional packages to install or remove, and ask for confirmation to proceed.
Check htop and click the Apply button. You will then be prompted for the superuser password; enter it, and PackageKit will install htop. One nice feature of PackageKit is that, following installation, it sometimes presents you with a list of your newly-installed applications and offer you the choice of running them immediately. Alternatively, you will remember that finding a package and selecting it in the Add/Remove Software window shows you the Location of where in the GNOME menus its application shortcut is located, which is helpful when you want to run it.
Once it is installed, you can run htop, a colorful and enhanced version of the top process viewer, by opening a shell prompt and entering:
htop
htop is nifty, but we decide that top is good enough for us and we want to uninstall it. Remembering that we need to change the Only Available filter we recently used to install it to Only Installed in FiltersInstalled, we search for htop again and uncheck it. The program did not install any dependencies of its own; if it had, those would be automatically removed as well, as long as they were not also dependencies of any other packages still installed on our system.

Removing a package when other packages depend on it

Although PackageKit automatically resolves dependencies during package installation and removal, it is unable to remove a package without also removing packages which depend on it. This type of operation can only be performed by RPM, is not advised, and can potentially leave your system in a non-functioning state or cause applications to misbehave and/or crash.
Removing a package with PackageKit's Add/Remove Software window
Removing the htop package with PackageKit's Add/Remove Software window
Figure 6.8. Removing a package with PackageKit's Add/Remove Software window

6.2.4. Installing and Removing Package Groups

PackageKit also has the ability to install Yum package groups, which it calls Package collections. Clicking on Package collections in the top-left list of categories in the Software Updates window allows us to scroll through and find the package group we want to install. In this case, we want to install Czech language support (the Czech Support group). Checking the box and clicking Apply informs us how many additional packages must be installed in order to fulfill the dependencies of the package group.
Installing the Czech Support package group
Using PackageKit to install Czech language support with PackageKit's Add/Remove Software window
Figure 6.9. Installing the Czech Support package group

Similarly, installed package groups can be uninstalled by selecting Package collections, unchecking the appropriate checkbox, and applying.

6.2.5. Viewing the Transaction Log

PackageKit maintains a log of the transactions that it performs. To view the log, from the Add/Remove Software window, click SystemSoftware Log, or run the gpk-log command at the shell prompt.
The Software Log Viewer shows the Action, such as Updated Packages or Installed Packages, the Date on which that action was performed, the Username of the user who performed the action, and the front end Application the user used (such as Add/Remove Software, or Update System). The Details column provides the types of the transactions, such as Updated, Installed, or Removed, as well as the list of packages the transactions were performed on.
Viewing the log of package management transactions with the Software Log Viewer
Viewing the log of package management transactions with PackageKit's Software Log Viewer window
Figure 6.10. Viewing the log of package management transactions with the Software Log Viewer

Typing the name of a package in the top text entry field filters the list of transactions to those which affected that package.

6.3. PackageKit Architecture

Fedora provides the PackageKit suite of applications for viewing, updating, installing and uninstalling packages and package groups compatible with your system. Architecturally, PackageKit consists of several graphical front ends that communicate with the packagekitd daemon back end, which communicates with a package manager-specific back end that utilizes Yum to perform the actual transactions, such as installing and removing packages, etc.
Table 6.1, “PackageKit GUI windows, menu locations, and shell prompt commands” shows the name of the GUI window, how to start the window from the GNOME desktop or from the Add/Remove Software window, and the name of the command line application that opens that window.
Table 6.1. PackageKit GUI windows, menu locations, and shell prompt commands
Window Title Function How to Open Shell Command
Add/Remove Software Install, remove or view package info
From the GNOME panel: SystemAdministrationAdd/Remove Software
gpk-application
Software Update Perform package updates
From the GNOME panel: SystemAdministrationSoftware Update
gpk-update-viewer
Software Sources Enable and disable Yum repositories
From Add/Remove Software: SystemSoftware Sources
gpk-repo
Software Log Viewer View the transaction log
From Add/Remove Software: SystemSoftware Log
gpk-log
Software Update Preferences Set PackageKit preferences gpk-prefs
(Notification Area Alert) Alerts you when updates are available
From the GNOME panel: SystemPreferencesStartup Applications, Startup Programs tab
gpk-update-icon

The packagekitd daemon runs outside the user session and communicates with the various graphical front ends. The packagekitd daemon[1] communicates via the DBus system message bus with another back end, which utilizes Yum's Python API to perform queries and make changes to the system. On Linux systems other than Red Hat Enterprise Linux and Fedora, packagekitd can communicate with other back ends that are able to utilize the native package manager for that system. This modular architecture provides the abstraction necessary for the graphical interfaces to work with many different package managers to perform essentially the same types of package management tasks. Learning how to use the PackageKit front ends means that you can use the same familiar graphical interface across many different Linux distributions, even when they utilize a native package manager other than Yum.
In addition, PackageKit's separation of concerns provides reliability in that a crash of one of the GUI windows—or even the user's X Window session—will not affect any package management tasks being supervised by the packagekitd daemon, which runs outside of the user session.
All of the front end graphical applications discussed in this chapter are provided by the gnome-packagekit package instead of by PackageKit and its dependencies. Users working in a KDE environment may prefer to install the kpackagekit package, which provides a KDE interface for PackageKit.
Finally, PackageKit also comes with a console-based front end called pkcon.

6.4. Additional Resources

PackageKit home page — http://www.packagekit.org/index.html
Information about and mailing lists for PackageKit.
PackageKit FAQ — http://www.packagekit.org/pk-faq.html
An informative list of Frequently Asked Questions for the PackageKit software suite.
PackageKit Feature Matrix — http://www.packagekit.org/pk-matrix.html
Cross-reference PackageKit-provided features with the long list of package manager back ends.


[1] System daemons are typically long-running processes that provide services to the user or to other programs, and which are started, often at boot time. Daemons respond to the systemctl command and can be turned on or off permanently by using the systemctl enable or systemctl disablecommands. They can typically be recognized by a d appended to their name, such as the packagekitd daemon. Refer to Chapter 9, Services and Daemons for information about system services.

Part III. Networking

This part describes how to configure the network on Fedora.

Chapter 7. NetworkManager

NetworkManager is a dynamic network control and configuration system that attempts to keep network devices and connections up and active when they are available. NetworkManager consists of a core daemon, a GNOME Notification Area applet that provides network status information, and graphical configuration tools that can create, edit and remove connections and interfaces. NetworkManager can be used to configure the following types of connections: Ethernet, wireless, mobile broadband (such as cellular 3G), and DSL and PPPoE (Point-to-Point over Ethernet). In addition, NetworkManager allows for the configuration of network aliases, static routes, DNS information and VPN connections, as well as many connection-specific parameters. Finally, NetworkManager provides a rich API via D-Bus which allows applications to query and control network configuration and state.
Previous versions of Fedora included the Network Administration Tool, which was commonly known as system-config-network after its command line invocation. In Fedora 18, NetworkManager replaces the former Network Administration Tool while providing enhanced functionality, such as user-specific and mobile broadband configuration. It is also possible to configure the network in Fedora 18 by editing interface configuration files; refer to Chapter 8, Network Interfaces for more information.
NetworkManager may be installed by default on Fedora. To ensure that it is, first run the following command as the root user:
~]# yum install NetworkManager

7.1. The NetworkManager Daemon

The NetworkManager daemon runs with root privileges and is usually configured to start up at boot time. You can determine whether the NetworkManager daemon is running by entering this command:
~]$ systemctl status NetworkManager.service
NetworkManager.service - Network Manager
	  Loaded: loaded (/lib/systemd/system/NetworkManager.service; enabled)
	  Active: active (running) since Fri, 08 Mar 2013 12:50:04 +0100; 3 days ago      
The systemctl status command will report NetworkManager as Active: inactive (dead) if the NetworkManager service is not running. To start it for the current session run the following command as the root user:
~]# systemctl start NetworkManager.service
Run the systemctl enable command to ensure that NetworkManager starts up every time the system boots:
~]# systemctl enable NetworkManager.service
For more information on starting, stopping and managing services and runlevels, refer to Chapter 9, Services and Daemons.

7.2. Interacting with NetworkManager

Users do not interact with the NetworkManager system service directly. Instead, you can perform network configuration tasks via NetworkManager's Notification Area applet. The applet has multiple states that serve as visual indicators for the type of connection you are currently using.
NetworkManager applet states
A row of four icons representing NetworkManager applet states
Figure 7.1. NetworkManager applet states

If you do not see the NetworkManager applet in the GNOME panel, and assuming that the NetworkManager package is installed on your system, you can start the applet by running the following command as a normal user (not root):
~]$ nm-applet &
After running this command, the applet appears in your Notification Area.

7.2.1. Connecting to a Network

When you click on the applet icon, you are presented with:
  • a list of categorized networks you are currently connected to (such as Wired and Wireless);
  • a list of all Available Networks that NetworkManager has detected;
  • options for connecting to any configured Virtual Private Networks (VPNs); and,
  • options for connecting to hidden or new wireless networks.
If you are connected to a network, its name is presented first under its network type, such as Wired or Wireless with a bulletpoint to the left. When many networks are available, such as wireless access points, the More networks expandable menu entry appears.
The NetworkManager applet's drop-down menu, showing all available and connected-to networks
A screen shot of the NetworkManager applet's drop-down menu, showing all available and connected-to networks
Figure 7.2. The NetworkManager applet's drop-down menu, showing all available and connected-to networks

7.2.2. Configuring New and Editing Existing Connections

Click on the NetworkManager applet to open the drop-down menu, this is the main point of entry for interacting with NetworkManager to configure connections.
If the system has detected a wired connection, the Wired menu entry will appear. If the system has detected a wireless card, then you will also see a Wireless menu entry. Clicking the Wired and Wireless labels or the associated ON OFF indicator to the right will toggle the status between ON and OFF.
Finally, clicking on the Network Settings menu entry opens the Network window, from where you can view some basic network configuration information and initiate configuration tasks.
Then, to configure:

7.2.3. Connecting to a Network Automatically

For any connection type you add or configure, you can choose whether you want NetworkManager to try to connect to that network automatically when it is available.
Procedure 7.1. Configuring NetworkManager to Connect to a Network Automatically When Detected
  1. Click on the NetworkManager applet icon in the Notification Area.
  2. Click Network Settings.
    The Network window appears.
  3. Select the type of connection from the left-hand-side menu.
  4. Click on Options. The Editing window appears.
  5. Click on the General tab.
  6. Select Automatically connect to this network when it is available to cause NetworkManager to auto-connect to the connection whenever NetworkManager detects that it is available. Unselect the checkbox if you do not want NetworkManager to connect automatically. If the box is unchecked, you will have to select that connection manually in the NetworkManager applet's initial menu to cause it to connect.

7.2.4. User and System Connections

NetworkManager connections are always either user connections or system connections. Depending on the system-specific policy that the administrator has configured, users may need root privileges to create and modify system connections. NetworkManager's default policy enables users to create and modify user connections, but requires them to have root privileges to add, modify or delete system connections.
User connections are so-called because they are specific to the user who creates them. In contrast to system connections, whose configurations are stored under the /etc/sysconfig/network-scripts/ directory (mainly in ifcfg-<network_type> interface configuration files), user connection settings are stored in the GConf configuration database and the GNOME keyring, and are only available during login sessions for the user who created them. Thus, logging out of the desktop session causes user-specific connections to become unavailable.

Increase security by making VPN connections user-specific

Because NetworkManager uses the GConf and GNOME keyring applications to store user connection settings, and because these settings are specific to your desktop session, it is highly recommended to configure your personal VPN connections as user connections. If you do so, other non-root users on the system cannot view or access these connections in any way.
System connections, on the other hand, become available at boot time and can be used by other users on the system without first logging in to a desktop session.
NetworkManager can quickly and conveniently convert user to system connections and vice versa. Converting a user connection to a system connection causes NetworkManager to create the relevant interface configuration files under the /etc/sysconfig/network-scripts/ directory, and to delete the GConf settings from the user's session. Conversely, converting a system to a user-specific connection causes NetworkManager to remove the system-wide configuration files and create the corresponding GConf/GNOME keyring settings.
Procedure 7.2. Changing a Connection to be User-Specific instead of System-Wide, or Vice-Versa

Root privileges may be required

Depending on the system's policy, you may need root privileges on the system in order to change whether a connection is user-specific or system-wide.
  1. Click on the NetworkManager applet icon in the Notification Area and click Network Settings. The Network window appears.
  2. Select the menu entry for the type of network connection you want to configure.
  3. Select the Options button.
  4. Select the General tab.
  5. Check the All users may connect to this network checkbox to ask NetworkManager to make the connection a system-wide connection. Depending on system policy, you may then be prompted for the root password by the PolicyKit application. If so, enter the root password to finalize the change.
    Conversely, uncheck the All users may connect to this network checkbox to make the connection user-specific.

7.3. Establishing Connections

7.3.1. Establishing a Wired (Ethernet) Connection

To establish a wired network connection, click on the NetworkManager applet to open its menu, then click on Network Settings. This opens the Network window.
Select the Wired menu entry and then click Options.
The system startup scripts create and configure a single wired connection called em1 by default on all systems. Although you can edit the default connection, creating a new wired connection for your custom settings is recommended. You can create a new wired connection by changing the connection name and clicking the Save button.

The dialog for adding and editing connections is the same

When you add a new connection by clicking the Save button, NetworkManager creates a new configuration file for that connection and uses the same dialog that is used for editing an existing connection. There is no difference between these dialogs. In effect, you are always editing a connection; the difference only lies in whether that connection previously existed or was just created by NetworkManager when you clicked Save.

Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings

Three settings in the Editing dialog are common to all connection types:
  • At the top of the window, above the tabs: Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the Network window.
  • On the General tab: Automatically connect to this network when it is available — Check this box if you want NetworkManager to auto-connect to this connection when it is available. Refer to Section 7.2.3, “Connecting to a Network Automatically” for more information.
  • On the General tab: All users may connect to this network — Check this box to create a connection available to all users on the system. Changing this setting may require root privileges. Refer to Section 7.2.4, “User and System Connections” for details.

Configuring the Wired Tab

The final three configurable settings are located within the Wired tab itself: the first is a text-entry field where you can specify a MAC (Media Access Control) address, manually or by using the dropdown list, in order to lock the connection to a specifc hardware device. The second alows specifying a cloned MAC address to be used, and the third allows you to specify the MTU (Maximum Transmission Unit) value. Normally, you can leave the MAC address field blank and the MTU set to automatic. These defaults will suffice unless you are associating a wired connection with a second or specific NIC, or performing advanced networking. In such cases, refer to the following descriptions:
MAC Address
Network hardware such as a Network Interface Card (NIC) has a unique MAC address (Media Access Control; also known as a hardware address) that identifies it to the system. Running the ip addr command will show the MAC address associated with each interface. For example, in the following ip addr output, the MAC address for the eth0 interface (which is 52:54:00:26:9e:f1) immediately follows the link/ether keyword:
~]# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000
    link/ether 52:54:00:26:9e:f1 brd ff:ff:ff:ff:ff:ff
    inet 192.168.122.251/24 brd 192.168.122.255 scope global eth0
    inet6 fe80::5054:ff:fe26:9ef1/64 scope link
       valid_lft forever preferred_lft forever
A single system can have one or more NICs installed on it. The MAC address field therefore allows you to associate a specific NIC with a specific connection (or connections). As mentioned, you can determine the MAC address using the ip addr command, and then copy and paste that value into the MAC address text-entry field.
MTU
The MTU (Maximum Transmission Unit) value represents the size in bytes of the largest packet that the connection will use to transmit. This value defaults to 1500 when using IPv4, or a variable number 1280 or higher for IPv6, and does not generally need to be specified or changed.

Saving Your New (or Modified) Connection and Making Further Configurations

Once you have finished editing your wired connection, click the Apply button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 7.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:

7.3.2. Establishing a Wireless Connection

This section explains how to use NetworkManager to configure a wireless (also known as Wi-Fi or 802.1a/b/g/n) connection to an Access Point.
To configure a mobile broadband (such as 3G) connection, refer to Section 7.3.3, “Establishing a Mobile Broadband Connection”.

Quickly Connecting to an Available Access Point

The easiest way to connect to an available access point is to click on the NetworkManager applet, locate the Service Set Identifier (SSID) of the access point in the list of available networks, and click on it. If the access point is secured, a dialog prompts you for authentication.
NetworkManager tries to auto-detect the type of security used by the access point. If there are multiple possibilities, NetworkManager guesses the security type and presents it in the Wireless security dropdown menu. To see if there are multiple choices, click the Wireless security dropdown menu and select the type of security the access point is using. If you are unsure, try connecting to each type in turn. Finally, enter the key or passphrase in the Password field. Certain password types, such as a 40-bit WEP or 128-bit WPA key, are invalid unless they are of a requisite length. The Connect button will remain inactive until you enter a key of the length required for the selected security type. To learn more about wireless security, refer to Section 7.4.2, “Configuring Wireless Security”.
If NetworkManager connects to the access point successfully, its applet icon will change into a graphical indicator of the wireless connection's signal strength.
Applet icon indicating wireless connection signal strength
A screen shot of the Signal Strength Applet icon indicating wireless connection signal strength
Figure 7.3. Applet icon indicating wireless connection signal strength

Connecting to a Hidden Wireless Network

All access points have a Service Set Identifier (SSID) to identify them. However, an access point may be configured not to broadcast its SSID, in which case it is hidden, and will not show up in NetworkManager's list of Available networks. You can still connect to a wireless access point that is hiding its SSID as long as you know its SSID, authentication method, and secrets.
To connect to a hidden wireless network, click NetworkManager's applet icon and then click Network Settings. The Network window appears. Select the Wireless menu entry and then the Wireless connections are displayed. Select Connect to a Hidden Network. The Hidden Wi-Fi network dialog window appears.
Hidden wireless network dialog window
A screen shot of NetworkManager's Hidden wireless network dialog window
Figure 7.4. Hidden wireless network dialog window

If you have connected to the hidden network before, use the Connection drop-down list to select it, and click Connect. If you have not, leave the Connection dropdown as New..., enter the SSID of the hidden network, select its Wireless security method, enter the correct authentication secrets, and click Connect.
For more information on wireless security settings, refer to Section 7.4.2, “Configuring Wireless Security”.

Editing a Connection, or Creating a Completely New One

You can create a new connection by clicking on the NetworkManager applet to open its menu.
  1. Click on the NetworkManager applet icon in the Notification Area and click Network Settings. The Network window appears.
  2. Select the Wireless menu entry. The list of available SSIDs appears
  3. Select the SSID you want to connect to. You may be prompted for a password or encryption keys.
  4. Click the arrow to the right of the SSID to configure the connection. The Network window displays connection status and essential information for the connection.
  5. Select the Settings button. The Editing window appears.

Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings

Three settings in the Editing dialog are common to all connection types:
  • At the top of the window, above the tabs: Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the Network window. By default, wireless connections are named the same as the SSID of the wireless access point. You can rename the wireless connection without affecting its ability to connect, as in the example above, but it is recommended to retain the SSID name.
  • On the General tab: Automatically connect to this network when it is available — Check this box if you want NetworkManager to auto-connect to this connection when it is available. Refer to Section 7.2.3, “Connecting to a Network Automatically” for more information.
  • On the General tab: All users may connect to this network — Check this box to create a connection available to all users on the system. Changing this setting may require root privileges. Refer to Section 7.2.4, “User and System Connections” for details.

Configuring the Wireless Tab

SSID
All access points have a Service Set identifier to identify them. However, an access point may be configured not to broadcast its SSID, in which case it is hidden, and will not show up in NetworkManager's list of Available networks. You can still connect to a wireless access point that is hiding its SSID as long as you know its SSID (and authentication secrets).
For information on connecting to a hidden wireless network, refer to the section called “Connecting to a Hidden Wireless Network”.
Mode
Infrastructure — Set Mode to Infrastructure if you are connecting to a dedicated wireless access point or one built into a network device such as a router or a switch.
Ad-hoc — Set Mode to Ad-hoc if you are creating a peer-to-peer network for two or more mobile devices to communicate directly with each other. If you use Ad-hoc mode, referred to as Independent Basic Service Set (IBSS) in the 802.11 standard, you must ensure that the same SSID is set for all participating wireless devices, and that they are all communicating over the same channel.
BSSID
The Basic Service Set Identifier (BSSID) is the MAC address of the specific wireless access point you are connecting to when in Infrastructure mode. This field is blank by default, and you are able to connect to a wireless access point by SSID without having to specify its BSSID. If the BSSID is specified, it will force the system to associate to a specific access point only.
For ad-hoc networks, the BSSID is generated randomly by the mac80211 subsystem when the ad-hoc network is created. It is not displayed by NetworkManager
MAC address
Like an Ethernet Network Interface Card (NIC), a wireless adapter has a unique MAC address (Media Access Control; also known as a hardware address) that identifies it to the system. Running the ip addr command will show the MAC address associated with each interface. For example, in the following ip addr output, the MAC address for the wlan0 interface (which is 00:1c:bf:02:f8:70) immediately follows the link/ether keyword:
~]# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 1000
    link/ether 52:54:00:26:9e:f1 brd ff:ff:ff:ff:ff:ff
    inet 192.168.122.251/24 brd 192.168.122.255 scope global eth0
    inet6 fe80::5054:ff:fe26:9ef1/64 scope link
       valid_lft forever preferred_lft forever
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP qlen 1000
    link/ether 00:1c:bf:02:f8:70 brd ff:ff:ff:ff:ff:ff
    inet 10.200.130.67/24 brd 10.200.130.255 scope global wlan0
    inet6 fe80::21c:bfff:fe02:f870/64 scope link
       valid_lft forever preferred_lft forever
A single system could have one or more wireless network adapters connected to it. The MAC address field therefore allows you to associate a specific wireless adapter with a specific connection (or connections). As mentioned, you can determine the MAC address using the ip addr command, and then copy and paste that value into the MAC address text-entry field.
MTU
The MTU (Maximum Transmission Unit) value represents the size in bytes of the largest packet that the connection will use to transmit. If set to a non-zero number, only packets of the specified size or smaller will be transmitted. Larger packets are broken up into multiple Ethernet frames. It is recommended to leave this setting on automatic.

Saving Your New (or Modified) Connection and Making Further Configurations

Once you have finished editing the wireless connection, click the Apply button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can successfully connect to your the modified connection by selecting it from the NetworkManager Notification Area applet. See Section 7.2.1, “Connecting to a Network” for details on selecting and connecting to a network.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:

7.3.3. Establishing a Mobile Broadband Connection

You can use NetworkManager's mobile broadband connection abilities to connect to the following 2G and 3G services:
  • 2G — GPRS (General Packet Radio Service) or EDGE (Enhanced Data Rates for GSM Evolution)
  • 3G — UMTS (Universal Mobile Telecommunications System) or HSPA (High Speed Packet Access)
Your computer must have a mobile broadband device (modem), which the system has discovered and recognized, in order to create the connection. Such a device may be built into your computer (as is the case on many notebooks and netbooks), or may be provided separately as internal or external hardware. Examples include PC card, USB Modem or Dongle, mobile or cellular telephone capable of acting as a modem.
Procedure 7.3. Adding a New Mobile Broadband Connection
You can configure a mobile broadband connection by opening the Network Connections window and selecting the Mobile Broadband tab.
  1. Open the Network Connections window by running, as a normal user:
    ~]$ nm-connection-editor &
    
    The Network Connections window appears.
  2. Select the Mobile Broadband tab.
  3. Click the Add button to open the Set up a Mobile Broadband Connection assistant.
  4. Under Create a connection for this mobile broadband device, choose the 2G- or 3G-capable device you want to use with the connection. If the dropdown menu is inactive, this indicates that the system was unable to detect a device capable of mobile broadband. In this case, click Cancel, ensure that you do have a mobile broadband-capable device attached and recognized by the computer and then retry this procedure. Click the Forward button.
  5. Select the country where your service provider is located from the list and click the Forward button.
  6. Select your provider from the list or enter it manually. Click the Forward button.
  7. Select your payment plan from the dropdown menu and confirm the Access Point Name (APN) is correct. Click the Forward button.
  8. Review and confirm the settings and then click the Apply button.
  9. Edit the mobile broadband-specific settings by referring to the Configuring the Mobile Broadband Tab description below .
Procedure 7.4. Editing an Existing Mobile Broadband Connection
Follow these steps to edit an existing mobile broadband connection.
  1. Open the Network Connections window by running, as a normal user:
    ~]$ nm-connection-editor &
    
    The Network Connections window appears.
  2. Select the Mobile Broadband tab.
  3. Select the connection you wish to edit and click the Edit button.
  4. Configure the connection name, auto-connect behavior, and availability settings.
    Three settings in the Editing dialog are common to all connection types:
    • Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the Mobile Broadband tab of the Network Connections window.
    • Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. Refer to Section 7.2.3, “Connecting to a Network Automatically” for more information.
    • Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require root privileges. Refer to Section 7.2.4, “User and System Connections” for details.
  5. Edit the mobile broadband-specific settings by referring to the Configuring the Mobile Broadband Tab description below .

Saving Your New (or Modified) Connection and Making Further Configurations

Once you have finished editing your mobile broadband connection, click the Apply button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 7.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:

Configuring the Mobile Broadband Tab

If you have already added a new mobile broadband connection using the assistant (refer to Procedure 7.3, “Adding a New Mobile Broadband Connection” for instructions), you can edit the Mobile Broadband tab to disable roaming if home network is not available, assign a network ID, or instruct NetworkManager to prefer a certain technology (such as 3G or 2G) when using the connection.
Number
The number that is dialed to establish a PPP connection with the GSM-based mobile broadband network. This field may be automatically populated during the initial installation of the broadband device. You can usually leave this field blank and enter the APN instead.
Username
Enter the username used to authenticate with the network. Some providers do not provide a username, or accept any username when connecting to the network.
Password
Enter the password used to authenticate with the network. Some providers do not provide a password, or accept any password.
APN
Enter the Access Point Name (APN) used to establish a connection with the GSM-based network. Entering the correct APN for a connection is important because it often determines:
  • how the user is billed for their network usage; and/or
  • whether the user has access to the Internet, an intranet, or a subnetwork.
Network ID
Entering a Network ID causes NetworkManager to force the device to register only to a specific network. This can be used to ensure the connection does not roam when it is not possible to control roaming directly.
Type
Any — The default value of Any leaves the modem to select the fastest network.
3G (UMTS/HSPA) — Force the connection to use only 3G network technologies.
2G (GPRS/EDGE) — Force the connection to use only 2G network technologies.
Prefer 3G (UMTS/HSPA) — First attempt to connect using a 3G technology such as HSPA or UMTS, and fall back to GPRS or EDGE only upon failure.
Prefer 2G (GPRS/EDGE) — First attempt to connect using a 2G technology such as GPRS or EDGE, and fall back to HSPA or UMTS only upon failure.
Allow roaming if home network is not available
Uncheck this box if you want NetworkManager to terminate the connection rather than transition from the home network to a roaming one, thereby avoiding possible roaming charges. If the box is checked, NetworkManager will attempt to maintain a good connection by transitioning from the home network to a roaming one, and vice versa.
PIN
If your device's SIM (Subscriber Identity Module) is locked with a PIN (Personal Identification Number), enter the PIN so that NetworkManager can unlock the device. NetworkManager must unlock the SIM if a PIN is required in order to use the device for any purpose.

7.3.4. Establishing a VPN Connection

Establishing an encrypted Virtual Private Network (VPN) enables you to communicate securely between your Local Area Network (LAN), and another, remote LAN. After successfully establishing a VPN connection, a VPN router or gateway performs the following actions upon the packets you transmit:
  1. it adds an Authentication Header for routing and authentication purposes;
  2. it encrypts the packet data; and,
  3. it encloses the data with an Encapsulating Security Payload (ESP), which constitutes the decryption and handling instructions.
The receiving VPN router strips the header information, decrypts the data, and routes it to its intended destination (either a workstation or other node on a network). Using a network-to-network connection, the receiving node on the local network receives the packets already decrypted and ready for processing. The encryption/decryption process in a network-to-network VPN connection is therefore transparent to clients.
Because they employ several layers of authentication and encryption, VPNs are a secure and effective means of connecting multiple remote nodes to act as a unified intranet.
Procedure 7.5. Adding a New VPN Connection
  1. You can configure a new VPN connection by opening the Network window and selecting the VPN menu entry.
  2. Click on the NetworkManager applet icon in the Notification Area. Clicking on the Network Settings menu entry opens the Network window, from where you can view some basic network configuration information and initiate configuration tasks.
  3. Click on the VPN menu entry followed by Configure and proceed to Section 7.3.4, “Establishing a VPN Connection”. If there is no VPN menu entry click on the plus sign at the bottom. A dialog box appears. Ensure the interface is set to VPN.

    A VPN plug-in is required

    The appropriate NetworkManager VPN plug-in for the VPN type you want to configure must be installed. (refer to Section 5.2.4, “Installing Packages” for more information on how to install new packages in Fedora 18).
  4. Click the Create button to open the Choose a VPN Connection Type assistant.
  5. Select the VPN protocol for the gateway you are connecting to from the dropdown menu. The VPN protocols available for selection in the dropdown menu corresponds to the NetworkManager VPN plug-ins installed. For example, if the NetworkManager VPN plug-in for openswanis installed then the IPsec based VPN will be selectable from the dropdown menu.
    After selecting the correct one, press the Create... button.
  6. The Editing VPN Connection 1 window then appears. This window presents settings customized for the type of VPN connection you selected in Step 5.
You can configure an existing VPN connection by opening the Network window and selecting the VPN menu entry.
  1. Click on the NetworkManager applet icon in the Notification Area and click Network Settings. The Network window appears.
  2. Select the VPN menu entry.
  3. Select the connection you wish to edit and click the Configure button.
Editing the newly-created VPN connection 1.
A screenshot of the Editing VPN connection 1 window. The VPN tab is on the left and in the foreground
Figure 7.5. Editing the newly-created VPN connection 1.

Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings

Three settings in the Editing dialog are common to all connection types:
  • Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the VPN tab of the Network Connections window.
  • Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. Refer to Section 7.2.3, “Connecting to a Network Automatically” for more information.
  • Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require root privileges. Refer to Section 7.2.4, “User and System Connections” for details.

Configuring the VPN Tab

Gateway
The name or IP address of the remote VPN gateway.
Group name
The name of a VPN group configured on the remote gateway.
User password
If required, enter the password used to authenticate with the VPN.
Group password
If required, enter the password used to authenticate with the VPN.
User name
If required, enter the username used to authenticate with the VPN.
Phase1 Algorithms
If required, enter the algorithms to be used to authenticate and set up an encrypted channel.
Phase2 Algorithms
If required, enter the algorithms to be used for the IPsec negotiations.
Domain
If required, enter the Domain Name.
NAT traversal
Cisco UDP (default) — IPsec over UDP.
NAT-T — ESP encapsulation and IKE extensions are used to handle NAT Traversal.
Disabled — No special NAT measures required.
Disable Dead Peer Detection — Disable the sending of probes to the remote gateway or endpoint.

Saving Your New (or Modified) Connection and Making Further Configurations

Once you have finished editing your new VPN connection, click the Apply button and NetworkManager will immediately save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 7.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:

7.3.5. Establishing a DSL Connection

This section is intended for those installations which have a DSL card fitted within a host rather than the external combined DSL modem router combinations typical of private consumer or SOHO installations.
Procedure 7.6. Adding a New DSL Connection
  1. Open the Network Connections window by running, as a normal user:
    ~]$ nm-connection-editor &
    
  2. Select the DSL tab and click Add.
  3. The Editing DSL Connection 1 window appears.
Procedure 7.7. Editing an Existing DSL Connection
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
  1. Open the Network Connections window by running, as a normal user:
    ~]$ nm-connection-editor &
    
  2. Select the DSL tab.
  3. Select the connection you wish to edit and click the Edit button.
  4. The Editing DSL Connection window appears.

Configuring the Connection Name, Auto-Connect Behavior, and Availability Settings

Three settings in the Editing dialog are common to all connection types:
  • Connection name — Enter a descriptive name for your network connection. This name will be used to list this connection in the DSL section of the Network Connections window.
  • Connect automatically — Check this box if you want NetworkManager to auto-connect to this connection when it is available. Refer to Section 7.2.3, “Connecting to a Network Automatically” for more information.
  • Available to all users — Check this box to create a connection available to all users on the system. Changing this setting may require root privileges. Refer to Section 7.2.4, “User and System Connections” for details.

Configuring the DSL Tab

Username
Enter the username used to authenticate with the service provider.
Service
Leave blank unless otherwise directed.
Password
Enter the password supplied by the service provider.

Saving Your New (or Modified) Connection and Making Further Configurations

Once you have finished editing your DSL connection, click the Save button to save your customized configuration. Given a correct configuration, you can connect to your new or customized connection by selecting it from the NetworkManager Notification Area applet. See Section 7.2.1, “Connecting to a Network” for information on using your new or altered connection.
You can further configure an existing connection by selecting it in the Network Connections window and clicking Edit to return to the Editing dialog.
Then, to configure:

7.4. Configuring Connection Settings

7.4.1. Configuring 802.1x Security

802.1x security is the name of the IEEE standard for port-based Network Access Control (PNAC). Simply put, 802.1x security is a way of defining a logical network out of a physical one. All clients who want to join the logical network must authenticate with the server (a router, for example) using the correct 802.1x authentication method.
802.1x security is most often associated with securing wireless networks (WLANs), but can also be used to prevent intruders with physical access to the network (LAN) from gaining entry. In the past, DHCP servers were configured not to lease IP addresses to unauthorized users, but for various reasons this practice is both impractical and insecure, and thus is no longer recommended. Instead, 802.1x security is used to ensure a logically-secure network through port-based authentication.
802.1x provides a framework for WLAN and LAN access control and serves as an envelope for carrying one of the Extensible Authentication Protocol (EAP) types. An EAP type is a protocol that defines how WLAN security is achieved on the network.
You can configure 802.1x security for a wired or wireless connection type by opening the Network Connections window (refer to Section 7.2.2, “Configuring New and Editing Existing Connections”) and following the applicable procedure:
Procedure 7.8. For a wired connection...
  1. Select the Wired tab.
  2. Either click on Add to add a new network connection for which you want to configure 802.1x security, or select an existing connection and click Edit.
  3. Then select the 802.1x Security tab and check the Use 802.1x security for this connection checkbox to enable settings configuration.
Procedure 7.9. For a wireless connection...
  1. Select the Wireless tab.
  2. Either click on Add to add a new network connection for which you want to configure 802.1x security, or select an existing connection and click Edit.
  3. Then click the Security dropdown and choose one of the following security methods: LEAP, Dynamic WEP (802.1x), or WPA & WPA2 Enterprise.
  4. Refer to Section 7.4.1.1, “Configuring TLS (Transport Layer Security) Settings” for descriptions of which EAP types correspond to your selection in the Security dropdown.

7.4.1.1. Configuring TLS (Transport Layer Security) Settings

With Transport Layer Security, the client and server mutually authenticate using the TLS protocol. The server demonstrates that it holds a digital certificate, the client proves its own identity using its client-side certificate, and key information is exchanged. Once authentication is complete, the TLS tunnel is no longer used. Instead, the client and server use the exchanged keys to encrypt data using AES, TKIP or WEP.
The fact that certificates must be distributed to all clients who want to authenticate means that the EAP-TLS authentication method is very strong, but also more complicated to set up. Using TLS security requires the overhead of a public key infrastructure (PKI) to manage certificates. The benefit of using TLS security is that a compromised password does not allow access to the (W)LAN: an intruder must also have access to the authenticating client's private key.
Network Manger does not determine the version of TLS supported. Network Manager gathers the parameters entered by the user and passes them to the daemon, wpa_supplicant, that handles the procedure. It, in turn, uses OpenSSL to establish the TLS tunnel. OpenSSL itself negotiates the SSL/TLS protocol version. It uses the highest version both ends support.
Identity
Identity string for EAP authentication methods, such as a username or login name.
User certificate
Click to browse for, and select, a user's certificate.
CA certificate
Click to browse for, and select, a Certificate Authority's certificate.
Private key
Click to browse for, and select, a user's private key file.
Private key password
Enter the user password corresponding to the user's private key.

7.4.1.2. Configuring Tunneled TLS Settings

Anonymous identity
This value is used as the unencrypted identity.
CA certificate
Click to browse for, and select, a Certificate Authority's certificate.
Inner authentication
PAP — Password Authentication Protocol.
MSCHAP — Challenge Handshake Authentication Protocol.
MSCHAPv2 — Microsoft Challenge Handshake Authentication Protocol version 2.
CHAP — Challenge Handshake Authentication Protocol.
Username
Enter the username to be used in the authentication process.
Password
Enter the password to be used in the authentication process.

7.4.1.3. Configuring Protected EAP (PEAP) Settings

Anonymous Identity
This value is used as the unencrypted identity.
CA certificate
Click to browse for, and select, a Certificate Authority's certificate.
PEAP version
The version of Protected EAP to use. Automatic, 0 or 1.
Inner authentication
MSCHAPv2 — Microsoft Challenge Handshake Authentication Protocol version 2.
MD5 — Message Digest 5, a cryptographic hash function.
GTC — Generic Token Card.
Username
Enter the username to be used in the authentication process.
Password
Enter the password to be used in the authentication process.

7.4.2. Configuring Wireless Security

Security
None — Do not encrypt the Wi-Fi connection.
WEP 40/128-bit Key — Wired Equivalent Privacy (WEP), from the IEEE 802.11 standard. Uses a single pre-shared key (PSK).
WEP 128-bit Passphrase — An MD5 hash of the passphrase will be used to derive a WEP key.
LEAP — Lightweight Extensible Authentication Protocol, from Cisco Systems.
Dynamic WEP (802.1x) — WEP keys are changed dynamically.
WPA & WPA2 Personal — Wi-Fi Protected Access (WPA), from the draft IEEE 802.11i standard. A replacement for WEP. Wi-Fi Protected Access II (WPA2), from the 802.11i-2004 standard. Personal mode uses a pre-shared key (WPA-PSK).
WPA & WPA2 Enterprise — WPA for use with a RADUIS authentication server to provide IEEE 802.1x network access control.
Password
Enter the password to be used in the authentication process.

7.4.3. Configuring PPP (Point-to-Point) Settings

Configure Methods
Use point-to-point encryption (MPPE)
Microsoft Point-To-Point Encryption protocol (RFC 3078).
Allow BSD data compression
PPP BSD Compression Protocol (RFC 1977).
Allow Deflate data compression
PPP Deflate Protocol (RFC 1979).
Use TCP header compression
Compressing TCP/IP Headers for Low-Speed Serial Links (RFC 1144).
Send PPP echo packets
LCP Echo-Request and Echo-Reply Codes for loopback tests (RFC 1661).

7.4.4. Configuring IPv4 Settings

Editing the IPv4 Settings Tab
A screen shot of NetworkManager's IPv4 Settings Tab
Figure 7.6. Editing the IPv4 Settings Tab

The IPv4 Settings tab allows you to configure the method by which you connect to the Internet and enter IP address, route, and DNS information as required. The IPv4 Settings tab is available when you create and modify one of the following connection types: wired, wireless, mobile broadband, VPN or DSL.
If you are using DHCP to obtain a dynamic IP address from a DHCP server, you can simply set Method to Automatic (DHCP).

Setting the Method

Available IPv4 Methods by Connection Type
When you click the Method dropdown menu, depending on the type of connection you are configuring, you are able to select one of the following IPv4 connection methods. All of the methods are listed here according to which connection type or types they are associated with.
Method
Automatic (DHCP) — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses. You do not need to fill in the DHCP client ID field.
Automatic (DHCP) addresses only — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.
Link-Local Only — Choose this option if the network you are connecting to does not have a DHCP server and you do not want to assign IP addresses manually. Random addresses will be selected as per RFC 3927.
Shared to other computers — Choose this option if the interface you are configuring is for sharing an Internet or WAN connection.
Wired, Wireless and DSL Connection Methods
Manual — Choose this option if the network you are connecting to does not have a DHCP server and you want to assign IP addresses manually.
Mobile Broadband Connection Methods
Automatic (PPP) — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.
Automatic (PPP) addresses only — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.
VPN Connection Methods
Automatic (VPN) — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.
Automatic (VPN) addresses only — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.
DSL Connection Methods
Automatic (PPPoE) — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.
Automatic (PPPoE) addresses only — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.

7.4.5. Configuring IPv6 Settings

Method
Ignore — Choose this option if you want to disable IPv6 settings.
Automatic — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses.
Automatic, addresses only — Choose this option if the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.
Manual — Choose this option if the network you are connecting to does not have a DHCP server and you want to assign IP addresses manually.
Link-Local Only — Choose this option if the network you are connecting to does not have a DHCP server and you do not want to assign IP addresses manually. Random addresses will be selected as per RFC 4862.
Shared to other computers — Choose this option if the interface you are configuring is for sharing an Internet or WAN connection.
Addresses
DNS servers — Enter a comma separated list of DNS servers.
Search domains — Enter a comma separated list of domain controllers.
For information on configuring static routes for the network connection, go to Section 7.4.6, “Configuring Routes”.

7.4.6. Configuring Routes

A hosts routing table will be automatically populated with routes to directly connected networks. The routes are learned by observing the network interfaces when they are up. This section is for entering static routes to networks or hosts which can be reached by traversing an intermediate network or connection, such as a VPN or leased line.
Configuring static network routes
A screen shot of the static routes window
Figure 7.7. Configuring static network routes

Addresses
Address — The IP address of a network, sub-net or host.
Netmask — The netmask or prefix length of the IP address just entered.
Gateway — The IP address of the gateway leading to the network, sub-net or host.
Metric — A network cost, that is to say a preference value to give to this route. Lower values will be preferred over higher values.
Ignore automatically obtained routes
Select this check box to only use manually entered routes for this conection.
Use this connection only for resources on its network
Select this checkbox to prevent the connection from becoming the default route. Typical examples are where a connection is a VPN or a leased line to a head office and you do not want any Internet bound traffic to pass over the connection. Selecting this option means that only traffic specifically destined for routes learned automatically over the connection or entered here manually will be routed over the connection.

7.5. NetworkManager Architecture

Chapter 8. Network Interfaces

Under Fedora, all network communications occur between configured software interfaces and physical networking devices connected to the system.
The configuration files for network interfaces are located in the /etc/sysconfig/network-scripts/ directory. The scripts used to activate and deactivate these network interfaces are also located here. Although the number and type of interface files can differ from system to system, there are three categories of files that exist in this directory:
  1. Interface configuration files
  2. Interface control scripts
  3. Network function files
The files in each of these categories work together to enable various network devices.
This chapter explores the relationship between these files and how they are used.

8.1. Network Configuration Files

Before delving into the interface configuration files, let us first itemize the primary configuration files used in network configuration. Understanding the role these files play in setting up the network stack can be helpful when customizing a Fedora system.
The primary network configuration files are as follows:
/etc/hosts
The main purpose of this file is to resolve hostnames that cannot be resolved any other way. It can also be used to resolve hostnames on small networks with no DNS server. Regardless of the type of network the computer is on, this file should contain a line specifying the IP address of the loopback device (127.0.0.1) as localhost.localdomain. For more information, refer to the hosts(5) manual page.
/etc/hostname
There are three separate classes of hostnames in use on a given system. The pretty hostname is the high level hostname often presented to users by their desktop environment or shell. The static hostname is used by the kernel at boot, and is usually the system's fully qualified domain name. A system may also have a transient hostname assigned by a dhcp server. hostnamectl is provided for administering these hostnames. For more information on hostnames, see man hostname and man hostnamectl.
/etc/resolv.conf
This file specifies the IP addresses of DNS servers and the search domain. Unless configured to do otherwise, the network initialization scripts populate this file. For more information about this file, refer to the resolv.conf(5) manual page.
/etc/sysconfig/network
This file specifies routing and host information for all network interfaces. For more information about this file and the directives it accepts, refer to Section D.1.13, “ /etc/sysconfig/network ”.
/etc/sysconfig/network-scripts/ifcfg-interface-name
For each network interface, there is a corresponding interface configuration script. Each of these files provide information specific to a particular network interface. Refer to Section 8.2, “Interface Configuration Files” for more information on this type of file and the directives it accepts.

Network interface names

Network interface names may be different on different hardware types. Refer to Appendix A, Consistent Network Device Naming for more information.

The /etc/sysconfig/networking/ directory

The /etc/sysconfig/networking/ directory is used by the now deprecated Network Administration Tool (system-config-network). Its contents should not be edited manually. Using only one method for network configuration is strongly encouraged, due to the risk of configuration deletion. For more information about configuring network interfaces using graphical configuration tools, refer to Chapter 7, NetworkManager.

8.2. Interface Configuration Files

Interface configuration files control the software interfaces for individual network devices. As the system boots, it uses these files to determine what interfaces to bring up and how to configure them. These files are usually named ifcfg-name , where name refers to the name of the device that the configuration file controls.

8.2.1. Ethernet Interfaces

One of the most common interface files is /etc/sysconfig/network-scripts/ifcfg-eth0, which controls the first Ethernet network interface card or NIC in the system. In a system with multiple NICs, there are multiple ifcfg-ethX files (where X is a unique number corresponding to a specific interface). Because each device has its own configuration file, an administrator can control how each interface functions individually.
The following is a sample ifcfg-eth0 file for a system using a fixed IP address:
DEVICE=eth0
BOOTPROTO=none
ONBOOT=yes
NETMASK=255.255.255.0
IPADDR=10.0.1.27
USERCTL=no
The values required in an interface configuration file can change based on other values. For example, the ifcfg-eth0 file for an interface using DHCP looks different because IP information is provided by the DHCP server:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
NetworkManager is graphical configuration tool which provides an easy way to make changes to the various network interface configuration files (refer to Chapter 7, NetworkManager for detailed instructions on using this tool).
However, it is also possible to manually edit the configuration files for a given network interface.
Below is a listing of the configurable parameters in an Ethernet interface configuration file:
BONDING_OPTS=parameters
sets the configuration parameters for the bonding device, and is used in /etc/sysconfig/network-scripts/ifcfg-bondN (see Section 8.2.3, “Channel Bonding Interfaces”). These parameters are identical to those used for bonding devices in /sys/class/net/bonding_device/bonding, and the module parameters for the bonding driver as described in bonding Module Directives.
This configuration method is used so that multiple bonding devices can have different configurations. It is highly recommended to place all of your bonding options after the BONDING_OPTS directive in ifcfg-name. Do not specify options for the bonding device in /etc/modprobe.d/bonding.conf, or in the deprecated /etc/modprobe.conf file.
BOOTPROTO=protocol
where protocol is one of the following:
  • none — No boot-time protocol should be used.
  • bootp — The BOOTP protocol should be used.
  • dhcp — The DHCP protocol should be used.
BROADCAST=address
where address is the broadcast address. This directive is deprecated, as the value is calculated automatically with ipcalc.
DEVICE=name
where name is the name of the physical device (except for dynamically-allocated PPP devices where it is the logical name).
DHCP_HOSTNAME=name
where name is a short hostname to be sent to the DHCP server. Use this option only if the DHCP server requires the client to specify a hostname before receiving an IP address.
DHCPV6C=answer
where answer is one of the following:
  • yes — Use DHCP to obtain an IPv6 address for this interface.
  • no — Do not use DHCP to obtain an IPv6 address for this interface. This is the default value.
An IPv6 link-local address will still be assigned by default. The link-local address is based on the MAC address of the interface as per RFC 4862.
DHCPV6C_OPTIONS=answer
where answer is one of the following:
  • -P — Enable IPv6 prefix delegation.
  • -S — Use DHCP to obtain stateless configuration only, not addresses, for this interface.
  • -N — Restore normal operation after using the -T or -P options.
  • -T — Use DHCP to obtain a temporary IPv6 address for this interface.
  • -D — Override the default when selecting the type of DHCP Unique Identifier (DUID) to use.
    By default, the DHCPv6 client (dhclient) creates a DHCP Unique Identifier (DUID) based on the link-layer address (DUID-LL) if it is running in stateless mode (with the -S option, to not request an address), or it creates an identifier based on the link-layer address plus a timestamp (DUID-LLT) if it is running in stateful mode (without -S, requesting an address). The -D option overrides this default, with a value of either LL or LLT.
DNS{1,2}=address
where address is a name server address to be placed in /etc/resolv.conf if the PEERDNS directive is set to yes.
ETHTOOL_OPTS=options
where options are any device-specific options supported by ethtool. For example, if you wanted to force 100Mb, full duplex:
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
Instead of a custom initscript, use ETHTOOL_OPTS to set the interface speed and duplex settings. Custom initscripts run outside of the network init script lead to unpredictable results during a post-boot network service restart.

Set autoneg off before changing speed or duplex settings

Changing speed or duplex settings almost always requires disabling autonegotiation with the autoneg off option. This option needs to be stated first, as the option entries are order-dependent.
Refer to Section 8.2.2, “Ethtool” for more Ethtool options.
HOTPLUG=answer
where answer is one of the following:
  • yes — This device should be activated when it is hot-plugged (this is the default option).
  • no — This device should not be activated when it is hot-plugged.
The HOTPLUG=no option can be used to prevent a channel bonding interface from being activated when a bonding kernel module is loaded.
Refer to Section 8.2.3, “Channel Bonding Interfaces” for more information about channel bonding interfaces.
HWADDR=MAC-address
where MAC-address is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF. This directive must be used in machines containing more than one NIC to ensure that the interfaces are assigned the correct device names regardless of the configured load order for each NIC's module. This directive should not be used in conjunction with MACADDR.

Note

Persistent device names are now handled by /etc/udev/rules.d/70-persistent-net.rules.
IPADDR=address
where address is the IPv4 address.
IPV6ADDR=address
where address is the first static, or primary, IPv6 address on an interface.
The format is Address/Prefix-length. If no prefix length is specified, /64 is assumed. Note that this setting depends on IPV6INIT being enabled.
IPV6ADDR_SECONDARIES=address
where address is one or more, space separated, additional IPv6 addresses.
The format is Address/Prefix-length. If no prefix length is specified, /64 is assumed. Note that this setting depends on IPV6INIT being enabled.
IPV6_PRIVACY_PREFER_PUBLIC_IP=answer
where answer is one of the following:
  • yes — Prefer the public IP address.
  • no — Do not prefer the public IP address, prefer a temporary address.
LINKDELAY=time
where time is the number of seconds to wait for link negotiation before configuring the device.
MACADDR=MAC-address
where MAC-address is the hardware address of the Ethernet device in the form AA:BB:CC:DD:EE:FF.
This directive is used to assign a MAC address to an interface, overriding the one assigned to the physical NIC. This directive should not be used in conjunction with the HWADDR directive.
MASTER=bond-interface
where bond-interface is the channel bonding interface to which the Ethernet interface is linked.
This directive is used in conjunction with the SLAVE directive.
Refer to Section 8.2.3, “Channel Bonding Interfaces” for more information about channel bonding interfaces.
NETMASK=mask
where mask is the netmask value.
NETWORK=address
where address is the network address. This directive is deprecated, as the value is calculated automatically with ipcalc.
NM_CONTROLLED=answer
where answer is one of the following:
  • yes — NetworkManager is permitted to configure this device.This is the default behavior and can be omitted.
  • no — NetworkManager is not permitted to configure this device.
ONBOOT=answer
where answer is one of the following:
  • yes — This device should be activated at boot-time.
  • no — This device should not be activated at boot-time.
PEERDNS=answer
where answer is one of the following:
  • yes — Modify /etc/resolv.conf if the DNS directive is set. If using DHCP, then yes is the default.
  • no — Do not modify /etc/resolv.conf.
SLAVE=answer
where answer is one of the following:
  • yes — This device is controlled by the channel bonding interface specified in the MASTER directive.
  • no — This device is not controlled by the channel bonding interface specified in the MASTER directive.
This directive is used in conjunction with the MASTER directive.
Refer to Section 8.2.3, “Channel Bonding Interfaces” for more about channel bonding interfaces.
SRCADDR=address
where address is the specified source IP address for outgoing packets.
USERCTL=answer
where answer is one of the following:
  • yes — Non-root users are allowed to control this device.
  • no — Non-root users are not allowed to control this device.

8.2.2. Ethtool

Ethtool is a utility for configuration of Network Interface Cards (NICs). This utility allows querying and changing settings such as speed, port, auto-negotiation, PCI locations and checksum offload on many network devices, especially Ethernet devices.
We present here a short selection of often used Ethtool commands together with some useful commands that are not well known. For a full list of commands type ethtool -h or refer to the man page, ethtool(8), for a more verbose list and explanation. The first two examples are information queries and show the use of the different formats of the command.
But first, the command structure:
ethtool [option...] devname
where option is none or more options, and devname is your Network Interface Card (NIC). For example eth0 or em1.
ethtool
The ethtool command with only a device name as an option is used to query a network device for NIC and driver statistics. It takes the following form:
ethtool -S, --statistics devname
where devname is your NIC. For example eth0 or em1.
Some values can only be obtained when the command is run as root. Here is an example of the output when the command is run as root:
~]# ethtool em1
Settings for em1:
	Supported ports: [ TP ]
	Supported link modes:   10baseT/Half 10baseT/Full 
	                        100baseT/Half 100baseT/Full 
	                        1000baseT/Full 
	Supported pause frame use: No
	Supports auto-negotiation: Yes
	Advertised link modes:  10baseT/Half 10baseT/Full 
	                        100baseT/Half 100baseT/Full 
	                        1000baseT/Full 
	Advertised pause frame use: No
	Advertised auto-negotiation: Yes
	Speed: 1000Mb/s
	Duplex: Full
	Port: Twisted Pair
	PHYAD: 2
	Transceiver: internal
	Auto-negotiation: on
	MDI-X: on
	Supports Wake-on: pumbg
	Wake-on: g
	Current message level: 0x00000007 (7)
			       drv probe link
	Link detected: yes
Issue the following command, using the short or long form of the argument, to query the specified network device for associated driver information:
ethtool -i, --driver devname
where devname is your Network Interface Card (NIC). For example eth0 or em1.
Here is an example of the output:
~]$ ethtool -i em1
driver: e1000e
version: 2.0.0-k
firmware-version: 0.13-3
bus-info: 0000:00:19.0
supports-statistics: yes
supports-test: yes
supports-eeprom-access: yes
supports-register-dump: yes
Here follows a list of commands to query, identify or reset the device. They are in the usual -short and --long form:
--statistics
The --statistics or -S queries the specified network device for NIC and driver statistics. It takes the following form:
ethtool -S, --statistics devname
where devname is your NIC.
--identify
The --identify or -p option initiates adapter-specific action intended to enable an operator to easily identify the adapter by sight. Typically this involves blinking one or more LEDs on the specified network port. It takes the following form:
-p, --identify devname integer
where integer is length of time in seconds to perform the action,
and devname is your NIC.
--test
The --test or -t option is used to perform tests on a Network Interface Card. It takes the following form:
-t, --test word devname
where word is one of the following:
  • offline — Perform a comprehensive set of tests. Service will be interrupted.
  • online — Perform a reduced set of tests. Service should not be interrupted.
  • external_lb — Perform full set of tests including loopback tests while fitted with a loopback cable.
and devname is your NIC.
Changing some or all settings of the specified network device requires the -s or --change option. All the following options are only applied if the -s or --change option is also specified. For the sake of clarity we will omit it here.
To make these settings permanent you can make use of the ETHTOOL_OPTS directive. It can be used in interface configuration files to set the desired options when the network interface is brought up. Refer to Section 8.2.1, “Ethernet Interfaces” for more details on how to use this directive.
--speed
The --speed option is used to set the speed in megabits per second (Mb/s). Omitting the speed value will show the supported device speeds. It takes the following form:
--speed number devname
where number is the speed in megabits per second (Mb/s),
and devname is your NIC.
--duplex
The --duplex option is used to set the transmit and receive mode of operation. It takes the following form:
 --duplex word devname
where word is one of the following:
  • half — Sets half-duplex mode. Usually used when connected to a hub.
  • full — Sets full-duplex mode. Usually used when connected to a switch or another host.
and devname is your NIC.
--port
The --port option is used to select the device port . It takes the following form:
--port value devname
where value is one of the following:
  • tp — An Ethernet interface using Twisted-Pair cable as the medium.
  • aui — Attachment Unit Interface (AUI). Normally used with hubs.
  • bnc — An Ethernet interface using BNC connectors and co-axial cable.
  • mii — An Ethernet interface using a Media Independent Interface (MII).
  • fibre — An Ethernet interface using Optical Fibre as the medium.
and devname is your NIC.
--autoneg
The --autoneg option is used to control auto-negotiation of network speed and mode of operation (full-duplex or half-duplex mode). If auto-negotiation is enabled you can initiate re-negotiation of network speeds and mode of operation by using the -r, --negotiate option. You can display the auto-negotiation state using the --a, --show-pause option.
It takes the following form:
--autoneg value devname
where value is one of the following:
  • yes — Allow auto-negotiating of network speed and mode of operation.
  • no — Do not allow auto-negotiating of network speed and mode of operation.
and devname is your NIC.
--advertise
The --advertise option is used to set what speeds and modes of operation (duplex mode) are advertised for auto-negotiation. The argument is one or more hexadecimal values from Table 8.1, “Ethtool advertise options: speed and mode of operation”.
It takes the following form:
--advertise HEX-VALUE devname
where HEX-VALUE is one or more of the hexadecimal values from the table below and devname is your NIC.
Table 8.1. Ethtool advertise options: speed and mode of operation
Hex Value Speed Duplex Mode IEEE standard?
0x001 10
Half
Yes
0x002 10
Full
Yes
0x004 100
Half
Yes
0x008 100
Full
Yes
0x010 1000 Half No
0x020 1000
Full
Yes
0x8000 2500 Full Yes
0x1000 10000 Full Yes
0x20000 20000MLD2 Full No
0x20000 20000MLD2 Full No
0x40000 20000KR2 Full No

--phyad
The --phyad option is used to change the physical address. Often referred to as the MAC or hardware address but in this context referred to as the physical address.
It takes the following form:
--phyad HEX-VALUE devname
where HEX-VALUE is the physical address in hexadecimal format and devname is your NIC.
--xcvr
The --xcvr option is used to select the transceiver type. Currently only internal and external can be specified. In the future other types might be added.
It takes the following form:
--xcvr word devname
where word is one of the following:
  • internal — Use internal transceiver.
  • external — Use external transceiver.
and devname is your NIC.
--wol
The --wol option is used to set Wake-on-LAN options. Not all devices support this. The argument to this option is a string of characters specifying which options to enable.
It takes the following form:
--wol value devname
where value is one or more of the following:
  • p — Wake on PHY activity.
  • u — Wake on unicast messages.
  • m — Wake on multicast messages.
  • b — Wake on broadcast messages.
  • g — Wake-on-Lan; wake on receipt of a "magic packet".
  • s — Enable security function using password for Wake-on-Lan.
  • d — Disable Wake-on-Lan and clear all settings.
and devname is your NIC.
--sopass
The --sopass option is used to set the SecureOn password. The argument to this option must be 6 bytes in Ethernet MAC hexadecimal format (xx:yy:zz:aa:bb:cc).
It takes the following form:
--sopass xx:yy:zz:aa:bb:cc devname
where xx:yy:zz:aa:bb:cc is the password in the same format as a MAC address and devname is your NIC.
--msglvl
The --msglvl option is used to set the driver message-type flags by name or number. The precise meanings of these type flags differ between drivers.
It takes the following form:
--msglvl value devname
where value is one of:
  • HEX-VALUE — Hexadecimal value indicating message type.
  • message-type — The message type name in plain text.
and devname is your NIC.
The defined message type names and numbers are shown in the table below:
Table 8.2. Driver message type
Message Type Hex Value Description
drv 0x0001
General driver status
probe 0x0002
Hardware probing
link 0x0004
Link state
timer 0x0008
Periodic status check
ifdown 0x0010 Interface being brought down
ifup 0x0020
Interface being brought up
rx_err 0x0040 Receive error
tx_err 0x0080 Transmit error
intr 0x0200 Interrupt handling
tx_done 0x0400 Transmit completion
rx_status 0x0800 Receive completion
pktdata 0x1000 Packet contents
hw 0x2000 Hardware status
wol 0x4000 Wake-on-LAN status

8.2.3. Channel Bonding Interfaces

Fedora allows administrators to bind multiple network interfaces together into a single channel using the bonding kernel module and a special network interface called a channel bonding interface. Channel bonding enables two or more network interfaces to act as one, simultaneously increasing the bandwidth and providing redundancy.
To create a channel bonding interface, create a file in the /etc/sysconfig/network-scripts/ directory called ifcfg-bondN, replacing N with the number for the interface, such as 0.
The contents of the file can be identical to whatever type of interface is getting bonded, such as an Ethernet interface. The only difference is that the DEVICE directive is bondN, replacing N with the number for the interface.
The following is a sample channel bonding configuration file:
Example 8.1. Sample ifcfg-bond0 interface configuration file
DEVICE=bond0
IPADDR=192.168.1.1
NETMASK=255.255.255.0
ONBOOT=yes
BOOTPROTO=none
USERCTL=no
BONDING_OPTS="bonding parameters separated by spaces"

After the channel bonding interface is created, the network interfaces to be bound together must be configured by adding the MASTER and SLAVE directives to their configuration files. The configuration files for each of the channel-bonded interfaces can be nearly identical.
For example, if two Ethernet interfaces are being channel bonded, both eth0 and eth1 may look like the following example:
DEVICE=ethN
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
In this example, replace N with the numerical value for the interface.
For a channel bonding interface to be valid, the kernel module must be loaded. To ensure that the module is loaded when the channel bonding interface is brought up, create a new file as root named bonding.conf in the /etc/modprobe.d/ directory. Note that you can name this file anything you like as long as it ends with a .conf extension. Insert the following line in this new file:
alias bondN bonding
Replace N with the interface number, such as 0. For each configured channel bonding interface, there must be a corresponding entry in your new /etc/modprobe.d/bonding.conf file.

Put all bonding module parameters in ifcfg-bondN files

Parameters for the bonding kernel module must be specified as a space-separated list in the BONDING_OPTS="bonding parameters" directive in the ifcfg-bondN interface file. Do not specify options for the bonding device in /etc/modprobe.d/bonding.conf, or in the deprecated /etc/modprobe.conf file. For further instructions and advice on configuring the bonding module and to view the list of bonding parameters, refer to Section 24.7.2, “Using Channel Bonding”.

8.2.4. Network Bridge

A network bridge is a Link Layer device which forwards traffic between networks based on MAC addresses and is therefore also referred to as a Layer 2 device. It makes forwarding decisions based on tables of MAC addresses which it builds by learning what hosts are connected to each network. A software bridge can be used within a Linux host in order to emulate a hardware bridge, for example in virtualization applications for sharing a NIC with one or more virtual NICs. This case will be illustrated here as an example.
To create a network bridge, create a file in the /etc/sysconfig/network-scripts/ directory called ifcfg-brN, replacing N with the number for the interface, such as 0.
The contents of the file is similar to whatever type of interface is getting bridged to, such as an Ethernet interface. The differences in this example are as follows:
  • The DEVICE directive is given an interface name as its argument in the format brN, where N is replaced with the number of the interface.
  • The TYPE directive is given an argument Bridge or Ethernet. This directive determines the device type and the argument is case sensitive.
  • The bridge interface configuration file now has the IP address and the physical interface has only a MAC address.
  • An extra directive, DELAY=0, is added to prevent the bridge from waiting while it monitors traffic, learns where hosts are located, and builds a table of MAC addresses on which to base its filtering decisions. The default delay of 30 seconds is not needed if no routing loops are possible.
  • The NM_CONTROLLED=no should be added to the Ethernet interface to prevent NetworkManager from altering the file. It can also be added to the bridge configuration file in case future versions of NetworkManager support bridge configuration.
The following is a sample bridge interface configuration file using a static IP address:
Example 8.2. Sample ifcfg-br0 interface configuration file
DEVICE=br0
TYPE=Bridge
IPADDR=192.168.1.1
NETMASK=255.255.255.0
ONBOOT=yes
BOOTPROTO=static
NM_CONTROLLED=no
DELAY=0

To complete the bridge another interface is created, or an existing interface is modified, and pointed to the bridge interface. The following is a sample Ethernet interface configuration file pointing to a bridge interface. Configure your physical interface in /etc/sysconfig/network-scripts/ifcfg-ethX, where X is a unique number corresponding to a specific interface, as follows:
Example 8.3. Sample ifcfg-ethX interface configuration file
DEVICE=ethX
TYPE=Ethernet
HWADDR=AA:BB:CC:DD:EE:FF
BOOTPROTO=none
ONBOOT=yes
NM_CONTROLLED=no
BRIDGE=br0

Note

For the DEVICE directive, almost any interface name could be used as it does not determine the device type. Other commonly used names include tap, dummy and bond for example. TYPE=Ethernet is not strictly required. If the TYPE directive is not set, the device is treated as an Ethernet device (unless it's name explicitly matches a different interface configuration file.)
You can refer to Section 8.2, “Interface Configuration Files” for a review of the directives and options used in network interface config files.

Warning

If you are configuring bridging on a remote host, and you are connected to that host over the physical NIC you are configuring, please consider the implications of losing connectivity before proceeding. You will lose connectivity when restarting the service and may not be able to regain connectivity if any errors have been made. Console, or out-of-band access is advised.
Restart the networking service, in order for the changes to take effect by running as root:
  systemctl restart network.service 
An example of a network bridge formed from two or more bonded Ethernet interfaces will now be given as this is another common application in a virtualization environment. If you are not very familiar with the configuration files for bonded interfaces then please refer to Section 8.2.3, “Channel Bonding Interfaces”
Create or edit two or more Ethernet interface configuration files, which are to be bonded, as follows:
DEVICE=ethX
TYPE=Ethernet
USERCTL=no
SLAVE=yes
MASTER=bond0
BOOTPROTO=none
HWADDR=AA:BB:CC:DD:EE:FF
NM_CONTROLLED=no

Note

Using ethX as the interface name is common practice but almost any name could be used. Names such as tap, dummy and bond are commonly used.
Create or edit one interface configuration file, /etc/sysconfig/network-scripts/ifcfg-bond0, as follows:
DEVICE=bond0
ONBOOT=yes
BONDING_OPTS='mode=1 miimon=100'
BRIDGE=brbond0
NM_CONTROLLED=no
For further instructions and advice on configuring the bonding module and to view the list of bonding parameters, refer to Section 24.7.2, “Using Channel Bonding”.
Create or edit one interface configuration file, /etc/sysconfig/network-scripts/ifcfg-brbond0, as follows:
DEVICE=brbond0
ONBOOT=yes
TYPE=Bridge
IPADDR=192.168.1.1
NETMASK=255.255.255.0
NM_CONTROLLED=no
A network bridge consisting of two bonded Ethernet interfaces.
A diagram of two Ethernet interfaces on the left feeding into a virtual interface labeled bond 0. This in turn leads to a virtual interface called BR Bond 0 on the right. From there a path leads to a virtual network below.
Figure 8.1. A network bridge consisting of two bonded Ethernet interfaces.

We now have two or more interface configuration files with the MASTER=bond0 directive. These point to the configuration file named /etc/sysconfig/network-scripts/ifcfg-bond0, which contains the DEVICE=bond0 directive. This ifcfg-bond0 in turn points to the /etc/sysconfig/network-scripts/ifcfg-brbond0 configuration file, which contains the IP address, and acts as an interface to the virtual networks inside the host.
Restart the networking service, in order for the changes to take effect by running as root:
  systemctl restart network.service 

8.2.5. Setting Up 802.1q VLAN Tagging

  1. Ensure that the module is loaded by entering the following command:
     lsmod | grep 8021q
  2. If the module is not loaded, load it with the following command:
    modprobe 8021q
  3. Configure your physical interface in /etc/sysconfig/network-scripts/ifcfg-ethX, where X is a unique number corresponding to a specific interface, as follows:
    DEVICE=ethX
    TYPE=Ethernet
    BOOTPROTO=none
    ONBOOT=yes
  4. Configure the VLAN interface configuration in /etc/sysconfig/network-scripts. The configuration filename should be the physical interface plus a . character plus the VLAN ID number. For example, if the VLAN ID is 192, and the physical interface is eth0, then the configuration filename should be ifcfg-eth0.192:
    DEVICE=ethX.192
    BOOTPROTO=static
    ONBOOT=yes
    IPADDR=192.168.1.1
    NETMASK=255.255.255.0
    USERCTL=no
    NETWORK=192.168.1.0
    VLAN=yes
    If there is a need to configure a second VLAN, with for example, VLAN ID 193, on the same interface, eth0 , add a new file with the name eth0.193 with the VLAN configuration details.
  5. Restart the networking service, in order for the changes to take effect by running as root:
      systemctl restart network.service 

8.2.6. Alias and Clone Files

Two lesser-used types of interface configuration files are alias and clone files. As the ip command of the iproute package now supports assigning multiple address to the same interface it is no longer necessary to use this method of binding multiple addresses to the same interface.

Note

At the time of writing, NetworkManager does not detect IP aliases in ifcfg files. For example, if ifcfg-eth0 and ifcfg-eth0:1 files are present, NetworkManager creates two connections, which will cause confusion.
For new installations, users should select the Manual method on the IPv4 or IPv6 tab in NetworkManager to assign multiple IP address to the same interface. For more information on using this tool, refer to Chapter 7, NetworkManager.
Alias interface configuration files, which are used to bind multiple addresses to a single interface, use the ifcfg-if-name:alias-value naming scheme.
For example, an ifcfg-eth0:0 file could be configured to specify DEVICE=eth0:0 and a static IP address of 10.0.0.2, serving as an alias of an Ethernet interface already configured to receive its IP information via DHCP in ifcfg-eth0. Under this configuration, eth0 is bound to a dynamic IP address, but the same physical network card can receive requests via the fixed, 10.0.0.2 IP address.

Warning

Alias interfaces do not support DHCP.
A clone interface configuration file should use the following naming convention: ifcfg-if-name-clone-name. While an alias file allows multiple addresses for an existing interface, a clone file is used to specify additional options for an interface. For example, a standard DHCP Ethernet interface called eth0, may look similar to this:
DEVICE=eth0
ONBOOT=yes
BOOTPROTO=dhcp
Since the default value for the USERCTL directive is no if it is not specified, users cannot bring this interface up and down. To give users the ability to control the interface, create a clone by copying ifcfg-eth0 to ifcfg-eth0-user and add the following line to ifcfg-eth0-user:
USERCTL=yes
This way a user can bring up the eth0 interface using the /sbin/ifup eth0-user command because the configuration options from ifcfg-eth0 and ifcfg-eth0-user are combined. While this is a very basic example, this method can be used with a variety of options and interfaces.
It is no longer possible to create alias and clone interface configuration files using a graphical tool. However, as explained at the beginning of this section, it is no longer necessary to use this method as it is now possible to directly assign multiple IP address to the same interface. For new installations, users should select the Manual method on the IPv4 or IPv6 tab in NetworkManager to assign multiple IP address to the same interface. For more information on using this tool, refer to Chapter 7, NetworkManager.

8.2.7. Dialup Interfaces

If you are connecting to the Internet via a dialup connection, a configuration file is necessary for the interface.
PPP interface files are named using the following format:
ifcfg-pppX
where X is a unique number corresponding to a specific interface.
The PPP interface configuration file is created automatically when wvdial, the Network Administration Tool or Kppp is used to create a dialup account. It is also possible to create and edit this file manually.
The following is a typical ifcfg-ppp0 file:
DEVICE=ppp0
NAME=test
WVDIALSECT=test
MODEMPORT=/dev/modem
LINESPEED=115200
PAPNAME=test
USERCTL=true
ONBOOT=no
PERSIST=no
DEFROUTE=yes
PEERDNS=yes
DEMAND=no
IDLETIMEOUT=600
Serial Line Internet Protocol (SLIP) is another dialup interface, although it is used less frequently. SLIP files have interface configuration file names such as ifcfg-sl0.
Other options that may be used in these files include:
DEFROUTE=answer
where answer is one of the following:
  • yes — Set this interface as the default route.
  • no — Do not set this interface as the default route.
DEMAND=answer
where answer is one of the following:
  • yes — This interface allows pppd to initiate a connection when someone attempts to use it.
  • no — A connection must be manually established for this interface.
IDLETIMEOUT=value
where value is the number of seconds of idle activity before the interface disconnects itself.
INITSTRING=string
where string is the initialization string passed to the modem device. This option is primarily used in conjunction with SLIP interfaces.
LINESPEED=value
where value is the baud rate of the device. Possible standard values include 57600, 38400, 19200, and 9600.
MODEMPORT=device
where device is the name of the serial device that is used to establish the connection for the interface.
MTU=value
where value is the Maximum Transfer Unit (MTU) setting for the interface. The MTU refers to the largest number of bytes of data a frame can carry, not counting its header information. In some dialup situations, setting this to a value of 576 results in fewer packets dropped and a slight improvement to the throughput for a connection.
NAME=name
where name is the reference to the title given to a collection of dialup connection configurations.
PAPNAME=name
where name is the username given during the Password Authentication Protocol (PAP) exchange that occurs to allow connections to a remote system.
PERSIST=answer
where answer is one of the following:
  • yes — This interface should be kept active at all times, even if deactivated after a modem hang up.
  • no — This interface should not be kept active at all times.
REMIP=address
where address is the IP address of the remote system. This is usually left unspecified.
WVDIALSECT=name
where name associates this interface with a dialer configuration in /etc/wvdial.conf. This file contains the phone number to be dialed and other important information for the interface.

8.2.8. Other Interfaces

Other common interface configuration files include the following:
ifcfg-lo
A local loopback interface is often used in testing, as well as being used in a variety of applications that require an IP address pointing back to the same system. Any data sent to the loopback device is immediately returned to the host's network layer.

Do not manually edit the ifcfg-lo script

The loopback interface script, /etc/sysconfig/network-scripts/ifcfg-lo, should never be edited manually. Doing so can prevent the system from operating correctly.
ifcfg-irlan0
An infrared interface allows information between devices, such as a laptop and a printer, to flow over an infrared link. This works in a similar way to an Ethernet device except that it commonly occurs over a peer-to-peer connection.
ifcfg-plip0
A Parallel Line Interface Protocol (PLIP) connection works much the same way as an Ethernet device, except that it utilizes a parallel port.

8.3. Interface Control Scripts

The interface control scripts activate and deactivate system interfaces. There are two primary interface control scripts that call on control scripts located in the /etc/sysconfig/network-scripts/ directory: /sbin/ifdown and /sbin/ifup.
The ifup and ifdown interface scripts are symbolic links to scripts in the /sbin/ directory. When either of these scripts are called, they require the value of the interface to be specified, such as:
ifup eth0

Use the ifup and ifdown interface scripts

The ifup and ifdown interface scripts are the only scripts that the user should use to bring up and take down network interfaces.
The following scripts are described for reference purposes only.
Two files used to perform a variety of network initialization tasks during the process of bringing up a network interface are /etc/rc.d/init.d/functions and /etc/sysconfig/network-scripts/network-functions. Refer to Section 8.6, “Network Function Files” for more information.
After verifying that an interface has been specified and that the user executing the request is allowed to control the interface, the correct script brings the interface up or down. The following are common interface control scripts found within the /etc/sysconfig/network-scripts/ directory:
ifup-aliases
Configures IP aliases from interface configuration files when more than one IP address is associated with an interface.
ifup-ippp and ifdown-ippp
Brings ISDN interfaces up and down.
ifup-ipv6 and ifdown-ipv6
Brings IPv6 interfaces up and down.
ifup-plip
Brings up a PLIP interface.
ifup-plusb
Brings up a USB interface for network connections.
ifup-post and ifdown-post
Contains commands to be executed after an interface is brought up or down.
ifup-ppp and ifdown-ppp
Brings a PPP interface up or down.
ifup-routes
Adds static routes for a device as its interface is brought up.
ifdown-sit and ifup-sit
Contains function calls related to bringing up and down an IPv6 tunnel within an IPv4 connection.
ifup-wireless
Brings up a wireless interface.

Be careful when removing or modifying network scripts!

Removing or modifying any scripts in the /etc/sysconfig/network-scripts/ directory can cause interface connections to act irregularly or fail. Only advanced users should modify scripts related to a network interface.
The easiest way to manipulate all network scripts simultaneously is to use the systemctl command on the network service (/etc/rc.d/init.d/network), as illustrated by the following command:
systemctl action network.service
Here, action can be either start, stop, or restart.
To view a list of configured devices and currently active network interfaces, use the following command:
 systemctl status network.service

Note

The older SysV service commands, such as service network status are considered deprecated but will still work. The SysV services can define their status in an arbitrary fashion so the output of the status command is not considered predictable over time. The SysV commands are retained for compatibility purposes. The /sbin/service utility will call systemctl when necessary.

8.4. Using Hostnamectl

The hostnamectl tool is provided for administering the three separate classes of host names in use on a given system. The three classes of hostname are: static, pretty, and transient.
The static host name is the traditional hostname, which can be chosen by the user, and is stored in the /etc/hostname file. The transient hostname is a dynamic host name maintained by the kernel. It is initialized to the static host name by default, whose value defaults to localhost. It can be changed by DHCP or mDNS at runtime. The pretty hostname is a free-form UTF8 host name for presentation to the user.

Note

A host name can be a free-form string up to 64 characters in length, however it is recommended that the static and transient names consists only of 7 bit ASCII lower-case characters, no spaces or dots, and limits itself to the format allowed for DNS domain name labels, even though this is not a strict requirement. The hostnamectl tool will enforce the following: Static and transient host names to consist of a-z, A-Z, 0-9, -, _ and . only, to not begin or end in a dot, and to not have two dots immediately following each other. The size limit of 64 charcters is enforced.

8.4.1. View all the Hostnames

To view all the current host names, enter the following command:
~]$ hostnamectl status
The status option is implied by default if no option is given.

8.4.2. Set All The Hostnames

To set all the host names on a system, enter the following command as root:
~]# hostnamectl set-hostname name
This will alter the pretty, static, and transient host names alike. The static and transient host names will be simplified forms of the pretty host name. Spaces will be replaced with “-” and special characters will be removed.

8.4.3. Set A Particular Hostname

To set a particular host name, enter the following command as root with the relevant option:
~]# hostnamectl set-hostname name option
Where option is one or more of: --pretty, --static, and --transient.
If the static or transient options are used together with the pretty option, the static and transient host names will be simplified forms of the pretty host name. Spaces will be replaced with - and special characters will be removed. If the --pretty option is not given, no simplification takes place.

8.4.4. Clear A Particular Hostname

To clear a particular host name and to allow it to revert to the default, enter the following command as root with the relevant option:
~]# hostnamectl set-hostname "" option
Where "" is a quoted empty string and where option is one or more of: --pretty, --static, and --transient.

8.4.5. Changing Host Names Remotely

To execute a hostnamectl command on a remote system, use the -H, --host option as follows:
~]# hostnamectl set-hostname -H username@hostname
Where hostname is the remote host you wish to configure. The username is optional. The hostnamectl tool will use SSH to connect to the remote system.

8.4.6. Additional Resources

The following sources of information provide additional resources regarding hostnamectl.

8.4.6.1. Installed Documentation

  • hostnamectl(1) man page — Describes hostnamectl including the commands and command options.
  • hostname(1) man page — Contains an explanation of the hostname and domainname command.
  • hostname(5) man page — Contains an explanation of the host name file, its contents, and use.
  • hostname(7) man page — Contains an explanation of host name resolution.
  • machine-info(5) man page — Describes the local machine information file and the environment variables it contains.
  • machine-id(5) man page — Describes the local machine ID configuration file.
  • systemd-hostnamed.service man page — Describes the systemd-hostnamed system service used by hostnamectl.

8.4.6.2. Useful Websites

8.5. Static Routes and the Default Gateway

Static routes are for traffic that must not, or should not, go through the default gateway. Routing is usually handled by routing devices and therefore it is often not necessary to configure static routes on Red Hat Enterprise Linux servers or clients. Exceptions include traffic that must pass through an encrypted VPN tunnel or traffic that should take a less costly route. The default gateway is for any and all traffic which is not destined for the local network and for which no preferred route is specified in the routing table. The default gateway is traditionally a dedicated network router.

Static Routes

Use the ip route command to display the IP routing table. If static routes are required, they can be added to the routing table by means of the ip route add command and removed using the ip route del command. To add a static route to a host address, that is to say to a single IP address, issue the following command as root:
ip route add X.X.X.X
where X.X.X.X is the IP address of the host in dotted decimal notation. To add a static route to a network, that is to say to an IP address representing a range of IP addresses, issue the following command as root:
ip route add X.X.X.X/Y
where X.X.X.X is the IP address of the network in dotted decimal notation and Y is the network prefix. The network prefix is the number of enabled bits in the subnet mask. This format of network address slash prefix length is referred to as CIDR notation.
Static route configuration is stored per-interface in a /etc/sysconfig/network-scripts/route-interface file. For example, static routes for the eth0 interface would be stored in the /etc/sysconfig/network-scripts/route-eth0 file. The route-interface file has two formats: IP command arguments and network/netmask directives. These are described below.

The Default Gateway

The default gateway is specified by means of the GATEWAY directive and can be specified either globally or in interface-specific configuration files. Specifying the default gateway globally has certain advantages especially if more than one network interface is present and it can make fault finding simpler if applied consistently. There is also the GATEWAYDEV directive, which is a global option. If multiple devices specify GATEWAY, and one interface uses the GATEWAYDEV directive, that directive will take precedence. This option is not recommend as it can have unexpected consequences if an interface goes down and it can complicate fault finding.
Global default gateway configuration is stored in the /etc/sysconfig/network file. This file specifies gateway and host information for all network interfaces. For more information about this file and the directives it accepts, refer to Section D.1.13, “ /etc/sysconfig/network ”.

IP Command Arguments Format

If required in a per-interface configuration file, define a default gateway on the first line. This is only required if the default gateway is not set via DHCP and is not set globally as mentioned above:
default via X.X.X.X dev interface
X.X.X.X is the IP address of the default gateway. The interface is the interface that is connected to, or can reach, the default gateway. The dev option can be omitted, it is optional.
Define a static route. Each line is parsed as an individual route:
X.X.X.X/Y via X.X.X.X dev interface
X.X.X.X/Y is the network address and netmask for the static route. X.X.X.X and interface are the IP address and interface for the default gateway respectively. The X.X.X.X address does not have to be the default gateway IP address. In most cases, X.X.X.X will be an IP address in a different subnet, and interface will be the interface that is connected to, or can reach, that subnet. Add as many static routes as required.
The following is a sample route-eth0 file using the IP command arguments format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks:
default via 192.168.0.1 dev eth0
10.10.10.0/24 via 192.168.0.1 dev eth0
172.16.1.0/24 via 192.168.0.1 dev eth0
Static routes should only be configured for other subnets. The above example is not necessary, since packets going to the 10.10.10.0/24 and 172.16.1.0/24 networks will use the default gateway anyway. Below is an example of setting static routes to a different subnet, on a machine in a 192.168.0.0/24 subnet. The example machine has an eth0 interface in the 192.168.0.0/24 subnet, and an eth1 interface (10.10.10.1) in the 10.10.10.0/24 subnet:
10.10.10.0/24 via 10.10.10.1 dev eth1
Specifying an exit interface is optional. It can be useful if you want to force traffic out of a specific interface. For example, in the case of a VPN, you can force traffic to a remote network to pass through a tun0 interface even when the interface is in a different sub-net to the destination network.

Duplicate default gateways

If the default gateway is already assigned from DHCP, the IP command arguments format can cause one of two errors during start-up, or when bringing up an interface from the down state using the ifup command: "RTNETLINK answers: File exists" or 'Error: either "to" is a duplicate, or "X.X.X.X" is a garbage.', where X.X.X.X is the gateway, or a different IP address. These errors can also occur if you have another route to another network using the default gateway. Both of these errors are safe to ignore.

Network/Netmask Directives Format

You can also use the network/netmask directives format for route-interface files. The following is a template for the network/netmask format, with instructions following afterwards:
 ADDRESS0=X.X.X.X NETMASK0=X.X.X.X GATEWAY0=X.X.X.X 
  • ADDRESS0=X.X.X.X is the network number for the static route.
  • NETMASK0=X.X.X.X is the netmask for the network number defined with ADDRESS0=X.X.X.X.
  • GATEWAY0=X.X.X.X is the default gateway, or an IP address that can be used to reach ADDRESS0=X.X.X.X
The following is a sample route-eth0 file using the network/netmask directives format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks. However, as mentioned before, this example is not necessary as the 10.10.10.0/24 and 172.16.1.0/24 networks would use the default gateway anyway:
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.0.1
ADDRESS1=172.16.1.0
NETMASK1=255.255.255.0
GATEWAY1=192.168.0.1
Subsequent static routes must be numbered sequentially, and must not skip any values. For example, ADDRESS0, ADDRESS1, ADDRESS2, and so on.
Below is an example of setting static routes to a different subnet, on a machine in the 192.168.0.0/24 subnet. The example machine has an eth0 interface in the 192.168.0.0/24 subnet, and an eth1 interface (10.10.10.1) in the 10.10.10.0/24 subnet:
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=10.10.10.1
Note that if DHCP is used, it can assign these settings automatically.

8.6. Network Function Files

Fedora makes use of several files that contain important common functions used to bring interfaces up and down. Rather than forcing each interface control file to contain these functions, they are grouped together in a few files that are called upon when necessary.
The /etc/sysconfig/network-scripts/network-functions file contains the most commonly used IPv4 functions, which are useful to many interface control scripts. These functions include contacting running programs that have requested information about changes in the status of an interface, setting hostnames, finding a gateway device, verifying whether or not a particular device is down, and adding a default route.
As the functions required for IPv6 interfaces are different from IPv4 interfaces, a /etc/sysconfig/network-scripts/network-functions-ipv6 file exists specifically to hold this information. The functions in this file configure and delete static IPv6 routes, create and remove tunnels, add and remove IPv6 addresses to an interface, and test for the existence of an IPv6 address on an interface.

8.7. Additional Resources

The following are resources which explain more about network interfaces.

8.7.1. Installed Documentation

/usr/share/doc/initscripts-version/sysconfig.txt
A guide to available options for network configuration files, including IPv6 options not covered in this chapter.

8.7.2. Useful Websites

http://linux-ip.net/gl/ip-cref/
This document contains a wealth of information about the ip command, which can be used to manipulate routing tables, among other things.

Part IV. Infrastructure Services

This part provides information how to configure services and daemons, configure authentication, and enable remote logins.

Table of Contents

9. Services and Daemons
9.1. Configuring Services
9.1.1. Enabling the Service
9.1.2. Disabling the Service
9.2. Running Services
9.2.1. Checking the Service Status
9.2.2. Running the Service
9.2.3. Stopping the Service
9.2.4. Restarting the Service
9.3. Additional Resources
9.3.1. Installed Documentation
9.3.2. Related Books
10. Configuring Authentication
10.1. Configuring System Authentication
10.1.1. Launching the Authentication Configuration Tool UI
10.1.2. Selecting the Identity Store for Authentication
10.1.3. Configuring Alternative Authentication Features
10.1.4. Configuring Authentication from the Command Line
10.1.5. Using Custom Home Directories
10.2. Using and Caching Credentials with SSSD
10.2.1. About the sssd.conf File
10.2.2. Starting and Stopping SSSD
10.2.3. Configuring SSSD to Work with System Services
10.2.4. Creating Domains
10.2.5. Configuring Access Control for SSSD Domains
10.2.6. Configuring Domain Failover
10.2.7. Managing the SSSD Cache
10.2.8. Configuring OpenSSH to Check SSSD for Cached Keys (TECH PREVIEW)
10.2.9. Using NSCD with SSSD
10.2.10. Troubleshooting SSSD
11. OpenSSH
11.1. The SSH Protocol
11.1.1. Why Use SSH?
11.1.2. Main Features
11.1.3. Protocol Versions
11.1.4. Event Sequence of an SSH Connection
11.2. An OpenSSH Configuration
11.2.1. Configuration Files
11.2.2. Starting an OpenSSH Server
11.2.3. Requiring SSH for Remote Connections
11.2.4. Using a Key-Based Authentication
11.3. OpenSSH Clients
11.3.1. Using the ssh Utility
11.3.2. Using the scp Utility
11.3.3. Using the sftp Utility
11.4. More Than a Secure Shell
11.4.1. X11 Forwarding
11.4.2. Port Forwarding
11.5. Additional Resources
11.5.1. Installed Documentation
11.5.2. Useful Websites

Chapter 9. Services and Daemons

Maintaining security on your system is extremely important, and one approach for this task is to manage access to system services carefully. Your system may need to provide open access to particular services (for example, httpd if you are running a web server). However, if you do not need to provide a service, you should turn it off to minimize your exposure to possible bug exploits.
This chapter covers the configuration of the services to be run when a system is started, and provides information on how to start, stop, and restart the services on the command line using the systemctl utility.

Keep the system secure

When you allow access for new services, always remember that both the firewall and SELinux need to be configured as well. One of the most common mistakes committed when configuring a new service is neglecting to implement the necessary firewall configuration and SELinux policies to allow access for it. For more information, refer to the Fedora 18 Security Guide.

9.1. Configuring Services

To allow you to configure which services are started at boot time, Fedora is shipped with the systemctl command line tool.

Do not use the ntsysv and chkconfig utilities

Although it is still possible to use the ntsysv and chkconfig utilities to manage services that have init scripts installed in the /etc/rc.d/init.d/ directory, it is advised that you use the systemctl utility.

Enabling the irqbalance service

To ensure optimal performance on POWER architecture, it is recommended that the irqbalance service is enabled. In most cases, this service is installed and configured to run during the Fedora 18 installation. To verify that irqbalance is running, type the following at a shell prompt:
systemctl status irqbalance.service

9.1.1. Enabling the Service

To configure a service to be automatically started at boot time, use the systemctl command in the following form:
systemctl enable service_name.service
The service will be started the next time you boot the system. For information on how to start the service immediately, refer to Section 9.2.2, “Running the Service”.
Example 9.1. Enabling the httpd service
Imagine you want to run the Apache HTTP Server on your system. Provided that you have the httpd package installed, you can enable the httpd service by typing the following at a shell prompt as root:
~]# systemctl enable httpd.service

9.1.2. Disabling the Service

To disable starting a service at boot time, use the systemctl command in the following form:
systemctl disable service_name.service
The next time you boot the system, the service will not be started. For information on how to stop the service immediately, refer to Section 9.2.3, “Stopping the Service”.
Example 9.2. Disabling the telnet service
In order to secure the system, users are advised to disable insecure connection protocols such as Telnet. You can make sure that the telnet service is disabled by running the following command as root:
~]# systemctl disable telnet.service

9.2. Running Services

The systemctl utility also allows you to determine the status of a particular service, as well as to start, stop, or restart a service.

Do not use the service utility

Although it is still possible to use the service utility to manage services that have init scripts installed in the /etc/rc.d/init.d/ directory, it is advised that you use the systemctl utility.

9.2.1. Checking the Service Status

To determine the status of a particular service, use the systemctl command in the following form:
systemctl status service_name.service
This command provides detailed information on the service's status. However, if you merely need to verify that a service is running, you can use the systemctl command in the following form instead:
systemctl is-active service_name.service
Example 9.3. Checking the status of the httpd service
Example 9.1, “Enabling the httpd service” illustrated how to enable starting the httpd service at boot time. Imagine that the system has been restarted and you need to verify that the service is really running. You can do so by typing the following at a shell prompt:
~]$ systemctl is-active httpd.service
active
You can also display detailed information about the service by running the following command:
~]$ systemctl status httpd.service
httpd.service - LSB: start and stop Apache HTTP Server
          Loaded: loaded (/etc/rc.d/init.d/httpd)
          Active: active (running) since Mon, 23 May 2011 21:38:57 +0200; 27s ago
         Process: 2997 ExecStart=/etc/rc.d/init.d/httpd start (code=exited, status=0/SUCCESS)
        Main PID: 3002 (httpd)
          CGroup: name=systemd:/system/httpd.service
                  ├ 3002 /usr/sbin/httpd
                  ├ 3004 /usr/sbin/httpd
                  ├ 3005 /usr/sbin/httpd
                  ├ 3006 /usr/sbin/httpd
                  ├ 3007 /usr/sbin/httpd
                  ├ 3008 /usr/sbin/httpd
                  ├ 3009 /usr/sbin/httpd
                  ├ 3010 /usr/sbin/httpd
                  └ 3011 /usr/sbin/httpd

To display a list of all active system services, use the following command:
systemctl list-units --type=service
This command provides a tabular output with each line consisting of the following columns:
  • UNIT — A systemd unit name. In this case, a service name.
  • LOAD — Information whether the systemd unit was properly loaded.
  • ACTIVE — A high-level unit activation state.
  • SUB — A low-level unit activation state.
  • JOB — A pending job for the unit.
  • DESCRIPTION — A brief description of the unit.
Example 9.4. Listing all active services
You can list all active services by using the following command:
~]$ systemctl list-units --type=service
UNIT                      LOAD   ACTIVE SUB     JOB DESCRIPTION
abrt-ccpp.service         loaded active exited      LSB: Installs coredump handler which saves segfault data
abrt-oops.service         loaded active running     LSB: Watches system log for oops messages, creates ABRT dump directories for each oops
abrtd.service             loaded active running     ABRT Automated Bug Reporting Tool
accounts-daemon.service   loaded active running     Accounts Service
atd.service               loaded active running     Job spooling tools
[output truncated]
In the example above, the abrtd service is loaded, active, and running, and it does not have any pending jobs.

9.2.2. Running the Service

To run a service, use the systemctl command in the following form:
systemctl start service_name.service
This will start the service in the current session. To configure the service to be started at boot time, refer to Section 9.1.1, “Enabling the Service”.
Example 9.5. Running the httpd service
Example 9.1, “Enabling the httpd service” illustrated how to run the httpd service at boot time. You can start the service immediately by typing the following at a shell prompt as root:
~]# systemctl start httpd.service

9.2.3. Stopping the Service

To stop a service, use the systemctl command in the following form:
systemctl stop service_name.service
This will stop the service in the current session. To disable starting the service at boot time, refer to Section 9.1.1, “Enabling the Service”.
Example 9.6. Stopping the telnet service
Example 9.2, “Disabling the telnet service” illustrated how to disable starting the telnet service at boot time. You can stop the service immediately by running the following command as root:
~]# systemctl stop telnet.service

9.2.4. Restarting the Service

To restart a service, use the systemctl command in the following form:
systemctl restart service_name.service
Example 9.7. Restarting the sshd service
For any changes in the /etc/ssh/sshd_config configuration file to take effect, it is required that you restart the sshd service. You can do so by typing the following at a shell prompt as root:
~]# systemctl restart sshd.service

9.3. Additional Resources

9.3.1. Installed Documentation

  • systemctl(1) — The manual page for the systemctl utility.

9.3.2. Related Books

Fedora 18 Security Guide
A guide to securing Fedora. It contains valuable information on how to set up the firewall, as well as the configuration of SELinux.

Chapter 10. Configuring Authentication

Authentication is the way that a user is identified and verified to a system. The authentication process requires presenting some sort of identity and credentials, like a username and password. The credentials are then compared to information stored in some data store on the system. In Fedora, the Authentication Configuration Tool helps configure what kind of data store to use for user credentials, such as LDAP.
For convenience and potentially part of single sign-on, Fedora can use a central daemon to store user credentials for a number of different data stores. The System Security Services Daemon (SSSD) can interact with LDAP, Kerberos, and external applications to verify user credentials. The Authentication Configuration Tool can configure SSSD along with NIS, Winbind, and LDAP, so that authentication processing and caching can be combined.

10.1. Configuring System Authentication

When a user logs into a Fedora system, that user presents some sort of credential to establish the user identity. The system then checks those credentials against the configured authentication service. If the credentials match and the user account is active, then the user is authenticated. (Once a user is authenticated, then the information is passed to the access control service to determine what the user is permitted to do. Those are the resources the user is authorized to access.)
The information to verify the user can be located on the local system or the local system can reference a user database on a remote system, such as LDAP or Kerberos.
The system must have a configured list of valid account databases for it to check for user authentication. On Fedora, the Authentication Configuration Tool has both GUI and command-line options to configure any user data stores.
A local system can use a variety of different data stores for user information, including Lightweight Directory Access Protocol (LDAP), Network Information Service (NIS), and Winbind. Additionally, both LDAP and NIS data stores can use Kerberos to authenticate users.

Important

If a medium or high security level is set during installation or with the Security Level Configuration Tool, then the firewall prevents NIS authentication. For more information about firewalls, see the "Firewalls" section of the Security Guide.

10.1.1. Launching the Authentication Configuration Tool UI

  1. Log into the system as root.
  2. Open the System.
  3. Select the Administration menu.
  4. Select the Authentication item.
Alternatively, run the system-config-authentication command.

Important

Any changes take effect immediately when the Authentication Configuration Tool UI is closed.
There are two configuration tabs in the Authentication dialog box:
  • Identity & Authentication, which configures the resource used as the identity store (the data repository where the user IDs and corresponding credentials are stored).
  • Advanced Options, which allows authentication methods other than passwords or certificates, like smart cards and fingerprint.

10.1.2. Selecting the Identity Store for Authentication

The Identity & Authentication tab sets how users should be authenticated. The default is to use local system authentication, meaning the users and their passwords are checked against local system accounts. A Fedora machine can also use external resources which contain the users and credentials, including LDAP, NIS, and Winbind.
Local Authentication
Figure 10.1. Local Authentication

10.1.2.1. Configuring LDAP Authentication

Either the openldap-clients package or the sssd package is used to configure an LDAP server for the user database. Both packages are installed by default.
  1. Open the Authentication Configuration Tool, as in Section 10.1.1, “Launching the Authentication Configuration Tool UI”.
  2. Select LDAP in the User Account Database drop-down menu.
  3. Set the information that is required to connect to the LDAP server.
    • LDAP Search Base DN gives the root suffix or distinguished name (DN) for the user directory. All of the user entries used for identity/authentication will exist below this parent entry. For example, ou=people,dc=example,dc=com.
      This field is optional. If it is not specified, then SSSD attempts to detect the search base using the namingContexts and defaultNamingContext attributes in the LDAP server's configuration entry.
    • LDAP Server gives the URL of the LDAP server. This usually requires both the hostname and port number of the LDAP server, such as ldap://ldap.example.com:389.
      Entering the secure protocol in the URL, ldaps://, enables the Download CA Certificate button.
    • Use TLS to encrypt connections sets whether to use Start TLS to encrypt the connections to the LDAP server. This enables a secure connection over a standard port.
      Selecting TLS enables the Download CA Certificate button, which retrieves the issuing CA certificate for the LDAP server from whatever certificate authority issued it. The CA certificate must be in the privacy enhanced mail (PEM) format.

      Important

      Do not select Use TLS to encrypt connections if the server URL uses a secure protocol (ldaps). This option uses Start TLS, which initiates a secure connection over a standard port; if a secure port is specified, then a protocol like SSL must be used instead of Start TLS.
  4. Select the authentication method. LDAP allows simple password authentication or Kerberos authentication.
    The LDAP password option uses PAM applications to use LDAP authentication. This option requires either a secure (ldaps://) URL or the TLS option to connect to the LDAP server.

10.1.2.2. Configuring NIS Authentication

  1. Install the ypbind package. This is required for NIS services, but is not installed by default.
    [root@server ~]# yum install ypbind
    When the ypbind service is installed, the portmap and ypbind services are started and enabled to start at boot time.
  2. Open the Authentication Configuration Tool, as in Section 10.1.1, “Launching the Authentication Configuration Tool UI”.
  3. Select NIS in the User Account Database drop-down menu.
  4. Set the information to connect to the NIS server, meaning the NIS domain name and the server hostname. If the NIS server is not specified, the authconfig daemon scans for the NIS server.
  5. Select the authentication method. NIS allows simple password authentication or Kerberos authentication.
For more information about NIS, see the "Securing NIS" section of the Security Guide.

10.1.2.3. Configuring Winbind Authentication

Using Winbind as an authentication provider requires the samba-winbind package, which is installed by default.
  1. Open the Authentication Configuration Tool, as in Section 10.1.1, “Launching the Authentication Configuration Tool UI”.
  2. Select Winbind in the User Account Database drop-down menu.
  3. Set the information that is required to connect to the Microsoft Active Directory domain controller.
    • Winbind Domain gives the Windows domain to connect to.
      This should be in the Windows 2000 format, such as DOMAIN.
    • Security Model sets the security model to use for Samba clients. authconfig supports four types of security models:
      • ads configures Samba to act as a domain member in an Active Directory Server realm. To operate in this mode, the krb5-server package must be installed and Kerberos must be configured properly.
      • domain has Samba validate the username/password by authenticating it through a Windows primary or backup domain controller, much like a Windows server.
      • server has a local Samba server validate the username/password by authenticating it through another server, such as a Windows server. If the server authentication attempt fails, the system then attempts to authentication using user mode.
      • user requires a client to log in with a valid username and password. This mode does support encrypted passwords.
        The username format must be domain\user, such as EXAMPLE\jsmith.

        Note

        When verifying that a given user exists in the Windows domain, always use Windows 2000-style formats and escape the backslash (\) character. For example:
        [root@server ~]# getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bash
        This is the default option.
    • Winbind ADS Realm gives the Active Directory realm that the Samba server will join. This is only used with the ads security model.
    • Winbind Domain Controllers gives the domain controller to use. For more information about domain controllers, refer to Section 17.1.6.3, “Domain Controller”.
    • Template Shell sets which login shell to use for Windows user account settings.
    • Allow offline login allows authentication information to be stored in a local cache. The cache is referenced when a user attempts to authenticate to system resources while the system is offline.
For more information about the winbindd service, refer to Section 17.1.2, “Samba Daemons and Related Services”.

10.1.2.4. Using Kerberos with LDAP or NIS Authentication

Both LDAP and NIS authentication stores support Kerberos authentication methods. Using Kerberos has a couple of benefits:
  • It uses a security layer for communication while still allowing connections over standard ports.
  • It automatically uses credentials caching with SSSD, which allows offline logins.
Using Kerberos authentication requires the krb5-libs and krb5-workstation packages.
The Kerberos password option from the Authentication Method drop-down menu automatically opens the fields required to connect to the Kerberos realm.
Kerberos Fields
Figure 10.2. Kerberos Fields

  • Realm gives the name for the realm for the Kerberos server. The realm is the network that uses Kerberos, composed of one or more key distribution centers (KDC) and a potentially large number of clients.
  • KDCs gives a comma-separated list of servers that issue Kerberos tickets.
  • Admin Servers gives a list of administration servers running the kadmind process in the realm.
  • Optionally, use DNS to resolve server hostname and to find additional KDCs within the realm.
For more information about Kerberos, refer to section "Using Kerberos" of the Fedora 18 Managing Single Sign-On and Smart Cards guide.

10.1.3. Configuring Alternative Authentication Features

The Authentication Configuration Tool also configures settings related to authentication behavior, apart from the identity store. This includes entirely different authentication methods (fingerprint scans and smart cards) or local authentication rules. These alternative authentication options are configured in the Advanced Options tab.
Advanced Options
Figure 10.3. Advanced Options

10.1.3.1. Using Fingerprint Authentication

When there is appropriate hardware available, the Enable fingerprint reader support option allows fingerprint scans to be used to authenticate local users in addition to other credentials.

10.1.3.2. Setting Local Authentication Parameters

There are two options in the Local Authentication Options area which define authentication behavior on the local system:
  • Enable local access control instructs the /etc/security/access.conf file to check for local user authorization rules.
  • Password Hashing Algorithm sets the hashing algorithm to use to encrypt locally-stored passwords.

10.1.3.3. Enabling Smart Card Authentication

When there are appropriate smart card readers available, a system can accept smart cards (or tokens) instead of other user credentials to authenticate.
Once the Enable smart card support option is selected, then the behaviors of smart card authentication can be defined:
  • Card Removal Action tells the system how to respond when the card is removed from the card reader during an active session. A system can either ignore the removal and allow the user to access resources as normal, or a system can immediately lock until the smart card is supplied.
  • Require smart card login sets whether a smart card is required for logins or simply allowed for logins. When this option is selected, all other methods of authentication are immediately blocked.

    Warning

    Do not select this option until you have successfully authenticated to the system using a smart card.
Using smart cards requires the pam_pkcs11 package.

10.1.3.4. Creating User Home Directories

There is an option (Create home directories on the first login) to create a home directory automatically the first time that a user logs in.
This option is beneficial with accounts that are managed centrally, such as with LDAP. However, this option should not be selected if a system like automount is used to manage user home directories.

10.1.4. Configuring Authentication from the Command Line

The authconfig command-line tool updates all of the configuration files and services required for system authentication, according to the settings passed to the script. Along with allowing all of the identity and authentication configuration options that can be set through the UI, the authconfig tool can also be used to create backup and kickstart files.
For a complete list of authconfig options, check the help output and the man page.

10.1.4.1. Tips for Using authconfig

There are some things to remember when running authconfig:
  • With every command, use either the --update or --test option. One of those options is required for the command to run successfully. Using --update writes the configuration changes. --test prints the changes to stdout but does not apply the changes to the configuration.
  • Each enable option has a corresponding disable option.

10.1.4.2. Configuring LDAP User Stores

To use an LDAP identity store, use the --enableldap. To use LDAP as the authentication source, use --enableldapauth and then the requisite connection information, like the LDAP server name, base DN for the user suffix, and (optionally) whether to use TLS. The authconfig command also has options to enable or disable RFC 2307bis schema for user entries, which is not possible through the Authentication Configuration UI.
Be sure to use the full LDAP URL, including the protocol (ldap or ldaps) and the port number. Do not use a secure LDAP URL (ldaps) with the --enableldaptls option.
authconfig --enableldap --enableldapauth --ldapserver=ldap://ldap.example.com:389,ldap://ldap2.example.com:389 --ldapbasedn="ou=people,dc=example,dc=com" --enableldaptls --ldaploadcacert=https://ca.server.example.com/caCert.crt --update
Instead of using --ldapauth for LDAP password authentication, it is possible to use Kerberos with the LDAP user store. These options are described in Section 10.1.4.5, “Configuring Kerberos Authentication”.

10.1.4.3. Configuring NIS User Stores

To use a NIS identity store, use the --enablenis. This automatically uses NIS authentication, unless the Kerberos parameters are explicitly set, so it uses Kerberos authentication (Section 10.1.4.5, “Configuring Kerberos Authentication”). The only parameters are to identify the NIS server and NIS domain; if these are not used, then the authconfig service scans the network for NIS servers.
authconfig --enablenis --nisdomain=EXAMPLE --nisserver=nis.example.com --update

10.1.4.4. Configuring Winbind User Stores

Windows domains have several different security models, and the security model used in the domain determines the authentication configuration for the local system.
For user and server security models, the Winbind configuration requires only the domain (or workgroup) name and the domain controller hostnames.
authconfig --enablewinbind --enablewinbindauth --smbsecurity=user|server  --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --update

Note

The username format must be domain\user, such as EXAMPLE\jsmith.
When verifying that a given user exists in the Windows domain, always use Windows 2000-style formats and escape the backslash (\) character. For example:
[root@server ~]# getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bash
For ads and domain security models, the Winbind configuration allows additional configuration for the template shell and realm (ads only). For example:
authconfig --enablewinbind --enablewinbindauth --smbsecurity ads  --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --smbrealm EXAMPLE.COM --winbindtemplateshell=/bin/sh --update
There are a lot of other options for configuring Windows-based authentication and the information for Windows user accounts, such as name formats, whether to require the domain name with the username, and UID ranges. These options are listed in the authconfig help.

10.1.4.5. Configuring Kerberos Authentication

Both LDAP and NIS allow Kerberos authentication to be used in place of their native authentication mechanisms. At a minimum, using Kerberos authentication requires specifying the realm, the KDC, and the administrative server. There are also options to use DNS to resolve client names and to find additional admin servers.
authconfig NIS or LDAP options --enablekrb5 --krb5realm EXAMPLE --krb5kdc kdc.example.com:88,server.example.com:88 --krb5adminserver server.example.com:749 --enablekrb5kdcdns --enablekrb5realmdns --update

10.1.4.6. Configuring Local Authentication Settings

The Authentication Configuration Tool can also control some user settings that relate to security, such as creating home directories, setting password hash algorithms, and authorization. These settings are done independently of identity/user store settings.
For example, to create user home directories:
authconfig --enablemkhomedir --update
To set or change the hash algorithm used to encrypt user passwords:
authconfig --passalgo=sha512 --update

10.1.4.7. Configuring Fingerprint Authentication

There is one option to enable support for fingerprint readers. This option can be used alone or in conjunction with other authconfig settings, like LDAP user stores.
authconfig --enablefingerprint --update

10.1.4.8. Configuring Smart Card Authentication

All that is required to use smart cards with a system is to set the --enablesmartcard option:
authconfig --enablesmartcard --update
There are other configuration options for smart cards, such as changing the default smart card module, setting the behavior of the system when the smart card is removed, and requiring smart cards for login.
For example, this command instructs the system to lock out a user immediately if the smart card is removed (a setting of 1 ignores it if the smart card is removed):
authconfig --enablesmartcard --smartcardaction=0 --update
Once smart card authentication has been successfully configured and tested, then the system can be configured to require smart card authentication for users rather than simple password-based authentication.
authconfig --enablerequiresmartcard --update

Warning

Do not use the --enablerequiresmartcard option until you have successfully authenticated to the system using a smart card. Otherwise, users may be unable to log into the system.

10.1.4.9. Managing Kickstart and Configuration Files

The --update option updates all of the configuration files with the configuration changes. There are a couple of alternative options with slightly different behavior:
  • --kickstart writes the updated configuration to a kickstart file.
  • --test prints the full configuration, with changes, to stdout but does not edit any configuration files.
Additionally, authconfig can be used to back up and restore previous configurations. All archives are saved to a unique subdirectory in the /var/lib/authconfig/ directory. For example, the --savebackup option gives the backup directory as 2011-07-01:
authconfig --savebackup=2011-07-01
This backs up all of the authentication configuration files beneath the /var/lib/authconfig/backup-2011-07-01 directory.
Any of the saved backups can be used to restore the configuration using the --restorebackup option, giving the name of the manually-saved configuration:
authconfig --restorebackup=2011-07-01
Additionally, authconfig automatically makes a backup of the configuration before it applies any changes (with the --update option). The configuration can be restored from the most recent automatic backup, without having to specify the exact backup, using the --restorelastbackup option.

10.1.5. Using Custom Home Directories

If LDAP users have home directories that are not in /home and the system is configured to create home directories the first time users log in, then these directories are created with the wrong permissions.
  1. Apply the correct SELinux context and permissions from the /home directory to the home directory that is created on the local system. For example:
    # semanage fcontext -a -e /home /home/locale
  2. Install the oddjob-mkhomedir package on the system.
    This package provides the pam_oddjob_mkhomedir.so library, which the Authentication Configuration Tool uses to create home directories. The pam_oddjob_mkhomedir.so library, unlike the default pam_mkhomedir.so library, can create SELinux labels.
    The Authentication Configuration Tool automatically uses the pam_oddjob_mkhomedir.so library if it is available. Otherwise, it will default to using pam_mkhomedir.so.
  3. Make sure the oddjobd service is running.
  4. Re-run the Authentication Configuration Tool and enable home directories, as in Section 10.1.3, “Configuring Alternative Authentication Features”.
If home directories were created before the home directory configuration was changed, then correct the permissions and SELinux contexts. For example:
# semanage fcontext -a -e /home /home/locale
# restorecon -R -v /home/locale

10.2. Using and Caching Credentials with SSSD

The System Security Services Daemon (SSSD) provides access to different identity and authentication providers. SSSD is an intermediary between local clients and any configured data store. The local clients connect to SSSD and then SSSD contacts the external providers. This brings a number of benefits for administrators:
  • Reducing the load on identification/authentication servers. Rather than having every client service attempt to contact the identification server directly, all of the local clients can contact SSSD which can connect to the identification server or check its cache.
  • Permitting offline authentication. SSSD can optionally keep a cache of user identities and credentials that it retrieves from remote services. This allows users to authenticate to resources successfully, even if the remote identification server is offline or the local machine is offline.
  • Using a single user account. Remote users frequently have two (or even more) user accounts, such as one for their local system and one for the organizational system. This is necessary to connect to a virtual private network (VPN). Because SSSD supports caching and offline authentication, remote users can connect to network resources simply by authenticating to their local machine and then SSSD maintains their network credentials.
The System Security Services Daemon does not require any additional configuration or tuning to work with the Authentication Configuration Tool. However, SSSD can work with other applications, and the daemon may require configuration changes to improve the performance of those applications.

10.2.1. About the sssd.conf File

SSSD services and domains are configured in a .conf file. The default file is /etc/sssd/sssd.conf, although alternative files can be passed to SSSD by using the -c option with the sssd command:
# sssd -c /etc/sssd/customfile.conf
Both services and domains are configured individually, in separate sections on the configuration identified by [type/name] divisions, such as [domain/LDAP]. The configuration file uses simple key = value lines to set the configuration. Comment lines are set by either a hash sign (#) or a semicolon (;)
For example:
[section]
# Comment line
key1 = val1
key10 = val1,val2

10.2.2. Starting and Stopping SSSD

Note

Configure at least one domain before starting SSSD for the first time. See Section 10.2.4, “Creating Domains”.
Either the service command or the /etc/init.d/sssd script can start SSSD. For example:
# service sssd start
By default, SSSD is configured not to start automatically. There are two ways to change this behavior:
  • Using the authconfig command:
    [root@server ~]# authconfig --enablesssd --enablesssdauth --update
  • Using the chkconfig command:
    [root@server ~]# chkconfig sssd on

10.2.3. Configuring SSSD to Work with System Services

SSSD worked with specialized services that run in tandem with the SSSD process itself. SSSD and its associated services are configured in the sssd.conf file. The [sssd] section also lists the services that are active and should be started when sssd starts within the services directive.
SSSD currently provides several services:
  • A Name Service Switch (NSS) provider service that answers name service requests from the sssd_nss module. This is configured in the [nss] section of the SSSD configuration.
  • A PAM provider service that manages a PAM conversation through the sssd_pam module. This is configured in the [pam] section of the configuration.
  • monitor, a special service that monitors and starts or restarts all other SSSD services. Its options are specified in the [sssd] section of the /etc/sssd/sssd.conf configuration file.

Note

If a DNS lookup fails to return an IPv4 address for a hostname, SSSD attempts to look up an IPv6 address before returning a failure. This only ensures that the asynchronous resolver identifies the correct address.
The hostname resolution behavior is configured in the lookup family order option in the sssd.conf configuration file.

10.2.3.1. Configuring NSS Services

SSSD provides an NSS module, sssd_nss, which instructs the system to use SSSD to retrieve user information. The NSS configuration must include a reference to the SSSD module, and then the SSSD configuration sets how SSSD interacts with NSS.
10.2.3.1.1. About NSS Service Maps and SSSD
The Name Service Switch (NSS) provides a central configuration for services to look up a number of configuration and name resolution services. NSS provides one method of mapping system identities and services with configuration sources.
SSSD works with NSS as a provider services for several types of NSS maps:
  • Passwords (passwd)
  • User groups (shadow)
  • Groups (groups)
  • Netgroups (netgroups)
  • Services (services)
10.2.3.1.2. Configuring NSS Services to Use SSSD
NSS can use multiple identity and configuration providers for any and all of its service maps. The default is to use system files for services; for SSSD to be included, the nss_sss module has to be included for the desired service type.
  1. Use the Authentication Configuration tool to enable SSSD. This automatically configured the nsswitch.conf file to use SSSD as a provider.
    [root@server ~]# authconfig --enablesssd --update
    This automatically configures the password, shadow, group, and netgroups services maps to use the SSSD module:
    passwd:     files sss
    shadow:     files sss
    group:      files sss
    
    netgroup:   files sss
  2. The services map is not enabled by default when SSSD is enabled with authconfig. To include that map, open the nsswitch.conf file and add the sss module to the services map:
    [root@server ~]# vim /etc/nsswitch.conf
    
    ...
    services: file sss
    ...
10.2.3.1.3. Configuring SSSD to Work with NSS
The options and configuration that SSSD uses to service NSS requests are configured in the SSSD configuration file, in the [nss] services section.
  1. Open the sssd.conf file.
    [root@server ~]# vim /etc/sssd/sssd.conf
  2. Make sure that NSS is listed as one of the services that works with SSSD.
    [sssd]
    config_file_version = 2
    reconnection_retries = 3
    sbus_timeout = 30
    services = nss, pam
  3. In the [nss] section, change any of the NSS parameters. These are listed in Table 10.1, “SSSD [nss] Configuration Parameters”.
    [nss]
    filter_groups = root
    filter_users = root
    reconnection_retries = 3
    entry_cache_timeout = 300
    entry_cache_nowait_percentage = 75
  4. Restart SSSD.
    [root@server ~]# service sssd restart
Table 10.1. SSSD [nss] Configuration Parameters
Parameter Value Format Description
enum_cache_timeout integer Specifies how long, in seconds, sssd_nss should cache requests for information about all users (enumerations).
entry_cache_nowait_percentage integer Specifies how long sssd_nss should return cached entries before refreshing the cache. Setting this to zero (0) disables the entry cache refresh.
This configures the entry cache to update entries in the background automatically if they are requested if the time before the next update is a certain percentage of the next interval. For example, if the interval is 300 seconds and the cache percentage is 75, then the entry cache will begin refreshing when a request comes in at 225 seconds — 75% of the interval.
The allowed values for this option are 0 to 99, which sets the percentage based on the entry_cache_timeout value. The default value is 50%.
entry_negative_timeout integer Specifies how long, in seconds, sssd_nss should cache negative cache hits. A negative cache hit is a query for an invalid database entries, including non-existent entries.
filter_users, filter_groups string Tells SSSD to exclude certain users from being fetched from the NSS database. This is particularly useful for system accounts such as root.
filter_users_in_groups Boolean Sets whether users listed in the filter_users list appear in group memberships when performing group lookups. If set to FALSE, group lookups return all users that are members of that group. If not specified, this value defaults to true, which filters the group member lists.
debug_level integer, 0 - 9 Sets a debug logging level.

10.2.3.2. Configuring the PAM Service

Warning

A mistake in the PAM configuration file can lock users out of the system completely. Always back up the configuration files before performing any changes, and keep a session open so that any changes can be reverted.
SSSD provides a PAM module, sssd_pam, which instructs the system to use SSSD to retrieve user information. The PAM configuration must include a reference to the SSSD module, and then the SSSD configuration sets how SSSD interacts with PAM.
To configure the PAM service:
  1. Use authconfig to enable SSSD for system authentication.
    # authconfig --update --enablesssd --enablesssdauth
    This automatically updates the PAM configuration to reference all of the SSSD modules:
    #%PAM-1.0
    # This file is auto-generated.
    # User changes will be destroyed the next time authconfig is run.
    auth        required      pam_env.so
    auth        sufficient    pam_unix.so nullok try_first_pass
    auth        requisite     pam_succeed_if.so uid >= 500 quiet
    auth sufficient pam_sss.so use_first_pass
    auth        required      pam_deny.so
    
    account     required      pam_unix.so 
    account     sufficient    pam_localuser.so
    account     sufficient    pam_succeed_if.so uid < 500 quiet
    account [default=bad success=ok user_unknown=ignore] pam_sss.so
    account     required      pam_permit.so
    
    password    requisite     pam_cracklib.so try_first_pass retry=3
    password    sufficient    pam_unix.so sha512 shadow nullok try_first_pass use_authtok
    password sufficient pam_sss.so use_authtok
    password    required      pam_deny.so
    
    session     optional      pam_keyinit.so revoke
    session     required      pam_limits.so
    session     [success=1 default=ignore] pam_succeed_if.so service in crond quiet use_uid
    session sufficient pam_sss.so
    session     required      pam_unix.so
    
    These modules can be set to include statements, as necessary.
  2. Open the sssd.conf file.
    # vim /etc/sssd/sssd.conf
  3. Make sure that PAM is listed as one of the services that works with SSSD.
    [sssd]
    config_file_version = 2
    reconnection_retries = 3
    sbus_timeout = 30
    services = nss, pam
  4. In the [pam] section, change any of the PAM parameters. These are listed in Table 10.2, “SSSD [pam] Configuration Parameters”.
    [pam]
    reconnection_retries = 3
    offline_credentials_expiration = 2
    offline_failed_login_attempts = 3
    offline_failed_login_delay = 5
  5. Restart SSSD.
    [root@server ~]# service sssd restart
Table 10.2. SSSD [pam] Configuration Parameters
Parameter Value Format Description
offline_credentials_expiration integer Sets how long, in days, to allow cached logins if the authentication provider is offline. This value is measured from the last successful online login. If not specified, this defaults to zero (0), which is unlimited.
offline_failed_login_attempts integer Sets how many failed login attempts are allowed if the authentication provider is offline. If not specified, this defaults to zero (0), which is unlimited.
offline_failed_login_delay integer Sets how long to prevent login attempts if a user hits the failed login attempt limit. If set to zero (0), the user cannot authenticate while the provider is offline once he hits the failed attempt limit. Only a successful online authentication can re-enable offline authentication. If not specified, this defaults to five (5).

10.2.4. Creating Domains

SSSD recognizes domains, which are associated with the different identity servers. Domains are a combination of an identity provider and an authentication method. SSSD works with LDAP identity providers (including OpenLDAP, Red Hat Directory Server, and Microsoft Active Directory) and can use native LDAP authentication or Kerberos authentication.
As long as they belong to different domains, SSSD can recognize different users with the same username. For example, SSSD can successfully authenticate both jsmith in the ldap.example.com domain and jsmith in the ldap.otherexample.com domain. SSSD allows requests using fully-qualified domain names, so requesting information for jsmith@ldap.example.com returns the proper user account. Specifying only the username returns the user for whichever domain comes first in the lookup order.

Tip

SSSD has a filter_users option, which excludes the specified users from being returned in a search.
Configuring a domain defines both where user information is stored and how those users are allowed to authenticate to the system. The possible combinations are listed in Table 10.3, “Identity Store and Authentication Type Combinations”.
Table 10.3. Identity Store and Authentication Type Combinations
Identification Provider Authentication Provider
LDAP LDAP
LDAP Kerberos
proxy LDAP
proxy Kerberos
proxy proxy

10.2.4.1. General Rules and Options for Configuring a Domain

A domain configuration defines the identity provider, the authentication provider, and any specific configuration to access the information in those providers. There are two types of identity providers — LDAP and proxy —three types of authentication providers — LDAP, Kerberos, and proxy. The identity and authentication providers can be configured in any combination in a domain entry.
Along with the domain entry itself, the domain name must be added to the list of domains that SSSD will query. For example:
domains = LOCAL,Name

[domain/Name]
id_provider = type
auth_provider = type
provider_specific = value
global = value
global attributes are available to any type of domain, such as cache and time out settings. Each identity and authentication provider has its own set of required and optional configuration parameters.
Table 10.4. General [domain] Configuration Parameters
Parameter Value Format Description
id_provider string Specifies the data provider identity backend to use for this domain. The supported identity backends are:
  • ldap
  • ipa, compatible with FreeIPA version 2.x and Identity Management in Fedora
  • proxy for a legacy NSS provider, such as nss_nis. Using a proxy ID provider also requires specifying the legacy NSS library to load to start successfully, set in the proxy_lib_name option.
  • local, the SSSD internal local provider
auth_provider string Sets the authentication provider used for the domain. The default value for this option is the value of id_provider. The supported authentication providers are ldap, ipa, krb5 (Kerberos), proxy, and none.
min_id,max_id integer Optional. Specifies the UID and GID range for the domain. If a domain contains entries that are outside that range, they are ignored. The default value for min_id is 1; the default value for max_id is 0, which is unlimited.

Important

The default min_id value is the same for all types of identity provider. If LDAP directories are using UID numbers that start at one, it could cause conflicts with users in the local /etc/passwd file. To avoid these conflicts, set min_id to 1000 or higher as possible.
enumerate Boolean Optional. Specifies whether to list the users and groups of a domain. Enumeration means that the entire set of available users and groups on the remote source is cached on the local machine. When enumeration is disabled, users and groups are only cached as they are requested.

Warning

When enumeration is enabled, reinitializing a client results in a complete refresh of the entire set of available users and groups from the remote source. Similarly, when SSSD is connected to a new server, the entire set of available users and groups from the remote source is pulled and cached on the local machine. In a domain with a large number of clients connected to a remote source, this refresh process can harm the network performance because of frequent queries from the clients. If the set of available users and groups is large enough, it degrades client performance as well.
The default value for this parameter is false, which disables enumeration.
cache_credentials Boolean Optional. Specifies whether to store user credentials in the local SSSD domain database cache. The default value for this parameter is false. Set this value to true for domains other than the LOCAL domain to enable offline authentication.
entry_cache_timeout integer Optional. Specifies how long, in seconds, SSSD should cache positive cache hits. A positive cache hit is a successful query.
use_fully_qualified_names Boolean Optional. Specifies whether requests to this domain require fully-qualified domain names. If set to true, all requests to this domain must use fully-qualified domain names. It also means that the output from the request displays the fully-qualified name. Restricting requests to fully-qualified user names allows SSSD to differentiate between domains with users with conflicting usernames.
If use_fully_qualified_names is set to false, it is possible to use the fully-qualified name in the requests, but only the simplified version is displayed in the output.
SSSD can only parse names based on the domain name, not the realm name. The same name can be used for both domains and realms, however.

10.2.4.2. Configuring an LDAP Domain

An LDAP domain simply means that SSSD uses an LDAP directory as the identity provider (and, optionally, also as an authentication provider). SSSD supports several major directory services:
  • Red Hat Directory Server
  • OpenLDAP
  • Microsoft Active Directory 2008, with Subsystem for UNIX-based Applications

Note

DNS service discovery allows the LDAP backend to find the appropriate DNS servers to connect to automatically using a special DNS query.
10.2.4.2.1. Parameters for Configuring an LDAP Domain
An LDAP directory can function as both an identity provider and an authentication provider. The configuration requires enough information to identify and connect to the user directory in the LDAP server, but the way that those connection parameters are defined is flexible.
Other options are available to provide more fine-grained control, like specifying a user account to use to connect to the LDAP server or using different LDAP servers for password operations. The most common options are listed in Table 10.5, “LDAP Domain Configuration Parameters”. All of the options listed in Section 10.2.4.1, “General Rules and Options for Configuring a Domain” are also available for LDAP domains.

Tip

Many other options are listed in the man page for LDAP domain configuration, sssd-ldap(5).
Table 10.5. LDAP Domain Configuration Parameters
Parameter Description
ldap_uri Gives a comma-separated list of the URIs of the LDAP servers to which SSSD will connect. The list is given in order of preference, so the first server in the list is tried first. Listing additional servers provides failover protection. This can be detected from the DNS SRV records if it is not given.
ldap_search_base Gives the base DN to use for performing LDAP user operations.
ldap_tls_reqcert Specifies how to check for SSL server certificates in a TLS session. There are four options:
  • never disables requests for certificates.
  • allow requests a certificate, but proceeds normally even if no certificate is given or a bad certificate is given.
  • try requests a certificate and proceeds normally if no certificate is given, If a bad certificate is given, the session terminates.
  • demand and hard are the same option. This requires a valid certificate or the session is terminated.
The default is hard.
ldap_tls_cacert Gives the full path and file name to the file that contains the CA certificates for all of the CAs that SSSD recognizes. SSSD will accept any certificate issued by these CAs.
This uses the OpenLDAP system defaults if it is not given explicitly.
ldap_referrals Sets whether SSSD will use LDAP referrals, meaning forwarding queries from one LDAP database to another. SSSD supports database-level and subtree referrals. For referrals within the same LDAP server, SSSD will adjust the DN of the entry being queried. For referrals that go to different LDAP servers, SSSD does an exact match on the DN. Setting this value to true enables referrals; this is the default.
ldap_schema Sets what version of schema to use when searching for user entries. This can be either rfc2307 or rfc2307bis. The default is rfc2307.
In RFC 2307, group objects use a multi-valued attribute, memberuid, which lists the names of the users that belong to that group. In RFC 2307bis, group objects use the member attribute, which contains the full distinguished name (DN) of a user or group entry. RFC 2307bis allows nested groups usning the member attribute. Because these different schema use different definitions for group membership, using the wrong LDAP schema with SSSD can affect both viewing and managing network resources, even if the appropriate permissions are in place.
For example, with RFC 2307bis, all groups are returned when using nested groups or primary/secondary groups.
$ id
uid=500(myserver) gid=500(myserver) groups=500(myserver),510(myothergroup)
If SSSD is using RFC 2307 schema, only the primary group is returned.
This setting only affects how SSSD determines the group members. It does not change the actual user data.
ldap_search_timeout Sets the time, in seconds, that LDAP searches are allowed to run before they are canceled and cached results are returned. This defaults to five when the enumerate value is false and defaults to 30 when enumerate is true.
When an LDAP search times out, SSSD automatically switches to offline mode.
ldap_network_timeout Sets the time, in seconds, SSSD attempts to poll an LDAP server after a connection attempt fails. The default is six seconds.
ldap_opt_timeout Sets the time, in seconds, to wait before aborting synchronous LDAP operations if no response is received from the server. This option also controls the timeout when communicating with the KDC in case of a SASL bind. The default is five seconds.

10.2.4.2.2. LDAP Domain Example
The LDAP configuration is very flexible, depending on your specific environment and the SSSD behavior. These are some common examples of an LDAP domain, but the SSSD configuration is not limited to these examples.

Note

Along with creating the domain entry, add the new domain to the list of domains for SSSD to query in the sssd.conf file. For example:
domains = LOCAL,LDAP1,AD,PROXYNIS
Example 10.1. A Basic LDAP Domain Configuration
An LDAP domain requires three things:
  • An LDAP server
  • The search base
  • A way to establish a secure connection
The last item depends on the LDAP environment. SSSD requires a secure connection since it handles sensitive information. This connection can be a dedicated TLS/SSL connection or it can use Start TLS.
Using a dedicated TLS/SSL connection simply uses an LDAPS connection to connect to the server and is therefore set as part of the ldap_uri option:
# An LDAP domain
[domain/LDAP]
enumerate = false
cache_credentials = true

id_provider = ldap
auth_provider = ldap

ldap_uri = ldaps://ldap.example.com:636
ldap_search_base = dc=example,dc=com
Using Start TLS requires a way to input the certificate information to establish a secure connection dynamically over an insecure port. This is done using the ldap_id_use_start_tls option to use Start TLS and then ldap_tls_cacert to identify the CA certificate which issued the SSL server certificates.
# An LDAP domain
[domain/LDAP]
enumerate = false
cache_credentials = true

id_provider = ldap
auth_provider = ldap

ldap_uri = ldap://ldap.example.com
ldap_search_base = dc=example,dc=com
ldap_id_use_start_tls = true
ldap_tls_reqcert = demand
ldap_tls_cacert = /etc/pki/tls/certs/ca-bundle.crt

10.2.4.2.3. Active Directory Domain Example
For SSSD to work with an Active Directory domain, both the Active Directory domain and the local system have to be configured specially to communicate with one another.

Note

The Microsoft Active Directory documentation has complete procedures for configuring the Active Directory domain.
  1. Using authconfig, set the Linux client to use Active Directory as its LDAP identity provider. For example:
    authconfig --enableldap --enableldapauth --ldapserver=ldap://ad.example.com:389 --enablekrb5 --krb5realm AD-REALM.EXAMPLE.COM --krb5kdc ad-kdc.example.com:88 --krb5adminserver ad-kdc.example.com:749 --update
    The authconfig command is described in Section 10.1, “Configuring System Authentication”.
  2. Create the Active Directory Domain Services role.
  3. Add the Identity Management for UNIX service to the Active Directory Domain Services role. Use the Unix NIS domain as the domain name in the configuration.
  4. On the Active Directory server, create a new Computer object with the name of the Linux client.
    1. In the Administrative Tools menu, select the Active Directory Users and Computers application.
    2. Expand the Active Directory root object, such as ad.example.com.
    3. Right-click Computers, and select the New and the Computer item.
    4. Enter the name for the Linux client, such as rhel-server, and click OK.
    5. Expand the Computers object.
    6. Right-click the rhel-server object, and select Properties.
    7. In the UNIX Attributes, enter the name of the Linux NIS domain and the IP address of the Linux server.
      Click OK.
  5. From the command prompt on the Active Directory server, create a machine account, password, and UPN for the Linux host principal.
    C:\> setspn -A host/rhel-server.example.com@AD-REALM.EXAMPLE.COM rhel-server
    Registering ServicePrincipalNames for CN=rhel server,CN=Computers,DC=ad,DC=example,DC=com
            host/rhel server.example.com@AD-REALM.EXAMPLE.COM
    Updated object						
    	
    C:\> setspn -L rhel-server 
    Registered ServicePrincipalNames for CN=rhel server,CN=Computers,DC=ad,DC=example,DC=com:
            host/rhel server.example.com@AD-REALM.EXAMPLE.COM
    
    C:\> ktpass /princ host/rhel-server.example.com@AD-REALM.EXAMPLE.COM /out rhel-server.keytab /crypto all /ptype KRB5_NT_PRINCIPAL -desonly /mapuser AD\rhel-server$ +rndPass
    
    Targeting domain controller: 
        ad.example.com
    Using legacy password setting method
    Successfully mapped host/rhel server.redhat.com 
    ... 8< ...
  6. Copy the keytab from the Active Directory server to the Linux client, and save it as /etc/krb5.keytab.
  7. On the Linux system, reset the permissions and owner for the keytab file.
    [root@rhel-server ~]# chown root:root /etc/krb5.keytab 
    	
    [root@rhel-server ~]# chmod 0600 /etc/krb5.keytab
  8. Restore the SELinux file permissions for the keytab.
    [root@rhel-server ~]# restorecon /etc/krb5.keytab
  9. Verify that the host can connect to the Active Directory domain.
    [root@rhel-server ~]# kinit -k -t /etc/krb5.keytab host/rhel-server.example.com@AD-REALM.EXAMPLE.COM
  10. On the Active Directory server, create a a group for the Linux users.
    1. Create a new group named unixusers.
    2. Open the unixusers group and open the Unix Attributes tab.
    3. Configure the Unix settings:
      • The NIS domain
      • The UID
      • The login shell, to /bin/bash
      • The home directory, to /home/aduser
      • The primary group name, to unixusers
  11. Then, configure the SSSD domain on the Linux machine.
    Example 10.2. An Active Directory 2008 Domain
    [root@rhel-server ~]# vim /etc/sssd/sssd.conf
    
    [sssd]
    config_file_version = 2
    domains = ad.example.com
    services = nss, pam
    
    [nss]
    
    [pam]
    
    [domain/ad.example.com]
    cache_credentials = true
    enumerate = false
    
    id_provider = ldap
    auth_provider = krb5
    chpass_provider = krb5
    access_provider = ldap
    
    ldap_sasl_mech = GSSAPI
    ldap_sasl_authid = host/rhel-server.example.com@AD-REALM.EXAMPLE.COM 
    
    
    ldap_schema = rfc2307bis
    
    ldap_user_search_base = ou=user accounts,dc=ad,dc=example,dc=com
    ldap_user_object_class = user
    ldap_user_home_directory = unixHomeDirectory
    ldap_user_principal = userPrincipalName
    ldap_user_name = sAMAccountName
    
    ldap_group_search_base = ou=groups,dc=ad,dc=example,dc=com
    ldap_group_object_class = group
    
    ldap_access_order = expire
    ldap_account_expire_policy = ad
    ldap_force_upper_case_realm = true
    ldap_disable_referrals = true
    
    #krb5_server = server.ad.example.com
    krb5_realm = AD-REALM.EXAMPLE.COM

    These options are described in the man page for LDAP domain configuration, sssd-ldap(5).
  12. Restart SSSD.
    [root@rhel-server ~]# service sssd restart
10.2.4.2.4. Using IP Addresses in Certificate Subject Names
Using an IP address in the ldap_uri option instead of the server name may cause the TLS/SSL connection to fail. TLS/SSL certificates contain the server name, not the IP address. However, the subject alternative name field in the certificate can be used to include the IP address of the server, which allows a successful secure connection using an IP address.
  1. Convert an existing certificate into a certificate request. The signing key (-signkey) is the key of the issuer of whatever CA originally issued the certificate. If this is done by an external CA, it requires a separate PEM file; if the certificate is self-signed, then this is the certificate itself. For example:
    openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey key.pem
    With a self-signed certificate:
    openssl x509 -x509toreq -in old_cert.pem -out req.pem -signkey old_cert.pem
  2. Edit the /etc/pki/tls/openssl.cnf configuration file to include the server's IP address under the [ v3_ca ] section:
    subjectAltName = IP:10.0.0.10
  3. Use the generated certificate request to generate a new self-signed certificate with the specified IP address:
    openssl x509 -req -in req.pem -out new_cert.pem -extfile ./openssl.cnf -extensions v3_ca -signkey old_cert.pem
    The -extensions option sets which extensions to use with the certificate. For this, it should be v3_ca to load the appropriate section.
  4. Copy the private key block from the old_cert.pem file into the new_cert.pem file to keep all relevant information in one file.
When creating a certificate through the certutil utility provided by the nss-utils package, note that certutil supports DNS subject alternative names for certificate creation only.

10.2.4.3. Configuring Kerberos Authentication with a Domain

Both LDAP and proxy identity providers can use a separate Kerberos domain to supply authentication. Configuring a Kerberos authentication provider requires the key distribution center (KDC) and the Kerberos domain. All of the principal names must be available in the specified identity provider; if they are not, SSSD constructs the principals using the format username@REALM.

Note

Kerberos can only provide authentication; it cannot provide an identity database.
SSSD assumes that the Kerberos KDC is also a Kerberos kadmin server. However, production environments commonly have multiple, read-only replicas of the KDC and only a single kadmin server. Use the krb5_kpasswd option to specify where the password changing service is running or if it is running on a non-default port. If the krb5_kpasswd option is not defined, SSSD tries to use the Kerberos KDC to change the password.
The basic Kerberos configuration options are listed in Table 10.6, “Kerberos Authentication Configuration Parameters”. The sssd-krb5(5) man page has more information about Kerberos configuration options.
Example 10.3. Basic Kerberos Authentication
# A domain with identities provided by LDAP and authentication by Kerberos
[domain/KRBDOMAIN]
enumerate = false
id_provider = ldap
chpass_provider = krb5
ldap_uri = ldap://ldap.example.com
ldap_search_base = dc=example,dc=com
ldap-tls_reqcert = demand
ldap_tls_cacert = /etc/pki/tls/certs/ca-bundle.crt

auth_provider = krb5
krb5_server = 192.168.1.1, kerberos.example.com
krb5_realm = EXAMPLE.COM
krb5_kpasswd = kerberos.admin.example.com
krb5_auth_timeout = 15

Table 10.6. Kerberos Authentication Configuration Parameters
Parameter Description
chpass_provider Specifies which service to use for password change operations. This is assumed to be the same as the authentication provider. To use Kerberos, set this to krb5.
krb5_server Gives a comma-separated list of IP addresses or hostnames of Kerberos servers to which SSSD will connect. The list is given in order of preference, so the first server in the list is tried first. Listing additional servers provides failover protection.
When using service discovery for KDC or kpasswd servers, SSSD first searches for DNS entries that specify UDP as the connection protocol, and then falls back to TCP.
krb5_realm Identies the Kerberos realm served by the KDC.
krb5_lifetime Requests a Kerberos ticket with the specified lifetime in seconds (s), minutes (m), hours (h) or days (d).
krb5_renewable_lifetime Requests a renewable Kerberos ticket with a total lifetime that is specified in seconds (s), minutes (m), hours (h) or days (d).
krb5_renew_interval Sets the time, in seconds, for SSSD to check if tickets should be renewed. Tickets are renewed automatically once they exceed half their lifetime. If this option is missing or set to zero, then automatic ticket renewal is disabled.
krb5_store_password_if_offline Sets whether to store user passwords if the Kerberos authentication provider is offline, and then to use that cache to request tickets when the provider is back online. The default is false, which does not store passwords.
krb5_kpasswd Lists alternate Kerberos kadmin servers to use if the change password service is not running on the KDC.
krb5_ccname_template Gives the directory to use to store the user's credential cache. This can be templatized, and the following tokens are supported:
  • %u, the user's login name
  • %U, the user's login UID
  • %p, the user's principal name
  • %r, the realm name
  • %h, the user's home directory
  • %d, the value of the krb5ccache_dir parameter
  • %P, the process ID of the SSSD client.
  • %%, a literal percent sign (%)
  • XXXXXX, a string at the end of the template which instructs SSSD to create a unique filename safely
For example:
krb5_ccname_template = FILE:%d/krb5cc_%U_XXXXXX
krb5_ccachedir Specifies the directory to store credential caches. This can be templatized, using the same tokens as krb5_ccname_template, except for %d and %P. If %u, %U, %p, or %h are used, then SSSD creates a private directory for each user; otherwise, it creates a public directory.
krb5_auth_timeout Gives the time, in seconds, before an online authentication or change password request is aborted. If possible, the authentication request is continued offline. The default is 15 seconds.

10.2.4.4. Configuring a Proxy Domain

A proxy with SSSD is just a relay, an intermediary configuration. SSSD connects to its proxy service, and then that proxy loads the specified libraries. This allows SSSD to use some resources that it otherwise would not be able to use. For example, SSSD only supports LDAP and Kerberos as authentication providers, but using a proxy allows SSSD to use alternative authentication methods like a fingerprint scanner or smart card.
Table 10.7. Proxy Domain Configuration Parameters
Parameter Description
proxy_pam_target Specifies the target to which PAM must proxy as an authentication provider. The PAM target is a file containing PAM stack information in the default PAM directory, /etc/pam.d/.
This is used to proxy an authentication provider.

Important

Ensure that the proxy PAM stack does not recursively include pam_sss.so.
proxy_lib_name Specifies which existing NSS library to proxy identity requests through.
This is used to proxy an identity provider.

Example 10.4. Proxy Identity and Kerberos Authentication
The proxy library is loaded using the proxy_lib_name parameter. This library can be anything as long as it is compatible with the given authentication service. For a Kerberos authentication provider, it must be a Kerberos-compatible library, like NIS.
[domain/PROXY_KRB5]
auth_provider = krb5
krb5_server = 192.168.1.1
krb5_realm = EXAMPLE.COM

id_provider = proxy
proxy_lib_name = nis
enumerate = true
cache_credentials = true

Example 10.5. LDAP Identity and Proxy Authentication
The proxy library is loaded using the proxy_pam_target parameter. This library must be a PAM module that is compatible with the given identity provider. For example, this uses a PAM fingerprint module with LDAP:
[domain/LDAP_PROXY]
id_provider = ldap
ldap_uri = ldap://example.com
ldap_search_base = dc=example,dc=com

auth_provider = proxy
proxy_pam_target = sssdpamproxy
enumerate = true
cache_credentials = true
After the SSSD domain is configured, make sure that the specified PAM files are configured. In this example, the target is sssdpamproxy, so create a /etc/pam.d/sssdpamproxy file and load the PAM/LDAP modules:
auth          required      pam_frprint.so
account       required      pam_frprint.so
password      required      pam_frprint.so
session       required      pam_frprint.so

Example 10.6. Proxy Identity and Authentication
SSSD can have a domain with both identity and authentication proxies. The only configuration given then are the proxy settings, proxy_pam_target for the authentication PAM module and proxy_lib_name for the service, like NIS or LDAP.
This example illustrates a possible configuration, but this is not a realistic configuration. If LDAP is used for identity and authentication, then both the identity and authentication providers should be set to the LDAP configuration, not a proxy.
[domain/PROXY_PROXY]
auth_provider = proxy
id_provider = proxy
proxy_lib_name = ldap
proxy_pam_target = sssdproxyldap
enumerate = true 
cache_credentials = true
Once the SSSD domain is added, then update the system settings to configure the proxy service:
  1. Create a /etc/pam.d/sssdproxyldap file which requires the pam_ldap.so module:
    auth          required      pam_ldap.so
    account       required      pam_ldap.so
    password      required      pam_ldap.so
    session       required      pam_ldap.so
  2. Make sure the nss-pam-ldap package is installed.
    [root@server ~]# yum install nss-pam-ldap
  3. Edit the /etc/nslcd.conf file, the configuration file for the LDAP name service daemon, to contain the information for the LDAP directory:
    uid nslcd
    gid ldap
    uri ldaps://ldap.example.com:636
    base dc=example,dc=com
    ssl on
    tls_cacertdir /etc/openldap/cacerts

10.2.5. Configuring Access Control for SSSD Domains

SSSD provides a rudimentary access control for domain configuration, allowing either simple user allow/deny lists or using the LDAP backend itself.

10.2.5.1. Using the Simple Access Provider

The Simple Access Provider allows or denies access based on a list of usernames or groups.
The Simple Access Provider is a way to restrict access to certain, specific machines. For example, if a company uses laptops, the Simple Access Provider can be used to restrict access to only a specific user or a specific group, even if a different user authenticated successfully against the same authentication provider.
The most common options are simple_allow_users and simple_allow_groups, which grant access explicitly to specific users (either the given users or group members) and deny access to everyone else. It is also possible to create deny lists (which deny access only to explicit people and implicitly allow everyone else access).
The Simple Access Provider adheres to the following four rules to determine which users should or should not be granted access:
  • If both the allow and deny lists are empty, access is granted.
  • If any list is provided, allow rules are evaluated first, and then deny rules. Practically, this means that deny rules supersede allow rules.
  • If an allowed list is provided, then all users are denied access unless they are in the list.
  • If only deny lists are provided, then all users are allowed access unless they are in the list.
This example grants access to two users and anyone who belongs to the IT group; implicitly, all other users are denied:
[domain/example.com]
access_provider = simple
simple_allow_users = jsmith,bjensen
simple_allow_groups = itgroup

Note

The LOCAL domain in SSSD does not support simple as an access provider.
Other options are listed in the sssd-simple man page, but these are rarely used.

10.2.5.2. Using the LDAP Access Filter

The LDAP server itself can provide the access control rules. The associated filter option (ldap_access_filter) specifies which users are granted access to the specified host. The user filter must be used or all users are denied access.
For example:
[domain/example.com]
access_provider = ldap
ldap_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com

Note

Offline caching for LDAP access providers is limited to determining whether the user's last online login attempt was successful. Users that were granted access during their last login will continue to be granted access while offline.
SSSD can also check results by the account expiration policy and the authorizedService attribute.

10.2.6. Configuring Domain Failover

SSSD attempts to connect to machines and to services separately.
When SSSD tries to connect to one of its domain backends, it first tries to resolve the hostname of a given machine. If this resolution attempt fails, the machine is considered offline, and SSSD no longer attempts to connect to this machine for any other service.
If the resolution attempt succeeds, the backend tries to connect to a service on this machine. If the service connection attempt fails, then only this particular service is considered offline and the backend automatically switches over to the next service. The machine is still considered online and might still be tried for another service.
SSSD only tries the first IP address given in the DNS A record. To find multiple servers with a single request, SSSD relies on SRV records.
Connections are retried to offline machines or services every 30 seconds, until SSSD can successfully connect to the backend.

10.2.6.1. Configuring Failover

Configuring failover allows SSSD to switch automatically to a different server if the primary server fails. These servers are entered as a case-insensitive, comma-separated list in the [domain/Name] sections of the /etc/sssd/sssd.conf file. The servers are listed in order of preference. This list can contain any number of servers.
For example, for a native LDAP domain:
ldap_uri = ldap://ldap0.example.com, ldap://ldap1.example.com, ldap://ldap2.example.com
The first entry, ldap://ldap0.example.com, is the primary server. If this server fails, SSSD first attempts to connect to ldap1.example.com and then ldap2.example.com.
If the server parameter is not specified, then SSSD uses service discovery to try to find another server on the network.

Important

The failover servers must be entered as a comma-separated list of values for a single key. If there are multiple keys, SSSD only recognizes the last entry.

10.2.6.2. Using SRV Records with Failover

SSSD supports SRV records in its failover configuration. The SSSD configuration can specify a server that is later resolved into a list of specific servers using SRV requests.
For every service with which to use service discovery, add a special DNS record to the DNS server:
_service._protocol._domain TTL priority weight port hostname
The priority and weight attributes of SRV records provide fine-grained control over which servers to contact first if the primary server fails.
A typical configuration contains multiple such records, each with a different priority for failover and different weights for load balancing.
For more information on SRV records, see RFC 2782.

10.2.7. Managing the SSSD Cache

SSSD can define multiple domains of the same type and different types of domain. SSSD maintains a separate database file for each domain, meaning each domain has its own cache. These cache files are stored in the /var/lib/sss/db/ directory.

10.2.7.1. Purging the SSSD Cache

As LDAP updates are made to the identity provider for the domains, it can be necessary to clear the cache to reload the new information quickly.
The cache purge utility, sss_cache, invalidates records in the SSSD cache for a user, a domain, or a group. Invalidating the current records forces the cache to retrieve the updated records from the identity provider, so changes can be realized quickly.
Most commonly, this is used to clear the cache and update the records for an entire domain:
Example 10.7. Purging Domain Records
[root@server ~]# sss_cache -d LDAP1

If the administrator knows that a specific record (user, group, or netgroup) has been updated, then sss_cache can purge the records for that specific account, and leave the rest of the cache intact.
Example 10.8. Purging a User Record
[root@server ~]# sss_cache -u jsmith

Table 10.8. sss_cache Options
Short Argument Long Argument Description
-d name --domain name Invalidates cache entries for users, groups, and other entries only within the specified domain.
-G --groups Invalidates all group records. If -g is also used, -G takes precedence and -g is ignored.
-g name --group name Invalidates the cache entry for the specified group.
-N --netgroups Invalidates cache entries for all netgroup cache records. If -n is also used, -N takes precedence and -n is ignored.
-n name --netgroup name Invalidates the cache entry for the specified netgroup.
-U --users Invalidates cache entries for all user records. If the -u option is also used, -U takes precedence and -u is ignored.
-u name --user name Invalidates the cache entry for the specified user.

10.2.7.2. Deleting Domain Cache Files

All cache files are named for the domain. For example, for a domain named exampleldap, the cache file is named cache_exampleldap.ldb.
Be careful when you delete a cache file. This operation has significant effects:
  • Deleting the cache file deletes all user data, both identification and cached credentials. Consequently, do not delete a cache file unless the system is online and can authenticate with a username against the domain's servers. Without a credentials cache, offline authentication will fail.
  • If the configuration is changed to reference a different identity provider, SSSD will recognize users from both providers until the cached entries from the original provider time out.
    It is possible to avoid this by purging the cache, but the better option is to use a different domain name for the new provider. When SSSD is restarted, it creates a new cache file with the new name and the old file is ignored.

10.2.8. Configuring OpenSSH to Check SSSD for Cached Keys (TECH PREVIEW)

OpenSSH creates secure, encrypted connections between two systems. One machine authenticates to another machine to allow access; the authentication can be of the machine itself for server connections or of a user on that machine. OpenSSH is described in more detail in Chapter 11, OpenSSH.
This authentication is performed through public-private key pairs that identify the authenticating user or machine. The remote machine or user attempting to access the machine presents a key pair. The local machine then elects whether to trust that remote entity; if it is trusted, the public key for that remote machine is stored in the known_hosts file or for the remote user in authorized_keys. Whenever that remote machine or user attempts to authenticate again, the local system simply checks the known_hosts or authorized_keys file first to see if that remote entity is recognized and trusted. If it is, then access is granted.
The first problem comes in verifying those identities reliably.
The known_hosts file is a triplet of the machine name, its IP address, and its public key:
server.example.com,255.255.255.255 ssh-rsa AbcdEfg1234ZYX098776/AbcdEfg1234ZYX098776/AbcdEfg1234ZYX098776=
The known_hosts file can quickly become outdated for a number of different reasons: systems using DHCP cycle through IP addresses, new keys can be re-issued periodically, or virtual machines or services can be brought online and removed. This changes the hostname, IP address, and key triplet.
Administrators have to clean and maintain a current known_hosts file to maintain security. (Or system users get in the habit of simply accepting any machine and key presented, which negates the security benefits of key-based security.)
Additionally, problem for both machines and users is distributing keys in a scalable way. Machines can send their keys are part of establishing an encrypted session, but users have to supply their keys in advance. Simply propagating and then updating keys consistently is a difficult administrative task.
Lastly, SSH key and machine information are only maintained locally. There may be machines or users on the network which are recognized and trusted by some systems and not by others because the known_hosts file has not been updated uniformly.
The goal of SSSD is to server as a credentials cache. This includes working as a credentials cache for SSH public keys for machines and users. OpenSSH is configured to reference SSSD to check for cached keys; SSSD uses Red Hat Linux's Identity Management (IPA) domain as an identity, and IPA actually stores the public keys and host information.

NOTE

Only Linux machines enrolled, or joined, in the IPA domain can use SSSD as a key cache for OpenSSH. Other Unix machines and Windows machines must use the regular authentication mechanisms with the known_hosts file.

10.2.8.1. Configuring OpenSSH to Use SSSD for Host Keys

OpenSSH is configured in either a user-specific configuration file (~/.ssh/config) or a system-wide configuration file (/etc/ssh/ssh_config). The user file has precedence over the system settings and the first obtained value for a paramter is used. The formatting and conventions for this file are covered in Chapter 11, OpenSSH.
In order to manage host keys, SSSD has a tool, sss_ssh_knownhostsproxy, which performs three operations:
  1. Retrieves the public host key from the enrolled Linux system.
  2. Stores the host key in a custom hosts file, .ssh/sss_known_hosts.
  3. Establishes a connection with the host machine, either a socket (the default) or a secure connection.
This tool has the format:
sss_ssh_knownhostsproxy [-d sssd_domain] [-p ssh_port] HOST [PROXY_COMMAND]
Table 10.9. sss_ssh_knownhostsproxy Options
Short Argument Long Argument Description
HOSTNAME Gives the hostname of the host to check and connect to. In the OpenSSH configuration file, this can be a token, %h.
PROXY_COMMAND Passes a proxy command to use to connect to the SSH client. This is similar to running ssh -o ProxyCommand=value. This option is used when running sss_ssh_knownhostsproxy from the command line or through another script, but is not necessary in the OpenSSH configuration file.
-d sssd_domain --domain sssd_domain Only searches for public keys in entries in the specified domain. If not given, SSSD searches for keys in all configured domains.
-p port --port port Uses this port to connect to the SSH client. By default, this is port 22.

To use this SSSD tool, add or edit two parameters to the ssh_config or ~/.ssh/config file:
  • Specify the command to use to connect to the SSH client (ProxyCommand). This is the sss_ssh_knownhostsproxy, with the desired arguments and hostname.
  • Specify the location of the SSSD hosts file, rather than the default known_hosts file (UserKnownHostsFile). The SSSD hosts file is .ssh/sss_known_hosts.
For example, this looks for public keys in the IPA1 SSSD domain and connects over whatever port and host are supplied:
ProxyCommand /usr/bin/sss_ssh_knownhostsproxy -p %p -d IPA1 %h
UserKnownHostsFile2 .ssh/sss_known_hosts

10.2.8.2. Configuring OpenSSH to Use SSSD for User Keys

User keys are stored on a local system in the authorized_keys file for OpenSSH. As with hosts, SSSD can maintain and automatically update a separate cache of user public keys for OpenSSH to refer to. This is kept in the .ssh/sss_authorized_keys file.
OpenSSH is configured in either a user-specific configuration file (~/.ssh/config) or a system-wide configuration file (/etc/ssh/ssh_config). The user file has precedence over the system settings and the first obtained value for a paramter is used. The formatting and conventions for this file are covered in Chapter 11, OpenSSH.
In order to manage user keys, SSSD has a tool, sss_ssh_authorizedkeys, which performs two operations:
  1. Retrieves the user's public key from the user entries in the Identity Management (IPA) domain.
  2. Stores the user key in a custom file, .ssh/sss_authorized_keys, in the standard authorized keys format.
This tool has the format:
sss_ssh_authorizedkeys [-d sssd_domain] USER
Table 10.10. sss_ssh_authorizedkeys Options
Short Argument Long Argument Description
USER Gives the username or account name for which to obtain the public key. In the OpenSSH configuration file, this can be represented by a token, %u.
-d sssd_domain --domain sssd_domain Only searches for public keys in entries in the specified domain. If not given, SSSD searches for keys in all configured domains.

There are two possible options for how to configure OpenSSH to use SSSD for user keys, depending on the SSH deployment:
  • Most commonly, SSH supports the authorized key command. In that case, it is necessary only to specify the command to run to retrieve user keys. For example:
    AuthorizedKeysCommand /usr/bin/sss_ssh_authorizedkeys
  • SSH can also support a public key agent. In that case, give the command to use to retrieve agent keys, including tokens for required arguments (such as the username):
    PubKeyAgent /usr/bin/sss_ssh_authorizedkeys %u

10.2.9. Using NSCD with SSSD

SSSD is not designed to be used with the NSCD daemon. Even though SSSD does not directly conflict with NSCD, using both services can result in unexpected behavior, especially with how long entries are cached.
The most common evidence of a problem is conflicts with NFS. When using Network Manager to manage network connections, it may take several minutes for the network interface to come up. During this time, various services attempt to start. If these services start before the network is up and the DNS servers are available, these services fail to identify the forward or reverse DNS entries they need. These services will read an incorrect or possibly empty resolv.conf file. This file is typically only read once, and so any changes made to this file are not automatically applied. This can cause NFS locking to fail on the machine where the NSCD service is running, unless that service is manually restarted.
To avoid this problem, enable caching for hosts and services in the /etc/nscd.conf file and rely on the SSSD cache for the passwd, group, and netgroup entries.
Change the /etc/nscd.conf file:
enable-cache hosts yes
enable-cache passwd no
enable-cache group no
enable-cache netgroup no
With NSCD answering hosts requests, these entries will be cached by NSCD and returned by NSCD during the boot process. All other entries are handled by SSSD.

10.2.10. Troubleshooting SSSD

10.2.10.1. Setting Debug Logs for SSSD Domains

Each domain sets its own debug log level. Increasing the log level can provide more information about problems with SSSD or with the domain configuration.
To change the log level, set the debug_level parameter for each section in the sssd.conf file for which to produce extra logs. For example:
[domain/LDAP]
enumerate = false
cache_credentials = true
debug_level = 9
Table 10.11. Debug Log Levels
Level Description
0 Fatal failures. Anything that would prevent SSSD from starting up or causes it to cease running.
1 Critical failures. An error that doesn't kill the SSSD, but one that indicates that at least one major feature is not going to work properly.
2 Serious failures. An error announcing that a particular request or operation has failed.
3 Minor failures. These are the errors that would percolate down to cause the operation failure of 2.
4 Configuration settings.
5 Function data.
6 Trace messages for operation functions.
7 Trace messages for internal control functions.
8 Contents of function-internal variables that may be interesting.
9 Extremely low-level tracing information.

NOTE

In versions of SSSD older than 1.8, debug log levels could be set globally in the [sssd] section. Now, each domain and service must configure its own debug log level.
To copy the global SSSD debug log levels into each configuration area in the SSSD configuration file, use the sssd_update_debug_levels.py script.
python /usr/lib/python2.6/site-packages/sssd_update_debug_levels.py

10.2.10.2. Checking SSSD Log Files

SSSD uses a number of log files to report information about its operation, located in the /var/log/sssd/ directory. SSSD produces a log file for each domain, as well as an sssd_pam.log and an sssd_nss.log file.
Additionally, the /var/log/secure file logs authentication failures and the reason for the failure.

10.2.10.3. Problems with SSSD Configuration

SSSD fails to start
SSSD requires that the configuration file be properly set up, with all the required entries, before the daemon will start.
  • SSSD requires at least one properly configured domain before the service will start. Without a domain, attempting to start SSSD returns an error that no domains are configured:
    # sssd -d4
    
    [sssd] [ldb] (3): server_sort:Unable to register control with rootdse!
    [sssd] [confdb_get_domains] (0): No domains configured, fatal error!
    [sssd] [get_monitor_config] (0): No domains configured.
    
    Edit the /etc/sssd/sssd.conf file and create at least one domain.
  • SSSD also requires at least one available service provider before it will start. If the problem is with the service provider configuration, the error message indicates that there are no services configured:
    [sssd] [get_monitor_config] (0): No services configured!
    
    Edit the /etc/sssd/sssd.conf file and configure at least one service provider.

    Important

    SSSD requires that service providers be configured as a comma-separated list in a single services entry in the /etc/sssd/sssd.conf file. If services are listed in multiple entries, only the last entry is recognized by SSSD.
I don't see any groups with 'id' or group members with 'getent group'.
This may be due to an incorrect ldap_schema setting in the [domain/DOMAINNAME] section of sssd.conf.
SSSD supports RFC 2307 and RFC 2307bis schema types. By default, SSSD uses the more common RFC 2307 schema.
The difference between RFC 2307 and RFC 2307bis is the way which group membership is stored in the LDAP server. In an RFC 2307 server, group members are stored as the multi-valued memberuid attribute, which contains the name of the users that are members. In an RFC2307bis server, group members are stored as the multi-valued member or uniqueMember attribute which contains the DN of the user or group that is a member of this group. RFC2307bis allows nested groups to be maintained as well.
If group lookups are not returning any information:
  1. Set ldap_schema to rfc2307bis.
  2. Delete /var/lib/sss/db/cache_DOMAINNAME.ldb.
  3. Restarting SSSD.
If that doesn't work, add this line to sssd.conf:
ldap_group_name = uniqueMember
Then delete the cache and restart SSSD again.
Authentication fails against LDAP.
To perform authentication, SSSD requires that the communication channel be encrypted. This means that if sssd.conf is configured to connect over a standard protocol (ldap://), it attempts to encrypt the communication channel with Start TLS. If sssd.conf is configured to connect over a secure protocol (ldaps://), then SSSD uses SSL.
This means that the LDAP server must be configured to run in SSL or TLS. TLS must be enabled for the standard LDAP port (389) or SSL enabled on the secure LDAPS port (636). With either SSL or TLS, the LDAP server must also be configured with a valid certificate trust.
An invalid certificate trust is one of the most common issues with authenticating against LDAP. If the client does not have proper trust of the LDAP server certificate, it is unable to validate the connection, and SSSD refuses to send the password. The LDAP protocol requires that the password be sent in plaintext to the LDAP server. Sending the password in plaintext over an unencrypted connection is a security problem.
If the certificate is not trusted, a syslog message is written, indicating that TLS encryption could not be started. The certificate configuration can be tested by checking if the LDAP server is accessible apart from SSSD. For example, this tests an anonymous bind over a TLS connection to test.example.com:
$ ldapsearch -x -ZZ -h test.example.com -b dc=example,dc=com
If the certificate trust is not properly configured, the test fails with this error:
ldap_start_tls: Connect error (-11) additional info: TLS error -8179:Unknown code ___f 13
To trust the certificate:
  1. Obtain a copy of the public CA certificate for the certificate authority used to sign the LDAP server certificate and save it to the local system.
  2. Add a line to the sssd.conf file that points to the CA certificate on the filesystem.
    ldap_tls_cacert = /path/to/cacert
  3. If the LDAP server uses a self-signed certificate, remove the ldap_tls_reqcert line from the sssd.conf file.
    This parameter directs SSSD to trust any certificate issued by the CA certificate, which is a security risk with a self-signed CA certificate.
Connecting to LDAP servers on non-standard ports fail.
When running SELinux in enforcing mode, the client's SELinux policy has to be modified to connect to the LDAP server over the non-standard port. For example:
# semanage port -a -t ldap_port_t -p tcp 1389
NSS fails to return user information
This usually means that SSSD cannot connect to the NSS service.
  • Ensure that NSS is running:
    # service sssd status
  • If NSS is running, make sure that the provider is properly configured in the [nss] section of the /etc/sssd/sssd.conf file. Especially check the filter_users and filter_groups attributes.
  • Make sure that NSS is included in the list of services that SSSD uses.
  • Check the configuration in the /etc/nsswitch.conf file.
NSS returns incorrect user information
If searches are returning the incorrect user information, check that there are not conflicting usernames in separate domains. When there are multiple domains, set the use_fully_qualified_domains attribute to true in the /etc/sssd/sssd.conf file. This differentiates between different users in different domains with the same name.
Setting the password for the local SSSD user prompts twice for the password
When attempting to change a local SSSD user's password, it may prompt for the password twice:
[root@clientF11 tmp]# passwd user1000
Changing password for user user1000.
New password:
Retype new password:
New Password:
Reenter new Password:
passwd: all authentication tokens updated successfully.
This is the result of an incorrect PAM configuration. Ensure that the use_authtok option is correctly configured in your /etc/pam.d/system-auth file.

Chapter 11. OpenSSH

SSH (Secure Shell) is a protocol which facilitates secure communications between two systems using a client/server architecture and allows users to log into server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the login session, rendering the connection difficult for intruders to collect unencrypted passwords.
The ssh program is designed to replace older, less secure terminal applications used to log into remote hosts, such as telnet or rsh. A related program called scp replaces older programs designed to copy files between hosts, such as rcp. Because these older applications do not encrypt passwords transmitted between the client and the server, avoid them whenever possible. Using secure methods to log into remote systems decreases the risks for both the client system and the remote host.
Fedora includes the general OpenSSH package (openssh) as well as the OpenSSH server (openssh-server) and client (openssh-clients) packages. Note that the OpenSSH packages require the OpenSSL package (openssl), which installs several important cryptographic libraries, enabling OpenSSH to provide encrypted communications.

11.1. The SSH Protocol

11.1.1. Why Use SSH?

Potential intruders have a variety of tools at their disposal enabling them to disrupt, intercept, and re-route network traffic in an effort to gain access to a system. In general terms, these threats can be categorized as follows:
Interception of communication between two systems
The attacker can be somewhere on the network between the communicating parties, copying any information passed between them. He may intercept and keep the information, or alter the information and send it on to the intended recipient.
This attack is usually performed using a packet sniffer, a rather common network utility that captures each packet flowing through the network, and analyzes its content.
Impersonation of a particular host
Attacker's system is configured to pose as the intended recipient of a transmission. If this strategy works, the user's system remains unaware that it is communicating with the wrong host.
This attack can be performed using a technique known as DNS poisoning, or via so-called IP spoofing. In the first case, the intruder uses a cracked DNS server to point client systems to a maliciously duplicated host. In the second case, the intruder sends falsified network packets that appear to be from a trusted host.
Both techniques intercept potentially sensitive information and, if the interception is made for hostile reasons, the results can be disastrous. If SSH is used for remote shell login and file copying, these security threats can be greatly diminished. This is because the SSH client and server use digital signatures to verify their identity. Additionally, all communication between the client and server systems is encrypted. Attempts to spoof the identity of either side of a communication does not work, since each packet is encrypted using a key known only by the local and remote systems.

11.1.2. Main Features

The SSH protocol provides the following safeguards:
No one can pose as the intended server
After an initial connection, the client can verify that it is connecting to the same server it had connected to previously.
No one can capture the authentication information
The client transmits its authentication information to the server using strong, 128-bit encryption.
No one can intercept the communication
All data sent and received during a session is transferred using 128-bit encryption, making intercepted transmissions extremely difficult to decrypt and read.
Additionally, it also offers the following options:
It provides secure means to use graphical applications over a network
Using a technique called X11 forwarding, the client can forward X11 (X Window System) applications from the server.
It provides a way to secure otherwise insecure protocols
The SSH protocol encrypts everything it sends and receives. Using a technique called port forwarding, an SSH server can become a conduit to securing otherwise insecure protocols, like POP, and increasing overall system and data security.
It can be used to create a secure channel
The OpenSSH server and client can be configured to create a tunnel similar to a virtual private network for traffic between server and client machines.
It supports the Kerberos authentication
OpenSSH servers and clients can be configured to authenticate using the GSSAPI (Generic Security Services Application Program Interface) implementation of the Kerberos network authentication protocol.

11.1.3. Protocol Versions

Two varieties of SSH currently exist: version 1, and newer version 2. The OpenSSH suite under Fedora uses SSH version 2, which has an enhanced key exchange algorithm not vulnerable to the known exploit in version 1. However, for compatibility reasons, the OpenSSH suite does support version 1 connections as well.

Avoid using SSH version 1

To ensure maximum security for your connection, it is recommended that only SSH version 2-compatible servers and clients are used whenever possible.

11.1.4. Event Sequence of an SSH Connection

The following series of events help protect the integrity of SSH communication between two hosts.
  1. A cryptographic handshake is made so that the client can verify that it is communicating with the correct server.
  2. The transport layer of the connection between the client and remote host is encrypted using a symmetric cipher.
  3. The client authenticates itself to the server.
  4. The remote client interacts with the remote host over the encrypted connection.

11.1.4.1. Transport Layer

The primary role of the transport layer is to facilitate safe and secure communication between the two hosts at the time of authentication and during subsequent communication. The transport layer accomplishes this by handling the encryption and decryption of data, and by providing integrity protection of data packets as they are sent and received. The transport layer also provides compression, speeding the transfer of information.
Once an SSH client contacts a server, key information is exchanged so that the two systems can correctly construct the transport layer. The following steps occur during this exchange:
  • Keys are exchanged
  • The public key encryption algorithm is determined
  • The symmetric encryption algorithm is determined
  • The message authentication algorithm is determined
  • The hash algorithm is determined
During the key exchange, the server identifies itself to the client with a unique host key. If the client has never communicated with this particular server before, the server's host key is unknown to the client and it does not connect. OpenSSH gets around this problem by accepting the server's host key. This is done after the user is notified and has both accepted and verified the new host key. In subsequent connections, the server's host key is checked against the saved version on the client, providing confidence that the client is indeed communicating with the intended server. If, in the future, the host key no longer matches, the user must remove the client's saved version before a connection can occur.

Always verify the integrity of a new SSH server

It is possible for an attacker to masquerade as an SSH server during the initial contact since the local system does not know the difference between the intended server and a false one set up by an attacker. To help prevent this, verify the integrity of a new SSH server by contacting the server administrator before connecting for the first time or in the event of a host key mismatch.
SSH is designed to work with almost any kind of public key algorithm or encoding format. After an initial key exchange creates a hash value used for exchanges and a shared secret value, the two systems immediately begin calculating new keys and algorithms to protect authentication and future data sent over the connection.
After a certain amount of data has been transmitted using a given key and algorithm (the exact amount depends on the SSH implementation), another key exchange occurs, generating another set of hash values and a new shared secret value. Even if an attacker is able to determine the hash and shared secret value, this information is only useful for a limited period of time.

11.1.4.2. Authentication

Once the transport layer has constructed a secure tunnel to pass information between the two systems, the server tells the client the different authentication methods supported, such as using a private key-encoded signature or typing a password. The client then tries to authenticate itself to the server using one of these supported methods.
SSH servers and clients can be configured to allow different types of authentication, which gives each side the optimal amount of control. The server can decide which encryption methods it supports based on its security model, and the client can choose the order of authentication methods to attempt from the available options.

11.1.4.3. Channels

After a successful authentication over the SSH transport layer, multiple channels are opened via a technique called multiplexing[2]. Each of these channels handles communication for different terminal sessions and for forwarded X11 sessions.
Both clients and servers can create a new channel. Each channel is then assigned a different number on each end of the connection. When the client attempts to open a new channel, the clients sends the channel number along with the request. This information is stored by the server and is used to direct communication to that channel. This is done so that different types of sessions do not affect one another and so that when a given session ends, its channel can be closed without disrupting the primary SSH connection.
Channels also support flow-control, which allows them to send and receive data in an orderly fashion. In this way, data is not sent over the channel until the client receives a message that the channel is open.
The client and server negotiate the characteristics of each channel automatically, depending on the type of service the client requests and the way the user is connected to the network. This allows great flexibility in handling different types of remote connections without having to change the basic infrastructure of the protocol.

11.2. An OpenSSH Configuration

In order to perform tasks described in this section, you must have superuser privileges. To obtain them, log in as root by typing:
su -

11.2.1. Configuration Files

There are two different sets of configuration files: those for client programs (that is, ssh, scp, and sftp), and those for the server (the sshd daemon).
System-wide SSH configuration information is stored in the /etc/ssh/ directory. See Table 11.1, “System-wide configuration files” for a description of its content.
Table 11.1. System-wide configuration files
Configuration File Description
/etc/ssh/moduli Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange which is critical for constructing a secure transport layer. When keys are exchanged at the beginning of an SSH session, a shared, secret value is created which cannot be determined by either party alone. This value is then used to provide host authentication.
/etc/ssh/ssh_config The default SSH client configuration file. Note that it is overridden by ~/.ssh/config if it exists.
/etc/ssh/sshd_config The configuration file for the sshd daemon.
/etc/ssh/ssh_host_dsa_key The DSA private key used by the sshd daemon.
/etc/ssh/ssh_host_dsa_key.pub The DSA public key used by the sshd daemon.
/etc/ssh/ssh_host_key The RSA private key used by the sshd daemon for version 1 of the SSH protocol.
/etc/ssh/ssh_host_key.pub The RSA public key used by the sshd daemon for version 1 of the SSH protocol.
/etc/ssh/ssh_host_rsa_key The RSA private key used by the sshd daemon for version 2 of the SSH protocol.
/etc/ssh/ssh_host_rsa_key.pub The RSA public key used by the sshd for version 2 of the SSH protocol.

User-specific SSH configuration information is stored in the user's home directory within the ~/.ssh/ directory. See Table 11.2, “User-specific configuration files” for a description of its content.
Table 11.2. User-specific configuration files
Configuration File Description
~/.ssh/authorized_keys Holds a list of authorized public keys for servers. When the client connects to a server, the server authenticates the client by checking its signed public key stored within this file.
~/.ssh/id_dsa Contains the DSA private key of the user.
~/.ssh/id_dsa.pub The DSA public key of the user.
~/.ssh/id_rsa The RSA private key used by ssh for version 2 of the SSH protocol.
~/.ssh/id_rsa.pub The RSA public key used by ssh for version 2 of the SSH protocol
~/.ssh/identity The RSA private key used by ssh for version 1 of the SSH protocol.
~/.ssh/identity.pub The RSA public key used by ssh for version 1 of the SSH protocol.
~/.ssh/known_hosts Contains DSA host keys of SSH servers accessed by the user. This file is very important for ensuring that the SSH client is connecting the correct SSH server.

Refer to the ssh_config and sshd_config man pages for information concerning the various directives available in the SSH configuration files.

11.2.2. Starting an OpenSSH Server

Make sure you have relevant packages installed

To run an OpenSSH server, you must have the openssh-server and openssh packages installed. Refer to Section 5.2.4, “Installing Packages” for more information on how to install new packages in Fedora.
To start the sshd daemon, type the following at a shell prompt:
systemctl start sshd.service
To stop the running sshd daemon, use the following command:
systemctl stop sshd.service
If you want the daemon to start automatically at the boot time, type:
systemctl enable sshd.service
Refer to Chapter 9, Services and Daemons for more information on how to configure services in Fedora.
Note that if you reinstall the system, a new set of identification keys will be created. As a result, clients who had connected to the system with any of the OpenSSH tools before the reinstall will see the following message:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
To prevent this, you can back up the relevant files from the /etc/ssh/ directory (see Table 11.1, “System-wide configuration files” for a complete list), and restore them whenever you reinstall the system.

11.2.3. Requiring SSH for Remote Connections

For SSH to be truly effective, using insecure connection protocols should be prohibited. Otherwise, a user's password may be protected using SSH for one session, only to be captured later while logging in using Telnet. Some services to disable include telnet, rsh, rlogin, and vsftpd.
To make sure these services are not running, type the following commands at a shell prompt:
systemctl stop telnet.service
systemctl stop rsh.service
systemctl stop rlogin.service
systemctl stop vsftpd.service
To disable running these services at startup, type:
systemctl disable telnet.service
systemctl disable rsh.service
systemctl disable rlogin.service
systemctl disable vsftpd.service
Refer to Chapter 9, Services and Daemons for more information on how to configure services in Fedora.

11.2.4. Using a Key-Based Authentication

To improve the system security even further, you can enforce the key-based authentication by disabling the standard password authentication. To do so, open the /etc/ssh/sshd_config configuration file in a text editor, and change the PasswordAuthentication option as follows:
PasswordAuthentication no
To be able to use ssh, scp, or sftp to connect to the server from a client machine, generate an authorization key pair by following the steps below. Note that keys must be generated for each user separately.
Fedora 18 uses SSH Protocol 2 and RSA keys by default (see Section 11.1.3, “Protocol Versions” for more information).

Do not generate key pairs as root

If you complete the steps as root, only root will be able to use the keys.

Backup your ~/.ssh/ directory

If you reinstall your system and want to keep previously generated key pair, backup the ~/.ssh/ directory. After reinstalling, copy it back to your home directory. This process can be done for all users on your system, including root.

11.2.4.1. Generating Key Pairs

To generate an RSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate an RSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t rsa
    Generating public/private rsa key pair.
    Enter file in which to save the key (/home/john/.ssh/id_rsa):
  2. Press Enter to confirm the default location (that is, ~/.ssh/id_rsa) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/id_rsa.
    Your public key has been saved in /home/john/.ssh/id_rsa.pub.
    The key fingerprint is:
    e7:97:c7:e2:0e:f9:0e:fc:c4:d7:cb:e5:31:11:92:14 john@penguin.example.com
    The key's randomart image is:
    +--[ RSA 2048]----+
    |             E.  |
    |            . .  |
    |             o . |
    |              . .|
    |        S .    . |
    |         + o o ..|
    |          * * +oo|
    |           O +..=|
    |           o*  o.|
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 755 ~/.ssh
  5. Copy the content of ~/.ssh/id_rsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 644 ~/.ssh/authorized_keys
To generate a DSA key pair for version 2 of the SSH protocol, follow these steps:
  1. Generate a DSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t dsa
    Generating public/private dsa key pair.
    Enter file in which to save the key (/home/john/.ssh/id_dsa):
  2. Press Enter to confirm the default location (that is, ~/.ssh/id_dsa) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log in to your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/id_dsa.
    Your public key has been saved in /home/john/.ssh/id_dsa.pub.
    The key fingerprint is:
    81:a1:91:a8:9f:e8:c5:66:0d:54:f5:90:cc:bc:cc:27 john@penguin.example.com
    The key's randomart image is:
    +--[ DSA 1024]----+
    |   .oo*o.        |
    |  ...o Bo        |
    | .. . + o.       |
    |.  .   E o       |
    | o..o   S        |
    |. o= .           |
    |. +              |
    | .               |
    |                 |
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 775 ~/.ssh
  5. Copy the content of ~/.ssh/id_dsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 644 ~/.ssh/authorized_keys
To generate an RSA key pair for version 1 of the SSH protocol, follow these steps:
  1. Generate an RSA key pair by typing the following at a shell prompt:
    ~]$ ssh-keygen -t rsa1
    Generating public/private rsa1 key pair.
    Enter file in which to save the key (/home/john/.ssh/identity):
  2. Press Enter to confirm the default location (that is, ~/.ssh/identity) for the newly created key.
  3. Enter a passphrase, and confirm it by entering it again when prompted to do so. For security reasons, avoid using the same password as you use to log into your account.
    After this, you will be presented with a message similar to this:
    Your identification has been saved in /home/john/.ssh/identity.
    Your public key has been saved in /home/john/.ssh/identity.pub.
    The key fingerprint is:
    cb:f6:d5:cb:6e:5f:2b:28:ac:17:0c:e4:62:e4:6f:59 john@penguin.example.com
    The key's randomart image is:
    +--[RSA1 2048]----+
    |                 |
    |     . .         |
    |    o o          |
    |     + o E       |
    |    . o S        |
    |       = +   .   |
    |      . = . o . .|
    |       . = o o..o|
    |       .o o  o=o.|
    +-----------------+
  4. Change the permissions of the ~/.ssh/ directory:
    ~]$ chmod 755 ~/.ssh
  5. Copy the content of ~/.ssh/identity.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect, appending it to its end if the file already exists.
  6. Change the permissions of the ~/.ssh/authorized_keys file using the following command:
    ~]$ chmod 644 ~/.ssh/authorized_keys
Refer to Section 11.2.4.2, “Configuring ssh-agent” for information on how to set up your system to remember the passphrase.

Never share your private key

The private key is for your personal use only, and it is important that you never give it to anyone.

11.2.4.2. Configuring ssh-agent

To store your passphrase so that you do not have to enter it each time you initiate a connection with a remote machine, you can use the ssh-agent authentication agent. To save your passphrase for a certain shell prompt, use the following command:
~]$ ssh-add
Enter passphrase for /home/john/.ssh/id_rsa:
Note that when you log out, your passphrase will be forgotten. You must execute the command each time you log in to a virtual console or a terminal window.

11.3. OpenSSH Clients

Make sure you have relevant packages installed

To connect to an OpenSSH server from a client machine, you must have the openssh-clients and openssh packages installed. Refer to Section 5.2.4, “Installing Packages” for more information on how to install new packages in Fedora.

11.3.1. Using the ssh Utility

ssh allows you to log in to a remote machine and execute commands there. It is a secure replacement for the rlogin, rsh, and telnet programs.
Similarly to telnet, to log in to a remote machine named penguin.example.com, type the following command at a shell prompt:
~]$ ssh penguin.example.com
This will log you in with the same username you are using on a local machine. If you want to specify a different one, use a command in the ssh username@hostname form. For example, to log in as john, type:
~]$ ssh john@penguin.example.com
The first time you initiate a connection, you will be presented with a message similar to this:
The authenticity of host 'penguin.example.com' can't be established.
RSA key fingerprint is 94:68:3a:3a:bc:f3:9a:9b:01:5d:b3:07:38:e2:11:0c.
Are you sure you want to continue connecting (yes/no)?
Type yes to confirm. You will see a notice that the server has been added to the list of known hosts, and a prompt asking for your password:
Warning: Permanently added 'penguin.example.com' (RSA) to the list of known hosts.
john@penguin.example.com's password:

Updating the host key of an SSH server

If the SSH server's host key changes, the client notifies the user that the connection cannot proceed until the server's host key is deleted from the ~/.ssh/known_hosts file. To do so, open the file in a text editor, and remove a line containing the remote machine name at the beginning. Before doing this, however, contact the system administrator of the SSH server to verify the server is not compromised.
After entering the password, you will be provided with a shell prompt for the remote machine.
Alternatively, the ssh program can be used to execute a command on the remote machine without logging in to a shell prompt. The syntax for that is ssh [username@]hostname command. For example, if you want to execute the whoami command on penguin.example.com, type:
~]$ ssh john@penguin.example.com whoami
john@penguin.example.com's password:
john
After you enter the correct password, the username will be displayed, and you will return to your local shell prompt.

11.3.2. Using the scp Utility

scp can be used to transfer files between machines over a secure, encrypted connection. In its design, it is very similar to rcp.
To transfer a local file to a remote system, use a command in the following form:
scp localfile username@hostname:remotefile
For example, if you want to transfer taglist.vim to a remote machine named penguin.example.com, type the following at a shell prompt:
~]$ scp taglist.vim john@penguin.example.com:.vim/plugin/taglist.vim
john@penguin.example.com's password:
taglist.vim                                   100%  144KB 144.5KB/s   00:00
Multiple files can be specified at once. To transfer the contents of .vim/plugin/ to the same directory on the remote machine penguin.example.com, type the following command:
~]$ scp .vim/plugin/* john@penguin.example.com:.vim/plugin/
john@penguin.example.com's password:
closetag.vim                                  100%   13KB  12.6KB/s   00:00    
snippetsEmu.vim                               100%   33KB  33.1KB/s   00:00    
taglist.vim                                   100%  144KB 144.5KB/s   00:00
To transfer a remote file to the local system, use the following syntax:
scp username@hostname:remotefile localfile
For instance, to download the .vimrc configuration file from the remote machine, type:
~]$ scp john@penguin.example.com:.vimrc .vimrc
john@penguin.example.com's password:
.vimrc                                        100% 2233     2.2KB/s   00:00

11.3.3. Using the sftp Utility

The sftp utility can be used to open a secure, interactive FTP session. In its design, it is similar to ftp except that it uses a secure, encrypted connection.
To connect to a remote system, use a command in the following form:
sftp username@hostname
For example, to log in to a remote machine named penguin.example.com with john as a username, type:
~]$ sftp john@penguin.example.com
john@penguin.example.com's password:
Connected to penguin.example.com.
sftp>
After you enter the correct password, you will be presented with a prompt. The sftp utility accepts a set of commands similar to those used by ftp (see Table 11.3, “A selection of available sftp commands”).
Table 11.3. A selection of available sftp commands
Command Description
ls [directory] List the content of a remote directory. If none is supplied, a current working directory is used by default.
cd directory Change the remote working directory to directory.
mkdir directory Create a remote directory.
rmdir path Remove a remote directory.
put localfile [remotefile] Transfer localfile to a remote machine.
get remotefile [localfile] Transfer remotefile from a remote machine.

For a complete list of available commands, refer to the sftp man page.

11.4. More Than a Secure Shell

A secure command line interface is just the beginning of the many ways SSH can be used. Given the proper amount of bandwidth, X11 sessions can be directed over an SSH channel. Or, by using TCP/IP forwarding, previously insecure port connections between systems can be mapped to specific SSH channels.

11.4.1. X11 Forwarding

To open an X11 session over an SSH connection, use a command in the following form:
ssh -Y username@hostname
For example, to log in to a remote machine named penguin.example.com with john as a username, type:
~]$ ssh -Y john@penguin.example.com
john@penguin.example.com's password:
When an X program is run from the secure shell prompt, the SSH client and server create a new secure channel, and the X program data is sent over that channel to the client machine transparently.
X11 forwarding can be very useful. For example, X11 forwarding can be used to create a secure, interactive session of the Printer Configuration utility. To do this, connect to the server using ssh and type:
~]$ system-config-printer &
The Printer Configuration Tool will appear, allowing the remote user to safely configure printing on the remote system.

11.4.2. Port Forwarding

SSH can secure otherwise insecure TCP/IP protocols via port forwarding. When using this technique, the SSH server becomes an encrypted conduit to the SSH client.
Port forwarding works by mapping a local port on the client to a remote port on the server. SSH can map any port from the server to any port on the client. Port numbers do not need to match for this technique to work.

Using reserved port numbers

Setting up port forwarding to listen on ports below 1024 requires root level access.
To create a TCP/IP port forwarding channel which listens for connections on the localhost, use a command in the following form:
ssh -L local-port:remote-hostname:remote-port username@hostname
For example, to check email on a server called mail.example.com using POP3 through an encrypted connection, use the following command:
~]$ ssh -L 1100:mail.example.com:110 mail.example.com
Once the port forwarding channel is in place between the client machine and the mail server, direct a POP3 mail client to use port 1100 on the localhost to check for new email. Any requests sent to port 1100 on the client system will be directed securely to the mail.example.com server.
If mail.example.com is not running an SSH server, but another machine on the same network is, SSH can still be used to secure part of the connection. However, a slightly different command is necessary:
~]$ ssh -L 1100:mail.example.com:110 other.example.com
In this example, POP3 requests from port 1100 on the client machine are forwarded through the SSH connection on port 22 to the SSH server, other.example.com. Then, other.example.com connects to port 110 on mail.example.com to check for new email. Note that when using this technique, only the connection between the client system and other.example.com SSH server is secure.
Port forwarding can also be used to get information securely through network firewalls. If the firewall is configured to allow SSH traffic via its standard port (that is, port 22) but blocks access to other ports, a connection between two hosts using the blocked ports is still possible by redirecting their communication over an established SSH connection.

A connection is only as secure as a client system

Using port forwarding to forward connections in this manner allows any user on the client system to connect to that service. If the client system becomes compromised, the attacker also has access to forwarded services.
System administrators concerned about port forwarding can disable this functionality on the server by specifying a No parameter for the AllowTcpForwarding line in /etc/ssh/sshd_config and restarting the sshd service.

11.5. Additional Resources

The OpenSSH and OpenSSL projects are in constant development, and the most up-to-date information for them is available from their websites. The man pages for OpenSSH and OpenSSL tools are also good sources of detailed information.

11.5.1. Installed Documentation

man ssh
The manual page for ssh containing the full documentation on its usage.
man scp
The manual page for scp containing the full documentation on its usage.
man sftp
The manual page for sftp containing the full documentation on its usage.
man sshd
The manual page for sshd containing the full documentation on its usage.
man ssh-keygen
The manual page for ssh-keygen containing the full documentation on its usage.
man ssh_config
The manual page with full description of available SSH client configuration options.
man sshd_config
The manual page with full description of available SSH daemon configuration options.

11.5.2. Useful Websites

http://www.openssh.com/
The OpenSSH home page containing further documentation, frequently asked questions, links to the mailing lists, bug reports, and other useful resources.
http://www.openssl.org/
The OpenSSL home page containing further documentation, frequently asked questions, links to the mailing lists, and other useful resources.
http://www.freesshd.com/
Another implementation of an SSH server.


[2] A multiplexed connection consists of several signals being sent over a shared, common medium. With SSH, different channels are sent over a common secure connection.

Part V. Servers

This part discusses various topics related to servers such as how to set up a Web server or share files and directories over the network.

Table of Contents

12. DHCP Servers
12.1. Why Use DHCP?
12.2. Configuring a DHCP Server
12.2.1. Configuration File
12.2.2. Lease Database
12.2.3. Starting and Stopping the Server
12.2.4. DHCP Relay Agent
12.3. Configuring a DHCP Client
12.4. Configuring a Multihomed DHCP Server
12.4.1. Host Configuration
12.5. DHCP for IPv6 (DHCPv6)
12.6. Additional Resources
12.6.1. Installed Documentation
13. DNS Servers
13.1. Introduction to DNS
13.1.1. Nameserver Zones
13.1.2. Nameserver Types
13.1.3. BIND as a Nameserver
13.2. BIND
13.2.1. Configuring the named Service
13.2.2. Editing Zone Files
13.2.3. Using the rndc Utility
13.2.4. Using the dig Utility
13.2.5. Advanced Features of BIND
13.2.6. Common Mistakes to Avoid
13.2.7. Additional Resources
14. Web Servers
14.1. The Apache HTTP Server
14.1.1. New Features
14.1.2. Notable Changes
14.1.3. Updating the Configuration
14.1.4. Running the httpd Service
14.1.5. Editing the Configuration Files
14.1.6. Working with Modules
14.1.7. Setting Up Virtual Hosts
14.1.8. Setting Up an SSL Server
14.1.9. Additional Resources
15. Mail Servers
15.1. Email Protocols
15.1.1. Mail Transport Protocols
15.1.2. Mail Access Protocols
15.2. Email Program Classifications
15.2.1. Mail Transport Agent
15.2.2. Mail Delivery Agent
15.2.3. Mail User Agent
15.3. Mail Transport Agents
15.3.1. Postfix
15.3.2. Sendmail
15.3.3. Fetchmail
15.3.4. Mail Transport Agent (MTA) Configuration
15.4. Mail Delivery Agents
15.4.1. Procmail Configuration
15.4.2. Procmail Recipes
15.5. Mail User Agents
15.5.1. Securing Communication
15.6. Additional Resources
15.6.1. Installed Documentation
15.6.2. Useful Websites
15.6.3. Related Books
16. Directory Servers
16.1. OpenLDAP
16.1.1. Introduction to LDAP
16.1.2. Installing the OpenLDAP Suite
16.1.3. Configuring an OpenLDAP Server
16.1.4. Running an OpenLDAP Server
16.1.5. Configuring a System to Authenticate Using OpenLDAP
16.1.6. Additional Resources
17. File and Print Servers
17.1. Samba
17.1.1. Introduction to Samba
17.1.2. Samba Daemons and Related Services
17.1.3. Connecting to a Samba Share
17.1.4. Configuring a Samba Server
17.1.5. Starting and Stopping Samba
17.1.6. Samba Server Types and the smb.conf File
17.1.7. Samba Security Modes
17.1.8. Samba Account Information Databases
17.1.9. Samba Network Browsing
17.1.10. Samba with CUPS Printing Support
17.1.11. Samba Distribution Programs
17.1.12. Additional Resources
17.2. FTP
17.2.1. The File Transfer Protocol
17.2.2. FTP Servers
17.2.3. Files Installed with vsftpd
17.2.4. Starting and Stopping vsftpd
17.2.5. vsftpd Configuration Options
17.2.6. Additional Resources
17.3. Printer Configuration
17.3.1. Starting the Printer Configuration Tool
17.3.2. Starting Printer Setup
17.3.3. Adding a Local Printer
17.3.4. Adding an AppSocket/HP JetDirect printer
17.3.5. Adding an IPP Printer
17.3.6. Adding an LPD/LPR Host or Printer
17.3.7. Adding a Samba (SMB) printer
17.3.8. Selecting the Printer Model and Finishing
17.3.9. Printing a test page
17.3.10. Modifying Existing Printers
17.3.11. Additional Resources

Chapter 12. DHCP Servers

Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns TCP/IP information to client machines. Each DHCP client connects to the centrally located DHCP server, which returns that client's network configuration (including the IP address, gateway, and DNS servers).

12.1. Why Use DHCP?

DHCP is useful for automatic configuration of client network interfaces. When configuring the client system, the administrator chooses DHCP instead of specifying an IP address, netmask, gateway, or DNS servers. The client retrieves this information from the DHCP server. DHCP is also useful if an administrator wants to change the IP addresses of a large number of systems. Instead of reconfiguring all the systems, he can just edit one DHCP configuration file on the server for the new set of IP addresses. If the DNS servers for an organization changes, the changes are made on the DHCP server, not on the DHCP clients. When the administrator restarts the network or reboots the clients, the changes will go into effect.
If an organization has a functional DHCP server properly connected to a network, laptops and other mobile computer users can move these devices from office to office.

12.2. Configuring a DHCP Server

The dhcp package contains an ISC DHCP server. First, install the package as root:
yum install dhcp
Installing the dhcp package creates a file, /etc/dhcp/dhcpd.conf, which is merely an empty configuration file:
#
# DHCP Server Configuration file.
#   see /usr/share/doc/dhcp*/dhcpd.conf.sample
#   see dhcpd.conf(5) man page
#
The sample configuration file can be found at /usr/share/doc/dhcp-version/dhcpd.conf.sample. You should use this file to help you configure /etc/dhcp/dhcpd.conf, which is explained in detail below.
DHCP also uses the file /var/lib/dhcpd/dhcpd.leases to store the client lease database. Refer to Section 12.2.2, “Lease Database” for more information.

12.2.1. Configuration File

The first step in configuring a DHCP server is to create the configuration file that stores the network information for the clients. Use this file to declare options and global options for client systems.
The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are case-insensitive and lines beginning with a hash sign (#) are considered comments.
There are two types of statements in the configuration file:
  • Parameters — State how to perform a task, whether to perform a task, or what network configuration options to send to the client.
  • Declarations — Describe the topology of the network, describe the clients, provide addresses for the clients, or apply a group of parameters to a group of declarations.
The parameters that start with the keyword option are referred to as options. These options control DHCP options; whereas, parameters configure values that are not optional or control how the DHCP server behaves.
Parameters (including options) declared before a section enclosed in curly brackets ({ }) are considered global parameters. Global parameters apply to all the sections below it.

Restart the DHCP daemon for the changes to take effect

If the configuration file is changed, the changes do not take effect until the DHCP daemon is restarted. To do so, type the following at a shell prompt as root:
systemctl restart dhcpd.service

Use the omshell command

Instead of changing a DHCP configuration file and restarting the service each time, using the omshell command provides an interactive way to connect to, query, and change the configuration of a DHCP server. By using omshell, all changes can be made while the server is running. For more information on omshell, refer to the omshell man page.
In Example 12.1, “Subnet declaration”, the routers, subnet-mask, domain-search, domain-name-servers, and time-offset options are used for any host statements declared below it.
Additionally, a subnet can be declared, a subnet declaration must be included for every subnet in the network. If it is not, the DHCP server fails to start.
In this example, there are global options for every DHCP client in the subnet and a range declared. Clients are assigned an IP address within the range.
Example 12.1. Subnet declaration
subnet 192.168.1.0 netmask 255.255.255.0 {
        option routers                  192.168.1.254;
        option subnet-mask              255.255.255.0;
        option domain-search              "example.com";
        option domain-name-servers       192.168.1.1;
        option time-offset              -18000;     # Eastern Standard Time
	range 192.168.1.10 192.168.1.100;
}

To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify Example 12.2, “Range parameter” with your values. It declares a default lease time, maximum lease time, and network configuration values for the clients. This example assigns IP addresses in the range 192.168.1.10 and 192.168.1.100 to client systems.
Example 12.2. Range parameter
default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.1.255;
option routers 192.168.1.254;
option domain-name-servers 192.168.1.1, 192.168.1.2;
option domain-search "example.com";
subnet 192.168.1.0 netmask 255.255.255.0 {
   range 192.168.1.10 192.168.1.100;
}

To assign an IP address to a client based on the MAC address of the network interface card, use the hardware ethernet parameter within a host declaration. As demonstrated in Example 12.3, “Static IP address using DHCP”, the host apex declaration specifies that the network interface card with the MAC address 00:A0:78:8E:9E:AA always receives the IP address 192.168.1.4.
Note that the optional parameter host-name can also be used to assign a host name to the client.
Example 12.3. Static IP address using DHCP
host apex {
   option host-name "apex.example.com";
   hardware ethernet 00:A0:78:8E:9E:AA;
   fixed-address 192.168.1.4;
}

All subnets that share the same physical network should be declared within a shared-network declaration as shown in Example 12.4, “Shared-network declaration”. Parameters within the shared-network, but outside the enclosed subnet declarations, are considered to be global parameters. The name of the shared-network must be a descriptive title for the network, such as using the title 'test-lab' to describe all the subnets in a test lab environment.
Example 12.4. Shared-network declaration
shared-network name {
    option domain-search              "test.redhat.com";
    option domain-name-servers      ns1.redhat.com, ns2.redhat.com;
    option routers                  192.168.0.254;
    more parameters for EXAMPLE shared-network
    subnet 192.168.1.0 netmask 255.255.252.0 {
        parameters for subnet
        range 192.168.1.1 192.168.1.254;
    }
    subnet 192.168.2.0 netmask 255.255.252.0 {
        parameters for subnet
        range 192.168.2.1 192.168.2.254;
    }
}

As demonstrated in Example 12.5, “Group declaration”, the group declaration is used to apply global parameters to a group of declarations. For example, shared networks, subnets, and hosts can be grouped.
Example 12.5. Group declaration
group {
   option routers                  192.168.1.254;
   option subnet-mask              255.255.255.0;
   option domain-search              "example.com";
   option domain-name-servers       192.168.1.1;
   option time-offset              -18000;     # Eastern Standard Time
   host apex {
      option host-name "apex.example.com";
      hardware ethernet 00:A0:78:8E:9E:AA;
      fixed-address 192.168.1.4;
   }
   host raleigh {
      option host-name "raleigh.example.com";
      hardware ethernet 00:A1:DD:74:C3:F2;
      fixed-address 192.168.1.6;
   }
}

Using the sample configuration file

The sample configuration file provided can be used as a starting point and custom configuration options can be added to it. To copy it to the proper location, use the following command:
cp /usr/share/doc/dhcp-version-number/dhcpd.conf.sample /etc/dhcp/dhcpd.conf
... where version-number is the DHCP version number.
For a complete list of option statements and what they do, refer to the dhcp-options man page.

12.2.2. Lease Database

On the DHCP server, the file /var/lib/dhcpd/dhcpd.leases stores the DHCP client lease database. Do not change this file. DHCP lease information for each recently assigned IP address is automatically stored in the lease database. The information includes the length of the lease, to whom the IP address has been assigned, the start and end dates for the lease, and the MAC address of the network interface card that was used to retrieve the lease.
All times in the lease database are in Coordinated Universal Time (UTC), not local time.
The lease database is recreated from time to time so that it is not too large. First, all known leases are saved in a temporary lease database. The dhcpd.leases file is renamed dhcpd.leases~ and the temporary lease database is written to dhcpd.leases.
The DHCP daemon could be killed or the system could crash after the lease database has been renamed to the backup file but before the new file has been written. If this happens, the dhcpd.leases file does not exist, but it is required to start the service. Do not create a new lease file. If you do, all old leases are lost which causes many problems. The correct solution is to rename the dhcpd.leases~ backup file to dhcpd.leases and then start the daemon.

12.2.3. Starting and Stopping the Server

Starting the DHCP server for the first time

When the DHCP server is started for the first time, it fails unless the dhcpd.leases file exists. Use the command touch /var/lib/dhcpd/dhcpd.leases to create the file if it does not exist.
If the same server is also running BIND as a DNS server, this step is not necessary, as starting the named service automatically checks for a dhcpd.leases file.
To start the DHCP service, use the following command:
systemctl start dhcpd.service
To stop the DHCP server, type:
systemctl stop dhcpd.service
By default, the DHCP service does not start at boot time. To configure the daemon to start automatically at boot time, run:
systemctl enable dhcpd.service
Refer to Chapter 9, Services and Daemons for more information on how to configure services in Fedora.
If more than one network interface is attached to the system, but the DHCP server should only be started on one of the interfaces, configure the DHCP server to start only on that device. In /etc/sysconfig/dhcpd, add the name of the interface to the list of DHCPDARGS:
# Command line options here
DHCPDARGS=eth0
This is useful for a firewall machine with two network cards. One network card can be configured as a DHCP client to retrieve an IP address to the Internet. The other network card can be used as a DHCP server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users can not connect to the daemon via the Internet.
Other command line options that can be specified in /etc/sysconfig/dhcpd include:
  • -p portnum — Specifies the UDP port number on which dhcpd should listen. The default is port 67. The DHCP server transmits responses to the DHCP clients at a port number one greater than the UDP port specified. For example, if the default port 67 is used, the server listens on port 67 for requests and responses to the client on port 68. If a port is specified here and the DHCP relay agent is used, the same port on which the DHCP relay agent should listen must be specified. Refer to Section 12.2.4, “DHCP Relay Agent” for details.
  • -f — Runs the daemon as a foreground process. This is mostly used for debugging.
  • -d — Logs the DHCP server daemon to the standard error descriptor. This is mostly used for debugging. If this is not specified, the log is written to /var/log/messages.
  • -cf filename — Specifies the location of the configuration file. The default location is /etc/dhcp/dhcpd.conf.
  • -lf filename — Specifies the location of the lease database file. If a lease database file already exists, it is very important that the same file be used every time the DHCP server is started. It is strongly recommended that this option only be used for debugging purposes on non-production machines. The default location is /var/lib/dhcpd/dhcpd.leases.
  • -q — Do not print the entire copyright message when starting the daemon.

12.2.4. DHCP Relay Agent

The DHCP Relay Agent (dhcrelay) allows for the relay of DHCP and BOOTP requests from a subnet with no DHCP server on it to one or more DHCP servers on other subnets.
When a DHCP client requests information, the DHCP Relay Agent forwards the request to the list of DHCP servers specified when the DHCP Relay Agent is started. When a DHCP server returns a reply, the reply is broadcast or unicast on the network that sent the original request.
The DHCP Relay Agent listens for DHCP requests on all interfaces unless the interfaces are specified in /etc/sysconfig/dhcrelay with the INTERFACES directive.
To start the DHCP Relay Agent, use the following command:
systemctl start dhcrelay.service

12.3. Configuring a DHCP Client

To configure a DHCP client manually, modify the /etc/sysconfig/network file to enable networking and the configuration file for each network device in the /etc/sysconfig/network-scripts directory. In this directory, each device should have a configuration file named ifcfg-eth0, where eth0 is the network device name.
The /etc/sysconfig/network-scripts/ifcfg-eth0 file should contain the following lines:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
A configuration file is needed for each device to be configured to use DHCP.
Other options for the network script includes:
  • DHCP_HOSTNAME — Only use this option if the DHCP server requires the client to specify a hostname before receiving an IP address. (The DHCP server daemon in Fedora does not support this feature.)
  • PEERDNS=answer , where answer is one of the following:
    • yes — Modify /etc/resolv.conf with information from the server. If using DHCP, then yes is the default.
    • no — Do not modify /etc/resolv.conf.

Advanced configurations

For advanced configurations of client DHCP options such as protocol timing, lease requirements and requests, dynamic DNS support, aliases, as well as a wide variety of values to override, prepend, or append to client-side configurations, refer to the dhclient and dhclient.conf man pages.

12.4. Configuring a Multihomed DHCP Server

A multihomed DHCP server serves multiple networks, that is, multiple subnets. The examples in these sections detail how to configure a DHCP server to serve multiple networks, select which network interfaces to listen on, and how to define network settings for systems that move networks.
Before making any changes, back up the existing /etc/sysconfig/dhcpd and /etc/dhcp/dhcpd.conf files.
The DHCP daemon listens on all network interfaces unless otherwise specified. Use the /etc/sysconfig/dhcpd file to specify which network interfaces the DHCP daemon listens on. The following /etc/sysconfig/dhcpd example specifies that the DHCP daemon listens on the eth0 and eth1 interfaces:
DHCPDARGS="eth0 eth1";
If a system has three network interfaces cards -- eth0, eth1, and eth2 -- and it is only desired that the DHCP daemon listens on eth0, then only specify eth0 in /etc/sysconfig/dhcpd:
DHCPDARGS="eth0";
The following is a basic /etc/dhcp/dhcpd.conf file, for a server that has two network interfaces, eth0 in a 10.0.0.0/24 network, and eth1 in a 172.16.0.0/24 network. Multiple subnet declarations allow different settings to be defined for multiple networks:
default-lease-time 600;
max-lease-time 7200;
subnet 10.0.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 10.0.0.1;
	range 10.0.0.5 10.0.0.15;
}
subnet 172.16.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 172.16.0.1;
	range 172.16.0.5 172.16.0.15;
}
subnet 10.0.0.0 netmask 255.255.255.0;
A subnet declaration is required for every network your DHCP server is serving. Multiple subnets require multiple subnet declarations. If the DHCP server does not have a network interface in a range of a subnet declaration, the DHCP server does not serve that network.
If there is only one subnet declaration, and no network interfaces are in the range of that subnet, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
dhcpd: No subnet declaration for eth0 (0.0.0.0).
dhcpd: ** Ignoring requests on eth0.  If this is not what
dhcpd:    you want, please write a subnet declaration
dhcpd:    in your dhcpd.conf file for the network segment
dhcpd:    to which interface eth1 is attached. **
dhcpd:
dhcpd:
dhcpd: Not configured to listen on any interfaces!
option subnet-mask 255.255.255.0;
The option subnet-mask option defines a subnet mask, and overrides the netmask value in the subnet declaration. In simple cases, the subnet and netmask values are the same.
option routers 10.0.0.1;
The option routers option defines the default gateway for the subnet. This is required for systems to reach internal networks on a different subnet, as well as external networks.
range 10.0.0.5 10.0.0.15;
The range option specifies the pool of available IP addresses. Systems are assigned an address from the range of specified IP addresses.
For further information, refer to the dhcpd.conf(5) man page.

12.4.1. Host Configuration

Before making any changes, back up the existing /etc/sysconfig/dhcpd and /etc/dhcp/dhcpd.conf files.
Configuring a single system for multiple networks
The following /etc/dhcp/dhcpd.conf example creates two subnets, and configures an IP address for the same system, depending on which network it connects to:
default-lease-time 600;
max-lease-time 7200;
subnet 10.0.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 10.0.0.1;
	range 10.0.0.5 10.0.0.15;
}
subnet 172.16.0.0 netmask 255.255.255.0 {
	option subnet-mask 255.255.255.0;
	option routers 172.16.0.1;
	range 172.16.0.5 172.16.0.15;
}
host example0 {
	hardware ethernet 00:1A:6B:6A:2E:0B;
	fixed-address 10.0.0.20;
}
host example1 {
	hardware ethernet 00:1A:6B:6A:2E:0B;
	fixed-address 172.16.0.20;
}
host example0
The host declaration defines specific parameters for a single system, such as an IP address. To configure specific parameters for multiple hosts, use multiple host declarations.
Most DHCP clients ignore the name in host declarations, and as such, this name can anything, as long as it is unique to other host declarations. To configure the same system for multiple networks, use a different name for each host declaration, otherwise the DHCP daemon fails to start. Systems are identified by the hardware ethernet option, not the name in the host declaration.
hardware ethernet 00:1A:6B:6A:2E:0B;
The hardware ethernet option identifies the system. To find this address, run the ip link command.
fixed-address 10.0.0.20;
The fixed-address option assigns a valid IP address to the system specified by the hardware ethernet option. This address must be outside the IP address pool specified with the range option.
If option statements do not end with a semicolon, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
/etc/dhcp/dhcpd.conf line 20: semicolon expected.
dhcpd: }
dhcpd: ^
dhcpd: /etc/dhcp/dhcpd.conf line 38: unexpected end of file
dhcpd:
dhcpd: ^
dhcpd: Configuration file errors encountered -- exiting
Configuring systems with multiple network interfaces
The following host declarations configure a single system, that has multiple network interfaces, so that each interface receives the same IP address. This configuration will not work if both network interfaces are connected to the same network at the same time:
host interface0 {
	hardware ethernet 00:1a:6b:6a:2e:0b;
	fixed-address 10.0.0.18;
}
host interface1 {
	hardware ethernet 00:1A:6B:6A:27:3A;
	fixed-address 10.0.0.18;
}
For this example, interface0 is the first network interface, and interface1 is the second interface. The different hardware ethernet options identify each interface.
If such a system connects to another network, add more host declarations, remembering to:
  • assign a valid fixed-address for the network the host is connecting to.
  • make the name in the host declaration unique.
When a name given in a host declaration is not unique, the DHCP daemon fails to start, and an error such as the following is logged to /var/log/messages:
dhcpd: /etc/dhcp/dhcpd.conf line 31: host interface0: already exists
dhcpd: }
dhcpd: ^
dhcpd: Configuration file errors encountered -- exiting
This error was caused by having multiple host interface0 declarations defined in /etc/dhcp/dhcpd.conf.

12.5. DHCP for IPv6 (DHCPv6)

The ISC DHCP includes support for IPv6 (DHCPv6) since the 4.x release with a DHCPv6 server, client and relay agent functionality. The server, client and relay agents support both IPv4 and IPv6. However, the client and the server can only manage one protocol at a time — for dual support they must be started separately for IPv4 and IPv6.
The DHCPv6 server configuration file can be found at /etc/dhcp/dhcpd6.conf.
The sample server configuration file can be found at /usr/share/doc/dhcp-version/dhcpd6.conf.sample.
To start the DHCPv6 service, use the following command:
systemctl start dhcpd6.service
A simple DHCPv6 server configuration file can look like this:
subnet6 2001:db8:0:1::/64 {
        range6 2001:db8:0:1::129 2001:db8:0:1::254;
        option dhcp6.name-servers fec0:0:0:1::1;
        option dhcp6.domain-search "domain.example";
}

12.6. Additional Resources

For additional information, refer to The DHCP Handbook; Ralph Droms and Ted Lemon; 2003 or the following resources.

12.6.1. Installed Documentation

  • dhcpd man page — Describes how the DHCP daemon works.
  • dhcpd.conf man page — Explains how to configure the DHCP configuration file; includes some examples.
  • dhcpd.leases man page — Describes a persistent database of leases.
  • dhcp-options man page — Explains the syntax for declaring DHCP options in dhcpd.conf; includes some examples.
  • dhcrelay man page — Explains the DHCP Relay Agent and its configuration options.
  • /usr/share/doc/dhcp-version/ — Contains sample files, README files, and release notes for current versions of the DHCP service.

Chapter 13. DNS Servers

DNS (Domain Name System), also known as a nameserver, is a network system that associates hostnames with their respective IP addresses. For users, this has the advantage that they can refer to machines on the network by names that are usually easier to remember than the numerical network addresses. For system administrators, using the nameserver allows them to change the IP address for a host without ever affecting the name-based queries, or to decide which machines handle these queries.

13.1. Introduction to DNS

DNS is usually implemented using one or more centralized servers that are authoritative for certain domains. When a client host requests information from a nameserver, it usually connects to port 53. The nameserver then attempts to resolve the name requested. If it does not have an authoritative answer, or does not already have the answer cached from an earlier query, it queries other nameservers, called root nameservers, to determine which nameservers are authoritative for the name in question, and then queries them to get the requested name.

13.1.1. Nameserver Zones

In a DNS server such as BIND, all information is stored in basic data elements called resource records (RR). The resource record is usually a fully qualified domain name (FQDN) of a host, and is broken down into multiple sections organized into a tree-like hierarchy. This hierarchy consists of a main trunk, primary branches, secondary branches, and so on. The following is an example of a resource record:
bob.sales.example.com
Each level of the hierarchy is divided by a period (that is, .). In the example above, com defines the top-level domain, example its subdomain, and sales the subdomain of example. In this case, bob identifies a resource record that is part of the sales.example.com domain. With the exception of the part furthest to the left (that is, bob), each of these sections is called a zone and defines a specific namespace.
Zones are defined on authoritative nameservers through the use of zone files, which contain definitions of the resource records in each zone. Zone files are stored on primary nameservers (also called master nameservers), where changes are made to the files, and secondary nameservers (also called slave nameservers), which receive zone definitions from the primary nameservers. Both primary and secondary nameservers are authoritative for the zone and look the same to clients. Depending on the configuration, any nameserver can also serve as a primary or secondary server for multiple zones at the same time.

13.1.2. Nameserver Types

There are two nameserver configuration types:
authoritative
Authoritative nameservers answer to resource records that are part of their zones only. This category includes both primary (master) and secondary (slave) nameservers.
recursive
Recursive nameservers offer resolution services, but they are not authoritative for any zone. Answers for all resolutions are cached in a memory for a fixed period of time, which is specified by the retrieved resource record.
Although a nameserver can be both authoritative and recursive at the same time, it is recommended not to combine the configuration types. To be able to perform their work, authoritative servers should be available to all clients all the time. On the other hand, since the recursive lookup takes far more time than authoritative responses, recursive servers should be available to a restricted number of clients only, otherwise they are prone to distributed denial of service (DDoS) attacks.

13.1.3. BIND as a Nameserver

BIND consists of a set of DNS-related programs. It contains a monolithic nameserver called named, an administration utility called rndc, and a debugging tool called dig. Refer to Chapter 9, Services and Daemons for more information on how to configure services in Fedora.

13.2. BIND

This chapter covers BIND (Berkeley Internet Name Domain), the DNS server included in Fedora. It focuses on the structure of its configuration files, and describes how to administer it both locally and remotely.

13.2.1. Configuring the named Service

When the named service is started, it reads the configuration from the files as described in Table 13.1, “The named service configuration files”.
Table 13.1. The named service configuration files
Path Description
/etc/named.conf The main configuration file.
/etc/named/ An auxiliary directory for configuration files that are included in the main configuration file.

The configuration file consists of a collection of statements with nested options surrounded by opening and closing curly brackets (that is, { and }). Note that when editing the file, you have to be careful not to make any syntax error, otherwise the named service will not start. A typical /etc/named.conf file is organized as follows:
statement-1 ["statement-1-name"] [statement-1-class] {
  option-1;
  option-2;
  option-N;
};
statement-2 ["statement-2-name"] [statement-2-class] {
  option-1;
  option-2;
  option-N;
};
statement-N ["statement-N-name"] [statement-N-class] {
  option-1;
  option-2;
  option-N;
};

Running BIND in a chroot environment

If you have installed the bind-chroot package, the BIND service will run in the /var/named/chroot environment. In that case, the initialization script will mount the above configuration files using the mount --bind command, so that you can manage the configuration outside this environment.

13.2.1.1. Installing BIND In A Chroot Environment

To install BIND to run in a chroot environment, issue the following commands as root:
~]# yum install bind-chroot
To enable the named-chroot service, first check if the named service is running by issuing the following command:
~]$ systemctl status named
If it is running, it must be disabled.
To disable named, issue the following commands as root:
~]# systemctl stop named
~]# systemctl disable named
Then, to enable the named-chroot service, issue the following commands as root:
~]# systemctl enable named-chroot
~]# systemctl start named-chroot
To check the status of the named-chroot service, issue the following command as root:
~]# systemctl status named-chroot

13.2.1.2. Common Statement Types

The following types of statements are commonly used in /etc/named.conf:
acl
The acl (Access Control List) statement allows you to define groups of hosts, so that they can be permitted or denied access to the nameserver. It takes the following form:
acl acl-name {
  match-element;
  ...
};
The acl-name statement name is the name of the access control list, and the match-element option is usually an individual IP address (such as 10.0.1.1) or a CIDR network notation (for example, 10.0.1.0/24). For a list of already defined keywords, see Table 13.2, “Predefined access control lists”.
Table 13.2. Predefined access control lists
Keyword Description
any Matches every IP address.
localhost Matches any IP address that is in use by the local system.
localnets Matches any IP address on any network to which the local system is connected.
none Does not match any IP address.

The acl statement can be especially useful with conjunction with other statements such as options. Example 13.1, “Using acl in conjunction with options” defines two access control lists, black-hats and red-hats, and adds black-hats on the blacklist while granting red-hats a normal access.
Example 13.1. Using acl in conjunction with options
acl black-hats {
  10.0.2.0/24;
  192.168.0.0/24;
  1234:5678::9abc/24;
};
acl red-hats {
  10.0.1.0/24;
};
options {
  blackhole { black-hats; };
  allow-query { red-hats; };
  allow-query-cache { red-hats; };
};

include
The include statement allows you to include files in the /etc/named.conf, so that potentially sensitive data can be placed in a separate file with restricted permissions. It takes the following form:
include "file-name"
The file-name statement name is an absolute path to a file.
Example 13.2. Including a file to /etc/named.conf
include "/etc/named.rfc1912.zones";

options
The options statement allows you to define global server configuration options as well as to set defaults for other statements. It can be used to specify the location of the named working directory, the types of queries allowed, and much more. It takes the following form:
options {
  option;
  ...
};
For a list of frequently used option directives, see Table 13.3, “Commonly used options” below.
Table 13.3. Commonly used options
Option Description
allow-query Specifies which hosts are allowed to query the nameserver for authoritative resource records. It accepts an access control lists, a collection of IP addresses, or networks in the CIDR notation. All hosts are allowed by default.
allow-query-cache Specifies which hosts are allowed to query the nameserver for non-authoritative data such as recursive queries. Only localhost and localnets are allowed by default.
blackhole Specifies which hosts are not allowed to query the nameserver. This option should be used when particular host or network floods the server with requests. The default option is none.
directory Specifies a working directory for the named service. The default option is /var/named/.
dnssec-enable Specifies whether to return DNSSEC related resource records. The default option is yes.
dnssec-validation Specifies whether to prove that resource records are authentic via DNSSEC. The default option is yes.
forwarders Specifies a list of valid IP addresses for nameservers to which the requests should be forwarded for resolution.
forward
Specifies the behavior of the forwarders directive. It accepts the following options:
  • first — The server will query the nameservers listed in the forwarders directive before attempting to resolve the name on its own.
  • only — When unable to query the nameservers listed in the forwarders directive, the server will not attempt to resolve the name on its own.
listen-on Specifies the IPv4 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv4 interfaces are used by default.
listen-on-v6 Specifies the IPv6 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv6 interfaces are used by default.
max-cache-size Specifies the maximum amount of memory to be used for server caches. When the limit is reached, the server causes records to expire prematurely so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the cache of each view. The default option is 32M.
notify
Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:
  • yes — The server will notify all secondary nameservers.
  • no — The server will not notify any secondary nameserver.
  • master-only — The server will notify primary server for the zone only.
  • explicit — The server will notify only the secondary servers that are specified in the also-notify list within a zone statement.
pid-file Specifies the location of the process ID file created by the named service.
recursion Specifies whether to act as a recursive server. The default option is yes.
statistics-file Specifies an alternate location for statistics files. The /var/named/named.stats file is used by default.

Restrict recursive servers to selected clients only

To prevent distributed denial of service (DDoS) attacks, it is recommended that you use the allow-query-cache option to restrict recursive DNS services for a particular subset of clients only.
Refer to the BIND 9 Administrator Reference Manual referenced in Section 13.2.7.1, “Installed Documentation”, and the named.conf manual page for a complete list of available options.
Example 13.3. Using the options statement
options {
  allow-query       { localhost; };
  listen-on port    53 { 127.0.0.1; };
  listen-on-v6 port 53 { ::1; };
  max-cache-size    256M;
  directory         "/var/named";
  statistics-file   "/var/named/data/named_stats.txt";

  recursion         yes;
  dnssec-enable     yes;
  dnssec-validation yes;
};

zone
The zone statement allows you to define the characteristics of a zone, such as the location of its configuration file and zone-specific options, and can be used to override the global options statements. It takes the following form:
zone zone-name [zone-class] {
  option;
  ...
};
The zone-name attribute is the name of the zone, zone-class is the optional class of the zone, and option is a zone statement option as described in Table 13.4, “Commonly used options”.
The zone-name attribute is particularly important, as it is the default value assigned for the $ORIGIN directive used within the corresponding zone file located in the /var/named/ directory. The named daemon appends the name of the zone to any non-fully qualified domain name listed in the zone file. For example, if a zone statement defines the namespace for example.com, use example.com as the zone-name so that it is placed at the end of hostnames within the example.com zone file.
For more information about zone files, refer to Section 13.2.2, “Editing Zone Files”.
Table 13.4. Commonly used options
Option Description
allow-query Specifies which clients are allowed to request information about this zone. This option overrides global allow-query option. All query requests are allowed by default.
allow-transfer Specifies which secondary servers are allowed to request a transfer of the zone's information. All transfer requests are allowed by default.
allow-update
Specifies which hosts are allowed to dynamically update information in their zone. The default option is to deny all dynamic update requests.
Note that you should be careful when allowing hosts to update information about their zone. Do not set IP addresses in this option unless the server is in the trusted network. Instead, use TSIG key as described in Section 13.2.5.3, “Transaction SIGnatures (TSIG)”.
file Specifies the name of the file in the named working directory that contains the zone's configuration data.
masters Specifies from which IP addresses to request authoritative zone information. This option is used only if the zone is defined as type slave.
notify
Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options:
  • yes — The server will notify all secondary nameservers.
  • no — The server will not notify any secondary nameserver.
  • master-only — The server will notify primary server for the zone only.
  • explicit — The server will notify only the secondary servers that are specified in the also-notify list within a zone statement.
type
Specifies the zone type. It accepts the following options:
  • delegation-only — Enforces the delegation status of infrastructure zones such as COM, NET, or ORG. Any answer that is received without an explicit or implicit delegation is treated as NXDOMAIN. This option is only applicable in TLDs or root zone files used in recursive or caching implementations.
  • forward — Forwards all requests for information about this zone to other nameservers.
  • hint — A special type of zone used to point to the root nameservers which resolve queries when a zone is not otherwise known. No configuration beyond the default is necessary with a hint zone.
  • master — Designates the nameserver as authoritative for this zone. A zone should be set as the master if the zone's configuration files reside on the system.
  • slave — Designates the nameserver as a slave server for this zone. Master server is specified in masters directive.

Most changes to the /etc/named.conf file of a primary or secondary nameserver involve adding, modifying, or deleting zone statements, and only a small subset of zone statement options is usually needed for a nameserver to work efficiently.
In Example 13.4, “A zone statement for a primary nameserver”, the zone is identified as example.com, the type is set to master, and the named service is instructed to read the /var/named/example.com.zone file. It also allows only a secondary nameserver (192.168.0.2) to transfer the zone.
Example 13.4. A zone statement for a primary nameserver
zone "example.com" IN {
  type master;
  file "example.com.zone";
  allow-transfer { 192.168.0.2; };
};

A secondary server's zone statement is slightly different. The type is set to slave, and the masters directive is telling named the IP address of the master server.
In Example 13.5, “A zone statement for a secondary nameserver”, the named service is configured to query the primary server at the 192.168.0.1 IP address for information about the example.com zone. The received information is then saved to the /var/named/slaves/example.com.zone file. Note that you have to put all slave zones to /var/named/slaves directory, otherwise the service will fail to transfer the zone.
Example 13.5. A zone statement for a secondary nameserver
zone "example.com" {
  type slave;
  file "slaves/example.com.zone";
  masters { 192.168.0.1; };
};

13.2.1.3. Other Statement Types

The following types of statements are less commonly used in /etc/named.conf:
controls
The controls statement allows you to configure various security requirements necessary to use the rndc command to administer the named service.
Refer to Section 13.2.3, “Using the rndc Utility” for more information on the rndc utility and its usage.
key
The key statement allows you to define a particular key by name. Keys are used to authenticate various actions, such as secure updates or the use of the rndc command. Two options are used with key:
  • algorithm algorithm-name — The type of algorithm to be used (for example, hmac-md5).
  • secret "key-value" — The encrypted key.
Refer to Section 13.2.3, “Using the rndc Utility” for more information on the rndc utility and its usage.
logging
The logging statement allows you to use multiple types of logs, so called channels. By using the channel option within the statement, you can construct a customized type of log with its own file name (file), size limit (size), versioning (version), and level of importance (severity). Once a customized channel is defined, a category option is used to categorize the channel and begin logging when the named service is restarted.
By default, named sends standard messages to the rsyslog daemon, which places them in /var/log/messages. Several standard channels are built into BIND with various severity levels, such as default_syslog (which handles informational logging messages) and default_debug (which specifically handles debugging messages). A default category, called default, uses the built-in channels to do normal logging without any special configuration.
Customizing the logging process can be a very detailed process and is beyond the scope of this chapter. For information on creating custom BIND logs, refer to the BIND 9 Administrator Reference Manual referenced in Section 13.2.7.1, “Installed Documentation”.
server
The server statement allows you to specify options that affect how the named service should respond to remote nameservers, especially with regard to notifications and zone transfers.
The transfer-format option controls the number of resource records that are sent with each message. It can be either one-answer (only one resource record), or many-answers (multiple resource records). Note that while the many-answers option is more efficient, it is not supported by older versions of BIND.
trusted-keys
The trusted-keys statement allows you to specify assorted public keys used for secure DNS (DNSSEC). Refer to Section 13.2.5.4, “DNS Security Extensions (DNSSEC)” for more information on this topic.
view
The view statement allows you to create special views depending upon which network the host querying the nameserver is on. This allows some hosts to receive one answer regarding a zone while other hosts receive totally different information. Alternatively, certain zones may only be made available to particular trusted hosts while non-trusted hosts can only make queries for other zones.
Multiple views can be used as long as their names are unique. The match-clients option allows you to specify the IP addresses that apply to a particular view. If the options statement is used within a view, it overrides the already configured global options. Finally, most view statements contain multiple zone statements that apply to the match-clients list.
Note that the order in which the view statements are listed is important, as the first statement that matches a particular client's IP address is used. For more information on this topic, refer to Section 13.2.5.1, “Multiple Views”.

13.2.1.4. Comment Tags

Additionally to statements, the /etc/named.conf file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to a user. The following are valid comment tags:
//
Any text after the // characters to the end of the line is considered a comment. For example:
notify yes;  // notify all secondary nameservers
#
Any text after the # character to the end of the line is considered a comment. For example:
notify yes;  # notify all secondary nameservers
/* and */
Any block of text enclosed in /* and */ is considered a comment. For example:
notify yes;  /* notify all secondary nameservers */

13.2.2. Editing Zone Files

As outlined in Section 13.1.1, “Nameserver Zones”, zone files contain information about a namespace. They are stored in the named working directory located in /var/named/ by default, and each zone file is named according to the file option in the zone statement, usually in a way that relates to the domain in question and identifies the file as containing zone data, such as example.com.zone.
Table 13.5. The named service zone files
Path Description
/var/named/ The working directory for the named service. The nameserver is not allowed to write to this directory.
/var/named/slaves/ The directory for secondary zones. This directory is writable by the named service.
/var/named/dynamic/ The directory for other files, such as dynamic DNS (DDNS) zones or managed DNSSEC keys. This directory is writable by the named service.
/var/named/data/ The directory for various statistics and debugging files. This directory is writable by the named service.

A zone file consists of directives and resource records. Directives tell the nameserver to perform tasks or apply special settings to the zone, resource records define the parameters of the zone and assign identities to individual hosts. While the directives are optional, the resource records are required in order to provide name service to a zone.
All directives and resource records should be entered on individual lines.

13.2.2.1. Common Directives

Directives begin with the dollar sign character (that is, $) followed by the name of the directive, and usually appear at the top of the file. The following directives are commonly used in zone files:
$INCLUDE
The $INCLUDE directive allows you to include another file at the place where it appears, so that other zone settings can be stored in a separate zone file.
Example 13.6. Using the $INCLUDE directive
$INCLUDE /var/named/penguin.example.com

$ORIGIN
The $ORIGIN directive allows you to append the domain name to unqualified records, such as those with the hostname only. Note that the use of this directive is not necessary if the zone is specified in /etc/named.conf, since the zone name is used by default.
In Example 13.7, “Using the $ORIGIN directive”, any names used in resource records that do not end in a trailing period (that is, the . character) are appended with example.com.
Example 13.7. Using the $ORIGIN directive
$ORIGIN example.com.

$TTL
The $TTL directive allows you to set the default Time to Live (TTL) value for the zone, that is, how long is a zone record valid. Each resource record can contain its own TTL value, which overrides this directive.
Increasing this value allows remote nameservers to cache the zone information for a longer period of time, reducing the number of queries for the zone and lengthening the amount of time required to proliferate resource record changes.
Example 13.8. Using the $TTL directive
$TTL 1D

13.2.2.2. Common Resource Records

The following resource records are commonly used in zone files:
A
The Address record specifies an IP address to be assigned to a name. It takes the following form:
hostname IN A IP-address
If the hostname value is omitted, the record will point to the last specified hostname.
In Example 13.9, “Using the A resource record”, the requests for server1.example.com are pointed to 10.0.1.3 or 10.0.1.5.
Example 13.9. Using the A resource record
server1  IN  A  10.0.1.3
         IN  A  10.0.1.5

CNAME
The Canonical Name record maps one name to another. Because of this, this type of record is sometimes referred to as an alias record. It takes the following form:
alias-name IN CNAME real-name
CNAME records are most commonly used to point to services that use a common naming scheme, such as www for Web servers. However, there are multiple restrictions for their usage:
  • CNAME records should not point to other CNAME records. This is mainly to avoid possible infinite loops.
  • CNAME records should not contain other resource record types (such as A, NS, MX, etc.). The only exception are DNSSEC related records (that is, RRSIG, NSEC, etc.) when the zone is signed.
  • Other resource record that point to the fully qualified domain name (FQDN) of a host (that is, NS, MX, PTR) should not point to a CNAME record.
In Example 13.10, “Using the CNAME resource record”, the A record binds a hostname to an IP address, while the CNAME record points the commonly used www hostname to it.
Example 13.10. Using the CNAME resource record
server1  IN  A      10.0.1.5
www      IN  CNAME  server1

MX
The Mail Exchange record specifies where the mail sent to a particular namespace controlled by this zone should go. It takes the following form:
IN MX preference-value email-server-name
The email-server-name is a fully qualified domain name (FQDN). The preference-value allows numerical ranking of the email servers for a namespace, giving preference to some email systems over others. The MX resource record with the lowest preference-value is preferred over the others. However, multiple email servers can possess the same value to distribute email traffic evenly among them.
In Example 13.11, “Using the MX resource record”, the first mail.example.com email server is preferred to the mail2.example.com email server when receiving email destined for the example.com domain.
Example 13.11. Using the MX resource record
example.com.  IN  MX  10  mail.example.com.
              IN  MX  20  mail2.example.com.

NS
The Nameserver record announces authoritative nameservers for a particular zone. It takes the following form:
IN NS nameserver-name
The nameserver-name should be a fully qualified domain name (FQDN). Note that when two nameservers are listed as authoritative for the domain, it is not important whether these nameservers are secondary nameservers, or if one of them is a primary server. They are both still considered authoritative.
Example 13.12. Using the NS resource record
IN  NS  dns1.example.com.
IN  NS  dns2.example.com.

PTR
The Pointer record points to another part of the namespace. It takes the following form:
last-IP-digit IN PTR FQDN-of-system
The last-IP-digit directive is the last number in an IP address, and the FQDN-of-system is a fully qualified domain name (FQDN).
PTR records are primarily used for reverse name resolution, as they point IP addresses back to a particular name. Refer to Section 13.2.2.4.2, “A Reverse Name Resolution Zone File” for more examples of PTR records in use.
SOA
The Start of Authority record announces important authoritative information about a namespace to the nameserver. Located after the directives, it is the first resource record in a zone file. It takes the following form:
@  IN  SOA  primary-name-server hostmaster-email (
       serial-number
       time-to-refresh
       time-to-retry
       time-to-expire
       minimum-TTL )
The directives are as follows:
  • The @ symbol places the $ORIGIN directive (or the zone's name if the $ORIGIN directive is not set) as the namespace being defined by this SOA resource record.
  • The primary-name-server directive is the hostname of the primary nameserver that is authoritative for this domain.
  • The hostmaster-email directive is the email of the person to contact about the namespace.
  • The serial-number directive is a numerical value incremented every time the zone file is altered to indicate it is time for the named service to reload the zone.
  • The time-to-refresh directive is the numerical value secondary nameservers use to determine how long to wait before asking the primary nameserver if any changes have been made to the zone.
  • The time-to-retry directive is a numerical value used by secondary nameservers to determine the length of time to wait before issuing a refresh request in the event that the primary nameserver is not answering. If the primary server has not replied to a refresh request before the amount of time specified in the time-to-expire directive elapses, the secondary servers stop responding as an authority for requests concerning that namespace.
  • In BIND 4 and 8, the minimum-TTL directive is the amount of time other nameservers cache the zone's information. In BIND 9, it defines how long negative answers are cached for. Caching of negative answers can be set to a maximum of 3 hours (that is, 3H).
When configuring BIND, all times are specified in seconds. However, it is possible to use abbreviations when specifying units of time other than seconds, such as minutes (M), hours (H), days (D), and weeks (W). Table 13.6, “Seconds compared to other time units” shows an amount of time in seconds and the equivalent time in another format.
Table 13.6. Seconds compared to other time units
Seconds Other Time Units
60 1M
1800 30M
3600 1H
10800 3H
21600 6H
43200 12H
86400 1D
259200 3D
604800 1W
31536000 365D

Example 13.13. Using the SOA resource record
@  IN  SOA  dns1.example.com.  hostmaster.example.com. (
       2001062501  ; serial
       21600       ; refresh after 6 hours
       3600        ; retry after 1 hour
       604800      ; expire after 1 week
       86400 )     ; minimum TTL of 1 day

13.2.2.3. Comment Tags

Additionally to resource records and directives, a zone file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to the user. Any text after the semicolon character (that is, ;) to the end of the line is considered a comment. For example:
   604800  ; expire after 1 week

13.2.2.4. Example Usage

The following examples show the basic usage of zone files.
13.2.2.4.1. A Simple Zone File
Example 13.14, “A simple zone file” demonstrates the use of standard directives and SOA values.
Example 13.14. A simple zone file
$ORIGIN example.com.
$TTL 86400
@         IN  SOA  dns1.example.com.  hostmaster.example.com. (
              2001062501  ; serial
              21600       ; refresh after 6 hours
              3600        ; retry after 1 hour
              604800      ; expire after 1 week
              86400 )     ; minimum TTL of 1 day
;
;
          IN  NS     dns1.example.com.
          IN  NS     dns2.example.com.
dns1      IN  A      10.0.1.1
          IN  AAAA   aaaa:bbbb::1
dns2      IN  A      10.0.1.2
          IN  AAAA   aaaa:bbbb::2
;
;
@         IN  MX     10  mail.example.com.
          IN  MX     20  mail2.example.com.
mail      IN  A      10.0.1.5
          IN  AAAA   aaaa:bbbb::5
mail2     IN  A      10.0.1.6
          IN  AAAA   aaaa:bbbb::6
;
;
; This sample zone file illustrates sharing the same IP addresses
; for multiple services:
;
services  IN  A      10.0.1.10
          IN  AAAA   aaaa:bbbb::10
          IN  A      10.0.1.11
          IN  AAAA   aaaa:bbbb::11

ftp       IN  CNAME  services.example.com.
www       IN  CNAME  services.example.com.
;
;

In this example, the authoritative nameservers are set as dns1.example.com and dns2.example.com, and are tied to the 10.0.1.1 and 10.0.1.2 IP addresses respectively using the A record.
The email servers configured with the MX records point to mail and mail2 via A records. Since these names do not end in a trailing period (that is, the . character), the $ORIGIN domain is placed after them, expanding them to mail.example.com and mail2.example.com.
Services available at the standard names, such as www.example.com (WWW), are pointed at the appropriate servers using the CNAME record.
This zone file would be called into service with a zone statement in the /etc/named.conf similar to the following:
zone "example.com" IN {
  type master;
  file "example.com.zone";
  allow-update { none; };
};
13.2.2.4.2. A Reverse Name Resolution Zone File
A reverse name resolution zone file is used to translate an IP address in a particular namespace into an fully qualified domain name (FQDN). It looks very similar to a standard zone file, except that the PTR resource records are used to link the IP addresses to a fully qualified domain name as shown in Example 13.15, “A reverse name resolution zone file”.
Example 13.15. A reverse name resolution zone file
$ORIGIN 1.0.10.in-addr.arpa.
$TTL 86400
@  IN  SOA  dns1.example.com.  hostmaster.example.com. (
       2001062501  ; serial
       21600       ; refresh after 6 hours
       3600        ; retry after 1 hour
       604800      ; expire after 1 week
       86400 )     ; minimum TTL of 1 day
;
@  IN  NS   dns1.example.com.
;
1  IN  PTR  dns1.example.com.
2  IN  PTR  dns2.example.com.
;
5  IN  PTR  server1.example.com.
6  IN  PTR  server2.example.com.
;
3  IN  PTR  ftp.example.com.
4  IN  PTR  ftp.example.com.

In this example, IP addresses 10.0.1.1 through 10.0.1.6 are pointed to the corresponding fully qualified domain name.
This zone file would be called into service with a zone statement in the /etc/named.conf file similar to the following:
zone "1.0.10.in-addr.arpa" IN {
  type master;
  file "example.com.rr.zone";
  allow-update { none; };
};
There is very little difference between this example and a standard zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone.

13.2.3. Using the rndc Utility

The rndc utility is a command line tool that allows you to administer the named service, both locally and from a remote machine. Its usage is as follows:
rndc [option...] command [command-option]

13.2.3.1. Configuring the Utility

To prevent unauthorized access to the service, named must be configured to listen on the selected port (that is, 953 by default), and an identical key must be used by both the service and the rndc utility.
Table 13.7. Relevant files
Path Description
/etc/named.conf The default configuration file for the named service.
/etc/rndc.conf The default configuration file for the rndc utility.
/etc/rndc.key The default key location.

The rndc configuration is located in /etc/rndc.conf. If the file does not exist, the utility will use the key located in /etc/rndc.key, which was generated automatically during the installation process using the rndc-confgen -a command.
The named service is configured using the controls statement in the /etc/named.conf configuration file as described in Section 13.2.1.3, “Other Statement Types”. Unless this statement is present, only the connections from the loopback address (that is, 127.0.0.1) will be allowed, and the key located in /etc/rndc.key will be used.
For more information on this topic, refer to manual pages and the BIND 9 Administrator Reference Manual listed in Section 13.2.7, “Additional Resources”.

Set the correct permissions

To prevent unprivileged users from sending control commands to the service, make sure only root is allowed to read the /etc/rndc.key file:
~]# chmod o-rwx /etc/rndc.key

13.2.3.2. Checking the Service Status

To check the current status of the named service, use the following command:
~]# rndc status
version: 9.7.0-P2-RedHat-9.7.0-5.P2.el6
CPUs found: 1
worker threads: 1
number of zones: 16
debug level: 0
xfers running: 0
xfers deferred: 0
soa queries in progress: 0
query logging is OFF
recursive clients: 0/0/1000
tcp clients: 0/100
server is up and running

13.2.3.3. Reloading the Configuration and Zones

To reload both the configuration file and zones, type the following at a shell prompt:
~]# rndc reload
server reload successful
This will reload the zones while keeping all previously cached responses, so that you can make changes to the zone files without losing all stored name resolutions.
To reload a single zone, specify its name after the reload command, for example:
~]# rndc reload localhost
zone reload up-to-date
Finally, to reload the configuration file and newly added zones only, type:
~]# rndc reconfig

Modifying zones with dynamic DNS

If you intend to manually modify a zone that uses Dynamic DNS (DDNS), make sure you run the freeze command first:
~]# rndc freeze localhost
Once you are finished, run the thaw command to allow the DDNS again and reload the zone:
~]# rndc thaw localhost
The zone reload and thaw was successful.

13.2.3.4. Updating Zone Keys

To update the DNSSEC keys and sign the zone, use the sign command. For example:
~]# rndc sign localhost
Note that to sign a zone with the above command, the auto-dnssec option has to be set to maintain in the zone statement. For instance:
zone "localhost" IN {
  type master;
  file "named.localhost";
  allow-update { none; };
  auto-dnssec maintain;
};

13.2.3.5. Enabling the DNSSEC Validation

To enable the DNSSEC validation, type the following at a shell prompt:
~]# rndc validation on
Similarly, to disable this option, type:
~]# rndc validation off
Refer to the options statement described in Section 13.2.1.2, “Common Statement Types” for information on how configure this option in /etc/named.conf.

13.2.3.6. Enabling the Query Logging

To enable (or disable in case it is currently enabled) the query logging, run the following command:
~]# rndc querylog
To check the current setting, use the status command as described in Section 13.2.3.2, “Checking the Service Status”.

13.2.4. Using the dig Utility

The dig utility is a command line tool that allows you to perform DNS lookups and debug a nameserver configuration. Its typical usage is as follows:
dig [@server] [option...] name type
Refer to Section 13.2.2.2, “Common Resource Records” for a list of common types.

13.2.4.1. Looking Up a Nameserver

To look up a nameserver for a particular domain, use the command in the following form:
dig name NS
In Example 13.16, “A sample nameserver lookup”, the dig utility is used to display nameservers for example.com.