red_hat_enterprise_linux-7-system_administrators_guide-en-us.pdf

Red Hat Enterprise Linux 7
System Administrator’s Guide
Deployment, configuration, and administration of RHEL 7
Last Updated: 2022-07-14

Red Hat Enterprise Linux 7 System Administrator’s Guide
Deployment, configuration, and administration of RHEL 7

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Abstract
The System Administrator's Guide documents relevant information regarding the deployment,
configuration, and administration of Red Hat Enterprise Linux 7. It is oriented towards system
administrators with a basic understanding of the system. To expand your expertise, you might also
be interested in the Red Hat System Administration I (RH124), Red Hat System Administration II
(RH134), Red Hat System Administration III (RH254), or RHCSA Rapid Track (RH199) training
courses. If you want to use Red Hat Enterprise Linux 7 with the Linux Containers functionality, see
Product Documentation for Red Hat Enterprise Linux Atomic Host. For an overview of general Linux
Containers concept and their current capabilities implemented in Red Hat Enterprise Linux 7, see

Overview of Containers in Red Hat Systems. The topics related to containers management and
administration are described in the Red Hat Enterprise Linux Atomic Host 7 Managing Containers
guide.

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Table of Contents
PART I. BASIC SYSTEM CONFIGURATION
CHAPTER 1. GETTING STARTED
What web console Is and Which Tasks It Can Be Used For
1.1. BASIC CONFIGURATION OF THE ENVIRONMENT
1.1.1. Introduction to Configuring the Date and Time
Displaying the Current Date and Time
1.1.2. Introduction to Configuring the System Locale
1.1.3. Introduction to Configuring the Keyboard Layout
1.2. CONFIGURING AND INSPECTING NETWORK ACCESS
1.2.1. Configuring Network Access During the Installation Process
1.2.2. Managing Network Connections After the Installation Process Using nmcli
1.2.3. Managing Network Connections After the Installation Process Using nmtui
1.2.4. Managing Networking in web console
1.3. THE BASICS OF REGISTERING THE SYSTEM AND MANAGING SUBSCRIPTIONS
1.3.1. What Red Hat Subscriptions Are and Which Tasks They Can Be Used For
1.3.2. Registering the System During the Installation
1.3.3. Registering the System after the Installation
1.3.4. Registering a System to EUS Content
1.3.5. Registering a System to E4S Content
1.4. INSTALLING SOFTWARE
1.4.1. Prerequisites for Software Installation
1.4.2. Introduction to the System of Software Packaging and Software Repositories
1.4.3. Managing Basic Software Installation Tasks with Subscription Manager and Yum
1.5. MAKING SYSTEMD SERVICES START AT BOOT TIME
1.5.1. Enabling or Disabling the Services
1.5.2. Managing Services in web console
1.5.3. Additional Resources on systemd Services
1.6. ENHANCING SYSTEM SECURITY WITH A FIREWALL, SELINUX AND SSH LOGINGS
1.6.1. Ensuring the Firewall Is Enabled and Running
1.6.1.1. What a Firewall Is and How It Enhances System Security
1.6.1.2. Re-enabling the firewalld Service
1.6.2. Ensuring the Appropriate State of SELinux
1.6.2.1. What SELinux Is and How It Enhances System Security
SELinux States
SELinux Modes
1.6.2.2. Ensuring the Required State of SELinux
1.6.2.3. Managing SELinux in web console
1.6.3. Using SSH-based Authentication
1.6.3.1. What SSH-based Authentication Is and How It Enhances System Security
1.6.3.2. Establishing an SSH Connection
1.6.3.3. Disabling SSH Root Login
1.7. THE BASICS OF MANAGING USER ACCOUNTS
Normal and System Accounts
What Groups Are and Which Purposes They Can Be Used For
1.7.1. The Most Basic Command-Line Tools to Manage User Accounts and Groups
1.7.2. Managing User Accounts in web console
1.8. DUMPING THE CRASHED KERNEL USING THE KDUMP MECHANISM
1.8.1. What kdump Is and Which Tasks It Can Be Used For
1.8.2. Enabling and Activating kdump During the Installation Process
1.8.3. Ensuring That kdump Is Installed and Enabled after the Installation Process
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1.8.4. Configuring kdump in web console
1.8.5. Additional Resources on kdump
1.9. PERFORMING SYSTEM RESCUE AND CREATING SYSTEM BACKUP WITH REAR
1.9.1. What ReaR Is and Which Tasks It Can Be Used For
1.9.2. Quickstart to Installation and Configuration of ReaR
1.9.3. Quickstart to Creation of the Rescue System with ReaR
1.9.4. Quickstart to Configuration of ReaR with the Backup Software
1.10. USING THE LOG FILES TO TROUBLESHOOT PROBLEMS
1.10.1. Services Handling the syslog Messages
1.10.2. Subdirectories Storing the syslog Messages
1.11. ACCESSING RED HAT SUPPORT
1.11.1. Obtaining Red Hat Support Through Red Hat Customer Portal
1.11.1.1. What the Red Hat Support Tool Is and Which Tasks It Can Be Used For
1.11.2. Using the SOS Report to Troubleshoot Problems
CHAPTER 2. SYSTEM LOCALE AND KEYBOARD CONFIGURATION
2.1. SETTING THE SYSTEM LOCALE
2.1.1. Displaying the Current Status
2.1.2. Listing Available Locales
2.1.3. Setting the Locale
2.1.4. Making System Locale Settings Permanent when Installing with Kickstart
2.2. CHANGING THE KEYBOARD LAYOUT
2.2.1. Displaying the Current Settings
2.2.2. Listing Available Keymaps
2.2.3. Setting the Keymap
2.3. ADDITIONAL RESOURCES
Installed Documentation
See Also
CHAPTER 3. CONFIGURING THE DATE AND TIME
3.1. USING THE TIMEDATECTL COMMAND
3.1.1. Displaying the Current Date and Time
3.1.2. Changing the Current Time
3.1.3. Changing the Current Date
3.1.4. Changing the Time Zone
3.1.5. Synchronizing the System Clock with a Remote Server
3.2. USING THE DATE COMMAND
3.3. USING THE HWCLOCK COMMAND
3.3.2. Setting the Date and Time
3.3.3. Synchronizing the Date and Time
See Also
CHAPTER 4. MANAGING USERS AND GROUPS
4.1. INTRODUCTION TO USERS AND GROUPS
Reserved User and Group IDs
4.1.1. User Private Groups
4.1.2. Shadow Passwords
4.2. MANAGING USERS IN A GRAPHICAL ENVIRONMENT
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4.2.1. Using the Users Settings Tool
4.3. USING COMMAND-LINE TOOLS
4.3.1. Adding a New User
4.3.2. Adding a New Group
4.3.3. Adding an Existing User to an Existing Group
4.3.4. Creating Group Directories
4.3.5. Setting Default Permissions for New Files Using umask
What umask consists of
How umask works
4.3.5.1. Managing umask in Shells
Displaying the current mask
Setting mask in shell using umask
Working with the default shell umask
Working with the default shell umask of a specific user
Setting default permissions for newly created home directories
Online Documentation
See Also
CHAPTER 5. ACCESS CONTROL LISTS
5.1. MOUNTING FILE SYSTEMS
5.1.1. NFS
5.2. SETTING ACCESS ACLS
5.3. SETTING DEFAULT ACLS
5.4. RETRIEVING ACLS
5.5. ARCHIVING FILE SYSTEMS WITH ACLS
5.6. COMPATIBILITY WITH OLDER SYSTEMS
5.7. ACL REFERENCES
CHAPTER 6. GAINING PRIVILEGES
6.1. CONFIGURING ADMINISTRATIVE ACCESS USING THE SU UTILITY
6.2. CONFIGURING ADMINISTRATIVE ACCESS USING THE SUDO UTILITY
See Also
PART II. SUBSCRIPTION AND SUPPORT
CHAPTER 7. REGISTERING THE SYSTEM AND MANAGING SUBSCRIPTIONS
7.1. REGISTERING THE SYSTEM AND ATTACHING SUBSCRIPTIONS
7.2. MANAGING SOFTWARE REPOSITORIES
7.3. REMOVING SUBSCRIPTIONS
Related Books
See Also
CHAPTER 8. ACCESSING SUPPORT USING THE RED HAT SUPPORT TOOL
8.1. INSTALLING THE RED HAT SUPPORT TOOL
8.2. REGISTERING THE RED HAT SUPPORT TOOL USING THE COMMAND LINE
8.3. USING THE RED HAT SUPPORT TOOL IN INTERACTIVE SHELL MODE
8.4. CONFIGURING THE RED HAT SUPPORT TOOL
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8.4.1. Saving Settings to the Configuration Files
8.5. OPENING AND UPDATING SUPPORT CASES USING INTERACTIVE MODE
8.6. VIEWING SUPPORT CASES ON THE COMMAND LINE
PART III. INSTALLING AND MANAGING SOFTWARE
CHAPTER 9. YUM
9.1. CHECKING FOR AND UPDATING PACKAGES
9.1.1. Checking For Updates
9.1.2. Updating Packages
Updating a Single Package
9.1.3. Upgrading the System Off-line with ISO and Yum
9.2. WORKING WITH PACKAGES
9.2.1. Searching Packages
9.2.2. Listing Packages
9.2.3. Displaying Package Information
9.2.4. Installing Packages
9.2.5. Downloading Packages
9.2.6. Removing Packages
9.3. WORKING WITH PACKAGE GROUPS
9.3.1. Listing Package Groups
9.3.2. Installing a Package Group
9.3.3. Removing a Package Group
9.4. WORKING WITH TRANSACTION HISTORY
9.4.1. Listing Transactions
9.4.2. Examining Transactions
9.4.3. Reverting and Repeating Transactions
9.4.4. Starting New Transaction History
9.5. CONFIGURING YUM AND YUM REPOSITORIES
9.5.1. Setting [main] Options
9.5.2. Setting [repository] Options
9.5.3. Using Yum Variables
9.5.4. Viewing the Current Configuration
9.5.5. Adding, Enabling, and Disabling a Yum Repository
9.5.6. Creating a Yum Repository
9.5.6.1. Adding packages to an already created yum repository
9.5.7. Adding the Optional and Supplementary Repositories
9.6. YUM PLUG-INS
9.6.1. Enabling, Configuring, and Disabling Yum Plug-ins
9.6.2. Installing Additional Yum Plug-ins
9.6.3. Working with Yum Plug-ins
9.7. AUTOMATICALLY REFRESHING PACKAGE DATABASE AND DOWNLOADING UPDATES WITH YUM-
CRON
9.7.1. Enabling Automatic Installation of Updates
9.7.2. Setting up Optional Email Notifications
9.7.3. Enabling or Disabling Specific Repositories
9.7.4. Testing Yum-cron Settings
9.7.5. Disabling Yum-cron messages
9.7.6. Automatically Cleaning Packages
Online Resources
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See Also
PART IV. INFRASTRUCTURE SERVICES
CHAPTER 10. MANAGING SERVICES WITH SYSTEMD
10.1. INTRODUCTION TO SYSTEMD
Overriding the Default systemd Configuration Using system.conf
10.1.1. Main Features
10.1.2. Compatibility Changes
10.2. MANAGING SYSTEM SERVICES
Specifying Service Units
Behavior of systemctl in a chroot Environment
10.2.1. Listing Services
10.2.2. Displaying Service Status
10.2.3. Starting a Service
10.2.4. Stopping a Service
10.2.5. Restarting a Service
10.2.6. Enabling a Service
10.2.7. Disabling a Service
10.2.8. Starting a Conflicting Service
10.3. WORKING WITH SYSTEMD TARGETS
10.3.1. Viewing the Default Target
10.3.2. Viewing the Current Target
10.3.3. Changing the Default Target
10.3.4. Changing the Current Target
10.3.5. Changing to Rescue Mode
10.3.6. Changing to Emergency Mode
10.4. SHUTTING DOWN, SUSPENDING, AND HIBERNATING THE SYSTEM
10.4.1. Shutting Down the System
Using systemctl Commands
Using the shutdown Command
10.4.2. Restarting the System
10.4.3. Suspending the System
10.4.4. Hibernating the System
10.5. CONTROLLING SYSTEMD ON A REMOTE MACHINE
10.6. CREATING AND MODIFYING SYSTEMD UNIT FILES
10.6.1. Understanding the Unit File Structure
10.6.2. Creating Custom Unit Files
10.6.3. Converting SysV Init Scripts to Unit Files
Finding the Service Description
Finding Service Dependencies
Finding Default Targets of the Service
Finding Files Used by the Service
10.6.4. Modifying Existing Unit Files
Extending the Default Unit Configuration
Overriding the Default Unit Configuration
Monitoring Overriden Units
10.6.5. Working with Instantiated Units
10.7. ADDITIONAL CONSIDERATIONS WHILE MANAGING SERVICES
See Also
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CHAPTER 11. CONFIGURING A SYSTEM FOR ACCESSIBILITY
11.1. CONFIGURING THE BRLTTY SERVICE
Enable the brltty Service
Authorize Users to Use the Braille Display
Set the Braille Driver
Set the Braille Device
Set Specific Parameters for Particular Braille Displays
Set the Text Table
Set the Contraction Table
11.2. SWITCH ON ALWAYS SHOW UNIVERSAL ACCESS MENU
11.3. ENABLING THE FESTIVAL SPEECH SYNTHESIS SYSTEM
Choose a Voice for Festival
CHAPTER 12. OPENSSH
12.1. THE SSH PROTOCOL
12.1.1. Why Use SSH?
12.1.3. Protocol Versions
12.1.4. Event Sequence of an SSH Connection
12.1.4.1. Transport Layer
12.1.4.2. Authentication
12.1.4.3. Channels
12.2. CONFIGURING OPENSSH
12.2.1. Configuration Files
12.2.2. Starting an OpenSSH Server
12.2.3. Requiring SSH for Remote Connections
12.2.4. Using Key-based Authentication
12.2.4.1. Generating Key Pairs
12.2.4.2. Configuring ssh-agent
12.3. OPENSSH CLIENTS
12.3.1. Using the ssh Utility
12.3.2. Using the scp Utility
12.3.3. Using the sftp Utility
12.4. MORE THAN A SECURE SHELL
12.4.1. X11 Forwarding
12.4.2. Port Forwarding
See Also
CHAPTER 13. TIGERVNC
13.1. VNC SERVER
13.1.1. Installing VNC Server
13.1.2. Configuring VNC Server
13.1.2.1. Configuring VNC Server for Two Users
13.1.3. Starting VNC Server
13.1.3.1. Configuring VNC Server for Two Users and Two Different Displays
13.1.4. VNC setup based on xinetd with XDMCP for GDM
13.1.5. Terminating a VNC Session
13.2. SHARING AN EXISTING DESKTOP
13.3. VNC VIEWER
13.3.1. Installing VNC Viewer
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13.3.2. Connecting to VNC Server
13.3.2.1. Configuring the Firewall for VNC
13.3.3. Connecting to VNC Server Using SSH
Restricting VNC Access
PART V. SERVERS
CHAPTER 14. WEB SERVERS
14.1. THE APACHE HTTP SERVER
14.1.1. Notable Changes
14.1.2. Updating the Configuration
14.1.3. Running the httpd Service
14.1.3.1. Starting the Service
14.1.3.2. Stopping the Service
14.1.3.3. Restarting the Service
14.1.3.4. Verifying the Service Status
14.1.4. Editing the Configuration Files
14.1.5. Working with Modules
14.1.5.1. Loading a Module
14.1.5.2. Writing a Module
14.1.6. Setting Up Virtual Hosts
14.1.7. Setting Up an SSL Server
14.1.7.1. An Overview of Certificates and Security
14.1.8. Enabling the mod_ssl Module
14.1.8.1. Enabling and Disabling SSL and TLS in mod_ssl
14.1.9. Enabling the mod_nss Module
14.1.9.1. Enabling and Disabling SSL and TLS in mod_nss
14.1.10. Using an Existing Key and Certificate
14.1.11. Generating a New Key and Certificate
14.1.12. Configure the Firewall for HTTP and HTTPS Using the Command Line
14.1.12.1. Checking Network Access for Incoming HTTPS and HTTPS Using the Command Line
14.1.13. Additional Resources
Installable Documentation
CHAPTER 15. MAIL SERVERS
15.1. EMAIL PROTOCOLS
15.1.1. Mail Transport Protocols
15.1.1.1. SMTP
15.1.2. Mail Access Protocols
15.1.2.1. POP
15.1.2.2. IMAP
15.1.2.3. Dovecot
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
15.3.1.1. The Default Postfix Installation
15.3.1.2. Upgrading From a Previous Release
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15.3.1.3. Basic Postfix Configuration
15.3.1.4. Using Postfix with LDAP
15.3.1.4.1. The /etc/aliases lookup example
15.3.2. Sendmail
15.3.2.1. Purpose and Limitations
15.3.2.2. The Default Sendmail Installation
15.3.2.3. Common Sendmail Configuration Changes
15.3.2.4. Masquerading
15.3.2.5. Stopping Spam
15.3.2.6. Using Sendmail with LDAP
15.3.3. Fetchmail
15.3.3.1. Fetchmail Configuration Options
15.3.3.2. Global Options
15.3.3.3. Server Options
15.3.3.4. User Options
15.3.3.5. Fetchmail Command Options
15.3.3.6. Informational or Debugging Options
15.3.3.7. Special Options
15.3.4. Mail Transport Agent (MTA) Configuration
15.4. MAIL DELIVERY AGENTS
15.4.1. Procmail Configuration
15.4.2. Procmail Recipes
15.4.2.1. Delivering vs. Non-Delivering Recipes
15.4.2.2. Flags
15.4.2.3. Specifying a Local Lockfile
15.4.2.4. Special Conditions and Actions
15.4.2.5. Recipe Examples
15.4.2.6. Spam Filters
15.5. MAIL USER AGENTS
15.5.1. Securing Communication
15.5.1.1. Secure Email Clients
15.5.1.2. Securing Email Client Communications
15.6. CONFIGURING MAIL SERVER WITH ANTISPAM AND ANTIVIRUS
15.6.1. Configuring Spam Filtering for Mail Transport Agent or Mail Delivery Agent
15.6.1.1. Configuring Spam Filtering in a Mail Transport Agent
15.6.1.2. Configuring Spam Filtering in a Mail Delivery Agent
15.6.2. Configuring Antivirus Protection
15.6.3. Using the EPEL Repository to install Antispam and Antivirus Software
15.7.1. Installed Documentation
15.7.2. Online Documentation
15.7.3. Related Books
CHAPTER 16. FILE AND PRINT SERVERS
16.1. SAMBA
16.1.1. The Samba Services
16.1.2. Verifying the smb.conf File by Using the testparm Utility
16.1.3. Understanding the Samba Security Modes
16.1.4. Setting up Samba as a Standalone Server
16.1.4.1. Setting up the Server Configuration for the Standalone Server
16.1.4.2. Creating and Enabling Local User Accounts
16.1.5. Setting up Samba as a Domain Member
16.1.5.1. Joining a Domain
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16.1.5.2. Verifying That Samba Was Correctly Joined As a Domain Member
Verifying That the Operating System Can Retrieve Domain User Accounts and Groups
Verifying If AD Domain Users Can Obtain a Kerberos Ticket
Listing the Available Domains
16.1.5.3. Understanding ID Mapping
16.1.5.3.1. Planning ID Ranges
16.1.5.3.2. The * Default Domain
16.1.5.4. The Different ID Mapping Back Ends
16.1.5.4.1. Using the tdb ID Mapping Back End
16.1.5.4.2. Using the ad ID Mapping Back End
16.1.5.4.3. Using the rid ID Mapping Back End
16.1.5.4.4. Using the autorid ID Mapping Back End
16.1.6. Configuring File Shares on a Samba Server
16.1.6.1. Setting up a Share That Uses POSIX ACLs
16.1.6.1.1. Adding a Share That Uses POSIX ACLs
16.1.6.1.2. Setting ACLs
16.1.6.1.3. Setting Permissions on a Share
16.1.6.2. Setting up a Share That Uses Windows ACLs
16.1.6.2.1. Granting the SeDiskOperatorPrivilege Privilege
16.1.6.2.2. Enabling Windows ACL Support
16.1.6.2.3. Adding a Share That Uses Windows ACLs
16.1.6.2.4. Managing Share Permissions and File System ACLs of a Share That Uses Windows ACLs
16.1.6.3. Managing ACLs on an SMB Share Using smbcacls
16.1.6.3.1. Understanding Access Control Entries
16.1.6.3.2. Displaying ACLs Using smbcacls
16.1.6.3.3. Calculating an ACE Mask
16.1.6.3.4. Adding, Updating, And Removing an ACL Using smbcacls
16.1.6.4. Enabling Users to Share Directories on a Samba Server
16.1.6.4.1. Enabling the User Shares Feature
16.1.6.4.2. Adding a User Share
16.1.6.4.3. Updating Settings of a User Share
16.1.6.4.4. Displaying Information About Existing User Shares
16.1.6.4.5. Listing User Shares
16.1.6.4.6. Deleting a User Share
16.1.6.5. Enabling Guest Access to a Share
16.1.7. Setting up a Samba Print Server
16.1.7.1. The Samba spoolssd Service
16.1.7.2. Enabling Print Server Support in Samba
16.1.7.3. Manually Sharing Specific Printers
16.1.7.4. Setting up Automatic Printer Driver Downloads for Windows Clients
16.1.7.4.1. Basic Information about Printer Drivers
16.1.7.4.2. Enabling Users to Upload and Preconfigure Drivers
16.1.7.4.3. Setting up the print$ Share
16.1.7.4.4. Creating a GPO to Enable Clients to Trust the Samba Print Server
16.1.7.4.5. Uploading Drivers and Preconfiguring Printers
16.1.8. Tuning the Performance of a Samba Server
16.1.8.1. Setting the SMB Protocol Version
16.1.8.2. Tuning Shares with Directories That Contain a Large Number of Files
16.1.8.3. Settings That Can Have a Negative Performance Impact
16.1.9. Frequently Used Samba Command-line Utilities
16.1.9.1. Using the net Utility
16.1.9.1.1. Using the net ads join and net rpc join Commands
16.1.9.1.2. Using the net rpc rights Command
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16.1.9.1.3. Using the net rpc share Command
16.1.9.1.4. Using the net user Command
16.1.9.1.5. Using the net usershare Command
16.1.9.2. Using the rpcclient Utility
16.1.9.3. Using the samba-regedit Application
16.1.9.4. Using the smbcacls Utility
16.1.9.5. Using the smbclient Utility
16.1.9.5.1. Using smbclient in Interactive Mode
16.1.9.5.2. Using smbclient in Scripting Mode
16.1.9.6. Using the smbcontrol Utility
16.1.9.7. Using the smbpasswd Utility
16.1.9.8. Using the smbstatus Utility
16.1.9.9. Using the smbtar Utility
16.1.9.10. Using the testparm Utility
16.1.9.11. Using the wbinfo Utility
16.2. FTP
16.2.1. The File Transfer Protocol
16.2.2. The vsftpd Server
16.2.2.1. Starting and Stopping vsftpd
16.2.2.2. Starting Multiple Copies of vsftpd
16.2.2.3. Encrypting vsftpd Connections Using TLS
16.2.2.4. SELinux Policy for vsftpd
16.2.3.1. Installed Documentation
16.2.3.2. Online Documentation
16.3. PRINT SETTINGS
16.3.1. Starting the Print Settings Configuration Tool
16.3.2. Starting Printer Setup
16.3.3. Adding a Local Printer
16.3.4. Adding an AppSocket/HP JetDirect printer
16.3.5. Adding an IPP Printer
16.3.6. Adding an LPD/LPR Host or Printer
16.3.7. Adding a Samba (SMB) printer
16.3.8. Selecting the Printer Model and Finishing
16.3.9. Printing a Test Page
16.3.10. Modifying Existing Printers
16.3.10.1. The Settings Page
16.3.10.2. The Policies Page
16.3.10.2.1. Sharing Printers
16.3.10.2.2. The Access Control Page
16.3.10.2.3. The Printer Options Page
16.3.10.2.4. Job Options Page
16.3.10.2.5. Ink/Toner Levels Page
16.3.10.3. Managing Print Jobs
CHAPTER 17. DATABASE SERVERS
17.1. MARIADB
17.1.1. Installing the MariaDB server
17.1.1.1. Improving MariaDB installation security
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17.1.2. Configuring the MariaDB server for networking
17.1.3. Backing up MariaDB data
17.1.3.1. Logical back up
17.1.3.2. Physical back up
CHAPTER 18. CONFIGURING NTP USING THE CHRONY SUITE
18.1. INTRODUCTION TO THE CHRONY SUITE
18.1.1. Differences Between ntpd and chronyd
18.1.2. Choosing Between NTP Daemons
18.2. UNDERSTANDING CHRONY AND ITS CONFIGURATION
18.2.1. Understanding chronyd and chronyc
18.2.2. Understanding the chrony Configuration Commands
18.2.3. Security with chronyc
18.3. USING CHRONY
18.3.1. Installing chrony
18.3.2. Checking the Status of chronyd
18.3.3. Starting chronyd
18.3.4. Stopping chronyd
18.3.5. Checking if chrony is Synchronized
18.3.5.1. Checking chrony Tracking
18.3.5.2. Checking chrony Sources
18.3.5.3. Checking chrony Source Statistics
18.3.6. Manually Adjusting the System Clock
18.4. SETTING UP CHRONY FOR DIFFERENT ENVIRONMENTS
18.4.1. Setting Up chrony for a System in an Isolated Network
18.5. USING CHRONYC
18.5.1. Using chronyc to Control chronyd
18.6. CHRONY WITH HW TIMESTAMPING
18.6.1. Understanding Hardware Timestamping
18.6.2. Verifying Support for Hardware Timestamping
18.6.3. Enabling Hardware Timestamping
18.6.4. Configuring Client Polling Interval
18.6.5. Enabling Interleaved Mode
18.6.6. Configuring Server for Large Number of Clients
18.6.7. Verifying Hardware Timestamping
18.6.8. Configuring PTP-NTP bridge
CHAPTER 19. CONFIGURING NTP USING NTPD
19.1. INTRODUCTION TO NTP
19.2. NTP STRATA
19.3. UNDERSTANDING NTP
19.4. UNDERSTANDING THE DRIFT FILE
19.5. UTC, TIMEZONES, AND DST
19.6. AUTHENTICATION OPTIONS FOR NTP
19.7. MANAGING THE TIME ON VIRTUAL MACHINES
19.8. UNDERSTANDING LEAP SECONDS
19.9. UNDERSTANDING THE NTPD CONFIGURATION FILE
19.10. UNDERSTANDING THE NTPD SYSCONFIG FILE
19.11. DISABLING CHRONY
19.12. CHECKING IF THE NTP DAEMON IS INSTALLED
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19.13. INSTALLING THE NTP DAEMON (NTPD)
19.14. CHECKING THE STATUS OF NTP
19.15. CONFIGURE THE FIREWALL TO ALLOW INCOMING NTP PACKETS
19.15.1. Change the Firewall Settings
19.15.2. Open Ports in the Firewall for NTP Packets
19.16. CONFIGURE NTPDATE SERVERS
19.17. CONFIGURE NTP
19.17.1. Configure Access Control to an NTP Service
19.17.2. Configure Rate Limiting Access to an NTP Service
19.17.3. Adding a Peer Address
19.17.4. Adding a Server Address
19.17.5. Adding a Broadcast or Multicast Server Address
19.17.6. Adding a Manycast Client Address
19.17.7. Adding a Broadcast Client Address
19.17.8. Adding a Manycast Server Address
19.17.9. Adding a Multicast Client Address
19.17.10. Configuring the Burst Option
19.17.11. Configuring the iburst Option
19.17.12. Configuring Symmetric Authentication Using a Key
19.17.13. Configuring the Poll Interval
19.17.14. Configuring Server Preference
19.17.15. Configuring the Time-to-Live for NTP Packets
19.17.16. Configuring the NTP Version to Use
19.18. CONFIGURING THE HARDWARE CLOCK UPDATE
19.19. CONFIGURING CLOCK SOURCES
19.20.2. Useful Websites
CHAPTER 20. CONFIGURING PTP USING PTP4L
20.1. INTRODUCTION TO PTP
20.1.1. Understanding PTP
20.1.2. Advantages of PTP
20.2. USING PTP
20.2.1. Checking for Driver and Hardware Support
20.2.2. Installing PTP
20.2.3. Starting ptp4l
Starting ptp4l as a Service
Using ptp4l From The Command Line
Logging Messages From ptp4l
20.2.3.1. Selecting a Delay Measurement Mechanism
20.3. USING PTP WITH MULTIPLE INTERFACES
20.4. SPECIFYING A CONFIGURATION FILE
20.5. USING THE PTP MANAGEMENT CLIENT
20.6. SYNCHRONIZING THE CLOCKS
20.7. VERIFYING TIME SYNCHRONIZATION
20.8. SERVING PTP TIME WITH NTP
20.9. SERVING NTP TIME WITH PTP
20.10. SYNCHRONIZE TO PTP OR NTP TIME USING TIMEMASTER
20.10.1. Starting timemaster as a Service
20.10.2. Understanding the timemaster Configuration File
20.10.3. Configuring timemaster Options
20.11. IMPROVING ACCURACY
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PART VI. MONITORING AND AUTOMATION
CHAPTER 21. SYSTEM MONITORING TOOLS
21.1. VIEWING SYSTEM PROCESSES
21.1.1. Using the ps Command
21.1.2. Using the top Command
21.1.3. Using the System Monitor Tool
21.2. VIEWING MEMORY USAGE
21.2.1. Using the free Command
21.3. VIEWING CPU USAGE
21.4. VIEWING BLOCK DEVICES AND FILE SYSTEMS
21.4.1. Using the lsblk Command
21.4.2. Using the blkid Command
21.4.3. Using the findmnt Command
21.4.4. Using the df Command
21.4.5. Using the du Command
21.5. VIEWING HARDWARE INFORMATION
21.5.1. Using the lspci Command
21.5.2. Using the lsusb Command
21.5.3. Using the lscpu Command
21.6. CHECKING FOR HARDWARE ERRORS
21.7. MONITORING PERFORMANCE WITH NET-SNMP
21.7.1. Installing Net-SNMP
21.7.2. Running the Net-SNMP Daemon
21.7.3. Configuring Net-SNMP
21.7.3.1. Setting System Information
21.7.3.2. Configuring Authentication
Configuring SNMP Version 2c Community
Configuring SNMP Version 3 User
21.7.4. Retrieving Performance Data over SNMP
21.7.4.1. Hardware Configuration
21.7.4.2. CPU and Memory Information
21.7.4.3. File System and Disk Information
21.7.4.4. Network Information
21.7.5. Extending Net-SNMP
21.7.5.1. Extending Net-SNMP with Shell Scripts
21.7.5.2. Extending Net-SNMP with Perl
CHAPTER 22. OPENLMI
22.1. ABOUT OPENLMI
22.1.2. Management Capabilities
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22.2. INSTALLING OPENLMI
22.2.1. Installing OpenLMI on a Managed System
22.2.2. Installing OpenLMI on a Client System
22.3. CONFIGURING SSL CERTIFICATES FOR OPENPEGASUS
22.3.1. Managing Self-signed Certificates
22.3.2. Managing Authority-signed Certificates with Identity Management (Recommended)
22.3.3. Managing Authority-signed Certificates Manually
22.4. USING LMISHELL
22.4.1. Starting, Using, and Exiting LMIShell
Starting LMIShell in Interactive Mode
Using Tab Completion
Browsing History
Handling Exceptions
Configuring a Temporary Cache
Exiting LMIShell
Running an LMIShell Script
22.4.2. Connecting to a CIMOM
Connecting to a Remote CIMOM
Connecting to a Local CIMOM
Verifying a Connection to a CIMOM
22.4.3. Working with Namespaces
Listing Available Namespaces
Accessing Namespace Objects
22.4.4. Working with Classes
Listing Available Classes
Accessing Class Objects
Examining Class Objects
Listing Available Methods
Listing Available Properties
Listing and Viewing ValueMap Properties
Fetching a CIMClass Object
22.4.5. Working with Instances
Accessing Instances
Examining Instances
Creating New Instances
Deleting Individual Instances
Listing and Accessing Available Properties
Listing and Using Available Methods
Listing and Viewing ValueMap Parameters
Refreshing Instance Objects
Displaying MOF Representation
22.4.6. Working with Instance Names
Accessing Instance Names
Examining Instance Names
Creating New Instance Names
Listing and Accessing Key Properties
Converting Instance Names to Instances
22.4.7. Working with Associated Objects
Accessing Associated Instances
Accessing Associated Instance Names
22.4.8. Working with Association Objects
Accessing Association Instances
Accessing Association Instance Names
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22.4.9. Working with Indications
Subscribing to Indications
Listing Subscribed Indications
Unsubscribing from Indications
Implementing an Indication Handler
22.4.10. Example Usage
Using the OpenLMI Service Provider
Using the OpenLMI Networking Provider
Using the OpenLMI Storage Provider
Using the OpenLMI Hardware Provider
22.5. USING OPENLMI SCRIPTS
See Also
CHAPTER 23. VIEWING AND MANAGING LOG FILES
23.1. LOCATING LOG FILES
23.2. BASIC CONFIGURATION OF RSYSLOG
23.2.1. Filters
23.2.2. Actions
Specifying Multiple Actions
23.2.3. Templates
Generating Dynamic File Names
Properties
Template Examples
23.2.4. Global Directives
23.2.5. Log Rotation
23.2.6. Increasing the Limit of Open Files
23.3. USING THE NEW CONFIGURATION FORMAT
23.3.1. Rulesets
23.3.2. Compatibility with sysklogd
23.4. WORKING WITH QUEUES IN RSYSLOG
23.4.1. Defining Queues
Direct Queues
Disk Queues
In-memory Queues
Disk-Assisted In-memory Queues
23.4.2. Creating a New Directory for rsyslog Log Files
23.4.3. Managing Queues
Limiting Queue Size
Discarding Messages
Using Timeframes
Configuring Worker Threads
Batch Dequeuing
Terminating Queues
23.4.4. Using the New Syntax for rsyslog queues
23.5. CONFIGURING RSYSLOG ON A LOGGING SERVER
23.5.1. Using The New Template Syntax on a Logging Server
23.6. USING RSYSLOG MODULES
23.6.1. Importing Text Files
23.6.2. Exporting Messages to a Database
23.6.3. Enabling Encrypted Transport
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Configuring Encrypted Message Transfer with TLS
Configuring Encrypted Message Transfer with GSSAPI
23.6.4. Using RELP
Configuring RELP
Configuring RELP with TLS
23.7. INTERACTION OF RSYSLOG AND JOURNAL
23.8. STRUCTURED LOGGING WITH RSYSLOG
23.8.1. Importing Data from Journal
23.8.2. Filtering Structured Messages
23.8.3. Parsing JSON
23.8.4. Storing Messages in the MongoDB
23.9. DEBUGGING RSYSLOG
23.10. USING THE JOURNAL
23.10.1. Viewing Log Files
23.10.2. Access Control
23.10.3. Using The Live View
23.10.4. Filtering Messages
Filtering by Priority
Filtering by Time
Advanced Filtering
23.10.5. Enabling Persistent Storage
23.11. MANAGING LOG FILES IN A GRAPHICAL ENVIRONMENT
23.11.2. Adding a Log File
23.11.3. Monitoring Log Files
See Also
CHAPTER 24. AUTOMATING SYSTEM TASKS
24.1. SCHEDULING A RECURRING JOB USING CRON
24.1.1. Prerequisites for Cron Jobs
24.1.2. Scheduling a Cron Job
Scheduling a Job as root User
Scheduling a Job as Non-root User
Scheduling Hourly, Daily, Weekly, and Monthly Jobs
24.2. SCHEDULING A RECURRING ASYNCHRONOUS JOB USING ANACRON
24.2.1. Prerequisites for Anacrob Jobs
24.2.2. Scheduling an Anacron Job
Scheduling an anacron Job as root User
Scheduling Hourly, Daily, Weekly, and Monthly Jobs
24.3. SCHEDULING A JOB TO RUN AT A SPECIFIC TIME USING AT
24.3.1. Prerequisites for At Jobs
24.3.2. Scheduling an At Job
Viewing Pending Jobs
Deleting a Scheduled Job
24.3.2.1. Controlling Access to At and Batch
24.4. SCHEDULING A JOB TO RUN ON SYSTEM LOAD DROP USING BATCH
24.4.1. Prerequisites for Batch Jobs
24.4.2. Scheduling a Batch Job
Changing the Default System Load Average Limit
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Viewing Pending Jobs
Deleting a Scheduled Job
Controlling Access to Batch
24.5. SCHEDULING A JOB TO RUN ON NEXT BOOT USING A SYSTEMD UNIT FILE
CHAPTER 25. AUTOMATIC BUG REPORTING TOOL (ABRT)
25.1. INTRODUCTION TO ABRT
25.2. INSTALLING ABRT AND STARTING ITS SERVICES
25.2.1. Installing the ABRT GUI
25.2.2. Installing ABRT for the Command Line
25.2.3. Installing Supplementary ABRT Tools
25.2.4. Starting the ABRT Services
25.2.5. Testing ABRT Crash Detection
25.3. CONFIGURING ABRT
25.3.1. Configuring Events
25.3.2. Creating Custom Events
25.3.3. Setting Up Automatic Reporting
25.4. DETECTING SOFTWARE PROBLEMS
25.4.1. Detecting C and C++ Crashes
25.4.2. Detecting Python Exceptions
25.4.3. Detecting Ruby Exceptions
25.4.4. Detecting Java Exceptions
25.4.5. Detecting X.Org Crashes
25.4.6. Detecting Kernel Oopses and Panics
25.5. HANDLING DETECTED PROBLEMS
25.5.1. Using the Command Line Tool
25.5.2. Using the GUI
See Also
PART VII. KERNEL CUSTOMIZATION WITH BOOTLOADER
CHAPTER 26. WORKING WITH GRUB 2
26.1. INTRODUCTION TO GRUB 2
Menu Entries in grub.cfg
26.2. CONFIGURING GRUB 2
26.3. MAKING TEMPORARY CHANGES TO A GRUB 2 MENU
26.4. MAKING PERSISTENT CHANGES TO A GRUB 2 MENU USING THE GRUBBY TOOL
Listing the Default Kernel
Changing the Default Boot Entry
Viewing the GRUB 2 Menu Entry for a Kernel
Adding and Removing Arguments from a GRUB 2 Menu Entry
Updating All Kernel Menus with the Same Arguments
Changing a Kernel Argument
26.5. CUSTOMIZING THE GRUB 2 CONFIGURATION FILE
26.5.1. Changing the Default Boot Entry
26.5.2. Editing a Menu Entry
26.5.3. Adding a new Entry
26.5.4. Creating a Custom Menu
26.6. PROTECTING GRUB 2 WITH A PASSWORD
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Configuring GRUB 2 to Require a Password only for Modifying Entries
Configuring GRUB 2 to Require a Password for Modifying and Booting Entries
Passwords Set Before Updating to Red Hat Enterprise Linux 7.2
Additional GRUB 2 Users
26.7. REINSTALLING GRUB 2
26.7.1. Reinstalling GRUB 2 on BIOS-Based Machines
26.7.2. Reinstalling GRUB 2 on UEFI-Based Machines
26.7.3. Resetting and Reinstalling GRUB 2
26.8. UPGRADING FROM GRUB LEGACY TO GRUB 2
Upgrading from GRUB Legacy to GRUB 2 after the in-place upgrade of the operating system
Generating the GRUB 2 configuration files
Testing GRUB 2 with GRUB Legacy bootloader still installed
Replacing GRUB Legacy bootloader on systems that use BIOS
Removing GRUB Legacy on systems that use EFI
26.9. GRUB 2 OVER A SERIAL CONSOLE
26.9.1. Configuring GRUB 2 for a single boot
26.9.2. Configuring GRUB 2 for a persistent change
26.9.3. Configuring a new GRUB 2 file
26.9.4. Using screen to Connect to the Serial Console
26.10. TERMINAL MENU EDITING DURING BOOT
26.10.1. Booting to Rescue Mode
26.10.2. Booting to Emergency Mode
26.10.3. Booting to the Debug Shell
26.10.4. Changing and Resetting the Root Password
26.11. UNIFIED EXTENSIBLE FIRMWARE INTERFACE (UEFI) SECURE BOOT
26.11.1. UEFI Secure Boot Support in Red Hat Enterprise Linux 7
Restrictions Imposed by UEFI Secure Boot
Installable and External Documentation
PART VIII. SYSTEM BACKUP AND RECOVERY
CHAPTER 27. RELAX-AND-RECOVER (REAR)
27.1. BASIC REAR USAGE
27.1.1. Installing ReaR
27.1.2. Configuring ReaR
ISO-specific Configuration
27.1.3. Creating a Rescue System
27.1.4. Scheduling ReaR
27.1.5. Performing a System Rescue
27.2. INTEGRATING REAR WITH BACKUP SOFTWARE
27.2.1. The Built-in Backup Method
27.2.1.1. Configuring the Internal Backup Method
27.2.1.2. Creating a Backup Using the Internal Backup Method
27.2.2. Supported Backup Methods
27.2.3. Unsupported Backup Methods
27.2.4. Creating Multiple Backups
CHAPTER 28. CHOOSING SUITABLE RED HAT PRODUCT
CHAPTER 29. RED HAT CUSTOMER PORTAL LABS RELEVANT TO SYSTEM ADMINISTRATION
iSCSI Helper
NTP Configuration
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Samba Configuration Helper
VNC Configurator
Bridge Configuration
Network Bonding Helper
LVM RAID Calculator
NFS Helper
Load Balancer Configuration Tool
Yum Repository Configuration Helper
File System Layout Calculator
RHEL Backup and Restore Assistant
DNS Helper
AD Integration Helper (Samba FS - winbind)
Red Hat Enterprise Linux Upgrade Helper
Registration Assistant
Rescue Mode Assistant
Kernel Oops Analyzer
Kdump Helper
SCSI decoder
Red Hat Memory Analyzer
Multipath Helper
Multipath Configuration Visualizer
Red Hat I/O Usage Visualizer
Storage / LVM configuration viewer
CHAPTER 30. REVISION HISTORY
30.1. ACKNOWLEDGMENTS
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20

This part covers basic post-installation tasks and basic system administration tasks such as keyboard
configuration, date and time configuration, managing users and groups, and gaining privileges.
21

This chapter covers the basic tasks that you might need to perform just after you have installed Red Hat
Enterprise Linux 7.
Note that these items may include tasks that are usually done already during the installation process,
but they do not have to be done necessarily, such as the registration of the system. The subchapters
dealing with such tasks provide a brief summary of how this can be achieved during the installation and
links to related documentation in a special section.
For detailed information on Red Hat Enterprise Linux 7 installation, consult Red Hat Enterprise Linux 7
Installation Guide.
NOTE
This chapter mentions some commands to be performed. The commands that need to be
entered by the root user have # in the prompt, while the commands that can be
performed by a regular user, have $ in their prompt.
For further information on common post-installation tasks, you can see also Red Hat Enterprise Linux 7
Installation Guide.
Although all post-installation tasks can be achieved through the command line, you can also use the web
console tool to perform some of them.
What web console Is and Which Tasks It Can Be Used For
web console is a system administration tool that provides a user interface for monitoring and
administering servers through a web browser.
web console enables to perform these tasks:
Monitoring basic system features, such as hardware, internet connection, or performance
characteristics
Analyzing the content of the system log files
Configuring basic networking features, such as interfaces, network logs, packet sizes
Managing user accounts
Monitoring and configuring system services
Creating diagnostic reports
Setting kernel dump configuration
Configuring SELinux
Managing system subscriptions
Accessing the terminal
For more information on installing and using web console, see Managing systems using the RHEL 7 web
console.
22

1.1. BASIC CONFIGURATION OF THE ENVIRONMENT
Basic configuration of the environment includes:
Date and Time
System Locales
Keyboard Layout
Setting of these items is normally a part of the installation process.
For more information, see the appropriate source according to the installation method:
When installing with the Anaconda installer, see:
Date&Time, Language Support and Keyboard Configuration in Red Hat Enterprise Linux 7
Installation Guide
When installing with the Kickstart file, consult:
Kickstart Commands and Options in Red Hat Enterprise Linux 7 Installation Guide.
If you need to reconfigure the basic characteristics of the environment after the installation, follow the
instructions in this section.
1.1.1. Introduction to Configuring the Date and Time
Accurate time keeping is important for a number of reasons. In Red Hat Enterprise Linux 7, time keeping
is ensured by the NTP protocol, which 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.
Red Hat Enterprise Linux 7 uses the following daemons to implement NTP:
chronyd
The chronyd daemon is used by default. It is available from the chrony package. For more
information on configuring and using NTP with chronyd, see Chapter 18, Configuring NTP Using
the chrony Suite.
ntpd
The ntpd daemon is available from the ntp package. For more information on configuring and
using NTP with ntpd, see Chapter 19, Configuring NTP Using ntpd .
If you want to use ntpd instead of default chronyd, you need to disable chronyd, install, enable and
configure ntpd as shown in Chapter 19, Configuring NTP Using ntpd .
Displaying the Current Date and Time
To display the current date and time, use one of the following commands:
~]$ date
~]$ timedatectl
Note that the timedatectl command provides more verbose output, including universal time, currently
used time zone, the status of the Network Time Protocol (NTP) configuration, and some additional
information.
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For more information on configuring the date and time, see Chapter 3, Configuring the Date and Time .
1.1.2. Introduction to Configuring the System Locale
System-wide locale settings are stored in the /etc/locale.conf file, which is read at early boot by the
systemd daemon. The locale settings configured in /etc/locale.conf are inherited by every service or
user, unless individual programs or individual users override them.
Basic tasks to handle the system locales:
Listing available system locale settings:
~]$ localectl list-locales
Displaying current status of the system locales settings:
~]$ localectl status
Setting or changing the default system locale settings:
~]# localectl set-locale LANG=locale
For more information on configuring the system locale, see Chapter 2, System Locale and Keyboard
Configuration.
1.1.3. Introduction to Configuring the Keyboard Layout
The keyboard layout settings control the layout used on the text console and graphical user interfaces.
Basic tasks to handle the keyboard layout include:
Listing available keymaps:
~]$ localectl list-keymaps
Displaying current status of keymap settings:
Setting or changing the default system keymap:
~]# localectl set-keymap
For more information on configuring the keyboard layout, see Chapter 2, System Locale and Keyboard
Configuration.
1.2. CONFIGURING AND INSPECTING NETWORK ACCESS
The network access is usually configured during the installation process. However, the installation
process does not prompt you to configure network interfaces on some common installation paths.
Consequently, it is possible that the network access is not configured after the installation. If this
happens, you can configure network access after installation.
24

For a quickstart to configuring network access during the installation, see Section 1.2.1, “Configuring
Network Access During the Installation Process”. To configure network access after the installation, you
can use either the nmcli command-line utility, described in Red Hat Enterprise Linux 7 Networking
Guide or the nmtui text user interface utility, described in Red Hat Enterprise Linux 7 Networking
Guide.
The nmcli and nmtui utilities also enable you to add one or more new network connections, as well as
modify and inspect the existing connections. If you want to create and manage network connections with
nmcli, see Section 1.2.2, “Managing Network Connections After the Installation Process Using nmcli” . If
you want to create and manage network connections with nmtui, see Section 1.2.3, “Managing Network
Connections After the Installation Process Using nmtui”.
1.2.1. Configuring Network Access During the Installation Process
Ways to configure network access during the installation proces:
The Network & Hostname menu at the Installation Summary screen in the graphical user
interface of the Anaconda installation program
The Network settings option in the text mode of the Anaconda installation program
The Kickstart file
When the system boots for the first time after the installation has finished, any network interfaces which
you configured during the installation are automatically activated.
For detailed information on configuration of network access during installation process, see Red Hat
Enterprise Linux 7 Installation Guide.
1.2.2. Managing Network Connections After the Installation Process Using nmcli
Run the following commands as the root user to manage network connections using the nmcli utility.
To create a new connection:
~]# nmcli con add type type of the connection "con-name" connection name ifname ifname interface-
name the name of the interface ipv4 address ipv4 address gw4 address gateway address
To modify the existing connection:
~]# nmcli con mod "con-name"
To display all connections:
~]# nmcli con show
To display the active connection:
~]# nmcli con show --active
To display all configuration settings of a particular connection:
~]# nmcli con show "con-name"
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For more information on the nmcli command-line utility, see Red Hat Enterprise Linux 7 Networking
Guide.
1.2.3. Managing Network Connections After the Installation Process Using nmtui
The NetworkManager text user interface (TUI) utility, nmtui, provides a text interface to configure
networking by controlling NetworkManager.
For more information about installing and using the nmtui text interface tool, see Red Hat Enterprise
Linux 7 Networking Guide.
1.2.4. Managing Networking in web console
In web console, the Networking menu enables you:
To display currently received and sent packets
To display the most important characteristics of available network interfaces
To display content of the networking logs.
To add various types of network interfaces (bond, team, bridge, VLAN)
Figure 1.1. Managing Networking in web console
1.3. THE BASICS OF REGISTERING THE SYSTEM AND MANAGING
SUBSCRIPTIONS
1.3.1. What Red Hat Subscriptions Are and Which Tasks They Can Be Used For
The products installed on Red Hat Enterprise Linux 7, including the operating system itself, are covered
by subscriptions.
26

A subscription to Red Hat Content Delivery Network is used to track:
Registered systems
Products installed on those system
Subscriptions attached to those product
1.3.2. Registering the System During the Installation
This section provides a brief summary of registering Red Hat Enterprise Linux 7 during the installation
process. If your operating system is not registered after the installation, you can find what might have
been missed during the installation by reading through this section. For detailed information, consult
Red Hat Enterprise Linux 7 Installation Guide .
Basically, there are two ways to register the system during the installation:
Normally, registration is a part of the Initial Setup configuration process. For more information,
see Red Hat Enterprise Linux 7 Installation Guide .
Another option is to run Subscription manager as a post-installation script
, which performs
the automatic registration at the moment when the installation is complete and before the
system is rebooted for the first time. To ensure this, modify the %post section of the Kickstart
file. For more detailed information on running Subscription manager as a post-installation script,
see Red Hat Enterprise Linux 7 Installation Guide .
1.3.3. Registering the System after the Installation
If you have not registered your system during installation process, you can do it afterwards by applying
the following procedure. Note that all commands in this procedure need to be performed as the root
user.
Registering and Subscribing Your System
1. Register your system:
~]# subscription-manager register
The command will prompt you to enter your Red Hat Customer Portal user name and password.
2. Determine the pool ID of a subscription that you require:
~]# subscription-manager list --available
This command displays all available subscriptions for your Red Hat account. For every
subscription, various characteristics are displayed, including the pool ID.
3. Attach the appropriate subscription to your system by replacing pool_id with the pool ID
determined in the previous step:
~]# subscription-manager attach --pool=pool_id
For more information on registration of your system and attachment of the Red Hat Content Delivery
Network subscriptions, see Chapter 7, Registering the System and Managing Subscriptions .
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1.3.4. Registering a System to EUS Content
To access the Extended Update Support (EUS) content, register your system as follows:
1. Verify that EUS entitlements are available:
~]# subscription-manager list --available --matches="*Extended Update Support"
+-------------------------------------------+
Available Subscriptions
+-------------------------------------------+
Subscription Name: Extended Update Support
Provides: Red Hat Enterprise Linux High Availability for x86_64 - Extended Update
Support
Red Hat Enterprise Linux Resilient Storage for x86_64 - Extended Update
Support
Red Hat Enterprise Linux for x86_64 - Extended Update Support
Red Hat EUCJP Support (for RHEL Server) - Extended Update Support
RHEL for SAP - Extended Update Support
Red Hat Enterprise Linux Load Balancer (for RHEL Server) - Extended
Update Support
Red Hat Enterprise Linux Scalable File System (for RHEL Server) - Extended
Update Support
Red Hat CodeReady Linux Builder for x86_64 - Extended Update Support
RHEL for SAP HANA - Extended Update Support
Red Hat Enterprise Linux High Performance Networking (for RHEL Server) -
Extended Update Support
Oracle Java (for RHEL Server) - Extended Update Support
Red Hat S-JIS Support (for RHEL Server) - Extended Update Support
SKU: RH00030
Contract: 12069074
Pool ID: 8a99f9ac7238188b01723d9c8a8a06a9
Provides Management: No
Available: 8
Suggested: 0
Service Level: Layered
Service Type: L1-L3
Subscription Type: Instance Based
Starts: 05/22/2020
Ends: 05/21/2021
System Type: Physical
2. Attach the applicable subscription using the Pool identifier:
~]# subscription-manager attach --pool 8a99f9ac7238188b01723d9c8a8a06a9
3. Replace the default repositories enabled for the system with the EUS variants:
~]# subscription-manager repos --disable *
4. Enable the repositories which represent the EUS content set for the RHEL revision in use:
~]# subscription-manager repos --enable rhel-7-server-eus-rpms
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5. Select the required and supported release for the end system:
~]# subscription-manager release --set 7.6
For currently supported EUS releases, see Extended Update Support Add-on.
1.3.5. Registering a System to E4S Content
The following procedure describes how to register a system and utilize the E4S content.
1. Register your system using the following command:
~]# subscription-manager register
2. Verify that E4S entitlements are available:
~]# subscription-manager list --available --matches="*Update Services for SAP Solutions*"
+-------------------------------------------+
Available Subscriptions
+-------------------------------------------+
Subscription Name: Red Hat Enterprise Linux for SAP Solutions, Standard (Physical or
Virtual Nodes)
Provides: dotNET on RHEL Beta (for RHEL Server)
Red Hat CodeReady Linux Builder for x86_64
Red Hat Enterprise Linux for SAP HANA for x86_64
Red Hat Ansible Engine
RHEL for SAP HANA - Update Services for SAP Solutions
Red Hat Enterprise Linux Scalable File System (for RHEL Server) - Extended
Update Support
RHEL for SAP HANA - Extended Update Support
Red Hat Enterprise Linux Atomic Host Beta
Red Hat Beta
Red Hat EUCJP Support (for RHEL Server) - Extended Update Support
Red Hat Enterprise Linux High Availability for x86_64
Red Hat Enterprise Linux Load Balancer (for RHEL Server) - Extended Update
Support
dotNET on RHEL (for RHEL Server)
Red Hat CodeReady Linux Builder for x86_64 - Extended Update Support
Red Hat Enterprise Linux High Availability - Update Services for SAP Solutions
Red Hat Enterprise Linux Resilient Storage for x86_64 - Extended Update
Support
Red Hat Enterprise Linux High Availability for x86_64 - Extended Update
Support
Oracle Java (for RHEL Server)
Red Hat Enterprise Linux Server - Update Services for SAP Solutions
Red Hat Software Collections (for RHEL Server)
Red Hat Enterprise Linux Scalable File System (for RHEL Server)
Red Hat Enterprise Linux High Performance Networking (for RHEL Server) -
Extended Update Support
RHEL for SAP - Update Services for SAP Solutions
Oracle Java (for RHEL Server) - Extended Update Support
Red Hat Enterprise Linux Atomic Host
Red Hat Developer Tools (for RHEL Server)
29

Red Hat Software Collections Beta (for RHEL Server)
Red Hat Enterprise Linux Server
Red Hat Enterprise Linux for SAP Applications for x86_64
Red Hat Developer Tools Beta (for RHEL Server)
Red Hat Enterprise Linux for x86_64
Red Hat Enterprise Linux for x86_64 - Extended Update Support
RHEL for SAP - Extended Update Support
Red Hat Developer Toolset (for RHEL Server)
Red Hat S-JIS Support (for RHEL Server) - Extended Update Support
SKU: RH00764
Contract: 11977725
Pool ID: 8a85f99c6c4825eb016c4a30d3493064
Provides Management: Yes
Available: 18
Suggested: 0
Service Level: Standard
Service Type: L1-L3
Subscription Type: Instance Based
Starts: 03/29/2020
Ends: 12/31/2021
System Type: Physical
3. Attach the applicable subscription using the Pool identifier:
~]# subscription-manager attach --pool=#################
4. Replace the default repositories enabled for the system with the EUS variants:
~]# subscription-manager repos --disable="*"
5. Enable the repositories which represent the E4S content set for the RHEL revision in use:
~]# subscription-manager --enable=rhel-7-server-e4s-rpms
6. Clear the repository cache and release lock the system to a valid release for E4S which supports
your SAP application:
~]# yum clean all && subscription-manager release --set=7.7
1.4. INSTALLING SOFTWARE
This section provides information to guide you through the basics of software installation on a Red Hat
Enterprise Linux 7 system. It mentions the prerequisites that you need to fulfil to be able to install
software in Section 1.4.1, “Prerequisites for Software Installation” , provides the basic information on
software packaging and software repositories in Section 1.4.2, “Introduction to the System of Software
Packaging and Software Repositories”, and references the ways to perform basic tasks related to
software installation in Section 1.4.3, “Managing Basic Software Installation Tasks with Subscription
Manager and Yum”.
1.4.1. Prerequisites for Software Installation
The Red Hat Content Delivery Network subscription service provides a mechanism to handle Red Hat
software inventory and enables you to install additional software or update already installed packages.
30

You can start installing software once you have registered your system and attached a subscription, as
described in Section 1.3, “The Basics of Registering the System and Managing Subscriptions” .
1.4.2. Introduction to the System of Software Packaging and Software Repositories
All software on a Red Hat Enterprise Linux system is divided into RPM packages, which are stored in
particular repositories. When a system is subscribed to the Red Hat Content Delivery Network, a
repository file is created in the /etc/yum.repos.d/ directory.
Use the yum utility to manage package operations:
Searching information about packages
Installing packages
Updating packages
Removing packages
Checking the list of currently available repositories
Adding or removing a repository
Enabling or disabling a repository
For information on basic tasks related to the installation of software, see Section 1.4.3, “Managing Basic
Software Installation Tasks with Subscription Manager and Yum”. For further information on managing
software repositories, see Section 7.2, “Managing Software Repositories” . For detailed information on
using the yum utility, see Chapter 9, Yum.
1.4.3. Managing Basic Software Installation Tasks with Subscription Manager and
Yum
The most basic software-installation tasks that you might need after the operating system has been
installed include:
Listing all available repositories:
~]# subscription-manager repos --list
Listing all currently enabled repositories:
~]$ yum repolist
Enabling or disabling a repository:
~]# subscription-manager repos --enable repository
~]# subscription-manager repos --disable repository
Searching for packages matching a specific string:
~]$ yum search string
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Installing a package:
~]# yum install package_name
Updating all packages and their dependencies:
~]# yum update
Updating a package:
~]# yum update package_name
Uninstalling a package and any packages that depend on it:
~]# yum remove package_name
Listing information on all installed and available packages:
~]$ yum list all
Listing information on all installed packages:
~]$ yum list installed
1.5. MAKING SYSTEMD SERVICES START AT BOOT TIME
Systemd is a system and service manager for Linux operating systems that introduces the concept of
systemd units. For more information on systemd, see Section 10.1, “Introduction to systemd” .
This section provides the information on how to ensure that a service is enabled or disabled at boot
time. It also explains how to manage the services through web console.
1.5.1. Enabling or Disabling the Services
You can determine services that are enabled or disabled at boot time already during the installation
process, or you can enable or disable a service on an installed operating system.
To create the list of services enabled or disabled at boot time during the installation process, use the
services option in the Kickstart file:
services [--disabled=list] [--enabled=list]
NOTE
The list of disabled services is processed before the list of enabled services. Therefore, if
a service appears on both lists, it will be enabled. The list of the services should be given
in the comma separated format. Do not include spaces in the list of services. For detailed
information, refer to Red Hat Enterprise Linux 7 Installation Guide.
To enable or disable a service on an already installed operating system:
32

~]# systemctl enableservice_name
~]# systemctl disableservice_name
For further details, see Section 10.2, “Managing System Services” .
1.5.2. Managing Services in web console
In web console, select Services to manage systemd targets, services, sockets, timers and paths. There
you can check their status, start or stop them, enable or disable them.
Figure 1.2. Managing Services in web console
1.5.3. Additional Resources on systemd Services
For more information on systemd, see Chapter 10, Managing Services with systemd .
1.6. ENHANCING SYSTEM SECURITY WITH A FIREWALL, SELINUX AND
SSH LOGINGS
Computer security is the protection of computer systems from the theft or damage to their hardware,
software, or information, as well as from disruption or misdirection of the services they provide. Ensuring
computer security is therefore an essential task not only in the enterprises processing sensitive data or
handling some business transactions.
Computer security includes a wide variety of features and tools. This section covers only the basic
security features that you need to configure after you have installed the operating system. For detailed
information on securing Red Hat Enterprise Linux 7, see Red Hat Enterprise Linux 7 Security Guide .
1.6.1. Ensuring the Firewall Is Enabled and Running
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1.6.1.1. What a Firewall Is and How It Enhances System Security
A firewall is a network security system that monitors and controls the incoming and outgoing network
traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted,
secure internal network and another outside network.
On Red Hat Enterprise Linux 7, the firewall is provided by the firewalld service, which is automatically
enabled during the installation of Red Hat Enterprise Linux. However, if you explicitly disabled the
service, for example in the kickstart configuration, you can re-enable it, as described in Section 1.6.1.2,
“Re-enabling the firewalld Service”. For overview of firewall setting options in the Kickstart file, see Red
Hat Enterprise Linux 7 Installation Guide.
1.6.1.2. Re-enabling the firewalld Service
In case that the firewalld service is disabled after the installation, Red Hat recommends to consider re-
enabling it.
You can display the current status of firewalld even as a regular user:
~]$ systemctl status firewalld
If firewalld is not enabled and running, switch to the root user, and change its status:
~]# systemctl start firewalld
~]# systemctl enable firewalld
For further information on post-installation procedures related to firewalld, see Red Hat Enterprise
Linux 7 Security Guide. For detailed information on configuring and using firewall, see Red Hat
Enterprise Linux 7 Security Guide
1.6.2. Ensuring the Appropriate State of SELinux
1.6.2.1. What SELinux Is and How It Enhances System Security
Security Enhanced Linux (SELinux)is an additional layer of system security that determines which
process can access which files, directories, and ports.
SELinux States
SELinux has two possible states:
Enabled
Disabled
When SELinux is disabled, only Discretionary Access Control (DAC) rules are used.
SELinux Modes
When SELinux is enabled, it can run in one of the following modes:
Enforcing
Permissive
Enforcing mode means that SELinux policies is enforced. SELinux denies access based on SELinux
34

Enforcing mode means that SELinux policies is enforced. SELinux denies access based on SELinux
policy rules, and enables only the interactions that are particularly allowed. Enforcing mode is the default
mode after the installation and it is also the safest SELinux mode.
Permissive mode means that SELinux policies is not enforced. SELinux does not deny access, but
denials are logged for actions that would have been denied if running in enforcing mode. Permissive
mode is the default mode during the installation. Operating in permissive mode is also useful in some
specific cases, for example if you require access to the Access Vector Cache (AVC) denials when
troubleshooting problems.
For further information on SELinux in Red Hat Enterprise Linux 7, see Red Hat Enterprise Linux 7
SELinux User’s and Administrator’s Guide.
1.6.2.2. Ensuring the Required State of SELinux
By default, SELinux operates in permissive mode during the installation and in enforcing mode when
the installation has finished.
However, in some specific situations, SELinux might be explicitly set to permissive mode or it might
even be disabled on the installed operating system. This can be set for example in the kickstart
configuration. For overview of SELinux setting options in the Kickstart file, see Red Hat Enterprise Linux
7 Installation Guide.
IMPORTANT
Red Hat recommends to keep your system in enforcing mode.
To display the current SELinux mode, and to set the mode as needed:
Ensuring the Required State of SELinux
1. Display the current SELinux mode in effect:
~]$ getenforce
2. If needed, switch between the SELinux modes.
The switch can be either temporary or permanent. A temporary switch is not persistent across
reboots, while permanent switch is.
To temporary switch to either enforcing or permissive mode:
~]# setenforce Enforcing
~]# setenforce Permissive
To permanently set the SELinux mode, modify the SELINUX variable in the
/etc/selinux/config configuration file.
For example, to switch SELinux to enforcing mode:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
35

# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=enforcing
1.6.2.3. Managing SELinux in web console
In web console, use the SELinux option to turn SELinux enforcing policy on or off.
By default, SELinux enforcing policy in web console is on, and SELinux operates in enforcing mode. By
turning it off, you can switch SELinux into permissive mode. Note that such deviation from the default
configuration in the /etc/sysconfig/selinux file is automatically reverted on the next boot.
Figure 1.3. Managing SELinux in web console
1.6.3. Using SSH-based Authentication
1.6.3.1. What SSH-based Authentication Is and How It Enhances System Security
If you want to secure your communication with another computer, you can use SSH-based
authentication.
Secure Shell (SSH) is a protocol which facilitates client-server communication and allows users to log in
to any host system running SSH remotely. SSH encrypts the connection. The client transmits its
authentication information to the server using encryption, and all data sent and received during a
session are transferred under the encryption.
SSH enables its users to authenticate without a password. To do so, SSH uses a private-public key
scheme.
For further information about SSH safeguards, see Section 12.1.2, “Main Features”.
1.6.3.2. Establishing an SSH Connection
To be able to use SSH connection, create a pair of two keys consisting of a public and a private key.
Creating the Key Files and Copying Them to the Server
36

1. Generate a public and a private key:
~]$ ssh-keygen
Both keys are stored in the ~/.ssh/ directory:
~/.ssh/id_rsa.pub - public key
~/.ssh/id_rsa - private key
The public key does not need to be secret. It is used to verify the private key. The private
key is secret. You can choose to protect the private key with the passphrase that you
specify during the key generation process. With the passphrase, authentication is even more
secure, but is no longer password-less. You can avoid this using the ssh-agent command. In
this case, you will enter the passphrase only once - at the beginning of a session. For more
information on ssh-agent configuration, see Section 12.2.4, “Using Key-based
Authentication”.
2. Copy the most recently modified public key to a remote machine you want to log into:
~]# ssh-copy-id USER@hostname
As a result, you are now able to enter the system in a secure way, but without entering a
password.
1.6.3.3. Disabling SSH Root Login
To increase system security, you can disable SSH access for the root user, which is enabled by default.
For more information on this topic, see Red Hat Enterprise Linux 7 Security Guide .
Disabling SSH Root Login
1. Access the /etc/ssh/sshd_config file:
~]# vi /etc/ssh/sshd_config
2. Change the line that reads #PermitRootLogin yes to:
PermitRootLogin no
3. Restart the sshd service:
~]# systemctl restart sshd
1.7. THE BASICS OF MANAGING USER ACCOUNTS
Red Hat Enterprise Linux 7 is a multi-user operating system, which enables multiple users on different
computers to access a single system installed on one machine. Every user operates under its own
account, and managing user accounts thus represents a core element of Red Hat Enterprise Linux
system administration.
Normal and System Accounts
Normal accounts are created for users of a particular system. Such accounts can be added, removed,
37

Normal accounts are created for users of a particular system. Such accounts can be added, removed,
and modified during normal system administration.
System accounts represent a particular applications identifier on a system. Such accounts are generally
added or manipulated only at software installation time, and they are not modified later.
WARNING
System accounts are presumed to be available locally on a system. If these accounts
are configured and provided remotely, such as in the instance of an LDAP
configuration, system breakage and service start failures can occur.
For system accounts, user IDs below 1000 are reserved. For normal accounts, you can use IDs starting
at 1000. However, the recommended practice is to assign IDs starting at 5000. See Section 4.1,
“Introduction to Users and Groups” for more information. The guidelines for assigning IDs can be found
in the /etc/login.defs file.
# Min/max values for automatic uid selection in useradd
#
UID_MIN 1000
UID_MAX 60000
# System accounts
SYS_UID_MIN 201
SYS_UID_MAX 999
What Groups Are and Which Purposes They Can Be Used For
A group is an entity which ties together multiple user accounts for a common purpose, such as granting
access to particular files.
1.7.1. The Most Basic Command-Line Tools to Manage User Accounts and Groups
The most basic tasks to manage user accounts and groups, and the appropriate command-line tools,
include:
Displaying user and group IDs:
~]$ id
Creating a new user account:
~]# useradd [options] user_name
Assigning a new password to a user account belonging to username:
~]# passwd user_name
Adding a user to a group:
~]# usermod -a -G group_name user_name

38

For detailed information on managing users and groups, see Chapter 4, Managing Users and Groups.
If you want to use graphical user interface to manage users and groups, see Section 4.2, “Managing
Users in a Graphical Environment”.
1.7.2. Managing User Accounts in web console
To manage accounts in web console, select the Accounts menu.
Figure 1.4. Managing User Accounts in web console
1.8. DUMPING THE CRASHED KERNEL USING THE KDUMP
MECHANISM
This section provides an introduction to the Kernel crash dump mechanism, also called kdump, and
briefly explains what kdump is used for in Section 1.8.1, “What kdump Is and Which Tasks It Can Be Used
For”.
Activation of the kdump service is a part of the installation process, and by default, kdump was enabled
during the installation. This section summarizes how to activate kdump during the installation in
Section 1.8.2, “Enabling and Activating kdump During the Installation Process” , and how to manually
enable the kdump service if it is disabled after the installation in Section 1.8.3, “Ensuring That kdump Is
Installed and Enabled after the Installation Process”.
You can also use web console to configure kdump. See Section 1.8.4, “Configuring kdump in web
console” for more information.
1.8.1. What kdump Is and Which Tasks It Can Be Used For
In case of a system crash, you can use the Kernel crash dump mechanism called kdump that enables you
to save the content of the system’s memory for later analysis. The kdump mechanism relies on the
kexec system call, which can be used to boot a Linux kernel from the context of another kernel, bypass
BIOS, and preserve the contents of the first kernel’s memory that would otherwise be lost.
When kernel crash occurs, kdump uses kexec to boot into a second kernel (a capture kernel), which
resides in a reserved part of the system memory that is inaccessible to the first kernel. The second
kernel captures the contents of the crashed kernel’s memory (a crash dump) and saves it.
39

1.8.2. Enabling and Activating kdump During the Installation Process
During the installation, enablement and activation of kdump can be done either in the Anaconda
installer or using the %addon com_redhat_kdump command in the Kickstart file.
For more information, see the appropriate source according to the installation method:
When installing with the Anaconda installer, see:
Installing Using Anaconda in Red Hat Enterprise Linux 7 Installation Guide.
When installing with the Kickstart file, see:
Kickstart Commands and Options in Red Hat Enterprise Linux 7 Installation Guide.
1.8.3. Ensuring That kdump Is Installed and Enabled after the Installation Process
To ensure that kdump is installed and to configure it:
Checking Whether kdump Is Installed and Configuring kdump
1. To check whether kdump is installed on your system:
~]$ rpm -q kexec-tools
2. If not installed, to install kdump, enter as the root user:
~]# yum install kexec-tools
3. To configure kdump:
Use either the command line or graphical user interface.
Both options are described in detail in Red Hat Enterprise Linux 7 Kernel Crash Dump Guide.
If you need to install the graphical configuration tool:
~]# yum install system-config-kdump
1.8.4. Configuring kdump in web console
In web console, select Kernel dump to verify:
the kdump status
the amount of memory reserved for kdump
the location of the crash dump files
Figure 1.5. Configuring kdump in web console
40

Figure 1.5. Configuring kdump in web console
1.8.5. Additional Resources on kdump
For more information on kdump, see Red Hat Enterprise Linux 7 Kernel Crash Dump Guide.
1.9. PERFORMING SYSTEM RESCUE AND CREATING SYSTEM BACKUP
WITH REAR
When a software or hardware failure breaks the operating system, you need a mechanism to rescue the
system. It is also useful to have the system backup saved. Red Hat recommends using the Relax-and-
Recover (ReaR) tool to fulfil both these needs.
1.9.1. What ReaR Is and Which Tasks It Can Be Used For
ReaR is a disaster recovery and system migration utility which enables you to create the complete rescue
system. By default, this rescue system restores only the storage layout and the boot loader, but not the
actual user and system files.
Additionally, certain backup software enables you to integrate ReaR for disaster recovery.
ReaR enables to perform the following tasks:
Booting a rescue system on the new hardware
Replicating the original storage layout
Restoring user and system files
1.9.2. Quickstart to Installation and Configuration of ReaR
To install ReaR, enter as the root user:
~]# yum install rear
Use the settings in the /etc/rear/local.conf file to configure ReaR.
For further information see, Section 27.1, “Basic ReaR Usage”.
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1.9.3. Quickstart to Creation of the Rescue System with ReaR
To create the rescue system, perform the following command as the
root user
~]# rear mkrescue
For further information on creating the rescue system with ReaR, see Section 27.1.3, “Creating a Rescue
System”.
1.9.4. Quickstart to Configuration of ReaR with the Backup Software
ReaR contains a fully-integrated built-in, or internal, backup method called NETFS.
To make ReaR use its internal backup method, add these lines to the /etc/rear/local.conf file:
BACKUP=NETFS
BACKUP_URL=backup location
You can also configure ReaR to keep the previous backup archives when the new ones are created by
adding the following line to /etc/rear/local.conf:
NETFS_KEEP_OLD_BACKUP_COPY=y
To make the backups incremental, meaning that only the changed files are backed up on each run, add
this line to /etc/rear/local.conf:
BACKUP_TYPE=incremental
For detailed information about using the ReaR NETFS internal backup method, see Section 27.2.1, “The
Built-in Backup Method”.
For information on supported external backup methods and unsupported backup methods, see
Section 27.2.2, “Supported Backup Methods” and Section 27.2.3, “Unsupported Backup Methods” .
1.10. USING THE LOG FILES TO TROUBLESHOOT PROBLEMS
When troubleshooting a problem, you may appreciate the log files that contain different information and
messages about the operating system. The logging system in Red Hat Enterprise Linux 7 is based on the
built-in syslog protocol. Particular programs use this system to record events and organize them into
log files, which are useful when auditing the operating system and troubleshooting various problems.
For more information on log files, see Chapter 23, Viewing and Managing Log Files .
1.10.1. Services Handling the syslog Messages
The syslog messages are handled by two services:
the systemd-journald daemon - Collects messages from the kernel, the early stages of the
boot process, standard output and error of daemons as they start up and run, and syslog, and
forwards the messages to the rsyslog service for further processing.
the rsyslog service - Sorts the syslog messages by type and priority, and writes them to the
files in the /var/log directory, where the logs are persistently stored.
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1.10.2. Subdirectories Storing the syslog Messages
The syslog messages are stored in various subdirectories under the /var/log directory according to what
kind of messages and logs they contain:
var/log/messages - all syslog messages except those mentioned below
var/log/secure - security and authentication-related messages and errors
var/log/maillog - mail server-related messages and errors
var/log/cron - log files related to periodically executed tasks
var/log/boot.log - log files related to system startup
1.11. ACCESSING RED HAT SUPPORT
To obtain support from Red Hat, use the Red Hat Customer Portal , which provides access to everything
available with your subscription.
This section describes:
Obtaining Red Hat support, see Section 1.11.1, “Obtaining Red Hat Support Through Red Hat
Customer Portal”
Using the SOS report to troubleshoot problems, see Section 1.11.2, “Using the SOS Report to
Troubleshoot Problems”
1.11.1. Obtaining Red Hat Support Through Red Hat Customer Portal
By using the Red Hat Customer Portal you can:
Open a new support case
Initiate a live chat with a Red Hat expert
Contact a Red Hat expert by making a call or sending an email
To access the Red Hat Customer Portal, go to https://access.redhat.com..
To work with Red Hat Customer Portal services related to Red Hat support, you can use:
Web browser
Red Hat Support Tool
1.11.1.1. What the Red Hat Support Tool Is and Which Tasks It Can Be Used For
The Red Hat Support Toolis a command-line based tool which provides a text console interface to the
subscription-based Red Hat access services. This tool is contained in the redhat-support-tool
package.
The Red Hat Support Toolenables you to perform support-related tasks, such as:
Opening or updating support cases
43

Searching in the Red Hat knowledge base solutions
Analyzing Python and Java errors
To start the tool in interactive mode:
~]$ redhat-support-tool
Welcome to the Red Hat Support Tool.
Command (? for help):
In interactive mode, enter ? to display the available commands:
Command (? for help): ?
For more information about the installation and use of the Red Hat Support Tool, see Chapter 8,
Accessing Support Using the Red Hat Support Tool , and the Red Hat Knowledgebase article Red Hat
Access: Red Hat Support Tool.
1.11.2. Using the SOS Report to Troubleshoot Problems
The SOS report collects configuration details, system information and diagnostic information from a
Red Hat Enterprise Linux system. Attach the report when you open a support case.
Note that the SOS report is provided in the sos package, which is not installed with the default minimal
installation of Red Hat Enterprise Linux 7.
To install the sos package:
~]# yum install sos
To generate an SOS report:
~]# sosreport
To attach the sos report to your support case, see the Red Hat Knowledgebase article How can I attach
a file to a Red Hat support case?. Note that you will be prompted to enter the number of the support
case, when attaching the sos report.
For more information on SOS report, see the Red Hat Knowledgebase article What is a sosreport and
how to create one in Red Hat Enterprise Linux 4.6 and later?.
44

CHAPTER 2. SYSTEM LOCALE AND KEYBOARD
CONFIGURATION
The system locale specifies the language settings of system services and user interfaces. The keyboard
layout settings control the layout used on the text console and graphical user interfaces.
These settings can be made by modifying the /etc/locale.conf configuration file or by using the localectl
utility. Also, you can use the graphical user interface to perform the task; for a description of this
method, see Red Hat Enterprise Linux 7 Installation Guide .
2.1. SETTING THE SYSTEM LOCALE
System-wide locale settings are stored in the /etc/locale.conf file, which is read at early boot by the
systemd daemon. The locale settings configured in /etc/locale.conf are inherited by every service or
user, unless individual programs or individual users override them.
The basic file format of /etc/locale.conf is a newline-separated list of variable assignments. For
example, German locale with English messages in /etc/locale.conf looks as follows:
LANG=de_DE.UTF-8
LC_MESSAGES=C
Here, the LC_MESSAGES option determines the locale used for diagnostic messages written to the
standard error output. To further specify locale settings in /etc/locale.conf, you can use several other
options, the most relevant are summarized in Table 2.1, “Options configurable in /etc/locale.conf” . See
the locale(7) manual page for detailed information on these options. Note that the LC_ALL option,
which represents all possible options, should not be configured in /etc/locale.conf.
Table 2.1. Options configurable in /etc/locale.conf
Option Description
LANG Provides a default value for the system locale.
LC_COLLATE Changes the behavior of functions which compare
strings in the local alphabet.
LC_CTYPE Changes the behavior of the character handling and
classification functions and the multibyte character
functions.
LC_NUMERIC Describes the way numbers are usually printed, with
details such as decimal point versus decimal comma.
LC_TIME Changes the display of the current time, 24-hour
versus 12-hour clock.
LC_MESSAGES Determines the locale used for diagnostic messages
written to the standard error output.
2.1.1. Displaying the Current Status
45

The localectl command can be used to query and change the system locale and keyboard layout
settings. To show the current settings, use the status option:
localectl status
Example 2.1. Displaying the Current Status
The output of the previous command lists the currently set locale, keyboard layout configured for
the console and for the X11 window system.
System Locale: LANG=en_US.UTF-8
VC Keymap: us
X11 Layout: n/a
2.1.2. Listing Available Locales
To list all locales available for your system, type:
localectl list-locales
Example 2.2. Listing Locales
Imagine you want to select a specific English locale, but you are not sure if it is available on the
system. You can check that by listing all English locales with the following command:
~]$ localectl list-locales | grep en_
en_AG
en_AG.utf8
en_AU
en_AU.iso88591
en_AU.utf8
en_BW
en_BW.iso88591
en_BW.utf8
output truncated
2.1.3. Setting the Locale
To set the default system locale, use the following command as root:
localectl set-locale LANG=locale
Replace locale with the locale name, found with the localectl list-locales command. The above syntax
can also be used to configure parameters from Table 2.1, “Options configurable in /etc/locale.conf” .
Example 2.3. Changing the Default Locale
For example, if you want to set British English as your default locale, first find the name of this locale
46

For example, if you want to set British English as your default locale, first find the name of this locale
by using list-locales. Then, as root, type the command in the following form:
~]# localectl set-locale LANG=en_GB.utf8
2.1.4. Making System Locale Settings Permanent when Installing with Kickstart
When Red Hat Enterprise Linux is installed with the Red Hat Kickstart installation method, setting of the
system locales might not be persistent after an upgrade of the operating system.
When the %packages section of the Kickstart file includes the --instLang option, the _install_langs
RPM macro is set to the particular value for this installation, and the set of installed locales is adjusted
accordingly. However, this adjustment affects only this installation, not subsequent upgrades. If an
upgrade reinstalls the glibc package, the entire set of locales is upgraded instead of only the locales you
requested during the installation.
To avoid this, make the choice of locales permanent. You have these options:
If you have not started the Kickstart installation, modify the Kickstart file to include instructions
for setting RPM macros globally by applying this procedure: Setting RPM macros during the
Kickstart installation
If you have already installed the system, set RPM macros globally on the system by applying this
procedure: Setting RPM macros globally
Setting RPM macros during the Kickstart installation
1. Modify the %post section of the Kickstart file:
LANG=en_US
echo "%_install_langs $LANG" > /etc/rpm/macros.language-conf
yum-config-manager --setopt=override_install_langs=$LANG --save
2. Modify the %packages section of the Kickstart file:
%packages
yum-utils*
%end
Setting RPM macros globally
1. Create the RPM configuration file at /etc/rpm/macros.language-conf with the following
contents:
%_install_langs LANG
LANG is the value of the instLang option.
2. Update the /etc/yum.conf file with:
override_install_langs=LANG
47

2.2. CHANGING THE KEYBOARD LAYOUT
The keyboard layout settings enable the user to control the layout used on the text console and
graphical user interfaces.
2.2.1. Displaying the Current Settings
As mentioned before, you can check your current keyboard layout configuration with the following
command:
localectl status
Example 2.4. Displaying the Keyboard Settings
In the following output, you can see the keyboard layout configured for the virtual console and for
the X11 window system.
System Locale: LANG=en_US.utf8
VC Keymap: us
X11 Layout: us
2.2.2. Listing Available Keymaps
To list all available keyboard layouts that can be configured on your system, type:
localectl list-keymaps
Example 2.5. Searching for a Particular Keymap
You can use grep to search the output of the previous command for a specific keymap name. There
are often multiple keymaps compatible with your currently set locale. For example, to find available
Czech keyboard layouts, type:
~]$ localectl list-keymaps | grep cz
cz
cz-cp1250
cz-lat2
cz-lat2-prog
cz-qwerty
cz-us-qwertz
sunt5-cz-us
sunt5-us-cz
2.2.3. Setting the Keymap
To set the default keyboard layout for your system, use the following command as root:
localectl set-keymap map
48

Replace map with the name of the keymap taken from the output of the localectl list-keymaps
command. Unless the --no-convert option is passed, the selected setting is also applied to the default
keyboard mapping of the X11 window system, after converting it to the closest matching X11 keyboard
mapping. This also applies in reverse, you can specify both keymaps with the following command as root:
localectl set-x11-keymap map
If you want your X11 layout to differ from the console layout, use the --no-convert option.
localectl --no-convert set-x11-keymap map
With this option, the X11 keymap is specified without changing the previous console layout setting.
Example 2.6. Setting the X11 Keymap Separately
Imagine you want to use German keyboard layout in the graphical interface, but for console
operations you want to retain the US keymap. To do so, type as root:
~]# localectl --no-convert set-x11-keymap de
Then you can verify if your setting was successful by checking the current status:
System Locale: LANG=de_DE.UTF-8
VC Keymap: us
X11 Layout: de
Apart from keyboard layout (map), three other options can be specified:
localectl set-x11-keymap map model variant options
Replace model with the keyboard model name, variant and options with keyboard variant and option
components, which can be used to enhance the keyboard behavior. These options are not set by default.
For more information on X11 Model, X11 Variant, and X11 Options see the kbd(4) man page.
For more information on how to configure the keyboard layout on Red Hat Enterprise Linux, see the
resources listed below:
localectl(1) — The manual page for the localectl command line utility documents how to use
this tool to configure the system locale and keyboard layout.
loadkeys(1) — The manual page for the loadkeys command provides more information on how
to use this tool to change the keyboard layout in a virtual console.
See Also
Chapter 6, Gaining Privileges documents how to gain administrative privileges by using the su
and sudo commands.
Chapter 10, Managing Services with systemd provides more information on systemd and
49

documents how to use the systemctl command to manage system services.
50

Modern operating systems distinguish between the following two types of clocks:
A real-time clock (RTC), commonly referred to as a hardware clock, (typically an integrated
circuit on the system board) that is completely independent of the current state of the
operating system and runs even when the computer is shut down.
A system clock, also known as a software clock, that is maintained by the kernel and its initial
value is based on the real-time clock. Once the system is booted and the system clock is
initialized, the system clock is completely independent of the real-time clock.
The system time is always kept in Coordinated Universal Time (UTC) and converted in applications to
local time as needed. Local time is the actual time in your current time zone, taking into account daylight
saving time (DST). The real-time clock can use either UTC or local time. UTC is recommended.
Red Hat Enterprise Linux 7 offers three command line tools that can be used to configure and display
information about the system date and time:
The timedatectl utility, which is new in Red Hat Enterprise Linux 7 and is part of systemd.
The traditional date command.
The hwclock utility for accessing the hardware clock.
3.1. USING THE TIMEDATECTL COMMAND
The timedatectl utility is distributed as part of the systemd system and service manager and allows you
to review and change the configuration of the system clock. You can use this tool to change the current
date and time, set the time zone, or enable automatic synchronization of the system clock with a remote
server.
For information on how to display the current date and time in a custom format, see also Section 3.2,
“Using the date Command”.
To display the current date and time along with detailed information about the configuration of the
system and hardware clock, run the timedatectl command with no additional command line options:
timedatectl
This displays the local and universal time, the currently used time zone, the status of the Network Time
Protocol (NTP) configuration, and additional information related to DST.
Example 3.1. Displaying the Current Date and Time
The following is an example output of the timedatectl command on a system that does not use NTP
to synchronize the system clock with a remote server:
~]$ timedatectl
Local time: Mon 2016-09-16 19:30:24 CEST
Universal time: Mon 2016-09-16 17:30:24 UTC
Timezone: Europe/Prague (CEST, +0200)
51

NTP enabled: no
NTP synchronized: no
RTC in local TZ: no
DST active: yes
Last DST change: DST began at
Sun 2016-03-31 01:59:59 CET
Sun 2016-03-31 03:00:00 CEST
Next DST change: DST ends (the clock jumps one hour backwards) at
Sun 2016-10-27 02:59:59 CEST
Sun 2016-10-27 02:00:00 CET
IMPORTANT
Changes to the status of chrony or ntpd will not be immediately noticed by timedatectl.
If changes to the configuration or status of these tools is made, enter the following
command:
~]# systemctl restart systemd-timedated.service
To change the current time, type the following at a shell prompt as root:
timedatectl set-time HH:MM:SS
Replace HH with an hour, MM with a minute, and SS with a second, all typed in two-digit form.
This command updates both the system time and the hardware clock. The result it is similar to using
both the date --set and hwclock --systohc commands.
The command will fail if an NTP service is enabled. See Section 3.1.5, “Synchronizing the System Clock
with a Remote Server” to temporally disable the service.
Example 3.2. Changing the Current Time
To change the current time to 11:26 p.m., run the following command as root:
~]# timedatectl set-time 23:26:00
By default, the system is configured to use UTC. To configure your system to maintain the clock in the
local time, run the timedatectl command with the set-local-rtc option as root:
timedatectl set-local-rtc boolean
To configure your system to maintain the clock in the local time, replace boolean with yes (or,
alternatively, y, true, t, or 1). To configure the system to use UTC, replace boolean with no (or,
alternatively, n, false, f, or 0). The default option is no.
52

To change the current date, type the following at a shell prompt as root:
timedatectl set-time YYYY-MM-DD
Replace YYYY with a four-digit year, MM with a two-digit month, and DD with a two-digit day of the
month.
Note that changing the date without specifying the current time results in setting the time to 00:00:00.
Example 3.3. Changing the Current Date
To change the current date to 2 June 2017 and keep the current time (11:26 p.m.), run the following
command as root:
~]# timedatectl set-time "2017-06-02 23:26:00"
3.1.4. Changing the Time Zone
To list all available time zones, type the following at a shell prompt:
timedatectl list-timezones
To change the currently used time zone, type as root:
timedatectl set-timezone time_zone
Replace time_zone with any of the values listed by the timedatectl list-timezones command.
Example 3.4. Changing the Time Zone
To identify which time zone is closest to your present location, use the timedatectl command with
the list-timezones command line option. For example, to list all available time zones in Europe, type:
~]# timedatectl list-timezones | grep Europe
Europe/Amsterdam
Europe/Andorra
Europe/Athens
Europe/Belgrade
Europe/Berlin
Europe/Bratislava
…
To change the time zone to Europe/Prague, type as root:
~]# timedatectl set-timezone Europe/Prague
3.1.5. Synchronizing the System Clock with a Remote Server
As opposed to the manual adjustments described in the previous sections, the timedatectl command
also allows you to enable automatic synchronization of your system clock with a group of remote servers
53

using the NTP protocol. Enabling NTP enables the chronyd or ntpd service, depending on which of
them is installed.
The NTP service can be enabled and disabled using a command as follows:
timedatectl set-ntp boolean
To enable your system to synchronize the system clock with a remote NTP server, replace boolean with
yes (the default option). To disable this feature, replace boolean with no.
Example 3.5. Synchronizing the System Clock with a Remote Server
To enable automatic synchronization of the system clock with a remote server, type:
~]# timedatectl set-ntp yes
The command will fail if an NTP service is not installed. See Section 18.3.1, “Installing chrony” for more
information.
3.2. USING THE DATE COMMAND
The date utility is available on all Linux systems and allows you to display and configure the current date
and time. It is frequently used in scripts to display detailed information about the system clock in a
custom format.
For information on how to change the time zone or enable automatic synchronization of the system
clock with a remote server, see Section 3.1, “Using the timedatectl Command”.
To display the current date and time, run the date command with no additional command line options:
date
This displays the day of the week followed by the current date, local time, abbreviated time zone, and
year.
By default, the date command displays the local time. To display the time in UTC, run the command with
the --utc or -u command line option:
date --utc
You can also customize the format of the displayed information by providing the +"format" option on
the command line:
date +"format"
Replace format with one or more supported control sequences as illustrated in Example 3.6, “Displaying
the Current Date and Time”. See Table 3.1, “Commonly Used Control Sequences” for a list of the most
frequently used formatting options, or the date(1) manual page for a complete list of these options.
Table 3.1. Commonly Used Control Sequences
54

Control Sequence Description
%H The hour in the HH format (for example,17).
%M The minute in the MM format (for example,30).
%S The second in the SS format (for example,24).
%d The day of the month in the DD format (for example,
16).
%m The month in the MM format (for example,09).
%Y The year in the YYYY format (for example,2016).
%Z The time zone abbreviation (for example, CEST).
%F The full date in the YYYY-MM-DD format (for
example, 2016-09-16). This option is equal to%Y-
%m-%d.
%T The full time in the HH:MM:SS format (for example,
17:30:24). This option is equal to %H:%M:%S
To display the current date and local time, type the following at a shell prompt:
~]$ date
Mon Sep 16 17:30:24 CEST 2016
To display the current date and time in UTC, type the following at a shell prompt:
~]$ date --utc
Mon Sep 16 15:30:34 UTC 2016
To customize the output of the date command, type:
~]$ date +"%Y-%m-%d %H:%M"
2016-09-16 17:30
To change the current time, run the date command with the --set or -s option as root:
date --set HH:MM:SS
Replace HH with an hour, MM with a minute, and SS with a second, all typed in two-digit form.
55

By default, the date command sets the system clock to the local time. To set the system clock in UTC,
run the command with the --utc or -u command line option:
date --set HH:MM:SS --utc
Example 3.7. Changing the Current Time
To change the current time to 11:26 p.m., run the following command as root:
~]# date --set 23:26:00
To change the current date, run the date command with the --set or -s option as root:
date --set YYYY-MM-DD
Replace YYYY with a four-digit year, MM with a two-digit month, and DD with a two-digit day of the
month.
Note that changing the date without specifying the current time results in setting the time to 00:00:00.
Example 3.8. Changing the Current Date
To change the current date to 2 June 2017 and keep the current time (11:26 p.m.), run the following
command as root:
~]# date --set "2017-06-02 23:26:00"
3.3. USING THE HWCLOCK COMMAND
hwclock is a utility for accessing the hardware clock, also referred to as the Real Time Clock (RTC). The
hardware clock is independent of the operating system you use and works even when the machine is
shut down. This utility is used for displaying the time from the hardware clock. hwclock also contains
facilities for compensating for systematic drift in the hardware clock.
The hardware clock stores the values of: year, month, day, hour, minute, and second. It is not able to
store the time standard, local time or Coordinated Universal Time (UTC), nor set the Daylight Saving
Time (DST).
The hwclock utility saves its settings in the /etc/adjtime file, which is created with the first change you
make, for example, when you set the time manually or synchronize the hardware clock with the system
time.
NOTE
For the changes in the hwclock behaviour between Red Hat Enterprise Linux version 6
and 7, see Red Hat Enterprise Linux 7 Migration Planning Guide guide.
56

Running hwclock with no command line options as the root user returns the date and time in local time
to standard output.
hwclock
Note that using the --utc or --localtime options with the hwclock command does not mean you are
displaying the hardware clock time in UTC or local time. These options are used for setting the hardware
clock to keep time in either of them. The time is always displayed in local time. Additionally, using the
hwclock --utc or hwclock --local commands does not change the record in the /etc/adjtime file. This
command can be useful when you know that the setting saved in /etc/adjtime is incorrect but you do not
want to change the setting. On the other hand, you may receive misleading information if you use the
command an incorrect way. See the hwclock(8) manual page for more details.
To display the current date and the current local time from the hardware clock, run as root:
~]# hwclock
Tue 15 Apr 2017 04:23:46 PM CEST -0.329272 seconds
CEST is a time zone abbreviation and stands for Central European Summer Time.
For information on how to change the time zone, see Section 3.1.4, “Changing the Time Zone”.
3.3.2. Setting the Date and Time
Besides displaying the date and time, you can manually set the hardware clock to a specific time.
When you need to change the hardware clock date and time, you can do so by appending the --set and -
-date options along with your specification:
hwclock --set --date "dd mmm yyyy HH:MM"
Replace dd with a day (a two-digit number), mmm with a month (a three-letter abbreviation), yyyy with
a year (a four-digit number), HH with an hour (a two-digit number), MM with a minute (a two-digit
number).
At the same time, you can also set the hardware clock to keep the time in either UTC or local time by
adding the --utc or --localtime options, respectively. In this case, UTC or LOCAL is recorded in the
/etc/adjtime file.
Example 3.10. Setting the Hardware Clock to a Specific Date and Time
If you want to set the date and time to a specific value, for example, to "21:17, October 21, 2016", and
keep the hardware clock in UTC, run the command as root in the following format:
~]# hwclock --set --date "21 Oct 2016 21:17" --utc
3.3.3. Synchronizing the Date and Time
57

You can synchronize the hardware clock and the current system time in both directions.
Either you can set the hardware clock to the current system time by using this command:
hwclock --systohc
Note that if you use NTP, the hardware clock is automatically synchronized to the system clock
every 11 minutes, and this command is useful only at boot time to get a reasonable initial system
time.
Or, you can set the system time from the hardware clock by using the following command:
hwclock --hctosys
When you synchronize the hardware clock and the system time, you can also specify whether you want
to keep the hardware clock in local time or UTC by adding the --utc or --localtime option. Similarly to
using --set, UTC or LOCAL is recorded in the /etc/adjtime file.
The hwclock --systohc --utc command is functionally similar to timedatectl set-local-rtc false and the
hwclock --systohc --local command is an alternative to timedatectl set-local-rtc true.
Example 3.11. Synchronizing the Hardware Clock with System Time
To set the hardware clock to the current system time and keep the hardware clock in local time, run
the following command as root:
~]# hwclock --systohc --localtime
To avoid problems with time zone and DST switching, it is recommended to keep the hardware clock
in UTC. The shown Example 3.11, “Synchronizing the Hardware Clock with System Time” is useful, for
example, in case of a multi boot with a Windows system, which assumes the hardware clock runs in
local time by default, and all other systems need to accommodate to it by using local time as well. It
may also be needed with a virtual machine; if the virtual hardware clock provided by the host is
running in local time, the guest system needs to be configured to use local time, too.
For more information on how to configure the date and time in Red Hat Enterprise Linux 7, see the
resources listed below.
timedatectl(1) — The manual page for the timedatectl command line utility documents how to
use this tool to query and change the system clock and its settings.
date(1) — The manual page for the date command provides a complete list of supported
command line options.
hwclock(8) — The manual page for the hwclock command provides a complete list of
supported command line options.
See Also
Chapter 2, System Locale and Keyboard Configuration documents how to configure the
keyboard layout.
58

keyboard layout.
and sudo commands.
59

The control of users and groups is a core element of Red Hat Enterprise Linux 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 creating group directories.
4.1. INTRODUCTION TO USERS AND GROUPS
While users can be either people (meaning accounts tied to physical users) or accounts that exist for
specific applications to use, groups are logical expressions of organization, tying users together for a
common purpose. Users within a group share the same permissions to 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, Red Hat Enterprise Linux supports access control lists (ACLs) for files and directories which
allow permissions for specific users outside of the owner to be set. For more information about this
feature, see Chapter 5, Access Control Lists .
Reserved User and Group IDs
Red Hat Enterprise Linux reserves user and group IDs below 1000 for system users and groups. By
default, the User Manager does not display the system users. Reserved user and group IDs are
documented in the setup package. To view the documentation, use this command:
cat /usr/share/doc/setup*/uidgid
The recommended practice is to assign IDs starting at 5,000 that were not already reserved, as the
reserved range can increase in the future. To make the IDs assigned to new users by default start at
5,000, change the UID_MIN and GID_MIN directives in the /etc/login.defs file:
[file contents truncated]
UID_MIN 5000
GID_MIN 5000
NOTE
For users created before you changed UID_MIN and GID_MIN directives, UIDs will still
start at the default 1000.
Even with new user and group IDs beginning with 5,000, it is recommended not to raise IDs reserved by
the system above 1000 to avoid conflict with systems that retain the 1000 limit.
4.1.1. User Private Groups
Red Hat Enterprise Linux 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.
60

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-based 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. See Section 4.3.5, “Setting Default Permissions for New Files Using
umask” for more information.
A list of all groups is stored in the /etc/group configuration file.
4.1.2. Shadow Passwords
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 to enforce some of the security policies set in the /etc/login.defs file.
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,
some utilities and commands do not work without first enabling shadow passwords:
The chage utility for setting password aging parameters. For details, see the Password Security
section in the Red Hat Enterprise Linux 7 Security Guide .
The gpasswd utility for administrating the /etc/group file.
The usermod command with the -e, --expiredate or -f, --inactive option.
The useradd command with the -e, --expiredate or -f, --inactive option.
4.2. MANAGING USERS IN A GRAPHICAL ENVIRONMENT
The Users utility allows you to view, modify, add, and delete local users in the graphical user interface.
4.2.1. Using the Users Settings Tool
Press the Super key to enter the Activities Overview, type Users and then press Enter. The Users
settings tool appears. The Super key appears in a variety of guises, depending on the keyboard and
other hardware, but often as either the Windows or Command key, and typically to the left of the Space
bar. Alternatively, you can open the Users utility from the Settings menu after clicking your user name in
the top right corner of the screen.
To make changes to the user accounts, first select the Unlock button and authenticate yourself as
indicated by the dialog box that appears. Note that unless you have superuser privileges, the application
61

will prompt you to authenticate as root. To add and remove users, select the + and - button respectively.
To add a user to the administrative group wheel, change the Account Type from Standard to
Administrator. To edit a user’s language setting, select the language and a drop-down menu appears.
Figure 4.1. The Users Settings Tool
When a new user is created, the account is disabled until a password is set. The Password drop-down
menu, shown in Figure 4.2, “The Password Menu”, contains the options to set a password by the
administrator immediately, choose a password by the user at the first login, or create a guest account
with no password required to log in. You can also disable or enable an account from this menu.
Figure 4.2. The Password Menu
62

Figure 4.2. The Password Menu
4.3. USING COMMAND-LINE TOOLS
Apart from the Users settings tool described in Section 4.2, “Managing Users in a Graphical
Environment”, which is designed for basic managing of users, you can use command line tools for
managing users and groups 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
id Displays user and group IDs.
useradd, usermod, userdel Standard utilities for adding, modifying, and deleting
user accounts.
groupadd, groupmod, groupdel Standard utilities for adding, modifying, and deleting
groups.
gpasswd Utility primarily used for modification of group
password in the /etc/gshadow file which is used by
the newgrp command.
63

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.
grpconv, grpunconv Similar to the previous, these utilities can be used for
conversion of shadowed information for group
accounts.
Utilities Description
4.3.1. Adding a New User
To add a new user to the system, type the following at a shell prompt as root:
useradd options username
…where options are command-line options as described in Table 4.2, “Common 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 a password aging policy. See the Password Security section in the Red Hat
Enterprise Linux 7 Security Guide.
Table 4.2. Common useradd command-line options
Option
-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 disabled after the
password expires.
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-g group_name Group name or group number for the user’s default
(primary) group. The group must exist prior to being
specified here.
-G group_list List of additional (supplementary, 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.
Option
IMPORTANT
The default range of IDs for system and normal users has been changed in Red Hat
Enterprise Linux 7 from earlier releases. Previously, UID 1-499 was used for system users
and values above for normal users. The default range for system users is now 1-999. This
change might cause problems when migrating to Red Hat Enterprise Linux 7 with existing
users having UIDs and GIDs between 500 and 999. The default ranges of UID and GID
can be changed in the /etc/login.defs file.
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:1001:1001::/home/juan:/bin/bash
The line has the following characteristics:
It begins with the user name juan.
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There is an x for the password field indicating that the system is using shadow passwords.
A UID greater than 999 is created. Under Red Hat Enterprise Linux 7, UIDs below 1000 are
reserved for system use and should not be assigned to users.
A GID greater than 999 is created. Under Red Hat Enterprise Linux 7, GIDs below 1000 are
reserved for system use and should not be assigned to users.
The optional GECOS information is left blank. The GECOS field can be used to provide
additional information about the user, such as their full name or phone number.
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:::
It begins with the user name 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:1001:
A group with the same name as a user is called a user private group. For more information on
user private groups, see 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 juan's primary group in /etc/passwd.
4. A new line for a group named juan is created in /etc/gshadow:
juan:!::
It begins with the group name juan.
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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 -ld /home/juan
drwx------. 4 juan juan 4096 Mar 3 18:23 /home/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. 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 (see the Password Security section in the Red Hat Enterprise Linux 7 Security Guide for
details).
4.3.2. 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, “Common groupadd command-line
options”.
Table 4.3. Common groupadd command-line options
Option Description
-f, --force When used with -g gid and gid already exists,
groupadd will choose another uniquegid 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 Allows creating groups with duplicate GID.
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-p, --password password Use this encrypted password for the new group.
-r Create a system group with a GID less than 1000.
Option Description
4.3.3. Adding an Existing User to an Existing Group
Use the usermod utility to add an already existing user to an already existing group.
Various options of usermod have different impact on user’s primary group and on his or her
supplementary groups.
To override user’s primary group, run the following command as root:
~]# usermod -g group_name user_name
To override user’s supplementary groups, run the following command as root:
~]# usermod -G group_name1,group_name2,... user_name
Note that in this case all previous supplementary groups of the user are replaced by the new group or
several new groups.
To add one or more groups to user’s supplementary groups, run one of the following commands as root:
~]# usermod -aG group_name1,group_name2,... user_name
~]# usermod --append -G group_name1,group_name2,... user_name
Note that in this case the new group is added to user’s current supplementary groups.
4.3.4. 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 management 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 that
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:
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groupadd myproject
3. Associate the contents of the /opt/myproject/ directory with the myproject group:
chown root:myproject /opt/myproject
4. Allow users in the group 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 -ld /opt/myproject
drwxrwsr-x. 3 root myproject 4096 Mar 3 18:31 /opt/myproject
5. Add users to the myproject group:
usermod -aG myproject username
4.3.5. Setting Default Permissions for New Files Using umask
When a process creates a file, the file has certain default permissions, for example, -rw-rw-r--. These
initial permissions are partially defined by the file mode creation mask , also called file permission mask or
umask. Every process has its own umask, for example, bash has umask 0022 by default. Process umask
can be changed.
What umask consists of
A umask consists of bits corresponding to standard file permissions. For example, for umask 0137, the
digits mean that:
0 = no meaning, it is always 0 (umask does not affect special bits)
1 = for owner permissions, the execute bit is set
3 = for group permissions, the execute and write bits are set
7 = for others permissions, the execute, write, and read bits are set
Umasks can be represented in binary, octal, or symbolic notation. For example, the octal representation
0137 equals symbolic representation u=rw-,g=r--,o=---. Symbolic notation specification is the reverse of
the octal notation specification: it shows the allowed permissions, not the prohibited permissions.
How umask works
Umask prohibits permissions from being set for a file:
When a bit is set in umask, it is unset in the file.
When a bit is not set in umask, it can be set in the file, depending on other factors.
The following figure shows how umask 0137 affects creating a new file.
Figure 4.3. Applying umask when creating a file
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Figure 4.3. Applying umask when creating a file
IMPORTANT
For security reasons, a regular file cannot have execute permissions by default. Therefore,
even if umask is 0000, which does not prohibit any permissions, a new regular file still does
not have execute permissions. However, directories can be created with execute
permissions:
[john@server tmp]$ umask 0000
[john@server tmp]$ touch file
[john@server tmp]$ mkdir directory
[john@server tmp]$ ls -lh .
total 0
drwxrwxrwx. 2 john john 40 Nov 2 13:17 directory
-rw-rw-rw-. 1 john john 0 Nov 2 13:17 file
4.3.5.1. Managing umask in Shells
For popular shells, such as bash, ksh, zsh and tcsh, umask is managed using the umask shell builtin.
Processes started from shell inherit its umask.
Displaying the current mask
To show the current umask in octal notation:
~]$ umask
0022
To show the current umask in symbolic notation:
~]$ umask -S
u=rwx,g=rx,o=rx
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Setting mask in shell using umask
To set umask for the current shell session using octal notation run:
~]$ umask octal_mask
Substitute octal_mask with four or less digits from 0 to 7. When three or less digits are provided,
permissions are set as if the command contained leading zeros. For example, umask 7 translates to 0007.
Example 4.1. Setting umask Using Octal Notation
To prohibit new files from having write and execute permissions for owner and group, and from
having any permissions for others:
~]$ umask 0337
Or simply:
~]$ umask 337
To set umask for the current shell session using symbolic notation:
~]$ umask -S symbolic_mask
Example 4.2. Setting umask Using Symbolic Notation
To set umask 0337 using symbolic notation:
~]$ umask -S u=r,g=r,o=
Working with the default shell umask
Shells usually have a configuration file where their default umask is set. For bash, it is /etc/bashrc. To
show the default bash umask:
~]$ grep -i -B 1 umask /etc/bashrc
The output shows if umask is set, either using the umask command or the UMASK variable. In the
following example, umask is set to 022 using the umask command:
~]$ grep -i -B 1 umask /etc/bashrc
# By default, we want umask to get set. This sets it for non-login shell.
--
if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then
umask 002
else
umask 022
To change the default umask for bash, change the umask command call or the UMASK variable
assignment in /etc/bashrc. This example changes the default umask to 0227:
if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then
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umask 002
else
umask 227
Working with the default shell umask of a specific user
By default, bash umask of a new user defaults to the one defined in /etc/bashrc.
To change bash umaskfor a particular user, add a call to the umask command in $HOME/.bashrc file of
that user. For example, to change bash umask of user john to 0227:
john@server ~]$ echo 'umask 227' >> /home/john/.bashrc
Setting default permissions for newly created home directories
To change permissions with which user home directories are created, change the UMASK variable in the
/etc/login.defs file:
# The permission mask is initialized to this value. If not specified,
# the permission mask will be initialized to 022.
UMASK 077
For more information on how to manage users and groups on Red Hat Enterprise Linux, see the
For information about various utilities for managing users and groups, see the following manual pages:
useradd(8) — The manual page for the useradd command documents how to use it to create
new users.
userdel(8) — The manual page for the userdel command documents how to use it to delete
users.
usermod(8) — The manual page for the usermod command documents how to use it to modify
users.
groupadd(8) — The manual page for the groupadd command documents how to use it to
create new groups.
groupdel(8) — The manual page for the groupdel command documents how to use it to delete
groups.
groupmod(8) — The manual page for the groupmod command documents how to use it to
modify group membership.
gpasswd(1) — The manual page for the gpasswd command documents how to manage the
/etc/group file.
grpck(8) — The manual page for the grpck command documents how to use it to verify the
integrity of the /etc/group file.
pwck(8) — The manual page for the pwck command documents how to use it to verify the
integrity of the /etc/passwd and /etc/shadow files.
pwconv(8) — The manual page for the pwconv, pwunconv, grpconv, and grpunconv
72

pwconv(8) — The manual page for the pwconv, pwunconv, grpconv, and grpunconv
commands documents how to convert shadowed information for passwords and groups.
id(1) — The manual page for the id command documents how to display user and group IDs.
umask(2) — The manual page for the umask command documents how to work with the file
mode creation mask.
For information about related configuration files, see:
group(5) — The manual page for the /etc/group file documents how to use this file to define
system groups.
passwd(5) — The manual page for the /etc/passwd file documents how to use this file to
define user information.
shadow(5) — The manual page for the /etc/shadow file documents how to use this file to set
passwords and account expiration information for the system.
Red Hat Enterprise Linux 7 Security Guide — The Security Guide for Red Hat Enterprise Linux 7
provides additional information how to ensure password security and secure the workstation by
enabling password aging and user account locking.
See Also
and sudo commands.
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Files and directories have permission sets for the owner of the file, the group associated with the file,
and all other users for the system. However, these permission sets have limitations. For example,
different permissions cannot be configured for different users. Thus, Access Control Lists (ACLs) were
implemented.
The Red Hat Enterprise Linux kernel provides ACL support for the ext3 file system and NFS-exported
file systems. ACLs are also recognized on ext3 file systems accessed via Samba.
Along with support in the kernel, the acl package is required to implement ACLs. It contains the utilities
used to add, modify, remove, and retrieve ACL information.
The cp and mv commands copy or move any ACLs associated with files and directories.
5.1. MOUNTING FILE SYSTEMS
Before using ACLs for a file or directory, the partition for the file or directory must be mounted with ACL
support. If it is a local ext3 file system, it can mounted with the following command:
mount -t ext3 -o acl device-name partition
For example:
mount -t ext3 -o acl /dev/VolGroup00/LogVol02 /work
Alternatively, if the partition is listed in the /etc/fstab file, the entry for the partition can include the acl
option:
LABEL=/work /work ext3 acl 1 2
If an ext3 file system is accessed via Samba and ACLs have been enabled for it, the ACLs are
recognized because Samba has been compiled with the --with-acl-support option. No special flags are
required when accessing or mounting a Samba share.
5.1.1. NFS
By default, if the file system being exported by an NFS server supports ACLs and the NFS client can
read ACLs, ACLs are utilized by the client system.
To disable ACLs on NFS shares when configuring the server, include the no_acl option in the
/etc/exports file. To disable ACLs on an NFS share when mounting it on a client, mount it with the
no_acl option via the command line or the /etc/fstab file.
5.2. SETTING ACCESS ACLS
There are two types of ACLs: access ACLs and default ACLs. An access ACL is the access control list for
a specific file or directory. A default ACL can only be associated with a directory; if a file within the
directory does not have an access ACL, it uses the rules of the default ACL for the directory. Default
ACLs are optional.
ACLs can be configured:
1. Per user
74

2. Per group
3. Via the effective rights mask
4. For users not in the user group for the file
The setfacl utility sets ACLs for files and directories. Use the -m option to add or modify the ACL of a
file or directory:
# setfacl -m rules files
Rules (rules) must be specified in the following formats. Multiple rules can be specified in the same
command if they are separated by commas.
u:uid:perms
Sets the access ACL for a user. The user name or UID may be specified. The user may be any valid
user on the system.
g:gid:perms
Sets the access ACL for a group. The group name or GID may be specified. The group may be any
valid group on the system.
m:perms
Sets the effective rights mask. The mask is the union of all permissions of the owning group and all of
the user and group entries.
o:perms
Sets the access ACL for users other than the ones in the group for the file.
Permissions (perms) must be a combination of the characters r, w, and x for read, write, and execute.
If a file or directory already has an ACL, and the setfacl command is used, the additional rules are added
to the existing ACL or the existing rule is modified.
Example 5.1. Give read and write permissions
For example, to give read and write permissions to user andrius:
# setfacl -m u:andrius:rw /project/somefile
To remove all the permissions for a user, group, or others, use the -x option and do not specify any
permissions:
# setfacl -x rules files
Example 5.2. Remove all permissions
For example, to remove all permissions from the user with UID 500:
# setfacl -x u:500 /project/somefile
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5.3. SETTING DEFAULT ACLS
To set a default ACL, add d: before the rule and specify a directory instead of a file name.
Example 5.3. Setting default ACLs
For example, to set the default ACL for the /share/ directory to read and execute for users not in the
user group (an access ACL for an individual file can override it):
# setfacl -m d:o:rx /share
5.4. RETRIEVING ACLS
To determine the existing ACLs for a file or directory, use the getfacl command. In the example below,
the getfacl is used to determine the existing ACLs for a file.
Example 5.4. Retrieving ACLs
# getfacl home/john/picture.png
The above command returns the following output:
# file: home/john/picture.png
# owner: john
# group: john
user::rw-
group::r--
other::r--
If a directory with a default ACL is specified, the default ACL is also displayed as illustrated below.
For example, getfacl home/sales/ will display similar output:
# file: home/sales/
# owner: john
# group: john
user::rw-
user:barryg:r--
group::r--
mask::r--
other::r--
default:user::rwx
default:user:john:rwx
default:group::r-x
default:mask::rwx
default:other::r-x
5.5. ARCHIVING FILE SYSTEMS WITH ACLS
By default, the dump command now preserves ACLs during a backup operation. When archiving a file or
file system with tar, use the --acls option to preserve ACLs. Similarly, when using cp to copy files with
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ACLs, include the --preserve=mode option to ensure that ACLs are copied across too. In addition, the -
a option (equivalent to -dR --preserve=all) of cp also preserves ACLs during a backup along with other
information such as timestamps, SELinux contexts, and the like. For more information about dump, tar,
or cp, refer to their respective man pages.
The star utility is similar to the tar utility in that it can be used to generate archives of files; however,
some of its options are different. Refer to Table 5.1, “Command Line Options for star” for a listing of
more commonly used options. For all available options, refer to man star. The star package is required
to use this utility.
Table 5.1. Command Line Options forstar
Option Description
-c Creates an archive file.
-n Do not extract the files; use in conjunction with -x to
show what extracting the files does.
-r Replaces files in the archive. The files are written to
the end of the archive file, replacing any files with the
same path and file name.
-t Displays the contents of the archive file.
-u Updates the archive file. The files are written to the
end of the archive if they do not exist in the archive,
or if the files are newer than the files of the same
name in the archive. This option only works if the
archive is a file or an unblocked tape that may
backspace.
-x Extracts the files from the archive. If used with -U
and a file in the archive is older than the
corresponding file on the file system, the file is not
extracted.
-help Displays the most important options.
-xhelp Displays the least important options.
-/ Do not strip leading slashes from file names when
extracting the files from an archive. By default, they
are stripped when files are extracted.
-acl When creating or extracting, archives or restores any
ACLs associated with the files and directories.
5.6. COMPATIBILITY WITH OLDER SYSTEMS
If an ACL has been set on any file on a given file system, that file system has the ext_attr attribute. This
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If an ACL has been set on any file on a given file system, that file system has the ext_attr attribute. This
attribute can be seen using the following command:
# tune2fs -l filesystem-device
A file system that has acquired the ext_attr attribute can be mounted with older kernels, but those
kernels do not enforce any ACLs which have been set.
Versions of the e2fsck utility included in version 1.22 and higher of the e2fsprogs package (including
the versions in Red Hat Enterprise Linux 2.1 and 4) can check a file system with the ext_attr attribute.
Older versions refuse to check it.
5.7. ACL REFERENCES
Refer to the following man pages for more information.
man acl — Description of ACLs
man getfacl — Discusses how to get file access control lists
man setfacl — Explains how to set file access control lists
man star — Explains more about the star utility and its many options
78

System administrators, and in some cases users, need to perform certain tasks with administrative
access. Accessing the system as the root user is potentially dangerous and can lead to widespread
damage to the system and data. This chapter covers ways to gain administrative privileges using the
setuid programs such as su and sudo. These programs allow specific users to perform tasks which
would normally be available only to the root user while maintaining a higher level of control and system
security.
See the Red Hat Enterprise Linux 7 Security Guide for more information on administrative controls,
potential dangers and ways to prevent data loss resulting from improper use of privileged access.
6.1. CONFIGURING ADMINISTRATIVE ACCESS USING THE SU UTILITY
When a user executes the su command, they are prompted for the root password and, after
authentication, are given a root shell prompt.
Once logged in using the su command, the user is the root user and has absolute administrative access
to the system. Note that this access is still subject to the restrictions imposed by SELinux, if it is enabled.
In addition, once a user has become root, it is possible for them to use the su command to change to
any other user on the system without being prompted for a password.
Because this program is so powerful, administrators within an organization may want to limit who has
access to the command.
One of the simplest ways to do this is to add users to the special administrative group called wheel. To
do this, type the following command as root:
~]# usermod -a -G wheel username
In the previous command, replace username with the user name you want to add to the wheel group.
You can also use the Users settings tool to modify group memberships, as follows. Note that you need
administrator privileges to perform this procedure.
1. Press the Super key to enter the Activities Overview, type Users and then press Enter. The
Users settings tool appears. The Super key appears in a variety of guises, depending on the
keyboard and other hardware, but often as either the Windows or Command key, and typically
to the left of the Spacebar.
2. To enable making changes, click the Unlock button, and enter a valid administrator password.
3. Click a user icon in the left column to display the user’s properties in the right pane.
4. Change the Account Type from Standard to Administrator. This will add the user to the wheel
group.
See Section 4.2, “Managing Users in a Graphical Environment” for more information about the Users
tool.
After you add the desired users to the wheel group, it is advisable to only allow these specific users to
use the su command. To do this, edit the Pluggable Authentication Module (PAM) configuration file for
su, /etc/pam.d/su. Open this file in a text editor and uncomment the following line by removing the #
character:
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#auth required pam_wheel.so use_uid
This change means that only members of the administrative group wheel can switch to another user
using the su command.
6.2. CONFIGURING ADMINISTRATIVE ACCESS USING THE SUDO
UTILITY
The sudo command offers another approach to giving users administrative access. When trusted users
precede an administrative command with sudo, they are prompted for their own password. Then, when
they have been authenticated and assuming that the command is permitted, the administrative
command is executed as if they were the root user.
The basic format of the sudo command is as follows:
sudo command
In the above example, command would be replaced by a command normally reserved for the root user,
such as mount.
The sudo command allows for a high degree of flexibility. For instance, only users listed in the
/etc/sudoers configuration file are allowed to use the sudo command and the command is executed in
the user’s shell, not a root shell. This means the root shell can be completely disabled as shown in the
Red Hat Enterprise Linux 7 Security Guide .
Each successful authentication using the sudo command is logged to the file /var/log/messages and
the command issued along with the issuer’s user name is logged to the file /var/log/secure. If additional
logging is required, use the pam_tty_audit module to enable TTY auditing for specified users by adding
the following line to your /etc/pam.d/system-auth file:
session required pam_tty_audit.so disable=pattern enable=pattern
where pattern represents a comma-separated listing of users with an optional use of globs. For
example, the following configuration will enable TTY auditing for the root user and disable it for all other
users:
session required pam_tty_audit.so disable=* enable=root
IMPORTANT
Configuring the pam_tty_audit PAM module for TTY auditing records only TTY input.
This means that, when the audited user logs in, pam_tty_audit records the exact
keystrokes the user makes into the /var/log/audit/audit.log file. For more information,
see the pam_tty_audit(8) manual page.
Another advantage of the sudo command is that an administrator can allow different users access to
specific commands based on their needs.
Administrators wanting to edit the sudo configuration file, /etc/sudoers, should use the visudo
command.
To give someone full administrative privileges, type visudo and add a line similar to the following in the
user privilege specification section:
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juan ALL=(ALL) ALL
This example states that the user, juan, can use sudo from any host and execute any command.
The example below illustrates the granularity possible when configuring sudo:
%users localhost=/usr/sbin/shutdown -h now
This example states that any member of the users system group can issue the command
/sbin/shutdown -h now as long as it is issued from the console.
The man page for sudoers has a detailed listing of options for this file.
You can also configure sudo users who do not need to provide any password by using the NOPASSWD
option in the /etc/sudoers file:
user_name ALL=(ALL) NOPASSWD: ALL
However, even for such users, sudo runs Pluggable Authentication Module (PAM) account management
modules, which enables checking for restrictions imposed by PAM modules outside of the authentication
phase. This ensures that PAM modules work properly. For example, in case of the pam_time module,
the time-based account restriction does not fail.
WARNING
Always include sudo in the list of allowed services in all PAM-based access control
rules. Otherwise, users will receive a "permission denied" error message when they
try to access sudo but access is forbidden based on current access control rules.
For more information, see the Red Hat Knowledgebase article After patching to Red
Hat Enterprise Linux 7.6, sudo gives a permission denied error..
IMPORTANT

81

IMPORTANT
There are several potential risks to keep in mind when using the sudo command. You can
avoid them by editing the /etc/sudoers configuration file using visudo as described
above. Leaving the /etc/sudoers file in its default state gives every user in the wheel
group unlimited root access.
By default, sudo stores the password for a five minute timeout period. Any
subsequent uses of the command during this period will not prompt the user for a
password. This could be exploited by an attacker if the user leaves his workstation
unattended and unlocked while still being logged in. This behavior can be
changed by adding the following line to the /etc/sudoers file:
Defaults timestamp_timeout=value
where value is the desired timeout length in minutes. Setting the value to 0
causes sudo to require a password every time.
If an account is compromised, an attacker can use sudo to open a new shell with
administrative privileges:
sudo /bin/bash
Opening a new shell as root in this or similar fashion gives the attacker
administrative access for a theoretically unlimited amount of time, bypassing the
timeout period specified in the /etc/sudoers file and never requiring the attacker
to input a password for sudo again until the newly opened session is closed.
While programs allowing users to gain administrative privileges are a potential security risk, security itself
is beyond the scope of this particular book. You should therefore refer to the resources listed below for
more information regarding security and privileged access.
su(1) — The manual page for su provides information regarding the options available with this
command.
sudo(8) — The manual page for sudo includes a detailed description of this command and lists
options available for customizing its behavior.
pam(8) — The manual page describing the use of Pluggable Authentication Modules (PAM) for
Linux.
provides a more detailed look at potential security issues pertaining to the setuid programs as
well as techniques used to alleviate these risks.
See Also
Chapter 4, Managing Users and Groups documents how to manage system users and groups in
the graphical user interface and on the command line.
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To receive updates to the software on a Red Hat Enterprise Linux system it must be subscribed to the
Red Hat Content Delivery Network (CDN) and the appropriate repositories enabled. This part describes
how to subscribe a system to the Red Hat Content Delivery Network.
Red Hat provides support via the Customer Portal, and you can access this support directly from the
command line using the Red Hat Support Tool. This part describes the use of this command-line tool.
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CHAPTER 7. REGISTERING THE SYSTEM AND MANAGING
SUBSCRIPTIONS
The subscription service provides a mechanism to handle Red Hat software inventory and allows you to
install additional software or update already installed programs to newer versions using the yum
package manager. In Red Hat Enterprise Linux 7 the recommended way to register your system and
attach subscriptions is to use Red Hat Subscription Management.
NOTE
It is also possible to register the system and attach subscriptions after installation during
the initial setup process. For detailed information about the initial setup see the Initial
Setup chapter in the Installation Guide for Red Hat Enterprise Linux 7. Note that the
Initial Setup application is only available on systems installed with the X Window System
at the time of installation.
7.1. REGISTERING THE SYSTEM AND ATTACHING SUBSCRIPTIONS
Complete the following steps to register your system and attach one or more subscriptions using Red
Hat Subscription Management. Note that all subscription-manager commands are supposed to be run
as root.
1. Run the following command to register your system. You will be prompted to enter your user
name and password. Note that the user name and password are the same as your login
credentials for Red Hat Customer Portal.
subscription-manager register
2. Determine the pool ID of a subscription that you require. To do so, type the following at a shell
prompt to display a list of all subscriptions that are available for your system:
subscription-manager list --available
For each available subscription, this command displays its name, unique identifier, expiration
date, and other details related to your subscription. To list subscriptions for all architectures,
add the --all option. The pool ID is listed on a line beginning with Pool ID.
3. Attach the appropriate subscription to your system by entering a command as follows:
subscription-manager attach --pool=pool_id
Replace pool_id with the pool ID you determined in the previous step.
To verify the list of subscriptions your system has currently attached, at any time, run:
subscription-manager list --consumed
For more details on how to register your system using Red Hat Subscription Management and associate
it with subscriptions, see the designated solution article. For comprehensive information about
subscriptions, see the Red Hat Subscription Management collection of guides.
7.2. MANAGING SOFTWARE REPOSITORIES
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When a system is subscribed to the Red Hat Content Delivery Network, a repository file is created in the
/etc/yum.repos.d/ directory. To verify that, use yum to list all enabled repositories:
yum repolist
Red Hat Subscription Management also allows you to manually enable or disable software repositories
provided by Red Hat. To list all available repositories, use the following command:
subscription-manager repos --list
The repository names depend on the specific version of Red Hat Enterprise Linux you are using and are
in the following format:
rhel-version-variant-rpms
rhel-version-variant-debug-rpms
rhel-version-variant-source-rpms
Where version is the Red Hat Enterprise Linux system version ( 6 or 7), and variant is the Red Hat
Enterprise Linux system variant (server or workstation), for example:
rhel-7-server-rpms
rhel-7-server-debug-rpms
rhel-7-server-source-rpms
To enable a repository, enter a command as follows:
subscription-manager repos --enable repository
Replace repository with the name of the repository to enable.
Similarly, to disable a repository, use the following command:
subscription-manager repos --disable repository
Section 9.5, “Configuring Yum and Yum Repositories” provides detailed information about managing
software repositories using yum.
If you want to update the repositories automatically, you can use the yum-cron service. For more
information, see Section 9.7, “Automatically Refreshing Package Database and Downloading Updates
with Yum-cron”.
7.3. REMOVING SUBSCRIPTIONS
To remove a particular subscription, complete the following steps.
1. Determine the serial number of the subscription you want to remove by listing information about
already attached subscriptions:
subscription-manager list --consumed
The serial number is the number listed as serial. For instance, 744993814251016831 in the
example below:
CHAPTER 7. REGISTERING THE SYSTEM AND MANAGING SUBSCRIPTIONS
85

SKU: ES0113909
Contract: 01234567
Account: 1234567
Serial: 744993814251016831
Pool ID: 8a85f9894bba16dc014bccdd905a5e23
Active: False
Quantity Used: 1
Service Level: SELF-SUPPORT
Service Type: L1-L3
Status Details:
Subscription Type: Standard
Starts: 02/27/2015
Ends: 02/27/2016
System Type: Virtual
2. Enter a command as follows to remove the selected subscription:
subscription-manager remove --serial=serial_number
Replace serial_number with the serial number you determined in the previous step.
To remove all subscriptions attached to the system, run the following command:
subscription-manager remove --all
For more information on how to register your system using Red Hat Subscription Management and
associate it with subscriptions, see the resources listed below.
subscription-manager(8) — the manual page for Red Hat Subscription Management provides
a complete list of supported options and commands.
Related Books
Red Hat Subscription Management collection of guides — These guides contain detailed
information how to use Red Hat Subscription Management.
Installation Guide — see the Initial Setup chapter for detailed information on how to register
during the initial setup process.
See Also
and sudo commands.
Chapter 9, Yum provides information about using the yum packages manager to install and
update software.
86

CHAPTER 8. ACCESSING SUPPORT USING THE RED HAT
SUPPORT TOOL
The Red Hat Support Tool, in the redhat-support-tool package, can function as both an interactive
shell and as a single-execution program. It can be run over SSH or from any terminal. It enables, for
example, searching the Red Hat Knowledgebase from the command line, copying solutions directly on
the command line, opening and updating support cases, and sending files to Red Hat for analysis.
8.1. INSTALLING THE RED HAT SUPPORT TOOL
The Red Hat Support Toolis installed by default on Red Hat Enterprise Linux. If required, to ensure that
it is, enter the following command as root:
~]# yum install redhat-support-tool
8.2. REGISTERING THE RED HAT SUPPORT TOOL USING THE
COMMAND LINE
To register the Red Hat Support Tool to the customer portal using the command line, run the following
commands:
~]# redhat-support-tool config user username
+ Where username is the user name of the Red Hat Customer Portal account.
~]# redhat-support-tool config password
Please enter the password for username:
8.3. USING THE RED HAT SUPPORT TOOL IN INTERACTIVE SHELL
MODE
To start the tool in interactive mode, enter the following command:
~]$ redhat-support-tool
Command (? for help):
The tool can be run as an unprivileged user, with a consequently reduced set of commands, or as root.
The commands can be listed by entering the ? character. The program or menu selection can be exited
by entering the q or e character. You will be prompted for your Red Hat Customer Portal user name and
password when you first search the Knowledgebase or support cases. Alternately, set the user name and
password for your Red Hat Customer Portal account using interactive mode, and optionally save it to the
configuration file.
8.4. CONFIGURING THE RED HAT SUPPORT TOOL
When in interactive mode, the configuration options can be listed by entering the command config --
help:
87

~]# redhat-support-tool
Command (? for help): config --help
Usage: config [options] config.option <new option value>
Use the 'config' command to set or get configuration file values.
Options:
-h, --help show this help message and exit
-g, --global Save configuration option in /etc/redhat-support-tool.conf.
-u, --unset Unset configuration option.
The configuration file options which can be set are:
user : The Red Hat Customer Portal user.
password : The Red Hat Customer Portal password.
debug : CRITICAL, ERROR, WARNING, INFO, or DEBUG
url : The support services URL. Default=https://api.access.redhat.com
proxy_url : A proxy server URL.
proxy_user: A proxy server user.
proxy_password: A password for the proxy server user.
ssl_ca : Path to certificate authorities to trust during communication.
kern_debug_dir: Path to the directory where kernel debug symbols should be downloaded and
cached. Default=/var/lib/redhat-support-tool/debugkernels
Examples:
- config user
- config user my-rhn-username
- config --unset user
Registering the Red Hat Support Tool Using Interactive Mode
To register the Red Hat Support Tool to the customer portal using interactive mode, proceed as follows:
1. Start the tool by entering the following command:
2. Enter your Red Hat Customer Portal user name:
Command (? for help): config user username
To save your user name to the global configuration file, add the -g option.
3. Enter your Red Hat Customer Portal password:
Command (? for help): config password
Please enter the password for username:
8.4.1. Saving Settings to the Configuration Files
The Red Hat Support Tool, unless otherwise directed, stores values and options locally in the home
directory of the current user, using the ~/.redhat-support-tool/redhat-support-tool.conf configuration
file. If required, it is recommended to save passwords to this file because it is only readable by that
88

particular user. When the tool starts, it will read values from the global configuration file /etc/redhat-
support-tool.conf and from the local configuration file. Locally stored values and options take
precedence over globally stored settings.
WARNING
It is recommended not to save passwords in the global /etc/redhat-support-
tool.conf configuration file because the password is just base64 encoded and can
easily be decoded. In addition, the file is world readable.
To save a value or option to the global configuration file, add the -g, --global option as follows:
Command (? for help): config setting -g value
NOTE
In order to be able to save settings globally, using the -g, --global option, the Red Hat
Support Tool must be run as root because normal users do not have the permissions
required to write to /etc/redhat-support-tool.conf.
To remove a value or option from the local configuration file, add the -u, --unset option as follows:
Command (? for help): config setting -u value
This will clear, unset, the parameter from the tool and fall back to the equivalent setting in the global
configuration file, if available.
NOTE
When running as an unprivileged user, values stored in the global configuration file cannot
be removed using the -u, --unset option, but they can be cleared, unset, from the current
running instance of the tool by using the -g, --global option simultaneously with the -u, --
unset option. If running as root, values and options can be removed from the global
configuration file using -g, --global simultaneously with the -u, --unset option.
8.5. OPENING AND UPDATING SUPPORT CASES USING INTERACTIVE
MODE
Opening a New Support Case Using Interactive Mode
To open a new support case using interactive mode, proceed as follows:
2. Enter the opencase command:

89

Command (? for help): opencase
3. Follow the on screen prompts to select a product and then a version.
4. Enter a summary of the case.
5. Enter a description of the case and press Ctrl+D on an empty line when complete.
6. Select a severity of the case.
7. Optionally chose to see if there is a solution to this problem before opening a support case.
8. Confirm you would still like to open the support case.
Support case 0123456789 has successfully been opened
9. Optionally chose to attach an SOS report.
10. Optionally chose to attach a file.
Viewing and Updating an Existing Support Case Using Interactive Mode
To view and update an existing support case using interactive mode, proceed as follows:
2. Enter the getcase command:
Command (? for help): getcase case-number
Where case-number is the number of the case you want to view and update.
3. Follow the on screen prompts to view the case, modify or add comments, and get or add
attachments.
Modifying an Existing Support Case Using Interactive Mode
To modify the attributes of an existing support case using interactive mode, proceed as follows:
2. Enter the modifycase command:
Command (? for help): modifycase case-number
Where case-number is the number of the case you want to view and update.
3. The modify selection list appears:
Type the number of the attribute to modify or 'e' to return to the previous menu.
1 Modify Type
90

2 Modify Severity
3 Modify Status
4 Modify Alternative-ID
5 Modify Product
6 Modify Version
End of options.
Follow the on screen prompts to modify one or more of the options.
4. For example, to modify the status, enter 3:
Selection: 3
1 Waiting on Customer
2 Waiting on Red Hat
3 Closed
Please select a status (or 'q' to exit):
8.6. VIEWING SUPPORT CASES ON THE COMMAND LINE
Viewing the contents of a case on the command line provides a quick and easy way to apply solutions
from the command line.
To view an existing support case on the command line, enter a command as follows:
~]# redhat-support-tool getcase case-number
Where case-number is the number of the case you want to download.
The Red Hat Knowledgebase article Red Hat Support Tool has additional information, examples, and
video tutorials.
91

PART III. INSTALLING AND MANAGING SOFTWARE
All software on a Red Hat Enterprise Linux system is divided into RPM packages, which can be installed,
upgraded, or removed. This part describes how to manage packages on Red Hat Enterprise Linux using
Yum.
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CHAPTER 9. YUM
Yum is the Red Hat package manager that is able to query for information about available packages,
fetch packages from repositories, install and uninstall them, and update an entire system to the latest
available version. Yum performs automatic dependency resolution when updating, installing, or removing
packages, 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.
The following sections assume your system was registered with Red Hat Subscription Management
during installation as described in the Red Hat Enterprise Linux 7 Installation Guide. If your system is not
subscribed, see Chapter 7, Registering the System and Managing Subscriptions .
IMPORTANT
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 (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 Red Hat, and were
not modified during transfer. See Section 9.5, “Configuring Yum and Yum Repositories”
for details on enabling signature-checking with yum.
Yum also enables you to easily set up your own repositories of RPM packages for download and
installation on other machines. When possible, yum uses parallel download of multiple packages and
metadata to speed up downloading.
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.
NOTE
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.
9.1. CHECKING FOR AND UPDATING PACKAGES
Yum enables you to check if your system has any updates waiting to be applied. You can list packages
that need to be updated and update them as a whole, or you can update a selected individual package.
9.1.1. Checking For Updates
To see which installed packages on your system have updates available, use the following command:
yum check-update
CHAPTER 9. YUM
93

Example 9.1. Example output of the yum check-update command
The output of yum check-update can look as follows:
~]# yum check-update
Loaded plugins: product-id, search-disabled-repos, subscription-manager
dracut.x86_64 033-360.el7_2 rhel-7-server-rpms
dracut-config-rescue.x86_64 033-360.el7_2 rhel-7-server-rpms
kernel.x86_64 3.10.0-327.el7 rhel-7-server-rpms
rpm.x86_64 4.11.3-17.el7 rhel-7-server-rpms
rpm-libs.x86_64 4.11.3-17.el7 rhel-7-server-rpms
rpm-python.x86_64 4.11.3-17.el7 rhel-7-server-rpms
yum.noarch 3.4.3-132.el7 rhel-7-server-rpms
The packages in the above output are listed as having updates available. The first package in the list
is dracut. Each line in the example output consists of several rows, in case of dracut:
dracut — the name of the package,
x86_64 — the CPU architecture the package was built for,
033 — the version of the updated package to be installed,
360.el7 — the release of the updated package,
_2 — a build version, added as part of a z-stream update,
rhel-7-server-rpms — the repository in which the updated package is located.
The output also shows 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 rpm-libs, and
rpm-python packages), all using the yum command.
9.1.2. 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
Example 9.2. Updating the rpm package
To update the rpm package, type:
~]# yum update rpm
Loaded plugins: langpacks, product-id, subscription-manager
Updating Red Hat repositories.
INFO:rhsm-app.repolib:repos updated: 0
Setting up Update Process
Resolving Dependencies
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--> Running transaction check
---> Package rpm.x86_64 0:4.11.1-3.el7 will be updated
--> Processing Dependency: rpm = 4.11.1-3.el7 for package: rpm-libs-4.11.1-3.el7.x86_64
--> Processing Dependency: rpm = 4.11.1-3.el7 for package: rpm-python-4.11.1-3.el7.x86_64
--> Processing Dependency: rpm = 4.11.1-3.el7 for package: rpm-build-4.11.1-3.el7.x86_64
---> Package rpm.x86_64 0:4.11.2-2.el7 will be an update
...
--> Finished Dependency Resolution
Dependencies Resolved
=============================================================================
Package Arch Version Repository Size
=============================================================================
Updating:
rpm x86_64 4.11.2-2.el7 rhel 1.1 M
Updating for dependencies:
rpm-build x86_64 4.11.2-2.el7 rhel 139 k
rpm-build-libs x86_64 4.11.2-2.el7 rhel 98 k
rpm-libs x86_64 4.11.2-2.el7 rhel 261 k
rpm-python x86_64 4.11.2-2.el7 rhel 74 k
Transaction Summary
=============================================================================
Upgrade 1 Package (+4 Dependent packages)
Total size: 1.7 M
Is this ok [y/d/N]:
This output contains several items of interest:
1. Loaded plugins: langpacks, product-id, subscription-manager — Yum always informs
you which yum plug-ins are installed and enabled. See Section 9.6, “Yum Plug-ins” for
general information on yum plug-ins, or Section 9.6.3, “Working with Yum Plug-ins” for
descriptions of specific plug-ins.
2. rpm.x86_64 — you can download and install a new rpm package as well as its dependencies.
Transaction check is performed for each of these packages.
3. Yum presents the update information and then prompts you for confirmation of the update;
yum runs interactively by default. If you already know which transactions the yum command
plans to perform, you can use the -y option to automatically answer yes to any questions
that yum asks (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. You can also choose to download the package without installing
it. To do so, select the d option at the download prompt. This launches a background
download of the selected package.
If a transaction fails, you can view yum transaction history by using the yum history
command as described in Section 9.4, “Working with Transaction History” .
IMPORTANT
CHAPTER 9. YUM
95

IMPORTANT
Yum always installs a new kernel 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.
Similarly, it is possible to update a package group. Type as root:
yum group update group_name
Here, replace group_name with a name of the package group you want to update. For more information
on package groups, see Section 9.3, “Working with Package Groups” .
Yum also offers the upgrade command that is equal to update with enabled obsoletes configuration
option (see Section 9.5.1, “Setting [main] Options” ). By default, obsoletes is turned on in /etc/yum.conf,
which makes these two commands equivalent.
Updating All Packages and Their Dependencies
To update all packages and their dependencies, use the yum update command without any arguments:
yum update
Updating Security-Related Packages
If packages have security updates available, you can update only these packages to their latest versions.
Type as root:
yum update --security
You can also update packages only to versions containing the latest security updates. Type as root:
yum update-minimal --security
For example, assume that:
the kernel-3.10.0-1 package is installed on your system;
the kernel-3.10.0-2 package was released as a security update;
the kernel-3.10.0-3 package was released as a bug fix update.
Then yum update-minimal --security updates the package to kernel-3.10.0-2, and yum update --
security updates the package to kernel-3.10.0-3.
Automating Package Updating
To refresh the package database and download updates automatically, you can use the yum-cron
service. For more information, see Section 9.7, “Automatically Refreshing Package Database and
Downloading Updates with Yum-cron”.
9.1.3. Upgrading the System Off-line with ISO and Yum
For systems that are disconnected from the Internet or Red Hat Network, using the yum update
96

For systems that are disconnected from the Internet or Red Hat Network, using the yum update
command with the Red Hat Enterprise Linux installation ISO image is an easy and quick way to upgrade
systems to the latest minor version. The following steps illustrate the upgrading process:
1. Create a target directory to mount your ISO image. This directory is not automatically created
when mounting, so create it before proceeding to the next step. As root, type:
mkdir mount_dir
Replace mount_dir with a path to the mount directory. Typically, users create it as a subdirectory
in the /media directory.
2. Mount the Red Hat Enterprise Linux 7 installation ISO image to the previously created target
directory. As root, type:
mount -o loop iso_name mount_dir
Replace iso_name with a path to your ISO image and mount_dir with a path to the target
directory. Here, the -o loop option is required to mount the file as a block device.
3. Copy the media.repo file from the mount directory to the /etc/yum.repos.d/ directory. Note
that configuration files in this directory must have the .repo extension to function properly.
cp mount_dir/media.repo /etc/yum.repos.d/new.repo
This creates a configuration file for the yum repository. Replace new.repo with the filename, for
example rhel7.repo.
4. Edit the new configuration file so that it points to the Red Hat Enterprise Linux installation ISO.
Add the following line into the /etc/yum.repos.d/new.repo file:
baseurl=file:///mount_dir
Replace mount_dir with a path to the mount point.
5. Update all yum repositories including /etc/yum.repos.d/new.repo created in previous steps. As
root, type:
yum update
This upgrades your system to the version provided by the mounted ISO image.
6. After successful upgrade, you can unmount the ISO image. As root, type:
umount mount_dir
where mount_dir is a path to your mount directory. Also, you can remove the mount directory
created in the first step. As root, type:
rmdir mount_dir
7. If you will not use the previously created configuration file for another installation or update, you
can remove it. As root, type:
CHAPTER 9. YUM
97

rm /etc/yum.repos.d/new.repo
Example 9.3. Upgrading from Red Hat Enterprise Linux 7.0 to 7.1
If required to upgrade a system without access to the Internet using an ISO image with the newer
version of the system, called for example rhel-server-7.1-x86_64-dvd.iso, create a target directory
for mounting, such as /media/rhel7/. As root, change into the directory with your ISO image and type:
~]# mount -o loop
rhel-server-7.1-x86_64-dvd.iso /media/rhel7/
Then set up a yum repository for your image by copying the media.repo file from the mount
directory:
~]# cp /media/rhel7/media.repo /etc/yum.repos.d/rhel7.repo
To make yum recognize the mount point as a repository, add the following line into the
/etc/yum.repos.d/rhel7.repo copied in the previous step:
baseurl=file:///media/rhel7/
Now, updating the yum repository will upgrade your system to a version provided by rhel-server-7.1-
x86_64-dvd.iso. As root, execute:
~]# yum update
When your system is successfully upgraded, you can unmount the image, remove the target directory
and the configuration file:
~]# umount /media/rhel7/
~]# rmdir /media/rhel7/
~]# rm
/etc/yum.repos.d/rhel7.repo
9.2. WORKING WITH PACKAGES
Yum enables you to perform a complete set of operations with software packages, including searching
for packages, viewing information about them, installing and removing.
9.2.1. Searching Packages
You can search all RPM package names, descriptions and summaries by using the following command:
yum search term…
Replace term with a package name you want to search.
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Example 9.4. Searching for packages matching a specific string
To list all packages that match "vim", "gvim", or "emacs", type:
~]$ yum search vim gvim emacs
Loaded plugins: langpacks, product-id, search-disabled-repos, subscription-manager
============================= N/S matched: vim ==============================
vim-X11.x86_64 : The VIM version of the vi editor for the X Window System
vim-common.x86_64 : The common files needed by any version of the VIM editor
[output truncated]
============================ N/S matched: emacs =============================
emacs.x86_64 : GNU Emacs text editor
emacs-auctex.noarch : Enhanced TeX modes for Emacs
[output truncated]
Name and summary matches mostly, use "search all" for everything.
Warning: No matches found for: gvim
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. Note that by default, yum search returns matches in package name and
summary, which makes the search faster. Use the yum search all command for a more exhaustive but
slower search.
Filtering the Results
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 subset) and ? (which expands to match any single
character).
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 global expressions to yum. To make sure the glob
expressions are passed to yum as intended, use one of the following methods:
escape the wildcard characters by preceding them with a backslash character
double-quote or single-quote the entire glob expression.
Examples in the following section demonstrate usage of both these methods.
9.2.2. Listing Packages
To list information on all installed and available packages type the following at a shell prompt:
yum list all
To list installed and available packages that match inserted glob expressions use the following
command:
yum list glob_expression…
CHAPTER 9. YUM
99

Example 9.5. Listing ABRT-related packages
Packages with various ABRT add-ons and plug-ins either begin with "abrt-addon-", or "abrt-plugin-".
To list these packages, type the following command at a shell prompt. Note how the wildcard
characters are escaped with a backslash character:
~]$ yum list abrt-addon* abrt-plugin*
Installed Packages
abrt-addon-ccpp.x86_64 2.1.11-35.el7 @rhel-7-server-rpms
abrt-addon-kerneloops.x86_64 2.1.11-35.el7 @rhel-7-server-rpms
abrt-addon-pstoreoops.x86_64 2.1.11-35.el7 @rhel-7-server-rpms
abrt-addon-python.x86_64 2.1.11-35.el7 @rhel-7-server-rpms
abrt-addon-vmcore.x86_64 2.1.11-35.el7 @rhel-7-server-rpms
abrt-addon-xorg.x86_64 2.1.11-35.el7 @rhel-7-server-rpms
To list all packages installed on your system use the installed keyword. The rightmost column in the
output lists the repository from which the package was retrieved.
yum list installed glob_expression…
Example 9.6. Listing all installed versions of the krb package
The following example shows how to list all installed packages that begin with "krb" followed by
exactly one character and a hyphen. This is useful when you want to list all versions of certain
component as these are distinguished by numbers. The entire glob expression is quoted to ensure
proper processing.
~]$ yum list installed "krb?-*"
Installed Packages
krb5-libs.x86_64 1.13.2-10.el7 @rhel-7-server-rpms
To list all packages in all enabled repositories that are available to install, use the command in the
following form:
yum list available glob_expression…
Example 9.7. Listing available gstreamer plug-ins
For instance, to list all available packages with names that contain "gstreamer" and then "plugin", run
the following command:
~]$ yum list available gstreamer*plugin*
Available Packages
gstreamer-plugins-bad-free.i686 0.10.23-20.el7 rhel-7-server-rpms
gstreamer-plugins-base.i686 0.10.36-10.el7 rhel-7-server-rpms
gstreamer-plugins-good.i686 0.10.31-11.el7 rhel-7-server-rpms
gstreamer1-plugins-bad-free.i686 1.4.5-3.el7 rhel-7-server-rpms
gstreamer1-plugins-base.i686 1.4.5-2.el7 rhel-7-server-rpms
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gstreamer1-plugins-base-devel.i686 1.4.5-2.el7 rhel-7-server-rpms
gstreamer1-plugins-base-devel.x86_64 1.4.5-2.el7 rhel-7-server-rpms
gstreamer1-plugins-good.i686 1.4.5-2.el7 rhel-7-server-rpms
Listing Repositories
To list the repository ID, name, and number of packages for each enabled repository on your system,
use the following command:
yum repolist
To list more information about these repositories, add the -v option. With this option enabled,
information including the file name, overall size, date of the last update, and base URL are displayed for
each listed repository. As an alternative, you can use the repoinfo command that produces the same
output.
yum repolist -v
yum repoinfo
To list both enabled and disabled repositories use the following command. A status column is added to
the output list to show which of the repositories are enabled.
yum repolist all
By passing disabled as a first argument, you can reduce the command output to disabled repositories.
For further specification you can pass the ID or name of repositories or related glob_expressions as
arguments. Note that if there is an exact match between the repository ID or name and the inserted
argument, this repository is listed even if it does not pass the enabled or disabled filter.
9.2.3. Displaying Package Information
To display information about one or more packages, use the following command (glob expressions are
valid here as well):
yum info package_name…
Replace package_name with the name of the package.
Example 9.8. Displaying information on the abrt package
To display information about the abrt package, type:
~]$ yum info abrt
Installed Packages
Name : abrt
Arch : x86_64
Version : 2.1.11
Release : 35.el7
Size : 2.3 M
Repo : installed
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From repo : rhel-7-server-rpms
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 information 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 name of the yum repository the RPM package was installed from
(look for the From repo: line in the output).
Using yumdb
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 check sum of the package
(and the 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 why 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).
Example 9.9. Querying yumdb for information on the yum package
To display additional information about the yum package, type:
~]$ yumdb info yum
Loaded plugins: langpacks, product-id
yum-3.4.3-132.el7.noarch
changed_by = 1000
checksum_data = a9d0510e2ff0d04d04476c693c0313a11379053928efd29561f9a837b3d9eb02
checksum_type = sha256
command_line = upgrade
from_repo = rhel-7-server-rpms
from_repo_revision = 1449144806
from_repo_timestamp = 1449144805
installed_by = 4294967295
origin_url = https://cdn.redhat.com/content/dist/rhel/server/7/7Server/x86_64/os/Packages/yum-
3.4.3-132.el7.noarch.rpm
reason = user
releasever = 7Server
var_uuid = 147a7d49-b60a-429f-8d8f-3edb6ce6f4a1
For more information on the yumdb command, see the yumdb(8) manual page.
9.2.4. Installing Packages
To install a single package and all of its non-installed dependencies, enter a command in the following
form as root:
102

yum install package_name
You can also install multiple packages simultaneously by appending their names as arguments. To do so,
type as root:
yum install package_name package_name…
If you are installing packages on a multilib system, such as an AMD64 or Intel 64 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:
yum install package_name.arch
Example 9.10. Installing packages on multilib system
To install the sqlite package for the i686 architecture, type:
~]# yum install sqlite.i686
You can use glob expressions to quickly install multiple similarly named packages. Execute as root:
yum install glob_expression…
Example 9.11. Installing all audacious plugins
Global expressions are useful when you want to install several packages with similar names. To install
all audacious plug-ins, use the command in the following form:
~]# 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. As root, type:
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.
As you can see in the above examples, the yum install command does not require strictly defined
arguments. It can process various formats of package names and glob expressions, which makes
installation easier for users. On the other hand, it takes some time until yum parses the input correctly,
especially if you specify a large number of packages. To optimize the package search, you can use the
following commands to explicitly define how to parse the arguments:
yum install-n name
yum install-na name.architecture
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yum install-nevra name-epoch:version-release.architecture
With install-n, yum interprets name as the exact name of the package. The install-na command tells
yum that the subsequent argument contains the package name and architecture divided by the dot
character. With install-nevra, yum will expect an argument in the form name-epoch:version-
release.architecture. Similarly, you can use yum remove-n, yum remove-na, and yum remove-nevra
when searching for packages to be removed.
NOTE
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 the file is installed, use the yum provides
command with a glob expression:
~]# yum provides "*bin/named"
Loaded plugins: langpacks, product-id, search-disabled-repos, subscription-
: manager
32:bind-9.9.4-14.el7.x86_64 : The Berkeley Internet Name Domain (BIND) DNS
: (Domain Name System) server
Repo : rhel-7-server-rpms
Matched from:
Filename : /usr/sbin/named
yum provides "*/file_name" is a useful way to find the packages that contain file_name.
Example 9.12. Installation Process
The following example provides an overview of installation with use of yum. To download and install
the latest version of the httpd package, execute as root:
~]# yum install httpd
Resolving Dependencies
---> Package httpd.x86_64 0:2.4.6-12.el7 will be updated
---> Package httpd.x86_64 0:2.4.6-13.el7 will be an update
--> Processing Dependency: 2.4.6-13.el7 for package: httpd-2.4.6-13.el7.x86_64
---> Package httpd-tools.x86_64 0:2.4.6-12.el7 will be updated
---> Package httpd-tools.x86_64 0:2.4.6-13.el7 will be an update
--> Finished Dependency Resolution
Dependencies Resolved
After executing the above command, yum loads the necessary plug-ins and runs the transaction
check. In this case, httpd is already installed. Since the installed package is older than the latest
currently available version, it will be updated. The same applies to the httpd-tools package that
httpd depends on. Then, a transaction summary is displayed:
===============================================================================
=
Package Arch Version Repository Size
===============================================================================
=
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Updating:
httpd x86_64 2.4.6-13.el7 rhel-x86_64-server-7 1.2 M
Updating for dependencies:
httpd-tools x86_64 2.4.6-13.el7 rhel-x86_64-server-7 77 k
Transaction Summary
===============================================================================
=
Upgrade 1 Package (+1 Dependent package)
Total size: 1.2 M
Is this ok [y/d/N]:
In this step yum prompts you to confirm the installation. Apart from y (yes) and N (no) options, you
can choose d (download only) to download the packages but not to install them directly. If you
choose y, the installation proceeds with the following messages until it is finished successfully.
Downloading packages:
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Updating : httpd-tools-2.4.6-13.el7.x86_64 1/4
Updating : httpd-2.4.6-13.el7.x86_64 2/4
Cleanup : httpd-2.4.6-12.el7.x86_64 3/4
Cleanup : httpd-tools-2.4.6-12.el7.x86_64 4/4
Verifying : httpd-2.4.6-13.el7.x86_64 1/4
Verifying : httpd-tools-2.4.6-13.el7.x86_64 2/4
Verifying : httpd-tools-2.4.6-12.el7.x86_64 3/4
Verifying : httpd-2.4.6-12.el7.x86_64 4/4
Updated:
httpd.x86_64 0:2.4.6-13.el7
Dependency Updated:
httpd-tools.x86_64 0:2.4.6-13.el7
Complete!
To install a previously downloaded package from the local directory on your system, use the following
command:
yum localinstall path
Replace path with the path to the package you want to install.
9.2.5. Downloading Packages
As shown in Example 9.12, “Installation Process”, at a certain point of installation process you are
prompted to confirm the installation with the following message:
...
Total size: 1.2 M
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Is this ok [y/d/N]:
...
With the d option, yum downloads the packages without installing them immediately. You can install
these packages later offline with the yum localinstall command or you can share them with a different
device. Downloaded packages are saved in one of the subdirectories of the cache directory, by default
/var/cache/yum/$basearch/$releasever/packages/. The downloading proceeds in background mode
so that you can use yum for other operations in parallel.
9.2.6. Removing Packages
Similarly to package installation, yum enables you to uninstall them. 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.
Example 9.13. Removing several packages
To remove totem, type the following at a shell prompt:
~]# yum remove totem
Similar to install, remove can take these arguments:
package names
glob expressions
file lists
package provides
WARNING
Yum is not able to remove a package without also removing packages which depend
on it. This type of operation, which can only be performed by RPM, is not advised,
and can potentially leave your system in a non-functioning state or cause
applications to not work correctly or crash.
9.3. WORKING WITH PACKAGE GROUPS
A package group is a collection of packages that serve a common purpose, for instance System Tools or
Sound and Video. Installing a package group pulls a set of dependent packages, saving time
considerably. The yum groups command is a top-level command that covers all the operations that act
on package groups in yum.

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9.3.1. Listing Package Groups
The summary option is used to view the number of installed groups, available groups, available
environment groups, and both installed and available language groups:
yum groups summary
Example 9.14. Example output of yum groups summary
~]$ yum groups summary
Available Environment Groups: 12
Installed Groups: 10
Available Groups: 12
To list all package groups from yum repositories add the list option. You can filter the command output
by group names.
yum group list glob_expression…
Several optional arguments can be passed to this command, including hidden to list also groups not
marked as user visible, and ids to list group IDs. You can add language, environment, installed, or
available options to reduce the command output to a specific group type.
To list mandatory and optional packages contained in a particular group, use the following command:
yum group info glob_expression…
Example 9.15. Viewing information on the LibreOffice package group
~]$ yum group info LibreOffice
Group: LibreOffice
Group-Id: libreoffice
Description: LibreOffice Productivity Suite
Mandatory Packages:
=libreoffice-calc
libreoffice-draw
-libreoffice-emailmerge
libreoffice-graphicfilter
=libreoffice-impress
=libreoffice-math
=libreoffice-writer
+libreoffice-xsltfilter
Optional Packages:
libreoffice-base
libreoffice-pyuno
As you can see in the above example, the packages included in the package group can have different
states that are marked with the following symbols:
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" - " — Package is not installed and it will not be installed as a part of the package group.
" + " — Package is not installed but it will be installed on the next yum upgrade or yum group
upgrade.
" = " — Package is installed and it was installed as a part of the package group.
no symbol — Package is installed but it was installed outside of the package group. This means
that the yum group remove will not remove these packages.
These distinctions take place only when the group_command configuration parameter is set to
objects, which is the default setting. Set this parameter to a different value if you do not want yum to
track if a package was installed as a part of the group or separately, which will make "no symbol"
packages equivalent to "=" packages.
You can alter the above package states using the yum group mark command. For example, yum
group mark packages marks any given installed packages as members of a specified group. To avoid
installation of new packages on group update, use yum group mark blacklist. See the yum(8) man
page for more information on capabilities of yum group mark.
NOTE
You can identify an environmental group with use of the @^ prefix and a package group
can be marked with @. When using yum group list, info, install, or remove, pass
@group_name to specify a package group, @^group_name to specify an environmental
group, or group_name to include both.
9.3.2. Installing a Package Group
Each package group has a name and a group ID (groupid). To list the names of all package groups, and
their group IDs, which are displayed in parentheses, type:
yum group list ids
Example 9.16. Finding name and groupid of a package group
To find the name or ID of a package group, for example a group related to the KDE desktop
environment, type:
~]$ yum group list ids kde*
Available environment groups:
KDE Plasma Workspaces (kde-desktop-environment)
Done
Some groups are hidden by settings in the configured repositories. For example, on a server, make
use of the hidden command option to list hidden groups too:
~]$ yum group list hidden ids kde*
Loaded plugins: product-id, subscription-manager
Available Groups:
KDE (kde-desktop)
Done
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You can install a package group by passing its full group name, without the groupid part, to the group
install command. As root, type:
yum group install "group name"
You can also install by groupid. As root, execute the following command:
yum group install groupid
You can pass the groupid or quoted group name to the install command if you prepend it with an @
symbol, which tells yum that you want to perform group install. As root, type:
yum install @group
Replace group with the groupid or quoted group name. The same logic applies to environmental groups:
yum install @^group
Example 9.17. Four equivalent ways of installing the KDE Desktop group
As mentioned before, you can use four alternative, but equivalent ways to install a package group.
For KDE Desktop, the commands look as follows:
~]# yum group install "KDE Desktop"
~]# yum group install kde-desktop
~]# yum install @"KDE Desktop"
~]# yum install @kde-desktop
9.3.3. Removing a Package Group
You can remove a package group using syntax similar to the install syntax, with use of either name of
the package group or its id. As root, type:
yum group remove group_name
yum group remove groupid
Also, you can pass the groupid or quoted name to the remove command if you prepend it with an @-
symbol, which tells yum that you want to perform group remove. As root, type:
yum remove @group
Replace group with the groupid or quoted group name. Similarly, you can replace an environmental
group:
yum remove @^group
Example 9.18. Four equivalent ways of removing theKDE Desktop group
Similarly to install, you can use four alternative, but equivalent ways to remove a package group. For
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Similarly to install, you can use four alternative, but equivalent ways to remove a package group. For
KDE Desktop, the commands look as follows:
~]# yum group remove "KDE Desktop"
~]# yum group remove kde-desktop
~]# yum remove @"KDE Desktop"
~]# yum remove @kde-desktop
9.4. WORKING WITH TRANSACTION HISTORY
The yum history command enables users to review information about a timeline of yum transactions,
the dates and times they occurred, the number of packages affected, whether these 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. All history data is stored in the history
DB in the /var/lib/yum/history/ directory.
9.4.1. Listing Transactions
To display a list of the 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…
Example 9.19. Listing the five oldest transactions
In the output of yum history list, the most recent transaction is displayed at the top of the list. To
display information about the five oldest transactions stored in the history data base, type:
~]# yum history list 1..5
ID | Login user | Date and time | Action(s) | Altered
-------------------------------------------------------------------------------
5 | User <user> | 2013-07-29 15:33 | Install | 1
4 | User <user> | 2013-07-21 15:10 | Install | 1
3 | User <user> | 2013-07-16 15:27 | I, U | 73
2 | System <unset> | 2013-07-16 15:19 | Update | 1
1 | System <unset> | 2013-07-16 14:38 | Install | 1106
history list
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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 9.1,
“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 9.2, “Possible values of the Altered field” .
Table 9.1. 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 9.2. 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.
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* 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.
Symbol Description
To synchronize the rpmdb or yumdb database contents for any installed package with the currently
used rpmdb or yumdb database, type the following:
yum history sync
To display some overall statistics about the currently used history database use the following command:
yum history stats
Example 9.20. Example output of yum history stats
~]# yum history stats
File : //var/lib/yum/history/history-2012-08-15.sqlite
Size : 2,766,848
Transactions: 41
Begin time : Wed Aug 15 16:18:25 2012
End time : Wed Feb 27 14:52:30 2013
Counts :
NEVRAC : 2,204
NEVRA : 2,204
NA : 1,759
NEVR : 2,204
rpm DB : 2,204
yum DB : 2,204
history stats
Yum also enables you to display a summary of all past transactions. To do so, run the command in the
following form as root:
yum history summary
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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…
Example 9.21. Summary of the five latest transactions
~]# yum history summary 1..5
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…
Example 9.22. Tracing the history of a package
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*
ID | Action(s) | Package
-------------------------------------------------------------------------------
2 | Updated | subscription-manager-1.13.22-1.el7.x86_64 EE
2 | Update | 1.15.9-15.el7.x86_64 EE
2 | Obsoleted | subscription-manager-firstboot-1.13.22-1.el7.x86_64 EE
2 | Updated | subscription-manager-gui-1.13.22-1.el7.x86_64 EE
2 | Update | 1.15.9-15.el7.x86_64 EE
2 | Obsoleting | subscription-manager-initial-setup-addon-1.15.9-15.el7.x86_64 EE
1 | Install | subscription-manager-1.13.22-1.el7.x86_64
1 | Install | subscription-manager-firstboot-1.13.22-1.el7.x86_64
1 | Install | subscription-manager-gui-1.13.22-1.el7.x86_64
history package-list
In this example, three packages were installed during the initial system installation: subscription-
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In this example, three packages were installed during the initial system installation: subscription-
manager, subscription-manager-firstboot, and subscription-manager-gui. In the third transaction,
all these packages were updated from version 1.10.11 to version 1.10.17.
9.4.2. 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
Here, id stands for the ID of the transaction.
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
Example 9.23. Example output of yum history info
The following is sample output for two transactions, each installing one new package:
~]# yum history info 4..5
Transaction ID : 4..5
Begin time : Mon Dec 7 16:51:07 2015
Begin rpmdb : 1252:d2b62b7b5768e855723954852fd7e55f641fbad9
End time : 17:18:49 2015 (27 minutes)
End rpmdb : 1253:cf8449dc4c53fc0cbc0a4c48e496a6c50f3d43c5
User : Maxim Svistunov <msvistun>
Return-Code : Success
Command Line : install tigervnc-server.x86_64
Command Line : reinstall tigervnc-server
Transaction performed with:
Installed rpm-4.11.3-17.el7.x86_64 @rhel-7-server-rpms
Installed subscription-manager-1.15.9-15.el7.x86_64 @rhel-7-server-rpms
Installed yum-3.4.3-132.el7.noarch @rhel-7-server-rpms
Packages Altered:
Reinstall tigervnc-server-1.3.1-3.el7.x86_64 @rhel-7-server-rpms
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
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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
Example 9.24. Example output ofyum history addon-info
For the fourth transaction in the history, the yum history addon-info command provides the
following output:
~]# yum history addon-info 4
Transaction ID: 4
Available additional history information:
config-main
config-repos
saved_tx
history addon-info
In the output of the yum history addon-info command, three types of information are available:
config-main — global yum options that were in use during the transaction. See Section 9.5.1,
“Setting [main] Options” for information on how to change global options.
config-repos — options for individual yum repositories. See Section 9.5.2, “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 a selected type of additional information, run the following command as root:
yum history addon-info id information
9.4.3. 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 only 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 if the transaction uninstalled a package the command will again
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install it. This command also attempts to downgrade all updated packages to their previous version, if
these older packages are still available.
When managing several identical systems, yum also enables 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
You can configure load-transaction to ignore missing packages or rpmdb version. For more information
on these configuration options see the yum.conf(5) man page.
9.4.4. Starting New Transaction History
Yum stores the transaction history in a single SQLite database file. To start new transaction history, run
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.
9.5. CONFIGURING YUM AND YUM REPOSITORIES
NOTE
To expand your expertise, you might also be interested in the Red Hat System
Administration III (RH254) and RHCSA Rapid Track (RH199) training courses.
The configuration information for yum and related utilities is located at /etc/yum.conf. This file contains
one mandatory [main] section, which enables you to set yum options that have global effect, and can
also contain one or more [repository] sections, which allow you to set repository-specific options.
However, it is recommended to define individual repositories in new or existing .repo files in the
/etc/yum.repos.d/ directory. The values you define in individual [repository] sections of the
/etc/yum.conf file override values set in the [main] section.
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
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set up your own custom yum repository.
9.5.1. 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
The assumeyes option determines whether or not yum prompts for confirmation of critical actions.
Replace value with one of:
0 (default) — yum prompts for confirmation of critical actions it performs.
1 — Do not prompt for confirmation of critical yum actions. If assumeyes=1 is set, yum behaves in
the same way as the command-line options -y and --assumeyes.
cachedir=directory
Use this option to set the directory where yum stores its cache and database files. Replace directory
with an absolute path to the directory. By default, yum’s cache directory is
/var/cache/yum/$basearch/$releasever/.
See Section 9.5.3, “Using Yum Variables” for descriptions of the $basearch and $releasever yum
variables.
debuglevel=value
This option specifies the detail of debugging output produced by yum. Here, value is an integer
between 1 and 10. Setting a higher debuglevel value causes yum to display more detailed debugging
output. debuglevel=2 is the default, while debuglevel=0 disables debugging output.
exactarch=value
With this option, you can set yum to consider the exact architecture when updating already installed
packages. Replace value with:
0 — Do not take into account the exact architecture when updating packages.
1 (default) — Consider the exact architecture when updating packages. With this setting, yum does
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1 (default) — Consider the exact architecture when updating packages. With this setting, yum does
not install a package for 32-bit architecture to update a package already installed on the system with
64-bit architecture.
exclude=package_name more_package_names
The exclude option enables you to exclude packages by keyword during installation or system
update. Listing multiple packages for exclusion can be accomplished by quoting a space-delimited
list of packages. Shell glob expressions using wildcards (for example, * and ?) are allowed.
gpgcheck=value
Use the gpgcheck option to specify if yum should perform a GPG signature check on packages.
Replace value with:
0 — Disable GPG signature-checking on packages in all repositories, including local package
installation.
1 (default) — Enable checking of GPG signature on all packages in all repositories, including local
package installation. With gpgcheck enabled, 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.
group_command=value
Use the group_command option to specify how the yum group install, yum group upgrade, and
yum group remove commands handle a package group. Replace value with on of:
simple — Install all members of a package group. Upgrade only previously installed packages, but do
not install packages that have been added to the group in the meantime.
compat — Similar to simple but yum upgrade also installs packages that were added to the group
since the previous upgrade.
objects — (default.) With this option, yum keeps track of the previously installed groups and
distinguishes between packages installed as a part of the group and packages installed separately.
See Example 9.15, “Viewing information on the LibreOffice package group”
group_package_types=package_typemore_package_types
Here you can specify which type of packages (optional, default or mandatory) is installed when the
yum group install command is called. The default and mandatory package types are chosen by
default.
history_record=value
With this option, you can set yum to record transaction history. Replace value with one of:
0 — yum should not record history entries for transactions.
1 (default) — yum should record history entries for transactions. This operation takes certain amount
of disk space, and some extra time in the transactions, but it provides a lot of information about past
operations, which can be displayed with the yum history command. history_record=1 is the default.
For more information on the yum history command, see Section 9.4, “Working with Transaction
History”.
NOTE
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NOTE
Yum uses history records to detect modifications to the rpmdb data base that have
been done outside of yum. In such case, yum displays a warning and automatically
searches for possible problems caused by altering rpmdb. With history_record turned
off, yum is not able to detect these changes and no automatic checks are performed.
installonlypkgs=spaceseparated list of packages
Here you can provide a space-separated list of packages which yum can install, but will never update.
See 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, 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, make sure that kernel packages are always listed in installonlypkgs (as
they are by default), and installonly_limit is always 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
This option sets how many packages listed in the installonlypkgs directive can be installed at the
same time. Replace value with an integer representing the maximum number of versions that can be
installed simultaneously for any single package listed in installonlypkgs.
The defaults for the installonlypkgs directive include several different kernel packages, so be aware
that changing the value of installonly_limit also affects the maximum number of installed versions of
any single kernel package. The default value listed in /etc/yum.conf is installonly_limit=3, and the
minimum possible value is installonly_limit=2.
You cannot set installonly_limit=1 because that would make yum remove the running kernel, which
is prohibited. If installonly_limit=1 is used, yum fails.
Using installonly_limit=2 ensures that one backup kernel is available. However, it is recommended
to keep the default setting installonly_limit=3, so that you have two backup kernels available.
keepcache=value
The keepcache option determines whether yum keeps the cache of headers and packages after
successful installation. Here, value is one of:
0 (default) — Do not retain the cache of headers and packages after a successful installation.
1 — Retain the cache after a successful installation.
logfile=file_name
To specify the location for logging output, replace file_name with an absolute path to the file in which
yum should write its logging output. By default, yum logs to /var/log/yum.log.
max_connenctions=number
Here value stands for the maximum number of simultaneous connections, default is 5.
multilib_policy=value
The multilib_policy option sets the installation behavior if several architecture versions are available
for package install. Here, value stands for:
best — install the best-choice architecture for this system. For example, setting
multilib_policy=best on an AMD64 system causes yum to install the 64-bit versions of all packages.
all — always install every possible architecture for every package. For example, with multilib_policy
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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 i686 and AMD64 versions of a package, if
both were available.
obsoletes=value
The obsoletes option enables the obsoletes process logic during updates.When one package
declares in its spec file that it obsoletes another package, the latter package is replaced by the
former package when the former package is installed. Obsoletes are declared, for example, when a
package is renamed. Replace value with one of:
0 — Disable yum’s obsoletes processing logic when performing updates.
1 (default) — Enable yum’s obsoletes processing logic when performing updates.
plugins=value
This is a global switch to enable or disable yum plug-ins, value is one of:
0 — Disable all yum plug-ins globally.
IMPORTANT
Disabling all plug-ins is not advised because certain plug-ins provide important yum
services. In particular, product-id and subscription-manager plug-ins provide
support for the certificate-based Content Delivery Network (CDN). Disabling plug-
ins globally is provided as a convenience option, and is generally only recommended
when diagnosing a potential problem with yum.
1 (default) — 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, see Section 9.6, “Yum Plug-ins” . For further
information on controlling plug-ins, see Section 9.6.1, “Enabling, Configuring, and Disabling Yum
Plug-ins”.
reposdir=directory
Here, 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
This option sets the number of times yum should attempt to retrieve a file before returning an error.
value is an integer 0 or greater. Setting value to 0 makes yum retry forever. The default value is 10.
For a complete list of available [main] options, see the [main] OPTIONS section of the yum.conf(5)
manual page.
9.5.2. 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. To avoid conflicts, custom
repositories should not use names used by Red Hat repositories.
The following is a bare minimum example of the form a [repository] section takes:
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[repository]
name=repository_name
baseurl=repository_url
Every [repository] section must contain the following directives:
name=repository_name
Here, repository_name is a human-readable string describing the repository.
baseurl=repository_url
Replace repository_url with 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 user
name 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 user name 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 $basearch variables in URLs.
For more information about yum variables, see Section 9.5.3, “Using Yum Variables”.
Other useful [repository] directive are:
enabled=value
This is a simple way to tell yum to use or ignore a particular repository, value is one of:
0 — Do not include this repository as a package source when performing updates and installs. This is
an easy way of quickly turning repositories on and off, which is useful when you desire a single
package from a repository that you do not want to enable for updates or installs.
1 — Include this repository as a package source.
Turning repositories on and off can also be performed by passing either the --
enablerepo=repo_name or --disablerepo=repo_name option to yum, or through the
Add/Remove Software window of the PackageKit utility.
async=value
Controls parallel downloading of repository packages. Here, value is one of:
auto (default) — parallel downloading is used if possible, which means that yum automatically
disables it for repositories created by plug-ins to avoid failures.
on — parallel downloading is enabled for the repository.
off — parallel downloading is disabled for the repository.
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Many more [repository] options exist, part of them have the same form and function as certain [main]
options. For a complete list, see the [repository] OPTIONS section of the yum.conf(5) manual page.
Example 9.25. A sample /etc/yum.repos.d/redhat.repo file
The following is a sample /etc/yum.repos.d/redhat.repo file:
#
# Red Hat Repositories
# Managed by (rhsm) subscription-manager
#
[red-hat-enterprise-linux-scalable-file-system-for-rhel-6-entitlement-rpms]
name = Red Hat Enterprise Linux Scalable File System (for RHEL 6 Entitlement) (RPMs)
baseurl = https://cdn.redhat.com/content/dist/rhel/entitlement-
6/releases/$releasever/$basearch/scalablefilesystem/os
enabled = 1
gpgcheck = 1
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release
sslverify = 1
sslcacert = /etc/rhsm/ca/redhat-uep.pem
sslclientkey = /etc/pki/entitlement/key.pem
sslclientcert = /etc/pki/entitlement/11300387955690106.pem
[red-hat-enterprise-linux-scalable-file-system-for-rhel-6-entitlement-source-rpms]
name = Red Hat Enterprise Linux Scalable File System (for RHEL 6 Entitlement) (Source RPMs)
6/releases/$releasever/$basearch/scalablefilesystem/source/SRPMS
enabled = 0
gpgcheck = 1
sslverify = 1
[red-hat-enterprise-linux-scalable-file-system-for-rhel-6-entitlement-debug-rpms]
name = Red Hat Enterprise Linux Scalable File System (for RHEL 6 Entitlement) (Debug RPMs)
6/releases/$releasever/$basearch/scalablefilesystem/debug
enabled = 0
gpgcheck = 1
sslverify = 1
9.5.3. 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
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You can use this variable to reference the release version of Red Hat Enterprise Linux. 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-releaseproduct package that provides the redhat-release file.
$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 Intel 64 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 $osname, create a new file with "Red Hat Enterprise Linux" on the first line and save it as
/etc/yum/vars/osname:
~]# echo "Red Hat Enterprise Linux 7" > /etc/yum/vars/osname
Instead of "Red Hat Enterprise Linux 7", you can now use the following in the .repo files:
name=$osname $releasever
9.5.4. Viewing the Current Configuration
To display the current values of global yum options (that is, the options specified in the [main] section of
the /etc/yum.conf file), execute the yum-config-manager command with no command-line options:
yum-config-manager
To list the content of a different configuration section or sections, use the command in the following
form:
yum-config-manager section…
You can also use a glob expression to display the configuration of all matching sections:
yum-config-manager glob_expression…
Example 9.26. Viewing configuration of the main section
To list all configuration options and their corresponding values for the main section, type the
following at a shell prompt:
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~]$ yum-config-manager main *
================================== 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]
9.5.5. Adding, Enabling, and Disabling a Yum Repository
NOTE
Administration III (RH254) training course.
Section 9.5.2, “Setting [repository] Options” describes 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.
IMPORTANT
When the system is registered with Red Hat Subscription Management to the certificate-
based Content Delivery Network (CDN), the Red Hat Subscription Managertools are
used to manage repositories in the /etc/yum.repos.d/redhat.repo file.
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 it is recommended to define your repositories here instead of in /etc/yum.conf.
WARNING
Obtaining and installing software packages from unverified or untrusted software
sources other than Red Hat’s certificate-based Content Delivery Network (CDN)
constitutes a potential security risk, and could lead to security, stability,
compatibility, and 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

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…where repository_url is a link to the .repo file.
Example 9.27. Adding example.repo
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
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…
Example 9.28. Enabling repositories defined in custom sections of /etc/yum.conf.
To enable repositories defined in the [example], [example-debuginfo], and [example-
source]sections, type:
~]# yum-config-manager --enable example*
============================== repo: example ==============================
[example]
bandwidth = 0
base_persistdir = /var/lib/yum/repos/x86_64/7Server
baseurl = http://www.example.com/repo/7Server/x86_64/
cache = 0
cachedir = /var/cache/yum/x86_64/7Server/example
[output truncated]
Example 9.29. Enabling all repositories
To enable all repositories defined both in the /etc/yum.conf file and in the /etc/yum.repos.d/
directory, type:
~]# yum-config-manager --enable *
============================== repo: example ==============================
[example]
bandwidth = 0
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cache = 0
[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…
Example 9.30. Disabling all repositories
To disable all repositories defined both in the /etc/yum.conf file and in the /etc/yum.repos.d/
directory, type:
~]# yum-config-manager --disable *
============================== repo: example ==============================
[example]
bandwidth = 0
cache = 0
[output truncated]
When successful, the yum-config-manager --disable command displays the current configuration.
9.5.6. Creating a Yum Repository
To set up a yum repository:
1. Install the createrepo package:
# yum install createrepo
2. Copy all packages for your new repository into one directory, such as /tmp/local_repo/:
cp /your/packages/*.rpm /tmp/local_repo/
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3. To create the repository run:
createrepo /tmp/local_repo/
This creates the necessary metadata for the yum repository and places metadata in a newly
created subdirectory repodata.
The repository is now ready to be consumed by yum. This repository can be shared over the
HTTP or FTP protocol, or refered directly from the local machine. See the Section 9.5.2,
“Setting [repository] Options” section for more details on how to configure a yum repository.
NOTE
When constructing the URL for a repository, refer to the /mnt/local_repo not to
/mnt/local_repo/repodata, as this directory contains only metadata. Actual yum
packages are in /mnt/local_repo.
9.5.6.1. Adding packages to an already created yum repository
To add packages to an already created yum repository:
1. Copy the new packages to your repository directory, such as /tmp/local_repo/:
cp /your/packages/*.rpm /tmp/local_repo/
2. To reflect the newly added packages in the metadata, run:
createrepo --update /tmp/local_repo/
3. Optional: If you have already used any yum command with newly updated repository, run:
yum clean expire-cache
9.5.7. Adding the Optional and Supplementary Repositories
The Optional and Supplementary subscription channels provide additional software packages for Red
Hat Enterprise Linux that cover open source licensed software (in the Optional channel) and proprietary
licensed software (in the Supplementary channel).
Before subscribing to the Optional and Supplementary channels see the Scope of Coverage Details. If
you decide to install packages from these channels, follow the steps documented in the article called
How to access Optional and Supplementary channels, and -devel packages using Red Hat Subscription
Manager (RHSM)? on the Red Hat Customer Portal.
9.6. 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
[output truncated]
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Note that the plug-in names which follow Loaded plugins are the names you can provide to the --
disableplugin=plugin_name option.
9.6.1. 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 1:
plugins=1
You can disable all plug-ins by changing this line to plugins=0.
IMPORTANT
Disabling all plug-ins is not advised because certain plug-ins provide important yum
services. In particular, the product-id and subscription-manager plug-ins provide
support for the certificate-based Content Delivery Network (CDN). 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 aliases plug-in’s aliases.conf
configuration file:
[main]
enabled=1
Similar to the /etc/yum.conf file, the plug-in configuration files always contain a [main] section where
the enabled= option controls whether the plug-in is enabled when you run yum commands. If this
option is missing, you can add it manually to the file.
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 aliases plug-in while
updating a system, type:
~]# yum update --disableplugin=aliases
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=aliases,lang*
9.6.2. 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 kabi plug-in is named kabi-yum-plugins, for example. You can
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install a yum plug-in in the same way you install other packages. For instance, to install the yum-aliases
plug-in, type the following at a shell prompt:
~]# yum install yum-plugin-aliases
9.6.3. Working with Yum Plug-ins
The following list provides descriptions and usage instructions for several useful yum plug-ins. Plug-ins
are listed by names, brackets contain the name of the package.
search-disabled-repos (subscription-manager)
The search-disabled-repos plug-in allows you to temporarily or permanently enable disabled
repositories to help resolve dependencies. With this plug-in enabled, when Yum fails to install a
package due to failed dependency resolution, it offers to temporarily enable disabled repositories
and try again. If the installation succeeds, Yum also offers to enable the used repositories
permanently. Note that the plug-in works only with the repositories that are managed by
subscription-manager and not with custom repositories.
IMPORTANT
If yum is executed with the --assumeyes or -y option, or if the assumeyes directive
is enabled in /etc/yum.conf, the plug-in enables disabled repositories, both
temporarily and permanently, without prompting for confirmation. This may lead to
problems, for example, enabling repositories that you do not want enabled.
To configure the search-disabled-repos plug-in, edit the configuration file located in
/etc/yum/pluginconf.d/search-disabled-repos.conf. For the list of directives you can use in the
[main] section, see the table below.
Table 9.3. Supported search-disabled-repos.conf directives
Directive Description
enabled=value Allows you to enable or disable the plug-in. The
value must be either1 (enabled), or0 (disabled).
The plug-in is enabled by default.
notify_only=value Allows you to restrict the behavior of the plug-in to
notifications only. The value must be either1
(notify only without modifying the behavior of
Yum), or 0 (modify the behavior of Yum). By
default the plug-in only notifies the user.
ignored_repos=repositories Allows you to specify the repositories that will not
be enabled by the plug-in.
kabi (kabi-yum-plugins)
The kabi plug-in checks whether a driver update package conforms with the official Red Hat kernel
Application Binary Interface (kABI). With this plug-in enabled, when a user attempts to install a
package that uses kernel symbols which are not on a whitelist, a warning message is written to the
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system log. Additionally, configuring the plug-in to run in enforcing mode prevents such packages
from being installed at all.
To configure the kabi plug-in, edit the configuration file located in /etc/yum/pluginconf.d/kabi.conf.
A list of directives that can be used in the [main] section is shown in the table below.
Table 9.4. Supported kabi.conf directives
Directive Description
enabled=value Allows you to enable or disable the plug-in. The
value must be either1 (enabled), or0 (disabled).
When installed, the plug-in is enabled by default.
whitelists=directory Allows you to specify the directory in which the files
with supported kernel symbols are located. By
default, the kabi plug-in uses files provided by the
kernel-abi-whitelists package (that is, the
/usr/lib/modules/kabi-rhel70/ directory).
enforce=value Allows you to enable or disable enforcing mode.
The value must be either1 (enabled), or0
(disabled). By default, this option is commented out
and the kabi plug-in only displays a warning
message.
product-id (subscription-manager)
The product-id plug-in manages product identity certificates for products installed from the
Content Delivery Network. The product-id plug-in is installed by default.
langpacks (yum-langpacks)
The langpacks plug-in is used to search for locale packages of a selected language for every
package that is installed. The langpacks plug-in is installed by default.
aliases (yum-plugin-aliases)
The aliases plug-in adds the alias command-line option which enables configuring and using aliases
for yum commands.
yum-changelog (yum-plugin-changelog)
The yum-changelog plug-in adds the --changelog command-line option that enables viewing
package change logs before and after updating.
yum-tmprepo (yum-plugin-tmprepo)
The yum-tmprepo plug-in adds the --tmprepo command-line option that takes the URL of a
repository file, downloads and enables it for only one transaction. This plug-in tries to ensure the safe
temporary usage of repositories. By default, it does not allow to disable the gpg check.
yum-verify (yum-plugin-verify)
The yum-verify plug-in adds the verify, verify-rpm, and verify-all command-line options for viewing
verification data on the system.
yum-versionlock (yum-plugin-versionlock)
The yum-versionlock plug-in excludes other versions of selected packages, which enables
protecting packages from being updated by newer versions. With the versionlock command-line
option, you can view and edit the list of locked packages.
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9.7. AUTOMATICALLY REFRESHING PACKAGE DATABASE AND
DOWNLOADING UPDATES WITH YUM-CRON
The yum-cron service checks and downloads package updates automatically. The cron jobs provided by
the yum-cron service are active immediately after installation of the yum-cron package. The yum-cron
service can also automatically install downloaded updates.
With default settings, the yum-cron service:
Updates the metadata in the yum cache once per hour.
Downloads pending package updates to the yum cache once per day. If new packages are
available in the repository, an email is sent. See chapter Section 9.7.2, “Setting up Optional
Email Notifications” for more information.
The yum-cron service has two configuration files:
/etc/yum/yum-cron.conf
For daily tasks.
/etc/yum/yum-cron-hourly.conf
For hourly tasks.
9.7.1. Enabling Automatic Installation of Updates
To enable automatic installation of downloaded updates, edit the daily configuration file for daily
installation or the hourly configuration file for hourly installation by setting the apply_updates option as
follows:
apply_updates = yes
9.7.2. Setting up Optional Email Notifications
By default, the yum-cron service uses cron to send emails containing an output of the executed
command. This email is sent according to cron configuration, typically to the local superuser and stored
in the /var/spool/mail/root file.
You can use specific email configuration different from the settings which affect all cron jobs. However,
this email configuration does not support TLS and overall email built-in logic is very basic.
To enable yum-cron built-in email notifications:
1. Open selected yum-cron configuration file:
For daily tasks.
For hourly tasks.
2. In the [emitters] section, set the following option:
emit_via = email
3. Set the email_from, email_to, email_host options as required
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9.7.3. Enabling or Disabling Specific Repositories
The yum-cron does not support specific configuration of repositories. As a workaround for enabling or
disabling specific repositories for yum-cron but not for yum in general follow the steps bellow:
1. Create an empty repository configuration directory anywhere on the system.
2. Copy all configuration files from the /etc/yum.repos.d/ directory to this newly created directory.
3. In the respective .repo configuration file within the /etc/yum.repos.d/, set the enabled option
as follows:
enabled = 1
To enable the repository.
enabled = 0
To disable the repository.
4. Add the following option, which points to the newly created repository directory, at the end of
the selected yum-cron configuration file:
reposdir=/path/to/new/reposdir
9.7.4. Testing Yum-cron Settings
To test yum-cron settings without waiting for the next scheduled yum-cron task:
For daily tasks.
For hourly tasks.
2. Set the random_sleep option in the selected configuration file as follows:
random_sleep = 0
3. Run the configuration files:
# yum-cron /etc/yum/yum-cron.conf
# yum-cron /etc/yum/yum-cron-hourly.conf
9.7.5. Disabling Yum-cron messages
The yum-cron messages cannot be entirely disabled, but can be limited to messages with critical
priority only. To limit the messages:
For daily tasks.
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For hourly tasks.
2. Set the following option in the [base] section of the configuration file:
debuglevel = -4
9.7.6. Automatically Cleaning Packages
The yum-cron service does not support any configuration option for removing packages similar to the
yum clean all command. To clean packages automatically, you can create a cron job as an executable
shell script:
1. Create a shell script in the /etc/cron.daily/ directory containing:
#!/bin/sh
yum clean all
2. Make the script executable:
# chmod +x /etc/cron.daily/script-name.sh
For more information on how to manage software packages on Red Hat Enterprise Linux, see the
yum(8) — The manual page for the yum command-line utility provides a complete list of
supported options and commands.
yumdb(8) — The manual page for the yumdb command-line utility documents how to use this
tool to query and, if necessary, alter the yum database.
yum.conf(5) — The manual page named yum.conf documents available yum configuration
options.
yum-utils(1) — The manual page named yum-utils lists and briefly describes additional utilities
for managing yum configuration, manipulating repositories, and working with yum database.
Online Resources
Yum Guides — The Yum Guides page on the project home page provides links to further
documentation.
Red Hat Customer Portal Labs — The Red Hat Customer Portal Labs includes a "Yum
Repository Configuration Helper".
See Also
and sudo commands.
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PART IV. INFRASTRUCTURE SERVICES
This part provides information on how to configure services and daemons and enable remote access to
a Red Hat Enterprise Linux machine.
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10.1. INTRODUCTION TO SYSTEMD
Systemd is a system and service manager for Linux operating systems. It is designed to be backwards
compatible with SysV init scripts, and provides a number of features such as parallel startup of system
services at boot time, on-demand activation of daemons, or dependency-based service control logic. In
Red Hat Enterprise Linux 7, systemd replaces Upstart as the default init system.
Systemd introduces the concept of systemd units. These units are represented by unit configuration
files located in one of the directories listed in Table 10.2, “Systemd Unit Files Locations” , and
encapsulate information about system services, listening sockets, and other objects that are relevant to
the init system. For a complete list of available systemd unit types, see Table 10.1, “Available systemd
Unit Types”.
Table 10.1. Available systemd Unit Types
Unit Type File Extension Description
Service unit .service A system service.
Target unit .target A group of systemd units.
Automount unit .automount A file system automount point.
Device unit .device A device file recognized by the
kernel.
Mount unit .mount A file system mount point.
Path unit .path A file or directory in a file system.
Scope unit .scope An externally created process.
Slice unit .slice A group of hierarchically
organized units that manage
system processes.
Snapshot unit .snapshot A saved state of the systemd
manager.
Socket unit .socket An inter-process communication
socket.
Swap unit .swap A swap device or a swap file.
Timer unit .timer A systemd timer.
Table 10.2. Systemd Unit Files Locations
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Directory Description
/usr/lib/systemd/system/ Systemd unit files distributed with installed RPM
packages.
/run/systemd/system/ Systemd unit files created at run time. This directory
takes precedence over the directory with installed
service unit files.
/etc/systemd/system/ Systemd unit files created by systemctl enable as
well as unit files added for extending a service. This
directory takes precedence over the directory with
runtime unit files.
Overriding the Default systemd Configuration Using system.conf
The default configuration of systemd is defined during the compilation and it can be found in systemd
configuration file at /etc/systemd/system.conf. Use this file if you want to deviate from those defaults
and override selected default values for systemd units globally.
For example, to override the default value of the timeout limit, which is set to 90 seconds, use the
DefaultTimeoutStartSec parameter to input the required value in seconds.
DefaultTimeoutStartSec=required value
See also Example 10.21, “Changing the timeout limit” .
In Red Hat Enterprise Linux 7, the systemd system and service manager provides the following main
features:
Socket-based activation — At boot time, systemd creates listening sockets for all system
services that support this type of activation, and passes the sockets to these services as soon as
they are started. This not only allows systemd to start services in parallel, but also makes it
possible to restart a service without losing any message sent to it while it is unavailable: the
corresponding socket remains accessible and all messages are queued.
Systemd uses socket units for socket-based activation.
Bus-based activation — System services that use D-Bus for inter-process communication can be
started on-demand the first time a client application attempts to communicate with them.
Systemd uses D-Bus service files for bus-based activation.
Device-based activation — System services that support device-based activation can be started
on-demand when a particular type of hardware is plugged in or becomes available. Systemd
uses device units for device-based activation.
Path-based activation — System services that support path-based activation can be started on-
demand when a particular file or directory changes its state. Systemd uses path units for path-
based activation.
Mount and automount point management — Systemd monitors and manages mount and
automount points. Systemd uses mount units for mount points and automount units for
automount points.
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Aggressive parallelization — Because of the use of socket-based activation, systemd can start
system services in parallel as soon as all listening sockets are in place. In combination with
system services that support on-demand activation, parallel activation significantly reduces the
time required to boot the system.
Transactional unit activation logic — Before activating or deactivating a unit, systemd calculates
its dependencies, creates a temporary transaction, and verifies that this transaction is
consistent. If a transaction is inconsistent, systemd automatically attempts to correct it and
remove non-essential jobs from it before reporting an error.
Backwards compatibility with SysV init — Systemd supports SysV init scripts as described in the
Linux Standard Base Core Specification , which eases the upgrade path to systemd service units.
10.1.2. Compatibility Changes
The systemd system and service manager is designed to be mostly compatible with SysV init and
Upstart. The following are the most notable compatibility changes with regards to the previous major
release of the Red Hat Enterprise Linux system:
Systemd has only limited support for runlevels. It provides a number of target units that can be
directly mapped to these runlevels and for compatibility reasons, it is also distributed with the
earlier runlevel command. Not all systemd targets can be directly mapped to runlevels,
however, and as a consequence, this command might return N to indicate an unknown runlevel.
It is recommended that you avoid using the runlevel command if possible.
For more information about systemd targets and their comparison with runlevels, see
Section 10.3, “Working with systemd Targets”.
The systemctl utility does not support custom commands. In addition to standard commands
such as start, stop, and status, authors of SysV init scripts could implement support for any
number of arbitrary commands in order to provide additional functionality. For example, the init
script for iptables in Red Hat Enterprise Linux 6 could be executed with the panic command,
which immediately enabled panic mode and reconfigured the system to start dropping all
incoming and outgoing packets. This is not supported in systemd and the systemctl only
accepts documented commands.
For more information about the systemctl utility and its comparison with the earlier service
utility, see Section 10.2, “Managing System Services” .
The systemctl utility does not communicate with services that have not been started by
systemd. When systemd starts a system service, it stores the ID of its main process in order to
keep track of it. The systemctl utility then uses this PID to query and manage the service.
Consequently, if a user starts a particular daemon directly on the command line, systemctl is
unable to determine its current status or stop it.
Systemd stops only running services. Previously, when the shutdown sequence was initiated,
Red Hat Enterprise Linux 6 and earlier releases of the system used symbolic links located in the
/etc/rc0.d/ directory to stop all available system services regardless of their status. With
systemd, only running services are stopped on shutdown.
System services are unable to read from the standard input stream. When systemd starts a
service, it connects its standard input to /dev/null to prevent any interaction with the user.
System services do not inherit any context (such as the HOME and PATH environment
variables) from the invoking user and their session. Each service runs in a clean execution
context.
When loading a SysV init script, systemd reads dependency information encoded in the Linux
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When loading a SysV init script, systemd reads dependency information encoded in the Linux
Standard Base (LSB) header and interprets it at run time.
All operations on service units are subject to a default timeout of 5 minutes to prevent a
malfunctioning service from freezing the system. This value is hardcoded for services that are
generated from initscripts and cannot be changed. However, individual configuration files can
be used to specify a longer timeout value per service, see Example 10.21, “Changing the timeout
limit”
For a detailed list of compatibility changes introduced with systemd, see the Migration Planning Guide
for Red Hat Enterprise Linux 7.
10.2. MANAGING SYSTEM SERVICES
NOTE
Administration II (RH134) training course.
Previous versions of Red Hat Enterprise Linux, which were distributed with SysV init or Upstart, used init
scripts located in the /etc/rc.d/init.d/ directory. These init scripts were typically written in Bash, and
allowed the system administrator to control the state of services and daemons in their system. In Red
Hat Enterprise Linux 7, these init scripts have been replaced with service units.
Service units end with the .service file extension and serve a similar purpose as init scripts. To view,
start, stop, restart, enable, or disable system services, use the systemctl command as described in
Table 10.3, “Comparison of the service Utility with systemctl” , Table 10.4, “Comparison of the chkconfig
Utility with systemctl”, and further in this section. The service and chkconfig commands are still
available in the system and work as expected, but are only included for compatibility reasons and should
be avoided.
Table 10.3. Comparison of the service Utility with systemctl
service systemctl Description
service name start systemctl start name.service Starts a service.
service name stop systemctl stop name.service Stops a service.
service name restart systemctl restart
name.service
Restarts a service.
service name condrestart systemctl try-restart
name.service
Restarts a service only if it is
running.
service name reload systemctl reload
name.service
Reloads configuration.
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service name status systemctl status
name.service
systemctl is-active
name.service
Checks if a service is running.
service --status-all systemctl list-units --type
service --all
Displays the status of all services.
service systemctl Description
Table 10.4. Comparison of the chkconfig Utility with systemctl
chkconfig systemctl Description
chkconfig name on systemctl enable
name.service
Enables a service.
chkconfig name off systemctl disable
name.service
Disables a service.
chkconfig --list name systemctl status
name.service
systemctl is-enabled
name.service
Checks if a service is enabled.
chkconfig --list systemctl list-unit-files --
type service
Lists all services and checks if
they are enabled.
chkconfig --list systemctl list-dependencies
--after
Lists services that are ordered to
start before the specified unit.
chkconfig --list systemctl list-dependencies
--before
Lists services that are ordered to
start after the specified unit.
Specifying Service Units
For clarity, all command examples in the rest of this section use full unit names with the .service file
extension, for example:
~]# systemctl stop nfs-server.service
However, the file extension can be omitted, in which case the systemctl utility assumes the argument is
a service unit. The following command is equivalent to the one above:
~]# systemctl stop nfs-server
Additionally, some units have alias names. Those names can have shorter names than units, which can be
used instead of the actual unit names. To find all aliases that can be used for a particular unit, use:
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~]# systemctl show nfs-server.service -p Names
Behavior of systemctl in a chroot Environment
If you change the root directory using the chroot command, most systemctl commands refuse to
perform any action. The reason for this is that the systemd process and the user that used the chroot
command do not have the same view of the filesystem. This happens, for example, when systemctl is
invoked from a kickstart file.
The exception to this are unit file commands such as the systemctl enable and systemctl disable
commands. These commands do not need a running system and do not affect running processes, but
they do affect unit files. Therefore, you can run these commands even in chroot environment. For
example, to enable the httpd service on a system under the /srv/website1/ directory:
~]# chroot /srv/website1
~]# systemctl enable httpd.service
Created symlink /etc/systemd/system/multi-user.target.wants/httpd.service, pointing to
/usr/lib/systemd/system/httpd.service.
10.2.1. Listing Services
To list all currently loaded service units, type the following at a shell prompt:
systemctl list-units --type service
For each service unit file, this command displays its full name (UNIT) followed by a note whether the unit
file has been loaded (LOAD), its high-level ( ACTIVE) and low-level (SUB) unit file activation state, and
a short description (DESCRIPTION).
By default, the systemctl list-units command displays only active units. If you want to list all loaded
units regardless of their state, run this command with the --all or -a command line option:
systemctl list-units --type service --all
You can also list all available service units to see if they are enabled. To do so, type:
systemctl list-unit-files --type service
For each service unit, this command displays its full name (UNIT FILE) followed by information whether
the service unit is enabled or not (STATE). For information on how to determine the status of individual
service units, see Section 10.2.2, “Displaying Service Status”.
Example 10.1. Listing Services
To list all currently loaded service units, run the following command:
~]$ systemctl list-units --type service
UNIT LOAD ACTIVE SUB DESCRIPTION
abrt-ccpp.service loaded active exited Install ABRT coredump hook
abrt-oops.service loaded active running ABRT kernel log watcher
abrt-vmcore.service loaded active exited Harvest vmcores for ABRT
abrt-xorg.service loaded active running ABRT Xorg log watcher
abrtd.service loaded active running ABRT Automated Bug Reporting Tool
...
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systemd-vconsole-setup.service loaded active exited Setup Virtual Console
tog-pegasus.service loaded active running OpenPegasus CIM Server
LOAD = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB = The low-level unit activation state, values depend on unit type.
46 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'
To list all installed service unit files to determine if they are enabled, type:
~]$ systemctl list-unit-files --type service
UNIT FILE STATE
abrt-ccpp.service enabled
abrt-oops.service enabled
abrt-vmcore.service enabled
abrt-xorg.service enabled
abrtd.service enabled
...
wpa_supplicant.service disabled
ypbind.service disabled
208 unit files listed.
10.2.2. Displaying Service Status
To display detailed information about a service unit that corresponds to a system service, type the
following at a shell prompt:
systemctl status name.service
Replace name with the name of the service unit you want to inspect (for example, gdm). This command
displays the name of the selected service unit followed by its short description, one or more fields
described in Table 10.5, “Available Service Unit Information”, and if it is executed by the root user, also
the most recent log entries.
Table 10.5. Available Service Unit Information
Field Description
Loaded Information whether the service unit has been
loaded, the absolute path to the unit file, and a note
whether the unit is enabled.
Active Information whether the service unit is running
followed by a time stamp.
Main PID The PID of the corresponding system service
followed by its name.
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Status Additional information about the corresponding
system service.
Process Additional information about related processes.
CGroup Additional information about related Control Groups
(cgroups).
Field Description
To only verify that a particular service unit is running, run the following command:
systemctl is-active name.service
Similarly, to determine whether a particular service unit is enabled, type:
systemctl is-enabled name.service
Note that both systemctl is-active and systemctl is-enabled return an exit status of 0 if the specified
service unit is running or enabled. For information on how to list all currently loaded service units, see
Section 10.2.1, “Listing Services”.
Example 10.2. Displaying Service Status
The service unit for the GNOME Display Manager is named gdm.service. To determine the current
status of this service unit, type the following at a shell prompt:
~]# systemctl status gdm.service
gdm.service - GNOME Display Manager
Loaded: loaded (/usr/lib/systemd/system/gdm.service; enabled)
Active: active (running) since Thu 2013-10-17 17:31:23 CEST; 5min ago
Main PID: 1029 (gdm)
CGroup: /system.slice/gdm.service
├─1029 /usr/sbin/gdm
├─1037 /usr/libexec/gdm-simple-slave --display-id /org/gno...
└─1047 /usr/bin/Xorg :0 -background none -verbose -auth /r...
Oct 17 17:31:23 localhost systemd[1]: Started GNOME Display Manager.
Example 10.3. Displaying Services Ordered to Start Before a Service
To determine what services are ordered to start before the specified service, type the following at a
shell prompt:
~]# systemctl list-dependencies --after gdm.service
gdm.service
├─dbus.socket
├─getty@tty1.service
├─livesys.service
├─plymouth-quit.service
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├─system.slice
├─systemd-journald.socket
├─systemd-user-sessions.service
└─basic.target
[output truncated]
Example 10.4. Displaying Services Ordered to Start After a Service
To determine what services are ordered to start after the specified service, type the following at a
shell prompt:
~]# systemctl list-dependencies --before gdm.service
gdm.service
├─dracut-shutdown.service
├─graphical.target
│ ├─systemd-readahead-done.service
│ ├─systemd-readahead-done.timer
│ └─systemd-update-utmp-runlevel.service
└─shutdown.target
├─systemd-reboot.service
└─final.target
└─systemd-reboot.service
10.2.3. Starting a Service
To start a service unit that corresponds to a system service, type the following at a shell prompt as root:
systemctl start name.service
Replace name with the name of the service unit you want to start (for example, gdm). This command
starts the selected service unit in the current session. For information on how to enable a service unit to
be started at boot time, see Section 10.2.6, “Enabling a Service” . For information on how to determine
the status of a certain service unit, see Section 10.2.2, “Displaying Service Status”.
Example 10.5. Starting a Service
The service unit for the Apache HTTP Server is named httpd.service. To activate this service unit
and start the httpd daemon in the current session, run the following command as root:
~]# systemctl start httpd.service
10.2.4. Stopping a Service
To stop a service unit that corresponds to a system service, type the following at a shell prompt as root:
systemctl stop name.service
Replace name with the name of the service unit you want to stop (for example, bluetooth). This
command stops the selected service unit in the current session. For information on how to disable a
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service unit and prevent it from being started at boot time, see Section 10.2.7, “Disabling a Service” . For
information on how to determine the status of a certain service unit, see Section 10.2.2, “Displaying
Service Status”.
Example 10.6. Stopping a Service
The service unit for the bluetoothd daemon is named bluetooth.service. To deactivate this service
unit and stop the bluetoothd daemon in the current session, run the following command as root:
~]# systemctl stop bluetooth.service
10.2.5. Restarting a Service
To restart a service unit that corresponds to a system service, type the following at a shell prompt as
root:
systemctl restart name.service
Replace name with the name of the service unit you want to restart (for example, httpd). This command
stops the selected service unit in the current session and immediately starts it again. Importantly, if the
selected service unit is not running, this command starts it too. To tell systemd to restart a service unit
only if the corresponding service is already running, run the following command as root:
systemctl try-restart name.service
Certain system services also allow you to reload their configuration without interrupting their execution.
To do so, type as root:
systemctl reload name.service
Note that system services that do not support this feature ignore this command altogether. For
convenience, the systemctl command also supports the reload-or-restart and reload-or-try-restart
commands that restart such services instead. For information on how to determine the status of a
certain service unit, see Section 10.2.2, “Displaying Service Status”.
Example 10.7. Restarting a Service
In order to prevent users from encountering unnecessary error messages or partially rendered web
pages, the Apache HTTP Server allows you to edit and reload its configuration without the need to
restart it and interrupt actively processed requests. To do so, type the following at a shell prompt as
root:
~]# systemctl reload httpd.service
10.2.6. Enabling a Service
To configure a service unit that corresponds to a system service to be automatically started at boot
time, type the following at a shell prompt as root:
systemctl enable name.service
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Replace name with the name of the service unit you want to enable (for example, httpd). This command
reads the [Install] section of the selected service unit and creates appropriate symbolic links to the
/usr/lib/systemd/system/name.service file in the /etc/systemd/system/ directory and its
subdirectories. This command does not, however, rewrite links that already exist. If you want to ensure
that the symbolic links are re-created, use the following command as root:
systemctl reenable name.service
This command disables the selected service unit and immediately enables it again. For information on
how to determine whether a certain service unit is enabled to start at boot time, see Section 10.2.2,
“Displaying Service Status”. For information on how to start a service in the current session, see
Section 10.2.3, “Starting a Service”.
Example 10.8. Enabling a Service
To configure the Apache HTTP Server to start automatically at boot time, run the following
command as root:
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to
10.2.7. Disabling a Service
To prevent a service unit that corresponds to a system service from being automatically started at boot
time, type the following at a shell prompt as root:
systemctl disable name.service
Replace name with the name of the service unit you want to disable (for example, bluetooth). This
command reads the [Install] section of the selected service unit and removes appropriate symbolic links
to the /usr/lib/systemd/system/name.service file from the /etc/systemd/system/ directory and its
subdirectories. In addition, you can mask any service unit to prevent it from being started manually or by
another service. To do so, run the following command as root:
systemctl mask name.service
This command replaces the /etc/systemd/system/name.service file with a symbolic link to /dev/null,
rendering the actual unit file inaccessible to systemd. To revert this action and unmask a service unit,
type as root:
systemctl unmask name.service
For information on how to determine whether a certain service unit is enabled to start at boot time, see
Section 10.2.2, “Displaying Service Status”. For information on how to stop a service in the current
session, see Section 10.2.4, “Stopping a Service” .
Example 10.9. Disabling a Service
Example 10.6, “Stopping a Service” illustrates how to stop the bluetooth.service unit in the current
session. To prevent this service unit from starting at boot time, type the following at a shell prompt
as root:
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~]# systemctl disable bluetooth.service
Removed symlink /etc/systemd/system/bluetooth.target.wants/bluetooth.service.
Removed symlink /etc/systemd/system/dbus-org.bluez.service.
10.2.8. Starting a Conflicting Service
In systemd, positive and negative dependencies between services exist. Starting particular service may
require starting one or more other services (positive dependency) or stopping one or more services
(negative dependency).
When you attempt to start a new service, systemd resolves all dependencies automatically. Note that
this is done without explicit notification to the user. If you are already running a service, and you attempt
to start another service with a negative dependency, the first service is automatically stopped.
For example, if you are running the postfix service, and you try to start the sendmail service, systemd
first automatically stops postfix, because these two services are conflicting and cannot run on the same
port.
10.3. WORKING WITH SYSTEMD TARGETS
Previous versions of Red Hat Enterprise Linux, which were distributed with SysV init or Upstart,
implemented a predefined set of runlevels that represented specific modes of operation. These
runlevels were numbered from 0 to 6 and were defined by a selection of system services to be run when
a particular runlevel was enabled by the system administrator. In Red Hat Enterprise Linux 7, the
concept of runlevels has been replaced with systemd targets.
Systemd targets are represented by target units. Target units end with the .target file extension and
their only purpose is to group together other systemd units through a chain of dependencies. For
example, the graphical.target unit, which is used to start a graphical session, starts system services such
as the GNOME Display Manager (gdm.service) or Accounts Service (accounts-daemon.service) and
also activates the multi-user.target unit. Similarly, the multi-user.target unit starts other essential
system services such as NetworkManager (NetworkManager.service) or D-Bus (dbus.service) and
activates another target unit named basic.target.
Red Hat Enterprise Linux 7 is distributed with a number of predefined targets that are more or less
similar to the standard set of runlevels from the previous releases of this system. For compatibility
reasons, it also provides aliases for these targets that directly map them to SysV runlevels. Table 10.6,
“Comparison of SysV Runlevels with systemd Targets” provides a complete list of SysV runlevels and
their corresponding systemd targets.
Table 10.6. Comparison of SysV Runlevels with systemd Targets
Runlevel Target Units Description
0 runlevel0.target,
poweroff.target
Shut down and power off the
system.
1 runlevel1.target,
rescue.target
Set up a rescue shell.
2 runlevel2.target, multi-
user.target
Set up a non-graphical multi-user
system.
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user.target
system.
user.target
system.
5 runlevel5.target,
graphical.target
Set up a graphical multi-user
system.
6 runlevel6.target,
reboot.target
Shut down and reboot the system.
Runlevel Target Units Description
To view, change, or configure systemd targets, use the systemctl utility as described in Table 10.7,
“Comparison of SysV init Commands with systemctl” and in the sections below. The runlevel and telinit
commands are still available in the system and work as expected, but are only included for compatibility
reasons and should be avoided.
Table 10.7. Comparison of SysV init Commands with systemctl
Old Command New Command Description
runlevel systemctl list-units --type
target
Lists currently loaded target units.
telinit runlevel systemctl isolate
name.target
Changes the current target.
10.3.1. Viewing the Default Target
To determine which target unit is used by default, run the following command:
systemctl get-default
This command resolves the symbolic link located at /etc/systemd/system/default.target and displays
the result. For information on how to change the default target, see Section 10.3.3, “Changing the
Default Target”. For information on how to list all currently loaded target units, see Section 10.3.2,
“Viewing the Current Target”.
Example 10.10. Viewing the Default Target
To display the default target unit, type:
~]$ systemctl get-default
graphical.target
10.3.2. Viewing the Current Target
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To list all currently loaded target units, type the following command at a shell prompt:
systemctl list-units --type target
For each target unit, this commands displays its full name (UNIT) followed by a note whether the unit has
been loaded (LOAD), its high-level ( ACTIVE) and low-level (SUB) unit activation state, and a short
description (DESCRIPTION).
By default, the systemctl list-units command displays only active units. If you want to list all loaded
units regardless of their state, run this command with the --all or -a command line option:
systemctl list-units --type target --all
See Section 10.3.1, “Viewing the Default Target” for information on how to display the default target. For
information on how to change the current target, see Section 10.3.4, “Changing the Current Target”.
Example 10.11. Viewing the Current Target
To list all currently loaded target units, run the following command:
~]$ systemctl list-units --type target
UNIT LOAD ACTIVE SUB DESCRIPTION
basic.target loaded active active Basic System
cryptsetup.target loaded active active Encrypted Volumes
getty.target loaded active active Login Prompts
graphical.target loaded active active Graphical Interface
local-fs-pre.target loaded active active Local File Systems (Pre)
local-fs.target loaded active active Local File Systems
multi-user.target loaded active active Multi-User System
network.target loaded active active Network
paths.target loaded active active Paths
remote-fs.target loaded active active Remote File Systems
sockets.target loaded active active Sockets
sound.target loaded active active Sound Card
spice-vdagentd.target loaded active active Agent daemon for Spice guests
swap.target loaded active active Swap
sysinit.target loaded active active System Initialization
time-sync.target loaded active active System Time Synchronized
timers.target loaded active active Timers
LOAD = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB = The low-level unit activation state, values depend on unit type.
17 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
10.3.3. Changing the Default Target
To configure the system to use a different target unit by default, type the following at a shell prompt as
root:
systemctl set-default name.target
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Replace name with the name of the target unit you want to use by default (for example, multi-user).
This command replaces the /etc/systemd/system/default.target file with a symbolic link to
/usr/lib/systemd/system/name.target, where name is the name of the target unit you want to use. For
information on how to change the current target, see Section 10.3.4, “Changing the Current Target”. For
information on how to list all currently loaded target units, see Section 10.3.2, “Viewing the Current
Target”.
Example 10.12. Changing the Default Target
To configure the system to use the multi-user.target unit by default, run the following command as
root:
~]# systemctl set-default multi-user.target
rm '/etc/systemd/system/default.target'
ln -s '/usr/lib/systemd/system/multi-user.target' '/etc/systemd/system/default.target'
10.3.4. Changing the Current Target
To change to a different target unit in the current session, type the following at a shell prompt as root:
systemctl isolate name.target
Replace name with the name of the target unit you want to use (for example, multi-user). This
command starts the target unit named name and all dependent units, and immediately stops all others.
For information on how to change the default target, see Section 10.3.3, “Changing the Default Target”.
For information on how to list all currently loaded target units, see Section 10.3.2, “Viewing the Current
Target”.
Example 10.13. Changing the Current Target
To turn off the graphical user interface and change to the multi-user.target unit in the current
session, run the following command as root:
~]# systemctl isolate multi-user.target
10.3.5. Changing to Rescue Mode
Rescue mode provides a convenient single-user environment and allows you to repair your system in
situations when it is unable to complete a regular booting process. In rescue mode, the system attempts
to mount all local file systems and start some important system services, but it does not activate
network interfaces or allow more users to be logged into the system at the same time. In Red Hat
Enterprise Linux 7, rescue mode is equivalent to single user mode and requires the root password.
To change the current target and enter rescue mode in the current session, type the following at a shell
prompt as root:
systemctl rescue
This command is similar to systemctl isolate rescue.target, but it also sends an informative message to
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This command is similar to systemctl isolate rescue.target, but it also sends an informative message to
all users that are currently logged into the system. To prevent systemd from sending this message, run
this command with the --no-wall command line option:
systemctl --no-wall rescue
For information on how to enter emergency mode, see Section 10.3.6, “Changing to Emergency Mode” .
Example 10.14. Changing to Rescue Mode
To enter rescue mode in the current session, run the following command as root:
~]# systemctl rescue
Broadcast message from root@localhost on pts/0 (Fri 2013-10-25 18:23:15 CEST):
The system is going down to rescue mode NOW!
10.3.6. Changing to Emergency Mode
Emergency mode provides the most minimal environment possible and allows you to repair your system
even in situations when the system is unable to enter rescue mode. In emergency mode, the system
mounts the root file system only for reading, does not attempt to mount any other local file systems,
does not activate network interfaces, and only starts a few essential services. In Red Hat Enterprise
Linux 7, emergency mode requires the root password.
To change the current target and enter emergency mode, type the following at a shell prompt as root:
systemctl emergency
This command is similar to systemctl isolate emergency.target, but it also sends an informative
message to all users that are currently logged into the system. To prevent systemd from sending this
message, run this command with the --no-wall command line option:
systemctl --no-wall emergency
For information on how to enter rescue mode, see Section 10.3.5, “Changing to Rescue Mode” .
Example 10.15. Changing to Emergency Mode
To enter emergency mode without sending a message to all users that are currently logged into the
system, run the following command as root:
~]# systemctl --no-wall emergency
10.4. SHUTTING DOWN, SUSPENDING, AND HIBERNATING THE
SYSTEM
In Red Hat Enterprise Linux 7, the systemctl utility replaces a number of power management
commands used in previous versions of the Red Hat Enterprise Linux system. The commands listed in
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Table 10.8, “Comparison of Power Management Commands with systemctl” are still available in the
system for compatibility reasons, but it is advised that you use systemctl when possible.
Table 10.8. Comparison of Power Management Commands with systemctl
Old Command New Command Description
halt systemctl halt Halts the system.
poweroff systemctl poweroff Powers off the system.
reboot systemctl reboot Restarts the system.
pm-suspend systemctl suspend Suspends the system.
pm-hibernate systemctl hibernate Hibernates the system.
pm-suspend-hybrid systemctl hybrid-sleep Hibernates and suspends the
system.
10.4.1. Shutting Down the System
The systemctl utility provides commands for shutting down the system, however the traditional
shutdown command is also supported. Although the shutdown command will call the systemctl utility
to perform the shutdown, it has an advantage in that it also supports a time argument. This is
particularly useful for scheduled maintenance and to allow more time for users to react to the warning
that a system shutdown has been scheduled. The option to cancel the shutdown can also be an
advantage.
Using systemctl Commands
To shut down the system and power off the machine, type the following at a shell prompt as root:
systemctl poweroff
To shut down and halt the system without powering off the machine, run the following command as
root:
systemctl halt
By default, running either of these commands causes systemd to send an informative message to all
users that are currently logged into the system. To prevent systemd from sending this message, run the
selected command with the --no-wall command line option, for example:
systemctl --no-wall poweroff
Using the shutdown Command
To shut down the system and power off the machine at a certain time, use a command in the following
format as root:
shutdown --poweroff hh:mm
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Where hh:mm is the time in 24 hour clock format. The /run/nologin file is created 5 minutes before
system shutdown to prevent new logins. When a time argument is used, an optional message, the wall
message, can be appended to the command.
To shut down and halt the system after a delay, without powering off the machine, use a command in
the following format as root:
shutdown --halt +m
Where +m is the delay time in minutes. The now keyword is an alias for +0.
A pending shutdown can be canceled by the root user as follows:
shutdown -c
See the shutdown(8) manual page for further command options.
10.4.2. Restarting the System
To restart the system, run the following command as root:
systemctl reboot
By default, this command causes systemd to send an informative message to all users that are currently
logged into the system. To prevent systemd from sending this message, run this command with the --
no-wall command line option:
systemctl --no-wall reboot
10.4.3. Suspending the System
To suspend the system, type the following at a shell prompt as root:
systemctl suspend
This command saves the system state in RAM and with the exception of the RAM module, powers off
most of the devices in the machine. When you turn the machine back on, the system then restores its
state from RAM without having to boot again. Because the system state is saved in RAM and not on the
hard disk, restoring the system from suspend mode is significantly faster than restoring it from
hibernation, but as a consequence, a suspended system state is also vulnerable to power outages.
For information on how to hibernate the system, see Section 10.4.4, “Hibernating the System”.
10.4.4. Hibernating the System
To hibernate the system, type the following at a shell prompt as root:
systemctl hibernate
This command saves the system state on the hard disk drive and powers off the machine. When you turn
the machine back on, the system then restores its state from the saved data without having to boot
again. Because the system state is saved on the hard disk and not in RAM, the machine does not have to
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maintain electrical power to the RAM module, but as a consequence, restoring the system from
hibernation is significantly slower than restoring it from suspend mode.
To hibernate and suspend the system, run the following command as root:
systemctl hybrid-sleep
For information on how to suspend the system, see Section 10.4.3, “Suspending the System”.
10.5. CONTROLLING SYSTEMD ON A REMOTE MACHINE
In addition to controlling the systemd system and service manager locally, the systemctl utility also
allows you to interact with systemd running on a remote machine over the SSH protocol. Provided that
the sshd service on the remote machine is running, you can connect to this machine by running the
systemctl command with the --host or -H command line option:
systemctl --host user_name@host_name command
Replace user_name with the name of the remote user, host_name with the machine’s host name, and
command with any of the systemctl commands described above. Note that the remote machine must
be configured to allow the selected user remote access over the SSH protocol. For more information on
how to configure an SSH server, see Chapter 12, OpenSSH.
Example 10.16. Remote Management
To log in to a remote machine named server-01.example.com as the root user and determine the
current status of the httpd.service unit, type the following at a shell prompt:
~]$ systemctl -H root@server-01.example.com status httpd.service
>>>>>>> systemd unit files -- update
root@server-01.example.com's password:
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
Active: active (running) since Fri 2013-11-01 13:58:56 CET; 2h 48min ago
Main PID: 649
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
10.6. CREATING AND MODIFYING SYSTEMD UNIT FILES
A unit file contains configuration directives that describe the unit and define its behavior. Several
systemctl commands work with unit files in the background. To make finer adjustments, system
administrator must edit or create unit files manually. Table 10.2, “Systemd Unit Files Locations” lists
three main directories where unit files are stored on the system, the /etc/systemd/system/ directory is
reserved for unit files created or customized by the system administrator.
Unit file names take the following form:
unit_name.type_extension
Here, unit_name stands for the name of the unit and type_extension identifies the unit type, see
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Here, unit_name stands for the name of the unit and type_extension identifies the unit type, see
Table 10.1, “Available systemd Unit Types” for a complete list of unit types. For example, there usually is
sshd.service as well as sshd.socket unit present on your system.
Unit files can be supplemented with a directory for additional configuration files. For example, to add
custom configuration options to sshd.service, create the sshd.service.d/custom.conf file and insert
additional directives there. For more information on configuration directories, see Section 10.6.4,
“Modifying Existing Unit Files”.
Also, the sshd.service.wants/ and sshd.service.requires/ directories can be created. These
directories contain symbolic links to unit files that are dependencies of the sshd service. The symbolic
links are automatically created either during installation according to [Install] unit file options (see
Table 10.11, “Important [Install] Section Options” ) or at runtime based on [Unit] options (see Table 10.9,
“Important [Unit] Section Options”). It is also possible to create these directories and symbolic links
manually.
Many unit file options can be set using the so called unit specifiers – wildcard strings that are dynamically
replaced with unit parameters when the unit file is loaded. This enables creation of generic unit files that
serve as templates for generating instantiated units. See Section 10.6.5, “Working with Instantiated
Units” for details.
10.6.1. Understanding the Unit File Structure
Unit files typically consist of three sections:
[Unit] — contains generic options that are not dependent on the type of the unit. These options
provide unit description, specify the unit’s behavior, and set dependencies to other units. For a
list of most frequently used [Unit] options, see Table 10.9, “Important [Unit] Section Options” .
[unit type] — if a unit has type-specific directives, these are grouped under a section named
after the unit type. For example, service unit files contain the [Service] section, see Table 10.10,
“Important [Service] Section Options” for most frequently used [Service] options.
[Install] — contains information about unit installation used by systemctl enable and disable
commands, see Table 10.11, “Important [Install] Section Options” for a list of [Install] options.
Table 10.9. Important [Unit] Section Options
Option[a] section, see the systemd.unit(5)
manual page.]
Description
Description A meaningful description of the unit. This text is
displayed for example in the output of the
systemctl status command.
Documentation Provides a list of URIs referencing documentation for
the unit.
After[b] Defines the order in which units are started. The unit
starts only after the units specified in After are
active. Unlike Requires, After does not explicitly
activate the specified units. The Before option has
the opposite functionality to After.
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Requires Configures dependencies on other units. The units
listed in Requires are activated together with the
unit. If any of the required units fail to start, the unit is
not activated.
Wants Configures weaker dependencies than Requires. If
any of the listed units does not start successfully, it
has no impact on the unit activation. This is the
recommended way to establish custom unit
dependencies.
Conflicts Configures negative dependencies, an opposite to
Requires.
[a] For a complete list of options configurable in the [Unit
[b] In most cases, it is sufficient to set only the ordering dependencies with After and Before unit file options. If you also
set a requirement dependency with Wants (recommended) or Requires, the ordering dependency still needs to be
specified. That is because ordering and requirement dependencies work independently from each other.
manual page.]
Description
Table 10.10. Important [Service] Section Options
Option[a] section, see the systemd.service(5)
manual page.]
Description
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Type Configures the unit process startup type that affects
the functionality of ExecStart and related options.
One of:
* simple – The default value. The process started
with ExecStart is the main process of the service.
* forking – The process started withExecStart
spawns a child process that becomes the main
process of the service. The parent process exits
when the startup is complete.
* oneshot – This type is similar tosimple, but the
process exits before starting consequent units.
* dbus – This type is similar tosimple, but
consequent units are started only after the main
process gains a D-Bus name.
* notify – This type is similar tosimple, but
consequent units are started only after a notification
message is sent via the sd_notify() function.
* idle – similar to simple, the actual execution of the
service binary is delayed until all jobs are finished,
which avoids mixing the status output with shell
output of services.
ExecStart Specifies commands or scripts to be executed when
the unit is started. ExecStartPre and
ExecStartPost specify custom commands to be
executed before and after ExecStart.
Type=oneshot enables specifying multiple custom
commands that are then executed sequentially.
ExecStop Specifies commands or scripts to be executed when
the unit is stopped.
ExecReload Specifies commands or scripts to be executed when
the unit is reloaded.
Restart With this option enabled, the service is restarted after
its process exits, with the exception of a clean stop
by the systemctl command.
RemainAfterExit If set to True, the service is considered active even
when all its processes exited. Default value is False.
This option is especially useful if Type=oneshot is
configured.
manual page.]
Description
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[a] For a complete list of options configurable in the [Service
manual page.]
Description
Table 10.11. Important [Install] Section Options
manual page.]
Description
Alias Provides a space-separated list of additional names
for the unit. Most systemctl commands, excluding
systemctl enable, can use aliases instead of the
actual unit name.
RequiredBy A list of units that depend on the unit. When this unit
is enabled, the units listed in RequiredBy gain a
Require dependency on the unit.
WantedBy A list of units that weakly depend on the unit. When
this unit is enabled, the units listed in WantedBy
gain a Want dependency on the unit.
Also Specifies a list of units to be installed or uninstalled
along with the unit.
DefaultInstance Limited to instantiated units, this option specifies the
default instance for which the unit is enabled. See
Section 10.6.5, “Working with Instantiated Units”
[a] For a complete list of options configurable in the [Install
A whole range of options that can be used to fine tune the unit configuration, Example 10.17,
“postfix.service Unit File” shows an example of a service unit installed on the system. Moreover, unit file
options can be defined in a way that enables dynamic creation of units as described in Section 10.6.5,
“Working with Instantiated Units”.
Example 10.17. postfix.service Unit File
What follows is the content of the /usr/lib/systemd/system/postfix.service unit file as currently
provided by the postfix package:
[Unit]
Description=Postfix Mail Transport Agent
After=syslog.target network.target
Conflicts=sendmail.service exim.service
[Service]
Type=forking
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PIDFile=/var/spool/postfix/pid/master.pid
EnvironmentFile=-/etc/sysconfig/network
ExecStartPre=-/usr/libexec/postfix/aliasesdb
ExecStartPre=-/usr/libexec/postfix/chroot-update
ExecStart=/usr/sbin/postfix start
ExecReload=/usr/sbin/postfix reload
ExecStop=/usr/sbin/postfix stop
[Install]
WantedBy=multi-user.target
The [Unit] section describes the service, specifies the ordering dependencies, as well as conflicting
units. In [Service], a sequence of custom scripts is specified to be executed during unit activation, on
stop, and on reload. EnvironmentFile points to the location where environment variables for the
service are defined, PIDFile specifies a stable PID for the main process of the service. Finally, the
[Install] section lists units that depend on the service.
10.6.2. Creating Custom Unit Files
There are several use cases for creating unit files from scratch: you could run a custom daemon, create a
second instance of some existing service (as in Example 10.19, “Creating a second instance of the sshd
service”), or import a SysV init script (more in Section 10.6.3, “Converting SysV Init Scripts to Unit
Files”). On the other hand, if you intend just to modify or extend the behavior of an existing unit, use the
instructions from Section 10.6.4, “Modifying Existing Unit Files” . The following procedure describes the
general process of creating a custom service:
1. Prepare the executable file with the custom service. This can be a custom-created script, or an
executable delivered by a software provider. If required, prepare a PID file to hold a constant
PID for the main process of the custom service. It is also possible to include environment files to
store shell variables for the service. Make sure the source script is executable (by executing the
chmod a+x) and is not interactive.
2. Create a unit file in the /etc/systemd/system/ directory and make sure it has correct file
permissions. Execute as root:
touch /etc/systemd/system/name.service
chmod 664 /etc/systemd/system/name.service
Replace name with a name of the service to be created. Note that file does not need to be
executable.
3. Open the name.service file created in the previous step, and add the service configuration
options. There is a variety of options that can be used depending on the type of service you wish
to create, see Section 10.6.1, “Understanding the Unit File Structure” . The following is an
example unit configuration for a network-related service:
[Unit]
Description=service_description
After=network.target
[Service]
ExecStart=path_to_executable
Type=forking
PIDFile=path_to_pidfile
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[Install]
WantedBy=default.target
Where:
service_description is an informative description that is displayed in journal log files and in
the output of the systemctl status command.
the After setting ensures that the service is started only after the network is running. Add a
space-separated list of other relevant services or targets.
path_to_executable stands for the path to the actual service executable.
Type=forking is used for daemons that make the fork system call. The main process of the
service is created with the PID specified in path_to_pidfile. Find other startup types in
Table 10.10, “Important [Service] Section Options” .
WantedBy states the target or targets that the service should be started under. Think of
these targets as of a replacement of the older concept of runlevels, see Section 10.3,
“Working with systemd Targets” for details.
4. Notify systemd that a new name.service file exists by executing the following command as
root:
systemctl daemon-reload
systemctl start name.service
WARNING
Always run the systemctl daemon-reload command after creating new
unit files or modifying existing unit files. Otherwise, the systemctl start or
systemctl enable commands could fail due to a mismatch between states
of systemd and actual service unit files on disk.
The name.service unit can now be managed as any other system service with commands
described in Section 10.2, “Managing System Services” .
Example 10.18. Creating the emacs.service File
When using the Emacs text editor, it is often faster and more convenient to have it running in the
background instead of starting a new instance of the program whenever editing a file. The following
steps show how to create a unit file for Emacs, so that it can be handled like a service.
1. Create a unit file in the /etc/systemd/system/ directory and make sure it has the correct file
permissions. Execute as root:
~]# touch /etc/systemd/system/emacs.service
~]# chmod 664 /etc/systemd/system/emacs.service

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2. Add the following content to the file:
[Unit]
Description=Emacs: the extensible, self-documenting text editor
[Service]
Type=forking
ExecStart=/usr/bin/emacs --daemon
ExecStop=/usr/bin/emacsclient --eval "(kill-emacs)"
Environment=SSH_AUTH_SOCK=%t/keyring/ssh
Restart=always
[Install]
WantedBy=default.target
With the above configuration, the /usr/bin/emacs executable is started in daemon mode on
service start. The SSH_AUTH_SOCK environment variable is set using the "%t" unit specifier
that stands for the runtime directory. The service also restarts the emacs process if it exits
unexpectedly.
3. Execute the following commands to reload the configuration and start the custom service:
~]# systemctl daemon-reload
~]# systemctl start emacs.service
As the editor is now registered as a systemd service, you can use all standard systemctl commands.
For example, run systemctl status emacs to display the editor’s status or systemctl enable emacs
to make the editor start automatically on system boot.
Example 10.19. Creating a second instance of the sshd service
System Administrators often need to configure and run multiple instances of a service. This is done
by creating copies of the original service configuration files and modifying certain parameters to
avoid conflicts with the primary instance of the service. The following procedure shows how to
create a second instance of the sshd service:
1. Create a copy of the sshd_config file that will be used by the second daemon:
~]# cp /etc/ssh/sshd{,-second}_config
2. Edit the sshd-second_config file created in the previous step to assign a different port
number and PID file to the second daemon:
Port 22220
PidFile /var/run/sshd-second.pid
See the sshd_config(5) manual page for more information on Port and PidFile options.
Make sure the port you choose is not in use by any other service. The PID file does not have
to exist before running the service, it is generated automatically on service start.
3. Create a copy of the systemd unit file for the sshd service:
~]# cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service
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4. Alter the sshd-second.service created in the previous step as follows:
a. Modify the Description option:
Description=OpenSSH server second instance daemon
b. Add sshd.service to services specified in the After option, so that the second instance
starts only after the first one has already started:
After=syslog.target network.target auditd.service sshd.service
c. The first instance of sshd includes key generation, therefore remove the
ExecStartPre=/usr/sbin/sshd-keygen line.
d. Add the -f /etc/ssh/sshd-second_config parameter to the sshd command, so that the
alternative configuration file is used:
ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS
e. After the above modifications, the sshd-second.service should look as follows:
[Unit]
Description=OpenSSH server second instance daemon
After=syslog.target network.target auditd.service sshd.service
[Service]
EnvironmentFile=/etc/sysconfig/sshd
ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process
Restart=on-failure
RestartSec=42s
[Install]
5. If using SELinux, add the port for the second instance of sshd to SSH ports, otherwise the
second instance of sshd will be rejected to bind to the port:
~]# semanage port -a -t ssh_port_t -p tcp 22220
6. Enable sshd-second.service, so that it starts automatically upon boot:
~]# systemctl enable sshd-second.service
Verify if the sshd-second.service is running by using the systemctl status command. Also,
verify if the port is enabled correctly by connecting to the service:
~]$ ssh -p 22220 user@server
If the firewall is in use, make sure that it is configured appropriately in order to allow
connections to the second instance of sshd.
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To learn how to properly choose a target for ordering and dependencies of your custom unit files,
see the following articles
How to write a service unit file which enforces that particular services have to be started
How to decide what dependencies a systemd service unit definition should have
Additional information with some real-world examples of cases triggered by the ordering and
dependencies in a unit file is available in the following article: Is there any useful information about
writing unit files?
If you want to set limits for services started by systemd, see the Red Hat Knowledgebase article How to
set limits for services in RHEL 7 and systemd. These limits need to be set in the service’s unit file. Note
that systemd ignores limits set in the /etc/security/limits.conf and /etc/security/limits.d/*.conf
configuration files. The limits defined in these files are set by PAM when starting a login session, but
daemons started by systemd do not use PAM login sessions.
10.6.3. Converting SysV Init Scripts to Unit Files
Before taking time to convert a SysV init script to a unit file, make sure that the conversion was not
already done elsewhere. All core services installed on Red Hat Enterprise Linux 7 come with default unit
files, and the same applies for many third-party software packages.
Converting an init script to a unit file requires analyzing the script and extracting the necessary
information from it. Based on this data you can create a unit file as described in Section 10.6.2, “Creating
Custom Unit Files”. As init scripts can vary greatly depending on the type of the service, you might need
to employ more configuration options for translation than outlined in this chapter. Note that some levels
of customization that were available with init scripts are no longer supported by systemd units, see
Section 10.1.2, “Compatibility Changes”.
The majority of information needed for conversion is provided in the script’s header. The following
example shows the opening section of the init script used to start the postfix service on Red Hat
Enterprise Linux 6:
#!/bin/bash
#
# postfix Postfix Mail Transfer Agent
#
# chkconfig: 2345 80 30
# description: Postfix is a Mail Transport Agent, which is the program
# that moves mail from one machine to another.
# processname: master
# pidfile: /var/spool/postfix/pid/master.pid
# config: /etc/postfix/main.cf
# config: /etc/postfix/master.cf
### BEGIN INIT INFO
# Provides: postfix MTA
# Required-Start: $local_fs $network $remote_fs
# Required-Stop: $local_fs $network $remote_fs
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: start and stop postfix
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# Description: Postfix is a Mail Transport Agent, which is the program that
# moves mail from one machine to another.
### END INIT INFO
In the above example, only lines starting with # chkconfig and # description are mandatory, so you might
not find the rest in different init files. The text enclosed between the # BEGIN INIT INFO and # END
INIT INFO lines is called Linux Standard Base (LSB) header . If specified, LSB headers contain directives
defining the service description, dependencies, and default runlevels. What follows is an overview of
analytic tasks aiming to collect the data needed for a new unit file. The postfix init script is used as an
example, see the resulting postfix unit file in Example 10.17, “postfix.service Unit File”.
Finding the Service Description
Find descriptive information about the script on the line starting with #description. Use this description
together with the service name in the Description option in the [Unit] section of the unit file. The LSB
header might contain similar data on the #Short-Description and #Description lines.
Finding Service Dependencies
The LSB header might contain several directives that form dependencies between services. Most of them
are translatable to systemd unit options, see Table 10.12, “Dependency Options from the LSB Header”
Table 10.12. Dependency Options from the LSB Header
LSB Option Description Unit File Equivalent
Provides Specifies the boot facility name
of the service, that can be
referenced in other init scripts
(with the "$" prefix). This is no
longer needed as unit files refer to
other units by their file names.
–
Required-Start Contains boot facility names of
required services. This is
translated as an ordering
dependency, boot facility names
are replaced with unit file names
of corresponding services or
targets they belong to. For
example, in case of postfix, the
Required-Start dependency on
$network was translated to the
After dependency on
network.target.
After, Before
Should-Start Constitutes weaker dependencies
than Required-Start. Failed
Should-Start dependencies do
not affect the service startup.
After, Before
Required-Stop, Should-Stop Constitute negative
dependencies.
Conflicts
Finding Default Targets of the Service
The line starting with #chkconfig contains three numerical values. The most important is the first number
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that represents the default runlevels in which the service is started. Use Table 10.6, “Comparison of SysV
Runlevels with systemd Targets” to map these runlevels to equivalent systemd targets. Then list these
targets in the WantedBy option in the [Install] section of the unit file. For example, postfix was
previously started in runlevels 2, 3, 4, and 5, which translates to multi-user.target and graphical.target on
Red Hat Enterprise Linux 7. Note that the graphical.target depends on multiuser.target, therefore it is not
necessary to specify both, as in Example 10.17, “postfix.service Unit File”. You might find information on
default and forbidden runlevels also at #Default-Start and #Default-Stop lines in the LSB header.
The other two values specified on the #chkconfig line represent startup and shutdown priorities of the init
script. These values are interpreted by systemd if it loads the init script, but there is no unit file equivalent.
Finding Files Used by the Service
Init scripts require loading a function library from a dedicated directory and allow importing configuration,
environment, and PID files. Environment variables are specified on the line starting with #config in the init
script header, which translates to the EnvironmentFile unit file option. The PID file specified on the
#pidfile init script line is imported to the unit file with the PIDFile option.
The key information that is not included in the init script header is the path to the service executable, and
potentially some other files required by the service. In previous versions of Red Hat Enterprise Linux, init
scripts used a Bash case statement to define the behavior of the service on default actions, such as start,
stop, or restart, as well as custom-defined actions. The following excerpt from the postfix init script shows
the block of code to be executed at service start.
conf_check() {
[ -x /usr/sbin/postfix ] || exit 5
[ -d /etc/postfix ] || exit 6
[ -d /var/spool/postfix ] || exit 5
}
make_aliasesdb() {
if [ "$(/usr/sbin/postconf -h alias_database)" == "hash:/etc/aliases" ]
then
# /etc/aliases.db might be used by other MTA, make sure nothing
# has touched it since our last newaliases call
[ /etc/aliases -nt /etc/aliases.db ] ||
[ "$ALIASESDB_STAMP" -nt /etc/aliases.db ] ||
[ "$ALIASESDB_STAMP" -ot /etc/aliases.db ] || return
/usr/bin/newaliases
touch -r /etc/aliases.db "$ALIASESDB_STAMP"
else
/usr/bin/newaliases
fi
}
start() {
[ "$EUID" != "0" ] && exit 4
# Check that networking is up.
[ ${NETWORKING} = "no" ] && exit 1
conf_check
# Start daemons.
echo -n $"Starting postfix: "
make_aliasesdb >/dev/null 2>&1
[ -x $CHROOT_UPDATE ] && $CHROOT_UPDATE
/usr/sbin/postfix start 2>/dev/null 1>&2 && success || failure $"$prog start"
RETVAL=$?
[ $RETVAL -eq 0 ] && touch $lockfile
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echo
return $RETVAL
}
The extensibility of the init script allowed specifying two custom functions, conf_check() and
make_aliasesdb(), that are called from the start() function block. On closer look, several external files
and directories are mentioned in the above code: the main service executable /usr/sbin/postfix, the
/etc/postfix/ and /var/spool/postfix/ configuration directories, as well as the /usr/sbin/postconf/
directory.
Systemd supports only the predefined actions, but enables executing custom executables with
ExecStart, ExecStartPre, ExecStartPost, ExecStop, and ExecReload options. In case of postfix on
Red Hat Enterprise Linux 7, the /usr/sbin/postfix together with supporting scripts are executed on
service start. Consult the postfix unit file at Example 10.17, “postfix.service Unit File”.
Converting complex init scripts requires understanding the purpose of every statement in the script.
Some of the statements are specific to the operating system version, therefore you do not need to
translate them. On the other hand, some adjustments might be needed in the new environment, both in
unit file as well as in the service executable and supporting files.
10.6.4. Modifying Existing Unit Files
Services installed on the system come with default unit files that are stored in the
/usr/lib/systemd/system/ directory. System Administrators should not modify these files directly,
therefore any customization must be confined to configuration files in the /etc/systemd/system/
directory. Depending on the extent of the required changes, pick one of the following approaches:
Create a directory for supplementary configuration files at /etc/systemd/system/unit.d/. This
method is recommended for most use cases. It enables extending the default configuration with
additional functionality, while still referring to the original unit file. Changes to the default unit
introduced with a package upgrade are therefore applied automatically. See the section called
“Extending the Default Unit Configuration” for more information.
Create a copy of the original unit file /usr/lib/systemd/system/ in /etc/systemd/system/ and
make changes there. The copy overrides the original file, therefore changes introduced with the
package update are not applied. This method is useful for making significant unit changes that
should persist regardless of package updates. See the section called “Overriding the Default Unit
Configuration” for details.
In order to return to the default configuration of the unit, just delete custom-created configuration files in
/etc/systemd/system/. To apply changes to unit files without rebooting the system, execute:
The daemon-reload option reloads all unit files and recreates the entire dependency tree, which is
needed to immediately apply any change to a unit file. As an alternative, you can achieve the same result
with the following command:
init q
Also, if the modified unit file belongs to a running service, this service must be restarted to accept new
settings:
IMPORTANT
165

IMPORTANT
To modify properties, such as dependencies or timeouts, of a service that is handled by a
SysV initscript, do not modify the initscript itself. Instead, create a systemd drop-in
configuration file for the service as described in the section called “Extending the Default
Unit Configuration” and the section called “Overriding the Default Unit Configuration” .
Then manage this service in the same way as a normal systemd service.
For example, to extend the configuration of the network service, do not modify the
/etc/rc.d/init.d/network initscript file. Instead, create new directory
/etc/systemd/system/network.service.d/ and a systemd drop-in file
/etc/systemd/system/network.service.d/my_config.conf. Then, put the modified values
into the drop-in file. Note: systemd knows the network service as network.service, which
is why the created directory must be called network.service.d
Extending the Default Unit Configuration
To extend the default unit file with additional configuration options, first create a configuration directory
in /etc/systemd/system/. If extending a service unit, execute the following command as root:
mkdir /etc/systemd/system/name.service.d/
Replace name with the name of the service you want to extend. The above syntax applies to all unit types.
Create a configuration file in the directory made in the previous step. Note that the file name must end
with the .conf suffix. Type:
touch /etc/systemd/system/name.service.d/config_name.conf
Replace config_name with the name of the configuration file. This file adheres to the normal unit file
structure, therefore all directives must be specified under appropriate sections, see Section 10.6.1,
“Understanding the Unit File Structure”.
For example, to add a custom dependency, create a configuration file with the following content:
[Unit]
Requires=new_dependency
After=new_dependency
Where new_dependency stands for the unit to be marked as a dependency. Another example is a
configuration file that restarts the service after its main process exited, with a delay of 30 seconds:
[Service]
Restart=always
RestartSec=30
It is recommended to create small configuration files focused only on one task. Such files can be easily
moved or linked to configuration directories of other services.
To apply changes made to the unit, execute as root:
Example 10.20. Extending the httpd.service Configuration
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To modify the httpd.service unit so that a custom shell script is automatically executed when starting
the Apache service, perform the following steps. First, create a directory and a custom configuration
file:
~]# mkdir /etc/systemd/system/httpd.service.d/
~]# touch /etc/systemd/system/httpd.service.d/custom_script.conf
Provided that the script you want to start automatically with Apache is located at
/usr/local/bin/custom.sh, insert the following text to the custom_script.conf file:
[Service]
ExecStartPost=/usr/local/bin/custom.sh
To apply the unit changes, execute:
~]# systemctl restart httpd.service
NOTE
The configuration files from configuration directories in /etc/systemd/system/ take
precedence over unit files in /usr/lib/systemd/system/. Therefore, if the configuration
files contain an option that can be specified only once, such as Description or ExecStart,
the default value of this option is overridden. Note that in the output of the systemd-
delta command, described in the section called “Monitoring Overriden Units” , such units
are always marked as [EXTENDED], even though in sum, certain options are actually
overridden.
Overriding the Default Unit Configuration
To make changes that will persist after updating the package that provides the unit file, first copy the file
to the /etc/systemd/system/ directory. To do so, execute the following command as root:
cp /usr/lib/systemd/system/name.service /etc/systemd/system/name.service
Where name stands for the name of the service unit you wish to modify. The above syntax applies to all
unit types.
Open the copied file with a text editor, and make the desired changes. To apply the unit changes, execute
as root:
Example 10.21. Changing the timeout limit
You can specify a timeout value per service to prevent a malfunctioning service from freezing the
system. Otherwise, timeout is set by default to 90 seconds for normal services and to 300 seconds for
SysV-compatible services.
For example, to extend timeout limit for the httpd service:
1. Copy the httpd unit file to the /etc/systemd/system/ directory:
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cp /usr/lib/systemd/system/httpd.service /etc/systemd/system/httpd.service
2. Open file /etc/systemd/system/httpd.service and specify the TimeoutStartUSec value in
the [Service] section:
...
[Service]
...
PrivateTmp=true
TimeoutStartSec=10
[Install]
...
3. Reload the systemd daemon:
4. Optional. Verify the new timeout value:
systemctl show httpd -p TimeoutStartUSec
NOTE
To change the timeout limit globally, input the DefaultTimeoutStartSec in the
/etc/systemd/system.conf file. See Section 10.1, “Introduction to systemd”.
Monitoring Overriden Units
To display an overview of overridden or modified unit files, use the following command:
systemd-delta
For example, the output of the above command can look as follows:
[EQUIVALENT] /etc/systemd/system/default.target → /usr/lib/systemd/system/default.target
[OVERRIDDEN] /etc/systemd/system/autofs.service → /usr/lib/systemd/system/autofs.service
--- /usr/lib/systemd/system/autofs.service 2014-10-16 21:30:39.000000000 -0400
+++ /etc/systemd/system/autofs.service 2014-11-21 10:00:58.513568275 -0500
@@ -8,7 +8,8 @@
EnvironmentFile=-/etc/sysconfig/autofs
ExecStart=/usr/sbin/automount $OPTIONS --pid-file /run/autofs.pid
ExecReload=/usr/bin/kill -HUP $MAINPID
-TimeoutSec=180
+TimeoutSec=240
+Restart=Always
[Install]
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[MASKED] /etc/systemd/system/cups.service → /usr/lib/systemd/system/cups.service
[EXTENDED] /usr/lib/systemd/system/sssd.service →
/etc/systemd/system/sssd.service.d/journal.conf
4 overridden configuration files found.
Table 10.13, “systemd-delta Difference Types” lists override types that can appear in the output of
systemd-delta. Note that if a file is overridden, systemd-delta by default displays a summary of changes
similar to the output of the diff command.
Table 10.13. systemd-delta Difference Types
Type Description
[MASKED] Masked unit files, see Section 10.2.7, “Disabling a
Service” for description of unit masking.
[EQUIVALENT] Unmodified copies that override the original files but
do not differ in content, typically symbolic links.
[REDIRECTED] Files that are redirected to another file.
[OVERRIDEN] Overridden and changed files.
[EXTENDED] Files that are extended with .conf files in the
/etc/systemd/system/unit.d/ directory.
[UNCHANGED] Unmodified files are displayed only when the --
type=unchanged option is used.
It is good practice to run systemd-delta after system update to check if there are any updates to the
default units that are currently overridden by custom configuration. It is also possible to limit the output
only to a certain difference type. For example, to view just the overridden units, execute:
systemd-delta --type=overridden
10.6.5. Working with Instantiated Units
It is possible to instantiate multiple units from a single template configuration file at runtime. The "@"
character is used to mark the template and to associate units with it. Instantiated units can be started
from another unit file (using Requires or Wants options), or with the systemctl start command.
Instantiated service units are named the following way:
template_name@instance_name.service
Where template_name stands for the name of the template configuration file. Replace instance_name
with the name for the unit instance. Several instances can point to the same template file with
configuration options common for all instances of the unit. Template unit name has the form of:
unit_name@.service
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For example, the following Wants setting in a unit file:
Wants=getty@ttyA.service,getty@ttyB.service
first makes systemd search for given service units. If no such units are found, the part between "@" and the
type suffix is ignored and systemd searches for the getty@.service file, reads the configuration from it,
and starts the services.
Wildcard characters, called unit specifiers, can be used in any unit configuration file. Unit specifiers
substitute certain unit parameters and are interpreted at runtime. Table 10.14, “Important Unit Specifiers”
lists unit specifiers that are particularly useful for template units.
Table 10.14. Important Unit Specifiers
Unit Specifier Meaning Description
%n Full unit name Stands for the full unit name
including the type suffix. %N has
the same meaning but also
replaces the forbidden characters
with ASCII codes.
%p Prefix name Stands for a unit name with type
suffix removed. For instantiated
units %p stands for the part of the
unit name before the "@"
character.
%i Instance name Is the part of the instantiated unit
name between the "@" character
and the type suffix. %I has the
same meaning but also replaces
the forbidden characters for ASCII
codes.
%H Host name Stands for the hostname of the
running system at the point in
time the unit configuration is
loaded.
%t Runtime directory Represents the runtime directory,
which is either /run for the root
user, or the value of the
XDG_RUNTIME_DIR variable for
unprivileged users.
For a complete list of unit specifiers, see the systemd.unit(5) manual page.
For example, the getty@.service template contains the following directives:
[Unit]
Description=Getty on %I
...
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[Service]
ExecStart=-/sbin/agetty --noclear %I $TERM
...
When the getty@ttyA.service and getty@ttyB.service are instantiated form the above template,
Description= is resolved as Getty on ttyA and Getty on ttyB.
10.7. ADDITIONAL CONSIDERATIONS WHILE MANAGING SERVICES
During normal operation, systemd maintains an association between a unit abstraction and the underlying
processes active on the system.
From: man systemd
Processes systemd spawns are placed in individual Linux control groups named after the unit which
they belong to in the private systemd hierarchy. (see cgroups.txt[1] for more information about control
groups, or short "cgroups"). systemd uses this to effectively keep track of processes. Control group
information is maintained in the kernel, and is accessible via the file system hierarchy (beneath
/sys/fs/cgroup/systemd/), or in tools such as ps(1) (ps xawf -eo pid,user,cgroup,args is particularly
useful to list all processes and the systemd units they belong to).
The cgroup hierarchy is critical to systemd’s view of process and service health. When a process forks
itself, it inherits the cgroup of the creating process. With this being the case, all processes associated with
a given unit can be verified by reading the contents of the applicable cgroup.procs file, such as:
~]# cat /sys/fs/cgroup/systemd/system.slice/httpd.service/cgroup.procs
11854
11855
11856
11857
11858
11859
The output matches the CGroup information returned during a systemctl status unit operation:
~]# systemctl status httpd
● httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: active (running) since Wed 2019-05-29 12:08:16 EDT; 45s ago
Docs: man:httpd(8)
man:apachectl(8)
Main PID: 11854 (httpd)
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
├─11854 /usr/sbin/httpd -DFOREGROUND
└─11859 /usr/sbin/httpd -DFOREGROUND
May 29 12:08:16 localhost systemd[1]: Starting The Apache HTTP Server...
May 29 12:08:16 localhost systemd[1]: Started The Apache HTTP Server.
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To directly view these groupings of processes system-wide, the systemd-cgls utility can be used:
~]# systemd-cgls | head -17
├─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 22
├─user.slice
│ └─user-0.slice
│ └─session-168.scope
│ ├─ 3049 login -- root
│ ├─11884 -bash
│ ├─11943 systemd-cgls
│ └─11944 head -17
└─system.slice
├─httpd.service
│ ├─11854 /usr/sbin/httpd -DFOREGROUND
│ └─11859 /usr/sbin/httpd -DFOREGROUND
├─rhnsd.service
In order for systemd to function properly, the service must be started or stopped through the systemd
system to maintain the correct process to unit grouping. Any operation that takes external action results in
the necessary cgroup structure not being created. This happens because systemd is not aware of the
special nature of the processes being started.
As an example of the above constraint, stopping the httpd service and then issuing /usr/sbin/httpd
directly results in the following:
~]# systemctl stop httpd
~]# /usr/sbin/httpd
# systemd-cgls | head -17
├─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 22
├─user.slice
│ └─user-0.slice
│ └─session-168.scope
│ ├─ 3049 login -- root
│ ├─11884 -bash
│ ├─11957 /usr/sbin/httpd
│ ├─11963 systemd-cgls
│ └─11964 head -17
└─system.slice
├─rhnsd.service
│ └─3261 rhnsd
Note that the httpd process is now visible under the user-0.slice and a session-168.scope. This service is
treated as a user started process, as opposed to a system service, that systemd should monitor and
manage directly. Some failures that can occur due to this misalignment include, but are not limited to:
Services are not properly shutdown during system shutdown or restart events.
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Unexpected signals are delivered during user logout such as SIGHUP and SIGTERM.
Processes that fail are not automatically restarted despite having a Restart= directive
NOTE
Non-graceful application shutdown events can result in a large number of subsequent
application failures, such as client-side failures, data loss, and on-disk corruption.
For more information on systemd and its usage on Red Hat Enterprise Linux 7, see the resources listed
below.
systemctl(1) — The manual page for the systemctl command line utility provides a complete list
of supported options and commands.
systemd(1) — The manual page for the systemd system and service manager provides more
information about its concepts and documents available command line options and environment
variables, supported configuration files and directories, recognized signals, and available kernel
options.
systemd-delta(1) — The manual page for the systemd-delta utility that allows to find extended
and overridden configuration files.
systemd.unit(5) — The manual page named systemd.unit provides detailed information about
systemd unit files and documents all available configuration options.
systemd.service(5) — The manual page named systemd.service documents the format of
service unit files.
systemd.target(5) — The manual page named systemd.target documents the format of target
unit files.
systemd.kill(5) — The manual page named systemd.kill documents the configuration of the
process killing procedure.
Red Hat Enterprise Linux 7 Networking Guide — The Networking Guide for Red Hat Enterprise
Linux 7 documents relevant information regarding the configuration and administration of
network interfaces, networks, and network services in this system. It provides an introduction to
the hostnamectl utility, explains how to use it to view and set host names on the command line,
both locally and remotely, and provides important information about the selection of host names
and domain names.
Red Hat Enterprise Linux 7 Desktop Migration and Administration Guide — The Desktop
Migration and Administration Guide for Red Hat Enterprise Linux 7 documents the migration
planning, deployment, configuration, and administration of the GNOME 3 desktop on this system.
It introduces the logind service, enumerates its most significant features, and explains how to use
the loginctl utility to list active sessions and enable multi-seat support.
Red Hat Enterprise Linux 7 SELinux User’s and Administrator’s Guide — The SELinux User’s and
Administrator’s Guide for Red Hat Enterprise Linux 7 describes the basic principles of SELinux
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and documents in detail how to configure and use SELinux with various services such as the
Apache HTTP Server, Postfix, PostgreSQL, or OpenShift. It explains how to configure SELinux
access permissions for system services managed by systemd.
Red Hat Enterprise Linux 7 Installation Guide — The Installation Guide for Red Hat Enterprise
Linux 7 documents how to install the system on AMD64 and Intel 64 systems, 64-bit IBM Power
Systems servers, and IBM Z. It also covers advanced installation methods such as Kickstart
installations, PXE installations, and installations over the VNC protocol. In addition, it describes
common post-installation tasks and explains how to troubleshoot installation problems, including
detailed instructions on how to boot into rescue mode or recover the root password.
assists users and administrators in learning the processes and practices of securing their
workstations and servers against local and remote intrusion, exploitation, and malicious activity. It
also explains how to secure critical system services.
systemd Home Page — The project home page provides more information about systemd.
See Also
Chapter 2, System Locale and Keyboard Configuration documents how to manage the system
locale and keyboard layouts. It explains how to use the localectl utility to view the current locale,
list available locales, and set the system locale on the command line, as well as to view the current
keyboard layout, list available keymaps, and enable a particular keyboard layout on the command
line.
Chapter 3, Configuring the Date and Time documents how to manage the system date and time.
It explains the difference between a real-time clock and system clock and describes how to use
the timedatectl utility to display the current settings of the system clock, configure the date and
time, change the time zone, and synchronize the system clock with a remote server.
and sudo commands.
Chapter 12, OpenSSH describes how to configure an SSH server and how to use the ssh, scp, and
sftp client utilities to access it.
Chapter 23, Viewing and Managing Log Files provides an introduction to journald. It describes
the journal, introduces the journald service, and documents how to use the journalctl utility to
view log entries, enter live view mode, and filter log entries. In addition, this chapter describes how
to give non-root users access to system logs and enable persistent storage for log files.
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Accessibility in Red Hat Enterprise Linux 7 is ensured by the Orca screen reader, which is included in the
default installation of the operating system. This chapter explains how a system administrator can
configure a system to support users with a visual impairment.
Orca reads information from the screen and communicates it to the user using:
a speech synthesizer, which provides a speech output
a braille display, which provides a tactile output
For more information on Orca settings, see its help page.
In order that Orca's communication outputs function properly, the system administrator needs to:
configure the brltty service, as described in Section 11.1, “Configuring the brltty Service”
switch on the Always Show Universal Access Menu, as described in Section 11.2, “Switch On
Always Show Universal Access Menu”
enable the Festival speech synthesizer, as described in Section 11.3, “Enabling the Festival
Speech Synthesis System”
11.1. CONFIGURING THE BRLTTY SERVICE
The Braille display uses the brltty service to provide tactile output for visually impaired users.
Enable the brltty Service
The braille display cannot work unless brltty is running. By default, brltty is disabled. Enable brltty to be
started on boot:
~]# systemctl enable brltty.service
Authorize Users to Use the Braille Display
To set the users who are authorized to use the braille display, choose one of the following procedures,
which have an equal effect. The procedure using the /etc/brltty.conf file is suitable even for the file
systems where users or groups cannot be assigned to a file. The procedure using the /etc/brlapi.key file is
suitable only for the file systems where users or groups can be assigned to a file.
Setting Access to Braille Display by Using/etc/brltty.conf
1. Open the /etc/brltty.conf file, and find the section called Application Programming Interface
Parameters.
2. Specify the users.
a. To specify one or more individual users, list the users on the following line:
api-parameters Auth=user:user_1, user_2, ... # Allow some local user
b. To specify a user group, enter its name on the following line:
api-parameters Auth=group:group # Allow some local group
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Setting Access to Braille Display by Using/etc/brlapi.key
1. Create the /etc/brlapi.key file.
~]# mcookie > /etc/brlapi.key
2. Change ownership of the /etc/brlapi.key to particular user or group.
a. To specify an individual user:
~]# chown user_1 /etc/brlapi.key
b. To specify a group:
~]# chown group_1 /etc/brlapi.key
3. Adjust the content of /etc/brltty.conf to include this:
api-parameters Auth=keyfile:/etc/brlapi.key
Set the Braille Driver
The braille-driver directive in /etc/brltty.conf specifies a two-letter driver identification code of the
driver for the braille display.
Setting the Braille Driver
1. Decide whether you want to use the autodetection for finding the appropriate braille driver.
a. If you want to use autodetection, leave braille driver specified to auto, which is the default
option.
braille-driver auto # autodetect
WARNING
Autodetection tries all drivers. Therefore, it might take a long time or
even fail. For this reason, setting up a particular braille driver is
recommended.
b. If you do not want to use the autodetection, specify the identification code of the required
braille driver in the braille-driver directive.
Choose the identification code of required braille driver from the list provided in
/etc/brltty.conf, for example:
braille-driver xw # XWindow
You can also set multiple drivers, separated by commas, and autodetection is then performed
among them.
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Set the Braille Device
The braille-device directive in /etc/brltty.conf specifies the device to which the braille display is
connected. The following device types are supported (see Table 11.1, “Braille Device Types and the
Corresponding Syntax”):
Table 11.1. Braille Device Types and the Corresponding Syntax
Braille Device Type Syntax of the Type
serial device serial:path [a]
USB device [serial-number] [b]
Bluetooth device bluetooth:address
[a] Relative paths are at /dev.
[b] The brackets here indicate optionality.
Examples of settings for particular devices:
braille-device serial:ttyS0 # First serial device
braille-device usb: # First USB device matching braille driver
braille-device usb:nnnnn # Specific USB device by serial number
braille-device bluetooth:xx:xx:xx:xx:xx:xx # Specific Bluetooth device by address
You can also set multiple devices, separated by commas, and each of them will be probed in turn.
WARNING
If the device is connected by a serial-to-USB adapter, setting braille-device to usb:
does not work. In this case, identify the virtual serial device that the kernel has
created for the adapter. The virtual serial device can look like this:
serial:ttyUSB0
You can find the actual device name in the kernel messages on the device plug
with the following command:
~]# dmesg | fgrep ttyUSB0
Set Specific Parameters for Particular Braille Displays
If you need to set specific parameters for particular braille displays, use the braille-parameters directive
in /etc/brltty.conf. The braille-parameters directive passes non-generic parameters through to the
braille driver. Choose the required parameters from the list in /etc/brltty.conf.
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Set the Text Table
The text-table directive in /etc/brltty.conf specifies which text table is used to encode the symbols.
Relative paths to text tables are in the /etc/brltty/Text/ directory.
Setting the Text Table
1. Decide whether you want to use the autoselection for finding the appropriate text table.
2. a. If you want to use the autoselection, leave text-table specified to auto, which is the default
option.
text-table auto # locale-based autoselection
This ensures that local-based autoselection with fallback to en-nabcc is performed.
b. If you do not want to use the autoselection, choose the required text-table from the list in
/etc/brltty.conf.
For example, to use the text table for American English:
text-table en_US # English (United States)
Set the Contraction Table
The contraction-table directive in /etc/brltty.conf specifies which table is used to encode the
abbreviations. Relative paths to particular contraction tables are in the /etc/brltty/Contraction/ directory.
Choose the required contraction-table from the list in /etc/brltty.conf.
For example, to use the contraction table for American English, grade 2:
contraction-table en-us-g2 # English (US, grade 2)
WARNING
If not specified, no contraction table is used.
11.2. SWITCH ON ALWAYS SHOW UNIVERSAL ACCESS MENU
To switch on the Orca screen reader, press the Super+Alt+S key combination. As a result, the Universal
Access Menu icon is displayed on the top bar.
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WARNING
The icon disappears in case that the user switches off all of the provided options
from the Universal Access Menu. Missing icon can cause difficulties to users with a
visual impairment. System administrators can prevent the inaccessibility of the icon
by switching on the Always Show Universal Access Menu. When the Always
Show Universal Access Menu is switched on, the icon is displayed on the top bar
even in the situation when all options from this menu are switched off.
Switching OnAlways Show Universal Access Menu
1. Open the Gnome settings menu, and click Universal Access.
2. Switch on Always Show Universal Access Menu.
3. Optional: Verify that the Universal Access Menu icon is displayed on the top bar even if all options
from this menu are switched off.
11.3. ENABLING THE FESTIVAL SPEECH SYNTHESIS SYSTEM
By default, Orca uses the eSpeak speech synthesizer, but it also supports the Festival Speech Synthesis
System. Both eSpeak and Festival Speech Synthesis System (Festival) synthesize voice differently.
Some users might prefer Festival to the default eSpeak synthesizer. To enable Festival, follow these steps:
Installing Festival and Making it Running on Boot
1. Install Festival:
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~]# yum install festival festival-freebsoft-utils
2. Make Festival running on boot:
a. Create a new systemd unit file:
Create a file in the /etc/systemd/system/ directory and make it executable.
~]# touch /etc/systemd/system/festival.service
~]# chmod 664 /etc/systemd/system/festival.service
b. Ensure that the script in the /usr/bin/festival_server file is used to run Festival. Add the
following content to the /etc/systemd/system/festival.service file:
[Unit]
Description=Festival speech synthesis server
[Service]
ExecStart=/usr/bin/festival_server
Type=simple
c. Notify systemd that a new festival.service file exists:
~]# systemctl start festival.service
d. Enable festival.service:
~]# systemctl enable festival.service
Choose a Voice for Festival
Festival provides multiples voices.
To make a voice available, install the relevant package from the following list:
festvox-awb-arctic-hts
festvox-bdl-arctic-hts
festvox-clb-arctic-hts
festvox-kal-diphone
festvox-ked-diphone
festvox-rms-arctic-hts
festvox-slt-arctic-hts
hispavoces-pal-diphone
hispavoces-sfl-diphone
To see detailed information about a particular voice:
~]# yum info package_name
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To make the required voice available, install the package with this voice and then reboot:
~]# yum install package_name
~]# reboot
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CHAPTER 12. OPENSSH
SSH (Secure Shell) is a protocol which facilitates secure communications between two systems using a
client-server architecture and allows users to log in to 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 in to 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 in to
remote systems decreases the risks for both the client system and the remote host.
Red Hat Enterprise Linux includes the general OpenSSH package, openssh, as well as the OpenSSH
server, openssh-server, and client, openssh-clients, packages. Note, the OpenSSH packages require the
OpenSSL package openssl-libs, which installs several important cryptographic libraries, enabling
OpenSSH to provide encrypted communications.
12.1. THE SSH PROTOCOL
12.1.1. 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.
The SSH protocol provides the following safeguards:
No one can pose as the intended server
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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.
12.1.3. Protocol Versions
Two varieties of SSH currently exist: version 1, and newer version 2. The OpenSSH suite under Red Hat
Enterprise Linux 7 uses SSH version 2, which has an enhanced key exchange algorithm not vulnerable to
the known exploit in version 1. In Red Hat Enterprise Linux 7, the OpenSSH suite does not support version
1 connections.
12.1.4. 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 client interacts with the remote host over the encrypted connection.
12.1.4.1. 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
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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.
WARNING
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.
12.1.4.2. 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
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security model, and the client can choose the order of authentication methods to attempt from the
available options.
12.1.4.3. Channels
After a successful authentication over the SSH transport layer, multiple channels are opened via a
technique called multiplexing[1]. 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.
12.2. CONFIGURING OPENSSH
12.2.1. 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 as described in Table 12.1,
“System-wide configuration files”. User-specific SSH configuration information is stored in ~/.ssh/ within
the user’s home directory as described in Table 12.2, “User-specific configuration files”.
Table 12.1. System-wide configuration files
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_ecdsa_key The ECDSA private key used by the sshd daemon.
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/etc/ssh/ssh_host_ecdsa_key.pub The ECDSA public key used by the sshd daemon.
/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 daemon for
version 2 of the SSH protocol.
/etc/pam.d/sshd The PAM configuration file for the sshd daemon.
/etc/sysconfig/sshd Configuration file for the sshd service.
File Description
Table 12.2. User-specific configuration files
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_ecdsa Contains the ECDSA private key of the user.
~/.ssh/id_ecdsa.pub The ECDSA 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/known_hosts Contains host keys of SSH servers accessed by the
user. This file is very important for ensuring that the
SSH client is connecting to the correct SSH server.
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WARNING
If setting up an SSH server, do not turn off the Privilege Separation feature by
using the UsePrivilegeSeparation no directive in the /etc/ssh/sshd_config file.
Turning off Privilege Separation disables many security features and exposes the
server to potential security vulnerabilities and targeted attacks. For more information
about UsePrivilegeSeparation, see the sshd_config(5) manual page or the What is
the significance of UsePrivilegeSeparation directive in /etc/ssh/sshd_config file and
how to test it ? Red Hat Knowledgebase article.
For information concerning various directives that can be used in the SSH configuration files, see the
ssh_config(5) and sshd_config(5) manual pages.
12.2.2. Starting an OpenSSH Server
In order to run an OpenSSH server, you must have the openssh-server package installed. For more
information on how to install new packages, see Section 9.2.4, “Installing Packages”.
To start the sshd daemon in the current session, type the following at a shell prompt as root:
~]# systemctl start sshd.service
To stop the running sshd daemon in the current session, use the following command as root:
~]# systemctl stop sshd.service
If you want the daemon to start automatically at boot time, type as root:
~]# systemctl enable sshd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/sshd.service to
/usr/lib/systemd/system/sshd.service.
The sshd daemon depends on the network.target target unit, which is sufficient for static configured
network interfaces and for default ListenAddress 0.0.0.0 options. To specify different addresses in the
ListenAddress directive and to use a slower dynamic network configuration, add dependency on the
network-online.target target unit to the sshd.service unit file. To achieve this, create the
/etc/systemd/system/sshd.service.d/local.conf file with the following options:
[Unit]
Wants=network-online.target
After=network-online.target
After this, reload the systemd manager configuration using the following command:
For more information on how to manage system services in Red Hat Enterprise Linux, see Chapter 10,
Managing Services with systemd .
Note that if you reinstall the system, a new set of identification keys will be created. As a result, clients who

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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:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@@@@@@@@@@@@@@@
@: 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 backup the relevant files from the /etc/ssh/ directory. See Table 12.1, “System-
wide configuration files” for a complete list, and restore the files whenever you reinstall the system.
12.2.3. 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.
For information on how to configure the vsftpd service, see Section 16.2, “FTP”. To learn how to manage
system services in Red Hat Enterprise Linux 7, read Chapter 10, Managing Services with systemd .
12.2.4. Using Key-based Authentication
To improve the system security even further, generate SSH key pairs and then enforce key-based
authentication by disabling password authentication. To do so, open the /etc/ssh/sshd_config
configuration file in a text editor such as vi or nano, and change the PasswordAuthentication option as
follows:
PasswordAuthentication no
If you are working on a system other than a new default installation, check that PubkeyAuthentication
no has not been set. If connected remotely, not using console or out-of-band access, testing the key-
based log in process before disabling password authentication is advised.
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.
To use key-based authentication with NFS-mounted home directories, enable the use_nfs_home_dirs
SELinux boolean first:
~]# setsebool -P use_nfs_home_dirs 1
Red Hat Enterprise Linux 7 uses SSH Protocol 2 and RSA keys by default (see Section 12.1.3, “Protocol
Versions” for more information).
IMPORTANT
If you complete the steps as root, only root will be able to use the keys.
NOTE
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NOTE
If you reinstall your system and want to keep previously generated key pairs, 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.
12.2.4.1. 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/USER/.ssh/id_rsa):
2. Press Enter to confirm the default location, ~/.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/USER/.ssh/id_rsa.
Your public key has been saved in /home/USER/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:UNIgIT4wfhdQH/K7yqmjsbZnnyGDKiDviv492U5z78Y
USER@penguin.example.com
The key's randomart image is:
+---[RSA 2048]----+
|o ..==o+. |
|.+ . .=oo |
| .o. ..o |
| ... .. |
| .S |
|o . . |
|o+ o .o+ .. |
|+.++=o*.o .E |
|BBBo+Bo. oo |
+----[SHA256]-----+
NOTE
To get an MD5 key fingerprint, which was the default fingerprint in previous
versions, use the ssh-keygen command with the -E md5 option.
4. By default, the permissions of the ~/.ssh/ directory are set to rwx------ or 700 expressed in octal
notation. This is to ensure that only the USER can view the contents. If required, this can be
confirmed with the following command:
~]$ ls -ld ~/.ssh
drwx------. 2 USER USER 54 Nov 25 16:56 /home/USER/.ssh/
5. To copy the public key to a remote machine, issue a command in the following format:
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ssh-copy-id user@hostname
This will copy the most recently modified ~/.ssh/id*.pub public key if it is not yet installed.
Alternatively, specify the public key’s file name as follows:
ssh-copy-id -i ~/.ssh/id_rsa.pub user@hostname
This will copy the content of ~/.ssh/id_rsa.pub into the ~/.ssh/authorized_keys file on the
machine to which you want to connect. If the file already exists, the keys are appended to its end.
To generate an ECDSA key pair for version 2 of the SSH protocol, follow these steps:
1. Generate an ECDSA key pair by typing the following at a shell prompt:
~]$ ssh-keygen -t ecdsa
Generating public/private ecdsa key pair.
Enter file in which to save the key (/home/USER/.ssh/id_ecdsa):
2. Press Enter to confirm the default location, ~/.ssh/id_ecdsa, 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/USER/.ssh/id_ecdsa.
Your public key has been saved in /home/USER/.ssh/id_ecdsa.pub.
The key fingerprint is:
SHA256:8BhZageKrLXM99z5f/AM9aPo/KAUd8ZZFPcPFWqK6+M
USER@penguin.example.com
The key's randomart image is:
+---[ECDSA 256]---+
| . . +=|
| . . . = o.o|
| + . * . o...|
| = . . * . + +..|
|. + . . So o * ..|
| . o . .+ = ..|
| o oo ..=. .|
| ooo...+ |
| .E++oo |
+----[SHA256]-----+
4. By default, the permissions of the ~/.ssh/ directory are set to rwx------ or 700 expressed in octal
notation. This is to ensure that only the USER can view the contents. If required, this can be
confirmed with the following command:
~]$ ls -ld ~/.ssh
~]$ ls -ld ~/.ssh/
drwx------. 2 USER USER 54 Nov 25 16:56 /home/USER/.ssh/
5. To copy the public key to a remote machine, issue a command in the following format:
ssh-copy-id USER@hostname
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This will copy the most recently modified ~/.ssh/id*.pub public key if it is not yet installed.
Alternatively, specify the public key’s file name as follows:
ssh-copy-id -i ~/.ssh/id_ecdsa.pub USER@hostname
This will copy the content of ~/.ssh/id_ecdsa.pub into the ~/.ssh/authorized_keys on the
machine to which you want to connect. If the file already exists, the keys are appended to its end.
See Section 12.2.4.2, “Configuring ssh-agent” for information on how to set up your system to remember
the passphrase.
IMPORTANT
The private key is for your personal use only, and it is important that you never give it to
anyone.
12.2.4.2. 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. If you are running GNOME, you can
configure it to prompt you for your passphrase whenever you log in and remember it during the whole
session. Otherwise you can store the passphrase for a certain shell prompt.
To save your passphrase during your GNOME session, follow these steps:
1. Make sure you have the openssh-askpass package installed. If not, see Section 9.2.4, “Installing
Packages” for more information on how to install new packages in Red Hat Enterprise Linux.
2. Press the Super key to enter the Activities Overview, type Startup Applications and then press
Enter. The Startup Applications Preferences tool appears. The tab containing a list of available
startup programs will be shown by default. The Super key appears in a variety of guises,
depending on the keyboard and other hardware, but often as either the Windows or Command
key, and typically to the left of the Space bar.
Figure 12.1. Startup Applications Preferences
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Figure 12.1. Startup Applications Preferences
3. Click the Add button on the right, and enter /usr/bin/ssh-add in the Command field.
Figure 12.2. Adding new application
4. Click Add and make sure the checkbox next to the newly added item is selected.
Figure 12.3. Enabling the application
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Figure 12.3. Enabling the application
5. Log out and then log back in. A dialog box will appear prompting you for your passphrase. From
this point on, you should not be prompted for a password by ssh, scp, or sftp.
Figure 12.4. Entering a passphrase
To save your passphrase for a certain shell prompt, use the following command:
~]$ ssh-add
Enter passphrase for /home/USER/.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.
12.3. OPENSSH CLIENTS
To connect to an OpenSSH server from a client machine, you must have the openssh-clients package
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To connect to an OpenSSH server from a client machine, you must have the openssh-clients package
installed (see Section 9.2.4, “Installing Packages” for more information on how to install new packages in
Red Hat Enterprise Linux).
12.3.1. Using the ssh Utility
The ssh utility 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 the telnet command, log in to a remote machine by using the following command:
ssh hostname
For example, to log in to a remote machine named penguin.example.com, type the following at a shell
prompt:
~]$ ssh penguin.example.com
This will log you in with the same user name you are using on the local machine. If you want to specify a
different user name, use a command in the following form:
ssh username@hostname
For example, to log in to penguin.example.com as USER, type:
~]$ ssh USER@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.
ECDSA key fingerprint is SHA256:vuGKK9dsW34zrZzwjl5g+vOE6EZQvHRQ8zObKYO2mW4.
ECDSA key fingerprint is MD5:7e:15:c3:03:4d:e1:dd:ee:99:dc:3e:f4:b9:67:6b:62.
Are you sure you want to continue connecting (yes/no)?
Users should always check if the fingerprint is correct before answering the question in this dialog. The
user can ask the administrator of the server to confirm the key is correct. This should be done in a secure
and previously agreed way. If the user has access to the server’s host keys, the fingerprint can be checked
by using the ssh-keygen command as follows:
~]# ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key.pub
SHA256:vuGKK9dsW34zrZzwjl5g+vOE6EZQvHRQ8zObKYO2mW4
NOTE
To get an MD5 key fingerprint, which was the default fingerprint in previous versions, use
the ssh-keygen command with the -E md5 option, for example:
~]# ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key.pub -EM md5
MD5:7e:15:c3:03:4d:e1:dd:ee:99:dc:3e:f4:b9:67:6b:62
Type yes to accept the key and confirm the connection. You will see a notice that the server has been
added to the list of known hosts, and a prompt asking for your password:
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Warning: Permanently added 'penguin.example.com' (ECDSA) to the list of known hosts.
USER@penguin.example.com's password:
IMPORTANT
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.
Before doing this, however, contact the system administrator of the SSH server to verify
the server is not compromised.
To remove a key from the ~/.ssh/known_hosts file, issue a command as follows:
~]# ssh-keygen -R penguin.example.com
# Host penguin.example.com found: line 15 type ECDSA
/home/USER/.ssh/known_hosts updated.
Original contents retained as /home/USER/.ssh/known_hosts.old
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:
ssh username@hostname command
For example, the /etc/redhat-release file provides information about the Red Hat Enterprise Linux
version. To view the contents of this file on penguin.example.com, type:
~]$ ssh USER@penguin.example.com cat /etc/redhat-release
Red Hat Enterprise Linux Server release 7.0 (Maipo)
After you enter the correct password, the user name will be displayed, and you will return to your local shell
prompt.
12.3.2. 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 USER@penguin.example.com:.vim/plugin/taglist.vim
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:
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~]$ scp .vim/plugin/* USER@penguin.example.com:.vim/plugin/
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 USER@penguin.example.com:.vimrc .vimrc
.vimrc 100% 2233 2.2KB/s 00:00
12.3.3. 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 USER as a user name,
type:
~]$ sftp USER@penguin.example.com
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 12.3, “A selection of available sftp commands” ).
Table 12.3. 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.
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getremotefile [localfile] Transfer remotefile from a remote machine.
Command Description
For a complete list of available commands, see the sftp(1) manual page.
12.4. 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.
12.4.1. 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 USER as a user name,
type:
~]$ ssh -Y USER@penguin.example.com
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.
Note that the X Window system must be installed on the remote system before X11 forwarding can take
place. Enter the following command as root to install the X11 package group:
~]# yum group install "X Window System"
For more information on package groups, see Section 9.3, “Working with Package Groups” .
X11 forwarding can be very useful. For example, X11 forwarding can be used to create a secure, interactive
session of the Print Settings utility. To do this, connect to the server using ssh and type:
~]$ system-config-printer &
The Print Settings tool will appear, allowing the remote user to safely configure printing on the remote
system.
12.4.2. 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.
NOTE
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197

NOTE
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.
The OpenSSH suite also provides local and remote port forwarding of UNIX domain sockets. To forward
UNIX domain sockets over the network to another machine, use the ssh -L local-socket:remote-socket
username@hostname command, for example:
~]$ ssh -L /var/mysql/mysql.sock:/var/mysql/mysql.sock username@hostname
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.
IMPORTANT
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.
For more information on how to configure or connect to an OpenSSH server on Red Hat Enterprise Linux,
see the resources listed below.
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sshd(8) — The manual page for the sshd daemon documents available command line options
and provides a complete list of supported configuration files and directories.
ssh(1) — The manual page for the ssh client application provides a complete list of available
command line options and supported configuration files and directories.
scp(1) — The manual page for the scp utility provides a more detailed description of this utility
and its usage.
sftp(1) — The manual page for the sftp utility.
ssh-keygen(1) — The manual page for the ssh-keygen utility documents in detail how to use it to
generate, manage, and convert authentication keys used by ssh.
ssh_config(5) — The manual page named ssh_config documents available SSH client
configuration options.
sshd_config(5) — The manual page named sshd_config provides a full description of available
SSH daemon configuration options.
OpenSSH Home Page — The OpenSSH home page containing further documentation, frequently
asked questions, links to the mailing lists, bug reports, and other useful resources.
OpenSSL Home Page — The OpenSSL home page containing further documentation, frequently
asked questions, links to the mailing lists, and other useful resources.
See Also
and sudo commands.
[1] 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.
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TigerVNC (Tiger Virtual Network Computing) is a system for graphical desktop sharing which allows you
to remotely control other computers.
TigerVNC works on the client-server principle: a server shares its output (vncserver) and a client
(vncviewer) connects to the server.
NOTE
Unlike in previous Red Hat Enterprise Linux distributions, TigerVNC in Red Hat Enterprise
Linux 7 uses the systemd system management daemon for its configuration. The
/etc/sysconfig/vncserver configuration file has been replaced by
/etc/systemd/system/vncserver@.service.
13.1. VNC SERVER
vncserver is a utility which starts a VNC (Virtual Network Computing) desktop. It runs Xvnc with
appropriate options and starts a window manager on the VNC desktop. vncserver allows users to run
separate sessions in parallel on a machine which can then be accessed by any number of clients from
anywhere.
13.1.1. Installing VNC Server
To install the TigerVNC server, issue the following command as root:
~]# yum install tigervnc-server
13.1.2. Configuring VNC Server
The VNC server can be configured to start a display for one or more users, provided that accounts for the
users exist on the system, with optional parameters such as for display settings, network address and port,
and security settings.
Configuring a VNC Display for a Single User
1. A configuration file named /etc/systemd/system/vncserver@.service is required. To create this
file, copy the /usr/lib/systemd/system/vncserver@.service file as root:
~]# cp /usr/lib/systemd/system/vncserver@.service
/etc/systemd/system/vncserver@.service
There is no need to include the display number in the file name because systemd automatically
creates the appropriately named instance in memory on demand, replacing '%i' in the service file
by the display number. For a single user it is not necessary to rename the file. For multiple users, a
uniquely named service file for each user is required, for example, by adding the user name to the
file name in some way. See Section 13.1.2.1, “Configuring VNC Server for Two Users” for details.
2. Edit /etc/systemd/system/vncserver@.service, replacing USER with the actual user name.
Leave the remaining lines of the file unmodified.
ExecStart=/usr/bin/vncserver_wrapper <USER> %i
NOTE
200

NOTE
The default size of the VNC desktop is 1024x768.
A user’s VNC session can be further configured using the ~/.vnc/config file.
For example, to change the VNC window size, add the following line:
geometry= <WIDTH> x <HEIGHT>
3. Save the changes.
4. To make the changes take effect immediately, issue the following command:
5. Set the password for the user or users defined in the configuration file. Note that you need to
switch from root to USER first.
~]# su - USER
~]$ vncpasswd
Password:
Verify:
IMPORTANT
The stored password is not encrypted; anyone who has access to the password file
can find the plain-text password.
Proceed to Section 13.1.3, “Starting VNC Server” .
13.1.2.1. Configuring VNC Server for Two Users
If you want to configure more than one user on the same machine, create different template-type service
files, one for each user.
1. Create two service files, for example vncserver-USER_1@.service and
vncserver-USER_2@.service. In both these files substitute USER with the correct user name.
2. Set passwords for both users:
~]$ su - USER_1
~]$ vncpasswd
Password:
Verify:
~]$ su - USER_2
~]$ vncpasswd
Password:
Verify:
13.1.3. Starting VNC Server
To start or enable the service, specify the display number directly in the command. The file configured
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To start or enable the service, specify the display number directly in the command. The file configured
above in Configuring a VNC Display for a Single User works as a template, in which %i is substituted with
the display number by systemd. With a valid display number, execute the following command:
~]# systemctl start vncserver@:display_number.service
You can also enable the service to start automatically at system start. Then, when you log in, vncserver is
automatically started. As root, issue a command as follows:
~]# systemctl enable vncserver@:display_number.service
At this point, other users are able to use a VNC viewer program to connect to the VNC server using the
display number and password defined. Provided a graphical desktop is installed, an instance of that
desktop will be displayed. It will not be the same instance as that currently displayed on the target
machine.
13.1.3.1. Configuring VNC Server for Two Users and Two Different Displays
For the two configured VNC servers, vncserver-USER_1@.service and vncserver-USER_2@.service, you
can enable different display numbers. For example, the following commands will cause a VNC server for
USER_1 to start on display 3, and a VNC server for USER_2 to start on display 5:
~]# systemctl start vncserver-USER_1@:3.service
~]# systemctl start vncserver-USER_2@:5.service
13.1.4. VNC setup based on xinetd with XDMCP for GDM
VNC setup based on xinetd with X Display Manager Control Protocol (XDMCP) for GDM is a useful setup
for client systems that consist mainly of thin clients. After the setup, clients are able to access the GDM
login window and log in to any system account. The prerequisite for the setup is that the gdm, vnc, vnc-
server & and xinetd packages are installed.
~]# yum install gdm tigervnc tigervnc-server xinetd
Service xinetd must be enabled.
~]# systemctl enable xinetd.service
System default target unit should be graphical.target. To get the currently set default target unit, use:
~]# systemctl get-default
The default target unit can be changed by using:
~]# systemctl set-default target_name
Accessing the GDM login window and logging in
1. Set up GDM to enable XDMCP by editing the /etc/gdm/custom.conf configuration file:
[xdmcp]
Enable=true
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2. Create a file called /etc/xinetd.d/xvncserver with the following content:
service service_name
{
disable = no
protocol = tcp
socket_type = stream
wait = no
user = nobody
server = /usr/bin/Xvnc
server_args = -inetd -query localhost -once -geometry selected_geometry -depth
selected_depth securitytypes=none
}
In the server_args section, the -query localhost option will make each Xvnc instance query
localhost for an xdmcp session. The -depth option specifies the pixel depth (in bits) of the VNC
desktop to be created. Acceptable values are 8, 15, 16 and 24 - any other values are likely to cause
unpredictable behavior of applications.
3. Edit file /etc/services to have the service defined. To do this, append the following snippet to the
/etc/services file:
# VNC xinetd GDM base
service_name 5950/tcp
4. To ensure that the configuration changes take effect, reboot the machine.
Alternatively, you can run the following. Change init levels to 3 and back to 5 to force gdm to
reload.
# init 3
# init 5
Verify that gdm is listening on UDP port 177.
# netstat -anu|grep 177
udp 0 0 0.0.0.0:177 0.0.0.0:*
Restart the xinetd service.
~]# systemctl restart xinetd.service
Verify that the xinetd service has loaded the new services.
# netstat -anpt|grep 595
tcp 0 0 :::5950 :::* LISTEN 3119/xinetd
5. Test the setup using a vncviewer command:
# vncviewer localhost:5950
The command will launch a VNC session to the localhost where no password is asked. You will see
a GDM login screen, and you will be able to log in to any user account on the system with a valid
user name and password. Then you can run the same test on remote connections.
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Configure firewall for the setup. Run the firewall configuration tool and add TCP port 5950 to allow
incoming connections to the system.
~]# firewall-cmd --permanent --zone=public --add-port=5950/tcp
~]# firewall-cmd --reload
13.1.5. Terminating a VNC Session
Similarly to enabling the vncserver service, you can disable the automatic start of the service at system
start:
~]# systemctl disable vncserver@:display_number.service
Or, when your system is running, you can stop the service by issuing the following command as root:
~]# systemctl stop vncserver@:display_number.service
13.2. SHARING AN EXISTING DESKTOP
By default a logged in user has a desktop provided by X Server on display 0. A user can share their
desktop using the TigerVNC server x0vncserver.
Sharing an X Desktop
To share the desktop of a logged in user, using the x0vncserver, proceed as follows:
1. Enter the following command as root
~]# yum install tigervnc-server
2. Set the VNC password for the user:
~]$ vncpasswd
Password:
Verify:
3. Enter the following command as that user:
~]$ x0vncserver -PasswordFile=.vnc/passwd -AlwaysShared=1
Provided the firewall is configured to allow connections to port 5900, the remote viewer can now connect
to display 0, and view the logged in users desktop. See Section 13.3.2.1, “Configuring the Firewall for VNC”
for information on how to configure the firewall.
13.3. VNC VIEWER
vncviewer is a program which shows the graphical user interfaces and controls the vncserver remotely.
For operating the vncviewer, there is a pop-up menu containing entries which perform various actions
such as switching in and out of full-screen mode or quitting the viewer. Alternatively, you can operate
vncviewer through the terminal. Enter vncviewer -h on the command line to list vncviewer's
parameters.
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13.3.1. Installing VNC Viewer
To install the TigerVNC client, vncviewer, issue the following command as root:
~]# yum install tigervnc
13.3.2. Connecting to VNC Server
Once the VNC server is configured, you can connect to it from any VNC viewer.
Connecting to a VNC Server Using a GUI
1. Enter the vncviewer command with no arguments, the VNC Viewer: Connection Details utility
appears. It prompts for a VNC server to connect to.
2. If required, to prevent disconnecting any existing VNC connections to the same display, select the
option to allow sharing of the desktop as follows:
a. Select the Options button.
b. Select the Misc. tab.
c. Select the Shared button.
d. Press OK to return to the main menu.
3. Enter an address and display number to connect to:
address:display_number
4. Press Connect to connect to the VNC server display.
5. You will be prompted to enter the VNC password. This will be the VNC password for the user
corresponding to the display number unless a global default VNC password was set.
A window appears showing the VNC server desktop. Note that this is not the desktop the normal
user sees, it is an Xvnc desktop.
Connecting to a VNC Server Using the CLI
1. Enter the viewer command with the address and display number as arguments:
vncviewer address:display_number
Where address is an IP address or host name.
2. Authenticate yourself by entering the VNC password. This will be the VNC password for the user
corresponding to the display number unless a global default VNC password was set.
3. A window appears showing the VNC server desktop. Note that this is not the desktop the normal
user sees, it is the Xvnc desktop.
13.3.2.1. Configuring the Firewall for VNC
When using a non-encrypted connection, firewalld might block the connection. To allow firewalld to pass
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When using a non-encrypted connection, firewalld might block the connection. To allow firewalld to pass
the VNC packets, you can open specific ports to TCP traffic. When using the -via option, traffic is
redirected over SSH which is enabled by default in firewalld.
NOTE
The default port of VNC server is 5900. To reach the port through which a remote desktop
will be accessible, sum the default port and the user’s assigned display number. For
example, for the second display: 2 + 5900 = 5902.
For displays 0 to 3, make use of firewalld's support for the VNC service by means of the service option
as described below. Note that for display numbers greater than 3, the corresponding ports will have to be
opened specifically as explained in Opening Ports in firewalld.
Enabling VNC Service in firewalld
1. Run the following command to see the information concerning firewalld settings:
~]$ firewall-cmd --list-all
2. To allow all VNC connections from a specific address, use a command as follows:
~]# firewall-cmd --add-rich-rule='rule family="ipv4" source address="192.168.122.116"
service name=vnc-server accept'
success
Note that these changes will not persist after the next system start. To make permanent changes
to the firewall, repeat the commands adding the --permanent option. See the Red Hat Enterprise
Linux 7 Security Guide for more information on the use of firewall rich language commands.
3. To verify the above settings, use a command as follows:
~]# firewall-cmd --list-all
public (default, active)
interfaces: bond0 bond0.192
sources:
services: dhcpv6-client ssh
ports:
masquerade: no
forward-ports:
icmp-blocks:
rich rules:
rule family="ipv4" source address="192.168.122.116" service name="vnc-server" accept
To open a specific port or range of ports make use of the --add-port option to the firewall-cmd
command Line tool. For example, VNC display 4 requires port 5904 to be opened for TCP traffic.
Opening Ports in firewalld
1. To open a port for TCP traffic in the public zone, issue a command as root as follows:
~]# firewall-cmd --zone=public --add-port=5904/tcp
success
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2. To view the ports that are currently open for the public zone, issue a command as follows:
~]# firewall-cmd --zone=public --list-ports
5904/tcp
A port can be removed using the firewall-cmd --zone=zone --remove-port=number/protocol
command.
Note that these changes will not persist after the next system start. To make permanent changes to the
firewall, repeat the commands adding the --permanent option. For more information on opening and
closing ports in firewalld, see the Red Hat Enterprise Linux 7 Security Guide .
13.3.3. Connecting to VNC Server Using SSH
VNC is a clear text network protocol with no security against possible attacks on the communication. To
make the communication secure, you can encrypt your server-client connection by using the -via option.
This will create an SSH tunnel between the VNC server and the client.
The format of the command to encrypt a VNC server-client connection is as follows:
vncviewer -via user@host:display_number
Example 13.1. Using the -via Option
1. To connect to a VNC server using SSH, enter a command as follows:
~]$ vncviewer -via USER_2@192.168.2.101:3
2. When you are prompted to, type the password, and confirm by pressing Enter.
3. A window with a remote desktop appears on your screen.
Restricting VNC Access
If you prefer only encrypted connections, you can prevent unencrypted connections altogether by using
the -localhost option in the systemd.service file, the ExecStart line:
ExecStart=/usr/sbin/runuser -l user -c "/usr/bin/vncserver -localhost %i"
This will stop vncserver from accepting connections from anything but the local host and port-forwarded
connections sent using SSH as a result of the -via option.
For more information on using SSH, see Chapter 12, OpenSSH.
For more information about TigerVNC, see the resources listed below.
vncserver(1) — The manual page for the VNC server utility.
vncviewer(1) — The manual page for the VNC viewer.
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vncpasswd(1) — The manual page for the VNC password command.
Xvnc(1) — The manual page for the Xvnc server configuration options.
x0vncserver(1) — The manual page for the TigerVNC server for sharing existing X servers.
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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 a network.
PART V. SERVERS
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A web server is a network service that serves content to a client over the web. This typically means web
pages, but any other documents can be served as well. Web servers are also known as HTTP servers, as
they use the hypertext transport protocol (HTTP).
The web servers available in Red Hat Enterprise Linux 7 are:
Apache HTTP Server
nginx
IMPORTANT
Note that the nginx web server is available only as a Software Collection for Red Hat
Enterprise Linux 7. See the Red Hat Software Collections Release Notes for information
regarding getting access to nginx, usage of Software Collections, and other.
14.1. THE APACHE HTTP SERVER
This section focuses on the Apache HTTP Server 2.4, httpd, an open source web server developed by the
Apache Software Foundation.
If you are upgrading from a previous release of Red Hat Enterprise Linux, you will need to update the
httpd service configuration accordingly. This section reviews some of the newly added features, outlines
important changes between Apache HTTP Server 2.4 and version 2.2, and guides you through the update
of older configuration files.
14.1.1. Notable Changes
The Apache HTTP Server in Red Hat Enterprise Linux 7 has the following changes compared to Red Hat
Enterprise Linux 6:
httpd Service Control
With the migration away from SysV init scripts, server administrators should switch to using the
apachectl and systemctl commands to control the service, in place of the service command. The
following examples are specific to the httpd service.
The command:
service httpd graceful
is replaced by
apachectl graceful
The systemd unit file for httpd has different behavior from the init script as follows:
A graceful restart is used by default when the service is reloaded.
A graceful stop is used by default when the service is stopped.
The command:
service httpd configtest
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is replaced by
apachectl configtest
Private /tmp
To enhance system security, the systemd unit file runs the httpd daemon using a private /tmp
directory, separate to the system /tmp directory.
Configuration Layout
Configuration files which load modules are now placed in the /etc/httpd/conf.modules.d/ directory.
Packages that provide additional loadable modules for httpd, such as php, will place a file in this
directory. An Include directive before the main section of the /etc/httpd/conf/httpd.conf file is used
to include files within the /etc/httpd/conf.modules.d/ directory. This means any configuration files
within conf.modules.d/ are processed before the main body of httpd.conf. An IncludeOptional
directive for files within the /etc/httpd/conf.d/ directory is placed at the end of the httpd.conf file.
This means the files within /etc/httpd/conf.d/ are now processed after the main body of httpd.conf.
Some additional configuration files are provided by the httpd package itself:
/etc/httpd/conf.d/autoindex.conf — This configures mod_autoindex directory indexing.
/etc/httpd/conf.d/userdir.conf — This configures access to user directories, for example,
http://example.com/~username/; such access is disabled by default for security reasons.
/etc/httpd/conf.d/welcome.conf — As in previous releases, this configures the welcome page
displayed for http://localhost/ when no content is present.
Default Configuration
A minimal httpd.conf file is now provided by default. Many common configuration settings, such as
Timeout or KeepAlive are no longer explicitly configured in the default configuration; hard-coded
settings will be used instead, by default. The hard-coded default settings for all configuration directives
are specified in the manual. See the section called “Installable Documentation” for more information.
Incompatible Syntax Changes
If migrating an existing configuration from httpd 2.2 to httpd 2.4, a number of backwards-
incompatible changes to the httpd configuration syntax were made which will require changes. See the
following Apache document for more information on upgrading
http://httpd.apache.org/docs/2.4/upgrading.html
Processing Model
In previous releases of Red Hat Enterprise Linux, different multi-processing models (MPM) were made
available as different httpd binaries: the forked model, "prefork", as /usr/sbin/httpd, and the thread-
based model "worker" as /usr/sbin/httpd.worker.
In Red Hat Enterprise Linux 7, only a single httpd binary is used, and three MPMs are available as
loadable modules: worker, prefork (default), and event. Edit the configuration file
/etc/httpd/conf.modules.d/00-mpm.conf as required, by adding and removing the comment
character # so that only one of the three MPM modules is loaded.
Packaging Changes
The LDAP authentication and authorization modules are now provided in a separate sub-package,
mod_ldap. The new module mod_session and associated helper modules are provided in a new sub-
package, mod_session. The new modules mod_proxy_html and mod_xml2enc are provided in a new
sub-package, mod_proxy_html. These packages are all in the Optional channel.
NOTE
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NOTE
Before subscribing to the Optional and Supplementary channels see the Scope of
Coverage Details. If you decide to install packages from these channels, follow the steps
documented in the article called How to access Optional and Supplementary channels,
and -devel packages using Red Hat Subscription Manager (RHSM)? on the Red Hat
Customer Portal.
Packaging Filesystem Layout
The /var/cache/mod_proxy/ directory is no longer provided; instead, the /var/cache/httpd/ directory
is packaged with a proxy and ssl subdirectory.
Packaged content provided with httpd has been moved from /var/www/ to /usr/share/httpd/:
/usr/share/httpd/icons/ — The directory containing a set of icons used with directory indices,
previously contained in /var/www/icons/, has moved to /usr/share/httpd/icons/. Available at
http://localhost/icons/ in the default configuration; the location and the availability of the
icons is configurable in the /etc/httpd/conf.d/autoindex.conf file.
/usr/share/httpd/manual/ — The /var/www/manual/ has moved to
/usr/share/httpd/manual/. This directory, contained in the httpd-manual package, contains
the HTML version of the manual for httpd. Available at http://localhost/manual/ if the
package is installed, the location and the availability of the manual is configurable in the
/etc/httpd/conf.d/manual.conf file.
/usr/share/httpd/error/ — The /var/www/error/ has moved to /usr/share/httpd/error/.
Custom multi-language HTTP error pages. Not configured by default, the example
configuration file is provided at /usr/share/doc/httpd-VERSION/httpd-multilang-
errordoc.conf.
Authentication, Authorization and Access Control
The configuration directives used to control authentication, authorization and access control have
changed significantly. Existing configuration files using the Order, Deny and Allow directives should
be adapted to use the new Require syntax. See the following Apache document for more information
http://httpd.apache.org/docs/2.4/howto/auth.html
suexec
To improve system security, the suexec binary is no longer installed as if by the root user; instead, it
has file system capability bits set which allow a more restrictive set of permissions. In conjunction with
this change, the suexec binary no longer uses the /var/log/httpd/suexec.log logfile. Instead, log
messages are sent to syslog; by default these will appear in the /var/log/secure log file.
Module Interface
Third-party binary modules built against httpd 2.2 are not compatible with httpd 2.4 due to changes
to the httpd module interface. Such modules will need to be adjusted as necessary for the httpd 2.4
module interface, and then rebuilt. A detailed list of the API changes in version 2.4 is available here:
http://httpd.apache.org/docs/2.4/developer/new_api_2_4.html.
The apxs binary used to build modules from source has moved from /usr/sbin/apxs to /usr/bin/apxs.
Removed modules
List of httpd modules removed in Red Hat Enterprise Linux 7:
mod_auth_mysql, mod_auth_pgsql
httpd 2.4 provides SQL database authentication support internally in the mod_authn_dbd module.
mod_perl
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mod_perl is not officially supported with httpd 2.4 by upstream.
mod_authz_ldap
httpd 2.4 provides LDAP support in sub-package mod_ldap using mod_authnz_ldap.
14.1.2. Updating the Configuration
To update the configuration files from the Apache HTTP Server version 2.2, take the following steps:
1. Make sure all module names are correct, since they may have changed. Adjust the LoadModule
directive for each module that has been renamed.
2. Recompile all third party modules before attempting to load them. This typically means
authentication and authorization modules.
3. If you use the mod_userdir module, make sure the UserDir directive indicating a directory name
(typically public_html) is provided.
4. If you use the Apache HTTP Secure Server, see Section 14.1.8, “Enabling the mod_ssl Module” for
important information on enabling the Secure Sockets Layer (SSL) protocol.
Note that you can check the configuration for possible errors by using the following command:
~]# apachectl configtest
Syntax OK
For more information on upgrading the Apache HTTP Server configuration from version 2.2 to 2.4, see
http://httpd.apache.org/docs/2.4/upgrading.html.
14.1.3. Running the httpd Service
This section describes how to start, stop, restart, and check the current status of the Apache HTTP
Server. To be able to use the httpd service, make sure you have the httpd installed. You can do so by
using the following command:
~]# yum install httpd
For more information on the concept of targets and how to manage system services in Red Hat Enterprise
Linux in general, see Chapter 10, Managing Services with systemd .
To run the httpd service, type the following at a shell prompt as root:
~]# systemctl start httpd.service
If you want the service to start automatically at boot time, use the following command:
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to
NOTE
213

NOTE
If running the Apache HTTP Server as a secure server, a password may be required after
the machine boots if using an encrypted private SSL key.
To stop the running httpd service, type the following at a shell prompt as root:
~]# systemctl stop httpd.service
To prevent the service from starting automatically at boot time, type:
~]# systemctl disable httpd.service
Removed symlink /etc/systemd/system/multi-user.target.wants/httpd.service.
There are three different ways to restart a running httpd service:
1. To restart the service completely, enter the following command as root:
~]# systemctl restart httpd.service
This stops the running httpd service and immediately starts it again. Use this command after
installing or removing a dynamically loaded module such as PHP.
2. To only reload the configuration, as root, type:
~]# systemctl reload httpd.service
This causes the running httpd service to reload its configuration file. Any requests currently being
processed will be interrupted, which may cause a client browser to display an error message or
render a partial page.
3. To reload the configuration without affecting active requests, enter the following command as
root:
~]# apachectl graceful
This causes the running httpd service to reload its configuration file. Any requests currently being
processed will continue to use the old configuration.
For more information on how to manage system services in Red Hat Enterprise Linux 7, see Chapter 10,
14.1.3.4. Verifying the Service Status
To verify that the httpd service is running, type the following at a shell prompt:
~]# systemctl is-active httpd.service
active
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14.1.4. Editing the Configuration Files
When the httpd service is started, by default, it reads the configuration from locations that are listed in
Table 14.1, “The httpd service configuration files” .
Table 14.1. The httpd service configuration files
Path Description
/etc/httpd/conf/httpd.conf The main configuration file.
/etc/httpd/conf.d/ An auxiliary directory for configuration files that are
included in the main configuration file.
Although the default configuration should be suitable for most situations, it is a good idea to become at
least familiar with some of the more important configuration options. Note that for any changes to take
effect, the web server has to be restarted first. See Section 14.1.3.3, “Restarting the Service” for more
information on how to restart the httpd service.
To check the configuration for possible errors, type the following at a shell prompt:
~]# apachectl configtest
Syntax OK
To make the recovery from mistakes easier, it is recommended that you make a copy of the original file
before editing it.
14.1.5. Working with Modules
Being a modular application, the httpd service is distributed along with a number of Dynamic Shared
Objects (DSOs), which can be dynamically loaded or unloaded at runtime as necessary. On Red Hat
Enterprise Linux 7, these modules are located in /usr/lib64/httpd/modules/.
14.1.5.1. Loading a Module
To load a particular DSO module, use the LoadModule directive. Note that modules provided by a
separate package often have their own configuration file in the /etc/httpd/conf.d/ directory.
Example 14.1. Loading the mod_ssl DSO
LoadModule ssl_module modules/mod_ssl.so
Once you are finished, restart the web server to reload the configuration. See Section 14.1.3.3, “Restarting
the Service” for more information on how to restart the httpd service.
14.1.5.2. Writing a Module
If you intend to create a new DSO module, make sure you have the httpd-devel package installed. To do
so, enter the following command as root:
~]# yum install httpd-devel
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This package contains the include files, the header files, and the APache eXtenSion (apxs) utility
required to compile a module.
Once written, you can build the module with the following command:
~]# apxs -i -a -c module_name.c
If the build was successful, you should be able to load the module the same way as any other module that
is distributed with the Apache HTTP Server.
14.1.6. Setting Up Virtual Hosts
The Apache HTTP Server’s built in virtual hosting allows the server to provide different information based
on which IP address, host name, or port is being requested.
To create a name-based virtual host, copy the example configuration file
/usr/share/doc/httpd-VERSION/httpd-vhosts.conf into the /etc/httpd/conf.d/ directory, and replace
the @@Port@@ and @@ServerRoot@@ placeholder values. Customize the options according to your
requirements as shown in Example 14.2, “Example virtual host configuration”.
Example 14.2. Example virtual host configuration
<VirtualHost *:80>
ServerAdmin webmaster@penguin.example.com
DocumentRoot "/www/docs/penguin.example.com"
ServerName penguin.example.com
ServerAlias www.penguin.example.com
ErrorLog "/var/log/httpd/dummy-host.example.com-error_log"
CustomLog "/var/log/httpd/dummy-host.example.com-access_log" common
</VirtualHost>
Note that ServerName must be a valid DNS name assigned to the machine. The <VirtualHost> container
is highly customizable, and accepts most of the directives available within the main server configuration.
Directives that are not supported within this container include User and Group, which were replaced by
SuexecUserGroup.
NOTE
If you configure a virtual host to listen on a non-default port, make sure you update the
Listen directive in the global settings section of the /etc/httpd/conf/httpd.conf file
accordingly.
To activate a newly created virtual host, the web server has to be restarted first. See Section 14.1.3.3,
“Restarting the Service” for more information on how to restart the httpd service.
14.1.7. Setting Up an SSL Server
Secure Sockets Layer (SSL) is a cryptographic protocol that allows a server and a client to communicate
securely. Along with its extended and improved version called Transport Layer Security (TLS), it ensures
both privacy and data integrity. The Apache HTTP Server in combination with mod_ssl, a module that
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uses the OpenSSL toolkit to provide the SSL/TLS support, is commonly referred to as the SSL server.
Red Hat Enterprise Linux also supports the use of Mozilla NSS as the TLS implementation. Support for
Mozilla NSS is provided by the mod_nss module.
Unlike an HTTP connection that can be read and possibly modified by anybody who is able to intercept it,
the use of SSL/TLS over HTTP, referred to as HTTPS, prevents any inspection or modification of the
transmitted content. This section provides basic information on how to enable this module in the Apache
HTTP Server configuration, and guides you through the process of generating private keys and self-
signed certificates.
14.1.7.1. An Overview of Certificates and Security
Secure communication is based on the use of keys. In conventional or symmetric cryptography, both ends
of the transaction have the same key they can use to decode each other’s transmissions. On the other
hand, in public or asymmetric cryptography, two keys co-exist: a private key that is kept a secret, and a
public key that is usually shared with the public. While the data encoded with the public key can only be
decoded with the private key, data encoded with the private key can in turn only be decoded with the
public key.
To provide secure communications using SSL, an SSL server must use a digital certificate signed by a
Certificate Authority (CA). The certificate lists various attributes of the server (that is, the server host
name, the name of the company, its location, etc.), and the signature produced using the CA’s private key.
This signature ensures that a particular certificate authority has signed the certificate, and that the
certificate has not been modified in any way.
When a web browser establishes a new SSL connection, it checks the certificate provided by the web
server. If the certificate does not have a signature from a trusted CA, or if the host name listed in the
certificate does not match the host name used to establish the connection, it refuses to communicate
with the server and usually presents a user with an appropriate error message.
By default, most web browsers are configured to trust a set of widely used certificate authorities. Because
of this, an appropriate CA should be chosen when setting up a secure server, so that target users can trust
the connection, otherwise they will be presented with an error message, and will have to accept the
certificate manually. Since encouraging users to override certificate errors can allow an attacker to
intercept the connection, you should use a trusted CA whenever possible. For more information on this,
see Table 14.2, “Information about CA lists used by common web browsers” .
Table 14.2. Information about CA lists used by common web browsers
Web Browser Link
Mozilla Firefox Mozilla root CA list.
Opera Information on root certificates used by Opera
.
Internet Explorer Information on root certificates used by Microsoft
Windows.
Chromium Information on root certificates used by the
Chromium project.
When setting up an SSL server, you need to generate a certificate request and a private key, and then
send the certificate request, proof of the company’s identity, and payment to a certificate authority. Once
the CA verifies the certificate request and your identity, it will send you a signed certificate you can use
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with your server. Alternatively, you can create a self-signed certificate that does not contain a CA
signature, and thus should be used for testing purposes only.
14.1.8. Enabling the mod_ssl Module
If you intend to set up an SSL or HTTPS server using mod_ssl, you cannot have the another application
or module, such as mod_nss configured to use the same port. Port 443 is the default port for HTTPS.
To set up an SSL server using the mod_ssl module and the OpenSSL toolkit, install the mod_ssl and
openssl packages. Enter the following command as root:
~]# yum install mod_ssl openssl
This will create the mod_ssl configuration file at /etc/httpd/conf.d/ssl.conf, which is included in the main
Apache HTTP Server configuration file by default. For the module to be loaded, restart the httpd service
as described in Section 14.1.3.3, “Restarting the Service”.
IMPORTANT
Due to the vulnerability described in POODLE: SSLv3 vulnerability (CVE-2014-3566), Red
Hat recommends disabling SSL and using only TLSv1.1 or TLSv1.2. Backwards
compatibility can be achieved using TLSv1.0. Many products Red Hat supports have the
ability to use SSLv2 or SSLv3 protocols, or enable them by default. However, the use of
SSLv2 or SSLv3 is now strongly recommended against.
14.1.8.1. Enabling and Disabling SSL and TLS in mod_ssl
To disable and enable specific versions of the SSL and TLS protocol, either do it globally by adding the
SSLProtocol directive in the " # SSL Global Context" section of the configuration file and removing it
everywhere else, or edit the default entry under " SSL Protocol support" in all "VirtualHost" sections. If you
do not specify it in the per-domain VirtualHost section then it will inherit the settings from the global
section. To make sure that a protocol version is being disabled the administrator should either only specify
SSLProtocol in the "SSL Global Context" section, or specify it in all per-domain VirtualHost sections.
Disable SSLv2 and SSLv3
To disable SSL version 2 and SSL version 3, which implies enabling everything except SSL version 2 and
SSL version 3, in all VirtualHost sections, proceed as follows:
1. As root, open the /etc/httpd/conf.d/ssl.conf file and search for all instances of the SSLProtocol
directive. By default, the configuration file contains one section that looks as follows:
~]# vi /etc/httpd/conf.d/ssl.conf
# SSL Protocol support:
# List the enable protocol levels with which clients will be able to
# connect. Disable SSLv2 access by default:
SSLProtocol all -SSLv2
This section is within the VirtualHost section.
2. Edit the SSLProtocol line as follows:
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SSLProtocol all -SSLv2 -SSLv3
Repeat this action for all VirtualHost sections. Save and close the file.
3. Verify that all occurrences of the SSLProtocol directive have been changed as follows:
~]# grep SSLProtocol /etc/httpd/conf.d/ssl.conf
SSLProtocol all -SSLv2 -SSLv3
This step is particularly important if you have more than the one default VirtualHost section.
4. Restart the Apache daemon as follows:
~]# systemctl restart httpd
Note that any sessions will be interrupted.
Disable All SSL and TLS Protocols Except TLS 1 and Up
To disable all SSL and TLS protocol versions except TLS version 1 and higher, proceed as follows:
1. As root, open the /etc/httpd/conf.d/ssl.conf file and search for all instances of SSLProtocol
directive. By default the file contains one section that looks as follows:
~]# vi /etc/httpd/conf.d/ssl.conf
SSLProtocol all -SSLv2
2. Edit the SSLProtocol line as follows:
SSLProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2
Save and close the file.
3. Verify the change as follows:
~]# grep SSLProtocol /etc/httpd/conf.d/ssl.conf
SSLProtocol -all +TLSv1 +TLSv1.1 +TLSv1.2
~]# systemctl restart httpd
Testing the Status of SSL and TLS Protocols
To check which versions of SSL and TLS are enabled or disabled, make use of the openssl s_client -
connect command. The command has the following form:
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openssl s_client -connect hostname:port -protocol
Where port is the port to test and protocol is the protocol version to test for. To test the SSL server
running locally, use localhost as the host name. For example, to test the default port for secure HTTPS
connections, port 443 to see if SSLv3 is enabled, issue a command as follows:
~]# openssl s_client -connect localhost:443 -ssl3
CONNECTED(00000003)
139809943877536:error:14094410:SSL routines:SSL3_READ_BYTES:sslv3 alert handshake
failure:s3_pkt.c:1257:SSL alert number 40
139809943877536:error:1409E0E5:SSL routines:SSL3_WRITE_BYTES:ssl handshake
failure:s3_pkt.c:596:
output omitted
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : SSLv3
output truncated
+ The above output indicates that the handshake failed and therefore no cipher was negotiated.
~]$ openssl s_client -connect localhost:443 -tls1_2
CONNECTED(00000003)
depth=0 C = --, ST = SomeState, L = SomeCity, O = SomeOrganization, OU =
SomeOrganizationalUnit, CN = localhost.localdomain, emailAddress = root@localhost.localdomain
output omitted
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : TLSv1.2
output truncated
+ The above output indicates that no failure of the handshake occurred and a set of ciphers was
negotiated.
The openssl s_client command options are documented in the s_client(1) manual page.
For more information on the SSLv3 vulnerability and how to test for it, see the Red Hat Knowledgebase
article POODLE: SSLv3 vulnerability (CVE-2014-3566).
14.1.9. Enabling the mod_nss Module
If you intend to set up an HTTPS server using mod_nss, you cannot have the mod_ssl package installed
with its default settings as mod_ssl will use port 443 by default, however this is the default HTTPS port. If
at all possible, remove the package.
To remove mod_ssl, enter the following command as root:
~]# yum remove mod_ssl
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NOTE
If mod_ssl is required for other purposes, modify the /etc/httpd/conf.d/ssl.conf file to
use a port other than 443 to prevent mod_ssl conflicting with mod_nss when its port to
listen on is changed to 443.
Only one module can own a port, therefore mod_nss and mod_ssl can only co-exist at
the same time if they use unique ports. For this reason mod_nss by default uses 8443, but
the default port for HTTPS is port 443. The port is specified by the Listen directive as well
as in the VirtualHost name or address.
Everything in NSS is associated with a "token". The software token exists in the NSS database but you can
also have a physical token containing certificates. With OpenSSL, discrete certificates and private keys are
held in PEM files. With NSS, these are stored in a database. Each certificate and key is associated with a
token and each token can have a password protecting it. This password is optional, but if a password is
used then the Apache HTTP server needs a copy of it in order to open the database without user
intervention at system start.
Configuring mod_nss
1. Install mod_nss as root:
~]# yum install mod_nss
This will create the mod_nss configuration file at /etc/httpd/conf.d/nss.conf. The
/etc/httpd/conf.d/ directory is included in the main Apache HTTP Server configuration file by
default. For the module to be loaded, restart the httpd service as described in Section 14.1.3.3,
“Restarting the Service”.
2. As root, open the /etc/httpd/conf.d/nss.conf file and search for all instances of the Listen
directive.
Edit the Listen 8443 line as follows:
Listen 443
Port 443 is the default port for HTTPS.
3. Edit the default VirtualHost default:8443 line as follows:
VirtualHost default:443
Edit any other non-default virtual host sections if they exist. Save and close the file.
4. Mozilla NSS stores certificates in a server certificate database indicated by the
NSSCertificateDatabase directive in the /etc/httpd/conf.d/nss.conf file. By default the path is
set to /etc/httpd/alias, the NSS database created during installation.
To view the default NSS database, issue a command as follows:
~]# certutil -L -d /etc/httpd/alias
Certificate Nickname Trust Attributes
SSL,S/MIME,JAR/XPI
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cacert CTu,Cu,Cu
Server-Cert u,u,u
alpha u,pu,u
In the above command output, Server-Cert is the default NSSNickname. The -L option lists all
the certificates, or displays information about a named certificate, in a certificate database. The -
d option specifies the database directory containing the certificate and key database files. See
the certutil(1) man page for more command line options.
5. To configure mod_nss to use another database, edit the NSSCertificateDatabase line in the
/etc/httpd/conf.d/nss.conf file. The default file has the following lines within the VirtualHost
section.
# Server Certificate Database:
# The NSS security database directory that holds the certificates and
# keys. The database consists of 3 files: cert8.db, key3.db and secmod.db.
# Provide the directory that these files exist.
NSSCertificateDatabase /etc/httpd/alias
In the above command output, alias is the default NSS database directory, /etc/httpd/alias/.
6. To apply a password to the default NSS certificate database, use the following command as root:
~]# certutil -W -d /etc/httpd/alias
Enter Password or Pin for "NSS Certificate DB":
Enter a password which will be used to encrypt your keys.
The password should be at least 8 characters long,
and should contain at least one non-alphabetic character.
Enter new password:
Re-enter password:
Password changed successfully.
7. Before deploying the HTTPS server, create a new certificate database using a certificate signed
by a certificate authority (CA).
Example 14.3. Adding a Certificate to the Mozilla NSS database
The certutil command is used to add a CA certificate to the NSS database files:
certutil -d /etc/httpd/nss-db-directory/ -A -n "CA_certificate" -t CT,, -a -i certificate.pem
The above command adds a CA certificate stored in a PEM-formatted file named
certificate.pem. The -d option specifies the NSS database directory containing the certificate
and key database files, the -n option sets a name for the certificate, -t CT,, means that the
certificate is trusted to be used in TLS clients and servers. The -A option adds an existing
certificate to a certificate database. If the database does not exist it will be created. The -a
option allows the use of ASCII format for input or output, and the -i option passes the
certificate.pem input file to the command.
See the certutil(1) man page for more command line options.
8. The NSS database should be password protected to safeguard the private key.
Example 14.4. Setting a Password for a Mozilla NSS database
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The certutil tool can be used set a password for an NSS database as follows:
certutil -W -d /etc/httpd/nss-db-directory/
For example, for the default database, issue a command as root as follows:
~]# certutil -W -d /etc/httpd/alias
Enter Password or Pin for "NSS Certificate DB":
Enter a password which will be used to encrypt your keys.
The password should be at least 8 characters long,
and should contain at least one non-alphabetic character.
Enter new password:
Re-enter password:
Password changed successfully.
9. Configure mod_nss to use the NSS internal software token by changing the line with the
NSSPassPhraseDialog directive as follows:
~]# vi /etc/httpd/conf.d/nss.conf
NSSPassPhraseDialog file:/etc/httpd/password.conf
This is to avoid manual password entry on system start. The software token exists in the NSS
database but you can also have a physical token containing your certificates.
10. If the SSL Server Certificate contained in the NSS database is an RSA certificate, make certain
that the NSSNickname parameter is uncommented and matches the nickname displayed in step
4 above:
NSSNickname Server-Cert
If the SSL Server Certificate contained in the NSS database is an ECC certificate, make certain
that the NSSECCNickname parameter is uncommented and matches the nickname displayed in
step 4 above:
NSSECCNickname Server-Cert
Make certain that the NSSCertificateDatabase parameter is uncommented and points to the
NSS database directory displayed in step 4 or configured in step 5 above:
NSSCertificateDatabase /etc/httpd/alias
Replace /etc/httpd/alias with the path to the certificate database to be used.
11. Create the /etc/httpd/password.conf file as root:
~]# vi /etc/httpd/password.conf
Add a line with the following form:
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internal:password
Replacing password with the password that was applied to the NSS security databases in step 6
above.
12. Apply the appropriate ownership and permissions to the /etc/httpd/password.conf file:
~]# chgrp apache /etc/httpd/password.conf
~]# chmod 640 /etc/httpd/password.conf
~]# ls -l /etc/httpd/password.conf
-rw-r-----. 1 root apache 10 Dec 4 17:13 /etc/httpd/password.conf
13. To configure mod_nss to use the NSS the software token in /etc/httpd/password.conf, edit
/etc/httpd/conf.d/nss.conf as follows:
14. Restart the Apache server for the changes to take effect as described in Section 14.1.3.3,
“Restarting the Service”
IMPORTANT
Hat recommends disabling SSL and using only TLSv1.1 or TLSv1.2. Backwards
compatibility can be achieved using TLSv1.0. Many products Red Hat supports have the
ability to use SSLv2 or SSLv3 protocols, or enable them by default. However, the use of
SSLv2 or SSLv3 is now strongly recommended against.
14.1.9.1. Enabling and Disabling SSL and TLS in mod_nss
To disable and enable specific versions of the SSL and TLS protocol, either do it globally by adding the
NSSProtocol directive in the " # SSL Global Context" section of the configuration file and removing it
everywhere else, or edit the default entry under " SSL Protocol" in all "VirtualHost" sections. If you do not
specify it in the per-domain VirtualHost section then it will inherit the settings from the global section. To
make sure that a protocol version is being disabled the administrator should either only specify
NSSProtocol in the "SSL Global Context" section, or specify it in all per-domain VirtualHost sections.
Disable All SSL and TLS Protocols Except TLS 1 and Up in mod_nss
To disable all SSL and TLS protocol versions except TLS version 1 and higher, proceed as follows:
1. As root, open the /etc/httpd/conf.d/nss.conf file and search for all instances of the
NSSProtocol directive. By default, the configuration file contains one section that looks as
follows:
# SSL Protocol:
output omitted
# Since all protocol ranges are completely inclusive, and no protocol in the
# middle of a range may be excluded, the entry "NSSProtocol SSLv3,TLSv1.1"
# is identical to the entry "NSSProtocol SSLv3,TLSv1.0,TLSv1.1".
NSSProtocol SSLv3,TLSv1.0,TLSv1.1
This section is within the VirtualHost section.
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2. Edit the NSSProtocol line as follows:
# SSL Protocol:
NSSProtocol TLSv1.0,TLSv1.1
Repeat this action for all VirtualHost sections.
3. Edit the Listen 8443 line as follows:
Listen 443
4. Edit the default VirtualHost default:8443 line as follows:
VirtualHost default:443
Edit any other non-default virtual host sections if they exist. Save and close the file.
5. Verify that all occurrences of the NSSProtocol directive have been changed as follows:
~]# grep NSSProtocol /etc/httpd/conf.d/nss.conf
# middle of a range may be excluded, the entry "NSSProtocol SSLv3,TLSv1.1"
# is identical to the entry "NSSProtocol SSLv3,TLSv1.0,TLSv1.1".
NSSProtocol TLSv1.0,TLSv1.1
This step is particularly important if you have more than one VirtualHost section.
~]# service httpd restart
Testing the Status of SSL and TLS Protocols in mod_nss
To check which versions of SSL and TLS are enabled or disabled in mod_nss, make use of the openssl
s_client -connect command. Install the openssl package as root:
~]# yum install openssl
The openssl s_client -connect command has the following form:
openssl s_client -connect hostname:port -protocol
Where port is the port to test and protocol is the protocol version to test for. To test the SSL server
running locally, use localhost as the host name. For example, to test the default port for secure HTTPS
connections, port 443 to see if SSLv3 is enabled, issue a command as follows:
~]# openssl s_client -connect localhost:443 -ssl3
CONNECTED(00000003)
3077773036:error:1408F10B:SSL routines:SSL3_GET_RECORD:wrong version
number:s3_pkt.c:337:
output omitted
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
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Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : SSLv3
output truncated
+ The above output indicates that the handshake failed and therefore no cipher was negotiated.
~]$ openssl s_client -connect localhost:443 -tls1
CONNECTED(00000003)
depth=1 C = US, O = example.com, CN = Certificate Shack
output omitted
New, TLSv1/SSLv3, Cipher is AES128-SHA
Server public key is 1024 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : TLSv1
output truncated
+ The above output indicates that no failure of the handshake occurred and a set of ciphers was
negotiated.
The openssl s_client command options are documented in the s_client(1) manual page.
For more information on the SSLv3 vulnerability and how to test for it, see the Red Hat Knowledgebase
article POODLE: SSLv3 vulnerability (CVE-2014-3566).
14.1.10. Using an Existing Key and Certificate
If you have a previously created key and certificate, you can configure the SSL server to use these files
instead of generating new ones. There are only two situations where this is not possible:
1. You are changing the IP address or domain name.
Certificates are issued for a particular IP address and domain name pair. If one of these values
changes, the certificate becomes invalid.
2. You have a certificate from VeriSign, and you are changing the server software.
VeriSign, a widely used certificate authority, issues certificates for a particular software product,
IP address, and domain name. Changing the software product renders the certificate invalid.
In either of the above cases, you will need to obtain a new certificate. For more information on this topic,
see Section 14.1.11, “Generating a New Key and Certificate” .
If you want to use an existing key and certificate, move the relevant files to the /etc/pki/tls/private/ and
/etc/pki/tls/certs/ directories respectively. You can do so by issuing the following commands as root:
~]# mv key_file.key /etc/pki/tls/private/hostname.key
~]# mv certificate.crt /etc/pki/tls/certs/hostname.crt
Then add the following lines to the /etc/httpd/conf.d/ssl.conf configuration file:
SSLCertificateFile /etc/pki/tls/certs/hostname.crt
SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
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To load the updated configuration, restart the httpd service as described in Section 14.1.3.3, “Restarting
the Service”.
Example 14.5. Using a key and certificate from the Red Hat Secure Web Server
~]# mv /etc/httpd/conf/httpsd.key /etc/pki/tls/private/penguin.example.com.key
~]# mv /etc/httpd/conf/httpsd.crt /etc/pki/tls/certs/penguin.example.com.crt
14.1.11. Generating a New Key and Certificate
In order to generate a new key and certificate pair, the crypto-utils package must be installed on the
system. To install it, enter the following command as root:
~]# yum install crypto-utils
This package provides a set of tools to generate and manage SSL certificates and private keys, and
includes genkey, the Red Hat Keypair Generation utility that will guide you through the key generation
process.
IMPORTANT
If the server already has a valid certificate and you are replacing it with a new one, specify a
different serial number. This ensures that client browsers are notified of this change,
update to this new certificate as expected, and do not fail to access the page. To create a
new certificate with a custom serial number, as root, use the following command instead of
genkey:
~]# openssl req -x509 -new -set_serial number -key hostname.key -out hostname.crt
NOTE
If there already is a key file for a particular host name in your system, genkey will refuse to
start. In this case, remove the existing file using the following command as root:
~]# rm /etc/pki/tls/private/hostname.key
To run the utility enter the genkey command as root, followed by the appropriate host name (for
example, penguin.example.com):
~]# genkey hostname
To complete the key and certificate creation, take the following steps:
1. Review the target locations in which the key and certificate will be stored.
Figure 14.1. Running the genkey utility
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Figure 14.1. Running the genkey utility
Use the Tab key to select the Next button, and press Enter to proceed to the next screen.
2. Using the up and down arrow keys, select a suitable key size. Note that while a larger key
increases the security, it also increases the response time of your server. The NIST recommends
using 2048 bits. See NIST Special Publication 800-131A.
Figure 14.2. Selecting the key size
Once finished, use the Tab key to select the Next button, and press Enter to initiate the random
bits generation process. Depending on the selected key size, this may take some time.
3. Decide whether you want to send a certificate request to a certificate authority.
Figure 14.3. Generating a certificate request
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Figure 14.3. Generating a certificate request
Use the Tab key to select Yes to compose a certificate request, or No to generate a self-signed
certificate. Then press Enter to confirm your choice.
4. Using the Spacebar key, enable ([*]) or disable ([ ]) the encryption of the private key.
Figure 14.4. Encrypting the private key
5. If you have enabled the private key encryption, enter an adequate passphrase. Note that for
security reasons, it is not displayed as you type, and it must be at least five characters long.
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IMPORTANT
Entering the correct passphrase is required in order for the server to start. If you
lose it, you will need to generate a new key and certificate.
6. Customize the certificate details.
Figure 14.6. Specifying certificate information
Use the Tab key to select the Next button, and press Enter to finish the key generation.
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7. If you have previously enabled the certificate request generation, you will be prompted to send it
to a certificate authority.
Figure 14.7. Instructions on how to send a certificate request
Press Enter to return to a shell prompt.
Once generated, add the key and certificate locations to the /etc/httpd/conf.d/ssl.conf configuration
file:
SSLCertificateFile /etc/pki/tls/certs/hostname.crt
SSLCertificateKeyFile /etc/pki/tls/private/hostname.key
Finally, restart the httpd service as described in Section 14.1.3.3, “Restarting the Service”, so that the
updated configuration is loaded.
14.1.12. Configure the Firewall for HTTP and HTTPS Using the Command Line
Red Hat Enterprise Linux does not allow HTTP and HTTPS traffic by default. To enable the system to act
as a web server, make use of firewalld's supported services to enable HTTP and HTTPS traffic to pass
through the firewall as required.
To enable HTTP using the command line, issue the following command as root:
~]# firewall-cmd --add-service http
success
To enable HTTPS using the command line, issue the following command as root:
~]# firewall-cmd --add-service https
success
Note that these changes will not persist after the next system start. To make permanent changes to the
firewall, repeat the commands adding the --permanent option.
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14.1.12.1. Checking Network Access for Incoming HTTPS and HTTPS Using the Command Line
To check what services the firewall is configured to allow, using the command line, issue the following
command as root:
interfaces: em1
sources:
output truncated
In this example taken from a default installation, the firewall is enabled but HTTP and HTTPS have not
been allowed to pass through.
Once the HTTP and HTTP firewall services are enabled, the services line will appear similar to the
following:
services: dhcpv6-client http https ssh
For more information on enabling firewall services, or opening and closing ports with firewalld, see the
Red Hat Enterprise Linux 7 Security Guide .
To learn more about the Apache HTTP Server, see the following resources.
httpd(8) — The manual page for the httpd service containing the complete list of its command-
line options.
genkey(1) — The manual page for genkey utility, provided by the crypto-utils package.
apachectl(8) — The manual page for the Apache HTTP Server Control Interface.
http://localhost/manual/ — The official documentation for the Apache HTTP Server with the full
description of its directives and available modules. Note that in order to access this
documentation, you must have the httpd-manual package installed, and the web server must be
running.
Before accessing the documentation, issue the following commands as root:
~] yum install httpd-manual ~] apachectl graceful
http://httpd.apache.org/ — The official website for the Apache HTTP Server with documentation
on all the directives and default modules.
http://www.openssl.org/ — The OpenSSL home page containing further documentation,
frequently asked questions, links to the mailing lists, and other useful resources.
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Red Hat Enterprise Linux offers many advanced applications to serve and access email. This chapter
describes modern email protocols in use today, and some of the programs designed to send and receive
email.
15.1. EMAIL PROTOCOLS
Today, email is delivered using a client/server architecture. An email message is created using a mail client
program. This program then sends the message to a server. The server then forwards the message to the
recipient’s email server, where the message is then supplied to the recipient’s email client.
To enable this process, a variety of standard network protocols allow different machines, often running
different operating systems and using different email programs, to send and receive email.
The following protocols discussed are the most commonly used in the transfer of email.
15.1.1. Mail Transport Protocols
Mail delivery from a client application to the server, and from an originating server to the destination
server, is handled by the Simple Mail Transfer Protocol (SMTP).
15.1.1.1. SMTP
The primary purpose of SMTP is to transfer email between mail servers. However, it is critical for email
clients as well. To send email, the client sends the message to an outgoing mail server, which in turn
contacts the destination mail server for delivery. But more intermediate SMTP servers may be included in
this chain. This concept is called a mail relaying. For this reason, it is necessary to specify an SMTP server
when configuring an email client.
Under Red Hat Enterprise Linux, a user can configure an SMTP server on the local machine to handle mail
delivery. However, it is also possible to configure remote SMTP servers for outgoing mail.
One important point to make about the SMTP protocol is that it does not require authentication. This
allows anyone on the Internet to send email to anyone else or even to large groups of people. It is this
characteristic of SMTP that makes junk email or spam possible. Imposing relay restrictions limits random
users on the Internet from sending email through your SMTP server, to other servers on the internet.
Servers that do not impose such restrictions are called open relay servers.
Red Hat Enterprise Linux 7 provides the Postfix and Sendmail SMTP programs.
15.1.2. Mail Access Protocols
There are two primary protocols used by email client applications to retrieve email from mail servers: the
Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP).
15.1.2.1. POP
The default POP server under Red Hat Enterprise Linux is Dovecot and is provided by the dovecot
package.
NOTE
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NOTE
To install Dovecot run the following command:
~]# yum install dovecot
For more information on installing packages with Yum, see Section 9.2.4, “Installing
Packages”.
When using a POP server, email messages are downloaded by email client applications. By default, most
POP email clients are automatically configured to delete the message on the email server after it has
been successfully transferred, however this setting usually can be changed.
POP is fully compatible with important Internet messaging standards, such as Multipurpose Internet Mail
Extensions (MIME), which allow for email attachments.
POP works best for users who have one system on which to read email. It also works well for users who do
not have a persistent connection to the Internet or the network containing the mail server. Unfortunately
for those with slow network connections, POP requires client programs upon authentication to download
the entire content of each message. This can take a long time if any messages have large attachments.
The most current version of the standard POP protocol is POP3.
There are, however, a variety of lesser-used POP protocol variants:
APOP — POP3 with MD5 authentication. An encoded hash of the user’s password is sent from
the email client to the server rather than sending an unencrypted password.
KPOP — POP3 with Kerberos authentication.
RPOP — POP3 with RPOP authentication. This uses a per-user ID, similar to a password, to
authenticate POP requests. However, this ID is not encrypted, so RPOP is no more secure than
standard POP.
To improve security, you can use Secure Socket Layer (SSL) encryption for client authentication and data
transfer sessions. To enable SSL encryption, use:
The pop3s service
The stunnel application
The starttls command
For more information on securing email communication, see Section 15.5.1, “Securing Communication”.
15.1.2.2. IMAP
The default IMAP server under Red Hat Enterprise Linux is Dovecot and is provided by the dovecot
package. See Section 15.1.2.1, “POP” for information on how to install Dovecot.
When using an IMAP mail server, email messages remain on the server where users can read or delete
them. IMAP also allows client applications to create, rename, or delete mail directories on the server to
organize and store email.
IMAP is particularly useful for users who access their email using multiple machines. The protocol is also
convenient for users connecting to the mail server via a slow connection, because only the email header
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information is downloaded for messages until opened, saving bandwidth. The user also has the ability to
delete messages without viewing or downloading them.
For convenience, IMAP client applications are capable of caching copies of messages locally, so the user
can browse previously read messages when not directly connected to the IMAP server.
IMAP, like POP, is fully compatible with important Internet messaging standards, such as MIME, which
allow for email attachments.
For added security, it is possible to use SSL encryption for client authentication and data transfer
sessions. This can be enabled by using the imaps service, or by using the stunnel program.
The pop3s service
The stunnel application
The starttls command
For more information on securing email communication, see Section 15.5.1, “Securing Communication”.
Other free, as well as commercial, IMAP clients and servers are available, many of which extend the IMAP
protocol and provide additional functionality.
15.1.2.3. Dovecot
The imap-login and pop3-login processes which implement the IMAP and POP3 protocols are spawned
by the master dovecot daemon included in the dovecot package. The use of IMAP and POP is
configured through the /etc/dovecot/dovecot.conf configuration file; by default dovecot runs IMAP and
POP3 together with their secure versions using SSL. To configure dovecot to use POP, complete the
following steps:
1. Edit the /etc/dovecot/dovecot.conf configuration file to make sure the protocols variable is
uncommented (remove the hash sign (#) at the beginning of the line) and contains the pop3
argument. For example:
protocols = imap pop3 lmtp
When the protocols variable is left commented out, dovecot will use the default values as
described above.
2. Make the change operational for the current session by running the following command as root:
~]# systemctl restart dovecot
3. Make the change operational after the next reboot by running the command:
~]# systemctl enable dovecot
Created symlink from /etc/systemd/system/multi-user.target.wants/dovecot.service to
/usr/lib/systemd/system/dovecot.service.
NOTE
Please note that dovecot only reports that it started the IMAP server, but also
starts the POP3 server.
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Unlike SMTP, both IMAP and POP3 require connecting clients to authenticate using a user name and
password. By default, passwords for both protocols are passed over the network unencrypted.
To configure SSL on dovecot:
Edit the /etc/dovecot/conf.d/10-ssl.conf configuration to make sure the ssl_protocols variable
is uncommented and contains the !SSLv2 !SSLv3 arguments:
ssl_protocols = !SSLv2 !SSLv3
These values ensure that dovecot avoids SSL versions 2 and also 3, which are both known to be
insecure. This is due to the vulnerability described in POODLE: SSLv3 vulnerability (CVE-2014-
3566). See Resolution for POODLE SSL 3.0 vulnerability (CVE-2014-3566) in Postfix and
Dovecot for details.
Make sure that /etc/dovecot/conf.d/10-ssl.conf contains the following option:
ssl=required
Edit the /etc/pki/dovecot/dovecot-openssl.cnf configuration file as you prefer. However, in a
typical installation, this file does not require modification.
Rename, move or delete the files /etc/pki/dovecot/certs/dovecot.pem and
/etc/pki/dovecot/private/dovecot.pem.
Execute the /usr/libexec/dovecot/mkcert.sh script which creates the dovecot self signed
certificates. These certificates are copied in the /etc/pki/dovecot/certs and
/etc/pki/dovecot/private directories. To implement the changes, restart dovecot by issuing the
following command as root:
More details on dovecot can be found online at http://www.dovecot.org.
15.2. EMAIL PROGRAM CLASSIFICATIONS
In general, all email applications fall into at least one of three classifications. Each classification plays a
specific role in the process of moving and managing email messages. While most users are only aware of
the specific email program they use to receive and send messages, each one is important for ensuring that
email arrives at the correct destination.
15.2.1. Mail Transport Agent
A Mail Transport Agent (MTA) transports email messages between hosts using SMTP. A message may
involve several MTAs as it moves to its intended destination.
While the delivery of messages between machines may seem rather straightforward, the entire process of
deciding if a particular MTA can or should accept a message for delivery is quite complicated. In addition,
due to problems from spam, use of a particular MTA is usually restricted by the MTA’s configuration or the
access configuration for the network on which the MTA resides.
Some email client programs, can act as an MTA when sending an email. However, such email client
programs do not have the role of a true MTA, because they can only send outbound messages to an MTA
they are authorized to use, but they cannot directly deliver the message to the intended recipient’s email
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server. This functionality is useful if host running the application does not have its own MTA.
Since Red Hat Enterprise Linux offers two MTAs, Postfix and Sendmail, email client programs are often
not required to act as an MTA. Red Hat Enterprise Linux also includes a special purpose MTA called
Fetchmail.
For more information on Postfix, Sendmail, and Fetchmail, see Section 15.3, “Mail Transport Agents”.
15.2.2. Mail Delivery Agent
A Mail Delivery Agent (MDA) is invoked by the MTA to file incoming email in the proper user’s mailbox. In
many cases, the MDA is actually a Local Delivery Agent (LDA), such as mail or Procmail.
Any program that actually handles a message for delivery to the point where it can be read by an email
client application can be considered an MDA. For this reason, some MTAs (such as Sendmail and Postfix)
can fill the role of an MDA when they append new email messages to a local user’s mail spool file. In
general, MDAs do not transport messages between systems nor do they provide a user interface; MDAs
distribute and sort messages on the local machine for an email client application to access.
15.2.3. Mail User Agent
A Mail User Agent (MUA) is synonymous with an email client application. MUA is a program that, at a
minimum, allows a user to read and compose email messages. MUAs can handle these tasks:
Retrieving messages via the POP or IMAP protocols
Setting up mailboxes to store messages.
Sending outbound messages to an MTA.
MUAs may be graphical, such as Thunderbird, Evolution, or have simple text-based interfaces, such as
mail or Mutt.
15.3. MAIL TRANSPORT AGENTS
Red Hat Enterprise Linux 7 offers two primary MTAs: Postfix and Sendmail. Postfix is configured as the
default MTA and Sendmail is considered deprecated. If required to switch the default MTA to Sendmail,
you can either uninstall Postfix or use the following command as root to switch to Sendmail:
~]# alternatives --config mta
You can also use the following command to enable the desired service:
~]# systemctl enable service
Similarly, to disable the service, type the following at a shell prompt:
~]# systemctl disable service
For more information on how to manage system services in Red Hat Enterprise Linux 7, see Chapter 10,
15.3.1. Postfix
Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a Sendmail-
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Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a Sendmail-
compatible MTA that is designed to be secure, fast, and easy to configure.
To improve security, Postfix uses a modular design, where small processes with limited privileges are
launched by a master daemon. The smaller, less privileged processes perform very specific tasks related to
the various stages of mail delivery and run in a changed root environment to limit the effects of attacks.
Configuring Postfix to accept network connections from hosts other than the local computer takes only a
few minor changes in its configuration file. Yet for those with more complex needs, Postfix provides a
variety of configuration options, as well as third party add-ons that make it a very versatile and full-
featured MTA.
The configuration files for Postfix are human readable and support upward of 250 directives. Unlike
Sendmail, no macro processing is required for changes to take effect and the majority of the most
commonly used options are described in the heavily commented files.
15.3.1.1. The Default Postfix Installation
The Postfix executable is postfix. This daemon launches all related processes needed to handle mail
delivery.
Postfix stores its configuration files in the /etc/postfix/ directory. The following is a list of the more
commonly used files:
access — Used for access control, this file specifies which hosts are allowed to connect to
Postfix.
main.cf — The global Postfix configuration file. The majority of configuration options are
specified in this file.
master.cf — Specifies how Postfix interacts with various processes to accomplish mail delivery.
transport — Maps email addresses to relay hosts.
The aliases file can be found in the /etc directory. This file is shared between Postfix and Sendmail. It is a
configurable list required by the mail protocol that describes user ID aliases.
IMPORTANT
The default /etc/postfix/main.cf file does not allow Postfix to accept network connections
from a host other than the local computer. For instructions on configuring Postfix as a
server for other clients, see Section 15.3.1.3, “Basic Postfix Configuration”.
Restart the postfix service after changing any options in the configuration files under the /etc/postfix/
directory in order for those changes to take effect. To do so, run the following command as root:
~]# systemctl restart postfix
15.3.1.2. Upgrading From a Previous Release
The following settings in Red Hat Enterprise Linux 7 are different to previous releases:
disable_vrfy_command = no — This is disabled by default, which is different to the default for
Sendmail. If changed to yes it can prevent certain email address harvesting methods.
allow_percent_hack = yes — This is enabled by default. It allows removing % characters in email
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allow_percent_hack = yes — This is enabled by default. It allows removing % characters in email
addresses. The percent hack is an old workaround that allowed sender-controlled routing of email
messages. DNS and mail routing are now much more reliable, but Postfix continues to support the
hack. To turn off percent rewriting, set allow_percent_hack to no.
smtpd_helo_required = no — This is disabled by default, as it is in Sendmail, because it can
prevent some applications from sending mail. It can be changed to yes to require clients to send
the HELO or EHLO commands before attempting to send the MAIL, FROM, or ETRN commands.
15.3.1.3. Basic Postfix Configuration
By default, Postfix does not accept network connections from any host other than the local host. Perform
the following steps as root to enable mail delivery for other hosts on the network:
Edit the /etc/postfix/main.cf file with a text editor, such as vi.
Uncomment the mydomain line by removing the hash sign ( #), and replace domain.tld with the
domain the mail server is servicing, such as example.com.
Uncomment the myorigin = $mydomain line.
Uncomment the myhostname line, and replace host.domain.tld with the host name for the
machine.
Uncomment the mydestination = $myhostname, localhost.$mydomain line.
Uncomment the mynetworks line, and replace 168.100.189.0/28 with a valid network setting for
hosts that can connect to the server.
Uncomment the inet_interfaces = all line.
Comment the inet_interfaces = localhost line.
Restart the postfix service.
Once these steps are complete, the host accepts outside emails for delivery.
Postfix has a large assortment of configuration options. One of the best ways to learn how to configure
Postfix is to read the comments within the /etc/postfix/main.cf configuration file. Additional resources
including information about Postfix configuration, SpamAssassin integration, or detailed descriptions of
the /etc/postfix/main.cf parameters are available online at http://www.postfix.org/.
IMPORTANT
Hat recommends disabling SSL and using only TLSv1.1 or TLSv1.2. See Resolution for
POODLE SSL 3.0 vulnerability (CVE-2014-3566) in Postfix and Dovecot for details.
15.3.1.4. Using Postfix with LDAP
Postfix can use an LDAP directory as a source for various lookup tables (for example, aliases, virtual,
canonical, and so on). This allows LDAP to store hierarchical user information and Postfix to only be
given the result of LDAP queries when needed. By not storing this information locally, administrators can
easily maintain it.
15.3.1.4.1. The /etc/aliases lookup example
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The following is a basic example for using LDAP to look up the /etc/aliases file. Make sure your
/etc/postfix/main.cf file contains the following:
alias_maps = hash:/etc/aliases, ldap:/etc/postfix/ldap-aliases.cf
Create a /etc/postfix/ldap-aliases.cf file if you do not have one already and make sure it contains the
following:
server_host = ldap.example.com
search_base = dc=example, dc=com
where ldap.example.com, example, and com are parameters that need to be replaced with specification
of an existing available LDAP server.
NOTE
The /etc/postfix/ldap-aliases.cf file can specify various parameters, including parameters
that enable LDAP SSL and STARTTLS. For more information, see the ldap_table(5) man
page.
For more information on LDAP, see OpenLDAP in the System-Level Authentication Guide.
15.3.2. Sendmail
Sendmail’s core purpose, like other MTAs, is to safely transfer email between hosts, usually using the
SMTP protocol. Note that Sendmail is considered deprecated and administrators are encouraged to use
Postfix when possible. See Section 15.3.1, “Postfix” for more information.
15.3.2.1. Purpose and Limitations
It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not. In these
days of monolithic applications that fulfill multiple roles, Sendmail may seem like the only application
needed to run an email server within an organization. Technically, this is true, as Sendmail can spool mail to
each users' directory and deliver outbound mail for users. However, most users actually require much
more than simple email delivery. Users usually want to interact with their email using an MUA, that uses
POP or IMAP, to download their messages to their local machine. Or, they may prefer a Web interface to
gain access to their mailbox. These other applications can work in conjunction with Sendmail, but they
actually exist for different reasons and can operate separately from one another.
It is beyond the scope of this section to go into all that Sendmail should or could be configured to do. With
literally hundreds of different options and rule sets, entire volumes have been dedicated to helping
explain everything that can be done and how to fix things that go wrong. See the Section 15.7, “Additional
Resources” for a list of Sendmail resources.
This section reviews the files installed with Sendmail by default and reviews basic configuration changes,
including how to stop unwanted email (spam) and how to extend Sendmail with the Lightweight Directory
Access Protocol (LDAP).
15.3.2.2. The Default Sendmail Installation
In order to use Sendmail, first ensure the sendmail package is installed on your system by running, as root:
~]# yum install sendmail
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In order to configure Sendmail, ensure the sendmail-cf package is installed on your system by running, as
root:
~]# yum install sendmail-cf
For more information on installing packages with Yum, see Section 9.2.4, “Installing Packages”.
Before using Sendmail, the default MTA has to be switched from Postfix. For more information how to
switch the default MTA refer to Section 15.3, “Mail Transport Agents”.
The Sendmail executable is sendmail.
Sendmail configuration file is located at /etc/mail/sendmail.cf. Avoid editing the sendmail.cf file directly.
To make configuration changes to Sendmail, edit the /etc/mail/sendmail.mc file, back up the original
/etc/mail/sendmail.cf file, and restart the sendmail service. As a part of the restart, the sendmail.cf file
and all binary representations of the databases are rebuild:
# systemctl restart sendmail
More information on configuring Sendmail can be found in Section 15.3.2.3, “Common Sendmail
Configuration Changes”.
Various Sendmail configuration files are installed in the /etc/mail/ directory including:
access — Specifies which systems can use Sendmail for outbound email.
domaintable — Specifies domain name mapping.
local-host-names — Specifies aliases for the host.
mailertable — Specifies instructions that override routing for particular domains.
virtusertable — Specifies a domain-specific form of aliasing, allowing multiple virtual domains to
be hosted on one machine.
Several configuration files in the /etc/mail/ directory, such as access, domaintable, mailertable and
virtusertable, store their information in database files before Sendmail can use any configuration
changes.
To include any changes made to these configurations in their database files, run the following command:
15.3.2.3. Common Sendmail Configuration Changes
When altering the Sendmail configuration file, it is best not to edit an existing file, but to generate an
entirely new /etc/mail/sendmail.cf file.
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WARNING
Before replacing or making any changes to the sendmail.cf file, create a backup
copy.
To add the desired functionality to Sendmail, edit the /etc/mail/sendmail.mc file as root. Once you are
finished, restart the sendmail service and, if the m4 package is installed, the m4 macro processor will
automatically generate a new sendmail.cf configuration file:
~]# systemctl restart sendmail
IMPORTANT
The default sendmail.cf file does not allow Sendmail to accept network connections from
any host other than the local computer. To configure Sendmail as a server for other clients,
edit the /etc/mail/sendmail.mc file, and either change the address specified in the Addr=
option of the DAEMON_OPTIONS directive from 127.0.0.1 to the IP address of an active
network device or comment out the DAEMON_OPTIONS directive all together by placing
dnl at the beginning of the line. When finished, regenerate /etc/mail/sendmail.cf by
restarting the service:
The default configuration in Red Hat Enterprise Linux works for most SMTP-only sites.
Consult the /usr/share/sendmail-cf/README file before editing any files in the directories under the
/usr/share/sendmail-cf/ directory, as they can affect the future configuration of the
/etc/mail/sendmail.cf file.
15.3.2.4. Masquerading
One common Sendmail configuration is to have a single machine act as a mail gateway for all machines on
the network. For example, a company may want to have a machine called mail.example.com that
handles all of their email and assigns a consistent return address to all outgoing mail.
In this situation, the Sendmail server must masquerade the machine names on the company network so
that their return address is user@example.com instead of user@host.example.com.
To do this, add the following lines to /etc/mail/sendmail.mc:
FEATURE(always_add_domain)dnl
FEATURE(masquerade_entire_domain)dnl
FEATURE(masquerade_envelope)dnl
FEATURE(allmasquerade)dnl
MASQUERADE_DOMAIN(`example.com.')dnl
MASQUERADE_AS(`example.com')dnl
After generating a new sendmail.cf file from the changed configuration in sendmail.mc, restart the
sendmail service by a following command:

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Note that administrators of mail servers, DNS and DHCP servers, as well as any provisioning applications,
should agree on the host name format used in an organization. See the Red Hat Enterprise Linux 7
Networking Guide for more information on recommended naming practices.
15.3.2.5. Stopping Spam
Email spam can be defined as unnecessary and unwanted email received by a user who never requested
the communication. It is a disruptive, costly, and widespread abuse of Internet communication standards.
Sendmail makes it relatively easy to block new spamming techniques being employed to send junk email. It
even blocks many of the more usual spamming methods by default. Main anti-spam features available in
sendmail are header checks, relaying denial (default from version 8.9), access database and sender
information checks.
For example, forwarding of SMTP messages, also called relaying, has been disabled by default since
Sendmail version 8.9. Before this change occurred, Sendmail directed the mail host (x.edu) to accept
messages from one party (y.com) and sent them to a different party ( z.net). Now, however, Sendmail
must be configured to permit any domain to relay mail through the server. To configure relay domains,
edit the /etc/mail/relay-domains file and restart Sendmail
However, servers on the Internet can also send spam messages. In these instances, Sendmail’s access
control features available through the /etc/mail/access file can be used to prevent connections from
unwanted hosts. The following example illustrates how this file can be used to both block and specifically
allow access to the Sendmail server:
badspammer.com ERROR:550 "Go away and do not spam us anymore" tux.badspammer.com
OK 10.0 RELAY
This example shows that any email sent from badspammer.com is blocked with a 550 RFC-821 compliant
error code, with a message sent back. Emails sent from the tux.badspammer.com sub-domain are
accepted. The last line shows that any email sent from the 10.0.. network can be relayed through the mail
server.
Because the /etc/mail/access.db file is a database, use the following command to update any changes:
The above examples only represent a small part of what Sendmail can do in terms of allowing or blocking
access. See the /usr/share/sendmail-cf/README file for more information and examples.
Since Sendmail calls the Procmail MDA when delivering mail, it is also possible to use a spam filtering
program, such as SpamAssassin, to identify and file spam for users. See Section 15.4.2.6, “Spam Filters” for
more information about using SpamAssassin.
15.3.2.6. Using Sendmail with LDAP
Using LDAP is a very quick and powerful way to find specific information about a particular user from a
much larger group. For example, an LDAP server can be used to look up a particular email address from a
common corporate directory by the user’s last name. In this kind of implementation, LDAP is largely
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separate from Sendmail, with LDAP storing the hierarchical user information and Sendmail only being
given the result of LDAP queries in pre-addressed email messages.
However, Sendmail supports a much greater integration with LDAP, where it uses LDAP to replace
separately maintained files, such as /etc/aliases and /etc/mail/virtusertables, on different mail servers
that work together to support a medium- to enterprise-level organization. In short, LDAP abstracts the
mail routing level from Sendmail and its separate configuration files to a powerful LDAP cluster that can
be leveraged by many different applications.
The current version of Sendmail contains support for LDAP. To extend the Sendmail server using LDAP,
first get an LDAP server, such as OpenLDAP, running and properly configured. Then edit the
/etc/mail/sendmail.mc to include the following:
LDAPROUTE_DOMAIN('yourdomain.com')dnl
FEATURE('ldap_routing')dnl
NOTE
This is only for a very basic configuration of Sendmail with LDAP. The configuration can
differ greatly from this depending on the implementation of LDAP, especially when
configuring several Sendmail machines to use a common LDAP server.
Consult /usr/share/sendmail-cf/README for detailed LDAP routing configuration
instructions and examples.
Next, recreate the /etc/mail/sendmail.cf file by running the m4 macro processor and again restarting
Sendmail. See Section 15.3.2.3, “Common Sendmail Configuration Changes” for instructions.
For more information on LDAP, see OpenLDAP in the System-Level Authentication Guide.
15.3.3. Fetchmail
Fetchmail is an MTA which retrieves email from remote servers and delivers it to the local MTA. Many
users appreciate the ability to separate the process of downloading their messages located on a remote
server from the process of reading and organizing their email in an MUA. Designed with the needs of dial-
up users in mind, Fetchmail connects and quickly downloads all of the email messages to the mail spool
file using any number of protocols, including POP3 and IMAP. It can even forward email messages to an
SMTP server, if necessary.
NOTE
In order to use Fetchmail, first ensure the fetchmail package is installed on your system
by running, as root:
~]# yum install fetchmail
Packages”.
Fetchmail is configured for each user through the use of a .fetchmailrc file in the user’s home directory. If
it does not already exist, create the .fetchmailrc file in your home directory
Using preferences in the .fetchmailrc file, Fetchmail checks for email on a remote server and downloads
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it. It then delivers it to port 25 on the local machine, using the local MTA to place the email in the correct
user’s spool file. If Procmail is available, it is launched to filter the email and place it in a mailbox so that it
can be read by an MUA.
15.3.3.1. Fetchmail Configuration Options
Although it is possible to pass all necessary options on the command line to check for email on a remote
server when executing Fetchmail, using a .fetchmailrc file is much easier. Place any desired configuration
options in the .fetchmailrc file for those options to be used each time the fetchmail command is issued.
It is possible to override these at the time Fetchmail is run by specifying that option on the command line.
A user’s .fetchmailrc file contains three classes of configuration options:
global options — Gives Fetchmail instructions that control the operation of the program or
provide settings for every connection that checks for email.
server options — Specifies necessary information about the server being polled, such as the host
name, as well as preferences for specific email servers, such as the port to check or number of
seconds to wait before timing out. These options affect every user using that server.
user options — Contains information, such as user name and password, necessary to authenticate
and check for email using a specified email server.
Global options appear at the top of the .fetchmailrc file, followed by one or more server options, each of
which designate a different email server that Fetchmail should check. User options follow server options
for each user account checking that email server. Like server options, multiple user options may be
specified for use with a particular server as well as to check multiple email accounts on the same server.
Server options are called into service in the .fetchmailrc file by the use of a special option verb, poll or
skip, that precedes any of the server information. The poll action tells Fetchmail to use this server option
when it is run, which checks for email using the specified user options. Any server options after a skip
action, however, are not checked unless this server’s host name is specified when Fetchmail is invoked.
The skip option is useful when testing configurations in the .fetchmailrc file because it only checks
skipped servers when specifically invoked, and does not affect any currently working configurations.
The following is an example of a .fetchmailrc file:
set postmaster "user1"
set bouncemail
poll pop.domain.com proto pop3
user 'user1' there with password 'secret' is user1 here
poll mail.domain2.com
user 'user5' there with password 'secret2' is user1 here
user 'user7' there with password 'secret3' is user1 here
In this example, the global options specify that the user is sent email as a last resort (postmaster option)
and all email errors are sent to the postmaster instead of the sender (bouncemail option). The set action
tells Fetchmail that this line contains a global option. Then, two email servers are specified, one set to
check using POP3, the other for trying various protocols to find one that works. Two users are checked
using the second server option, but all email found for any user is sent to user1's mail spool. This allows
multiple mailboxes to be checked on multiple servers, while appearing in a single MUA inbox. Each user’s
specific information begins with the user action.
NOTE
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NOTE
Users are not required to place their password in the .fetchmailrc file. Omitting the with
password 'password' section causes Fetchmail to ask for a password when it is launched.
Fetchmail has numerous global, server, and local options. Many of these options are rarely used or only
apply to very specific situations. The fetchmail man page explains each option in detail, but the most
common ones are listed in the following three sections.
15.3.3.2. Global Options
Each global option should be placed on a single line after a set action.
daemon seconds — Specifies daemon-mode, where Fetchmail stays in the background. Replace
seconds with the number of seconds Fetchmail is to wait before polling the server.
postmaster — Specifies a local user to send mail to in case of delivery problems.
syslog — Specifies the log file for errors and status messages. By default, this is /var/log/maillog.
15.3.3.3. Server Options
Server options must be placed on their own line in .fetchmailrc after a poll or skip action.
auth auth-type — Replace auth-type with the type of authentication to be used. By default,
password authentication is used, but some protocols support other types of authentication,
including kerberos_v5, kerberos_v4, and ssh. If the any authentication type is used, Fetchmail
first tries methods that do not require a password, then methods that mask the password, and
finally attempts to send the password unencrypted to authenticate to the server.
interval number — Polls the specified server every number of times that it checks for email on all
configured servers. This option is generally used for email servers where the user rarely receives
messages.
port port-number — Replace port-number with the port number. This value overrides the default
port number for the specified protocol.
proto protocol — Replace protocol with the protocol, such as pop3 or imap, to use when
checking for messages on the server.
timeout seconds — Replace seconds with the number of seconds of server inactivity after which
Fetchmail gives up on a connection attempt. If this value is not set, a default of 300 seconds is
used.
15.3.3.4. User Options
User options may be placed on their own lines beneath a server option or on the same line as the server
option. In either case, the defined options must follow the user option (defined below).
fetchall — Orders Fetchmail to download all messages in the queue, including messages that have
already been viewed. By default, Fetchmail only pulls down new messages.
fetchlimit number — Replace number with the number of messages to be retrieved before
stopping.
flush — Deletes all previously viewed messages in the queue before retrieving new messages.
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limit max-number-bytes — Replace max-number-bytes with the maximum size in bytes that
messages are allowed to be when retrieved by Fetchmail. This option is useful with slow network
links, when a large message takes too long to download.
password 'password' — Replace password with the user’s password.
preconnect "command" — Replace command with a command to be executed before retrieving
messages for the user.
postconnect "command" — Replace command with a command to be executed after retrieving
messages for the user.
ssl — Activates SSL encryption. At the time of writing, the default action is to use the best
available from SSL2, SSL3, SSL23, TLS1, TLS1.1 and TLS1.2. Note that SSL2 is considered
obsolete and due to the POODLE: SSLv3 vulnerability (CVE-2014-3566), SSLv3 should not be
used. However there is no way to force the use of TLS1 or newer, therefore ensure the mail server
being connected to is configured not to use SSLv2 and SSLv3. Use stunnel where the server
cannot be configured not to use SSLv2 and SSLv3.
sslproto — Defines allowed SSL or TLS protocols. Possible values are SSL2, SSL3, SSL23, and
TLS1. The default value, if sslproto is omitted, unset, or set to an invalid value, is SSL23. The
default action is to use the best from SSLv2, SSLv3, TLSv1, TLS1.1 and TLS1.2. Note that
setting any other value for SSL or TLS will disable all the other protocols. Due to the POODLE:
SSLv3 vulnerability (CVE-2014-3566), it is recommend to omit this option, or set it to SSLv23,
and configure the corresponding mail server not to use SSLv2 and SSLv3. Use stunnel where
the server cannot be configured not to use SSLv2 and SSLv3.
user "username" — Replace username with the user name used by Fetchmail to retrieve
messages. This option must precede all other user options.
15.3.3.5. Fetchmail Command Options
Most Fetchmail options used on the command line when executing the fetchmail command mirror the
.fetchmailrc configuration options. In this way, Fetchmail may be used with or without a configuration file.
These options are not used on the command line by most users because it is easier to leave them in the
.fetchmailrc file.
There may be times when it is desirable to run the fetchmail command with other options for a particular
purpose. It is possible to issue command options to temporarily override a .fetchmailrc setting that is
causing an error, as any options specified at the command line override configuration file options.
15.3.3.6. Informational or Debugging Options
Certain options used after the fetchmail command can supply important information.
--configdump — Displays every possible option based on information from .fetchmailrc and
Fetchmail defaults. No email is retrieved for any users when using this option.
-s — Executes Fetchmail in silent mode, preventing any messages, other than errors, from
appearing after the fetchmail command.
-v — Executes Fetchmail in verbose mode, displaying every communication between Fetchmail
and remote email servers.
-V — Displays detailed version information, lists its global options, and shows settings to be used
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-V — Displays detailed version information, lists its global options, and shows settings to be used
with each user, including the email protocol and authentication method. No email is retrieved for
any users when using this option.
15.3.3.7. Special Options
These options are occasionally useful for overriding defaults often found in the .fetchmailrc file.
-a — Fetchmail downloads all messages from the remote email server, whether new or previously
viewed. By default, Fetchmail only downloads new messages.
-k — Fetchmail leaves the messages on the remote email server after downloading them. This
option overrides the default behavior of deleting messages after downloading them.
-l max-number-bytes — Fetchmail does not download any messages over a particular size and
leaves them on the remote email server.
--quit — Quits the Fetchmail daemon process.
More commands and .fetchmailrc options can be found in the fetchmail man page.
15.3.4. Mail Transport Agent (MTA) Configuration
A Mail Transport Agent (MTA) is essential for sending email. A Mail User Agent (MUA) such as Evolution
or Mutt, is used to read and compose email. When a user sends an email from an MUA, the message is
handed off to the MTA, which sends the message through a series of MTAs until it reaches its destination.
Even if a user does not plan to send email from the system, some automated tasks or system programs
might use the mail command to send email containing log messages to the root user of the local system.
Red Hat Enterprise Linux 7 provides two MTAs: Postfix and Sendmail. If both are installed, Postfix is the
default MTA.
15.4. MAIL DELIVERY AGENTS
Red Hat Enterprise Linux includes Procmail as primary MDA. Both applications are considered LDAs and
both move email from the MTA’s spool file into the user’s mailbox. However, Procmail provides a robust
filtering system.
This section details only Procmail. For information on the mail command, consult its man page ( man
mail).
Procmail delivers and filters email as it is placed in the mail spool file of the localhost. It is powerful, gentle
on system resources, and widely used. Procmail can play a critical role in delivering email to be read by
email client applications.
Procmail can be invoked in several different ways. Whenever an MTA places an email into the mail spool
file, Procmail is launched. Procmail then filters and files the email for the MUA and quits. Alternatively, the
MUA can be configured to execute Procmail any time a message is received so that messages are moved
into their correct mailboxes. By default, the presence of /etc/procmailrc or of a ~/.procmailrc file (also
called an rc file) in the user’s home directory invokes Procmail whenever an MTA receives a new message.
By default, no system-wide rc files exist in the /etc directory and no .procmailrc files exist in any user’s
home directory. Therefore, to use Procmail, each user must construct a .procmailrc file with specific
environment variables and rules.
Whether Procmail acts upon an email message depends upon whether the message matches a specified
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Whether Procmail acts upon an email message depends upon whether the message matches a specified
set of conditions or recipes in the rc file. If a message matches a recipe, then the email is placed in a
specified file, is deleted, or is otherwise processed.
When Procmail starts, it reads the email message and separates the body from the header information.
Next, Procmail looks for a /etc/procmailrc file and rc files in the /etc/procmailrcs/ directory for default,
system-wide, Procmail environmental variables and recipes. Procmail then searches for a .procmailrc file
in the user’s home directory. Many users also create additional rc files for Procmail that are referred to
within the .procmailrc file in their home directory.
15.4.1. Procmail Configuration
The Procmail configuration file contains important environmental variables. These variables specify things
such as which messages to sort and what to do with the messages that do not match any recipes.
These environmental variables usually appear at the beginning of the ~/.procmailrc file in the following
format:
env-variable="value"
In this example, env-variable is the name of the variable and value defines the variable.
There are many environment variables not used by most Procmail users and many of the more important
environment variables are already defined by a default value. Most of the time, the following variables are
used:
DEFAULT — Sets the default mailbox where messages that do not match any recipes are placed.
The default DEFAULT value is the same as $ORGMAIL.
INCLUDERC — Specifies additional rc files containing more recipes for messages to be checked
against. This breaks up the Procmail recipe lists into individual files that fulfill different roles, such
as blocking spam and managing email lists, that can then be turned off or on by using comment
characters in the user’s ~/.procmailrc file.
For example, lines in a user’s ~/.procmailrc file may look like this:
MAILDIR=$HOME/Msgs
INCLUDERC=$MAILDIR/lists.rc
INCLUDERC=$MAILDIR/spam.rc
To turn off Procmail filtering of email lists but leaving spam control in place, comment out the
first INCLUDERC line with a hash sign ( #). Note that it uses paths relative to the current
directory.
LOCKSLEEP — Sets the amount of time, in seconds, between attempts by Procmail to use a
particular lockfile. The default is 8 seconds.
LOCKTIMEOUT — Sets the amount of time, in seconds, that must pass after a lockfile was last
modified before Procmail assumes that the lockfile is old and can be deleted. The default is 1024
seconds.
LOGFILE — The file to which any Procmail information or error messages are written.
MAILDIR — Sets the current working directory for Procmail. If set, all other Procmail paths are
relative to this directory.
ORGMAIL — Specifies the original mailbox, or another place to put the messages if they cannot
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ORGMAIL — Specifies the original mailbox, or another place to put the messages if they cannot
be placed in the default or recipe-required location.
By default, a value of /var/spool/mail/$LOGNAME is used.
SUSPEND — Sets the amount of time, in seconds, that Procmail pauses if a necessary resource,
such as swap space, is not available.
SWITCHRC — Allows a user to specify an external file containing additional Procmail recipes,
much like the INCLUDERC option, except that recipe checking is actually stopped on the
referring configuration file and only the recipes on the SWITCHRC-specified file are used.
VERBOSE — Causes Procmail to log more information. This option is useful for debugging.
Other important environmental variables are pulled from the shell, such as LOGNAME, the login name;
HOME, the location of the home directory; and SHELL, the default shell.
A comprehensive explanation of all environments variables, and their default values, is available in the
procmailrc man page.
15.4.2. Procmail Recipes
New users often find the construction of recipes the most difficult part of learning to use Procmail. This
difficulty is often attributed to recipes matching messages by using regular expressions which are used to
specify qualifications for string matching. However, regular expressions are not very difficult to construct
and even less difficult to understand when read. Additionally, the consistency of the way Procmail recipes
are written, regardless of regular expressions, makes it easy to learn by example. To see example Procmail
recipes, see Section 15.4.2.5, “Recipe Examples”.
Procmail recipes take the following form:
:0 flags : lockfile-name
* condition_1_special-condition-character condition_1_regular_expression
* condition_2_special-condition-character condition-2_regular_expression
* condition_N_special-condition-character condition-N_regular_expression
special-action-character
action-to-perform
The first two characters in a Procmail recipe are a colon and a zero. Various flags can be placed after the
zero to control how Procmail processes the recipe. A colon after the flags section specifies that a lockfile
is created for this message. If a lockfile is created, the name can be specified by replacing lockfile-name.
A recipe can contain several conditions to match against the message. If it has no conditions, every
message matches the recipe. Regular expressions are placed in some conditions to facilitate message
matching. If multiple conditions are used, they must all match for the action to be performed. Conditions
are checked based on the flags set in the recipe’s first line. Optional special characters placed after the
asterisk character (*) can further control the condition.
The action-to-perform argument specifies the action taken when the message matches one of the
conditions. There can only be one action per recipe. In many cases, the name of a mailbox is used here to
direct matching messages into that file, effectively sorting the email. Special action characters may also be
used before the action is specified. See Section 15.4.2.4, “Special Conditions and Actions” for more
information.
15.4.2.1. Delivering vs. Non-Delivering Recipes
The action used if the recipe matches a particular message determines whether it is considered a
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delivering or non-delivering recipe. A delivering recipe contains an action that writes the message to a file,
sends the message to another program, or forwards the message to another email address. A non-
delivering recipe covers any other actions, such as a nesting block. A nesting block is a set of actions,
contained in braces {}, that are performed on messages which match the recipe’s conditions. Nesting
blocks can be nested inside one another, providing greater control for identifying and performing actions
on messages.
When messages match a delivering recipe, Procmail performs the specified action and stops comparing
the message against any other recipes. Messages that match non-delivering recipes continue to be
compared against other recipes.
15.4.2.2. Flags
Flags are essential to determine how or if a recipe’s conditions are compared to a message. The egrep
utility is used internally for matching of the conditions. The following flags are commonly used:
A — Specifies that this recipe is only used if the previous recipe without an A or a flag also
matched this message.
a — Specifies that this recipe is only used if the previous recipe with an A or a flag also matched
this message and was successfully completed.
B — Parses the body of the message and looks for matching conditions.
b — Uses the body in any resulting action, such as writing the message to a file or forwarding it.
This is the default behavior.
c — Generates a carbon copy of the email. This is useful with delivering recipes, since the required
action can be performed on the message and a copy of the message can continue being
processed in the rc files.
D — Makes the egrep comparison case-sensitive. By default, the comparison process is not case-
sensitive.
E — While similar to the A flag, the conditions in the recipe are only compared to the message if
the immediately preceding recipe without an E flag did not match. This is comparable to an else
action.
e — The recipe is compared to the message only if the action specified in the immediately
preceding recipe fails.
f — Uses the pipe as a filter.
H — Parses the header of the message and looks for matching conditions. This is the default
behavior.
h — Uses the header in a resulting action. This is the default behavior.
w — Tells Procmail to wait for the specified filter or program to finish, and reports whether or not
it was successful before considering the message filtered.
W — Is identical to w except that "Program failure" messages are suppressed.
For a detailed list of additional flags, see the procmailrc man page.
15.4.2.3. Specifying a Local Lockfile
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Lockfiles are very useful with Procmail to ensure that more than one process does not try to alter a
message simultaneously. Specify a local lockfile by placing a colon (:) after any flags on a recipe’s first
line. This creates a local lockfile based on the destination file name plus whatever has been set in the
LOCKEXT global environment variable.
Alternatively, specify the name of the local lockfile to be used with this recipe after the colon.
15.4.2.4. Special Conditions and Actions
Special characters used before Procmail recipe conditions and actions change the way they are
interpreted.
The following characters may be used after the asterisk character (*) at the beginning of a recipe’s
condition line:
! — In the condition line, this character inverts the condition, causing a match to occur only if the
condition does not match the message.
< — Checks if the message is under a specified number of bytes.
> — Checks if the message is over a specified number of bytes.
The following characters are used to perform special actions:
! — In the action line, this character tells Procmail to forward the message to the specified email
addresses.
$ — Refers to a variable set earlier in the rc file. This is often used to set a common mailbox that is
referred to by various recipes.
| — Starts a specified program to process the message.
{ and } — Constructs a nesting block, used to contain additional recipes to apply to matching
messages.
If no special character is used at the beginning of the action line, Procmail assumes that the action line is
specifying the mailbox in which to write the message.
15.4.2.5. Recipe Examples
Procmail is an extremely flexible program, but as a result of this flexibility, composing Procmail recipes
from scratch can be difficult for new users.
The best way to develop the skills to build Procmail recipe conditions stems from a strong understanding
of regular expressions combined with looking at many examples built by others. A thorough explanation of
regular expressions is beyond the scope of this section. The structure of Procmail recipes and useful
sample Procmail recipes can be found at various places on the Internet. The proper use and adaptation of
regular expressions can be derived by viewing these recipe examples. In addition, introductory information
about basic regular expression rules can be found in the grep(1) man page.
The following simple examples demonstrate the basic structure of Procmail recipes and can provide the
foundation for more intricate constructions.
A basic recipe may not even contain conditions, as is illustrated in the following example:
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:0:
new-mail.spool
The first line specifies that a local lockfile is to be created but does not specify a name, so Procmail uses
the destination file name and appends the value specified in the LOCKEXT environment variable. No
condition is specified, so every message matches this recipe and is placed in the single spool file called
new-mail.spool, located within the directory specified by the MAILDIR environment variable. An MUA
can then view messages in this file.
A basic recipe, such as this, can be placed at the end of all rc files to direct messages to a default location.
The following example matched messages from a specific email address and throws them away.
:0
* ^From: spammer@domain.com
/dev/null
With this example, any messages sent by spammer@domain.com are sent to the /dev/null device,
deleting them.
WARNING
Be certain that rules are working as intended before sending messages to /dev/null
for permanent deletion. If a recipe inadvertently catches unintended messages, and
those messages disappear, it becomes difficult to troubleshoot the rule.
A better solution is to point the recipe’s action to a special mailbox, which can be
checked from time to time to look for false positives. Once satisfied that no
messages are accidentally being matched, delete the mailbox and direct the action to
send the messages to /dev/null.
The following recipe grabs email sent from a particular mailing list and places it in a specified folder.
:0:
* ^(From|Cc|To).*tux-lug
tuxlug
Any messages sent from the tux-lug@domain.com mailing list are placed in the tuxlug mailbox
automatically for the MUA. Note that the condition in this example matches the message if it has the
mailing list’s email address on the From, Cc, or To lines.
Consult the many Procmail online resources available in Section 15.7, “Additional Resources” for more
detailed and powerful recipes.
15.4.2.6. Spam Filters
Because it is called by Sendmail, Postfix, and Fetchmail upon receiving new emails, Procmail can be used
as a powerful tool for combating spam.
This is particularly true when Procmail is used in conjunction with SpamAssassin. When used together,
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This is particularly true when Procmail is used in conjunction with SpamAssassin. When used together,
these two applications can quickly identify spam emails, and sort or destroy them.
SpamAssassin uses header analysis, text analysis, blacklists, a spam-tracking database, and self-learning
Bayesian spam analysis to quickly and accurately identify and tag spam.
NOTE
In order to use SpamAssassin, first ensure the spamassassin package is installed on your
system by running, as root:
~]# yum install spamassassin
Packages”.
The easiest way for a local user to use SpamAssassin is to place the following line near the top of the
~/.procmailrc file:
INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc
The /etc/mail/spamassassin/spamassassin-default.rc contains a simple Procmail rule that activates
SpamAssassin for all incoming email. If an email is determined to be spam, it is tagged in the header as
such and the title is prepended with the following pattern:
SPAM
The message body of the email is also prepended with a running tally of what elements caused it to be
diagnosed as spam.
To file email tagged as spam, a rule similar to the following can be used:
:0 Hw
* ^X-Spam-Status: Yes
spam
This rule files all email tagged in the header as spam into a mailbox called spam.
Since SpamAssassin is a Perl script, it may be necessary on busy servers to use the binary
SpamAssassin daemon (spamd) and the client application (
spamc). Configuring SpamAssassin this
way, however, requires root access to the host.
To start the spamd daemon, type the following command:
~]# systemctl start spamassassin
To start the SpamAssassin daemon when the system is booted, run:
systemctl enable spamassassin.service
See Chapter 10, Managing Services with systemdfor more information about starting and stopping
services.
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To configure Procmail to use the SpamAssassin client application instead of the Perl script, place the
following line near the top of the ~/.procmailrc file. For a system-wide configuration, place it in
/etc/procmailrc:
INCLUDERC=/etc/mail/spamassassin/spamassassin-spamc.rc
15.5. MAIL USER AGENTS
Red Hat Enterprise Linux offers a variety of email programs, both graphical email client programs,
such as Evolution, and text-based email programs such asmutt.
The remainder of this section focuses on securing communication between a client and a server.
15.5.1. Securing Communication
MUAs included with Red Hat Enterprise Linux, such as Thunderbird, Evolution and Mutt offer SSL-
encrypted email sessions.
Like any other service that flows over a network unencrypted, important email information, such as
user names, passwords, and entire messages, may be intercepted and viewed by users on the
network. Additionally, since the standard POP and IMAP protocols pass authentication information
unencrypted, it is possible for an attacker to gain access to user accounts by collecting user names
and passwords as they are passed over the network.
15.5.1.1. Secure Email Clients
Most Linux MUAs designed to check email on remote servers support SSL encryption. To use SSL
when retrieving email, it must be enabled on both the email client and the server.
SSL is easy to enable on the client-side, often done with the click of a button in the MUA’s
configuration window or via an option in the MUA’s configuration file. Secure IMAP and POP have
known port numbers (993 and 995, respectively) that the MUA uses to authenticate and download
messages.
15.5.1.2. Securing Email Client Communications
Offering SSL encryption to IMAP and POP users on the email server is a simple matter.
First, create an SSL certificate. This can be done in two ways: by applying to a Certificate Authority
(CA) for an SSL certificate or by creating a self-signed certificate.
WARNING
Self-signed certificates should be used for testing purposes only. Any server used
in a production environment should use an SSL certificate signed by a CA.
To create a self-signed SSL certificate for IMAP or POP, change to the/etc/pki/dovecot/ directory,
edit the certificate parameters in the /etc/pki/dovecot/dovecot-openssl.cnf configuration file as you
prefer, and type the following commands, as root:
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dovecot]# rm -f certs/dovecot.pem private/dovecot.pem
dovecot]# /usr/libexec/dovecot/mkcert.sh
Once finished, make sure you have the following configurations in your /etc/dovecot/conf.d/10-
ssl.conf file:
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_key = </etc/pki/dovecot/private/dovecot.pem
Issue the following command to restart the dovecot daemon:
Alternatively, the stunnel command can be used as an encryption wrapper around the standard, non-
secure connections to IMAP or POP services.
The stunnel utility uses external OpenSSL libraries included with Red Hat Enterprise Linux to provide
strong cryptography and to protect the network connections. It is recommended to apply to a CA to
obtain an SSL certificate, but it is also possible to create a self-signed certificate.
See Using stunnel in the Red Hat Enterprise Linux 7 Security Guide for instructions on how to install
stunnel and create its basic configuration. To configurestunnel as a wrapper forIMAPS and POP3S,
add the following lines to the /etc/stunnel/stunnel.conf configuration file:
[pop3s]
accept = 995
connect = 110
[imaps]
accept = 993
connect = 143
The Security Guide also explains how to start and stop stunnel. Once you start it, it is possible to use
an IMAP or aPOP email client and connect to the email server using SSL encryption.
15.6. CONFIGURING MAIL SERVER WITH ANTISPAM AND ANTIVIRUS
Once your email delivery works, incoming emails may contain unsolicited messages also known as
spam. These messages can also contain harmful viruses and malware, posing security risk and
potential production loss on your systems.
To avoid these risks, you can filter the incoming messages and check them against viruses by using an
antispam and antivirus solution.
15.6.1. Configuring Spam Filtering for Mail Transport Agent or Mail Delivery Agent
You can filter spam in a Mail Transport Agent (MTA), Mail Delivery Agent (MDA), or Mail User Agent
(MUA). This chapter describes spam filtering in MTAs and MDAs.
15.6.1.1. Configuring Spam Filtering in a Mail Transport Agent
Red Hat Enterprise Linux 7 offers two primary MTAs: Postfix and Sendmail.
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For details on how to install and configure an MTA, see Section 15.3, “Mail Transport Agents”.
Stopping spam in a MTA side is possible with the use of Sendmail, which has several anti-spam
features: header checks, relaying denial, access database and sender information checks. Fore more
information, see Section 15.3.2.5, “Stopping Spam”.
Moreover, both Postfix and Sendmail can work with third-party mail filters (milters) to filter spam and
viruses in the mail-processing chain. In case of Postfix, the support for milters is included directly in
the postfix package. In case of Sendmail, you need to install the
sendmail-milter package, to be able
to use milters.
15.6.1.2. Configuring Spam Filtering in a Mail Delivery Agent
Red Hat Enterprise Linux includes two primary MDAs, Procmail and the mail utility. SeeSection 15.2.2,
“Mail Delivery Agent” for more information.
To stop spam in an MDA, users of Procmail can install third-party software named SpamAssassin
available in the spamassassin package. SpamAssassin is a spam detection system that uses a variety
of methods to identify spam in incoming mail. For further information on Spamassassin installation,
configuration and deployment, see Section 15.4.2.6, “Spam Filters” or theHow can I configure
Spamassassin to filter all the incoming mail on my server? Red Hat Knowledgebase article. For
additional information on SpamAssassin, see the SpamAssassin project website.
WARNING
Note that SpamAssasin is a third-party software, and Red Hat does not support
its use.
The spamassassin package is available only through the Extra Packages for
Enterprise Linux (EPEL) repository. To learn more about using the EPEL
repository, see Section 15.6.3, “Using the EPEL Repository to install Antispam and
Antivirus Software”.
To learn more about how Red Hat handles the third party software and what level
of support for it Red Hat provides, see How does Red Hat Global Support
Services handle third-party software, drivers, and/or uncertified
hardware/hypervisors or guest operating systems? Red Hat Knowledgebase
article.
15.6.2. Configuring Antivirus Protection
To protect your system against viruses, you can install ClamAV, an open source antivirus engine for
detecting trojans, viruses, malware, and other malicious software. For additional information about
ClamAV, see the ClamAV project website.
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WARNING
Note that ClamAV is a third-party software, and Red Hat does not support its use.
The clamav, clamav-data, clamav-server and clamav-update packages are only
available in the Extra Packages for Enterprise Linux (EPEL) repository. To learn
more about using the EPEL repository, see Section 15.6.3, “Using the EPEL
Repository to install Antispam and Antivirus Software”.
To learn more about how Red Hat handles the third party software and what level
of support for it Red Hat provides, see How does Red Hat Global Support
Services handle third-party software, drivers, and/or uncertified
hardware/hypervisors or guest operating systems? Red Hat Knowledgebase
article.
Once you have enabled the EPEL repository, install ClamAV by running the following command as the
root user:
~]# yum install clamav clamav-data clamav-server clamav-update
15.6.3. Using the EPEL Repository to install Antispam and Antivirus Software
EPEL is a Fedora Special Interest Group that creates, maintains, and manages a high quality set of
additional packages for Red Hat Enterprise Linux. For more information, see the Fedora EPEL
website.
To use the EPEL repository, download the latest version of the epel-release package for Red Hat
Enterprise Linux 7. You can also run the following command as theroot user:
~]# yum install https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpmzu
When using the EPEL repository for the first time, you need to authenticate with a public GPG key.
For more information, see Fedora Package Signing Keys.
The following is a list of additional documentation about email applications.
Information on configuring Sendmail is included with the sendmail and sendmail-cf packages.
/usr/share/sendmail-cf/README — Contains information on them4 macro processor, file
locations for Sendmail, supported mailers, how to access enhanced features, and more.
In addition, the sendmail and aliases man pages contain helpful information covering
various Sendmail options and the proper configuration of the Sendmail /etc/mail/aliases
file.
/usr/share/doc/postfix-version-number/ — Contains a large amount of information on how to
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/usr/share/doc/postfix-version-number/ — Contains a large amount of information on how to
configure Postfix. Replace version-number with the version number of Postfix.
/usr/share/doc/fetchmail-version-number/ — Contains a full list of Fetchmail features in the
FEATURES file and an introductoryFAQ document. Replaceversion-number with the version
number of Fetchmail.
/usr/share/doc/procmail-version-number/ — Contains aREADME file that provides an
overview of Procmail, a FEATURES file that explores every program feature, and an
FAQ file
with answers to many common configuration questions. Replace version-number with the
version number of Procmail.
When learning how Procmail works and creating new recipes, the following Procmail man
pages are invaluable:
procmail — Provides an overview of how Procmail works and the steps involved with
filtering email.
procmailrc — Explains therc file format used to construct recipes.
procmailex — Gives a number of useful, real-world examples of Procmail recipes.
procmailsc — Explains the weighted scoring technique used by Procmail to match a
particular recipe to a message.
/usr/share/doc/spamassassin-version-number/ — Contains a large amount of
information pertaining to SpamAssassin. Replace version-number with the version
number of the spamassassin package.
How to configure postfix with TLS?— A Red Hat Knowledgebase article that describes
configuring postfix to use TLS.
How to configure a Sendmail Smart Host— A Red Hat Knowledgebase solution that describes
configuring a sendmail Smart Host.
http://www.sendmail.org/ — Offers a thorough technical breakdown of Sendmail features,
documentation and configuration examples.
http://www.sendmail.com/ — Contains news, interviews and articles concerning Sendmail,
including an expanded view of the many options available.
http://www.postfix.org/ — The Postfix project home page contains a wealth of information
about Postfix. The mailing list is a particularly good place to look for information.
http://www.fetchmail.info/fetchmail-FAQ.html — A thorough FAQ about Fetchmail.
http://www.spamassassin.org/ — The official site of the SpamAssassin project.
15.7.3. Related Books
Sendmail Milters: A Guide for Fighting Spamby Bryan Costales and Marcia Flynt; Addison-
Wesley — A good Sendmail guide that can help you customize your mail filters.
Sendmail by Bryan Costales with Eric Allman et al.; O’Reilly & Associates — A good Sendmail
reference written with the assistance of the original creator of Delivermail and Sendmail.
Removing the Spam: Email Processing and Filtering by Geoff Mulligan; Addison-Wesley
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Removing the Spam: Email Processing and Filtering by Geoff Mulligan; Addison-Wesley
Publishing Company — A volume that looks at various methods used by email administrators
using established tools, such as Sendmail and Procmail, to manage spam problems.
Internet Email Protocols: A Developer’s Guideby Kevin Johnson; Addison-Wesley Publishing
Company — Provides a very thorough review of major email protocols and the security they
provide.
Managing IMAP by Dianna Mullet and Kevin Mullet; O’Reilly & Associates — Details the steps
required to configure an IMAP server.
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This chapter guides you through the installation and configuration of Samba, an open source
implementation of the Server Message Block(SMB) andcommon Internet file system(CIFS)
protocol, and vsftpd, the primary FTP server shipped with Red Hat Enterprise Linux. Additionally, it
explains how to use the Print Settings tool to configure printers.
16.1. SAMBA
Samba implements the Server Message Block (SMB) protocol in Red Hat Enterprise Linux. The SMB
protocol is used to access resources on a server, such as file shares and shared printers. Additionally,
Samba implements the Distributed Computing Environment Remote Procedure Call (DCE RPC)
protocol used by Microsoft Windows.
You can run Samba as:
An Active Directory (AD) or NT4 domain member
A standalone server
An NT4 Primary Domain Controller (PDC) or Backup Domain Controller (BDC)
NOTE
Red Hat supports these modes only in existing installations with Windows
versions which support NT4 domains. Red Hat recommends not setting up a
new Samba NT4 domain, because Microsoft operating systems later than
Windows 7 and Windows Server 2008 R2 do not support NT4 domains.
Independently of the installation mode, you can optionally share directories and printers. This enables
Samba to act as a file and print server.
NOTE
Red Hat does not support running Samba as an AD domain controller (DC).
16.1.1. The Samba Services
Samba provides the following services:
smbd
This service provides file sharing and printing services using the SMB protocol. Additionally, the
service is responsible for resource locking and for authenticating connecting users. The smb
systemd service starts and stops thesmbd daemon.
To use the smbd service, install thesamba package.
nmbd
This service provides host name and IP resolution using the NetBIOS over IPv4 protocol.
Additionally to the name resolution, the nmbd service enables browsing the SMB network to locate
domains, work groups, hosts, file shares, and printers. For this, the service either reports this
information directly to the broadcasting client or forwards it to a local or master browser. The nmb
systemd service starts and stops thenmbd daemon.
Note that modern SMB networks use DNS to resolve clients and IP addresses.
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To use the nmbd service, install thesamba package.
winbindd
The winbindd service provides an interface for the Name Service Switch (NSS) to use AD or NT4
domain users and groups on the local system. This enables, for example, domain users to
authenticate to services hosted on a Samba server or to other local services. The winbind
systemd service starts and stops thewinbindd daemon.
If you set up Samba as a domain member, winbindd must be started before thesmbd service.
Otherwise, domain users and groups are not available to the local system.
To use the winbindd service, install thesamba-winbind package.
IMPORTANT
Red Hat only supports running Samba as a server with the winbindd service to
provide domain users and groups to the local system. Due to certain limitations,
such as missing Windows access control list (ACL) support and NT LAN Manager
(NTLM) fallback, use of the System Security Services Daemon (SSSD) with Samba
is currently not supported for these use cases. For further details, see the Red Hat
Knowledgebase article What is the support status for Samba file server running on
IdM clients or directly enrolled AD clients where SSSD is used as the client daemon.
16.1.2. Verifying the smb.conf File by Using the testparm Utility
The testparm utility verifies that the Samba configuration in the
/etc/samba/smb.conf file is correct.
The utility detects invalid parameters and values, but also incorrect settings, such as for ID mapping.
If testparm reports no problem, the Samba services will successfully load the/etc/samba/smb.conf
file. Note that testparm cannot verify that the configured services will be available or work as
expected.
IMPORTANT
Red Hat recommends that you verify the /etc/samba/smb.conf file by usingtestparm
after each modification of this file.
To verify the /etc/samba/smb.conf file, run thetestparm utility as the root user. Iftestparm reports
incorrect parameters, values, or other errors in the configuration, fix the problem and run the utility
again.
Example 16.1. Usingtestparm
The following output reports a non-existent parameter and an incorrect ID mapping configuration:
~]# testparm
Load smb config files from /etc/samba/smb.conf
rlimit_max: increasing rlimit_max (1024) to minimum Windows limit (16384)
Unknown parameter encountered: "log levell"
Processing section "[example_share]"
Loaded services file OK.
ERROR: The idmap range for the domain * (tdb) overlaps with the range of DOMAIN (ad)!
Server role: ROLE_DOMAIN_MEMBER
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Press enter to see a dump of your service definitions
# Global parameters
[global]
...
[example_share]
...
16.1.3. Understanding the Samba Security Modes
The security parameter in the[global] section in the/etc/samba/smb.conf file manages how Samba
authenticates users that are connecting to the service. Depending on the mode you install Samba in,
the parameter must be set to different values:
On an AD domain member, set security = ads.
In this mode, Samba uses Kerberos to authenticate AD users.
For details about setting up Samba as a domain member, see Section 16.1.5, “Setting up
Samba as a Domain Member”.
On a standalone server, set security = user.
In this mode, Samba uses a local database to authenticate connecting users.
For details about setting up Samba as a standalone server, see Section 16.1.4, “Setting up
Samba as a Standalone Server”.
On an NT4 PDC or BDC, set security = user.
In this mode, Samba authenticates users to a local or LDAP database.
On an NT4 domain member, set security = domain.
In this mode, Samba authenticates connecting users to an NT4 PDC or BDC. You cannot use
this mode on AD domain members.
For details about setting up Samba as a domain member, see Section 16.1.5, “Setting up
Samba as a Domain Member”.
For further details, see the description of the security parameter in thesmb.conf(5) man page.
16.1.4. Setting up Samba as a Standalone Server
In certain situations, administrators want to set up a Samba server that is not a member of a domain.
In this installation mode, Samba authenticates users to a local database instead of to a central DC.
Additionally, you can enable guest access to allow users to connect to one or multiple services without
authentication.
16.1.4.1. Setting up the Server Configuration for the Standalone Server
To set up Samba as a standalone server:
Setting up Samba as a Standalone Server
1. Install the samba package:
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~]# yum install samba
2. Edit the /etc/samba/smb.conf file and set the following parameters:
[global]
workgroup = Example-WG
netbios name = Server
security = user
log file = /var/log/samba/%m.log
log level = 1
This configuration defines a standalone server named Server within theExample-WG work
group. Additionally, this configuration enables logging on a minimal level (1) and log files will
be stored in the /var/log/samba/ directory. Samba will expand the%m macro in thelog file
parameter to the NetBIOS name of connecting clients. This enables individual log files for
each client.
For further details, see the parameter descriptions in the smb.conf(5) man page.
3. Configure file or printer sharing. See:
Section 16.1.6, “Configuring File Shares on a Samba Server”
Section 16.1.7, “Setting up a Samba Print Server”
4. Verify the /etc/samba/smb.conf file:
~]# testparm
For details, see Section 16.1.2, “Verifying thesmb.conf File by Using thetestparm Utility”.
5. If you set up shares that require authentication, create the user accounts. For details, see
Section 16.1.4.2, “Creating and Enabling Local User Accounts”
.
6. Open the required ports and reload the firewall configuration by using the firewall-cmd utility:
~]# firewall-cmd --permanent --add-port={139/tcp,445/tcp}
7. Start the smb service:
~]# systemctl start smb
8. Optionally, enable the smb service to start automatically when the system boots:
~]# systemctl enable smb
16.1.4.2. Creating and Enabling Local User Accounts
To enable users to authenticate when they connect to a share, you must create the accounts on the
Samba host both in the operating system and in the Samba database. Samba requires the operating
system account to validate the Access Control Lists (ACL) on file system objects and the Samba
account to authenticate connecting users.
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If you use the passdb backend = tdbsam default setting, Samba stores user accounts in the
/var/lib/samba/private/passdb.tdb database.
For example, to create the example Samba user:
Creating a Samba User
1. Create the operating system account:
~]# useradd -M -s /sbin/nologin example
The previous command adds the example account without creating a home directory. If the
account is only used to authenticate to Samba, assign the /sbin/nologin command as shell to
prevent the account from logging in locally.
2. Set a password to the operating system account to enable it:
~]# passwd example
Enter new UNIX password: password
Retype new UNIX password: password
passwd: password updated successfully
Samba does not use the password set on the operating system account to authenticate.
However, you need to set a password to enable the account. If an account is disabled, Samba
denies access if this user connects.
3. Add the user to the Samba database and set a password to the account:
~]# smbpasswd -a example
New SMB password: password
Retype new SMB password: password
Added user example.
Use this password to authenticate when using this account to connect to a Samba share.
4. Enable the Samba account:
~]# smbpasswd -e example
Enabled user example.
16.1.5. Setting up Samba as a Domain Member
Administrators running an AD or NT4 domain often want to use Samba to join their Red Hat
Enterprise Linux server as a member to the domain. This enables you to:
Access domain resources on other domain members
Authenticate domain users to local services, such as sshd
Share directories and printers hosted on the server to act as a file and print server
16.1.5.1. Joining a Domain
To join a Red Hat Enterprise Linux system to a domain:
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Joining a Red Hat Enterprise Linux System to a Domain
1. Install the following packages:
~]# yum install realmd oddjob-mkhomedir oddjob samba-winbind-clients
samba-winbind samba-common-tools
2. To share directories or printers on the domain member, install the samba package:
~]# yum install samba
3. If you join an AD, additionally install the samba-winbind-krb5-locator package:
~]# yum install samba-winbind-krb5-locator
This plug-in enables Kerberos to locate the Key Distribution Center (KDC) based on AD sites
using DNS service records.
4. Optionally, rename the existing /etc/samba/smb.conf Samba configuration file:
~]# mv /etc/samba/smb.conf /etc/samba/smb.conf.old
5. Join the domain. For example, to join a domain named ad.example.com
~]# realm join --membership-software=samba --client-software=winbind
ad.example.com
Using the previous command, the realm utility automatically:
Creates a /etc/samba/smb.conf file for a membership in thead.example.com domain
Adds the winbind module for user and group lookups to the/etc/nsswitch.conf file
Updates the Pluggable Authentication Module (PAM) configuration files in the
/etc/pam.d/ directory
Starts the winbind service and enables the service to start when the system boots
For further details about the realm utility, see therealm(8) man page and the
corresponding section in the Red Hat Windows Integration Guide
.
6. Optionally, set an alternative ID mapping back end or customized ID mapping settings in the
/etc/samba/smb.conf file. For details, seeSection 16.1.5.3, “Understanding ID Mapping”.
7. Optionally, verify the configuration. See Section 16.1.5.2, “Verifying That Samba Was
Correctly Joined As a Domain Member”.
8. Verify that the winbindd is running:
~]# systemctl status winbind
IMPORTANT
To enable Samba to query domain user and group information, the winbindd
service must be running before you start smbd.
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9. If you installed the samba package to share directories and printers, start thesmbd service:
~]# systemctl start smb
16.1.5.2. Verifying That Samba Was Correctly Joined As a Domain Member
After you joined a Red Hat Enterprise Linux as a domain member, you can run different tests to verify
that the join succeeded. See:
the section called “Verifying That the Operating System Can Retrieve Domain User Accounts
and Groups”
the section called “Verifying If AD Domain Users Can Obtain a Kerberos Ticket”
the section called “Listing the Available Domains”
Verifying That the Operating System Can Retrieve Domain User Accounts and Groups
Use the getent utility to verify that the operating system can retrieve domain users and groups. For
example:
To query the administrator account in theAD domain:
~]# getent passwd ADadministrator
ADadministrator:*:10000:10000::/home/administrator@AD:/bin/bash
To query the members of the Domain Users group in theAD domain:
~]# getent group "ADDomain Users"
ADdomain users:x:10000:user
If the command works correctly, verify that you can use domain users and groups when you set
permissions on files and directories. For example, to set the owner of the /srv/samba/example.txt file
to ADadministrator and the group toADDomain Users:
~]# chown "ADadministrator":"ADDomain Users" /srv/samba/example.txt
Verifying If AD Domain Users Can Obtain a Kerberos Ticket
In an AD environment, users can obtain a Kerberos ticket from the DC. For example, to verify if the
administrator user can obtain a Kerberos ticket:
NOTE
To use the kinit and klist utilities, install thekrb5-workstation package on the Samba
domain member.
Obtaining a Kerberos Ticket
1. Obtain a ticket for the administrator@AD.EXAMPLE.COM principal:
~]# kinit administrator@AD.EXAMPLE.COM
2. Display the cached Kerberos ticket:
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~]# klist
Ticket cache: KEYRING:persistent:0:0
Default principal: administrator@AD.EXAMPLE.COM
Valid starting Expires Service principal
11.09.2017 14:46:21 12.09.2017 00:46:21
krbtgt/AD.EXAMPLE.COM@AD.EXAMPLE.COM
renew until 18.09.2017 14:46:19
Listing the Available Domains
To list all domains available through the winbindd service, enter:
~]# wbinfo --all-domains
If Samba was successfully joined as a domain member, the command displays the built-in and local
host name, as well as the domain Samba is a member of including trusted domains.
Example 16.2. Displaying the Available Domains
~]# wbinfo --all-domains
BUILTIN
SAMBA-SERVER
AD
16.1.5.3. Understanding ID Mapping
Windows domains distinguish users and groups by unique Security Identifiers (SID). However, Linux
requires unique UIDs and GIDs for each user and group. If you run Samba as a domain member, the
winbindd service is responsible for providing information about domain users and groups to the
operating system.
To enable the winbindd service to provide unique IDs for users and groups to Linux, you must
configure ID mapping in the /etc/samba/smb.conf file for:
The local database (default domain)
The AD or NT4 domain the Samba server is a member of
Each trusted domain from which users must be able to access resources on this Samba server
16.1.5.3.1. Planning ID Ranges
Regardless of whether you store the Linux UIDs and GIDs in AD or if you configure Samba to generate
them, each domain configuration requires a unique ID range that must notoverlap with any of the
other domains.
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WARNING
If you set overlapping ID ranges, Samba fails to work correctly.
Example 16.3. Unique ID Ranges
The following shows non-overlapping ID mapping ranges for the default (*), AD-DOM, and the
TRUST-DOM domains.
[global]
...
idmap config * : backend = tdb
idmap config * : range = 10000-999999
idmap config AD-DOM:backend = rid
idmap config AD-DOM:range = 2000000-2999999
idmap config TRUST-DOM:backend = rid
idmap config TRUST-DOM:range = 4000000-4999999
IMPORTANT
You can only assign one range per domain. Therefore, leave enough space between the
domains ranges. This enables you to extend the range later if your domain grows.
If you later assign a different range to a domain, the ownership of files and directories
previously created by these users and groups will be lost.
16.1.5.3.2. The * Default Domain
In a domain environment, you add one ID mapping configuration for each of the following:
The domain the Samba server is a member of
Each trusted domain that should be able to access the Samba server
However, for all other objects, Samba assigns IDs from the default domain. This includes:
Local Samba users and groups
Samba built-in accounts and groups, such as BUILTINAdministrators
IMPORTANT
You must configure the default domain as described in this section to enable Samba to
operate correctly.
The default domain back end must be writable to permanently store the assigned IDs.

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For the default domain, you can use one of the following back ends:
tdb
When you configure the default domain to use the tdb back end, set an ID range that is big enough
to include objects that will be created in the future and that are not part of a defined domain ID
mapping configuration.
For example, set the following in the [global] section in the/etc/samba/smb.conf file:
For further details, see Section 16.1.5.4.1, “Using thetdb ID Mapping Back End”.
autorid
When you configure the default domain to use the autorid back end, adding additional ID mapping
configurations for domains is optional.
For example, set the following in the [global] section in the/etc/samba/smb.conf file:
idmap config * : backend = autorid
For further details, see Configuring theautorid Back End.
16.1.5.4. The Different ID Mapping Back Ends
Samba provides different ID mapping back ends for specific configurations. The most frequently
used back ends are:
Table 16.1. Frequently Used ID Mapping Back Ends
Back End Use Case
tdb The * default domain only
ad AD domains only
rid AD and NT4 domains
autorid AD, NT4, and the * default domain
The following sections describe the benefits, recommended scenarios where to use the back end, and
how to configure it.
16.1.5.4.1. Using thetdb ID Mapping Back End
The winbindd service uses the writabletdb ID mapping back end by default to store Security Identifier
(SID), UID, and GID mapping tables. This includes local users, groups, and built-in principals.
Use this back end only for the * default domain. For example:
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For further details about the * default domain, seeSection 16.1.5.3.2, “The* Default Domain”.
16.1.5.4.2. Using the ad ID Mapping Back End
The ad ID mapping back end implements a read-only API to read account and group information from
AD. This provides the following benefits:
All user and group settings are stored centrally in AD.
User and group IDs are consistent on all Samba servers that use this back end.
The IDs are not stored in a local database which can corrupt, and therefore file ownerships
cannot be lost.
The ad back end reads the following attributes from AD:
Table 16.2. Attributes thead Back End Reads from User and Group Objects
AD Attribute Name Object Type Mapped to
sAMAccountName User and group User or group name, depending
on the object
uidNumber User User ID (UID)
gidNumber Group Group ID (GID)
loginShell [a] User Path to the shell of the user
unixHomeDirectory User Path to the home directory of the
user
primaryGroupID [b] User Primary group ID
[a] Samba only reads this attribute if you set idmap config DOMAIN:unix_nss_info = yes.
[b] Samba only reads this attribute if you set idmap config DOMAIN:unix_primary_group = yes.
Prerequisites of the ad Back End
To use the ad ID mapping back end:
Both users and groups must have unique IDs set in AD, and the IDs must be within the range
configured in the /etc/samba/smb.conf file. Objects whose IDs are outside of the range will
not be available on the Samba server.
Users and groups must have all required attributes set in AD. If required attributes are
missing, the user or group will not be available on the Samba server. The required attributes
depend on your configuration. See Table 16.2, “Attributes the ad Back End Reads from User
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and Group Objects”.
Configuring the ad Back End
To configure a Samba AD member to use the ad ID mapping back end:
Configuring thead Back End on a Domain Member
1. Edit the [global] section in the/etc/samba/smb.conf file:
a. Add an ID mapping configuration for the default domain (*) if it does not exist. For
example:
For further details about the default domain configuration, see Section 16.1.5.3.2, “The*
Default Domain”.
b. Enable the ad ID mapping back end for the AD domain:
idmap config DOMAIN : backend = ad
c. Set the range of IDs that is assigned to users and groups in the AD domain. For example:
idmap config DOMAIN : range = 2000000-2999999
IMPORTANT
The range must not overlap with any other domain configuration on this
server. Additionally, the range must be set big enough to include all IDs
assigned in the future. For further details, see Section 16.1.5.3.1, “Planning
ID Ranges”.
d. Set that Samba uses the RFC 2307 schema when reading attributes from AD:
idmap config DOMAIN : schema_mode = rfc2307
e. To enable Samba to read the login shell and the path to the users home directory from the
corresponding AD attribute, set:
idmap config DOMAIN : unix_nss_info = yes
Alternatively, you can set a uniform domain-wide home directory path and login shell that
is applied to all users. For example:
template shell = /bin/bash
template homedir = /home/%U
For details about variable substitution, see the VARIABLE SUBSTITUTIONS section in the
smb.conf(5) man page.
f. By default, Samba uses the primaryGroupID attribute of a user object as the user’s
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f. By default, Samba uses the primaryGroupID attribute of a user object as the user’s
primary group on Linux. Alternatively, you can configure Samba to use the value set in the
gidNumber attribute instead:
idmap config DOMAIN : unix_primary_group = yes
~]# testparm
3. Reload the Samba configuration:
~]# smbcontrol all reload-config
4. Verify that the settings work as expected. See the section called “Verifying That the
Operating System Can Retrieve Domain User Accounts and Groups”.
For further details, see the smb.conf(5) andidmap_ad(8) man pages.
16.1.5.4.3. Using therid ID Mapping Back End
Samba can use the relative identifier (RID) of a Windows SID to generate an ID on Red Hat Enterprise
Linux.
NOTE
The RID is the last part of a SID. For example, if the SID of a user is S-1-5-21-
5421822485-1151247151-421485315-30014, then 30014 is the corresponding RID. For
details, how Samba calculates the local ID, see the idmap_rid(8) man page.
The rid ID mapping back end implements a read-only API to calculate account and group information
based on an algorithmic mapping scheme for AD and NT4 domains. When you configure the back end,
you must set the lowest and highest RID in the idmap config DOMAIN : range parameter. Samba will
not map users or groups with a lower or higher RID than set in this parameter.
IMPORTANT
As a read-only back end, rid cannot assign new IDs, such as forBUILTIN groups.
Therefore, do not use this back end for the * default domain.
Benefits
All domain users and groups that have an RID within the configured range are automatically
available on the domain member.
You do not need to manually assign IDs, home directories, and login shells.
Drawbacks
All domain users get the same login shell and home directory assigned. However, you can use
variables.
User and group IDs are only the same across Samba domain members if all use the rid back
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User and group IDs are only the same across Samba domain members if all use the rid back
end with the same ID range settings.
You cannot exclude individual users or groups from being available on the domain member.
Only users and groups outside of the configured range are excluded.
Based on the formulas the winbindd service uses to calculate the IDs, duplicate IDs can occur
in multi-domain environments if objects in different domains have the same RID.
Configuring the rid Back End
To configure a Samba domain member to use the rid ID mapping back end:
Configuring therid Back End on a Domain Member
a. Add an ID mapping configuration for the default domain (*) if it does not exist. For
example:
For further details about the default domain configuration, see Section 16.1.5.3.2, “The*
Default Domain”.
b. Enable the rid ID mapping back end for the domain:
idmap config DOMAIN : backend = rid
c. Set a range that is big enough to include all RIDs that will be assigned in the future. For
example:
idmap config DOMAIN : range = 2000000-2999999
Samba ignores users and groups whose RIDs in this domain are not within the range.
IMPORTANT
The range must not overlap with any other domain configuration on this
server. For further details, see Section 16.1.5.3.1, “Planning ID Ranges”.
d. Set a shell and home directory path that will be assigned to all mapped users. For
example:
~]# testparm
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16.1.5.4.4. Using the autorid ID Mapping Back End
The autorid back end works similar to therid ID mapping back end, but can automatically assign IDs
for different domains. This enables you to use the autorid back end in the following situations:
Only for the * default domain.
For the * default domain and additional domains, without the need to create ID mapping
configurations for each of the additional domains.
Only for specific domains.
Benefits
All domain users and groups whose calculated UID and GID is within the configured range are
automatically available on the domain member.
You do not need to manually assign IDs, home directories, and login shells.
No duplicate IDs, even if multiple objects in a multi-domain environment have the same RID.
Drawbacks
User and group IDs are not the same across Samba domain members.
All domain users get the same login shell and home directory assigned. However, you can use
variables.
You cannot exclude individual users or groups from being available on the domain member.
Only users and groups whose calculated UID or GID is outside of the configured range are
excluded.
Configuring the autorid Back End
To configure a Samba domain member to use the autorid ID mapping back end for the* default
domain:
NOTE
If you use autorid for the default domain, adding additional ID mapping configuration
for domains is optional.
Configuring theautorid Back End on a Domain Member
a. Enable the autorid ID mapping back end for the* default domain:
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idmap config * : backend = autorid
b. Set a range that is big enough to assign IDs for all existing and future objects. For
example:
Samba ignores users and groups whose calculated IDs in this domain are not within the
range. For details about how the back end calculated IDs, see the THE MAPPING
FORMULAS section in theidmap_autorid(8) man page.
WARNING
After you set the range and Samba starts using it, you can only
increase the upper limit of the range. Any other change to the range
can result in new ID assignments, and thus in loosing file ownerships.
c. Optionally, set a range size. For example:
idmap config * : rangesize = 200000
Samba assigns this number of continuous IDs for each domain’s object until all IDs from
the range set in the idmap config * : range parameter are taken. For further details, see
the rangesize parameter description in theidmap_autorid(8) man page.
d. Set a shell and home directory path that will be assigned to all mapped users. For
example:
e. Optionally, add additional ID mapping configuration for domains. If no configuration for
an individual domain is available, Samba calculates the ID using the autorid back end
settings in the previously configured * default domain.
IMPORTANT
If you configure additional back ends for individual domains, the ranges for
all ID mapping configuration must not overlap. For further details, see
Section 16.1.5.3.1, “Planning ID Ranges”.
~]# testparm

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16.1.6. Configuring File Shares on a Samba Server
To use Samba as a file server, add shares to the /etc/samba/smb.conf file of your standalone or
domain member configuration.
You can add shares that uses either:
POSIX ACLs. See Section 16.1.6.1, “Setting up a Share That Uses POSIX ACLs”
.
Fine-granular Windows ACLs. See Section 16.1.6.2, “Setting up a Share That Uses Windows
ACLs”.
16.1.6.1. Setting up a Share That Uses POSIX ACLs
As a Linux service, Samba supports shares with POSIX ACLs. They enable you to manage permissions
locally on the Samba server using utilities, such as chmod. If the share is stored on a file system that
supports extended attributes, you can define ACLs with multiple users and groups.
NOTE
If you need to use fine-granular Windows ACLs instead, see Section 16.1.6.2, “Setting
up a Share That Uses Windows ACLs”.
Before you can add a share, set up Samba. See:
Section 16.1.4, “Setting up Samba as a Standalone Server”
Section 16.1.5, “Setting up Samba as a Domain Member”
16.1.6.1.1. Adding a Share That Uses POSIX ACLs
To create a share named example, that provides the content of the/srv/samba/example/ directory,
and uses POSIX ACLs:
Adding a Share That Uses POSIX ACLs
1. Optionally, create the folder if it does not exist. For example:
~]# mkdir -p /srv/samba/example/
2. If you run SELinux in enforcing mode, set thesamba_share_t context on the directory:
~]# semanage fcontext -a -t samba_share_t "/srv/samba/example(/.*)?"
~]# restorecon -Rv /srv/samba/example/
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3. Set file system ACLs on the directory. For details, see Section 16.1.6.1.2, “Setting ACLs”.
4. Add the example share to the/etc/samba/smb.conf file. For example, to add the share write-
enabled:
[example]
path = /srv/samba/example/
read only = no
NOTE
Regardless of the file system ACLs; if you do not set read only = no, Samba
shares the directory in read-only mode.
~]# testparm
6. Open the required ports and reload the firewall configuration using the firewall-cmd utility:
~]# firewall-cmd --permanent --add-service=samba
7. Restart the smb service:
~]# systemctl restart smb
8. Optionally, enable the smb service to start automatically at boot time:
16.1.6.1.2. Setting ACLs
Shares that use POSIX ACLs support:
Standard Linux ACLs. For details, see Setting Standard Linux ACLs.
Extended ACLs. For details, see Setting Extended ACLs.
Setting Standard Linux ACLs
The standard ACLs on Linux support setting permissions for one owner, one group, and for all other
undefined users. You can use the chown, chgrp, and chmod utility to update the ACLs. If you require
precise control, then you use the more complex POSIX ACLs, see Setting Extended ACLs.
For example, to set the owner of the /srv/samba/example/ directory to theroot user, grant read and
write permissions to the Domain Users group, and deny access to all other users:
~]# chown root:"Domain Users" /srv/samba/example/
~]# chmod 2770 /srv/samba/example/
NOTE
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NOTE
Enabling the set-group-ID (SGID) bit on a directory automatically sets the default
group for all new files and subdirectories to that of the directory group, instead of the
usual behavior of setting it to the primary group of the user who created the new
directory entry.
For further details about permissions, see the chown(1) and chmod(1) man pages.
Setting Extended ACLs
If the file system the shared directory is stored on supports extended ACLs, you can use them to set
complex permissions. Extended ACLs can contain permissions for multiple users and groups.
Extended POSIX ACLs enable you to configure complex ACLs with multiple users and groups.
However, you can only set the following permissions:
No access
Read access
Write access
Full control
If you require the fine-granular Windows permissions, such as Create folder / append data, configure
the share to use Windows ACLs. See Section 16.1.6.2, “Setting up a Share That Uses Windows ACLs”
.
To use extended POSIX ACLs on a share:
Enabling Extended POSIX ACLs on a Share
1. Enable the following parameter in the share’s section in the /etc/samba/smb.conf file to
enable ACL inheritance of extended ACLs:
inherit acls = yes
For details, see the parameter description in the smb.conf(5) man page.
4. Set the ACLs on the directory. For details about using extended ACLs, see Chapter 5, Access
Control Lists.
Example 16.4. Setting Extended ACLs
The following procedure sets read, write, and execute permissions for the Domain
Admins group, read, and execute permissions for theDomain Users group, and deny
access to everyone else on the /srv/samba/example/ directory:
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Setting Extended ACLs
a. Disable auto-granting permissions to the primary group of user accounts:
~]# setfacl -m group::--- /srv/samba/example/
~]# setfacl -m default:group::--- /srv/samba/example/
The primary group of the directory is additionally mapped to the dynamic CREATOR
GROUP principal. When you use extended POSIX ACLs on a Samba share, this
principal is automatically added and you cannot remove it.
b. Set the permissions on the directory:
i. Grant read, write, and execute permissions to the Domain Admins group:
~]# setfacl -m group:"DOMAINDomain Admins":rwx /srv/samba/example/
ii. Grant read and execute permissions to the Domain Users group:
~]# setfacl -m group:"DOMAINDomain Users":r-x /srv/samba/example/
iii. Set permissions for the other ACL entry to deny access to users that do not match
the other ACL entries:
~]# setfacl -R -m other::--- /srv/samba/example/
These settings apply only to this directory. In Windows, these ACLs are mapped to
the This folder only mode.
c. To enable the permissions set in the previous step to be inherited by new file system
objects created in this directory:
~]# setfacl -m default:group:"DOMAINDomain Admins":rwx
/srv/samba/example/
~]# setfacl -m default:group:"DOMAINDomain Users":r-x /srv/samba/example/
~]# setfacl -m default:other::--- /srv/samba/example/
With these settings, the This folder only mode for the principals is now set toThis
folder, subfolders, and files.
Samba maps the previously set permissions to the following Windows ACLs:
Principal Access Applies to
DOMAINDomain Admins Full control This folder, subfolders, and
files
DOMAINDomain Users Read & execute This folder, subfolders, and
files
Everyone [a] None This folder, subfolders, and
files
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owner (Unix
Userpass:attributes[]owner)
[b]
Full control This folder only
primary_group (Unix
Userpass:attributes[]primary_
group) [c]
None This folder only
CREATOR OWNER [d] [e] Full control Subfolders and files only
CREATOR GROUP [f] None Subfolders and files only
[a] Samba maps the permissions for this principal from the otherACL entry.
[b] Samba maps the owner of the directory to this entry.
[c] Samba maps the primary group of the directory to this entry.
[d] On new file system objects, the creator inherits automatically the permissions of this principal.
[e] Configuring or removing these principals from the ACLs not supported on shares that use POSIX ACLs.
[f] On new file system objects, the creator’s primary group inherits automatically the permissions of this
principal.
Principal Access Applies to
16.1.6.1.3. Setting Permissions on a Share
Optionally, to limit or grant access to a Samba share, you can set certain parameters in the share’s
section in the /etc/samba/smb.conf file.
NOTE
Share-based permissions manage if a user, group, or host is able to access a share.
These settings do not affect file system ACLs.
Use share-based settings to restrict access to shares. For example, to deny access from specific
hosts.
Configuring User and Group-based Share Access
User and group-based access control enables you to grant or deny access to a share for certain users
and groups. For example, to enable all members of the Domain Users group to access a share while
access is denied for the user account, add the following parameters to the share’s configuration:
valid users = +DOMAIN"Domain Users"
invalid users = DOMAINuser
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The invalid users parameter has a higher priority thanvalid users parameter. For example, if the
user account is a member of theDomain Users group, access is denied to this account when you use
the previous example.
For further details, see the parameter descriptions in the smb.conf(5) man page.
Configuring Host-based Share Access
Host-based access control enables you to grant or deny access to a share based on client’s host
names, IP addresses, or IP ranges.
For example, to enable the 127.0.0.1 IP address, the192.0.2.0/24 IP range, and the
client1.example.com host to access a share, and additionally deny access for the
client2.example.com host:
Configuring Host-based Share Access
1. Add the following parameters to the configuration of the share in the /etc/samba/smb.conf:
hosts allow = 127.0.0.1 192.0.2.0/24 client1.example.com
hosts deny = client2.example.com
2. Reload the Samba configuration
The hosts deny parameter has a higher priority thanhosts allow. For example, if
client1.example.com resolves to an IP address that is listed in the
hosts allow parameter, access for
this host is denied.
For further details, see the parameter description in the smb.conf(5) man page.
16.1.6.2. Setting up a Share That Uses Windows ACLs
Samba supports setting Windows ACLs on shares and file system object. This enables you to:
Use the fine-granular Windows ACLs
Manage share permissions and file system ACLs using Windows
Alternatively, you can configure a share to use POSIX ACLs. For details, see Section 16.1.6.1, “Setting
up a Share That Uses POSIX ACLs”.
16.1.6.2.1. Granting the SeDiskOperatorPrivilege Privilege
Only users and groups having the SeDiskOperatorPrivilege privilege granted can configure
permissions on shares that use Windows ACLs. For example, to grant the privilege to the
DOMAINDomain Admins group:
~]# net rpc rights grant "DOMAINDomain Admins" SeDiskOperatorPrivilege
-U "DOMAINadministrator"
Enter DOMAINadministrator's password:
Successfully granted rights.
NOTE
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NOTE
In a domain environment, grant SeDiskOperatorPrivilege to a domain group. This
enables you to centrally manage the privilege by updating a user’s group membership.
To list all users and groups having SeDiskOperatorPrivilege granted:
~]# net rpc rights list privileges SeDiskOperatorPrivilege
Enter administrator's password:
SeDiskOperatorPrivilege:
BUILTINAdministrators
DOMAINDomain Admins
16.1.6.2.2. Enabling Windows ACL Support
To configure shares that support Windows ACLs, you must enable this feature in Samba. To enable it
globally for all shares, add the following settings to the [global] section of the/etc/samba/smb.conf
file:
vfs objects = acl_xattr
map acl inherit = yes
store dos attributes = yes
Alternatively, you can enable Windows ACL support for individual shares, by adding the same
parameters to a share’s section instead.
16.1.6.2.3. Adding a Share That Uses Windows ACLs
To create a share named example, that shares the content of the/srv/samba/example/ directory, and
uses Windows ACLs:
Adding a Share That Uses Windows ACLs
1. Optionally, create the folder if it does not exists. For example:
~]# mkdir -p /srv/samba/example/
~]# semanage fcontext -a -t samba_share_t "/srv/samba/example(/.*)?"
~]# restorecon -Rv /srv/samba/example/
3. Add the example share to the/etc/samba/smb.conf file. For example, to add the share write-
enabled:
[example]
path = /srv/samba/example/
read only = no
NOTE
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NOTE
Regardless of the file system ACLs; if you do not set read only = no, Samba
shares the directory in read-only mode.
4. If you have not enabled Windows ACL support in the [global] section for all shares, add the
following parameters to the [example] section to enable this feature for this share:
vfs objects = acl_xattr
map acl inherit = yes
store dos attributes = yes
~]# testparm
16.1.6.2.4. Managing Share Permissions and File System ACLs of a Share That Uses Windows ACLs
To manage share and file system ACLs on a Samba share that uses Windows ACLs, use a Windows
applications, such as Computer Management. For details, see your Windows documentation.
Alternatively, use the smbcacls utility to manage ACLs. For details, seeSection 16.1.6.3, “Managing
ACLs on an SMB Share Using smbcacls”.
NOTE
To modify the file system permissions from Windows, you must use an account that
has the SeDiskOperatorPrivilege privilege granted. SeeSection 16.1.6.2.1, “Granting
the SeDiskOperatorPrivilege Privilege”.
16.1.6.3. Managing ACLs on an SMB Share Using smbcacls
The smbcacls utility can list, set, and delete ACLs of files and directories stored on an SMB share.
You can use smbcacls to manage file system ACLs:
On a local or remote Samba server that uses advanced Windows ACLs or POSIX ACLs.
On Red Hat Enterprise Linux to remotely manage ACLs on a share hosted on Windows.
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16.1.6.3.1. Understanding Access Control Entries
Each ACL entry of a file system object contains Access Control Entries (ACE) in the following format:
security_principal:access_right/inheritance_information/permissions
Example 16.5. Access Control Entries
If the ADDomain Users group hasModify permissions that apply toThis folder, subfolders, and
files on Windows, the ACL contains the following ACEs:
ADDomain Users:ALLOWED/OI|CI/CHANGE
The following describes the individual ACEs:
Security principal
The security principal is the user, group, or SID the permissions in the ACL are applied to.
Access right
Defines if access to an object is granted or denied. The value can be ALLOWED or DENIED.
Inheritance information
The following values exist:
Table 16.3. Inheritance Settings
Value Description Maps to
OI Object Inherit This folder and files
CI Container Inherit This folder and subfolders
IO Inherit Only The ACE does not apply to the
current file or directory.
ID Inherited The ACE was inherited from the
parent directory.
Additionally, the values can be combined as follows:
Table 16.4. Inheritance Settings Combinations
Value Combinations Maps to the WindowsApplies to Setting
[command]`OI CI`
This folder, subfolders, and files [command]`OI
CI IO`
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Subfolders and files only [command]`CI
IO` Subfolders only
[command]`OI IO`
Value Combinations Maps to the WindowsApplies to Setting
Permissions
This value can be either a hex value that represents one or more Windows permissions or an
smbcacls alias:
A hex value that represents one or more Windows permissions.
The following table displays the advanced Windows permissions and their corresponding
value in hex format:
Table 16.5. Windows Permissions and Their Correspondingsmbcacls Value in Hex Format
Windows Permissions Hex Values
Full control 0x001F01FF
Traverse folder / execute file 0x00100020
List folder / read data 0x00100001
Read attributes 0x00100080
Read extended attributes 0x00100008
Create files / write data 0x00100002
Create folders / append data 0x00100004
Write attributes 0x00100100
Write extended attributes 0x00100010
Delete subfolders and files 0x00100040
Delete 0x00110000
Read permissions 0x00120000
Change permissions 0x00140000
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Take ownership 0x00180000
Windows Permissions Hex Values
Multiple permissions can be combined as a single hex value using the bit-wise OR
operation. For details, see Section 16.1.6.3.3, “Calculating an ACE Mask”
.
An smbcacls alias. The following table displays the available aliases:
Table 16.6. Existingsmbcacls Aliases and Their Corresponding Windows Permission
smbcacls Alias Maps to Windows Permission
R Read
READ Read & execute
W Special +
+ * Create files / write data +* Create folders
/ append data + * Write attributes + * Write
extended attributes + * Read permissions
D Delete
P Change permissions
O Take ownership
X Traverse / execute
CHANGE Modify
FULL Full control
NOTE
You can combine single-letter aliases when you set permissions. For
example, you can set RD to apply the Windows permissionRead and Delete.
However, you can neither combine multiple non-single-letter aliases nor
combine aliases and hex values.
16.1.6.3.2. Displaying ACLs Usingsmbcacls
If you run smbcacls without any operation parameter, such as--add, the utility displays the ACLs of a
file system object.
For example, to list the ACLs of the root directory of the //server/example share:
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~]# smbcacls //server/example / -U "DOMAINpass:quotes[administrator]"
Enter DOMAINpass:quotes[administrator]'s password:
REVISION:1
CONTROL:SR|PD|DI|DP
OWNER:ADAdministrators
GROUP:ADDomain Users
ACL:ADAdministrator:ALLOWED/OI|CI/FULL
ACL:ADDomain Users:ALLOWED/OI|CI/CHANGE
ACL:ADDomain Guests:ALLOWED/OI|CI/0x00100021
The output of the command displays:
REVISION: The internal Windows NT ACL revision of the security descriptor
CONTROL: Security descriptor control
OWNER: Name or SID of the security descriptor’s owner
GROUP: Name or SID of the security descriptor’s group
ACL entries. For details, seeSection 16.1.6.3.1, “Understanding Access Control Entries”
.
16.1.6.3.3. Calculating an ACE Mask
In most situations, when you add or update an ACE, you use the smbcacls aliases listed inTable 16.6,
“Existing smbcacls Aliases and Their Corresponding Windows Permission”
.
However, if you want to set advanced Windows permissions as listed in Table 16.5, “Windows
Permissions and Their Corresponding smbcacls Value in Hex Format”, you must use the bit-wiseOR
operation to calculate the correct value. You can use the following shell command to calculate the
value:
~]# echo $(printf '0x%X' $
hex_value_1 | hex_value_2 | ...)
Example 16.6. Calculating an ACE Mask
You want set following permissions:
Traverse folder / execute file (0x00100020)
List folder / read data (0x00100001)
Read attributes (0x00100080)
To calculate the hex value for the previous permissions, enter:
~]# echo $(printf '0x%X' $(( 0x00100020 | 0x00100001 | 0x00100080 )))
0x1000A1
Use the returned value when you set or update an ACE.
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16.1.6.3.4. Adding, Updating, And Removing an ACL Usingsmbcacls
Depending on the parameter you pass to the smbcacls utility, you can add, update, and remove ACLs
from a file or directory.
Adding an ACL
To add an ACL to the root of the //server/example share that grantsCHANGE permissions forThis
folder, subfolders, and files to theADDomain Users group:
~]# smbcacls //server/example / -U "DOMAINadministrator
--add ACL:"ADDomain Users":ALLOWED/OI|CI/CHANGE
Updating an ACL
Updating an ACL is similar to adding a new ACL. You update an ACL by overriding the ACL using the -
-modify parameter with an existing security principal. If
smbcacls finds the security principal in the
ACL list, the utility updates the permissions. Otherwise the command fails with an error:
ACL for SID principal_name not found
For example, to update the permissions of the ADDomain Users group and set them toREAD for
This folder, subfolders, and files:
--modify ACL:"ADDomain Users":ALLOWED/OI|CI/READ
Deleting an ACL
To delete an ACL, pass the --delete with the exact ACL to thesmbcacls utility. For example:
--delete ACL:"ADDomain Users":ALLOWED/OI|CI/READ
16.1.6.4. Enabling Users to Share Directories on a Samba Server
On a Samba server, you can configure that users can share directories without root permissions.
16.1.6.4.1. Enabling the User Shares Feature
Before users can share directories, the administrator must enable user shares in Samba. For example,
to enable only members of the local example group to create user shares:
Enabling User Shares
1. Create the local example group, if it does not exist:
~]# groupadd example
2. Prepare the directory for Samba to store the user share definitions and set its permissions
properly. For example:
a. Create the directory:
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~]# mkdir -p /var/lib/samba/usershares/
b. Set write permissions for the example group:
~]# chgrp example /var/lib/samba/usershares/
~]# chmod 1770 /var/lib/samba/usershares/
Set the sticky bit to prevent users to rename or delete files stored by other users in this
directory.
3. Edit the /etc/samba/smb.conf file and add the following to the[global] section:
a. Set the path to the directory you configured to store the user share definitions. For
example:
usershare path = /var/lib/samba/usershares/
b. Set how many user shares Samba allows to be created on this server. For example:
usershare max shares = 100
If you use the default of 0 for theusershare max shares parameter, user shares are
disabled.
c. Optionally, set a list of absolute directory paths. For example, to configure that Samba
only allows to share subdirectories of the /data and /srv directory to be shared, set:
usershare prefix allow list = /data /srv
For a list of further user share-related parameters you can set, see the USERSHARES
section in the smb.conf(5) man page.
~]# testparm
Users are now able to create user shares. For details, see Section 16.1.6.4.2, “Adding a User Share”
.
16.1.6.4.2. Adding a User Share
After you configured Samba according to Section 16.1.6.4.1, “Enabling the User Shares Feature”, users
can share directories on the Samba server without root permissions by running thenet usershare add
command.
Synopsis of the net usershare add command:net usershare
addshare_namepathcommentACLsguest_ok=y|n
IMPORTANT
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IMPORTANT
If you set ACLs when you create a user share, you must specify the comment
parameter prior to the ACLs. To set an empty comment, use an empty string in double
quotes.
Note that users can only enable guest access on a user share, if the administrator set usershare allow
guests = yes in the[global] section in the/etc/samba/smb.conf file.
Example 16.7. Adding a User Share
A user wants to share the /srv/samba/ directory on a Samba server. The share should be named
example, have no comment set, and should be accessible by guest users. Additionally, the share
permissions should be set to full access for the ADDomain Users group and read permissions for
other users. To add this share, run as the user:
~]$ net usershare add example /srv/samba/ ""
"ADDomain Users":F,Everyone:R guest_ok=yes
16.1.6.4.3. Updating Settings of a User Share
If you want to update settings of a user share, override the share by using the net usershare add
command with the same share name and the new settings. See Section 16.1.6.4.2, “Adding a User
Share”.
16.1.6.4.4. Displaying Information About Existing User Shares
Users can enter the net usershare info command on a Samba server to display user shares and their
settings.
To display all user shares created by any user:
~]$ net usershare info -l
[share_1]
path=/srv/samba/
comment=
usershare_acl=Everyone:R,host_nameuser:F,
guest_ok=y
...
To list only shares created by the user who runs the command, omit the -l parameter.
To display only the information about specific shares, pass the share name or wild cards to the
command. For example, to display the information about shares whose name starts with share_:
~]$ net usershare info -l share*_
16.1.6.4.5. Listing User Shares
If you want to list only the available user shares without their settings on a Samba server, use the net
usershare list command.
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To list the shares created by any user:
~]$ net usershare list -l
share_1
share_2
...
To list only shares created by the user who runs the command, omit the -l parameter.
To list only specific shares, pass the share name or wild cards to the command. For example, to list
only shares whose name starts with share_:
~]$ net usershare list -l share_*
16.1.6.4.6. Deleting a User Share
To delete a user share, enter as the user who created the share or as the root user:
~]$ net usershare delete share_name
16.1.6.5. Enabling Guest Access to a Share
In certain situations, you want to share a directory to which users can connect without authentication.
To configure this, enable guest access on a share.
WARNING
Shares that do not require authentication can be a security risk.
If guest access is enabled on a share, Samba maps guest connections to the operating system
account set in the guest account parameter. Guest users can access these files if at least one of the
following conditions is satisfied:
The account is listed in file system ACLs
The POSIX permissions for other users allow it
Example 16.8. Guest Share Permissions
If you configured Samba to map the guest account to nobody, which is the default, the ACLs in the
following example:
Allow guest users to read file1.txt
Allow guest users to read and modify file2.txt.
Prevent guest users to read or modify file3.txt

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-rw-r--r--. 1 root root 1024 1. Sep 10:00 file1.txt
-rw-r-----. 1 nobody root 1024 1. Sep 10:00 file2.txt
-rw-r-----. 1 root root 1024 1. Sep 10:00 file3.txt
For example, to enable guest access for the existing [example] share:
Setting up a Guest Share
1. Edit the /etc/samba/smb.conf file:
a. If this is the first guest share you set up on this server:
i. Set map to guest = Bad User in the[global] section:
[global]
...
map to guest = Bad User
With this setting, Samba rejects login attempts that use an incorrect password unless
the user name does not exist. If the specified user name does not exist and guest
access is enabled on a share, Samba treats the connection as a guest log in.
ii. By default, Samba maps the guest account to the nobody account on Red Hat
Enterprise Linux. Optionally, you can set a different account. For example:
[global]
...
guest account = user_name
The account set in this parameter must exist locally on the Samba server. For security
reasons, Red Hat recommends using an account that does not have a valid shell
assigned.
b. Add the guest ok = yes setting to the[example] section:
[example]
...
guest ok = yes
~]# testparm
16.1.7. Setting up a Samba Print Server
If you set up Samba as a print server, clients in your network can use Samba to print. Additionally,
Windows clients can, if configured, download the driver from the Samba server.
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Before you can share a printer, set up Samba:
Section 16.1.4, “Setting up Samba as a Standalone Server”
Section 16.1.5, “Setting up Samba as a Domain Member”
16.1.7.1. The Samba spoolssd Service
The Samba spoolssd is a service that is integrated into thesmbd service. Enable spoolssd in the
Samba configuration to significantly increase the performance on print servers with a high number of
jobs or printers.
Without spoolssd, Samba forks thesmbd process and initializes theprintcap cache for each print
job. In case of a large number of printers, the smbd service can become unresponsive for multiple
seconds while the cache is initialized. The spoolssd service enables you to start pre-forkedsmbd
processes that are processing print jobs without any delays. The main spoolssd smbd process uses a
low amount of memory, and forks and terminates child processes.
To enable the spoolssd service:
Enabling the spoolssd Service
a. Add the following parameters:
rpc_server:spoolss = external
rpc_daemon:spoolssd = fork
b. Optionally, you can set the following parameters:
Parameter Default Description
spoolssd:prefork_min_childre
n
5 Minimum number of child
processes
spoolssd:prefork_max_childre
n
25 Maximum number of child
processes
spoolssd:prefork_spawn_rate 5 Samba forks the number of
new child processes set in this
parameter, up to the value
set in
spoolssd:prefork_max_c
hildren, if a new connection
is established
spoolssd:prefork_max_allowe
d_clients
100 Number of clients, a child
process serves
spoolssd:prefork_child_min_lif
e
60 Minimum lifetime of a child
process in seconds. 60
seconds is the minimum.
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~]# testparm
After you restarted the service, Samba automatically starts smbd child processes:
~]# ps axf
...
30903 smbd
30912 _ smbd
30913 _ smbd
30914 _ smbd
30915 _ smbd
...
16.1.7.2. Enabling Print Server Support in Samba
To enable the print server support:
Enabling Print Server Support in Samba
1. On the Samba server, set up CUPS and add the printer to the CUPS back end. For details, see
Section 16.3, “Print Settings”.
NOTE
Samba can only forward the print jobs to CUPS if CUPS is installed locally on
the Samba print server.
a. If you want to enable the spoolssd service, add the following parameters to the[global]
section:
rpc_server:spoolss = external
rpc_daemon:spoolssd = fork
For further details, see Section 16.1.7.1, “The Sambaspoolssd Service”.
b. To configure the printing back end, add the [printers] section:
[printers]
comment = All Printers
path = /var/tmp/
printable = yes
create mask = 0600
IMPORTANT
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IMPORTANT
The printers share name is hard-coded and cannot be changed.
~]# testparm
After restarting the service, Samba automatically shares all printers that are configured in the CUPS
back end. If you want to manually share only specific printers, see Section 16.1.7.3, “Manually Sharing
Specific Printers”.
16.1.7.3. Manually Sharing Specific Printers
If you configured Samba as a print server, by default, Samba shares all printers that are configured in
the CUPS back end. To share only specific printers:
Manually Sharing a Specific Printer
a. In the [global] section, disable automatic printer sharing by setting:
load printers = no
b. Add a section for each printer you want to share. For example, to share the printer named
example in the CUPS back end asExample-Printer in Samba, add the following section:
[Example-Printer]
path = /var/tmp/
printable = yes
printer name = example
You do not need individual spool directories for each printer. You can set the same spool
directory in the path parameter for the printer as you set in the[printers] section.
~]# testparm
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16.1.7.4. Setting up Automatic Printer Driver Downloads for Windows Clients
If you are running a Samba print server for Windows clients, you can upload drivers and preconfigure
printers. If a user connects to a printer, Windows automatically downloads and installs the driver
locally on the client. The user does not require local administrator permissions for the installation.
Additionally, Windows applies preconfigured driver settings, such as the number of trays.
NOTE
Before setting up automatic printer driver download, must configure Samba as a print
server and share a printer. For details, see Section 16.1.7, “Setting up a Samba Print
Server”.
16.1.7.4.1. Basic Information about Printer Drivers
This section provides general information about printer drivers.
Supported Driver Model Version
Samba only supports the printer driver model version 3 which is supported in Windows 2000 and
later, and Windows Server 2000 and later. Samba does not support the driver model version 4,
introduced in Windows 8 and Windows Server 2012. However, these and later Windows versions also
support version 3 drivers.
Package-aware Drivers
Samba does not support package-aware drivers.
Preparing a Printer Driver for Being Uploaded
Before you can upload a driver to a Samba print server:
Unpack the driver if it is provided in a compressed format.
Some drivers require to start a setup application that installs the driver locally on a Windows
host. In certain situations, the installer extracts the individual files into the operating
system’s temporary folder during the setup runs. To use the driver files for uploading:
a. Start the installer.
b. Copy the files from the temporary folder to a new location.
c. Cancel the installation.
Ask your printer manufacturer for drivers that support uploading to a print server.
Providing 32-bit and 64-bit Drivers for a Printer to a Client
To provide the driver for a printer for both 32-bit and 64-bit Windows clients, you must upload a
driver with exactly the same name for both architectures. For example, if you are uploading the 32-
bit driver named Example PostScript and the 64-bit driver namedExample PostScript (v1.0), the
names do not match. Consequently, you can only assign one of the drivers to a printer and the driver
will not be available for both architectures.
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16.1.7.4.2. Enabling Users to Upload and Preconfigure Drivers
To be able to upload and preconfigure printer drivers, a user or a group needs to have the
SePrintOperatorPrivilege privilege granted. A user must be added into theprintadmin group. Red
Hat Enterprise Linux creates this group automatically when you install the samba package. The
printadmin group gets assigned the lowest available dynamic system GID that is lower than 1000.
To grant the SePrintOperatorPrivilege privilege to theprintadmin group:
~]# net rpc rights grant "printadmin" SePrintOperatorPrivilege
NOTE
In a domain environment, grant SePrintOperatorPrivilege to a domain group. This
enables you to centrally manage the privilege by updating a user’s group membership.
To list all users and groups having SePrintOperatorPrivilege granted:
~]# net rpc rights list privileges SePrintOperatorPrivilege
Enter administrator's password:
SePrintOperatorPrivilege:
BUILTINAdministrators
DOMAINprintadmin
16.1.7.4.3. Setting up theprint$ Share
Windows operating systems download printer drivers from a share named print$ from a print server.
This share name is hard-coded in Windows and cannot be changed.
To share the /var/lib/samba/drivers/ directory asprint$, and enable members of the localprintadmin
group to upload printer drivers:
Setting up theprint$ Share
1. Add the [print$] section to the/etc/samba/smb.conf file:
[print$]
path = /var/lib/samba/drivers/
read only = no
write list = @printadmin
force group = @printadmin
create mask = 0664
directory mask = 2775
Using these settings:
Only members of the printadmin group can upload printer drivers to the share.
The group of new created files and directories will be set to printadmin.
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The permissions of new files will be set to 664.
The permissions of new directories will be set to 2775.
2. To upload only 64-bit drivers for a printer, include this setting in the [global] section in the
/etc/samba/smb.conf file:
spoolss: architecture = Windows x64
Without this setting, Windows only displays drivers for which you have uploaded at least the
32-bit version.
~]# testparm
4. Reload the Samba configuration
5. Create the printadmin group if it does not exists:
~]# groupadd printadmin
6. Grant the SePrintOperatorPrivilege privilege to theprintadmin group.
~]# net rpc rights grant "printadmin" SePrintOperatorPrivilege
For further details, see Section 16.1.7.4.2, “Enabling Users to Upload and Preconfigure
Drivers”.
~]# semanage fcontext -a -t samba_share_t "/var/lib/samba/drivers(/.*)?"
~]# restorecon -Rv /var/lib/samba/drivers/
8. Set the permissions on the /var/lib/samba/drivers/ directory:
If you use POSIX ACLs, set:
~]# chgrp -R "printadmin" /var/lib/samba/drivers/
~]# chmod -R 2775 /var/lib/samba/drivers/
If you use Windows ACLs, set:
Principal Access Apply to
CREATOR OWNER Full control Subfolders and files only
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Authenticated Users Read & execute, List folder
contents, Read
This folder, subfolders and
files
printadmin Full control This folder, subfolders and
files
Principal Access Apply to
For details about setting ACLs on Windows, see your Windows documentation.
16.1.7.4.4. Creating a GPO to Enable Clients to Trust the Samba Print Server
For security reasons, recent Windows operating systems prevent clients from downloading non-
package-aware printer drivers from an untrusted server. If your print server is a member in an AD, you
can create a Group Policy Object (GPO) in your domain to trust the Samba server.
To create GPOs, the Windows computer you are using must have the Windows Remote Server
Administration Tools (RSAT) installed. For details, see your Windows documentation.
Creating a GPO to Enable Clients to Trust the Samba Print Server
1. Log into a Windows computer using an account that is allowed to edit group policies, such as
the AD domain Administrator user.
2. Open the Group Policy Management Console.
3. Right-click to your AD domain and select Create a GPO in this domain, and Link it here
4. Enter a name for the GPO, such as Legacy printer Driver Policy and clickOK. The new GPO
will be displayed under the domain entry.
5. Right-click to the newly-created GPO and select Edit to open theGroup Policy Management
Editor.
6. Navigate to Computer Configuration →Policies →Administrative Templates →Printers.
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7. On the right side of the window, double-click Point and Print Restriction to edit the policy:
a. Enable the policy and set the following options:
i. Select Users can only point and print to these servers and enter the fully-qualified
domain name (FQDN) of the Samba print server to the field next to this option.
ii. In both check boxes under Security Prompts, selectDo not show warning or
elevation prompt.
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b. Click OK.
8. Double-click Package Point and Print - Approved servers to edit the policy:
a. Enable the policy and click the Show button.
b. Enter the FQDN of the Samba print server.
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c. Close both the Show Contents and policy properties window by clickingOK.
9. Close the Group Policy Management Editor.
10. Close the Group Policy Management Console.
After the Windows domain members applied the group policy, printer drivers are automatically
downloaded from the Samba server when a user connects to a printer.
For further details about using group policies, see your Windows documentation.
16.1.7.4.5. Uploading Drivers and Preconfiguring Printers
Use the Print Management application on a Windows client to upload drivers and preconfigure
printers hosted on the Samba print server. For further details, see your Windows documentation.
16.1.8. Tuning the Performance of a Samba Server
This section describes what settings can improve the performance of Samba in certain situations, and
which settings can have a negative performance impact.
16.1.8.1. Setting the SMB Protocol Version
Each new SMB version adds features and improves the performance of the protocol. The recent
Windows and Windows Server operating systems always supports the latest protocol version. If
Samba also uses the latest protocol version, Windows clients connecting to Samba benefit from the
performance improvements. In Samba, the default value of the server max protocol is set to the
latest supported stable SMB protocol version.
To always have the latest stable SMB protocol version enabled, do not set the server max protocol
parameter. If you set the parameter manually, you will need to modify the setting with each new
version of the SMB protocol, to have the latest protocol version enabled.
To unset, remove the server max protocol parameter from the[global] section in the
/etc/samba/smb.conf file.
16.1.8.2. Tuning Shares with Directories That Contain a Large Number of Files
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To improve the performance of shares that contain directories with more than 100.000 files:
Tuning Shares with Directories That Contain a Large Number of Files
1. Rename all files on the share to lowercase.
NOTE
Using the settings in this procedure, files with names other than in lowercase
will no longer be displayed.
2. Set the following parameters in the share’s section:
case sensitive = true
default case = lower
preserve case = no
short preserve case = no
For details about the parameters, see their descriptions in the smb.conf(5) man page.
After you applied these settings, the names of all newly created files on this share use lowercase.
Because of these settings, Samba no longer needs to scan the directory for uppercase and lowercase,
which improves the performance.
16.1.8.3. Settings That Can Have a Negative Performance Impact
By default, the kernel in Red Hat Enterprise Linux is tuned for high network performance. For
example, the kernel uses an auto-tuning mechanism for buffer sizes. Setting the socket options
parameter in the /etc/samba/smb.conf file overrides these kernel settings. As a result, setting this
parameter decreases the Samba network performance in most cases.
To use the optimized settings from the Kernel, remove the socket options parameter from the
[global] section in the/etc/samba/smb.conf.
16.1.9. Frequently Used Samba Command-line Utilities
This section describes frequently used commands when working with a Samba server.
16.1.9.1. Using the net Utility
The net utility enables you to perform several administration tasks on a Samba server. This section
describes the most frequently used subcommands of the net utility.
For further details, see the net(8) man page.
16.1.9.1.1. Using the net ads join and net rpc join Commands
Using the join subcommand of thenet utility, you can join Samba to an AD or NT4 domain. To join the
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Using the join subcommand of thenet utility, you can join Samba to an AD or NT4 domain. To join the
domain, you must create the /etc/samba/smb.conf file manually, and optionally update additional
configurations, such as PAM.
IMPORTANT
Red Hat recommends using the realm utility to join a domain. Therealm utility
automatically updates all involved configuration files. For details, see Section 16.1.5.1,
“Joining a Domain”.
To join a domain using the net command:
Joining a Domain Using thenet Command
1. Manually create the /etc/samba/smb.conf file with the following settings:
For an AD domain member:
[global]
workgroup = domain_name
security = ads
passdb backend = tdbsam
realm = AD_REALM
For an NT4 domain member:
[global]
workgroup = domain_name
security = user
passdb backend = tdbsam
2. Add an ID mapping configuration for the * default domain and for the domain you want to join
to the [global] section in the/etc/samba/smb.conf. For details, seeSection 16.1.5.3,
“Understanding ID Mapping”.
~]# testparm
4. Join the domain as the domain administrator:
To join an AD domain:
~]# net ads join -U "DOMAINpass:quotes[administrator]"
To join an NT4 domain:
~]# net rpc join -U "DOMAINpass:quotes[administrator]"
5. Append the winbind source to thepasswd and group database entry in the
/etc/nsswitch.conf file:
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passwd: files winbind
group: files winbind
6. Enable and start the winbind service:
~]# systemctl enable winbind
~]# systemctl start winbind
7. Optionally, configure PAM using the authconf utility.
For details, see the Using Pluggable Authentication Modules (PAM)section in theRed Hat
System-Level Authentication Guide.
8. Optionally for AD environments, configure the Kerberos client.
For details, see the Configuring a Kerberos Clientsection in theRed Hat System-Level
Authentication Guide.
16.1.9.1.2. Using the net rpc rights Command
In Windows, you can assign privileges to accounts and groups to perform special operations, such as
setting ACLs on a share or upload printer drivers. On a Samba server, you can use the net rpc rights
command to manage privileges.
Listing Privileges
To list all available privileges and their owners, use the net rpc rights list command. For example:
net rpc rights list -U "DOMAINpass:attributes[{blank}]administrator"
Enter DOMAINpass:attributes[{blank}]administrator's password:
SeMachineAccountPrivilege Add machines to domain
SeTakeOwnershipPrivilege Take ownership of files or other objects
SeBackupPrivilege Back up files and directories
SeRestorePrivilege Restore files and directories
SeRemoteShutdownPrivilege Force shutdown from a remote system
SePrintOperatorPrivilege Manage printers
SeAddUsersPrivilege Add users and groups to the domain
SeDiskOperatorPrivilege Manage disk shares
SeSecurityPrivilege System security
Granting Privileges
To grant a privilege to an account or group, use the net rpc rights grant command.
For example, grant the SePrintOperatorPrivilege privilege to theDOMAINprintadmin group:
~]# net rpc rights grant "DOMAINprintadmin" SePrintOperatorPrivilege
Revoking Privileges
To revoke a privilege from an account or group, use the net rpc rights revoke.
For example, to revoke the SePrintOperatorPrivilege privilege from theDOMAINprintadmin group:
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~]# net rpc rights remoke "DOMAINprintadmin" SePrintOperatorPrivilege
Successfully revoked rights.
16.1.9.1.3. Using thenet rpc share Command
The net rpc share command provides the capability to list, add, and remove shares on a local or
remote Samba or Windows server.
Listing Shares
To list the shares on an SMB server, use the net rpc share list command. Optionally, pass the-S
server_name parameter to the command to list the shares of a remote server. For example:
~]# net rpc share list -U "DOMAINadministrator" -S example
IPC$
share_1
share_2
...
NOTE
Shares hosted on a Samba server that have browseable = no set in their section in the
/etc/samba/smb.conf file are not displayed in the output.
Adding a Share
The net rpc share add command enables you to add a share to an SMB server.
For example, to add a share named example on a remote Windows server that shares theC:example
directory:
~]# net rpc share add example="C:example" -U "DOMAINadministrator" -S server
NOTE
You must omit the trailing backslash in the path when specifying a Windows directory
name.
To use the command to add a share to a Samba server:
The user specified in the -U parameter must have theSeDiskOperatorPrivilege privilege
granted.
You must write a script that adds a share section to the /etc/samba/smb.conf file and reloads
Samba. The script must be set in the add share command parameter in the[global] section in
/etc/samba/smb.conf. For further details, see theadd share command description in the
Removing a Share
The net rpc share delete command enables you to remove a share from an SMB server.
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For example, to remove the share named example from a remote Windows server:
~]# net rpc share delete example -U "DOMAINadministrator" -S server
To use the command to remove a share from a Samba server:
The user specified in the -U parameter must have theSeDiskOperatorPrivilege privilege
granted.
You must write a script that removes the share’s section from the /etc/samba/smb.conf file
and reloads Samba. The script must be set in the delete share command parameter in the
[global] section in/etc/samba/smb.conf. For further details, see thedelete share command
description in the smb.conf(5) man page.
16.1.9.1.4. Using the net user Command
The net user command enables you to perform the following actions on an AD DC or NT4 PDC:
List all user accounts
Add users
Remove Users
NOTE
Specifying a connection method, such as ads for AD domains orrpc for NT4 domains,
is only required when you list domain user accounts. Other user-related subcommands
can auto-detect the connection method.
Pass the -U user_name parameter to the command to specify a user that is allowed to perform the
requested action.
Listing Domain User Accounts
To list all users in an AD domain:
~]# net ads user -U "DOMAINadministrator"
To list all users in an NT4 domain:
~]# net rpc user -U "DOMAINadministrator"
Adding a User Account to the Domain
On a Samba domain member, you can use the net user add command to add a user account to the
domain.
For example, add the user account to the domain:
Adding a User Account to the Domain
1. Add the account:
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~]# net user add user password -U "DOMAINadministrator"
User user added
2. Optionally, use the remote procedure call (RPC) shell to enable the account on the AD DC or
NT4 PDC. For example:
~]# net rpc shell -U DOMAINadministrator -S DC_or_PDC_name
Talking to domain DOMAIN (S-1-5-21-1424831554-512457234-5642315751)
net rpc> user edit disabled user no
Set user's disabled flag from [yes] to [no]
net rpc> exit
Deleting a User Account from the Domain
On a Samba domain member, you can use the net user delete command to remove a user account
from the domain.
For example, to remove the user account from the domain:
~]# net user delete user -U "DOMAINadministrator"
User user deleted
16.1.9.1.5. Using thenet usershare Command
See Section 16.1.6.4, “Enabling Users to Share Directories on a Samba Server”
.
16.1.9.2. Using the rpcclient Utility
The rpcclient utility enables you to manually execute client-side Microsoft Remote Procedure Call
(MS-RPC) functions on a local or remote SMB server. However, most of the features are integrated
into separate utilities provided by Samba. Use rpcclient only for testing MS-PRC functions.
For example, you can use the utility to:
Manage the printer Spool Subsystem (SPOOLSS).
Example 16.9. Assigning a Driver to a Printer
~]# rpcclient server_name -U "DOMAINpass:quotes[administrator]"
-c 'setdriver "printer_name" "driver_name"'
Enter DOMAINpass:quotes[administrator]s password:
Successfully set printer_name to driver driver_name.
Retrieve information about an SMB server.
Example 16.10. Listing all File Shares and Shared Printers
~]# rpcclient server_name -U "DOMAINpass:quotes[administrator]" -c
'netshareenum'
netname: Example_Share
remark:
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path: C:srvsambaexample_share
password:
netname: Example_Printer
remark:
path: C:varspoolsamba
password:
Perform actions using the Security Account Manager Remote (SAMR) protocol.
Example 16.11. Listing Users on an SMB Server
~]# rpcclient server_name -U "DOMAINpass:quotes[administrator]" -c
'enumdomusers'
user:[user1] rid:[0x3e8]
user:[user2] rid:[0x3e9]
If you run the command against a standalone server or a domain member, it lists the users in
the local database. Running the command against an AD DC or NT4 PDC lists the domain
users.
For a complete list of supported subcommands, see COMMANDS section in therpcclient(1) man
page.
16.1.9.3. Using the samba-regedit Application
Certain settings, such as printer configurations, are stored in the registry on the Samba server. You
can use the ncurses-based samba-regedit application to edit the registry of a Samba server.
To start the application, enter:
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~]# samba-regedit
Use the following keys:
Cursor up and cursor down: Navigate through the registry tree and the values.
Enter: Opens a key or edits a value.
Tab: Switches between theKey and Value pane.
Ctrl+C: Closes the application.
16.1.9.4. Using the smbcacls Utility
See Section 16.1.6.3, “Managing ACLs on an SMB Share Usingsmbcacls”.
16.1.9.5. Using the smbclient Utility
The smbclient utility enables you to access file shares on an SMB server, similarly to a command-line
FTP client. You can use it, for example, to upload and download files to and from a share.
For example, to authenticate to the example share hosted onserver using theDOMAINuser account:
~]# smbclient -U "DOMAINuser" //server/example
Enter domainuser's password:
Domain=[SERVER] OS=[Windows 6.1] Server=[Samba 4.6.2]
smb: >
After smbclient connected successfully to the share, the utility enters the interactive mode and
shows the following prompt:
smb: >
To display all available commands in the interactive shell, enter:
smb: > help
To display the help for a specific command, enter:
smb: > help command_name
For further details and descriptions of the commands available in the interactive shell, see the
smbclient(1) man page.
16.1.9.5.1. Using smbclient in Interactive Mode
If you use smbclient without the-c parameter, the utility enters the interactive mode.
The following procedure shows how to connect to an SMB share and download a file from a
subdirectory:
Downloading a File from an SMB Share Usingsmbclient
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1. Connect to the share:
~]# smbclient -U "DOMAINpass:quotes[user_name]" //server_name/share_name
2. Change into the /example/ directory:
smb: > cd /example/
3. List the files in the directory:
smb: example> ls
. D 0 Mon Sep 1 10:00:00 2017
.. D 0 Mon Sep 1 10:00:00 2017
example.txt N 1048576 Mon Sep 1 10:00:00 2017
9950208 blocks of size 1024. 8247144 blocks available
4. Download the example.txt file:
smb: example> get example.txt
getting file directorysubdirectoryexample.txt of size 1048576 as example.txt
(511975,0 KiloBytes/sec) (average 170666,7 KiloBytes/sec)
5. Disconnect from the share:
smb: example> exit
16.1.9.5.2. Using smbclient in Scripting Mode
If you pass the -c commands parameter tosmbclient, you can automatically execute the commands
on the remote SMB share. This enables you to use smbclient in scripts.
The following command shows how to connect to an SMB share and download a file from a
subdirectory:
~]# smbclient -U DOMAINpass:quotes[user_name] //server_name/share_name
-c "cd /example/ ; get example.txt ; exit"
16.1.9.6. Using the smbcontrol Utility
The smbcontrol utility enables you to send command messages to thesmbd, nmbd, winbindd, or all
of these services. These control messages instruct the service, for example, to reload its
configuration.
Example 16.12. Reloading the Configuration of thesmbd, nmbd, and winbindd Service
For example, to reload the configuration of the smbd, nmbd, winbindd, send thereload-config
message-type to the all destination:
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For further details and a list of available command message types, see the smbcontrol(1) man page.
16.1.9.7. Using the smbpasswd Utility
The smbpasswd utility manages user accounts and passwords in the local Samba database.
If you run the command as a user, smbpasswd changes the Samba password of the user. For example:
[user@server ~]$ smbpasswd
New SMB password:
Retype new SMB password:
If you run smbpasswd as theroot user, you can use the utility, for example, to:
Create a new user:
[root@server ~]# smbpasswd -a user_name
New SMB password:
Retype new SMB password:
Added user user_name.
NOTE
Before you can add a user to the Samba database, you must create the account
in the local operating system. See Section 4.3.1, “Adding a New User”
Enable a Samba user:
[root@server ~]# smbpasswd -e user_name
Enabled user user_name.
Disable a Samba user:
[root@server ~]# smbpasswd -x user_name
Disabled user user_name.
Delete a user:
[root@server ~]# smbpasswd -x user_name
Deleted user user_name.
For further details, see the smbpasswd(8) man page.
16.1.9.8. Using the smbstatus Utility
The smbstatus utility reports on:
Connections per PID of each smbd daemon to the Samba server. This report includes the user
name, primary group, SMB protocol version, encryption, and signing information.
Connections per Samba share. This report includes the PID of the smbd daemon, the IP of the
connecting machine, the time stamp when the connection was established, encryption, and
signing information.
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A list of locked files. The report entries include further details, such as opportunistic lock
(oplock) types
Example 16.13. Output of thesmbstatus Utility
~]# smbstatus
Samba version 4.6.2
PID Username Group Machine Protocol Version Encryption Signing
-----------------------------------------------------------------------------------------------------------------------
------
963 DOMAINadministrator DOMAINdomain users client-pc (ipv4:192.0.2.1:57786) SMB3_02
- AES-128-CMAC
Service pid Machine Connected at Encryption Signing:
-------------------------------------------------------------------------------
example 969 192.0.2.1 Mo Sep 1 10:00:00 2017 CEST - AES-128-CMAC
Locked files:
Pid Uid DenyMode Access R/W Oplock SharePath Name Time
------------------------------------------------------------------------------------------------------------
969 10000 DENY_WRITE 0x120089 RDONLY LEASE(RWH) /srv/samba/example file.txt Mon
Sep 1 10:00:00 2017
For further details, see the smbstatus(1) man page.
16.1.9.9. Using the smbtar Utility
The smbtar utility backs up the content of an SMB share or a subdirectory of it and stores the content
in a tar archive. Alternatively, you can write the content to a tape device.
For example, to back up the content of the demo directory on the//server/example/ share and store
the content in the /root/example.tar archive:
~]# smbtar -s server -x example -u user_name -p password -t /root/example.tar
For further details, see the smbtar(1) man page.
16.1.9.10. Using the testparm Utility
See Section 16.1.2, “Verifying thesmb.conf File by Using thetestparm Utility”.
16.1.9.11. Using the wbinfo Utility
The wbinfo utility queries and returns information created and used by the
winbindd service.
NOTE
The winbindd service must be configured and running to usewbinfo.
You can use wbinfo, for example, to:
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List domain users:
~]# wbinfo -u
ADadministrator
ADguest
...
List domain groups:
~]# wbinfo -g
ADdomain computers
ADdomain admins
ADdomain users
...
Display the SID of a user:
~]# wbinfo --name-to-sid="ADadministrator"
S-1-5-21-1762709870-351891212-3141221786-500 SID_USER (1)
Display information about domains and trusts:
~]# wbinfo --trusted-domains --verbose
Domain Name DNS Domain Trust Type Transitive In Out
BUILTIN None Yes Yes Yes
server None Yes Yes Yes
DOMAIN1 domain1.example.com None Yes Yes Yes
DOMAIN2 domain2.example.com External No Yes Yes
For further details, see the wbinfo(1) man page.
The Red Hat Samba packages include manual pages for all Samba commands and
configuration files the package installs. For example, to display the man page of the
/etc/samba/smb.conf file that explains all configuration parameters you can set in this file:
~]# man 5 smb.conf
/usr/share/docs/samba-version/: Contains general documentation, example scripts, and
LDAP schema files, provided by the Samba project.
Red Hat Cluster Storage Administration Guide
: Provides information about setting up Samba
and the Clustered Trivial Database (CDTB) to share directories stored on an GlusterFS
volume.
The An active/active Samba Server in a Red Hat High Availability Clusterchapter in theRed
Hat Enterprise Linux High Availability Add-on Administration guide describes how to up a
Samba high-availability installation.
For details about mounting an SMB share on Red Hat Enterprise Linux, see the corresponding
section in the Red Hat Storage Administration Guide.
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16.2. FTP
The File Transfer Protocol(FTP) is one of the oldest and most commonly used protocols found on the
Internet today. Its purpose is to reliably transfer files between computer hosts on a network without
requiring the user to log directly in to the remote host or to have knowledge of how to use the remote
system. It allows users to access files on remote systems using a standard set of simple commands.
This section outlines the basics of the FTP protocol and introducesvsftpd, which is the preferredFTP
server in Red Hat Enterprise Linux.
16.2.1. The File Transfer Protocol
FTP uses a client-server architecture to transfer files using the TCP network protocol. BecauseFTP is
a rather old protocol, it uses unencrypted user name and password authentication. For this reason, it
is considered an insecure protocol and should not be used unless absolutely necessary. However,
because FTP is so prevalent on the Internet, it is often required for sharing files to the public. System
administrators, therefore, should be aware of FTP's unique characteristics.
This section describes how to configure vsftpd to establish connections secured byTLS and how to
secure an FTP server with the help ofSELinux. A good substitute forFTP is sftp from the OpenSSH
suite of tools. For information about configuring OpenSSH and about theSSH protocol in general,
refer to Chapter 12,OpenSSH.
Unlike most protocols used on the Internet, FTP requires multiple network ports to work properly.
When an FTP client application initiates a connection to anFTP server, it opens port 21 on the server
— known as the command port. This port is used to issue all commands to the server. Any data
requested from the server is returned to the client via a data port. The port number for data
connections, and the way in which data connections are initialized, vary depending upon whether the
client requests the data in active or passive mode.
The following defines these modes:
active mode
Active mode is the original method used by the FTP protocol for transferring data to the client
application. When an active-mode data transfer is initiated by the FTP client, the server opens a
connection from port 20 on the server to the IP address and a random, unprivileged port (greater
than 1024) specified by the client. This arrangement means that the client machine must be
allowed to accept connections over any port above 1024. With the growth of insecure networks,
such as the Internet, the use of firewalls for protecting client machines is now prevalent. Because
these client-side firewalls often deny incoming connections from active-mode FTP servers,
passive mode was devised.
passive mode
Passive mode, like active mode, is initiated by the FTP client application. When requesting data
from the server, the FTP client indicates it wants to access the data in passive mode and the server
provides the IP address and a random, unprivileged port (greater than 1024) on the server. The
client then connects to that port on the server to download the requested information.
While passive mode does resolve issues for client-side firewall interference with data connections,
it can complicate administration of the server-side firewall. You can reduce the number of open
ports on a server by limiting the range of unprivileged ports on the FTP server. This also simplifies
the process of configuring firewall rules for the server.
16.2.2. The vsftpd Server
The Very Secure FTP Daemon(vsftpd) is designed from the ground up to be fast, stable, and, most
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The Very Secure FTP Daemon(vsftpd) is designed from the ground up to be fast, stable, and, most
importantly, secure. vsftpd is the only stand-aloneFTP server distributed with Red Hat Enterprise
Linux, due to its ability to handle large numbers of connections efficiently and securely.
The security model used by vsftpd has three primary aspects:
Strong separation of privileged and non-privileged processes— Separate processes handle
different tasks, and each of these processes runs with the minimal privileges required for the
task.
Tasks requiring elevated privileges are handled by processes with the minimal privilege
necessary — By taking advantage of compatibilities found in thelibcap library, tasks that
usually require full root privileges can be executed more safely from a less privileged process.
Most processes run in achroot jail — Whenever possible, processes are change-rooted to the
directory being shared; this directory is then considered a chroot jail. For example, if the
/var/ftp/ directory is the primary shared directory,vsftpd reassigns/var/ftp/ to the new root
directory, known as /. This disallows any potential malicious hacker activities for any
directories not contained in the new root directory.
Use of these security practices has the following effect on how vsftpd deals with requests:
The parent process runs with the least privileges required— The parent process dynamically
calculates the level of privileges it requires to minimize the level of risk. Child processes
handle direct interaction with the FTP clients and run with as close to no privileges as
possible.
All operations requiring elevated privileges are handled by a small parent process
— Much like
the Apache HTTP Server, vsftpd launches unprivileged child processes to handle incoming
connections. This allows the privileged, parent process to be as small as possible and handle
relatively few tasks.
All requests from unprivileged child processes are distrusted by the parent process—
Communication with child processes is received over a socket, and the validity of any
information from child processes is checked before being acted on.
Most interactions withFTP clients are handled by unprivileged child processes in achroot jail
— Because these child processes are unprivileged and only have access to the directory being
shared, any crashed processes only allow the attacker access to the shared files.
16.2.2.1. Starting and Stopping vsftpd
To start the vsftpd service in the current session, type the following at a shell prompt as
root:
~]# systemctl start vsftpd.service
To stop the service in the current session, type as root:
~]# systemctl stop vsftpd.service
To restart the vsftpd service, run the following command asroot:
~]# systemctl restart vsftpd.service
This command stops and immediately starts the vsftpd service, which is the most efficient way to
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make configuration changes take effect after editing the configuration file for this FTP server.
Alternatively, you can use the following command to restart the vsftpd service only if it is already
running:
~]# systemctl try-restart vsftpd.service
By default, the vsftpd service doesnot start automatically at boot time. To configure the
vsftpd
service to start at boot time, type the following at a shell prompt as root:
~]# systemctl enable vsftpd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/vsftpd.service to
/usr/lib/systemd/system/vsftpd.service.
For more information on how to manage system services in Red Hat Enterprise Linux 7, see
Chapter 10, Managing Services with systemd.
16.2.2.2. Starting Multiple Copies of vsftpd
Sometimes, one computer is used to serve multiple FTP domains. This is a technique called
multihoming. One way to multihome usingvsftpd is by running multiple copies of the daemon, each
with its own configuration file.
To do this, first assign all relevant IP addresses to network devices or alias network devices on the
system. For more information about configuring network devices, device aliases, and additional
information about network configuration scripts, see the Red Hat Enterprise Linux 7 Networking
Guide.
Next, the DNS server for theFTP domains must be configured to reference the correct machine. For
information about BIND, the DNS protocol implementation used in Red Hat Enterprise Linux, and its
configuration files, see the Red Hat Enterprise Linux 7 Networking Guide
.
For vsftpd to answer requests on differentIP addresses, multiple copies of the daemon must be
running. To facilitate launching multiple instances of the vsftpd daemon, a specialsystemd service
unit (vsftpd@.service) for launchingvsftpd as an instantiated service is supplied in thevsftpd
package.
In order to make use of this service unit, a separate vsftpd configuration file for each required
instance of the FTP server must be created and placed in the/etc/vsftpd/ directory. Note that each of
these configuration files must have a unique name (such as /etc/vsftpd/vsftpd-site-2.conf) and must
be readable and writable only by the root user.
Within each configuration file for each FTP server listening on anIPv4 network, the following
directive must be unique:
listen_address=N.N.N.N
Replace N.N.N.N with aunique IP address for theFTP site being served. If the site is usingIPv6, use
the listen_address6 directive instead.
Once there are multiple configuration files present in the /etc/vsftpd/ directory, individual instances
of the vsftpd daemon can be started by executing the following command as
root:
~]# systemctl start vsftpd@configuration-file-name.service
In the above command, replace configuration-file-name with the unique name of the requested
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In the above command, replace configuration-file-name with the unique name of the requested
server’s configuration file, such as vsftpd-site-2. Note that the configuration file’s.conf extension
should not be included in the command.
If you want to start several instances of the vsftpd daemon at once, you can make use of a systemd
target unit file (vsftpd.target), which is supplied in thevsftpd package. This systemd target causes an
independent vsftpd daemon to be launched for each availablevsftpd configuration file in the
/etc/vsftpd/ directory. Execute the following command asroot to enable the target:
~]# systemctl enable vsftpd.target
Created symlink from /etc/systemd/system/multi-user.target.wants/vsftpd.target to
/usr/lib/systemd/system/vsftpd.target.
The above command configures the systemd service manager to launch the vsftpd service (along with
the configured vsftpd server instances) at boot time. To start the service immediately, without
rebooting the system, execute the following command as root:
~]# systemctl start vsftpd.target
See Section 10.3, “Working with systemd Targets”for more information on how to use systemd
targets to manage services.
Other directives to consider altering on a per-server basis are:
anon_root
local_root
vsftpd_log_file
xferlog_file
16.2.2.3. Encrypting vsftpd Connections Using TLS
In order to counter the inherently insecure nature of FTP, which transmits user names, passwords, and
data without encryption by default, the vsftpd daemon can be configured to utilize theTLS protocol
to authenticate connections and encrypt all transfers. Note that an FTP client that supportsTLS is
needed to communicate with vsftpd withTLS enabled.
NOTE
SSL (Secure Sockets Layer) is the name of an older implementation of the security
protocol. The new versions are called TLS (Transport Layer Security). Only the newer
versions (TLS) should be used asSSL suffers from serious security vulnerabilities. The
documentation included with the vsftpd server, as well as the configuration directives
used in the vsftpd.conf file, use theSSL name when referring to security-related
matters, but TLS is supported and used by default when thessl_enable directive is set
to YES.
Set the ssl_enable configuration directive in thevsftpd.conf file toYES to turn onTLS support. The
default settings of other TLS-related directives that become automatically active when the
ssl_enable option is enabled provide for a reasonably well-configuredTLS set up. This includes,
among other things, the requirement to only use the TLS v1 protocol for all connections (the use of
the insecure SSL protocol versions is disabled by default) or forcing all non-anonymous logins to use
TLS for sending passwords and data transfers.
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Example 16.14. Configuring vsftpd to Use TLS
In this example, the configuration directives explicitly disable the older SSL versions of the
security protocol in the vsftpd.conf file:
ssl_enable=YES
ssl_tlsv1=YES
ssl_sslv2=NO
ssl_sslv3=NO
Restart the vsftpd service after you modify its configuration:
~]# systemctl restart vsftpd.service
See the vsftpd.conf(5) manual page for otherTLS-related configuration directives for fine-tuning
the use of TLS byvsftpd.
16.2.2.4. SELinux Policy for vsftpd
The SELinux policy governing the vsftpd daemon (as well as otherftpd processes), defines a
mandatory access control, which, by default, is based on least access required. In order to allow the
FTP daemon to access specific files or directories, appropriate labels need to be assigned to them.
For example, in order to be able to share files anonymously, the public_content_t label must be
assigned to the files and directories to be shared. You can do this using the chcon command asroot:
~]# chcon -R -t public_content_t /path/to/directory
In the above command, replace /path/to/directory with the path to the directory to which you want
to assign the label. Similarly, if you want to set up a directory for uploading files, you need to assign
that particular directory the public_content_rw_t label. In addition to that, the
allow_ftpd_anon_write SELinux Boolean option must be set to1. Use thesetsebool command asroot
to do that:
~]# setsebool -P allow_ftpd_anon_write=1
If you want local users to be able to access their home directories through FTP, which is the default
setting on Red Hat Enterprise Linux 7, the ftp_home_dir Boolean option needs to be set to1. Ifvsftpd
is to be allowed to run in standalone mode, which is also enabled by default on Red Hat Enterprise
Linux 7, the ftpd_is_daemon option needs to be set to1 as well.
See the ftpd_selinux(8) manual page for more information, including examples of other useful labels
and Boolean options, on how to configure the SELinux policy pertaining to FTP. Also, see theRed Hat
Enterprise Linux 7 SELinux User’s and Administrator’s Guide for more detailed information about
SELinux in general.
For more information about vsftpd, see the following resources.
16.2.3.1. Installed Documentation
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The /usr/share/doc/vsftpd-version-number/ directory — Replaceversion-number with the
installed version of the vsftpd package. This directory contains aREADME file with basic
information about the software. The TUNING file contains basic performance-tuning tips and
the SECURITY/ directory contains information about the security model employed by
vsftpd.
vsftpd-related manual pages — There are a number of manual pages for the daemon and the
configuration files. The following lists some of the more important manual pages.
Server Applications
{blank}
vsftpd(8) — Describes available command-line options forvsftpd.
Configuration Files
{blank}
vsftpd.conf(5) — Contains a detailed list of options available within the configuration
file for vsftpd.
hosts_access(5) — Describes the format and options available within theTCP wrappers
configuration files: hosts.allow and hosts.deny.
Interaction with SELinux
{blank}
ftpd_selinux(8) — Contains a description of theSELinux policy governingftpd
processes as well as an explanation of the way SELinux labels need to be assigned and
Booleans set.
16.2.3.2. Online Documentation
About vsftpd and FTP in General
{blank}
http://vsftpd.beasts.org/ — Thevsftpd project page is a great place to locate the latest
documentation and to contact the author of the software.
http://slacksite.com/other/ftp.html — This website provides a concise explanation of the
differences between active and passive-mode FTP.
Red Hat Enterprise Linux Documentation
{blank}
Red Hat Enterprise Linux 7 Networking Guide— TheNetworking Guide for Red Hat
Enterprise Linux 7 documents relevant information regarding the configuration and
administration of network interfaces, networks, and network services in this system. It
provides an introduction to the hostnamectl utility and explains how to use it to view and
set host names on the command line, both locally and remotely.
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Red Hat Enterprise Linux 7 SELinux User’s and Administrator’s Guide— TheSELinux
User’s and Administrator’s Guide for Red Hat Enterprise Linux 7 describes the basic
principles of SELinux and documents in detail how to configure and useSELinux with
various services such as the Apache HTTP Server, Postfix, PostgreSQL, or OpenShift. It
explains how to configure SELinux access permissions for system services managed by
systemd.
Red Hat Enterprise Linux 7 Security Guide— TheSecurity Guidefor Red Hat Enterprise
Linux 7 assists users and administrators in learning the processes and practices of securing
their workstations and servers against local and remote intrusion, exploitation, and
malicious activity. It also explains how to secure critical system services.
Relevant RFC Documents
{blank}
RFC 0959 — The originalRequest for Comments(RFC) of theFTP protocol from theIETF.
RFC 1123 — The smallFTP-related section extends and clarifies RFC 0959.
RFC 2228 — FTP security extensions.vsftpd implements the small subset needed to
support TLS and SSL connections.
RFC 2389 — Proposes FEAT and OPTS commands.
RFC 2428 — IPv6 support.
16.3. PRINT SETTINGS
The Print Settings tool serves for printer configuring, maintenance of printer configuration files, print
spool directories and print filters, and printer classes management.
The tool is based on the Common Unix Printing System (CUPS). If you upgraded the system from a
previous Red Hat Enterprise Linux version that used CUPS, the upgrade process preserved the
configured printers.
IMPORTANT
The cupsd.conf man page documents configuration of a CUPS server. It includes
directives for enabling SSL support. However, CUPS does not allow control of the
protocol versions used. Due to the vulnerability described in Resolution for POODLE
SSLv3.0 vulnerability (CVE-2014-3566) for components that do not allow SSLv3 to be
disabled via configuration settings, Red Hat recommends that you do not rely on this
for security. It is recommend that you use stunnel to provide a secure tunnel and
disable SSLv3. For more information on usingstunnel, see the Red Hat Enterprise
Linux 7 Security Guide.
For ad-hoc secure connections to a remote system’s Print Settings tool, use X11
forwarding over SSH as described inSection 12.4.1, “X11 Forwarding”.
NOTE
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NOTE
You can perform the same and additional operations on printers directly from the
CUPS web application or command line. To access the application, in a web browser, go
to http://localhost:631/. For CUPS manuals refer to the links on theHome tab of the
web site.
16.3.1. Starting the Print Settings Configuration Tool
With the Print Settings configuration tool you can perform various operations on existing printers
and set up new printers. You can also use CUPS directly (go to http://localhost:631/ to access the
CUPS web application).
To start the Print Settings tool from the command line, typesystem-config-printer at a shell prompt.
The Print Settings tool appears. Alternatively, if using the GNOME desktop, press the
Super key to
enter the Activities Overview, type Print Settings and then pressEnter. ThePrint Settings tool
appears. The Super key appears in a variety of guises, depending on the keyboard and other
hardware, but often as either the Windows or Command key, and typically to the left of the Spacebar.
The Print Settings window depicted inFigure 16.1, “Print Settings window” appears.
Figure 16.1. Print Settings window
16.3.2. Starting Printer Setup
Printer setup process varies depending on the printer queue type.
If you are setting up a local printer connected with USB, the printer is discovered and added
automatically. You will be prompted to confirm the packages to be installed and provide an
administrator or the root user password. Local printers connected with other port types and network
printers need to be set up manually.
Follow this procedure to start a manual printer setup:
1. Start the Print Settings tool (refer to Section 16.3.1, “Starting the Print Settings
Configuration Tool”).
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2. Go to Server →New →Printer.
3. In the Authenticate dialog box, enter an administrator orroot user password. If this is the first
time you have configured a remote printer you will be prompted to authorize an adjustment
to the firewall.
4. Select the printer connection type and provide its details in the area on the right.
16.3.3. Adding a Local Printer
Follow this procedure to add a local printer connected with other than a serial port:
1. Open the Add printer dialog (refer toSection 16.3.2, “Starting Printer Setup”
).
2. If the device does not appear automatically, select the port to which the printer is connected
in the list on the left (such as Serial Port #1 or LPT #1).
3. On the right, enter the connection properties:
for Other
URI (for example file:/dev/lp0)
for Serial Port
Baud Rate
Parity
Data Bits
Flow Control
Figure 16.2. Adding a local printer
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Figure 16.2. Adding a local printer
4. Click Forward.
5. Select the printer model. See Section 16.3.8, “Selecting the Printer Model and Finishing”for
details.
16.3.4. Adding an AppSocket/HP JetDirect printer
Follow this procedure to add an AppSocket/HP JetDirect printer:
1. Open the New Printer dialog (refer toSection 16.3.1, “Starting the Print Settings
Configuration Tool”).
2. In the list on the left, select Network Printer→AppSocket/HP JetDirect.
3. On the right, enter the connection settings:
Hostname
Printer host name or IP address.
Port Number
Printer port listening for print jobs (9100 by default).
Figure 16.3. Adding a JetDirect printer
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Figure 16.3. Adding a JetDirect printer
4. Click Forward.
details.
16.3.5. Adding an IPP Printer
An IPP printer is a printer attached to a different system on the same TCP/IP network. The system
this printer is attached to may either be running CUPS or simply configured to use IPP.
If a firewall is enabled on the printer server, then the firewall must be configured to allow incoming
TCP connections on port631. Note that the CUPS browsing protocol allows client machines to
discover shared CUPS queues automatically. To enable this, the firewall on the client machine must
be configured to allow incoming UDP packets on port631.
Follow this procedure to add an IPP printer:
1. Open the New Printer dialog (refer toSection 16.3.2, “Starting Printer Setup”
).
2. In the list of devices on the left, select Network Printer and Internet Printing Protocol (ipp)
or Internet Printing Protocol (https).
Host
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The host name of the IPP printer.
Queue
The queue name to be given to the new queue (if the box is left empty, a name based on
the device node will be used).
Figure 16.4. Adding an IPP printer
4. Click Forward to continue.
details.
16.3.6. Adding an LPD/LPR Host or Printer
Follow this procedure to add an LPD/LPR host or printer:
).
2. In the list of devices on the left, select Network Printer→LPD/LPR Host or Printer.
Host
The host name of the LPD/LPR printer or host.
Optionally, click Probe to find queues on the LPD host.
Queue
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the device node will be used).
Figure 16.5. Adding an LPD/LPR printer
details.
16.3.7. Adding a Samba (SMB) printer
Follow this procedure to add a Samba printer:
NOTE
Note that in order to add a Samba printer, you need to have the samba-client package
installed. You can do so by running, as root:
yum install samba-client
For more information on installing packages with Yum, refer to Section 9.2.4,
“Installing Packages”.
).
2. In the list on the left, select Network Printer→Windows Printer via SAMBA.
3. Enter the SMB address in the smb:// field. Use the formatcomputer name/printer share. In
328

3. Enter the SMB address in the smb:// field. Use the formatcomputer name/printer share. In
Figure 16.6, “Adding a SMB printer”, the computer name is dellbox and theprinter share is r2.
Figure 16.6. Adding a SMB printer
4. Click Browse to see the available workgroups/domains. To display only queues of a particular
host, type in the host name (NetBios name) and click Browse.
5. Select either of the options:
a. Prompt user if authentication is required: user name and password are collected from
the user when printing a document.
b. Set authentication details now: provide authentication information now so it is not
required later. In the Username field, enter the user name to access the printer. This user
must exist on the SMB system, and the user must have permission to access the printer.
The default user name is typically guest for Windows servers, ornobody for Samba
servers.
6. Enter the Password (if required) for the user specified in theUsername field.
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WARNING
Samba printer user names and passwords are stored in the printer server
as unencrypted files readable by root and the Linux Printing Daemon,lpd.
Thus, other users that have root access to the printer server can view the
user name and password you use to access the Samba printer.
Therefore, when you choose a user name and password to access a Samba
printer, it is advisable that you choose a password that is different from
what you use to access your local Red Hat Enterprise Linux system.
If there are files shared on the Samba print server, it is recommended
that they also use a password different from what is used by the print
queue.
7. Click Verify to test the connection. Upon successful verification, a dialog box appears
confirming printer share accessibility.
8. Click Forward.
details.
16.3.8. Selecting the Printer Model and Finishing
Once you have properly selected a printer connection type, the system attempts to acquire a driver.
If the process fails, you can locate or search for the driver resources manually.
Follow this procedure to provide the printer driver and finish the installation:
1. In the window displayed after the automatic driver detection has failed, select one of the
following options:
a. Select a Printer from database — the system chooses a driver based on the selected
make of your printer from the list of Makes. If your printer model is not listed, choose
Generic.
b. Provide PPD file — the system uses the providedPostScript Printer Description(PPD)
file for installation. A PPD file may also be delivered with your printer as being normally
provided by the manufacturer. If the PPD file is available, you can choose this option and
use the browser bar below the option description to select the PPD file.
c. Search for a printer driver to download — enter the make and model of your printer into
the Make and model field to search on OpenPrinting.org for the appropriate packages.
Figure 16.7. Selecting a printer brand

330

Figure 16.7. Selecting a printer brand
2. Depending on your previous choice provide details in the area displayed below:
Printer brand for the Select printer from database option.
PPD file location for the Provide PPD file option.
Printer make and model for the Search for a printer driver to download option.
4. If applicable for your option, window shown in Figure 16.8, “Selecting a printer model”
appears. Choose the corresponding model in the Models column on the left.
NOTE
On the right, the recommended printer driver is automatically selected;
however, you can select another available driver. The print driver processes the
data that you want to print into a format the printer can understand. Since a
local printer is attached directly to your computer, you need a printer driver to
process the data that is sent to the printer.
Figure 16.8. Selecting a printer model
331

Figure 16.8. Selecting a printer model
5. Click Forward.
6. Under the Describe Printer enter a unique name for the printer in thePrinter Name field. The
printer name can contain letters, numbers, dashes (-), and underscores (); it _must not
contain any spaces. You can also use the Description and Location fields to add further
printer information. Both fields are optional, and may contain spaces.
Figure 16.9. Printer setup
332

Figure 16.9. Printer setup
7. Click Apply to confirm your printer configuration and add the print queue if the settings are
correct. Click Back to modify the printer configuration.
8. After the changes are applied, a dialog box appears allowing you to print a test page. Click
Yes to print a test page now. Alternatively, you can print a test page later as described in
Section 16.3.9, “Printing a Test Page”.
16.3.9. Printing a Test Page
After you have set up a printer or changed a printer configuration, print a test page to make sure the
printer is functioning properly:
1. Right-click the printer in the Printing window and clickProperties.
2. In the Properties window, click Settings on the left.
3. On the displayed Settings tab, click thePrint Test Page button.
16.3.10. Modifying Existing Printers
To delete an existing printer, in the Print Settings window, select the printer and go toPrinter →
Delete. Confirm the printer deletion. Alternatively, press theDelete key.
To set the default printer, right-click the printer in the printer list and click the Set as Default button
in the context menu.
16.3.10.1. The Settings Page
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To change printer driver configuration, double-click the corresponding name in the Printer list and
click the Settings label on the left to display theSettings page.
You can modify printer settings such as make and model, print a test page, change the device
location (URI), and more.
Figure 16.10. Settings page
16.3.10.2. The Policies Page
Click the Policies button on the left to change settings in printer state and print output.
You can select the printer states, configure the Error Policy of the printer (you can decide to abort
the print job, retry, or stop it if an error occurs).
You can also create a banner page(a page that describes aspects of the print job such as the
originating printer, the user name from the which the job originated, and the security status of the
document being printed): click the Starting Banner or Ending Banner drop-down menu and choose
the option that best describes the nature of the print jobs (for example, confidential).
16.3.10.2.1. Sharing Printers
On the Policies page, you can mark a printer as shared: if a printer is shared, users published on the
network can use it. To allow the sharing function for printers, go to Server →Settings and select
Publish shared printers connected to this system.
Figure 16.11. Policies page
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Figure 16.11. Policies page
Make sure that the firewall allows incoming TCP connections to port631, the port for the Network
Printing Server (IPP) protocol. To allowIPP traffic through the firewall on Red Hat Enterprise Linux 7,
make use of firewalld's IPP service. To do so, proceed as follows:
Enabling IPP Service in firewalld
1. To start the graphical firewall-config tool, press theSuper key to enter the Activities
Overview, type firewall and then pressEnter. TheFirewall Configuration window opens. You
will be prompted for an administrator or root password.
Alternatively, to start the graphical firewall configuration tool using the command line, enter
the following command as root user:
~]# firewall-config
The Firewall Configuration window opens.
Look for the word "Connected" in the lower left corner. This indicates that the firewall-config
tool is connected to the user space daemon, firewalld.
To immediately change the current firewall settings, ensure the drop-down selection menu
labeled Configuration is set toRuntime. Alternatively, to edit the settings to be applied at the
next system start, or firewall reload, select Permanent from the drop-down list.
2. Select the Zones tab and then select the firewall zone to correspond with the network
interface to be used. The default is the public zone. TheInterfaces tab shows what interfaces
have been assigned to a zone.
3. Select the Services tab and then select theipp service to enable sharing. Theipp-client
service is required for accessing network printers.
4. Close the firewall-config tool.
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For more information on opening and closing ports in firewalld, see the Red Hat Enterprise Linux 7
Security Guide.
16.3.10.2.2. The Access Control Page
You can change user-level access to the configured printer on the Access Control page. Click the
Access Control label on the left to display the page. Select eitherAllow printing for everyone except
these users or Deny printing for everyone except these users and define the user set below: enter
the user name in the text box and click the Add button to add the user to the user set.
Figure 16.12. Access Control page
16.3.10.2.3. The Printer Options Page
The Printer Options page contains various configuration options for the printer media and output,
and its content may vary from printer to printer. It contains general printing, paper, quality, and
printing size settings.
Figure 16.13. Printer Options page
336

Figure 16.13. Printer Options page
16.3.10.2.4. Job Options Page
On the Job Options page, you can detail the printer job options. Click theJob Options label on the
left to display the page. Edit the default settings to apply custom job options, such as number of
copies, orientation, pages per side, scaling (increase or decrease the size of the printable area, which
can be used to fit an oversize print area onto a smaller physical sheet of print medium), detailed text
options, and custom job options.
Figure 16.14. Job Options page
337

Figure 16.14. Job Options page
16.3.10.2.5. Ink/Toner Levels Page
The Ink/Toner Levels page contains details on toner status if available and printer status messages.
Click the Ink/Toner Levels label on the left to display the page.
Figure 16.15. Ink/Toner Levels page
338

Figure 16.15. Ink/Toner Levels page
16.3.10.3. Managing Print Jobs
When you send a print job to the printer daemon, such as printing a text file from Emacs or printing an
image from GIMP, the print job is added to the print spool queue. The print spool queue is a list of
print jobs that have been sent to the printer and information about each print request, such as the
status of the request, the job number, and more.
During the printing process, the Printer Status icon appears in theNotification Area on the panel. To
check the status of a print job, click the Printer Status, which displays a window similar toFigure 16.16,
“GNOME Print Status”.
Figure 16.16. GNOME Print Status
To cancel, hold, release, reprint or authenticate a print job, select the job in the GNOME Print Status
and on the Job menu, click the respective command.
To view the list of print jobs in the print spool from a shell prompt, type the command lpstat -o. The
last few lines look similar to the following:
Example 16.15. Example of lpstat -o output
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$ lpstat -o
Charlie-60 twaugh 1024 Tue 08 Feb 2011 16:42:11 GMT
Aaron-61 twaugh 1024 Tue 08 Feb 2011 16:42:44 GMT
Ben-62 root 1024 Tue 08 Feb 2011 16:45:42 GMT
If you want to cancel a print job, find the job number of the request with the command lpstat -o and
then use the command cancel job number. For example,cancel 60 would cancel the print job in
Example 16.15, “Example oflpstat -o output”. You cannot cancel print jobs that were started by other
users with the cancel command. However, you can enforce deletion of such job by issuing the
cancel -
U root job_number command. To prevent such canceling, change the printer operation policy to
Authenticated to forceroot authentication.
You can also print a file directly from a shell prompt. For example, the command lp sample.txt prints
the text file sample.txt. The print filter determines what type of file it is and converts it into a format
the printer can understand.
To learn more about printing on Red Hat Enterprise Linux, see the following resources.
lp(1) — The manual page for thelp command that allows you to print files from the command
line.
lpr(1) — The manual page for thelpr command that allows you to print files from the
command line.
cancel(1) — The manual page for the command-line utility to remove print jobs from the print
queue.
mpage(1) — The manual page for the command-line utility to print multiple pages on one
sheet of paper.
cupsd(8) — The manual page for the CUPS printer daemon.
cupsd.conf(5) — The manual page for the CUPS printer daemon configuration file.
classes.conf(5) — The manual page for the class configuration file for CUPS.
lpstat(1) — The manual page for thelpstat command, which displays status information about
classes, jobs, and printers.
http://www.linuxprinting.org/ — The OpenPrinting group on the Linux Foundation website
contains a large amount of information about printing in Linux.
http://www.cups.org/ — The CUPS website provides documentation, FAQs, and newsgroups
about CUPS.
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This chapter guides you through the installation and configuration of the MariaDB server, which is an
open source fast and robust database server that is based on MySQL technology. The chapter also
describes how to back up MariaDB data.
17.1. MARIADB
MariaDB is a relational database which converts data into structured information and provides an
SQL interface for accessing data. It includes multiple storage engines and plug-ins, as well as
geographic information system (GIS).
Red Hat Enterprise Linux 7 contains MariaDB 5.5 as the default implementation of a server from the
MySQL databases family. Later versions of the MariaDB database server are available as Software
Collections for Red Hat Enterprise Linux 6 and Red Hat Enterprise Linux 7. For more information
about the latest versions, see the Release Notes for Red Hat Software Collections
.
17.1.1. Installing the MariaDB server
To install the MariaDB server, follow this procedure:
Installing the MariaDB server
1. Ensure that the mariadb and mariadb-server packages are installed on the required server:
~]# yum install mariadb mariadb-server
2. Start the mariadb service:
~]# systemctl start mariadb.service
3. Enable the mariadb service to start at boot:
~]# systemctl enable mariadb.service
17.1.1.1. Improving MariaDB installation security
You can improve security when installing the MariaDB server by running the
mysql_secure_installation command:
~]# mysql_secure_installation
This command launches a fully interactive script, which prompts for each step in the process. The
script enables to improve security in the following ways:
Setting a password for root accounts
Removing anonymous users
Disallowing remote (outside the local host) root logins
Removing test database
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17.1.2. Configuring the MariaDB server for networking
To configure the MariaDB server for networking, use the[mysqld] section of the
/etc/my.cnf.d/server.cnf file, where you can set the following configuration directives:
bind-address
Bind-address is the address on which the server will listen.
Possible options are: a host name, an IPv4 address, or an IPv6 address.
skip-networking
Possible values are:
0 - to listen for all clients
1 - to listen for local clients only
port
The port on which MariaDB listens for TCP/IP connections.
17.1.3. Backing up MariaDB data
There are two main ways to back up data from a MariaDB database:
Logical backup
Physical backup
17.1.3.1. Logical back up
Logical backup consists of the SQL statements necessary to restore the data. This type of backup
exports information and records in plain text files.
The main advantage of logical backup over physical backup is portability and flexibility. The data can
be restored on other hardware configurations, MariaDB versions or Database Management System
(DBMS), which is not possible with physical backups.
WARNING
Logical backup can be performed only if the mariadb.service is running. Logical
backup does not include log and configuration files.
17.1.3.2. Physical back up
Physical backup consists of copies of files and directories that store the content.
Physical backup has the following advantages compared to logical backup:
Output is more compact.
Backup is smaller in size.

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Backup and restore are faster.
Backup includes log and configuration files.
WARNING
Physical backup must be performed when the mariadb.service is not running or all
tables in the database are locked to prevent changes during the backup.

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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 interruptions.
There is a choice between the daemons ntpd and chronyd, available from the repositories in thentp
and chrony packages respectively.
This chapter describes the use of the chrony suite.
18.1. INTRODUCTION TO THE CHRONY SUITE
Chrony is an implementation of the Network Time Protocol (NTP). You can use
Chrony:
to synchronize the system clock with NTP servers,
to synchronize the system clock with a reference clock, for example a GPS receiver,
to synchronize the system clock with a manual time input,
as an NTPv4(RFC 5905) server or peer to provide a time service to other computers in the
network.
Chrony performs well in a wide range of conditions, including intermittent network connections,
heavily congested networks, changing temperatures (ordinary computer clocks are sensitive to
temperature), and systems that do not run continuously, or run on a virtual machine.
Typical accuracy between two machines synchronized over the Internet is within a few milliseconds,
and for machines on a LAN within tens of microseconds. Hardware timestamping or a hardware
reference clock may improve accuracy between two machines synchronized to a sub-microsecond
level.
Chrony consists ofchronyd, a daemon that runs in user space, andchronyc, a command line program
which can be used to monitor the performance of chronyd and to change various operating
parameters when it is running.
18.1.1. Differences Between ntpd and chronyd
Things chronyd can do better thanntpd:
chronyd can work well in an environment where access to the time reference is intermittent,
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 synchronize 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.
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In the default configuration, chronyd never steps the time after the clock has been
synchronized at system start, 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 including negative effect on accuracy of the clock.
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.
chronyd is smaller, it uses less memory and it wakes up the CPU only when necessary, which is
better for power saving.
Things chronyd can do thatntpd 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 examine the
errors corrected at different updates to estimate the rate at which the computer gains or
loses time, and use this estimate to adjust the computer clock subsequently.
chronyd provides support to work out the rate of gain or loss of the real-time clock, for
example the 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 adapted value of time taken
from the real-time clock. These real-time clock facilities are currently only available on Linux
systems.
chronyd supports hardware timestamping on Linux, which allows extremely accurate
synchronization on local networks.
Things ntpd can do thatchronyd cannot do:
ntpd supports all operating modes fromNTP version 4 (RFC 5905), including broadcast,
multicast and manycast clients and servers. Note that the broadcast and multicast modes are,
even with authentication, inherently less accurate and less secure than the ordinary server
and client mode, and should generally be avoided.
ntpd supports the Autokey protocol (
RFC 5906) to authenticate servers with public-key
cryptography. Note that the protocol has proven to be insecure and will be probably replaced
with an implementation of the Network Time Security (NTS) specification.
ntpd includes drivers for many reference clocks, whereaschronyd relies on other programs,
for example gpsd, to access the data from the reference clocks using shared memory (SHM)
or Unix domain socket (SOCK).
18.1.2. Choosing Between NTP Daemons
Chrony should be preferred for all systems except for the systems that are managed or monitored by
tools that do not support chrony, or the systems that have a hardware reference clock which cannot
be used with chrony.
NOTE
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NOTE
Systems which are required to perform authentication of packets with the Autokey
protocol, can only be used with ntpd, because chronyd does not support this protocol.
The Autokey protocol has serious security issues, and thus using this protocol should
be avoided. Instead of Autokey, use authentication with symmetric keys, which is
supported by both chronyd and ntpd. Chrony supports stronger hash functions like
SHA256 and SHA512, while ntpd can use only MD5 and SHA1.
18.2. UNDERSTANDING CHRONY AND ITS CONFIGURATION
18.2.1. Understanding chronyd and chronyc
The chrony daemon,chronyd, can be monitored and controlled by the command line utilitychronyc.
This utility provides a command prompt, which allows entering a number of commands to query the
current state of chronyd and make changes to its configuration. By default,
chronyd accepts only
commands from a local instance of chronyc, but it can be configured to accept monitoring commands
also from remote hosts. The remote access should be restricted.
18.2.2. Understanding the chrony Configuration Commands
The default configuration file for chronyd is /etc/chrony.conf. The-f option can be used to specify an
alternate configuration file path. See the chronyd man page for further options. For a complete list
of the directives that can be used see http://chrony.tuxfamily.org/manual.html#Configuration-file.
Below is a selection of chronyd configuration options:
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.
Example 18.1. Granting access with theallow option:
Use this this command to grant access to an IPv4:
allow 192.0.2.0/24
Use this this command to grant access to an IPv6:
allow 2001:0db8:85a3::8a2e:0370:7334
NOTE
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NOTE
The UDP port number 123 needs to be open in the firewall in order to allow the client
access:
~]# firewall-cmd --zone=public --add-port=123/udp
If you want to open port 123 permanently, use the --permanent option:
~]# firewall-cmd --permanent --zone=public --add-port=123/udp
cmdallow
This is similar to the allow directive (see sectionallow), 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 behavior to thecmdallow 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 behavior whilst it is not running). If this capability is to be
used (via the dumponexit command in the configuration file, or thedump command inchronyc),
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).
hwtimestamp
The hwtimestamp directive enables hardware timestamping for extremely accurate
synchronization. For more details, see chrony.conf(5) manual page.
local
The local keyword is used to allowchronyd 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 the "master" computer in an isolated network, where several computers are required to
synchronize to one another, and the "master" is kept 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 unreliable. 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
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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.
The log 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 clock, because the
initstepslew directive only works withNTP sources.
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.
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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 indicates that the measurements have not settled down yet,
and that the estimated gain or loss rate is not very reliable.
The maxupdateskew parameter is the threshold for determining whether an estimate is too
unreliable to 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.
minsources
The minsources directive sets the minimum number of sources that need to be considered as
selectable in the source selection algorithm before the local clock is updated.
minsources number-of-sources
By default, number-of-sources is 1. Setting minsources to a larger number can be used to improve
the reliability, because multiple sources will need to correspond with each other.
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 synchronization 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.
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reselectdist dist-in-seconds
stratumweight
The stratumweight directive sets how much distance should be added per stratum to the
synchronization distance when chronyd selects the synchronization source from available sources.
stratumweight dist-in-seconds
By default, dist-in-seconds is 1 millisecond. This means that sources with lower stratum are usually
preferred to sources with higher stratum even when their distance is significantly worse. Setting
stratumweight to 0 makeschronyd ignore stratum when selecting the source.
rtcfile
The rtcfile directive defines the name of the file in whichchronyd can save parameters associated
with tracking the accuracy of the system’s real-time clock (RTC).
rtcfile /var/lib/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 is system-specific. Note that if this directive is used then 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.
rtcsync
The rtcsync directive is present in the/etc/chrony.conf file by default. This will inform the kernel
the system clock is kept synchronized and the kernel will update the real-time clock every 11
minutes.
18.2.3. Security with chronyc
Chronyc can accesschronyd in two ways:
Internet Protocol (IPv4 or IPv6,
Unix domain socket, which is accessible locally by the root or chrony user.
By default, chronyc connects to the Unix domain socket. The default path is
/var/run/chrony/chronyd.sock. If this connection fails, which can happen for example whenchronyc is
running under a non-root user, chronyc tries to connect to 127.0.0.1 and then ::1.
Only the following monitoring commands, which do not affect the behavior of chronyd, are allowed
from the network:
activity
manual list
rtcdata
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smoothing
sources
sourcestats
tracking
waitsync
The set of hosts from which chronyd accepts these commands can be configured with thecmdallow
directive in the configuration file of chronyd, or the cmdallow command inchronyc. By default, the
commands are accepted only from localhost (127.0.0.1 or ::1).
All other commands are allowed only through the Unix domain socket. When sent over the network,
chronyd responds with aNot authorised error, even if it is from localhost.
Accessing chronyd remotely withchronyc
1. Allow access from both IPv4 and IPv6 addresses by adding the following to the
/etc/chrony.conf file:
bindcmdaddress 0.0.0.0
or
bindcmdaddress :
2. Allow commands from the remote IP address, network, or subnet by using the cmdallow
directive.
Add the following content to the /etc/chrony.conf file:
cmdallow 192.168.1.0/24
3. Open port 323 in the firewall to connect from a remote system.
~]# firewall-cmd --zone=public --add-port=323/udp
If you want to open port 323 permanently, use the --permanent.
~]# firewall-cmd --permanent --zone=public --add-port=323/udp
Note that the allow directive is forNTP access whereas thecmdallow directive is to enable receiving
of remote commands. It is possible to make these changes temporarily using chronyc running locally.
Edit the configuration file to make permanent changes.
18.3. USING CHRONY
18.3.1. Installing chrony
The chrony suite is installed by default on some versions of Red Hat Enterprise Linux 7. If required, to
ensure that it is, run the following command as root:
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~]# yum install chrony
The default location for the chrony daemon is/usr/sbin/chronyd. The command line utility will be
installed to /usr/bin/chronyc.
18.3.2. 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
18.3.3. Starting chronyd
To start chronyd, issue the following command asroot:
~]# systemctl start chronyd
To ensure chronyd starts automatically at system start, issue the following command as
root:
~]# systemctl enable chronyd
18.3.4. Stopping chronyd
To stop chronyd, issue the following command asroot:
~]# systemctl stop chronyd
To prevent chronyd from starting automatically at system start, issue the following command as
root:
~]# systemctl disable chronyd
18.3.5. Checking if chrony is Synchronized
To check if chrony is synchronized, make use of thetracking, sources, and sourcestats commands.
18.3.5.1. Checking chrony Tracking
To check chrony tracking, issue the following command:
~]$ chronyc tracking
Reference ID : CB00710F (foo.example.net)
Stratum : 3
Ref time (UTC) : Fri Jan 27 09:49:17 2017
System time : 0.000006523 seconds slow of NTP time
Last offset : -0.000006747 seconds
RMS offset : 0.000035822 seconds
Frequency : 3.225 ppm slow
Residual freq : 0.000 ppm
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Skew : 0.129 ppm
Root delay : 0.013639022 seconds
Root dispersion : 0.001100737 seconds
Update interval : 64.2 seconds
Leap status : Normal
The fields are as follows:
Reference ID
This is the reference ID and name (or IP address) if available, of the server to which the computer
is currently synchronized. Reference ID is a hexadecimal number to avoid confusion with IPv4
addresses.
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
(that is to say, a.b.c is a stratum-2 and is synchronized 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 wrong if chronyd was not
correcting it. It is expressed in ppm (parts per million). For example, a value of 1 ppm 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
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This is the total of the network path delays to the stratum-1 computer from which the computer is
ultimately synchronized. Root delay values are printed in nanosecond resolution. 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 synchronized. Dispersion is due to system clock resolution,
statistical measurement variations etc. Root dispersion values are printed in nanosecond
resolution.
Leap status
This is the leap status, which can be Normal, Insert second, Delete second or Not synchronized.
18.3.5.2. 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
currently synchronized. "+" indicates acceptable sources which are combined with the selected
source. "-" indicates acceptable sources which are excluded by the combining algorithm. "?"
indicates sources to which connectivity has been lost or whose packets do not pass all tests. "x"
indicates a clock which chronyd thinks is afalseticker (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 reference ID for reference clock.
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 synchronized to a
stratum 1 computer is at stratum 2. A computer that is synchronized 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
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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 reach register printed as an 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 of 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), ors
(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 ahead of the source.
18.3.5.3. 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
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
===============================================================================
abc.def.ghi 11 5 46m -0.001 0.045 1us 25us
The columns are as follows:
Name/IP address
This is the name or IP address of theNTP server (or peer) or reference ID of the reference clock 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
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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 109 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.
18.3.6. Manually Adjusting the System Clock
To step the system clock immediately, bypassing any adjustments in progress by slewing, issue the
~]# chronyc makestep
If the rtcfile directive is used, the real-time clock should not be manually adjusted. Random
adjustments would interfere with chrony's need to measure the rate at which the real-time clock
drifts.
18.4. SETTING UP CHRONY FOR DIFFERENT ENVIRONMENTS
18.4.1. 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 calculate 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
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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, andmaster is the host name of the master. Clients
with this configuration 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.
In an Isolated Network, you can also use the local directive that enables a local reference mode, which
allows chronyd operating as an NTP server to appear synchronized to real time, even when it was
never synchronized or the last update of the clock happened a long time ago.
To allow multiple servers in the network to use the same local configuration and to be synchronized to
one another, without confusing clients that poll more than one server, use the orphan option of the
local directive which enables the orphan mode. Each server needs to be configured to poll all other
servers with local. This ensures that only the server with the smallest reference ID has the local
reference active and other servers are synchronized to it. When the server fails, another one will take
over.
18.5. USING CHRONYC
18.5.1. Using chronyc to Control chronyd
To make changes to the local instance of chronyd using the command line utilitychronyc in
interactive mode, enter the following command as root:
~]# chronyc
chronyc must run asroot 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 a command
as follows:
chronyc command
NOTE
Changes made using chronyc are not permanent, they will be lost after a
chronyd
restart. For permanent changes, modify /etc/chrony.conf.
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18.6. CHRONY WITH HW TIMESTAMPING
18.6.1. Understanding Hardware Timestamping
Hardware timestamping is a feature supported in some Network Interface Controller (NICs) which
provides accurate timestamping of incoming and outgoing packets. NTP timestamps are usually
created by the kernel and chronyd with the use of the system clock. However, when HW timestamping
is enabled, the NIC uses its own clock to generate the timestamps when packets are entering or
leaving the link layer or the physical layer. When used with NTP, hardware timestamping can
significantly improve the accuracy of synchronization. For best accuracy, both NTP servers andNTP
clients need to use hardware timestamping. Under ideal conditions, a sub-microsecond accuracy may
be possible.
Another protocol for time synchronization that uses hardware timestamping is PTP. For further
information about PTP, see Chapter 20, Configuring PTP Using ptp4l. Unlike NTP, PTP relies on
assistance in network switches and routers. If you want to reach the best accuracy of synchronization,
use PTP on networks that have switches and routers withPTP support, and preferNTP on networks
that do not have such switches and routers.
18.6.2. Verifying Support for Hardware Timestamping
To verify that hardware timestamping with NTP is supported by an interface, use theethtool -T
command. An interface can be used for hardware timestamping with NTP ifethtool lists the
SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE capabilities
and also the HWTSTAMP_FILTER_ALL filter mode.
Example 18.2. Verifying Support for Hardware Timestamping on a Specific Interface
~]# ethtool -T eth0
Output:
Timestamping parameters for eth0:
Capabilities:
hardware-transmit (SOF_TIMESTAMPING_TX_HARDWARE)
software-transmit (SOF_TIMESTAMPING_TX_SOFTWARE)
hardware-receive (SOF_TIMESTAMPING_RX_HARDWARE)
software-receive (SOF_TIMESTAMPING_RX_SOFTWARE)
software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
hardware-raw-clock (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
off (HWTSTAMP_TX_OFF)
on (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
none (HWTSTAMP_FILTER_NONE)
all (HWTSTAMP_FILTER_ALL)
ptpv1-l4-sync (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
ptpv1-l4-delay-req (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
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ptpv2-event (HWTSTAMP_FILTER_PTP_V2_EVENT)
ptpv2-sync (HWTSTAMP_FILTER_PTP_V2_SYNC)
ptpv2-delay-req (HWTSTAMP_FILTER_PTP_V2_DELAY_REQ)
18.6.3. Enabling Hardware Timestamping
To enable hardware timestamping, use the hwtimestamp directive in the/etc/chrony.conf file. The
directive can either specify a single interface, or a wildcard character () can be used to enable
hardware timestamping on all interfaces that support it. Use the wildcard specification in case that no
other application, like [application]*ptp4l from the linuxptp package, is using hardware timestamping
on an interface. Multiple hwtimestamp directives are allowed in the chrony configuration file.
Example 18.3. Enabling Hardware Timestamping by Using the hwtimestamp Directive
hwtimestamp eth0
hwtimestamp eth1
hwtimestamp *
18.6.4. Configuring Client Polling Interval
The default range of a polling interval (64-1024 seconds) is recommended for servers on the Internet.
For local servers and hardware timestamping, a shorter polling interval needs to be configured in
order to minimize offset of the system clock.
The following directive in /etc/chrony.conf specifies a localNTP server using one second polling
interval:
server ntp.local minpoll 0 maxpoll 0
18.6.5. Enabling Interleaved Mode
NTP servers that are not hardwareNTP appliances, but rather general purpose computers running a
software NTP implementation, likechrony, will get a hardware transmit timestamp only after sending
a packet. This behavior prevents the server from saving the timestamp in the packet to which it
corresponds. In order to enable NTP clients receiving transmit timestamps that were generated after
the transmission, configure the clients to use the NTP interleaved mode by adding thexleave option
to the server directive in /etc/chrony.conf:
server ntp.local minpoll 0 maxpoll 0 xleave
18.6.6. Configuring Server for Large Number of Clients
The default server configuration allows a few thousands of clients at most to use the interleaved
mode concurrently. To configure the server for a larger number of clients, increase the clientloglimit
directive in /etc/chrony.conf. This directive specifies the maximum size of memory allocated for
logging of clients' access on the server:
clientloglimit 100000000
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18.6.7. Verifying Hardware Timestamping
To verify that the interface has successfully enabled hardware timestamping, check the system log.
The log should contain a message from chronyd for each interface with successfully enabled
hardware timestamping.
Example 18.4. Log Messages for Interfaces with Enabled Hardware Timestamping
chronyd[4081]: Enabled HW timestamping on eth0
chronyd[4081]: Enabled HW timestamping on eth1
When chronyd is configured as anNTP client or peer, you can have the transmit and receive
timestamping modes and the interleaved mode reported for each NTP source by thechronyc
ntpdata command:
Example 18.5. Reporting the Transmit, Receive Timestamping and Interleaved Mode for Each NTP
Source
~]# chronyc ntpdata
Output:
Remote address : 203.0.113.15 (CB00710F)
Remote port : 123
Local address : 203.0.113.74 (CB00714A)
Leap status : Normal
Version : 4
Mode : Server
Stratum : 1
Poll interval : 0 (1 seconds)
Precision : -24 (0.000000060 seconds)
Root delay : 0.000015 seconds
Root dispersion : 0.000015 seconds
Reference ID : 47505300 (GPS)
Reference time : Wed May 03 13:47:45 2017
Offset : -0.000000134 seconds
Peer delay : 0.000005396 seconds
Peer dispersion : 0.000002329 seconds
Response time : 0.000152073 seconds
Jitter asymmetry: +0.00
NTP tests : 111 111 1111
Interleaved : Yes
Authenticated : No
TX timestamping : Hardware
RX timestamping : Hardware
Total TX : 27
Total RX : 27
Total valid RX : 27
Example 18.6. Reporting the Stability of NTP Measurements
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# chronyc sourcestats
With hardware timestamping enabled, stability of NTP measurements should be in tens or
hundreds of nanoseconds, under normal load. This stability is reported in the Std Dev column of
the output of the chronyc sourcestats command:
Output:
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
ntp.local 12 7 11 +0.000 0.019 +0ns 49ns
18.6.8. Configuring PTP-NTP bridge
If a highly accurate Precision Time Protocol (PTP) grandmaster is available in a network that does not
have switches or routers with PTP support, a computer may be dedicated to operate as a
PTP slave
and a stratum-1 NTP server. Such a computer needs to have two or more network interfaces, and be
close to the grandmaster or have a direct connection to it. This will ensure highly accurate
synchronization in the network.
Configure the ptp4l and phc2sys programs from the linuxptp packages to use one interface to
synchronize the system clock using PTP. The configuration is described in theChapter 20,
Configuring PTP Using ptp4l. Configure chronyd to provide the system time using the other
interface:
Example 18.7. Configuring chronyd to Provide the System Time Using the Other Interface
bindaddress 203.0.113.74
hwtimestamp eth1
local stratum 1
The following sources of information provide additional resources regarding chrony.
chronyc(1) man page — Describes thechronyc command-line interface tool including
commands and command options.
chronyd(8) man page — Describes thechronyd daemon including commands and command
options.
chrony.conf(5) man page — Describes thechrony configuration file.
http://chrony.tuxfamily.org/doc/3.1/chronyc.html
http://chrony.tuxfamily.org/doc/3.1/chronyd.html
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http://chrony.tuxfamily.org/doc/3.1/chrony.conf.html
For answers to FAQs, see http://chrony.tuxfamily.org/faq.html
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19.1. INTRODUCTION TO NTP
The Network Time Protocol(NTP) enables the accurate dissemination of time and date information in
order to keep the time clocks on networked computer systems synchronized to a common reference
over the network or the Internet. Many standards bodies around the world have atomic clocks which
may be made available as a reference. The satellites that make up the Global Position System contain
more than one atomic clock, making their time signals potentially very accurate. Their signals can be
deliberately degraded for military reasons. An ideal situation would be where each site has a server,
with its own reference clock attached, to act as a site-wide time server. Many devices which obtain
the time and date via low frequency radio transmissions or the Global Position System (GPS) exist.
However for most situations, a range of publicly accessible time servers connected to the Internet at
geographically dispersed locations can be used. These NTP servers provide "Coordinated Universal
Time" (UTC). Information about these time servers can found atwww.pool.ntp.org.
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. Logs are used to investigate service and security issues
and so time stamps made on different systems must be made by synchronized clocks to be of real
value. As systems and networks become increasingly faster, there is a corresponding need for clocks
with greater accuracy and resolution. In some countries there are legal obligations to keep accurately
synchronized clocks. See www.ntp.org for more information. In Linux systems,NTP is implemented by
a daemon running in user space. The default NTP user space daemon in Red Hat Enterprise Linux 7 is
chronyd. It must be disabled if you want to use thentpd daemon. SeeChapter 18, Configuring NTP
Using the chrony Suite for information onchrony.
The user space daemon updates the system clock, which is a software clock running in the kernel.
Linux uses a software clock as its system clock for better resolution than the typical embedded
hardware clock referred to as the "Real Time Clock" (RTC). See thertc(4) and hwclock(8) man pages
for information on hardware clocks. 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. On
system start, the system clock reads the time and date from the RTC. The time kept by the RTC will
drift away from actual time by up to 5 minutes per month due to temperature variations. Hence the
need for the system clock to be constantly synchronized with external time references. When the
system clock is being synchronized by ntpd, the kernel will in turn update the RTC every 11 minutes
automatically.
19.2. NTP STRATA
NTP servers are classified according to their synchronization distance from the atomic clocks which
are the source of the time signals. The servers are thought of as being arranged in layers, or strata,
from 1 at the top down to 15. Hence the word stratum is used when referring to a specific layer. Atomic
clocks are referred to as Stratum 0 as this is the source, but no Stratum 0 packet is sent on the
Internet, all stratum 0 atomic clocks are attached to a server which is referred to as stratum 1. These
servers send out packets marked as Stratum 1. A server which is synchronized by means of packets
marked stratum n belongs to the next, lower, stratum and will mark its packets as stratum
n+1.
Servers of the same stratum can exchange packets with each other but are still designated as
belonging to just the one stratum, the stratum one below the best reference they are synchronized to.
The designation Stratum 16 is used to indicate that the server is not currently synchronized to a
reliable time source.
Note that by default NTP clients act as servers for those systems in the stratum below them.
Here is a summary of the NTP Strata:
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Stratum 0
Atomic Clocks and their signals broadcast over Radio and GPS
GPS (Global Positioning System)
Mobile Phone Systems
Low Frequency Radio Broadcasts WWVB (Colorado, USA.), JJY-40 and JJY-60 (Japan),
DCF77 (Germany), and MSF (United Kingdom)
These signals can be received by dedicated devices and are usually connected by RS-232
to a system used as an organizational or site-wide time server.
Stratum 1
Computer with radio clock, GPS clock, or atomic clock attached
Stratum 2
Reads from stratum 1; Serves to lower strata
Stratum 3
Reads from stratum 2; Serves to lower strata
Stratum n+1
Reads from stratum n; Serves to lower strata
Stratum 15
Reads from stratum 14; This is the lowest stratum.
This process continues down to Stratum 15 which is the lowest valid stratum. The label Stratum 16 is
used to indicated an unsynchronized state.
19.3. UNDERSTANDING NTP
The version of NTP used by Red Hat Enterprise Linux is as described in
RFC 1305 Network Time
Protocol (Version 3) Specification, Implementation and Analysis and RFC 5905 Network Time
Protocol Version 4: Protocol and Algorithms Specification
This implementation of NTP enables sub-second accuracy to be achieved. Over the Internet, accuracy
to 10s of milliseconds is normal. On a Local Area Network (LAN), 1 ms accuracy is possible under ideal
conditions. This is because clock drift is now accounted and corrected for, which was not done in
earlier, simpler, time protocol systems. A resolution of 233 picoseconds is provided by using 64-bit
time stamps. The first 32-bits of the time stamp is used for seconds, the last 32-bits are used for
fractions of seconds.
NTP represents the time as a count of the number of seconds since 00:00 (midnight) 1 January, 1900
GMT. As 32-bits is used to count the seconds, this means the time will "roll over" in 2036. However
NTP works on the difference between time stamps so this does not present the same level of problem
as other implementations of time protocols have done. If a hardware clock that is within 68 years of
the correct time is available at boot time then NTP will correctly interpret the current date. TheNTP4
specification provides for an "Era Number" and an "Era Offset" which can be used to make software
more robust when dealing with time lengths of more than 68 years. Do not confuse this with the Unix
Year 2038 problem.
The NTP protocol provides additional information to improve accuracy. Four time stamps are used to
allow the calculation of round-trip time and server response time. In order for a system in its role as
NTP client to synchronize with a reference time server, a packet is sent with an "originate time
stamp". When the packet arrives, the time server adds a "receive time stamp". After processing the
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request for time and date information and just before returning the packet, it adds a "transmit time
stamp". When the returning packet arrives at the NTP client, a "receive time stamp" is generated. The
client can now calculate the total round trip time and by subtracting the processing time derive the
actual traveling time. By assuming the outgoing and return trips take equal time, the single-trip delay
in receiving the NTP data is calculated. The fullNTP algorithm is much more complex than presented
here.
When a packet containing time information is received it is not immediately responded to, but is first
subject to validation checks and then processed together with several other time samples to arrive at
an estimate of the time. This is then compared to the system clock to determine the time offset, the
difference between the system clock’s time and what ntpd has determined the time should be. The
system clock is adjusted slowly, at most at a rate of 0.5 ms per second, to reduce this offset by
changing the frequency of the counter being used. It will take at least 2000 seconds to adjust the
clock by 1 second using this method. This slow change is referred to as slewing and cannot go
backwards. If the time offset of the clock is more than 128 ms (the default setting), ntpd can "step"
the clock forwards or backwards. If the time offset at system start is greater than 1000 seconds then
the user, or an installation script, should make a manual adjustment. See Chapter 3, Configuring the
Date and Time. With the -g option to thentpd command (used by default), any offset at system start
will be corrected, but during normal operation only offsets of up to 1000 seconds will be corrected.
Some software may fail or produce an error if the time is changed backwards. For systems that are
sensitive to step changes in the time, the threshold can be changed to 600 s instead of 128 ms using
the -x option (unrelated to the-g option). Using the-x option to increase the stepping limit from 0.128
s to 600 s has a drawback because a different method of controlling the clock has to be used. It
disables the kernel clock discipline and may have a negative impact on the clock accuracy. The -x
option can be added to the /etc/sysconfig/ntpd configuration file.
19.4. UNDERSTANDING THE DRIFT FILE
The drift file is used to store the frequency offset between the system clock running at its nominal
frequency and the frequency required to remain in synchronization with UTC. If present, the value
contained in the drift file is read at system start and used to correct the clock source. Use of the drift
file reduces the time required to achieve a stable and accurate time. The value is calculated, and the
drift file replaced, once per hour by ntpd. The drift file is replaced, rather than just updated, and for
this reason the drift file must be in a directory for which the ntpd has write permissions.
19.5. UTC, TIMEZONES, AND DST
As NTP is entirely in UTC (Universal Time, Coordinated), Timezones and DST (Daylight Saving Time)
are applied locally by the system. The file /etc/localtime is a copy of, or symlink to, a zone information
file from /usr/share/zoneinfo. The RTC may be in localtime or in UTC, as specified by the 3rd line of
/etc/adjtime, which will be one of LOCAL or UTC to indicate how the RTC clock has been set. Users
can easily change this setting using the checkbox System Clock Uses UTC in theDate and Time
graphical configuration tool. See Chapter 3, Configuring the Date and Timefor information on how to
use that tool. Running the RTC in UTC is recommended to avoid various problems when daylight
saving time is changed.
The operation of ntpd is explained in more detail in the man pagentpd(8). The resources section lists
useful sources of information. See Section 19.20, “Additional Resources”.
19.6. AUTHENTICATION OPTIONS FOR NTP
NTPv4 NTPv4 added support for the Autokey Security Architecture, which is based on public
asymmetric cryptography while retaining support for symmetric key cryptography. The Autokey
protocol is described in RFC 5906 Network Time Protocol Version 4: Autokey Specification.
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Unfortunately, it was found later that the protocol has serious security issues, and thus Red Hat
strongly recommends to use symmetric keys instead. The man page ntp_auth(5) describes the
authentication options and commands for ntpd.
An attacker on the network can attempt to disrupt a service by sending NTP packets with incorrect
time information. On systems using the public pool of NTP servers, this risk is mitigated by having
more than three NTP servers in the list of publicNTP servers in/etc/ntp.conf. If only one time source is
compromised or spoofed, ntpd will ignore that source. You should conduct a risk assessment and
consider the impact of incorrect time on your applications and organization. If you have internal time
sources you should consider steps to protect the network over which the NTP packets are distributed.
If you conduct a risk assessment and conclude that the risk is acceptable, and the impact to your
applications minimal, then you can choose not to use authentication.
The broadcast and multicast modes require authentication by default. If you have decided to trust
the network then you can disable authentication by using disable auth directive in thentp.conf file.
Alternatively, authentication needs to be configured by using SHA1 or MD5 symmetric keys, or by
public (asymmetric) key cryptography using the Autokey scheme. The Autokey scheme for
asymmetric cryptography is explained in the ntp_auth(8) man page and the generation of keys is
explained in ntp-keygen(8). To implement symmetric key cryptography, seeSection 19.17.12,
“Configuring Symmetric Authentication Using a Key” for an explanation of thekey option.
19.7. MANAGING THE TIME ON VIRTUAL MACHINES
Virtual machines cannot access a real hardware clock and a virtual clock is not stable enough as the
stability is dependent on the host systems work load. For this reason, para-virtualized clocks should
be provided by the virtualization application in use. On Red Hat Enterprise Linux with KVM the default
clock source is kvm-clock. See theKVM guest timing managementchapter of theRed Hat Enterprise
Linux 7 Virtualization Deployment and Administration Guide.
19.8. UNDERSTANDING LEAP SECONDS
Greenwich Mean Time (GMT) was derived by measuring the solar day, which is dependent on the
Earth’s rotation. When atomic clocks were first made, the potential for more accurate definitions of
time became possible. In 1958, International Atomic Time (TAI) was introduced based on the more
accurate and very stable atomic clocks. A more accurate astronomical time, Universal Time 1 (UT1),
was also introduced to replace GMT. The atomic clocks are in fact far more stable than the rotation of
the Earth and so the two times began to drift apart. For this reason UTC was introduced as a practical
measure. It is kept within one second of UT1 but to avoid making many small trivial adjustments it was
decided to introduce the concept of a leap secondin order to reconcile the difference in a manageable
way. The difference between UT1 and UTC is monitored until they drift apart by more than half a
second. Then only is it deemed necessary to introduce a one second adjustment, forward or
backward. Due to the erratic nature of the Earth’s rotational speed, the need for an adjustment
cannot be predicted far into the future. The decision as to when to make an adjustment is made by
the International Earth Rotation and Reference Systems Service (IERS)
. However, these
announcements are important only to administrators of Stratum 1 servers because NTP transmits
information about pending leap seconds and applies them automatically.
19.9. UNDERSTANDING THE NTPD CONFIGURATION FILE
The daemon, ntpd, reads the configuration file at system start or when the service is restarted. The
default location for the file is /etc/ntp.conf and you can view the file by entering the following
command:
~]$ less /etc/ntp.conf
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The configuration commands are explained briefly later in this chapter, see Section 19.17, “Configure
NTP”, and more verbosely in thentp.conf(5) man page.
Here follows a brief explanation of the contents of the default configuration file:
The driftfile entry
A path to the drift file is specified, the default entry on Red Hat Enterprise Linux is:
driftfile /var/lib/ntp/drift
If you change this be certain that the directory is writable by ntpd. The file contains one value used
to adjust the system clock frequency after every system or service start. See Understanding the
Drift File for more information.
The access control entries
The following line sets the default access control restriction:
restrict default nomodify notrap nopeer noquery
The nomodify options prevents any changes to the configuration.
The notrap option preventsntpdc control message protocol traps.
The nopeer option prevents a peer association being formed.
The noquery option preventsntpq and ntpdc queries, but not time queries, from being
answered.
IMPORTANT
The ntpq and ntpdc queries can be used in amplification attacks, therefore
do not remove the noquery option from therestrict default command on
publicly accessible systems.
See CVE-2013-5211 for more details.
Addresses within the range 127.0.0.0/8 are sometimes required by various processes or
applications. As the "restrict default" line above prevents access to everything not
explicitly allowed, access to the standard loopback address for IPv4 and IPv6 is permitted
by means of the following lines:
# the administrative functions.
restrict 127.0.0.1
restrict ::1
Addresses can be added underneath if specifically required by another application.
Hosts on the local network are not permitted because of the "restrict default" line above.
To change this, for example to allow hosts from the 192.0.2.0/24 network to query the time
and statistics but nothing more, a line in the following format is required:
restrict 192.0.2.0 mask 255.255.255.0 nomodify notrap nopeer
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To allow unrestricted access from a specific host, for example 192.0.2.250/32, a line in the
following format is required:
restrict 192.0.2.250
A mask of 255.255.255.255 is applied if none is specified.
The restrict commands are explained in the ntp_acc(5) man page.
The public servers entry
By default, the ntp.conf file contains four public server entries:
server 0.rhel.pool.ntp.org iburst
The broadcast multicast servers entry
By default, the ntp.conf file contains some commented out examples. These are largely self
explanatory. See Section 19.17, “Configure NTP” for the explanation of the specific commands. If
required, add your commands just below the examples.
NOTE
When the DHCP client program,dhclient, receives a list ofNTP servers from theDHCP
server, it adds them to ntp.conf and restarts the service. To disable that feature, add
PEERNTP=no to /etc/sysconfig/network.
19.10. UNDERSTANDING THE NTPD SYSCONFIG FILE
The file will be read by the ntpd init script on service start. The default contents is as follows:
# Command line options for ntpd
OPTIONS="-g"
The -g option enablesntpd to ignore the offset limit of 1000 s and attempt to synchronize the time
even if the offset is larger than 1000 s, but only on system start. Without that option ntpd will exit if
the time offset is greater than 1000 s. It will also exit after system start if the service is restarted and
the offset is greater than 1000 s even with the -g option.
19.11. DISABLING CHRONY
In order to use ntpd the default user space daemon,chronyd, must be stopped and disabled. Issue the
~]# systemctl stop chronyd
To prevent it restarting at system start, issue the following command as root:
~]# systemctl disable chronyd
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To check the status of chronyd, issue the following command:
~]$ systemctl status chronyd
19.12. CHECKING IF THE NTP DAEMON IS INSTALLED
To check if ntpd is installed, enter the following command asroot:
~]# yum install ntp
NTP is implemented by means of the daemon or servicentpd, which is contained within thentp
package.
19.13. INSTALLING THE NTP DAEMON (NTPD)
To install ntpd, enter the following command asroot:
~]# yum install ntp
To enable ntpd at system start, enter the following command as
root:
~]# systemctl enable ntpd
19.14. CHECKING THE STATUS OF NTP
To check if ntpd is running and configured to run at system start, issue the following command:
~]$ systemctl status ntpd
To obtain a brief status report from ntpd, issue the following command:
~]$ ntpstat
unsynchronised
time server re-starting
polling server every 64 s
~]$ ntpstat
synchronised to NTP server (10.5.26.10) at stratum 2
time correct to within 52 ms
polling server every 1024 s
19.15. CONFIGURE THE FIREWALL TO ALLOW INCOMING NTP
PACKETS
The NTP traffic consists ofUDP packets on port123 and needs to be permitted through network and
host-based firewalls in order for NTP to function.
Check if the firewall is configured to allow incoming NTP traffic for clients using the graphicalFirewall
Configuration tool.
To start the graphical firewall-config tool, press theSuper key to enter the Activities Overview, type
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To start the graphical firewall-config tool, press theSuper key to enter the Activities Overview, type
firewall and then pressEnter. TheFirewall Configuration window opens. You will be prompted for
your user password.
To start the graphical firewall configuration tool using the command line, enter the following
command as root user:
~]# firewall-config
The Firewall Configuration window opens. Note, this command can be run as normal user but you will
then be prompted for the root password from time to time.
Look for the word "Connected" in the lower left corner. This indicates that the firewall-config tool is
connected to the user space daemon, firewalld.
19.15.1. Change the Firewall Settings
To immediately change the current firewall settings, ensure the drop-down selection menu labeled
Configuration is set toRuntime. Alternatively, to edit the settings to be applied at the next system
start, or firewall reload, select Permanent from the drop-down list.
NOTE
When making changes to the firewall settings in Runtime mode, your selection takes
immediate effect when you set or clear the check box associated with the service. You
should keep this in mind when working on a system that may be in use by other users.
When making changes to the firewall settings in Permanent mode, your selection will
only take effect when you reload the firewall or the system restarts. To reload the
firewall, select the Options menu and select Reload Firewall.
19.15.2. Open Ports in the Firewall for NTP Packets
To permit traffic through the firewall to a certain port, start the firewall-config tool and select the
network zone whose settings you want to change. Select the Ports tab and then click theAdd button.
The Port and Protocol window opens.
Enter the port number 123 and selectudp from the drop-down list.
19.16. CONFIGURE NTPDATE SERVERS
The purpose of the ntpdate service is to set the clock during system boot. This was used previously to
ensure that the services started after ntpdate would have the correct time and not observe a jump in
the clock. The use of ntpdate and the list of step-tickers is considered deprecated and so Red Hat
Enterprise Linux 7 uses the -g option to thentpd command and notntpdate by default.
The ntpdate service in Red Hat Enterprise Linux 7 is beneficial if it is used without the
ntpd service or
when the -x option is specified for thentpd command. Ifntpd is used with-x but without thentpdate
service enabled, the clock is corrected by step only if the time difference is larger than 600 seconds.
With a smaller offset than 600 seconds, the clock is adjusted slowly, approximately 2000 seconds for
every corrected second.
To check if the ntpdate service is enabled to run at system start, issue the following command:
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~]$ systemctl status ntpdate
To enable the service to run at system start, issue the following command as root:
~]# systemctl enable ntpdate
In Red Hat Enterprise Linux 7 the default /etc/ntp/step-tickers file contains0.rhel.pool.ntp.org. To
configure additional ntpdate servers, using a text editor running asroot, edit /etc/ntp/step-tickers.
The number of servers listed is not very important as ntpdate will only use this to obtain the date
information once when the system is starting. If you have an internal time server then use that host
name for the first line. An additional host on the second line as a backup is sensible. The selection of
backup servers and whether the second host is internal or external depends on your risk assessment.
For example, what is the chance of any problem affecting the first server also affecting the second
server? Would connectivity to an external server be more likely to be available than connectivity to
internal servers in the event of a network failure disrupting access to the first server?
19.17. CONFIGURE NTP
To change the default configuration of the NTP service, use a text editor running asroot user to edit
the /etc/ntp.conf file. This file is installed together withntpd and is configured to use time servers
from the Red Hat pool by default. The man page ntp.conf(5) describes the command options that can
be used in the configuration file apart from the access and rate limiting commands which are
explained in the ntp_acc(5) man page.
19.17.1. Configure Access Control to an NTP Service
To restrict or control access to the NTP service running on a system, make use of therestrict
command in the ntp.conf file. See the commented out example:
# Hosts on local network are less restricted.
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
The restrict command takes the following form:
restrict address [mask mask] option
where address and mask specify the IP addresses to which you want to apply the restriction, and
option is one or more of:
ignore — All packets will be ignored, includingntpq and ntpdc queries.
kod — a "Kiss-o'-death" packet is to be sent to reduce unwanted queries.
limited — do not respond to time service requests if the packet violates the rate limit default
values or those specified by the discard command.ntpq and ntpdc queries are not affected.
For more information on the discard command and the default values, seeSection 19.17.2,
“Configure Rate Limiting Access to an NTP Service”.
lowpriotrap — traps set by matching hosts to be low priority.
nomodify — prevents any changes to the configuration.
noquery — prevents ntpq and ntpdc queries, but not time queries, from being answered.
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nopeer — prevents a peer association being formed.
noserve — deny all packets exceptntpq and ntpdc queries.
notrap — prevents ntpdc control message protocol traps.
notrust — deny packets that are not cryptographically authenticated.
ntpport — modify the match algorithm to only apply the restriction if the source port is the
standard NTP UDP port 123.
version — deny packets that do not match the currentNTP version.
To configure rate limit access to not respond at all to a query, the respective restrict command has to
have the limited option. Ifntpd should reply with aKoD packet, therestrict command needs to have
both limited and kod options.
The ntpq and ntpdc queries can be used in amplification attacks (seeCVE-2013-5211 for more
details), do not remove the noquery option from therestrict default command on publicly accessible
systems.
19.17.2. Configure Rate Limiting Access to an NTP Service
To enable rate limiting access to the NTP service running on a system, add thelimited option to the
restrict command as explained inSection 19.17.1, “Configure Access Control to an NTP Service”
. If you
do not want to use the default discard parameters, then also use the discard command as explained
here.
The discard command takes the following form:
discard average value minimum value monitor value
average — specifies the minimum average packet spacing to be permitted, it accepts an
argument in log2 seconds. The default value is 3 (
23 equates to 8 seconds).
minimum — specifies the minimum packet spacing to be permitted, it accepts an argument in
log2 seconds. The default value is 1 (
21 equates to 2 seconds).
monitor — specifies the discard probability for packets once the permitted rate limits have
been exceeded. The default value is 3000 seconds. This option is intended for servers that
receive 1000 or more requests per second.
Examples of the discard command are as follows:
discard average 4
discard average 4 minimum 2
19.17.3. Adding a Peer Address
To add the address of a peer, that is to say, the address of a server running an NTP service of the same
stratum, make use of the peer command in thentp.conf file.
The peer command takes the following form:
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peer address
where address is anIP unicast address or aDNS resolvable name. The address must only be that of a
system known to be a member of the same stratum. Peers should have at least one time source that is
different to each other. Peers are normally systems under the same administrative control.
19.17.4. Adding a Server Address
To add the address of a server, that is to say, the address of a server running an NTP service of a
higher stratum, make use of the server command in thentp.conf file.
The server command takes the following form:
server address
where address is anIP unicast address or aDNS resolvable name. The address of a remote reference
server or local reference clock from which packets are to be received.
19.17.5. Adding a Broadcast or Multicast Server Address
To add a broadcast or multicast address for sending, that is to say, the address to broadcast or
multicast NTP packets to, make use of thebroadcast command in thentp.conf file.
The broadcast and multicast modes require authentication by default. See Section 19.6,
“Authentication Options for NTP”.
The broadcast command takes the following form:
broadcast address
where address is anIP broadcast or multicast address to which packets are sent.
This command configures a system to act as an NTP broadcast server. The address used must be a
broadcast or a multicast address. Broadcast address implies the IPv4 address 255.255.255.255. By
default, routers do not pass broadcast messages. The multicast address can be an IPv4 Class D
address, or an IPv6 address. The IANA has assignedIPv4 multicast address224.0.1.1 and IPv6
address FF05::101 (site local) toNTP. Administratively scoped IPv4 multicast addresses can also be
used, as described in RFC 2365 Administratively Scoped IP Multicast
.
19.17.6. Adding a Manycast Client Address
To add a manycast client address, that is to say, to configure a multicast address to be used for NTP
server discovery, make use of the manycastclient command in thentp.conf file.
The manycastclient command takes the following form:
manycastclient address
where address is anIP multicast address from which packets are to be received. The client will send a
request to the address and select the best servers from the responses and ignore other servers. NTP
communication then uses unicast associations, as if the discovered NTP servers were listed in
ntp.conf.
This command configures a system to act as an NTP client. Systems can be both client and server at
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the same time.
19.17.7. Adding a Broadcast Client Address
To add a broadcast client address, that is to say, to configure a broadcast address to be monitored for
broadcast NTP packets, make use of thebroadcastclient command in thentp.conf file.
The broadcastclient command takes the following form:
broadcastclient
Enables the receiving of broadcast messages. Requires authentication by default. See Section 19.6,
“Authentication Options for NTP”.
the same time.
19.17.8. Adding a Manycast Server Address
To add a manycast server address, that is to say, to configure an address to allow the clients to
discover the server by multicasting NTP packets, make use of themanycastserver command in the
ntp.conf file.
The manycastserver command takes the following form:
manycastserver address
Enables the sending of multicast messages. Where address is the address to multicast to. This should
be used together with authentication to prevent service disruption.
This command configures a system to act as an NTP server. Systems can be both client and server at
the same time.
19.17.9. Adding a Multicast Client Address
To add a multicast client address, that is to say, to configure a multicast address to be monitored for
multicast NTP packets, make use of themulticastclient command in thentp.conf file.
The multicastclient command takes the following form:
multicastclient address
Enables the receiving of multicast messages. Where address is the address to subscribe to. This
should be used together with authentication to prevent service disruption.
the same time.
19.17.10. Configuring the Burst Option
Using the burst option against a public server is considered abuse. Do not use this option with public
NTP servers. Use it only for applications within your own organization.
To increase the average quality of time offset statistics, add the following option to the end of a
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To increase the average quality of time offset statistics, add the following option to the end of a
server command:
burst
At every poll interval, when the server responds, the system will send a burst of up to eight packets
instead of the usual one packet. For use with the server command to improve the average quality of
the time-offset calculations.
19.17.11. Configuring the iburst Option
To improve the time taken for initial synchronization, add the following option to the end of a server
command:
iburst
When the server is unreachable, send a burst of eight packets instead of the usual one packet. The
packet spacing is normally 2 s; however, the spacing between the first and second packets can be
changed with the calldelay command to allow additional time for a modem or ISDN call to complete.
For use with the server command to reduce the time taken for initial synchronization. This is now a
default option in the configuration file.
19.17.12. Configuring Symmetric Authentication Using a Key
To configure symmetric authentication using a key, add the following option to the end of a server or
peer command:
key number
where number is in the range1 to 65534 inclusive. This option enables the use of amessage
authentication code (MAC) in packets. This option is for use with the
peer, server, broadcast, and
manycastclient commands.
The option can be used in the /etc/ntp.conf file as follows:
server 192.168.1.1 key 10
broadcast 192.168.1.255 key 20
manycastclient 239.255.254.254 key 30
See also Section 19.6, “Authentication Options for NTP”
.
19.17.13. Configuring the Poll Interval
To change the default poll interval, add the following options to the end of a server or peer command:
minpoll value and maxpoll value
Options to change the default poll interval, where the interval in seconds will be calculated by raising
2 to the power of value, in other words, the interval is expressed inlog2 seconds. The defaultminpoll
value is 6, 26 equates to 64 s. The default value formaxpoll is 10, which equates to 1024 s. Allowed
values are in the range 3 to 17 inclusive, which equates to 8 s to 36.4 h respectively. These options are
for use with the peer or server. Setting a shortermaxpoll may improve clock accuracy.
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19.17.14. Configuring Server Preference
To specify that a particular server should be preferred above others of similar statistical quality, add
the following option to the end of a server or peer command:
prefer
Use this server for synchronization in preference to other servers of similar statistical quality. This
option is for use with the peer or server commands.
19.17.15. Configuring the Time-to-Live for NTP Packets
To specify that a particular time-to-live (TTL) value should be used in place of the default, add the
following option to the end of a server or peer command:
ttl value
Specify the time-to-live value to be used in packets sent by broadcast servers and multicast NTP
servers. Specify the maximum time-to-live value to use for the "expanding ring search" by a
manycast client. The default value is 127.
19.17.16. Configuring the NTP Version to Use
To specify that a particular version of NTP should be used in place of the default, add the following
option to the end of a server or peer command:
version value
Specify the version of NTP set in createdNTP packets. The value can be in the range1 to 4. The
default is 4.
19.18. CONFIGURING THE HARDWARE CLOCK UPDATE
The system clock can be used to update the hardware clock, also known as the real-time clock (RTC).
This section shows three approaches to the task:
Instant one-time update
To perform an instant one-time update of the hardware clock, run this command as root:
~]# hwclock --systohc
Update on every boot
To make the hardware clock update on every boot after executing the ntpdate synchronization
utility, do the following:
a. Add the following line to the /etc/sysconfig/ntpdate file:
SYNC_HWCLOCK=yes
b. Enable the ntpdate service as root:
~]# systemctl enable ntpdate.service
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Note that the ntpdate service uses the NTP servers defined in the/etc/ntp/step-tickers file.
NOTE
On virtual machines, the hardware clock will be updated on the next boot of
the host machine, not of the virtual machine.
Update via NTP
You can make the hardware clock update every time the system clock is updated by the ntpd or
chronyd service:
Start the ntpd service as root:
~]# systemctl start ntpd.service
To make the behavior persistent across boots, make the service start automatically at the boot
time:
~]# systemctl enable ntpd.service
or
Start the chronyd service as root:
~]# systemctl start chronyd.service
To make the behavior persistent across boots, make the service start automatically at the boot
time:
~]# systemctl enable chronyd.service
As a result, every time the system clock is synchronized by ntpd or chronyd, the kernel
automatically updates the hardware clock in 11 minutes.
WARNING
This approach might not always work because the above mentioned 11-minute
mode is not always enabled. As a consequence, the hardware clock does not
necessarily get updated on the system clock update.
To check the synchronization of the software clock with the hardware clock, use the ntpdc -c
kerninfo or thentptime command asroot:
~]# ntpdc -c kerninfo
The result may look like this:
pll offset: 0 s

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pll frequency: 0.000 ppm
maximum error: 8.0185 s
estimated error: 0 s
status: 2001 pll nano
pll time constant: 6
precision: 1e-09 s
frequency tolerance: 500 ppm
or
~]# ntptime
The result may look like this:
ntp_gettime() returns code 0 (OK)
time dcba5798.c3dfe2e0 Mon, May 8 2017 11:34:00.765, (.765135199),
maximum error 8010000 us, estimated error 0 us, TAI offset 0
ntp_adjtime() returns code 0 (OK)
modes 0x0 (),
offset 0.000 us, frequency 0.000 ppm, interval 1 s,
maximum error 8010000 us, estimated error 0 us,
status 0x2001 (PLL,NANO),
time constant 6, precision 0.001 us, tolerance 500 ppm,
To recognize whether the hardware clock is synchronized to the system clock, see the status line in
the output. If the line contains the word unsync or UNSYNC, the hardware clock is not
synchronized to the system clock.
The hardware clock is synchronized to the system clock.
status 0x2001 (PLL,NANO)
The hardware clock is not synchronized to the system clock.
status 0x41 (PLL,UNSYNC)
19.19. CONFIGURING CLOCK SOURCES
To list the available clock sources on your system, issue the following commands:
~]$ cd /sys/devices/system/clocksource/clocksource0/
clocksource0]$ cat available_clocksource
kvm-clock tsc hpet acpi_pm
clocksource0]$ cat current_clocksource
kvm-clock
In the above example, the kernel is using kvm-clock. This was selected at boot time as this is a virtual
machine. Note that the available clock source is architecture dependent.
To override the default clock source, append the clocksource directive to the end of the kernel’s
GRUB 2 menu entry. Use the grubby tool to make the change. For example, to force the default
kernel on a system to use the tsc clock source, enter a command as follows:
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~]# grubby --args=clocksource=tsc --update-kernel=DEFAULT
The --update-kernel parameter also accepts the keywordALL, or a comma separated list of kernel
index numbers.
See Chapter 26, Working with GRUB 2for more information on making changes to the GRUB 2 menu.
The following sources of information provide additional resources regarding NTP and ntpd.
ntpd(8) man page — Describesntpd in detail, including the command-line options.
ntp.conf(5) man page — Contains information on how to configure associations with servers
and peers.
ntpq(8) man page — Describes theNTP query utility for monitoring and querying anNTP
server.
ntpdc(8) man page — Describes thentpd utility for querying and changing the state ofntpd.
ntp_auth(5) man page — Describes authentication options, commands, and key management
for ntpd.
ntp_keygen(8) man page — Describes generating public and private keys for
ntpd.
ntp_acc(5) man page — Describes access control options using therestrict command.
ntp_mon(5) man page — Describes monitoring options for the gathering of statistics.
ntp_clock(5) man page — Describes commands for configuring reference clocks.
ntp_misc(5) man page — Describes miscellaneous options.
ntp_decode(5) man page — Lists the status words, event messages and error codes used for
ntpd reporting and monitoring.
ntpstat(8) man page — Describes a utility for reporting the synchronization state of the
NTP
daemon running on the local machine.
ntptime(8) man page — Describes a utility for reading and setting kernel time variables.
tickadj(8) man page — Describes a utility for reading, and optionally setting, the length of the
tick.
http://doc.ntp.org/
The NTP Documentation Archive
http://www.eecis.udel.edu/~mills/ntp.html
Network Time Synchronization Research Project.
http://www.eecis.udel.edu/~mills/ntp/html/manyopt.html
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Information on Automatic Server Discovery in NTPv4.
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20.1. INTRODUCTION TO PTP
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks in a network. When used in
conjunction with hardware support, PTP is capable of sub-microsecond accuracy, which is far better
than is normally obtainable with NTP. PTP support is divided between the kernel and user space. The
kernel in Red Hat Enterprise Linux includes support for PTP clocks, which are provided by network
drivers. The actual implementation of the protocol is known as linuxptp, a PTPv2 implementation
according to the IEEE standard 1588 for Linux.
The linuxptp package includes the ptp4l and phc2sys programs for clock synchronization. Theptp4l
program implements the PTP boundary clock and ordinary clock. With hardware time stamping, it is
used to synchronize the PTP hardware clock to the master clock, and with software time stamping it
synchronizes the system clock to the master clock. The phc2sys program is needed only with
hardware time stamping, for synchronizing the system clock to the PTP hardware clock on the
network interface card(NIC).
20.1.1. Understanding PTP
The clocks synchronized by PTP are organized in a master-slave hierarchy. The slaves are
synchronized to their masters which may be slaves to their own masters. The hierarchy is created and
updated automatically by the best master clock(BMC) algorithm, which runs on every clock. When a
clock has only one port, it can be master or slave, such a clock is called anordinary clock (OC). A clock
with multiple ports can be master on one port and slave on another, such a clock is called a boundary
clock (BC). The top-level master is called thegrandmaster clock, which can be synchronized by using
a Global Positioning System(GPS) time source. By using a GPS-based time source, disparate
networks can be synchronized with a high-degree of accuracy.
Figure 20.1. PTP grandmaster, boundary, and slave Clocks
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Figure 20.1. PTP grandmaster, boundary, and slave Clocks
20.1.2. Advantages of PTP
One of the main advantages that PTP has over theNetwork Time Protocol(NTP) is hardware support
present in various network interface controllers(NIC) and network switches. This specialized
hardware allows PTP to account for delays in message transfer, and greatly improves the accuracy of
time synchronization. While it is possible to use non-PTP enabled hardware components within the
network, this will often cause an increase in jitter or introduce an asymmetry in the delay resulting in
synchronization inaccuracies, which add up with multiple non-PTP aware components used in the
communication path. To achieve the best possible accuracy, it is recommended that all networking
components between PTP clocks arePTP hardware enabled. Time synchronization in larger networks
where not all of the networking hardware supports PTP might be better suited forNTP.
With hardware PTP support, the NIC has its own on-board clock, which is used to time stamp the
received and transmitted PTP messages. It is this on-board clock that is synchronized to the
PTP
master, and the computer’s system clock is synchronized to the PTP hardware clock on the NIC. With
software PTP support, the system clock is used to time stamp the
PTP messages and it is
synchronized to the PTP master directly. HardwarePTP support provides better accuracy since the
NIC can time stamp the PTP packets at the exact moment they are sent and received while software
PTP support requires additional processing of thePTP packets by the operating system.
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20.2. USING PTP
In order to use PTP, the kernel network driver for the intended interface has to support either
software or hardware time stamping capabilities.
20.2.1. Checking for Driver and Hardware Support
In addition to hardware time stamping support being present in the driver, the NIC must also be
capable of supporting this functionality in the physical hardware. The best way to verify the time
stamping capabilities of a particular driver and NIC is to use the ethtool utility to query the interface
as follows:
~]# ethtool -T eth3
Time stamping parameters for eth3:
Capabilities:
hardware-transmit (SOF_TIMESTAMPING_TX_HARDWARE)
software-transmit (SOF_TIMESTAMPING_TX_SOFTWARE)
hardware-receive (SOF_TIMESTAMPING_RX_HARDWARE)
software-receive (SOF_TIMESTAMPING_RX_SOFTWARE)
software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
hardware-raw-clock (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
off (HWTSTAMP_TX_OFF)
on (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
none (HWTSTAMP_FILTER_NONE)
all (HWTSTAMP_FILTER_ALL)
Where eth3 is the interface you want to check.
For software time stamping support, the parameters list should include:
SOF_TIMESTAMPING_SOFTWARE
SOF_TIMESTAMPING_TX_SOFTWARE
SOF_TIMESTAMPING_RX_SOFTWARE
For hardware time stamping support, the parameters list should include:
SOF_TIMESTAMPING_RAW_HARDWARE
SOF_TIMESTAMPING_TX_HARDWARE
SOF_TIMESTAMPING_RX_HARDWARE
20.2.2. Installing PTP
The kernel in Red Hat Enterprise Linux includes support for PTP. User space support is provided by
the tools in the linuxptp package. To installlinuxptp, issue the following command asroot:
~]# yum install linuxptp
This will install ptp4l and phc2sys.
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Do not run more than one service to set the system clock’s time at the same time. If you intend to
serve PTP time usingNTP, see Section 20.8, “Serving PTP Time with NTP”
.
20.2.3. Starting ptp4l
The ptp4l program can be started from the command line or it can be started as a service. When
running as a service, options are specified in the /etc/sysconfig/ptp4l file. Options required for use
both by the service and on the command line should be specified in the /etc/ptp4l.conf file. The
/etc/sysconfig/ptp4l file includes the-f /etc/ptp4l.conf command line option, which causes theptp4l
program to read the /etc/ptp4l.conf file and process the options it contains. The use of the
/etc/ptp4l.conf is explained inSection 20.4, “Specifying a Configuration File”. More information on
the different ptp4l options and the configuration file settings can be found in the
ptp4l(8) man page.
Starting ptp4l as a Service
To start ptp4l as a service, issue the following command asroot:
~]# systemctl start ptp4l
For more information on managing system services in Red Hat Enterprise Linux 7, see Chapter 10,
Managing Services with systemd.
Using ptp4l From The Command Line
The ptp4l program tries to use hardware time stamping by default. To use
ptp4l with hardware time
stamping capable drivers and NICs, you must provide the network interface to use with the -i option.
Enter the following command as root:
~]# ptp4l -i eth3 -m
Where eth3 is the interface you want to configure. Below is example output fromptp4l when thePTP
clock on the NIC is synchronized to a master:
~]# ptp4l -i eth3 -m
selected eth3 as PTP clock
port 1: INITIALIZING to LISTENING on INITIALIZE
port 0: INITIALIZING to LISTENING on INITIALIZE
port 1: new foreign master 00a069.fffe.0b552d-1
selected best master clock 00a069.fffe.0b552d
port 1: LISTENING to UNCALIBRATED on RS_SLAVE
master offset -23947 s0 freq +0 path delay 11350
master offset -7275 s2 freq -30575 path delay 10583
port 1: UNCALIBRATED to SLAVE on MASTER_CLOCK_SELECTED
master offset -4552 s2 freq -30035 path delay 10385
The master offset value is the measured offset from the master in nanoseconds. The s0, s1, s2 strings
indicate the different clock servo states: s0 is unlocked,s1 is clock step ands2 is locked. Once the
servo is in the locked state (s2), the clock will not be stepped (only slowly adjusted) unless the
pi_offset_const option is set to a positive value in the configuration file (described in the
ptp4l(8)
man page). The adj value is the frequency adjustment of the clock in parts per billion (ppb). The path
delay value is the estimated delay of the synchronization messages sent from the master in
nanoseconds. Port 0 is a Unix domain socket used for local PTP management. Port 1 is theeth3
interface (based on the example above.) INITIALIZING, LISTENING, UNCALIBRATED and SLAVE are
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some of possible port states which change on the INITIALIZE, RS_SLAVE,
MASTER_CLOCK_SELECTED events. In the last state change message, the port state changed from
UNCALIBRATED to SLAVE indicating successful synchronization with a PTP master clock.
Logging Messages From ptp4l
By default, messages are sent to /var/log/messages. However, specifying the-m option enables
logging to standard output which can be useful for debugging purposes.
To enable software time stamping, the -S option needs to be used as follows:
~]# ptp4l -i eth3 -m -S
20.2.3.1. Selecting a Delay Measurement Mechanism
There are two different delay measurement mechanisms and they can be selected by means of an
option added to the ptp4l command as follows:
-P
The -P selects thepeer-to-peer (P2P) delay measurement mechanism.
The P2P mechanism is preferred as it reacts to changes in the network topology faster, and may
be more accurate in measuring the delay, than other mechanisms. The P2P mechanism can only be
used in topologies where each port exchanges PTP messages with at most one otherP2P port. It
must be supported and used by all hardware, including transparent clocks, on the communication
path.
-E
The -E selects theend-to-end (E2E) delay measurement mechanism. This is the default.
The E2E mechanism is also referred to as the delay "request-response" mechanism.
-A
The -A enables automatic selection of the delay measurement mechanism.
The automatic option starts ptp4l inE2E mode. It will change toP2P mode if a peer delay request
is received.
NOTE
All clocks on a single PTP communication path must use the same mechanism to
measure the delay. Warnings will be printed in the following circumstances:
When a peer delay request is received on a port using the E2E mechanism.
When a E2E delay request is received on a port using theP2P mechanism.
20.3. USING PTP WITH MULTIPLE INTERFACES
When using PTP with multiple interfaces in different networks, it is necessary to change the reverse
path forwarding mode to loose mode. Red Hat Enterprise Linux 7 defaults to using
Strict Reverse
Path Forwarding following the Strict Reverse Path recommendation fromRFC 3704, Ingress Filtering
for Multihomed Networks. See theReverse Path Forwarding section in theRed Hat Enterprise Linux 7
Security Guide for more details.
The sysctl utility is used to read and write values to tunables in the kernel. Changes to a running
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The sysctl utility is used to read and write values to tunables in the kernel. Changes to a running
system can be made using sysctl commands directly on the command line and permanent changes
can be made by adding lines to the /etc/sysctl.conf file.
To change to loose mode filtering globally, enter the following commands as root:
~]# sysctl -w net.ipv4.conf.default.rp_filter=2
~]# sysctl -w net.ipv4.conf.all.rp_filter=2
To change the reverse path filtering mode per network interface, use the
net.ipv4.interface.rp_filter command on all PTP interfaces. For example, for an interface with
device name em1:
~]# sysctl -w net.ipv4.conf.em1.rp_filter=2
To make these settings persistent across reboots, modify the /etc/sysctl.conf file. You can change
the mode for all interfaces, or for a particular interface.
To change the mode for all interfaces, open the /etc/sysctl.conf file with an editor running as theroot
user and add a line as follows:
net.ipv4.conf.all.rp_filter=2
To change only certain interfaces, add multiple lines in the following format:
net.ipv4.conf.interface.rp_filter=2
NOTE
When using the settings for all and particular interfaces as well, maximum value from
conf/{all,interface}/rp_filter is used when doing source validation on each interface.
You can also change the mode by using the default setting, which means that it applies only to the
newly created interfaces.
For more information on using the all, default, or a specific device settings in thesysctl parameters,
see the Red Hat Knowledgebase article What is the difference between "all", "default" and a specific
device in a sysctl parameter?.
Note that you might experience issues of two types due to the timing of the sysctl service run during
the boot process:
1. Drivers are loaded before the sysctl service runs.
In this case, affected network interfaces use the mode preset from the kernel, and sysctl
defaults are ignored.
For solution of this problem, see the Red Hat Knowledgebase article What is the difference
between "all", "default" and a specific device in a sysctl parameter?.
2. Drivers are loaded or reloaded after the sysctl service runs.
In this case, it is possible that some sysctl.conf parameters are not used after reboot. These
settings may not be available or they may return to defaults.
For solution of this problem, see the Red Hat Knowledgebase article Some sysctl.conf
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For solution of this problem, see the Red Hat Knowledgebase article Some sysctl.conf
parameters are not used after reboot, manually adjusting the settings works as expected.
20.4. SPECIFYING A CONFIGURATION FILE
The command line options and other options, which cannot be set on the command line, can be set in
an optional configuration file.
No configuration file is read by default, so it needs to be specified at runtime with the -f option. For
example:
~]# ptp4l -f /etc/ptp4l.conf
A configuration file equivalent to the -i eth3 -m -S options shown above would look as follows:
~]# cat /etc/ptp4l.conf
[global]
verbose 1
time_stamping software
[eth3]
20.5. USING THE PTP MANAGEMENT CLIENT
The PTP management client, pmc, can be used to obtain additional information fromptp4l as follows:
~]# pmc -u -b 0 'GET CURRENT_DATA_SET'
sending: GET CURRENT_DATA_SET
90e2ba.fffe.20c7f8-0 seq 0 RESPONSE MANAGMENT CURRENT_DATA_SET
stepsRemoved 1
offsetFromMaster -142.0
meanPathDelay 9310.0
~]# pmc -u -b 0 'GET TIME_STATUS_NP'
sending: GET TIME_STATUS_NP
90e2ba.fffe.20c7f8-0 seq 0 RESPONSE MANAGMENT TIME_STATUS_NP
master_offset 310
ingress_time 1361545089345029441
cumulativeScaledRateOffset +1.000000000
scaledLastGmPhaseChange 0
gmTimeBaseIndicator 0
lastGmPhaseChange 0x0000'0000000000000000.0000
gmPresent true
gmIdentity 00a069.fffe.0b552d
Setting the -b option tozero limits the boundary to the locally runningptp4l instance. A larger
boundary value will retrieve the information also from PTP nodes further from the local clock. The
retrievable information includes:
stepsRemoved is the number of communication paths to the grandmaster clock.
offsetFromMaster and master_offset is the last measured offset of the clock from the master
in nanoseconds.
meanPathDelay is the estimated delay of the synchronization messages sent from the
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meanPathDelay is the estimated delay of the synchronization messages sent from the
master in nanoseconds.
if gmPresent is true, thePTP clock is synchronized to a master, the local clock is not the
grandmaster clock.
gmIdentity is the grandmaster’s identity.
For a full list of pmc commands, type the following asroot:
~]# pmc help
Additional information is available in the pmc(8) man page.
20.6. SYNCHRONIZING THE CLOCKS
The phc2sys program is used to synchronize the system clock to the
PTP hardware clock (
PHC) on
the NIC. The phc2sys service is configured in the/etc/sysconfig/phc2sys configuration file. The
default setting in the /etc/sysconfig/phc2sys file is as follows:
OPTIONS="-a -r"
The -a option causesphc2sys to read the clocks to be synchronized from theptp4l application. It will
follow changes in the PTP port states, adjusting the synchronization between the NIC hardware clocks
accordingly. The system clock is not synchronized, unless the -r option is also specified. If you want
the system clock to be eligible to become a time source, specify the -r option twice.
After making changes to /etc/sysconfig/phc2sys, restart the phc2sys service from the command line
by issuing a command as root:
~]# systemctl restart phc2sys
Under normal circumstances, use systemctl commands to start, stop, and restart thephc2sys service.
When you do not want to start phc2sys as a service, you can start it from the command line. For
example, enter the following command as root:
~]# phc2sys -a -r
The -a option causesphc2sys to read the clocks to be synchronized from theptp4l application. If you
want the system clock to be eligible to become a time source, specify the -r option twice.
Alternately, use the -s option to synchronize the system clock to a specific interface’s
PTP hardware
clock. For example:
~]# phc2sys -s eth3 -w
The -w option waits for the runningptp4l application to synchronize thePTP clock and then retrieves
the TAI to UTC offset fromptp4l.
Normally, PTP operates in theInternational Atomic Time(TAI) timescale, while the system clock is
kept in Coordinated Universal Time (UTC). The current offset between the TAI and UTC timescales is
36 seconds. The offset changes when leap seconds are inserted or deleted, which typically happens
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every few years. The -O option needs to be used to set this offset manually when the
-w is not used,
as follows:
~]# phc2sys -s eth3 -O -36
Once the phc2sys servo is in a locked state, the clock will not be stepped, unless the
-S option is used.
This means that the phc2sys program should be started after theptp4l program has synchronized the
PTP hardware clock. However, with-w, it is not necessary to startphc2sys afterptp4l as it will wait
for it to synchronize the clock.
The phc2sys program can also be started as a service by running:
~]# systemctl start phc2sys
When running as a service, options are specified in the /etc/sysconfig/phc2sys file. More information
on the different phc2sys options can be found in thephc2sys(8) man page.
Note that the examples in this section assume the command is run on a slave system or slave port.
20.7. VERIFYING TIME SYNCHRONIZATION
When PTP time synchronization is working correctly, new messages with offsets and frequency
adjustments are printed periodically to the ptp4l and phc2sys outputs if hardware time stamping is
used. The output values converge shortly. You can see these messages in the /var/log/messages file.
The following examples of the ptp4l and thephc2sys output contain:
offset (in nanoseconds)
frequency offset (in parts per billion (ppb))
path delay (in nanoseconds)
Example of the ptp4l output:
ptp4l[352.359]: selected /dev/ptp0 as PTP clock
ptp4l[352.361]: port 1: INITIALIZING to LISTENING on INITIALIZE
ptp4l[352.361]: port 0: INITIALIZING to LISTENING on INITIALIZE
ptp4l[353.210]: port 1: new foreign master 00a069.fffe.0b552d-1
ptp4l[357.214]: selected best master clock 00a069.fffe.0b552d
ptp4l[357.214]: port 1: LISTENING to UNCALIBRATED on RS_SLAVE
ptp4l[359.224]: master offset 3304 s0 freq +0 path delay 9202
ptp4l[360.224]: master offset 3708 s1 freq -29492 path delay 9202
ptp4l[361.224]: master offset -3145 s2 freq -32637 path delay 9202
ptp4l[361.224]: port 1: UNCALIBRATED to SLAVE on MASTER_CLOCK_SELECTED
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Example of the phc2sys output:
phc2sys[526.527]: Waiting for ptp4l...
phc2sys[527.528]: Waiting for ptp4l...
phc2sys[528.528]: phc offset 55341 s0 freq +0 delay 2729
phc2sys[529.528]: phc offset 54658 s1 freq -37690 delay 2725
phc2sys[533.528]: phc offset -73 s2 freq -37026 delay 2764
To reduce the ptp4l output and print only the values, use thesummary_interval directive. The
summary_interval directive is specified as 2 to the power of
n in seconds. For example, to reduce the
output to every 1024 seconds, add the following line to the/etc/ptp4l.conf file:
summary_interval 10
An example of the ptp4l output, withsummary_interval set to 6:
ptp4l: [615.253] selected /dev/ptp0 as PTP clock
ptp4l: [615.255] port 1: INITIALIZING to LISTENING on INITIALIZE
ptp4l: [615.255] port 0: INITIALIZING to LISTENING on INITIALIZE
ptp4l: [615.564] port 1: new foreign master 00a069.fffe.0b552d-1
ptp4l: [619.574] selected best master clock 00a069.fffe.0b552d
ptp4l: [619.574] port 1: LISTENING to UNCALIBRATED on RS_SLAVE
ptp4l: [623.573] port 1: UNCALIBRATED to SLAVE on MASTER_CLOCK_SELECTED
ptp4l: [684.649] rms 669 max 3691 freq -29383 ± 3735 delay 9232 ± 122
By default, summary_interval is set to 0, so messages are printed once per second, which is the
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By default, summary_interval is set to 0, so messages are printed once per second, which is the
maximum frequency. The messages are logged at the LOG_INFO level. To disable messages, use the -
l option to set the maximum log level to 5 or lower:
~]# phc2sys -l 5
You can use the -u option to reduce thephc2sys output:
~]# phc2sys -u summary-updates
Where summary-updates is the number of clock updates to include in summary statistics. An example
follows:
~]# phc2sys -s eth3 -w -m -u 60
phc2sys[700.948]: rms 1837 max 10123 freq -36474 ± 4752 delay 2752 ± 16
When used with these options, the interval for updating the statistics is set to 60 seconds (-u),
phc2sys waits untilptp4l is in synchronized state (
-w), and messages are printed to the standard
output (-m). For further details about thephc2sys options, see thephc2sys(5) man page.
The output includes:
offset root mean square (rms)
maximum absolute offset (max)
frequency offset (freq): its mean, and standard deviation
path delay (delay): its mean, and standard deviation
20.8. SERVING PTP TIME WITH NTP
The ntpd daemon can be configured to distribute the time from the system clock synchronized by
ptp4l or phc2sys by using the LOCAL reference clock driver. To prevent
ntpd from adjusting the
system clock, the ntp.conf file must not specify anyNTP servers. The following is a minimal example
of ntp.conf:
~]# cat /etc/ntp.conf
server 127.127.1.0
fudge 127.127.1.0 stratum 0
NOTE
When the DHCP client program,dhclient, receives a list ofNTP servers from theDHCP
server, it adds them to ntp.conf and restarts the service. To disable that feature, add
PEERNTP=no to /etc/sysconfig/network.
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20.9. SERVING NTP TIME WITH PTP
NTP to PTP synchronization in the opposite direction is also possible. Whenntpd is used to
synchronize the system clock, ptp4l can be configured with thepriority1 option (or other clock
options included in the best master clock algorithm) to be the grandmaster clock and distribute the
time from the system clock via PTP:
~]# cat /etc/ptp4l.conf
[global]
priority1 127
[eth3]
# ptp4l -f /etc/ptp4l.conf
With hardware time stamping, phc2sys needs to be used to synchronize thePTP hardware clock to
the system clock. If running phc2sys as a service, edit the/etc/sysconfig/phc2sys configuration file.
The default setting in the /etc/sysconfig/phc2sys file is as follows:
OPTIONS="-a -r"
As root, edit that line as follows:
~]# vi /etc/sysconfig/phc2sys
OPTIONS="-a -r -r"
The -r option is used twice here to allow synchronization of thePTP hardware clock on the NIC from
the system clock. Restart the phc2sys service for the changes to take effect:
To prevent quick changes in the PTP clock’s frequency, the synchronization to the system clock can
be loosened by using smaller P (proportional) andI (integral) constants for the PI servo:
~]# phc2sys -a -r -r -P 0.01 -I 0.0001
20.10. SYNCHRONIZE TO PTP OR NTP TIME USING TIMEMASTER
When there are multiple PTP domains available on the network, or fallback toNTP is needed, the
timemaster program can be used to synchronize the system clock to all available time sources. The
PTP time is provided byphc2sys and ptp4l via shared memory driver (SHM reference clocks to
chronyd or ntpd (depending on theNTP daemon that has been configured on the system). The
NTP
daemon can then compare all time sources, both PTP and NTP, and use the best sources to
synchronize the system clock.
On start, timemaster reads a configuration file that specifies theNTP and PTP time sources, checks
which network interfaces have their own or share a PTP hardware clock (PHC), generates
configuration files for ptp4l and chronyd or ntpd, and starts theptp4l, phc2sys, and chronyd or ntpd
processes as needed. It will remove the generated configuration files on exit. It writes configuration
files for chronyd, ntpd, and ptp4l to /var/run/timemaster/.
20.10.1. Starting timemaster as a Service
To start timemaster as a service, issue the following command asroot:
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~]# systemctl start timemaster
This will read the options in /etc/timemaster.conf. For more information on managing system services
in Red Hat Enterprise Linux 7, see Chapter 10, Managing Services with systemd.
20.10.2. Understanding the timemaster Configuration File
Red Hat Enterprise Linux provides a default /etc/timemaster.conf file with a number of sections
containing default options. The section headings are enclosed in brackets.
To view the default configuration, issue a command as follows:
~]$ less /etc/timemaster.conf
# Configuration file for timemaster
#[ntp_server ntp-server.local]
#minpoll 4
#maxpoll 4
#[ptp_domain 0]
#interfaces eth0
[timemaster]
ntp_program chronyd
[chrony.conf]
include /etc/chrony.conf
[ntp.conf]
includefile /etc/ntp.conf
[ptp4l.conf]
[chronyd]
path /usr/sbin/chronyd
options -u chrony
[ntpd]
path /usr/sbin/ntpd
options -u ntp:ntp -g
[phc2sys]
path /usr/sbin/phc2sys
[ptp4l]
path /usr/sbin/ptp4l
Notice the section named as follows:
[ntp_server address]
This is an example of an NTP server section, "ntp-server.local" is an example of a host name for an
NTP server on the local LAN. Add more sections as required using a host name or
IP address as part
of the section name. Note that the short polling values in that example section are not suitable for a
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public server, see Chapter 19, Configuring NTP Using ntpdfor an explanation of suitableminpoll and
maxpoll values.
[ptp_domain number]
A "PTP domain" is a group of one or more PTP clocks that synchronize to each other. They may or
may not be synchronized to clocks in another domain. Clocks that are configured with the same
domain number make up the domain. This includes a PTP grandmaster clock. The domain number in
each "PTP domain" section needs to correspond to one of the PTP domains configured on the
network.
An instance of ptp4l is started for every interface which has its own
PTP clock and hardware time
stamping is enabled automatically. Interfaces that support hardware time stamping have a PTP clock
(PHC) attached, however it is possible for a group of interfaces on a NIC to share a PHC. A separate
ptp4l instance will be started for each group of interfaces sharing the same PHC and for each
interface that supports only software time stamping. All ptp4l instances are configured to run as a
slave. If an interface with hardware time stamping is specified in more than one PTP domain, then
only the first ptp4l instance created will have hardware time stamping enabled.
[timemaster]
The default timemaster configuration includes the systemntpd and chrony configuration
(/etc/ntp.conf or /etc/chronyd.conf) in order to include the configuration of access restrictions and
authentication keys. That means any NTP servers specified there will be used withtimemaster too.
The section headings are as follows:
[ntp_server ntp-server.local] — Specify polling intervals for this server. Create additional
sections as required. Include the host name or IP address in the section heading.
[ptp_domain 0] — Specify interfaces that havePTP clocks configured for this domain. Create
additional sections with, the appropriate domain number, as required.
[timemaster] — Specify theNTP daemon to be used. Possible values arechronyd and ntpd.
[chrony.conf] — Specify any additional settings to be copied to the configuration file
generated for chronyd.
[ntp.conf] — Specify any additional settings to be copied to the configuration file generated
for ntpd.
[ptp4l.conf] — Specify options to be copied to the configuration file generated for
ptp4l.
[chronyd] — Specify any additional settings to be passed on the command line to
chronyd.
[ntpd] — Specify any additional settings to be passed on the command line to
ntpd.
[phc2sys] — Specify any additional settings to be passed on the command line to
phc2sys.
[ptp4l] — Specify any additional settings to be passed on the command line to all instances of
ptp4l.
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The section headings and there contents are explained in detail in the timemaster(8) manual page.
20.10.3. Configuring timemaster Options
Editing the timemaster Configuration File
1. To change the default configuration, open the /etc/timemaster.conf file for editing asroot:
~]# vi /etc/timemaster.conf
2. For each NTP server you want to control usingtimemaster, create [ntp_server address]
sections. Note that the short polling values in the example section are not suitable for a
public server, see Chapter 19, Configuring NTP Using ntpdfor an explanation of suitable
minpoll and maxpoll values.
3. To add interfaces that should be used in a domain, edit the #[ptp_domain 0] section and add
the interfaces. Create additional domains as required. For example:
[ptp_domain 0]
interfaces eth0
[ptp_domain 1]
interfaces eth1
4. If required to use ntpd as theNTP daemon on this system, change the default entry in the
[timemaster] section from chronyd to ntpd. See Chapter 18, Configuring NTP Using the
chrony Suite for information on the differences between ntpd and chronyd.
5. If using chronyd as theNTP server on this system, add any additional options below the
default include /etc/chrony.conf entry in the[chrony.conf] section. Edit the defaultinclude
entry if the path to /etc/chrony.conf is known to have changed.
6. If using ntpd as theNTP server on this system, add any additional options below the default
include /etc/ntp.conf entry in the[ntp.conf] section. Edit the defaultinclude entry if the path
to /etc/ntp.conf is known to have changed.
7. In the [ptp4l.conf] section, add any options to be copied to the configuration file generated
for ptp4l. This chapter documents common options and more information is available in the
ptp4l(8) manual page.
8. In the [chronyd] section, add any command line options to be passed tochronyd when called
by timemaster. See Chapter 18, Configuring NTP Using the chrony Suitefor information on
using chronyd.
9. In the [ntpd] section, add any command line options to be passed tontpd when called by
timemaster. See Chapter 19, Configuring NTP Using ntpdfor information on usingntpd.
10. In the [phc2sys] section, add any command line options to be passed tophc2sys when called
by timemaster. This chapter documents common options and more information is available in
the phy2sys(8) manual page.
11. In the [ptp4l] section, add any command line options to be passed toptp4l when called by
timemaster. This chapter documents common options and more information is available in
the ptp4l(8) manual page.
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12. Save the configuration file and restart timemaster by issuing the following command asroot:
~]# systemctl restart timemaster
20.11. IMPROVING ACCURACY
Previously, test results indicated that disabling the tickless kernel capability could significantly
improve the stability of the system clock, and thus improve the PTP synchronization accuracy (at the
cost of increased power consumption). The kernel tickless mode can be disabled by adding nohz=off
to the kernel boot option parameters. However, recent improvements applied to kernel-3.10.0-
197.el7 have greatly improved the stability of the system clock and the difference in stability of the
clock with and without nohz=off should be much smaller now for most users.
The ptp4l and phc2sys applications can be configured to use a new adaptive servo. The advantage
over the PI servo is that it does not require configuration of the PI constants to perform well. To
make use of this for ptp4l, add the following line to the/etc/ptp4l.conf file:
clock_servo linreg
After making changes to /etc/ptp4l.conf, restart the ptp4l service from the command line by issuing
~]# systemctl restart ptp4l
To make use of this for phc2sys, add the following line to the/etc/sysconfig/phc2sys file:
-E linreg
After making changes to /etc/sysconfig/phc2sys, restart the phc2sys service from the command line
by issuing the following command as root:
The following sources of information provide additional resources regarding PTP and theptp4l tools.
ptp4l(8) man page — Describesptp4l options including the format of the configuration file.
pmc(8) man page — Describes thePTP management client and its command options.
phc2sys(8) man page — Describes a tool for synchronizing the system clock to a
PTP
hardware clock (PHC).
timemaster(8) man page — Describes a program that usesptp4l and phc2sys to synchronize
the system clock using chronyd or ntpd.
http://www.nist.gov/el/isd/ieee/ieee1588.cfm
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The IEEE 1588 Standard.
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PART VI. MONITORING AND AUTOMATION
This part describes various tools that allow system administrators to monitor system performance,
automate system tasks, and report bugs.
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In order to configure the system, system administrators often need to determine the amount of free
memory, how much free disk space is available, how the hard drive is partitioned, or what processes
are running.
21.1. VIEWING SYSTEM PROCESSES
21.1.1. Using the ps Command
The ps command allows you to display information about running processes. It produces a static list,
that is, a snapshot of what is running when you execute the command. If you want a constantly
updated list of running processes, use the top command or theSystem Monitor application instead.
To list all processes that are currently running on the system including processes owned by other
users, type the following at a shell prompt:
ps ax
For each listed process, the ps ax command displays the process ID (
PID), the terminal that is
associated with it (TTY), the current status (
STAT), the cumulated CPU time (
TIME), and the name of
the executable file (COMMAND). For example:
~]$ ps ax
PID TTY STAT TIME COMMAND
1 ? Ss 0:01 /usr/lib/systemd/systemd --switched-root --system --deserialize 23
2 ? S 0:00 [kthreadd]
3 ? S 0:00 [ksoftirqd/0]
5 ? S> 0:00 [kworker/0:0H]
[output truncated]
To display the owner alongside each process, use the following command:
ps aux
Apart from the information provided by the ps ax command,ps aux displays the effective user name
of the process owner (USER), the percentage of the CPU (
%CPU) and memory (%MEM) usage, the
virtual memory size in kilobytes (VSZ), the non-swapped physical memory size in kilobytes (
RSS), and
the time or date the process was started. For example:
~]$ ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.3 0.3 134776 6840 ? Ss 09:28 0:01 /usr/lib/systemd/systemd --switched-root --
system --d
root 2 0.0 0.0 0 0 ? S 09:28 0:00 [kthreadd]
root 3 0.0 0.0 0 0 ? S 09:28 0:00 [ksoftirqd/0]
root 5 0.0 0.0 0 0 ? S> 09:28 0:00 [kworker/0:0H]
[output truncated]
You can also use the ps command in a combination withgrep to see if a particular process is running.
For example, to determine if Emacs is running, type:
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~]$ ps ax | grep emacs
12056 pts/3 S+ 0:00 emacs
12060 pts/2 S+ 0:00 grep --color=auto emacs
For a complete list of available command line options, see the ps(1) manual page.
21.1.2. Using the top Command
The top command displays a real-time list of processes that are running on the system. It also
displays additional information about the system uptime, current CPU and memory usage, or total
number of running processes, and allows you to perform actions such as sorting the list or killing a
process.
To run the top command, type the following at a shell prompt:
top
For each listed process, the top command displays the process ID (
PID), the effective user name of
the process owner (USER), the priority (PR), the nice value (NI), the amount of virtual memory the
process uses (VIRT), the amount of non-swapped physical memory the process uses (
RES), the
amount of shared memory the process uses (SHR), the process status fieldS), the percentage of the
CPU (%CPU) and memory (%MEM) usage, the cumulated CPU time (
TIME+), and the name of the
executable file (COMMAND). For example:
~]$ top
top - 16:42:12 up 13 min, 2 users, load average: 0.67, 0.31, 0.19
Tasks: 165 total, 2 running, 163 sleeping, 0 stopped, 0 zombie
%Cpu(s): 37.5 us, 3.0 sy, 0.0 ni, 59.5 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 1016800 total, 77368 free, 728936 used, 210496 buff/cache
KiB Swap: 839676 total, 776796 free, 62880 used. 122628 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3168 sjw 20 0 1454628 143240 15016 S 20.3 14.1 0:22.53 gnome-shell
4006 sjw 20 0 1367832 298876 27856 S 13.0 29.4 0:15.58 firefox
1683 root 20 0 242204 50464 4268 S 6.0 5.0 0:07.76 Xorg
4125 sjw 20 0 555148 19820 12644 S 1.3 1.9 0:00.48 gnome-terminal-
10 root 20 0 0 0 0 S 0.3 0.0 0:00.39 rcu_sched
3091 sjw 20 0 37000 1468 904 S 0.3 0.1 0:00.31 dbus-daemon
3096 sjw 20 0 129688 2164 1492 S 0.3 0.2 0:00.14 at-spi2-registr
3925 root 20 0 0 0 0 S 0.3 0.0 0:00.05 kworker/0:0
1 root 20 0 126568 3884 1052 S 0.0 0.4 0:01.61 systemd
2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd
3 root 20 0 0 0 0 S 0.0 0.0 0:00.00 ksoftirqd/0
6 root 20 0 0 0 0 S 0.0 0.0 0:00.07 kworker/u2:0
[output truncated]
Table 21.1, “Interactive top commands” contains useful interactive commands that you can use with
top. For more information, see thetop(1) manual page.
Table 21.1. Interactive top commands
Command Description
400

Enter, Space Immediately refreshes the display.
h Displays a help screen for interactive commands.
h, ? Displays a help screen for windows and field groups.
k Kills a process. You are prompted for the process ID
and the signal to send to it.
n Changes the number of displayed processes. You are
prompted to enter the number.
u Sorts the list by user.
M Sorts the list by memory usage.
P Sorts the list by CPU usage.
q Terminates the utility and returns to the shell prompt.
Command Description
The Processes tab of theSystem Monitor tool allows you to view, search for, change the priority of,
and kill processes from the graphical user interface.
To start the System Monitor tool from the command line, typegnome-system-monitor at a shell
prompt. The System Monitor tool appears. Alternatively, if using the GNOME desktop, press the
Super key to enter the Activities Overview, typeSystem Monitor and then pressEnter. TheSystem
Monitor tool appears. TheSuper key appears in a variety of guises, depending on the keyboard and
other hardware, but often as either the Windows or Command key, and typically to the left of the
Spacebar.
Click the Processes tab to view the list of running processes.
Figure 21.1. System Monitor — Processes
401

Figure 21.1. System Monitor — Processes
For each listed process, the System Monitor tool displays its name (
Process Name), current status
(Status), percentage of the CPU usage (
% CPU), nice value (Nice), process ID (
ID), memory usage
(Memory), the channel the process is waiting in (
Waiting Channel), and additional details about the
session (Session). To sort the information by a specific column in ascending order, click the name of
that column. Click the name of the column again to toggle the sort between ascending and
descending order.
By default, the System Monitor tool displays a list of processes that are owned by the current user.
Selecting various options from the View menu allows you to:
view only active processes,
view all processes,
view your processes,
view process dependencies,
Additionally, two buttons enable you to:
refresh the list of processes,
end a process by selecting it from the list and then clicking the End Process button.
21.2. VIEWING MEMORY USAGE
21.2.1. Using the free Command
The free command allows you to display the amount of free and used memory on the system. To do so,
free
The free command provides information about both the physical memory (
Mem) and swap space
402

The free command provides information about both the physical memory (
Mem) and swap space
(Swap). It displays the total amount of memory (
total), as well as the amount of memory that is in use
(used), free (free), shared (shared), sum of buffers and cached (
buff/cache), and available
(available). For example:
~]$ free
total used free shared buff/cache available
Mem: 1016800 727300 84684 3500 204816 124068
Swap: 839676 66920 772756
By default, free displays the values in kilobytes. To display the values in megabytes, supply the
-m
command line option:
free -m
For instance:
~]$ free -m
total used free shared buff/cache available
Mem: 992 711 81 3 200 120
Swap: 819 65 754
For a complete list of available command line options, see the free(1) manual page.
The Resources tab of theSystem Monitor tool allows you to view the amount of free and used
memory on the system.
Spacebar.
Click the Resources tab to view the system’s memory usage.
Figure 21.2. System Monitor — Resources
403

Figure 21.2. System Monitor — Resources
In the Memory and Swap History section, theSystem Monitor tool displays a graphical
representation of the memory and swap usage history, as well as the total amount of the physical
memory (Memory) and swap space (
Swap) and how much of it is in use.
21.3. VIEWING CPU USAGE
The Resources tab of theSystem Monitor tool allows you to view the current CPU usage on the
system.
Spacebar.
Click the Resources tab to view the system’s CPU usage.
In the CPU History section, theSystem Monitor tool displays a graphical representation of the CPU
usage history and shows the percentage of how much CPU is currently in use.
21.4. VIEWING BLOCK DEVICES AND FILE SYSTEMS
21.4.1. Using the lsblk Command
The lsblk command allows you to display a list of available block devices. It provides more information
and better control on output formatting than the blkid command. It reads information fromudev,
therefore it is usable by non-root users. To display a list of block devices, type the following at a shell
prompt:
404

lsblk
For each listed block device, the lsblk command displays the device name (
NAME), major and minor
device number (MAJ:MIN), if the device is removable (
RM), its size (
SIZE), if the device is read-only
(RO), what type it is (
TYPE), and where the device is mounted (
MOUNTPOINT). For example:
~]$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sr0 11:0 1 1024M 0 rom
vda 252:0 0 20G 0 rom
|-vda1 252:1 0 500M 0 part /boot
`-vda2 252:2 0 19.5G 0 part
|-vg_kvm-lv_root (dm-0) 253:0 0 18G 0 lvm /
`-vg_kvm-lv_swap (dm-1) 253:1 0 1.5G 0 lvm [SWAP]
By default, lsblk lists block devices in a tree-like format. To display the information as an ordinary list,
add the -l command line option:
lsblk -l
For instance:
~]$ lsblk -l
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sr0 11:0 1 1024M 0 rom
vda 252:0 0 20G 0 rom
vda1 252:1 0 500M 0 part /boot
vda2 252:2 0 19.5G 0 part
vg_kvm-lv_root (dm-0) 253:0 0 18G 0 lvm /
vg_kvm-lv_swap (dm-1) 253:1 0 1.5G 0 lvm [SWAP]
For a complete list of available command line options, see the lsblk(8) manual page.
21.4.2. Using the blkid Command
The blkid command allows you to display low-level information about available block devices. It
requires root privileges, therefore non-root users should use thelsblk command. To do so, type the
following at a shell prompt as root:
blkid
For each listed block device, the blkid command displays available attributes such as itsuniversally
unique identifier (UUID), file system type (
TYPE), or volume label (LABEL). For example:
~]# blkid
/dev/vda1: UUID="7fa9c421-0054-4555-b0ca-b470a97a3d84" TYPE="ext4"
/dev/vda2: UUID="7IvYzk-TnnK-oPjf-ipdD-cofz-DXaJ-gPdgBW" TYPE="LVM2_member"
/dev/mapper/vg_kvm-lv_root: UUID="a07b967c-71a0-4925-ab02-aebcad2ae824" TYPE="ext4"
/dev/mapper/vg_kvm-lv_swap: UUID="d7ef54ca-9c41-4de4-ac1b-4193b0c1ddb6" TYPE="swap"
By default, the blkid command lists all available block devices. To display information about a
particular device only, specify the device name on the command line:
405

blkid device_name
For instance, to display information about /dev/vda1, type asroot:
~]# blkid /dev/vda1
/dev/vda1: UUID="7fa9c421-0054-4555-b0ca-b470a97a3d84" TYPE="ext4"
You can also use the above command with the -p and -o udev command line options to obtain more
detailed information. Note that root privileges are required to run this command:
blkid -po udev device_name
For example:
~]# blkid -po udev /dev/vda1
ID_FS_UUID=7fa9c421-0054-4555-b0ca-b470a97a3d84
ID_FS_UUID_ENC=7fa9c421-0054-4555-b0ca-b470a97a3d84
ID_FS_VERSION=1.0
ID_FS_TYPE=ext4
ID_FS_USAGE=filesystem
For a complete list of available command line options, see the blkid(8) manual page.
21.4.3. Using the findmnt Command
The findmnt command allows you to display a list of currently mounted file systems. To do so, type
the following at a shell prompt:
findmnt
For each listed file system, the findmnt command displays the target mount point (
TARGET), source
device (SOURCE), file system type (
FSTYPE), and relevant mount options (
OPTIONS). For example:
~]$ findmnt
TARGET SOURCE FSTYPE OPTIONS
/ /dev/mapper/rhel-root
xfs rw,relatime,seclabel,attr2,inode64,noquota
├─/proc proc proc rw,nosuid,nodev,noexec,relatime
│ ├─/proc/sys/fs/binfmt_misc systemd-1 autofs
rw,relatime,fd=32,pgrp=1,timeout=300,minproto=5,maxproto=5,direct
│ └─/proc/fs/nfsd sunrpc nfsd rw,relatime
├─/sys sysfs sysfs rw,nosuid,nodev,noexec,relatime,seclabel
│ ├─/sys/kernel/security securityfs securityfs rw,nosuid,nodev,noexec,relatime
│ ├─/sys/fs/cgroup tmpfs tmpfs rw,nosuid,nodev,noexec,seclabel,mode=755
[output truncated]
By default, findmnt lists file systems in a tree-like format. To display the information as an ordinary
list, add the -l command line option:
findmnt -l
For instance:
406

~]$ findmnt -l
/proc proc proc rw,nosuid,nodev,noexec,relatime
/sys sysfs sysfs rw,nosuid,nodev,noexec,relatime,seclabel
/dev devtmpfs devtmpfs
rw,nosuid,seclabel,size=933372k,nr_inodes=233343,mode=755
/sys/kernel/security securityfs securityfs rw,nosuid,nodev,noexec,relatime
/dev/shm tmpfs tmpfs rw,nosuid,nodev,seclabel
/dev/pts devpts devpts
rw,nosuid,noexec,relatime,seclabel,gid=5,mode=620,ptmxmode=000
/run tmpfs tmpfs rw,nosuid,nodev,seclabel,mode=755
/sys/fs/cgroup tmpfs tmpfs rw,nosuid,nodev,noexec,seclabel,mode=755
[output truncated]
You can also choose to list only file systems of a particular type. To do so, add the -t command line
option followed by a file system type:
findmnt -t type
For example, to all list xfs file systems, type:
~]$ findmnt -t xfs
/ /dev/mapper/rhel-root xfs rw,relatime,seclabel,attr2,inode64,noquota
└─/boot /dev/vda1 xfs rw,relatime,seclabel,attr2,inode64,noquota
For a complete list of available command line options, see the findmnt(8) manual page.
21.4.4. Using the df Command
The df command allows you to display a detailed report on the system’s disk space usage. To do so,
df
For each listed file system, the df command displays its name (
Filesystem), size (1K-blocks or Size),
how much space is used (Used), how much space is still available (
Available), the percentage of
space usage (Use%), and where is the file system mounted (
Mounted on). For example:
~]$ df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/mapper/vg_kvm-lv_root 18618236 4357360 13315112 25% /
tmpfs 380376 288 380088 1% /dev/shm
/dev/vda1 495844 77029 393215 17% /boot
By default, the df command shows the partition size in 1 kilobyte blocks and the amount of used and
available disk space in kilobytes. To view the information in megabytes and gigabytes, supply the -h
command line option, which causes df to display the values in a human-readable format:
df -h
For instance:
407

~]$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_kvm-lv_root 18G 4.2G 13G 25% /
tmpfs 372M 288K 372M 1% /dev/shm
/dev/vda1 485M 76M 384M 17% /boot
For a complete list of available command line options, see the df(1) manual page.
21.4.5. Using the du Command
The du command allows you to displays the amount of space that is being used by files in a directory.
To display the disk usage for each of the subdirectories in the current working directory, run the
command with no additional command line options:
du
For example:
~]$ du
14972 ./Downloads
4 ./.mozilla/extensions
4 ./.mozilla/plugins
12 ./.mozilla
15004 .
By default, the du command displays the disk usage in kilobytes. To view the information in
megabytes and gigabytes, supply the -h command line option, which causes the utility to display the
values in a human-readable format:
du -h
For instance:
~]$ du -h
15M ./Downloads
4.0K ./.mozilla/extensions
4.0K ./.mozilla/plugins
12K ./.mozilla
15M .
At the end of the list, the du command always shows the grand total for the current directory. To
display only this information, supply the -s command line option:
du -sh
For example:
~]$ du -sh
15M .
For a complete list of available command line options, see the du(1) manual page.
408

The File Systems tab of theSystem Monitor tool allows you to view file systems and disk space usage
in the graphical user interface.
Spacebar.
Click the File Systems tab to view a list of file systems.
Figure 21.3. System Monitor — File Systems
For each listed file system, the System Monitor tool displays the source device (
Device), target
mount point (Directory), and file system type (
Type), as well as its size (
Total), and how much space
is available (Available), and used (Used).
21.5. VIEWING HARDWARE INFORMATION
21.5.1. Using the lspci Command
The lspci command allows you to display information about PCI buses and devices that are attached
to them. To list all PCI devices that are in the system, type the following at a shell prompt:
lspci
This displays a simple list of devices, for example:
~]$ lspci
00:00.0 Host bridge: Intel Corporation 82X38/X48 Express DRAM Controller
00:01.0 PCI bridge: Intel Corporation 82X38/X48 Express Host-Primary PCI Express Bridge
00:1a.0 USB Controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #4 (rev
02)
02)
409

02)
[output truncated]
You can also use the -v command line option to display more verbose output, or-vv for very verbose
output:
lspci -v|-vv
For instance, to determine the manufacturer, model, and memory size of a system’s video card, type:
~]$ lspci -v
[output truncated]
01:00.0 VGA compatible controller: nVidia Corporation G84 [Quadro FX 370] (rev a1) (prog-if
00 [VGA controller])
Subsystem: nVidia Corporation Device 0491
Physical Slot: 2
Flags: bus master, fast devsel, latency 0, IRQ 16
Memory at f2000000 (32-bit, non-prefetchable) [size=16M]
Memory at e0000000 (64-bit, prefetchable) [size=256M]
Memory at f0000000 (64-bit, non-prefetchable) [size=32M]
I/O ports at 1100 [size=128]
Expansion ROM at <unassigned> [disabled]
Capabilities: <access denied>
Kernel driver in use: nouveau
Kernel modules: nouveau, nvidiafb
[output truncated]
For a complete list of available command line options, see the lspci(8) manual page.
21.5.2. Using the lsusb Command
The lsusb command allows you to display information about USB buses and devices that are
attached to them. To list all USB devices that are in the system, type the following at a shell prompt:
lsusb
This displays a simple list of devices, for example:
~]$ lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
[output truncated]
Bus 001 Device 002: ID 0bda:0151 Realtek Semiconductor Corp. Mass Storage Device
(Multicard Reader)
Bus 008 Device 002: ID 03f0:2c24 Hewlett-Packard Logitech M-UAL-96 Mouse
Bus 008 Device 003: ID 04b3:3025 IBM Corp.
You can also use the -v command line option to display more verbose output:
lsusb -v
410

For instance:
~]$ lsusb -v
[output truncated]
Bus 008 Device 002: ID 03f0:2c24 Hewlett-Packard Logitech M-UAL-96 Mouse
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 2.00
bDeviceClass 0 (Defined at Interface level)
bDeviceSubClass 0
bDeviceProtocol 0
bMaxPacketSize0 8
idVendor 0x03f0 Hewlett-Packard
idProduct 0x2c24 Logitech M-UAL-96 Mouse
bcdDevice 31.00
iManufacturer 1
iProduct 2
iSerial 0
bNumConfigurations 1
Configuration Descriptor:
bLength 9
bDescriptorType 2
[output truncated]
For a complete list of available command line options, see the lsusb(8) manual page.
21.5.3. Using the lscpu Command
The lscpu command allows you to list information about CPUs that are present in the system,
including the number of CPUs, their architecture, vendor, family, model, CPU caches, etc. To do so,
lscpu
For example:
~]$ lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 4
On-line CPU(s) list: 0-3
Thread(s) per core: 1
Core(s) per socket: 4
Socket(s): 1
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 23
Stepping: 7
CPU MHz: 1998.000
BogoMIPS: 4999.98
Virtualization: VT-x
411

L1d cache: 32K
L1i cache: 32K
L2 cache: 3072K
NUMA node0 CPU(s): 0-3
For a complete list of available command line options, see the lscpu(1) manual page.
21.6. CHECKING FOR HARDWARE ERRORS
Red Hat Enterprise Linux 7 introduced the new hardware event report mechanism(HERM.) This
mechanism gathers system-reported memory errors as well as errors reported by the error detection
and correction (EDAC) mechanism for dual in-line memory modules (
DIMMs) and reports them to
user space. The user-space daemon rasdaemon, catches and handles allreliability, availability, and
serviceability (RAS) error events that come from the kernel tracing mechanism, and logs them. The
functions previously provided by edac-utils are now replaced byrasdaemon.
To install rasdaemon, enter the following command asroot:
~]# yum install rasdaemon
Start the service as follows:
~]# systemctl start rasdaemon
To make the service run at system start, enter the following command:
~]# systemctl enable rasdaemon
The ras-mc-ctl utility provides a means to work withEDAC drivers. Enter the following command to
see a list of command options:
~]$ ras-mc-ctl --help
Usage: ras-mc-ctl [OPTIONS...]
--quiet Quiet operation.
--mainboard Print mainboard vendor and model for this hardware.
--status Print status of EDAC drivers.
output truncated
To view a summary of memory controller events, run as root:
~]# ras-mc-ctl --summary
Memory controller events summary:
Corrected on DIMM Label(s): 'CPU_SrcID#0_Ha#0_Chan#0_DIMM#0' location: 0:0:0:-1
errors: 1
No PCIe AER errors.
No Extlog errors.
MCE records summary:
1 MEMORY CONTROLLER RD_CHANNEL0_ERR Transaction: Memory read error errors
2 No Error errors
To view a list of errors reported by the memory controller, run as root:
412

~]# ras-mc-ctl --errors
Memory controller events:
1 3172-02-17 00:47:01 -0500 1 Corrected error(s): memory read error at
CPU_SrcID#0_Ha#0_Chan#0_DIMM#0 location: 0:0:0:-1, addr 65928, grain 7, syndrome 0
area:DRAM err_code:0001:0090 socket:0 ha:0 channel_mask:1 rank:0
No PCIe AER errors.
No Extlog errors.
MCE events:
1 3171-11-09 06:20:21 -0500 error: MEMORY CONTROLLER RD_CHANNEL0_ERR Transaction:
Memory read error, mcg mcgstatus=0, mci Corrected_error, n_errors=1, mcgcap=0x01000c16,
status=0x8c00004000010090, addr=0x1018893000, misc=0x15020a086, walltime=0x57e96780,
cpuid=0x00050663, bank=0x00000007
2 3205-06-22 00:13:41 -0400 error: No Error, mcg mcgstatus=0, mci Corrected_error
Error_enabled, mcgcap=0x01000c16, status=0x9400000000000000, addr=0x0000abcd,
walltime=0x57e967ea, cpuid=0x00050663, bank=0x00000001
3 3205-06-22 00:13:41 -0400 error: No Error, mcg mcgstatus=0, mci Corrected_error
Error_enabled, mcgcap=0x01000c16, status=0x9400000000000000, addr=0x00001234,
walltime=0x57e967ea, cpu=0x00000001, cpuid=0x00050663, apicid=0x00000002,
bank=0x00000002
These commands are also described in the ras-mc-ctl(8) manual page.
21.7. MONITORING PERFORMANCE WITH NET-SNMP
Red Hat Enterprise Linux 7 includes the Net-SNMP software suite, which includes a flexible and
extensible simple network management protocol(SNMP) agent. This agent and its associated
utilities can be used to provide performance data from a large number of systems to a variety of tools
which support polling over the SNMP protocol.
This section provides information on configuring the Net-SNMP agent to securely provide
performance data over the network, retrieving the data using the SNMP protocol, and extending the
SNMP agent to provide custom performance metrics.
21.7.1. Installing Net-SNMP
The Net-SNMP software suite is available as a set of RPM packages in the Red Hat Enterprise Linux
software distribution. Table 21.2, “Available Net-SNMP packages”summarizes each of the packages
and their contents.
Table 21.2. Available Net-SNMP packages
Package Provides
net-snmp The SNMP Agent Daemon and documentation. This
package is required for exporting performance data.
net-snmp-libs The netsnmp library and the bundled management
information bases (MIBs). This package is required
for exporting performance data.
413

net-snmp-utils SNMP clients such as snmpget and snmpwalk.
This package is required in order to query a system’s
performance data over SNMP.
net-snmp-perl The mib2c utility and the NetSNMP Perl module.
Note that this package is provided by the Optional
channel. See Section 9.5.7, “Adding the Optional and
Supplementary Repositories” for more information
on Red Hat additional channels.
net-snmp-python An SNMP client library for Python. Note that this
package is provided by the Optional channel. See
Section 9.5.7, “Adding the Optional and
Supplementary Repositories” for more information
on Red Hat additional channels.
Package Provides
To install any of these packages, use the yum command in the following form:
yum install package…
For example, to install the SNMP Agent Daemon and SNMP clients used in the rest of this section,
type the following at a shell prompt as root:
~]# yum install net-snmp net-snmp-libs net-snmp-utils
For more information on how to install new packages in Red Hat Enterprise Linux, see Section 9.2.4,
“Installing Packages”.
21.7.2. Running the Net-SNMP Daemon
The net-snmp package contains snmpd, the SNMP Agent Daemon. This section provides information
on how to start, stop, and restart the snmpd service. For more information on managing system
services in Red Hat Enterprise Linux 7, see Chapter 10, Managing Services with systemd.
To run the snmpd service in the current session, type the following at a shell prompt as
root:
systemctl start snmpd.service
To configure the service to be automatically started at boot time, use the following command:
systemctl enable snmpd.service
414

To stop the running snmpd service, type the following at a shell prompt asroot:
systemctl stop snmpd.service
To disable starting the service at boot time, use the following command:
systemctl disable snmpd.service
To restart the running snmpd service, type the following at a shell prompt:
systemctl restart snmpd.service
This command stops the service and starts it again in quick succession. To only reload the
configuration without stopping the service, run the following command instead:
systemctl reload snmpd.service
This causes the running snmpd service to reload its configuration.
21.7.3. Configuring Net-SNMP
To change the Net-SNMP Agent Daemon configuration, edit the /etc/snmp/snmpd.conf
configuration file. The default snmpd.conf file included with Red Hat Enterprise Linux 7 is heavily
commented and serves as a good starting point for agent configuration.
This section focuses on two common tasks: setting system information and configuring
authentication. For more information about available configuration directives, see the
snmpd.conf(5) manual page. Additionally, there is a utility in thenet-snmp package named
snmpconf which can be used to interactively generate a valid agent configuration.
Note that the net-snmp-utils package must be installed in order to use thesnmpwalk utility
described in this section.
NOTE
For any changes to the configuration file to take effect, force the snmpd service to re-
read the configuration by running the following command as root:
systemctl reload snmpd.service
21.7.3.1. Setting System Information
Net-SNMP provides some rudimentary system information via the system tree. For example, the
following snmpwalk command shows thesystem tree with a default agent configuration.
~]# snmpwalk -v2c -c public localhost system
SNMPv2-MIB::sysDescr.0 = STRING: Linux localhost.localdomain 3.10.0-123.el7.x86_64 #1
SMP Mon May 5 11:16:57 EDT 2014 x86_64
SNMPv2-MIB::sysObjectID.0 = OID: NET-SNMP-MIB::netSnmpAgentOIDs.10
DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (464) 0:00:04.64
415

SNMPv2-MIB::sysContact.0 = STRING: Root <root@localhost> (configure
/etc/snmp/snmp.local.conf)
[output truncated]
By default, the sysName object is set to the host name. The
sysLocation and sysContact objects can
be configured in the /etc/snmp/snmpd.conf file by changing the value of thesyslocation and
syscontact directives, for example:
syslocation Datacenter, Row 4, Rack 3
syscontact UNIX Admin <admin@example.com>
After making changes to the configuration file, reload the configuration and test it by running the
snmpwalk command again:
~]# systemctl reload snmp.service
~]# snmpwalk -v2c -c public localhost system
SMP Mon May 5 11:16:57 EDT 2014 x86_64
SNMPv2-MIB::sysContact.0 = STRING: UNIX Admin <admin@example.com>
SNMPv2-MIB::sysName.0 = STRING: localhost.localdomain
SNMPv2-MIB::sysLocation.0 = STRING: Datacenter, Row 4, Rack 3
[output truncated]
21.7.3.2. Configuring Authentication
The Net-SNMP Agent Daemon supports all three versions of the SNMP protocol. The first two
versions (1 and 2c) provide for simple authentication using a community string. This string is a shared
secret between the agent and any client utilities. The string is passed in clear text over the network
however and is not considered secure. Version 3 of the SNMP protocol supports user authentication
and message encryption using a variety of protocols. The Net-SNMP agent also supports tunneling
over SSH, and TLS authentication with X.509 certificates.
Configuring SNMP Version 2c Community
To configure an SNMP version 2c community, use either therocommunity or rwcommunity directive
in the /etc/snmp/snmpd.conf configuration file. The format of the directives is as follows:
directive community source OID
… where community is the community string to use,source is an IP address or subnet, andOID is the
SNMP tree to provide access to. For example, the following directive provides read-only access to the
system tree to a client using the community string "redhat" on the local machine:
rocommunity redhat 127.0.0.1 .1.3.6.1.2.1.1
To test the configuration, use the snmpwalk command with the-v and -c options.
~]# snmpwalk -v2c -c redhat localhost system
SMP Mon May 5 11:16:57 EDT 2014 x86_64
SNMPv2-MIB::sysContact.0 = STRING: UNIX Admin <admin@example.com>
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SNMPv2-MIB::sysName.0 = STRING: localhost.localdomain
SNMPv2-MIB::sysLocation.0 = STRING: Datacenter, Row 4, Rack 3
[output truncated]
Configuring SNMP Version 3 User
To configure an SNMP version 3 user, use the net-snmp-create-v3-user command. This command
adds entries to the /var/lib/net-snmp/snmpd.conf and /etc/snmp/snmpd.conf files which create the
user and grant access to the user. Note that the net-snmp-create-v3-user command may only be run
when the agent is not running. The following example creates the "admin" user with the password
"redhatsnmp":
~]# systemctl stop snmpd.service
~]# net-snmp-create-v3-user
Enter a SNMPv3 user name to create:
admin
Enter authentication pass-phrase:
redhatsnmp
Enter encryption pass-phrase:
[press return to reuse the authentication pass-phrase]
adding the following line to /var/lib/net-snmp/snmpd.conf:
createUser admin MD5 "redhatsnmp" DES
adding the following line to /etc/snmp/snmpd.conf:
rwuser admin
~]# systemctl start snmpd.service
The rwuser directive (orrouser when the-ro command line option is supplied) thatnet-snmp-create-
v3-user adds to/etc/snmp/snmpd.conf has a similar format to therwcommunity and rocommunity
directives:
directive user noauth|auth|priv OID
… where user is a user name andOID is the SNMP tree to provide access to. By default, the Net-SNMP
Agent Daemon allows only authenticated requests (the auth option). Thenoauth option allows you to
permit unauthenticated requests, and the priv option enforces the use of encryption. Theauthpriv
option specifies that requests must be authenticated and replies should be encrypted.
For example, the following line grants the user "admin" read-write access to the entire tree:
rwuser admin authpriv .1
To test the configuration, create a .snmp/ directory in your user’s home directory and a configuration
file named snmp.conf in that directory (
~/.snmp/snmp.conf) with the following lines:
defVersion 3
defSecurityLevel authPriv
defSecurityName admin
defPassphrase redhatsnmp
The snmpwalk command will now use these authentication settings when querying the agent:
~]$ snmpwalk -v3 localhost system
SMP Mon May 5 11:16:57 EDT 2014 x86_64
417

[output truncated]
21.7.4. Retrieving Performance Data over SNMP
The Net-SNMP Agent in Red Hat Enterprise Linux provides a wide variety of performance
information over the SNMP protocol. In addition, the agent can be queried for a listing of the installed
RPM packages on the system, a listing of currently running processes on the system, or the network
configuration of the system.
This section provides an overview of OIDs related to performance tuning available over SNMP. It
assumes that the net-snmp-utils package is installed and that the user is granted access to the SNMP
tree as described in Section 21.7.3.2, “Configuring Authentication”.
21.7.4.1. Hardware Configuration
The Host Resources MIB included with Net-SNMP presents information about the current hardware
and software configuration of a host to a client utility. Table 21.3, “Available OIDs”summarizes the
different OIDs available under that MIB.
Table 21.3. Available OIDs
OID Description
HOST-RESOURCES-MIB::hrSystem Contains general system information such as uptime,
number of users, and number of running processes.
HOST-RESOURCES-MIB::hrStorage Contains data on memory and file system usage.
HOST-RESOURCES-MIB::hrDevices Contains a listing of all processors, network devices,
and file systems.
HOST-RESOURCES-MIB::hrSWRun Contains a listing of all running processes.
HOST-RESOURCES-MIB::hrSWRunPerf Contains memory and CPU statistics on the process
table from HOST-RESOURCES-MIB::hrSWRun.
HOST-RESOURCES-MIB::hrSWInstalled Contains a listing of the RPM database.
There are also a number of SNMP tables available in the Host Resources MIB which can be used to
retrieve a summary of the available information. The following example displays HOST-RESOURCES-
MIB::hrFSTable:
~]$ snmptable -Cb localhost HOST-RESOURCES-MIB::hrFSTable
SNMP table: HOST-RESOURCES-MIB::hrFSTable
Index MountPoint RemoteMountPoint Type
Access Bootable StorageIndex LastFullBackupDate LastPartialBackupDate
1 "/" "" HOST-RESOURCES-TYPES::hrFSLinuxExt2
readWrite true 31 0-1-1,0:0:0.0 0-1-1,0:0:0.0
5 "/dev/shm" "" HOST-RESOURCES-TYPES::hrFSOther
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readWrite false 35 0-1-1,0:0:0.0 0-1-1,0:0:0.0
6 "/boot" "" HOST-RESOURCES-TYPES::hrFSLinuxExt2
readWrite false 36 0-1-1,0:0:0.0 0-1-1,0:0:0.0
For more information about HOST-RESOURCES-MIB, see the /usr/share/snmp/mibs/HOST-
RESOURCES-MIB.txt file.
21.7.4.2. CPU and Memory Information
Most system performance data is available in the UCD SNMP MIB. ThesystemStats OID provides a
number of counters around processor usage:
~]$ snmpwalk localhost UCD-SNMP-MIB::systemStats
UCD-SNMP-MIB::ssIndex.0 = INTEGER: 1
UCD-SNMP-MIB::ssErrorName.0 = STRING: systemStats
UCD-SNMP-MIB::ssSwapIn.0 = INTEGER: 0 kB
UCD-SNMP-MIB::ssSwapOut.0 = INTEGER: 0 kB
UCD-SNMP-MIB::ssIOSent.0 = INTEGER: 0 blocks/s
UCD-SNMP-MIB::ssIOReceive.0 = INTEGER: 0 blocks/s
UCD-SNMP-MIB::ssSysInterrupts.0 = INTEGER: 29 interrupts/s
UCD-SNMP-MIB::ssSysContext.0 = INTEGER: 18 switches/s
UCD-SNMP-MIB::ssCpuUser.0 = INTEGER: 0
UCD-SNMP-MIB::ssCpuSystem.0 = INTEGER: 0
UCD-SNMP-MIB::ssCpuIdle.0 = INTEGER: 99
UCD-SNMP-MIB::ssCpuRawUser.0 = Counter32: 2278
UCD-SNMP-MIB::ssCpuRawNice.0 = Counter32: 1395
UCD-SNMP-MIB::ssCpuRawSystem.0 = Counter32: 6826
UCD-SNMP-MIB::ssCpuRawIdle.0 = Counter32: 3383736
UCD-SNMP-MIB::ssCpuRawWait.0 = Counter32: 7629
UCD-SNMP-MIB::ssCpuRawKernel.0 = Counter32: 0
UCD-SNMP-MIB::ssCpuRawInterrupt.0 = Counter32: 434
UCD-SNMP-MIB::ssIORawSent.0 = Counter32: 266770
UCD-SNMP-MIB::ssIORawReceived.0 = Counter32: 427302
UCD-SNMP-MIB::ssRawInterrupts.0 = Counter32: 743442
UCD-SNMP-MIB::ssRawContexts.0 = Counter32: 718557
UCD-SNMP-MIB::ssCpuRawSoftIRQ.0 = Counter32: 128
UCD-SNMP-MIB::ssRawSwapIn.0 = Counter32: 0
UCD-SNMP-MIB::ssRawSwapOut.0 = Counter32: 0
In particular, the ssCpuRawUser, ssCpuRawSystem, ssCpuRawWait, and ssCpuRawIdle OIDs
provide counters which are helpful when determining whether a system is spending most of its
processor time in kernel space, user space, or I/O. ssRawSwapIn and ssRawSwapOut can be helpful
when determining whether a system is suffering from memory exhaustion.
More memory information is available under the UCD-SNMP-MIB::memory OID, which provides
similar data to the free command:
~]$ snmpwalk localhost UCD-SNMP-MIB::memory
UCD-SNMP-MIB::memIndex.0 = INTEGER: 0
UCD-SNMP-MIB::memErrorName.0 = STRING: swap
UCD-SNMP-MIB::memTotalSwap.0 = INTEGER: 1023992 kB
UCD-SNMP-MIB::memAvailSwap.0 = INTEGER: 1023992 kB
UCD-SNMP-MIB::memTotalReal.0 = INTEGER: 1021588 kB
UCD-SNMP-MIB::memAvailReal.0 = INTEGER: 634260 kB
UCD-SNMP-MIB::memTotalFree.0 = INTEGER: 1658252 kB
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UCD-SNMP-MIB::memMinimumSwap.0 = INTEGER: 16000 kB
UCD-SNMP-MIB::memBuffer.0 = INTEGER: 30760 kB
UCD-SNMP-MIB::memCached.0 = INTEGER: 216200 kB
UCD-SNMP-MIB::memSwapError.0 = INTEGER: noError(0)
UCD-SNMP-MIB::memSwapErrorMsg.0 = STRING:
Load averages are also available in the UCD SNMP MIB. The SNMP tableUCD-SNMP-MIB::laTable
has a listing of the 1, 5, and 15 minute load averages:
~]$ snmptable localhost UCD-SNMP-MIB::laTable
SNMP table: UCD-SNMP-MIB::laTable
laIndex laNames laLoad laConfig laLoadInt laLoadFloat laErrorFlag laErrMessage
1 Load-1 0.00 12.00 0 0.000000 noError
2 Load-5 0.00 12.00 0 0.000000 noError
3 Load-15 0.00 12.00 0 0.000000 noError
21.7.4.3. File System and Disk Information
The Host Resources MIB provides information on file system size and usage. Each file system (and
also each memory pool) has an entry in the HOST-RESOURCES-MIB::hrStorageTable table:
~]$ snmptable -Cb localhost HOST-RESOURCES-MIB::hrStorageTable
SNMP table: HOST-RESOURCES-MIB::hrStorageTable
Index Type Descr
AllocationUnits Size Used AllocationFailures
1 HOST-RESOURCES-TYPES::hrStorageRam Physical memory
1024 Bytes 1021588 388064 ?
3 HOST-RESOURCES-TYPES::hrStorageVirtualMemory Virtual memory
1024 Bytes 2045580 388064 ?
6 HOST-RESOURCES-TYPES::hrStorageOther Memory buffers
1024 Bytes 1021588 31048 ?
7 HOST-RESOURCES-TYPES::hrStorageOther Cached memory
1024 Bytes 216604 216604 ?
10 HOST-RESOURCES-TYPES::hrStorageVirtualMemory Swap space
1024 Bytes 1023992 0 ?
31 HOST-RESOURCES-TYPES::hrStorageFixedDisk /
4096 Bytes 2277614 250391 ?
35 HOST-RESOURCES-TYPES::hrStorageFixedDisk /dev/shm
4096 Bytes 127698 0 ?
36 HOST-RESOURCES-TYPES::hrStorageFixedDisk /boot
1024 Bytes 198337 26694 ?
The OIDs under HOST-RESOURCES-MIB::hrStorageSize and HOST-RESOURCES-
MIB::hrStorageUsed can be used to calculate the remaining capacity of each mounted file system.
I/O data is available both in UCD-SNMP-MIB::systemStats (ssIORawSent.0 and ssIORawRecieved.0)
and in UCD-DISKIO-MIB::diskIOTable. The latter provides much more granular data. Under this table
are OIDs for diskIONReadX and diskIONWrittenX, which provide counters for the number of bytes
read from and written to the block device in question since the system boot:
~]$ snmptable -Cb localhost UCD-DISKIO-MIB::diskIOTable
SNMP table: UCD-DISKIO-MIB::diskIOTable
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Index Device NRead NWritten Reads Writes LA1 LA5 LA15 NReadX NWrittenX
...
25 sda 216886272 139109376 16409 4894 ? ? ? 216886272 139109376
26 sda1 2455552 5120 613 2 ? ? ? 2455552 5120
27 sda2 1486848 0 332 0 ? ? ? 1486848 0
28 sda3 212321280 139104256 15312 4871 ? ? ? 212321280 139104256
21.7.4.4. Network Information
The Interfaces MIB provides information on network devices.IF-MIB::ifTable provides an SNMP table
with an entry for each interface on the system, the configuration of the interface, and various packet
counters for the interface. The following example shows the first few columns of ifTable on a system
with two physical network interfaces:
~]$ snmptable -Cb localhost IF-MIB::ifTable
SNMP table: IF-MIB::ifTable
Index Descr Type Mtu Speed PhysAddress AdminStatus
1 lo softwareLoopback 16436 10000000 up
2 eth0 ethernetCsmacd 1500 0 52:54:0:c7:69:58 up
3 eth1 ethernetCsmacd 1500 0 52:54:0:a7:a3:24 down
Network traffic is available under the OIDs IF-MIB::ifOutOctets and IF-MIB::ifInOctets. The following
SNMP queries will retrieve network traffic for each of the interfaces on this system:
~]$ snmpwalk localhost IF-MIB::ifDescr
IF-MIB::ifDescr.1 = STRING: lo
IF-MIB::ifDescr.2 = STRING: eth0
IF-MIB::ifDescr.3 = STRING: eth1
~]$ snmpwalk localhost IF-MIB::ifOutOctets
IF-MIB::ifOutOctets.1 = Counter32: 10060699
~]$ snmpwalk localhost IF-MIB::ifInOctets
IF-MIB::ifInOctets.1 = Counter32: 10060699
21.7.5. Extending Net-SNMP
The Net-SNMP Agent can be extended to provide application metrics in addition to raw system
metrics. This allows for capacity planning as well as performance issue troubleshooting. For example,
it may be helpful to know that an email system had a 5-minute load average of 15 while being tested,
but it is more helpful to know that the email system has a load average of 15 while processing 80,000
messages a second. When application metrics are available via the same interface as the system
metrics, this also allows for the visualization of the impact of different load scenarios on system
performance (for example, each additional 10,000 messages increases the load average linearly until
100,000).
A number of the applications included in Red Hat Enterprise Linux extend the Net-SNMP Agent to
provide application metrics over SNMP. There are several ways to extend the agent for custom
applications as well. This section describes extending the agent with shell scripts and the Perl plug-
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ins from the Optional channel. It assumes that the net-snmp-utils and net-snmp-perl packages are
installed, and that the user is granted access to the SNMP tree as described in Section 21.7.3.2,
“Configuring Authentication”.
21.7.5.1. Extending Net-SNMP with Shell Scripts
The Net-SNMP Agent provides an extension MIB (NET-SNMP-EXTEND-MIB) that can be used to
query arbitrary shell scripts. To specify the shell script to run, use the extend directive in the
/etc/snmp/snmpd.conf file. Once defined, the Agent will provide the exit code and any output of the
command over SNMP. The example below demonstrates this mechanism with a script which
determines the number of httpd processes in the process table.
NOTE
The Net-SNMP Agent also provides a built-in mechanism for checking the process
table via the proc directive. See thesnmpd.conf(5) manual page for more information.
The exit code of the following shell script is the number of httpd processes running on the system at a
given point in time:
#!/bin/sh
NUMPIDS=pgrep httpd | wc -l
exit $NUMPIDS
To make this script available over SNMP, copy the script to a location on the system path, set the
executable bit, and add an extend directive to the/etc/snmp/snmpd.conf file. The format of the
extend directive is the following:
extend name prog args
… where name is an identifying string for the extension,prog is the program to run, andargs are the
arguments to give the program. For instance, if the above shell script is copied to
/usr/local/bin/check_apache.sh, the following directive will add the script to the SNMP tree:
extend httpd_pids /bin/sh /usr/local/bin/check_apache.sh
The script can then be queried at NET-SNMP-EXTEND-MIB::nsExtendObjects:
~]$ snmpwalk localhost NET-SNMP-EXTEND-MIB::nsExtendObjects
NET-SNMP-EXTEND-MIB::nsExtendNumEntries.0 = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendCommand."httpd_pids" = STRING: /bin/sh
NET-SNMP-EXTEND-MIB::nsExtendArgs."httpd_pids" = STRING:
/usr/local/bin/check_apache.sh
NET-SNMP-EXTEND-MIB::nsExtendInput."httpd_pids" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendCacheTime."httpd_pids" = INTEGER: 5
NET-SNMP-EXTEND-MIB::nsExtendExecType."httpd_pids" = INTEGER: exec(1)
NET-SNMP-EXTEND-MIB::nsExtendRunType."httpd_pids" = INTEGER: run-on-read(1)
NET-SNMP-EXTEND-MIB::nsExtendStorage."httpd_pids" = INTEGER: permanent(4)
NET-SNMP-EXTEND-MIB::nsExtendStatus."httpd_pids" = INTEGER: active(1)
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."httpd_pids" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendOutputFull."httpd_pids" = STRING:
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NET-SNMP-EXTEND-MIB::nsExtendOutNumLines."httpd_pids" = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendResult."httpd_pids" = INTEGER: 8
NET-SNMP-EXTEND-MIB::nsExtendOutLine."httpd_pids".1 = STRING:
Note that the exit code ("8" in this example) is provided as an INTEGER type and any output is
provided as a STRING type. To expose multiple metrics as integers, supply different arguments to
the script using the extend directive. For example, the following shell script can be used to determine
the number of processes matching an arbitrary string, and will also output a text string giving the
number of processes:
#!/bin/sh
PATTERN=$1
NUMPIDS=pgrep $PATTERN | wc -l
echo "There are $NUMPIDS $PATTERN processes."
exit $NUMPIDS
The following /etc/snmp/snmpd.conf directives will give both the number ofhttpd PIDs as well as the
number of snmpd PIDs when the above script is copied to/usr/local/bin/check_proc.sh:
extend httpd_pids /bin/sh /usr/local/bin/check_proc.sh httpd
extend snmpd_pids /bin/sh /usr/local/bin/check_proc.sh snmpd
The following example shows the output of an snmpwalk of thensExtendObjects OID:
~]$ snmpwalk localhost NET-SNMP-EXTEND-MIB::nsExtendObjects
NET-SNMP-EXTEND-MIB::nsExtendNumEntries.0 = INTEGER: 2
NET-SNMP-EXTEND-MIB::nsExtendCommand."httpd_pids" = STRING: /bin/sh
NET-SNMP-EXTEND-MIB::nsExtendCommand."snmpd_pids" = STRING: /bin/sh
NET-SNMP-EXTEND-MIB::nsExtendArgs."httpd_pids" = STRING: /usr/local/bin/check_proc.sh
httpd
NET-SNMP-EXTEND-MIB::nsExtendArgs."snmpd_pids" = STRING:
/usr/local/bin/check_proc.sh snmpd
NET-SNMP-EXTEND-MIB::nsExtendInput."httpd_pids" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendInput."snmpd_pids" = STRING:
...
NET-SNMP-EXTEND-MIB::nsExtendResult."snmpd_pids" = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendOutLine."httpd_pids".1 = STRING: There are 8 httpd
processes.
NET-SNMP-EXTEND-MIB::nsExtendOutLine."snmpd_pids".1 = STRING: There are 1 snmpd
processes.
WARNING
Integer exit codes are limited to a range of 0–255. For values that are likely to
exceed 256, either use the standard output of the script (which will be typed as a
string) or a different method of extending the agent.

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This last example shows a query for the free memory of the system and the number of httpd
processes. This query could be used during a performance test to determine the impact of the
number of processes on memory pressure:
~]$ snmpget localhost
'NET-SNMP-EXTEND-MIB::nsExtendResult."httpd_pids"'
UCD-SNMP-MIB::memAvailReal.0
UCD-SNMP-MIB::memAvailReal.0 = INTEGER: 799664 kB
21.7.5.2. Extending Net-SNMP with Perl
Executing shell scripts using the extend directive is a fairly limited method for exposing custom
application metrics over SNMP. The Net-SNMP Agent also provides an embedded Perl interface for
exposing custom objects. The net-snmp-perl package in the Optional channel provides the
NetSNMP::agent Perl module that is used to write embedded Perl plug-ins on Red Hat Enterprise
Linux.
NOTE
Before subscribing to the Optional and Supplementary channels see the Scope of
Coverage Details. If you decide to install packages from these channels, follow the
steps documented in the article called How to access Optional and Supplementary
channels, and -devel packages using Red Hat Subscription Manager (RHSM)? on the
Red Hat Customer Portal.
The NetSNMP::agent Perl module provides anagent object which is used to handle requests for a
part of the agent’s OID tree. The agent object’s constructor has options for running the agent as a
sub-agent of snmpd or a standalone agent. No arguments are necessary to create an embedded
agent:
use NetSNMP::agent (':all');
my $agent = new NetSNMP::agent();
The agent object has aregister method which is used to register a callback function with a particular
OID. The register function takes a name, OID, and pointer to the callback function. The following
example will register a callback function named hello_handler with the SNMP Agent which will handle
requests under the OID .1.3.6.1.4.1.8072.9999.9999:
$agent->register("hello_world", ".1.3.6.1.4.1.8072.9999.9999",
&hello_handler);
NOTE
The OID .1.3.6.1.4.1.8072.9999.9999 (NET-SNMP-MIB::netSnmpPlaypen) is typically
used for demonstration purposes only. If your organization does not already have a
root OID, you can obtain one by contacting an ISO Name Registration Authority (ANSI
in the United States).
The handler function will be called with four parameters, HANDLER, REGISTRATION_INFO,
REQUEST_INFO, and REQUESTS. TheREQUESTS parameter contains a list of requests in the
current call and should be iterated over and populated with data. The request objects in the list have
424

get and set methods which allow for manipulating the OID and value of the request. For example, the
following call will set the value of a request object to the string "hello world":
$request->setValue(ASN_OCTET_STR, "hello world");
The handler function should respond to two types of SNMP requests: the GET request and the
GETNEXT request. The type of request is determined by calling the getMode method on the
request_info object passed as the third parameter to the handler function. If the request is a GET
request, the caller will expect the handler to set the value of therequest object, depending on the
OID of the request. If the request is a GETNEXT request, the caller will also expect the handler to set
the OID of the request to the next available OID in the tree. This is illustrated in the following code
example:
my $request;
my $string_value = "hello world";
my $integer_value = "8675309";
for($request = $requests; $request; $request = $request->next()) {
my $oid = $request->getOID();
if ($request_info->getMode() == MODE_GET) {
if ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) {
$request->setValue(ASN_OCTET_STR, $string_value);
}
elsif ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.1")) {
$request->setValue(ASN_INTEGER, $integer_value);
}
} elsif ($request_info->getMode() == MODE_GETNEXT) {
$request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.1");
}
elsif ($oid < new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) {
$request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.0");
}
}
}
When getMode returns MODE_GET, the handler analyzes the value of thegetOID call on therequest
object. The value of therequest is set to eitherstring_value if the OID ends in ".1.0", or set to
integer_value if the OID ends in ".1.1". If thegetMode returns MODE_GETNEXT, the handler
determines whether the OID of the request is ".1.0", and then sets the OID and value for ".1.1". If the
request is higher on the tree than ".1.0", the OID and value for ".1.0" is set. This in effect returns the
"next" value in the tree so that a program like snmpwalk can traverse the tree without prior
knowledge of the structure.
The type of the variable is set using constants from NetSNMP::ASN. See theperldoc for
NetSNMP::ASN for a full list of available constants.
The entire code listing for this example Perl plug-in is as follows:
#!/usr/bin/perl
use NetSNMP::agent (':all');
use NetSNMP::ASN qw(ASN_OCTET_STR ASN_INTEGER);
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sub hello_handler {
my ($handler, $registration_info, $request_info, $requests) = @_;
my $request;
my $string_value = "hello world";
my $integer_value = "8675309";
for($request = $requests; $request; $request = $request->next()) {
my $oid = $request->getOID();
if ($request_info->getMode() == MODE_GET) {
}
elsif ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.1")) {
}
} elsif ($request_info->getMode() == MODE_GETNEXT) {
$request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.1");
}
elsif ($oid < new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) {
$request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.0");
}
}
}
}
my $agent = new NetSNMP::agent();
$agent->register("hello_world", ".1.3.6.1.4.1.8072.9999.9999",
&hello_handler);
To test the plug-in, copy the above program to /usr/share/snmp/hello_world.pl and add the
following line to the /etc/snmp/snmpd.conf configuration file:
perl do "/usr/share/snmp/hello_world.pl"
The SNMP Agent Daemon will need to be restarted to load the new Perl plug-in. Once it has been
restarted, an snmpwalk should return the new data:
~]$ snmpwalk localhost NET-SNMP-MIB::netSnmpPlaypen
NET-SNMP-MIB::netSnmpPlaypen.1.0 = STRING: "hello world"
NET-SNMP-MIB::netSnmpPlaypen.1.1 = INTEGER: 8675309
The snmpget should also be used to exercise the other mode of the handler:
~]$ snmpget localhost
NET-SNMP-MIB::netSnmpPlaypen.1.0
NET-SNMP-MIB::netSnmpPlaypen.1.1
NET-SNMP-MIB::netSnmpPlaypen.1.0 = STRING: "hello world"
NET-SNMP-MIB::netSnmpPlaypen.1.1 = INTEGER: 8675309
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To learn more about gathering system information, see the following resources.
lscpu(1) — The manual page for thelscpu command.
lsusb(8) — The manual page for thelsusb command.
findmnt(8) — The manual page for thefindmnt command.
blkid(8) — The manual page for theblkid command.
lsblk(8) — The manual page for thelsblk command.
ps(1) — The manual page for theps command.
top(1) — The manual page for thetop command.
free(1) — The manual page for thefree command.
df(1) — The manual page for thedf command.
du(1) — The manual page for thedu command.
lspci(8) — The manual page for thelspci command.
snmpd(8) — The manual page for thesnmpd service.
snmpd.conf(5) — The manual page for the/etc/snmp/snmpd.conf file containing full
documentation of available configuration directives.
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CHAPTER 22. OPENLMI
The Open Linux Management Infrastructure, commonly abbreviated asOpenLMI, is a common
infrastructure for the management of Linux systems. It builds on top of existing tools and serves as
an abstraction layer in order to hide much of the complexity of the underlying system from system
administrators. OpenLMI is distributed with a set of services that can be accessed locally or remotely
and provides multiple language bindings, standard APIs, and standard scripting interfaces that can
be used to manage and monitor hardware, operating systems, and system services.
22.1. ABOUT OPENLMI
OpenLMI is designed to provide a common management interface to production servers running the
Red Hat Enterprise Linux system on both physical and virtual machines. It consists of the following
three components:
1. System management agents — these agents are installed on a managed system and
implement an object model that is presented to a standard object broker. The initial agents
implemented in OpenLMI include storage configuration and network configuration, but later
work will address additional elements of system management. The system management
agents are commonly referred to as Common Information Model providers or CIM providers.
2. A standard object broker— the object broker manages system management agents and
provides an interface to them. The standard object broker is also known as a CIM Object
Monitor or CIMOM.
3. Client applications and scripts— the client applications and scripts call the system
management agents through the standard object broker.
The OpenLMI project complements existing management initiatives by providing a low-level
interface that can be used by scripts or system management consoles. Interfaces distributed with
OpenLMI include C, C++, Python, Java, and an interactive command line client, and all of them offer
the same full access to the capabilities implemented in each agent. This ensures that you always have
access to exactly the same capabilities no matter which programming interface you decide to use.
The following are key benefits of installing and using OpenLMI on your system:
OpenLMI provides a standard interface for configuration, management, and monitoring of
your local and remote systems.
It allows you to configure, manage, and monitor production servers running on both physical
and virtual machines.
It is distributed with a collection of CIM providers that allow you to configure, manage, and
monitor storage devices and complex networks.
It allows you to call system management functions from C, C++, Python, and Java programs,
and includes LMIShell, which provides a command line interface.
It is free software based on open industry standards.
22.1.2. Management Capabilities
Key capabilities of OpenLMI include the management of storage devices, networks, system services,
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user accounts, hardware and software configuration, power management, and interaction with Active
Directory. For a complete list of CIM providers that are distributed with Red Hat Enterprise Linux 7,
see Table 22.1, “Available CIM Providers”.
Table 22.1. Available CIM Providers
Package Name Description
openlmi-account A CIM provider for managing user accounts.
openlmi-logicalfile A CIM provider for reading files and directories.
openlmi-networking A CIM provider for network management.
openlmi-powermanagement A CIM provider for power management.
openlmi-service A CIM provider for managing system services.
openlmi-storage A CIM provider for storage management.
openlmi-fan A CIM provider for controlling computer fans.
openlmi-hardware A CIM provider for retrieving hardware information.
openlmi-realmd A CIM provider for configuring realmd.
openlmi-software[a] A CIM provider for software management.
[a] In Red Hat Enterprise Linux 7, the OpenLMI Software provider is included as a Technology Preview. This provider is fully
functional, but has a known performance scaling issue where listing large numbers of software packages may consume
excessive amount of memory and time. To work around this issue, adjust package searches to return as few packages as
possible.
22.2. INSTALLING OPENLMI
OpenLMI is distributed as a collection of RPM packages that include the CIMOM, individual CIM
providers, and client applications. This allows you distinguish between a managed and client system
and install only those components you need.
22.2.1. Installing OpenLMI on a Managed System
A managed system is the system you intend to monitor and manage by using the OpenLMI client
tools. To install OpenLMI on a managed system, complete the following steps:
1. Install the tog-pegasus package by typing the following at a shell prompt as
root:
yum install tog-pegasus
This command installs the OpenPegasus CIMOM and all its dependencies to the system and
creates a user account for the pegasus user.
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2. Install required CIM providers by running the following command as root:
yum install openlmi-{storage,networking,service,account,powermanagement}
This command installs the CIM providers for storage, network, service, account, and power
management. For a complete list of CIM providers distributed with Red Hat Enterprise Linux
7, see Table 22.1, “Available CIM Providers”.
3. Edit the /etc/Pegasus/access.conf configuration file to customize the list of users that are
allowed to connect to the OpenPegasus CIMOM. By default, only the pegasus user is allowed
to access the CIMOM both remotely and locally. To activate this user account, run the
following command as root to set the user’s password:
passwd pegasus
4. Start the OpenPegasus CIMOM by activating the tog-pegasus.service unit. To activate the
tog-pegasus.service unit in the current session, type the following at a shell prompt as
root:
systemctl start tog-pegasus.service
To configure the tog-pegasus.service unit to start automatically at boot time, type as
root:
systemctl enable tog-pegasus.service
5. If you intend to interact with the managed system from a remote machine, enable TCP
communication on port 5989 (wbem-https). To open this port in the current session, run the
firewall-cmd --add-port 5989/tcp
To open port 5989 for TCP communication permanently, type asroot:
firewall-cmd --permanent --add-port 5989/tcp
You can now connect to the managed system and interact with it by using the OpenLMI client tools as
described in Section 22.4, “Using LMIShell”. If you intend to perform OpenLMI operations directly on
the managed system, also complete the steps described in Section 22.2.2, “Installing OpenLMI on a
Client System”.
22.2.2. Installing OpenLMI on a Client System
A client system is the system from which you intend to interact with the managed system. In a typical
scenario, the client system and the managed system are installed on two separate machines, but you
can also install the client tools on the managed system and interact with it directly.
To install OpenLMI on a client system, complete the following steps:
1. Install the openlmi-tools package by typing the following at a shell prompt as
root:
yum install openlmi-tools
This command installs LMIShell, an interactive client and interpreter for accessing CIM
objects provided by OpenPegasus, and all its dependencies to the system.
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2. Configure SSL certificates for OpenPegasus as described in Section 22.3, “Configuring SSL
Certificates for OpenPegasus”.
You can now use the LMIShell client to interact with the managed system as described in
Section 22.4, “Using LMIShell”.
22.3. CONFIGURING SSL CERTIFICATES FOR OPENPEGASUS
OpenLMI uses the Web-Based Enterprise Management (WBEM) protocol that functions over an
HTTP transport layer. Standard HTTP Basic authentication is performed in this protocol, which means
that the user name and password are transmitted alongside the requests.
Configuring the OpenPegasus CIMOM to use HTTPS for communication is necessary to ensure
secure authentication. A Secure Sockets Layer (SSL) or Transport Layer Security (TLS) certificate is
required on the managed system to establish an encrypted channel.
There are two ways of managing SSL/TLS certificates on a system:
Self-signed certificates require less infrastructure to use, but are more difficult to deploy to
clients and manage securely.
Authority-signed certificates are easier to deploy to clients once they are set up, but may
require a greater initial investment.
When using an authority-signed certificate, it is necessary to configure a trusted certificate authority
on the client systems. The authority can then be used for signing all of the managed systems' CIMOM
certificates. Certificates can also be part of a certificate chain, so the certificate used for signing the
managed systems' certificates may in turn be signed by another, higher authority (such as Verisign,
CAcert, RSA and many others).
The default certificate and trust store locations on the file system are listed in Table 22.2, “Certificate
and Trust Store Locations”.
Table 22.2. Certificate and Trust Store Locations
Configuration Option Location Description
sslCertificateFilePath /etc/Pegasus/server.pem Public certificate of the CIMOM.
sslKeyFilePath /etc/Pegasus/file.pem Private key known only to the
CIMOM.
sslTrustStore /etc/Pegasus/client.pem The file or directory providing the
list of trusted certificate
authorities.
IMPORTANT
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IMPORTANT
If you modify any of the files mentioned in Table 22.2, “Certificate and Trust Store
Locations”, restart the tog-pegasus service to make sure it recognizes the new
certificates. To restart the service, type the following at a shell prompt as root:
systemctl restart tog-pegasus.service
For more information on how to manage system services in Red Hat Enterprise Linux 7,
see Chapter 10, Managing Services with systemd.
22.3.1. Managing Self-signed Certificates
A self-signed certificate uses its own private key to sign itself and it is not connected to any chain of
trust. On a managed system, if certificates have not been provided by the administrator prior to the
first time that the tog-pegasus service is started, a set of self-signed certificates will be
automatically generated using the system’s primary host name as the certificate subject.
IMPORTANT
The automatically generated self-signed certificates are valid by default for 10 years,
but they have no automatic-renewal capability. Any modification to these certificates
will require manually creating new certificates following guidelines provided by the
OpenSSL or Mozilla NSS documentation on the subject.
To configure client systems to trust the self-signed certificate, complete the following steps:
1. Copy the /etc/Pegasus/server.pem certificate from the managed system to the/etc/pki/ca-
trust/source/anchors/ directory on the client system. To do so, type the following at a shell
prompt as root:
scp root@hostname:/etc/Pegasus/server.pem /etc/pki/ca-
trust/source/anchors/pegasus-hostname.pem
Replace hostname with the host name of the managed system. Note that this command only
works if the sshd service is running on the managed system and is configured to allow the
root user to log in to the system over the SSH protocol. For more information on how to install
and configure the sshd service and use thescp command to transfer files over the SSH
protocol, see Chapter 12,OpenSSH.
2. Verify the integrity of the certificate on the client system by comparing its check sum with
the check sum of the original file. To calculate the check sum of the /etc/Pegasus/server.pem
file on the managed system, run the following command as root on that system:
sha1sum /etc/Pegasus/server.pem
To calculate the check sum of the /etc/pki/ca-trust/source/anchors/pegasus-hostname.pem
file on the client system, run the following command on this system:
sha1sum /etc/pki/ca-trust/source/anchors/pegasus-hostname.pem
Replace hostname with the host name of the managed system.
3. Update the trust store on the client system by running the following command as root:
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update-ca-trust extract
22.3.2. Managing Authority-signed Certificates with Identity Management
(Recommended)
The Identity Management feature of Red Hat Enterprise Linux provides a domain controller which
simplifies the management of SSL certificates within systems joined to the domain. Among others,
the Identity Management server provides an embedded Certificate Authority. See the Red Hat
Enterprise Linux 7 Linux Domain Identity, Authentication, and Policy Guide or the FreeIPA
documentation for information on how to join the client and managed systems to the domain.
It is necessary to register the managed system to Identity Management; for client systems the
registration is optional.
The following steps are required on the managed system:
1. Install the ipa-client package and register the system to Identity Management as described in
the Red Hat Enterprise Linux 7 Linux Domain Identity, Authentication, and Policy Guide
.
2. Copy the Identity Management signing certificate to the trusted store by typing the
cp /etc/ipa/ca.crt /etc/pki/ca-trust/source/anchors/ipa.crt
3. Update the trust store by running the following command as root:
4. Register Pegasus as a service in the Identity Management domain by running the following
command as a privileged domain user:
ipa service-add CIMOM/hostname
This command can be run from any system in the Identity Management domain that has the
ipa-admintools package installed. It creates a service entry in Identity Management that can
be used to generate signed SSL certificates.
5. Back up the PEM files located in the /etc/Pegasus/ directory (recommended).
6. Retrieve the signed certificate by running the following command as root:
ipa-getcert request -f /etc/Pegasus/server.pem -k /etc/Pegasus/file.pem -N
CN=hostname -K CIMOM/hostname
The certificate and key files are now kept in proper locations. The certmonger daemon
installed on the managed system by the ipa-client-install script ensures that the certificate is
kept up-to-date and renewed as necessary.
For more information, see the Red Hat Enterprise Linux 7 Linux Domain Identity,
Authentication, and Policy Guide.
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To register the client system and update the trust store, follow the steps below.
1. Install the ipa-client package and register the system to Identity Management as described in
the Red Hat Enterprise Linux 7 Linux Domain Identity, Authentication, and Policy Guide
.
2. Copy the Identity Management signing certificate to the trusted store by typing the
cp /etc/ipa/ca.crt /etc/pki/ca-trust/source/anchors/ipa.crt
If the client system is not meant to be registered in Identity Management, complete the following
steps to update the trust store.
1. Copy the /etc/ipa/ca.crt file securely from any other system joined to the same Identity
Management domain to the trusted store /etc/pki/ca-trust/source/anchors/ directory asroot.
22.3.3. Managing Authority-signed Certificates Manually
Managing authority-signed certificates with other mechanisms than Identity Management requires
more manual configuration.
It is necessary to ensure that all of the clients trust the certificate of the authority that will be signing
the managed system certificates:
If a certificate authority is trusted by default, it is not necessary to perform any particular
steps to accomplish this.
If the certificate authority is not trusted by default, the certificate has to be imported on the
client and managed systems.
a. Copy the certificate to the trusted store by typing the following command as root:
cp /path/to/ca.crt /etc/pki/ca-trust/source/anchors/ca.crt
b. Update the trust store by running the following command as root:
On the managed system, complete the following steps:
1. Create a new SSL configuration file /etc/Pegasus/ssl.cnf to store information about the
certificate. The contents of this file must be similar to the following example:
[ req ]
distinguished_name = req_distinguished_name
prompt = no
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[ req_distinguished_name ]
C = US
ST = Massachusetts
L = Westford
O = Fedora
OU = Fedora OpenLMI
CN = hostname
Replace hostname with the fully qualified domain name of the managed system.
2. Generate a private key on the managed system by using the following command as root:
openssl genrsa -out /etc/Pegasus/file.pem 1024
3. Generate a certificate signing request (CSR) by running this command as root:
openssl req -config /etc/Pegasus/ssl.cnf -new -key /etc/Pegasus/file.pem -out
/etc/Pegasus/server.csr
4. Send the /etc/Pegasus/server.csr file to the certificate authority for signing. The detailed
procedure of submitting the file depends on the particular certificate authority.
5. When the signed certificate is received from the certificate authority, save it as
/etc/Pegasus/server.pem.
6. Copy the certificate of the trusted authority to the Pegasus trust store to make sure that
Pegasus is capable of trusting its own certificate by running as root:
cp /path/to/ca.crt /etc/Pegasus/client.pem
After accomplishing all the described steps, the clients that trust the signing authority are able to
successfully communicate with the managed server’s CIMOM.
IMPORTANT
Unlike the Identity Management solution, if the certificate expires and needs to be
renewed, all of the described manual steps have to be carried out again. It is
recommended to renew the certificates before they expire.
22.4. USING LMISHELL
LMIShell is an interactive client and non-interactive interpreter that can be used to access CIM
objects provided by the OpenPegasus CIMOM. It is based on the Python interpreter, but also
implements additional functions and classes for interacting with CIM objects.
22.4.1. Starting, Using, and Exiting LMIShell
Similarly to the Python interpreter, you can use LMIShell either as an interactive client, or as a non-
interactive interpreter for LMIShell scripts.
Starting LMIShell in Interactive Mode
To start the LMIShell interpreter in interactive mode, run the lmishell command with no additional
arguments:
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lmishell
By default, when LMIShell attempts to establish a connection with a CIMOM, it validates the server-
side certificate against the Certification Authorities trust store. To disable this validation, run the
lmishell command with the--noverify or -n command line option:
lmishell --noverify
Using Tab Completion
When running in interactive mode, the LMIShell interpreter allows you press the Tab key to complete
basic programming structures and CIM objects, including namespaces, classes, methods, and object
properties.
Browsing History
By default, LMIShell stores all commands you type at the interactive prompt in the
~/.lmishell_history file. This allows you to browse the command history and re-use already entered
lines in interactive mode without the need to type them at the prompt again. To move backward in the
command history, press the Up Arrow key or theCtrl+p key combination. To move forward in the
command history, press the Down Arrow key or theCtrl+n key combination.
LMIShell also supports an incremental reverse search. To look for a particular line in the command
history, press Ctrl+r and start typing any part of the command. For example:
> (reverse-i-search)`connect': c = connect("server.example.com", "pegasus")
To clear the command history, use the clear_history() function as follows:
clear_history()
You can configure the number of lines that are stored in the command history by changing the value
of the history_length option in the~/.lmishellrc configuration file. In addition, you can change the
location of the history file by changing the value of the history_file option in this configuration file.
For example, to set the location of the history file to ~/.lmishell_history and configure LMIShell to
store the maximum of 1000 lines in it, add the following lines to the~/.lmishellrc file:
history_file = "~/.lmishell_history"
history_length = 1000
Handling Exceptions
By default, the LMIShell interpreter handles all exceptions and uses return values. To disable this
behavior in order to handle all exceptions in the code, use the use_exceptions() function as follows:
use_exceptions()
To re-enable the automatic exception handling, use:
use_exception(False)
You can permanently disable the exception handling by changing the value of the use_exceptions
option in the ~/.lmishellrc configuration file toTrue:
use_exceptions = True
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Configuring a Temporary Cache
With the default configuration, LMIShell connection objects use a temporary cache for storing CIM
class names and CIM classes in order to reduce network communication. To clear this temporary
cache, use the clear_cache() method as follows:
object_name.clear_cache()
Replace object_name with the name of a connection object.
To disable the temporary cache for a particular connection object, use the use_cache() method as
follows:
object_name.use_cache(False)
To enable it again, use:
object_name.use_cache(True)
You can permanently disable the temporary cache for connection objects by changing the value of
the use_cache option in the~/.lmishellrc configuration file toFalse:
use_cache = False
Exiting LMIShell
To terminate the LMIShell interpreter and return to the shell prompt, press the Ctrl+d key
combination or issue the quit() function as follows:
> quit()
~]$
Running an LMIShell Script
To run an LMIShell script, run the lmishell command as follows:
lmishell file_name
Replace file_name with the name of the script. To inspect an LMIShell script after its execution, also
specify the --interact or -i command line option:
lmishell --interact file_name
The preferred file extension of LMIShell scripts is .lmi.
22.4.2. Connecting to a CIMOM
LMIShell allows you to connect to a CIMOM that is running either locally on the same system, or on a
remote machine accessible over the network.
Connecting to a Remote CIMOM
To access CIM objects provided by a remote CIMOM, create a connection object by using the
connect() function as follows:
connect(host_name, user_name, password)
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Replace host_name with the host name of the managed system,user_name with the name of a user
that is allowed to connect to the OpenPegasus CIMOM running on that system, and password with the
user’s password. If the password is omitted, LMIShell prompts the user to enter it. The function
returns an LMIConnection object.
Example 22.1. Connecting to a Remote CIMOM
To connect to the OpenPegasus CIMOM running on server.example.com as userpegasus, type
the following at the interactive prompt:
> c = connect("server.example.com", "pegasus")
password:
>
Connecting to a Local CIMOM
LMIShell allows you to connect to a local CIMOM by using a Unix socket. For this type of connection,
you must run the LMIShell interpreter as the root user and the/var/run/tog-pegasus/cimxml.socket
socket must exist.
To access CIM objects provided by a local CIMOM, create a connection object by using the connect()
function as follows:
connect(host_name)
Replace host_name withlocalhost, 127.0.0.1, or ::1. The function returns anLMIConnection object or
None.
Example 22.2. Connecting to a Local CIMOM
To connect to the OpenPegasus CIMOM running on localhost as theroot user, type the following
at the interactive prompt:
> c = connect("localhost")
>
Verifying a Connection to a CIMOM
The connect() function returns either anLMIConnection object, or None if the connection could not
be established. In addition, when the connect() function fails to establish a connection, it prints an
error message to standard error output.
To verify that a connection to a CIMOM has been established successfully, use the isinstance()
function as follows:
isinstance(object_name, LMIConnection)
Replace object_name with the name of the connection object. This function returns
True if
object_name is anLMIConnection object, or False otherwise.
Example 22.3. Verifying a Connection to a CIMOM
To verify that the c variable created inExample 22.1, “Connecting to a Remote CIMOM”contains
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To verify that the c variable created inExample 22.1, “Connecting to a Remote CIMOM”contains
an LMIConnection object, type the following at the interactive prompt:
> isinstance(c, LMIConnection)
True
>
Alternatively, you can verify that c is notNone:
> c is None
False
>
22.4.3. Working with Namespaces
LMIShell namespaces provide a natural means of organizing available classes and serve as a hierarchic
access point to other namespaces and classes. The root namespace is the first entry point of a
connection object.
Listing Available Namespaces
To list all available namespaces, use the print_namespaces() method as follows:
object_name.print_namespaces()
Replace object_name with the name of the object to inspect. This method prints available
namespaces to standard output.
To get a list of available namespaces, access the object attribute namespaces:
object_name.namespaces
This returns a list of strings.
Example 22.4. Listing Available Namespaces
To inspect the root namespace object of thec connection object created inExample 22.1,
“Connecting to a Remote CIMOM” and list all available namespaces, type the following at the
interactive prompt:
> c.root.print_namespaces()
cimv2
interop
PG_InterOp
PG_Internal
>
To assign a list of these namespaces to a variable named root_namespaces, type:
> root_namespaces = c.root.namespaces
>
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Accessing Namespace Objects
To access a particular namespace object, use the following syntax:
object_name.namespace_name
Replace object_name with the name of the object to inspect andnamespace_name with the name of
the namespace to access. This returns an LMINamespace object.
Example 22.5. Accessing Namespace Objects
To access the cimv2 namespace of thec connection object created inExample 22.1, “Connecting
to a Remote CIMOM” and assign it to a variable namedns, type the following at the interactive
prompt:
> ns = c.root.cimv2
>
22.4.4. Working with Classes
LMIShell classes represent classes provided by a CIMOM. You can access and list their properties,
methods, instances, instance names, and ValueMap properties, print their documentation strings, and
create new instances and instance names.
Listing Available Classes
To list all available classes in a particular namespace, use the print_classes() method as follows:
namespace_object.print_classes()
Replace namespace_object with the namespace object to inspect. This method prints available
classes to standard output.
To get a list of available classes, use the classes() method:
namespace_object.classes()
This method returns a list of strings.
Example 22.6. Listing Available Classes
To inspect the ns namespace object created inExample 22.5, “Accessing Namespace Objects”and
list all available classes, type the following at the interactive prompt:
> ns.print_classes()
CIM_CollectionInSystem
CIM_ConcreteIdentity
CIM_ControlledBy
CIM_DeviceSAPImplementation
CIM_MemberOfStatusCollection
...
>
To assign a list of these classes to a variable named cimv2_classes, type:
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> cimv2_classes = ns.classes()
>
Accessing Class Objects
To access a particular class object that is provided by the CIMOM, use the following syntax:
namespace_object.class_name
Replace namespace_object with the name of the namespace object to inspect and
class_name with
the name of the class to access.
Example 22.7. Accessing Class Objects
To access the LMI_IPNetworkConnection class of thens namespace object created in
Example 22.5, “Accessing Namespace Objects”and assign it to a variable namedcls, type the
following at the interactive prompt:
> cls = ns.LMI_IPNetworkConnection
>
Examining Class Objects
All class objects store information about their name and the namespace they belong to, as well as
detailed class documentation. To get the name of a particular class object, use the following syntax:
class_object.classname
Replace class_object with the name of the class object to inspect. This returns a string representation
of the object name.
To get information about the namespace a class object belongs to, use:
class_object.namespace
This returns a string representation of the namespace.
To display detailed class documentation, use the doc() method as follows:
class_object.doc()
Example 22.8. Examining Class Objects
To inspect the cls class object created inExample 22.7, “Accessing Class Objects”and display its
name and corresponding namespace, type the following at the interactive prompt:
> cls.classname
'LMI_IPNetworkConnection'
> cls.namespace
'root/cimv2'
>
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To access class documentation, type:
> cls.doc()
Class: LMI_IPNetworkConnection
SuperClass: CIM_IPNetworkConnection
[qualifier] string UMLPackagePath: 'CIM::Network::IP'
[qualifier] string Version: '0.1.0'
...
Listing Available Methods
To list all available methods of a particular class object, use the print_methods() method as follows:
class_object.print_methods()
Replace class_object with the name of the class object to inspect. This method prints available
methods to standard output.
To get a list of available methods, use the methods() method:
class_object.methods()
Example 22.9. Listing Available Methods
To inspect the cls class object created inExample 22.7, “Accessing Class Objects”and list all
available methods, type the following at the interactive prompt:
> cls.print_methods()
RequestStateChange
>
To assign a list of these methods to a variable named service_methods, type:
> service_methods = cls.methods()
>
Listing Available Properties
To list all available properties of a particular class object, use the print_properties() method as
follows:
class_object.print_properties()
properties to standard output.
To get a list of available properties, use the properties() method:
class_object.properties()
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Example 22.10. Listing Available Properties
available properties, type the following at the interactive prompt:
> cls.print_properties()
RequestedState
HealthState
StatusDescriptions
TransitioningToState
Generation
...
>
To assign a list of these classes to a variable named service_properties, type:
> service_properties = cls.properties()
>
Listing and Viewing ValueMap Properties
CIM classes may contain ValueMap properties in their Managed Object Format (
MOF) definition.
ValueMap properties contain constant values, which may be useful when calling methods or checking
returned values.
To list all available ValueMap properties of a particular class object, use the
print_valuemap_properties() method as follows:
class_object.print_valuemap_properties()
ValueMap properties to standard output:
To get a list of available ValueMap properties, use the valuemap_properties() method:
class_object.valuemap_properties()
Example 22.11. Listing ValueMap Properties
available ValueMap properties, type the following at the interactive prompt:
> cls.print_valuemap_properties()
RequestedState
HealthState
DetailedStatus
OperationalStatus
...
>
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To assign a list of these ValueMap properties to a variable named service_valuemap_properties,
type:
> service_valuemap_properties = cls.valuemap_properties()
>
To access a particular ValueMap property, use the following syntax:
class_object.valuemap_propertyValues
Replace valuemap_property with the name of the ValueMap property to access.
To list all available constant values, use the print_values() method as follows:
class_object.valuemap_propertyValues.print_values()
This method prints available named constant values to standard output. You can also get a list of
available constant values by using the values() method:
class_object.valuemap_propertyValues.values()
Example 22.12. Accessing ValueMap Properties
Example 22.11, “Listing ValueMap Properties”mentions a ValueMap property named
RequestedState. To inspect this property and list available constant values, type the following at
the interactive prompt:
> cls.RequestedStateValues.print_values()
Reset
NoChange
NotApplicable
Quiesce
Unknown
...
>
To assign a list of these constant values to a variable named requested_state_values, type:
> requested_state_values = cls.RequestedStateValues.values()
>
To access a particular constant value, use the following syntax:
class_object.valuemap_propertyValues.constant_value_name
Replace constant_value_name with the name of the constant value. Alternatively, you can use the
value() method as follows:
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class_object.valuemap_propertyValues.value("constant_value_name")
To determine the name of a particular constant value, use the value_name() method:
class_object.valuemap_propertyValues.value_name("constant_value")
This method returns a string.
Example 22.13. Accessing Constant Values
Example 22.12, “Accessing ValueMap Properties”shows that theRequestedState property
provides a constant value named Reset. To access this named constant value, type the following
at the interactive prompt:
> cls.RequestedStateValues.Reset
11
> cls.RequestedStateValues.value("Reset")
11
>
To determine the name of this constant value, type:
> cls.RequestedStateValues.value_name(11)
u'Reset'
>
Fetching a CIMClass Object
Many class methods do not require access to a CIMClass object, which is why LMIShell only fetches
this object from the CIMOM when a called method actually needs it. To fetch the CIMClass object
manually, use the fetch() method as follows:
class_object.fetch()
Replace class_object with the name of the class object. Note that methods that require access to a
CIMClass object fetch it automatically.
22.4.5. Working with Instances
LMIShell instances represent instances provided by a CIMOM. You can get and set their properties,
list and call their methods, print their documentation strings, get a list of associated or association
objects, push modified objects to the CIMOM, and delete individual instances from the CIMOM.
Accessing Instances
To get a list of all available instances of a particular class object, use the instances() method as
follows:
class_object.instances()
Replace class_object with the name of the class object to inspect. This method returns a list of
LMIInstance objects.
To access the first instance of a class object, use the first_instance() method:
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class_object.first_instance()
This method returns an LMIInstance object.
In addition to listing all instances or returning the first one, both instances() and first_instance()
support an optional argument to allow you to filter the results:
class_object.instances(criteria)
class_object.first_instance(criteria)
Replace criteria with a dictionary consisting of key-value pairs, where keys represent instance
properties and values represent required values of these properties.
Example 22.14. Accessing Instances
To find the first instance of the cls class object created inExample 22.7, “Accessing Class Objects”
that has the ElementName property equal toeth0 and assign it to a variable nameddevice, type
the following at the interactive prompt:
> device = cls.first_instance({"ElementName": "eth0"})
>
Examining Instances
All instance objects store information about their class name and the namespace they belong to, as
well as detailed documentation about their properties and values. In addition, instance objects allow
you to retrieve a unique identification object.
To get the class name of a particular instance object, use the following syntax:
instance_object.classname
Replace instance_object with the name of the instance object to inspect. This returns a string
representation of the class name.
To get information about the namespace an instance object belongs to, use:
instance_object.namespace
To retrieve a unique identification object for an instance object, use:
instance_object.path
This returns an LMIInstanceName object.
Finally, to display detailed documentation, use the doc() method as follows:
instance_object.doc()
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Example 22.15. Examining Instances
To inspect the device instance object created inExample 22.14, “Accessing Instances” and display
its class name and the corresponding namespace, type the following at the interactive prompt:
> device.classname
u'LMI_IPNetworkConnection'
> device.namespace
'root/cimv2'
>
To access instance object documentation, type:
> device.doc()
Instance of LMI_IPNetworkConnection
[property] uint16 RequestedState = '12'
[property] uint16 HealthState
[property array] string [] StatusDescriptions
...
Creating New Instances
Certain CIM providers allow you to create new instances of specific classes objects. To create a new
instance of a class object, use the create_instance() method as follows:
class_object.create_instance(properties)
Replace class_object with the name of the class object andproperties with a dictionary that consists
of key-value pairs, where keys represent instance properties and values represent property values.
This method returns an LMIInstance object.
Example 22.16. Creating New Instances
The LMI_Group class represents system groups and theLMI_Account class represents user
accounts on the managed system. To use the ns namespace object created inExample 22.5,
“Accessing Namespace Objects”, create instances of these two classes for the system group
named pegasus and the user namedlmishell-user, and assign them to variables namedgroup and
user, type the following at the interactive prompt:
> group = ns.LMI_Group.first_instance({"Name" : "pegasus"})
> user = ns.LMI_Account.first_instance({"Name" : "lmishell-user"})
>
To get an instance of the LMI_Identity class for thelmishell-user user, type:
> identity = user.first_associator(ResultClass="LMI_Identity")
>
The LMI_MemberOfGroup class represents system group membership. To use the
LMI_MemberOfGroup class to add thelmishell-user to thepegasus group, create a new instance
of this class as follows:
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> ns.LMI_MemberOfGroup.create_instance({
... "Member" : identity.path,
... "Collection" : group.path})
LMIInstance(classname="LMI_MemberOfGroup", ...)
>
Deleting Individual Instances
To delete a particular instance from the CIMOM, use the delete() method as follows:
instance_object.delete()
Replace instance_object with the name of the instance object to delete. This method returns a
boolean. Note that after deleting an instance, its properties and methods become inaccessible.
Example 22.17. Deleting Individual Instances
The LMI_Account class represents user accounts on the managed system. To use thens
namespace object created in Example 22.5, “Accessing Namespace Objects”, create an instance
of the LMI_Account class for the user namedlmishell-user, and assign it to a variable nameduser,
type the following at the interactive prompt:
> user = ns.LMI_Account.first_instance({"Name" : "lmishell-user"})
>
To delete this instance and remove the lmishell-user from the system, type:
> user.delete()
True
>
Listing and Accessing Available Properties
To list all available properties of a particular instance object, use the print_properties() method as
follows:
instance_object.print_properties()
Replace instance_object with the name of the instance object to inspect. This method prints available
properties to standard output.
To get a list of available properties, use the properties() method:
instance_object.properties()
Example 22.18. Listing Available Properties
To inspect the device instance object created inExample 22.14, “Accessing Instances” and list all
available properties, type the following at the interactive prompt:
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> device.print_properties()
RequestedState
HealthState
StatusDescriptions
Generation
...
>
To assign a list of these properties to a variable named device_properties, type:
> device_properties = device.properties()
>
To get the current value of a particular property, use the following syntax:
instance_object.property_name
Replace property_name with the name of the property to access.
To modify the value of a particular property, assign a value to it as follows:
instance_object.property_name = value
Replace value with the new value of the property. Note that in order to propagate the change to the
CIMOM, you must also execute the push() method:
instance_object.push()
This method returns a three-item tuple consisting of a return value, return value parameters, and an
error string.
Example 22.19. Accessing Individual Properties
To inspect the device instance object created inExample 22.14, “Accessing Instances” and display
the value of the property named SystemName, type the following at the interactive prompt:
> device.SystemName
u'server.example.com'
>
Listing and Using Available Methods
To list all available methods of a particular instance object, use the print_methods() method as
follows:
instance_object.print_methods()
Replace instance_object with the name of the instance object to inspect. This method prints available
methods to standard output.
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To get a list of available methods, use the method() method:
instance_object.methods()
Example 22.20. Listing Available Methods
To inspect the device instance object created inExample 22.14, “Accessing Instances” and list all
available methods, type the following at the interactive prompt:
> device.print_methods()
RequestStateChange
>
To assign a list of these methods to a variable named network_device_methods, type:
> network_device_methods = device.methods()
>
To call a particular method, use the following syntax:
instance_object.method_name(
parameter=value,
...)
Replace instance_object with the name of the instance object to use,method_name with the name of
the method to call, parameter with the name of the parameter to set, and
value with the value of this
parameter. Methods return a three-item tuple consisting of a return value, return value parameters,
and an error string.
IMPORTANT
LMIInstance objects donot automatically refresh their contents (properties, methods,
qualifiers, and so on). To do so, use the refresh() method as described below.
Example 22.21. Using Methods
The PG_ComputerSystem class represents the system. To create an instance of this class by
using the ns namespace object created inExample 22.5, “Accessing Namespace Objects”and
assign it to a variable named sys, type the following at the interactive prompt:
> sys = ns.PG_ComputerSystem.first_instance()
>
The LMI_AccountManagementService class implements methods that allow you to manage users
and groups in the system. To create an instance of this class and assign it to a variable named acc,
type:
> acc = ns.LMI_AccountManagementService.first_instance()
>
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To create a new user named lmishell-user in the system, use theCreateAccount() method as
follows:
> acc.CreateAccount(Name="lmishell-user", System=sys)
LMIReturnValue(rval=0, rparams=NocaseDict({u'Account':
LMIInstanceName(classname="LMI_Account"...), u'Identities':
[LMIInstanceName(classname="LMI_Identity"...),
LMIInstanceName(classname="LMI_Identity"...)]}), errorstr='')
LMIShell support synchronous method calls: when you use a synchronous method, LMIShell waits for
the corresponding Job object to change its state to "finished" and then returns the return
parameters of this job. LMIShell is able to perform a synchronous method call if the given method
returns an object of one of the following classes:
LMI_StorageJob
LMI_SoftwareInstallationJob
LMI_NetworkJob
LMIShell first tries to use indications as the waiting method. If it fails, it uses a polling method instead.
To perform a synchronous method call, use the following syntax:
instance_object.Syncmethod_name(
parameter=value,
...)
Replace instance_object with the name of the instance object to use,method_name with the name of
the method to call, parameter with the name of the parameter to set, and
value with the value of this
parameter. All synchronous methods have the Sync prefix in their name and return a three-item tuple
consisting of the job’s return value, job’s return value parameters, and job’s error string.
You can also force LMIShell to use only polling method. To do so, specify the PreferPolling parameter
as follows:
instance_object.Syncmethod_name(
PreferPolling=True
parameter=value,
...)
Listing and Viewing ValueMap Parameters
CIM methods may contain ValueMap parameters in their Managed Object Format (
MOF) definition.
ValueMap parameters contain constant values.
To list all available ValueMap parameters of a particular method, use the
print_valuemap_parameters() method as follows:
instance_object.method_name.print_valuemap_parameters()
Replace instance_object with the name of the instance object andmethod_name with the name of the
method to inspect. This method prints available ValueMap parameters to standard output.
To get a list of available ValueMap parameters, use the valuemap_parameters() method:
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instance_object.method_name.valuemap_parameters()
Example 22.22. Listing ValueMap Parameters
To inspect the acc instance object created inExample 22.21, “Using Methods”and list all available
ValueMap parameters of the CreateAccount() method, type the following at the interactive
prompt:
> acc.CreateAccount.print_valuemap_parameters()
CreateAccount
>
To assign a list of these ValueMap parameters to a variable named create_account_parameters,
type:
> create_account_parameters = acc.CreateAccount.valuemap_parameters()
>
To access a particular ValueMap parameter, use the following syntax:
instance_object.method_name.valuemap_parameterValues
Replace valuemap_parameter with the name of the ValueMap parameter to access.
To list all available constant values, use the print_values() method as follows:
instance_object.method_name.valuemap_parameterValues.print_values()
This method prints available named constant values to standard output. You can also get a list of
available constant values by using the values() method:
instance_object.method_name.valuemap_parameterValues.values()
Example 22.23. Accessing ValueMap Parameters
Example 22.22, “Listing ValueMap Parameters”mentions a ValueMap parameter named
CreateAccount. To inspect this parameter and list available constant values, type the following at
the interactive prompt:
> acc.CreateAccount.CreateAccountValues.print_values()
Operationunsupported
Failed
Unabletosetpasswordusercreated
Unabletocreatehomedirectoryusercreatedandpasswordset
Operationcompletedsuccessfully
>
To assign a list of these constant values to a variable named create_account_values, type:
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> create_account_values = acc.CreateAccount.CreateAccountValues.values()
>
To access a particular constant value, use the following syntax:
instance_object.method_name.valuemap_parameterValues.constant_value_name
Replace constant_value_name with the name of the constant value. Alternatively, you can use the
value() method as follows:
instance_object.method_name.valuemap_parameterValues.value("constant_value_name")
To determine the name of a particular constant value, use the value_name() method:
instance_object.method_name.valuemap_parameterValues.value_name("constant_value")
This method returns a string.
Example 22.24. Accessing Constant Values
Example 22.23, “Accessing ValueMap Parameters” shows that theCreateAccount ValueMap
parameter provides a constant value named Failed. To access this named constant value, type the
> acc.CreateAccount.CreateAccountValues.Failed
2
> acc.CreateAccount.CreateAccountValues.value("Failed")
2
>
To determine the name of this constant value, type:
> acc.CreateAccount.CreateAccountValues.value_name(2)
u'Failed'
>
Refreshing Instance Objects
Local objects used by LMIShell, which represent CIM objects at CIMOM side, can get outdated, if
such objects change while working with LMIShell’s ones. To update the properties and methods of a
particular instance object, use the refresh() method as follows:
instance_object.refresh()
Replace instance_object with the name of the object to refresh. This method returns a three-item
tuple consisting of a return value, return value parameter, and an error string.
Example 22.25. Refreshing Instance Objects
To update the properties and methods of the device instance object created inExample 22.14,
“Accessing Instances”, type the following at the interactive prompt:
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> device.refresh()
LMIReturnValue(rval=True, rparams=NocaseDict({}), errorstr='')
>
Displaying MOF Representation
To display the Managed Object Format (MOF) representation of an instance object, use thetomof()
method as follows:
instance_object.tomof()
Replace instance_object with the name of the instance object to inspect. This method prints the MOF
representation of the object to standard output.
Example 22.26. Displaying MOF Representation
To display the MOF representation of the device instance object created inExample 22.14,
“Accessing Instances”, type the following at the interactive prompt:
> device.tomof()
instance of LMI_IPNetworkConnection {
RequestedState = 12;
HealthState = NULL;
StatusDescriptions = NULL;
TransitioningToState = 12;
...
22.4.6. Working with Instance Names
LMIShell instance names are objects that hold a set of primary keys and their values. This type of an
object exactly identifies an instance.
Accessing Instance Names
CIMInstance objects are identified byCIMInstanceName objects. To get a list of all available
instance name objects, use the instance_names() method as follows:
class_object.instance_names()
Replace class_object with the name of the class object to inspect. This method returns a list of
LMIInstanceName objects.
To access the first instance name object of a class object, use the first_instance_name() method:
class_object.first_instance_name()
This method returns an LMIInstanceName object.
In addition to listing all instance name objects or returning the first one, both instance_names() and
first_instance_name() support an optional argument to allow you to filter the results:
class_object.instance_names(criteria)
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class_object.first_instance_name(criteria)
Replace criteria with a dictionary consisting of key-value pairs, where keys represent key properties
and values represent required values of these key properties.
Example 22.27. Accessing Instance Names
To find the first instance name of the cls class object created inExample 22.7, “Accessing Class
Objects” that has theName key property equal toeth0 and assign it to a variable named
device_name, type the following at the interactive prompt:
> device_name = cls.first_instance_name({"Name": "eth0"})
>
Examining Instance Names
All instance name objects store information about their class name and the namespace they belong
to.
To get the class name of a particular instance name object, use the following syntax:
instance_name_object.classname
Replace instance_name_object with the name of the instance name object to inspect. This returns a
string representation of the class name.
To get information about the namespace an instance name object belongs to, use:
instance_name_object.namespace
Example 22.28. Examining Instance Names
To inspect the device_name instance name object created inExample 22.27, “Accessing Instance
Names” and display its class name and the corresponding namespace, type the following at the
interactive prompt:
> device_name.classname
u'LMI_IPNetworkConnection'
> device_name.namespace
'root/cimv2'
>
Creating New Instance Names
LMIShell allows you to create a new wrapped CIMInstanceName object if you know all primary keys of
a remote object. This instance name object can then be used to retrieve the whole instance object.
To create a new instance name of a class object, use the new_instance_name() method as follows:
class_object.new_instance_name(key_properties)
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Replace class_object with the name of the class object andkey_properties with a dictionary that
consists of key-value pairs, where keys represent key properties and values represent key property
values. This method returns an LMIInstanceName object.
Example 22.29. Creating New Instance Names
The LMI_Account class represents user accounts on the managed system. To use thens
namespace object created in Example 22.5, “Accessing Namespace Objects”and create a new
instance name of the LMI_Account class representing thelmishell-user user on the managed
system, type the following at the interactive prompt:
> instance_name = ns.LMI_Account.new_instance_name({
... "CreationClassName" : "LMI_Account",
... "Name" : "lmishell-user",
... "SystemCreationClassName" : "PG_ComputerSystem",
... "SystemName" : "server"})
>
Listing and Accessing Key Properties
To list all available key properties of a particular instance name object, use the print_key_properties()
method as follows:
instance_name_object.print_key_properties()
Replace instance_name_object with the name of the instance name object to inspect. This method
prints available key properties to standard output.
To get a list of available key properties, use the key_properties() method:
instance_name_object.key_properties()
Example 22.30. Listing Available Key Properties
Names” and list all available key properties, type the following at the interactive prompt:
> device_name.print_key_properties()
CreationClassName
SystemName
Name
SystemCreationClassName
>
To assign a list of these key properties to a variable named device_name_properties, type:
> device_name_properties = device_name.key_properties()
>
To get the current value of a particular key property, use the following syntax:
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instance_name_object.key_property_name
Replace key_property_name with the name of the key property to access.
Example 22.31. Accessing Individual Key Properties
Names” and display the value of the key property namedSystemName, type the following at the
interactive prompt:
> device_name.SystemName
u'server.example.com'
>
Converting Instance Names to Instances
Each instance name can be converted to an instance. To do so, use the to_instance() method as
follows:
instance_name_object.to_instance()
Replace instance_name_object with the name of the instance name object to convert. This method
returns an LMIInstance object.
Example 22.32. Converting Instance Names to Instances
To convert the device_name instance name object created inExample 22.27, “Accessing Instance
Names” to an instance object and assign it to a variable named
device, type the following at the
interactive prompt:
> device = device_name.to_instance()
>
22.4.7. Working with Associated Objects
The Common Information Model defines an association relationship between managed objects.
Accessing Associated Instances
To get a list of all objects associated with a particular instance object, use the associators() method
as follows:
instance_object.associators(
AssocClass=class_name,
ResultClass=class_name,
ResultRole=role,
IncludeQualifiers=include_qualifiers,
IncludeClassOrigin=include_class_origin,
PropertyList=property_list)
To access the first object associated with a particular instance object, use the first_associator()
method:
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instance_object.first_associator(
ResultRole=role,
Replace instance_object with the name of the instance object to inspect. You can filter the results by
specifying the following parameters:
AssocClass — Each returned object must be associated with the source object through an
instance of this class or one of its subclasses. The default value is None.
ResultClass — Each returned object must be either an instance of this class or one of its
subclasses, or it must be this class or one of its subclasses. The default value is None.
Role — Each returned object must be associated with the source object through an
association in which the source object plays the specified role. The name of the property in
the association class that refers to the source object must match the value of this parameter.
The default value is None.
ResultRole — Each returned object must be associated with the source object through an
association in which the returned object plays the specified role. The name of the property in
the association class that refers to the returned object must match the value of this
parameter. The default value is None.
The remaining parameters refer to:
IncludeQualifiers — A boolean indicating whether all qualifiers of each object (including
qualifiers on the object and on any returned properties) should be included as QUALIFIER
elements in the response. The default value is False.
IncludeClassOrigin — A boolean indicating whether the CLASSORIGIN attribute should be
present on all appropriate elements in each returned object. The default value is False.
PropertyList — The members of this list define one or more property names. Returned
objects will not include elements for any properties missing from this list. If PropertyList is an
empty list, no properties are included in returned objects. If it is None, no additional filtering is
defined. The default value is None.
Example 22.33. Accessing Associated Instances
The LMI_StorageExtent class represents block devices available in the system. To use thens
namespace object created in Example 22.5, “Accessing Namespace Objects”, create an instance
of the LMI_StorageExtent class for the block device named/dev/vda, and assign it to a variable
named vda, type the following at the interactive prompt:
> vda = ns.LMI_StorageExtent.first_instance({
... "DeviceID" : "/dev/vda"})
>
To get a list of all disk partitions on this block device and assign it to a variable named
vda_partitions, use the associators() method as follows:
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> vda_partitions = vda.associators(ResultClass="LMI_DiskPartition")
>
Accessing Associated Instance Names
To get a list of all associated instance names of a particular instance object, use the
associator_names() method as follows:
instance_object.associator_names(
Role=role,
ResultRole=role)
To access the first associated instance name of a particular instance object, use the
first_associator_name() method:
instance_object.first_associator_name(
AssocClass=class_object,
ResultClass=class_object,
Role=role,
ResultRole=role)
AssocClass — Each returned name identifies an object that must be associated with the
source object through an instance of this class or one of its subclasses. The default value is
None.
ResultClass — Each returned name identifies an object that must be either an instance of this
class or one of its subclasses, or it must be this class or one of its subclasses. The default
value is None.
Role — Each returned name identifies an object that must be associated with the source
object through an association in which the source object plays the specified role. The name of
the property in the association class that refers to the source object must match the value of
this parameter. The default value is None.
ResultRole — Each returned name identifies an object that must be associated with the
source object through an association in which the returned named object plays the specified
role. The name of the property in the association class that refers to the returned object must
match the value of this parameter. The default value is None.
Example 22.34. Accessing Associated Instance Names
To use the vda instance object created inExample 22.33, “Accessing Associated Instances”, get a
list of its associated instance names, and assign it to a variable named vda_partitions, type:
> vda_partitions = vda.associator_names(ResultClass="LMI_DiskPartition")
>
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22.4.8. Working with Association Objects
The Common Information Model defines an association relationship between managed objects.
Association objects define the relationship between two other objects.
Accessing Association Instances
To get a list of association objects that refer to a particular target object, use the references()
method as follows:
instance_object.references(
Role=role,
To access the first association object that refers to a particular target object, use the first_reference()
method:
instance_object.first_reference(
... ResultClass=class_name,
... Role=role,
... IncludeQualifiers=include_qualifiers,
... IncludeClassOrigin=include_class_origin,
... PropertyList=property_list)
>
ResultClass — Each returned object must be either an instance of this class or one of its
subclasses, or it must be this class or one of its subclasses. The default value is None.
Role — Each returned object must refer to the target object through a property with a name
that matches the value of this parameter. The default value is None.
The remaining parameters refer to:
IncludeQualifiers — A boolean indicating whether each object (including qualifiers on the
object and on any returned properties) should be included as a QUALIFIER element in the
response. The default value is False.
IncludeClassOrigin — A boolean indicating whether the CLASSORIGIN attribute should be
present on all appropriate elements in each returned object. The default value is False.
PropertyList — The members of this list define one or more property names. Returned
objects will not include elements for any properties missing from this list. If PropertyList is an
empty list, no properties are included in returned objects. If it is None, no additional filtering is
defined. The default value is None.
Example 22.35. Accessing Association Instances
The LMI_LANEndpoint class represents a communication endpoint associated with a certain
network interface device. To use the ns namespace object created inExample 22.5, “Accessing
Namespace Objects”, create an instance of theLMI_LANEndpoint class for the network interface
460

device named eth0, and assign it to a variable named lan_endpoint, type the following at the
interactive prompt:
> lan_endpoint = ns.LMI_LANEndpoint.first_instance({
... "Name" : "eth0"})
>
To access the first association object that refers to an LMI_BindsToLANEndpoint object and
assign it to a variable named bind, type:
> bind = lan_endpoint.first_reference(
... ResultClass="LMI_BindsToLANEndpoint")
>
You can now use the Dependent property to access the dependentLMI_IPProtocolEndpoint class
that represents the IP address of the corresponding network interface device:
> ip = bind.Dependent.to_instance()
> print ip.IPv4Address
192.168.122.1
>
Accessing Association Instance Names
To get a list of association instance names of a particular instance object, use the reference_names()
method as follows:
instance_object.reference_names(
Role=role)
To access the first association instance name of a particular instance object, use the
first_reference_name() method:
instance_object.first_reference_name(
Role=role)
ResultClass — Each returned object name identifies either an instance of this class or one of
its subclasses, or this class or one of its subclasses. The default value is None.
Role — Each returned object identifies an object that refers to the target instance through a
property with a name that matches the value of this parameter. The default value is None.
Example 22.36. Accessing Association Instance Names
To use the lan_endpoint instance object created inExample 22.35, “Accessing Association
Instances”, access the first association instance name that refers to an
LMI_BindsToLANEndpoint object, and assign it to a variable namedbind, type:
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> bind = lan_endpoint.first_reference_name(
... ResultClass="LMI_BindsToLANEndpoint")
You can now use the Dependent property to access the dependentLMI_IPProtocolEndpoint class
that represents the IP address of the corresponding network interface device:
> ip = bind.Dependent.to_instance()
> print ip.IPv4Address
192.168.122.1
>
22.4.9. Working with Indications
Indication is a reaction to a specific event that occurs in response to a particular change in data.
LMIShell can subscribe to an indication in order to receive such event responses.
Subscribing to Indications
To subscribe to an indication, use the subscribe_indication() method as follows:
connection_object.subscribe_indication(
QueryLanguage="WQL",
Query='SELECT * FROM CIM_InstModification',
Name="cpu",
CreationNamespace="root/interop",
SubscriptionCreationClassName="CIM_IndicationSubscription",
FilterCreationClassName="CIM_IndicationFilter",
FilterSystemCreationClassName="CIM_ComputerSystem",
FilterSourceNamespace="root/cimv2",
HandlerCreationClassName="CIM_IndicationHandlerCIMXML",
HandlerSystemCreationClassName="CIM_ComputerSystem",
Destination="http://host_name:5988")
Alternatively, you can use a shorter version of the method call as follows:
connection_object.subscribe_indication(
Query='SELECT * FROM CIM_InstModification',
Name="cpu",
Destination="http://host_name:5988")
Replace connection_object with a connection object andhost_name with the host name of the system
you want to deliver the indications to.
By default, all subscriptions created by the LMIShell interpreter are automatically deleted when the
interpreter terminates. To change this behavior, pass the Permanent=True keyword parameter to the
subscribe_indication() method call. This will prevent LMIShell from deleting the subscription.
Example 22.37. Subscribing to Indications
To use the c connection object created inExample 22.1, “Connecting to a Remote CIMOM”and
subscribe to an indication named cpu, type the following at the interactive prompt:
> c.subscribe_indication(
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... QueryLanguage="WQL",
... Query='SELECT * FROM CIM_InstModification',
... Name="cpu",
... CreationNamespace="root/interop",
... SubscriptionCreationClassName="CIM_IndicationSubscription",
... FilterCreationClassName="CIM_IndicationFilter",
... FilterSystemCreationClassName="CIM_ComputerSystem",
... FilterSourceNamespace="root/cimv2",
... HandlerCreationClassName="CIM_IndicationHandlerCIMXML",
... HandlerSystemCreationClassName="CIM_ComputerSystem",
... Destination="http://server.example.com:5988")
>
Listing Subscribed Indications
To list all the subscribed indications, use the print_subscribed_indications() method as follows:
connection_object.print_subscribed_indications()
Replace connection_object with the name of the connection object to inspect. This method prints
subscribed indications to standard output.
To get a list of subscribed indications, use the subscribed_indications() method:
connection_object.subscribed_indications()
Example 22.38. Listing Subscribed Indications
To inspect the c connection object created inExample 22.1, “Connecting to a Remote CIMOM”and
list all subscribed indications, type the following at the interactive prompt:
> c.print_subscribed_indications()
>
To assign a list of these indications to a variable named indications, type:
> indications = c.subscribed_indications()
>
Unsubscribing from Indications
By default, all subscriptions created by the LMIShell interpreter are automatically deleted when the
interpreter terminates. To delete an individual subscription sooner, use the
unsubscribe_indication() method as follows:
connection_object.unsubscribe_indication(indication_name)
Replace connection_object with the name of the connection object andindication_name with the
name of the indication to delete.
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To delete all subscriptions, use the unsubscribe_all_indications() method:
connection_object.unsubscribe_all_indications()
Example 22.39. Unsubscribing from Indications
To use the c connection object created inExample 22.1, “Connecting to a Remote CIMOM”and
unsubscribe from the indication created in Example 22.37, “Subscribing to Indications”, type the
> c.unsubscribe_indication('cpu')
>
Implementing an Indication Handler
The subscribe_indication() method allows you to specify the host name of the system you want to
deliver the indications to. The following example shows how to implement an indication handler:
> def handler(ind, arg1, arg2, kwargs): ... exported_objects = ind.exported_objects() ...
do_something_with(exported_objects) > listener = LmiIndicationListener("0.0.0.0",
listening_port) > listener.add_handler("indication-name-XXXXXXXX", handler, arg1, arg2,
kwargs)
> listener.start()
>
The first argument of the handler is an LmiIndication object, which contains a list of methods and
objects exported by the indication. Other parameters are user specific: those arguments need to be
specified when adding a handler to the listener.
In the example above, the add_handler() method call uses a special string with eight "X" characters.
These characters are replaced with a random string that is generated by listeners in order to avoid a
possible handler name collision. To use the random string, start the indication listener first and then
subscribe to an indication so that the Destination property of the handler object contains the
following value: schema://host_name/random_string.
Example 22.40. Implementing an Indication Handler
The following script illustrates how to write a handler that monitors a managed system located at
192.168.122.1 and calls theindication_callback() function whenever a new user account is
created:
#!/usr/bin/lmishell
import sys
from time import sleep
from lmi.shell.LMIUtil import LMIPassByRef
from lmi.shell.LMIIndicationListener import LMIIndicationListener
# These are passed by reference to indication_callback
var1 = LMIPassByRef("some_value")
var2 = LMIPassByRef("some_other_value")
def indication_callback(ind, var1, var2):
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# Do something with ind, var1 and var2
print ind.exported_objects()
print var1.value
print var2.value
c = connect("hostname", "username", "password")
listener = LMIIndicationListener("0.0.0.0", 65500)
unique_name = listener.add_handler(
"demo-XXXXXXXX", # Creates a unique name for me
indication_callback, # Callback to be called
var1, # Variable passed by ref
var2 # Variable passed by ref
)
listener.start()
print c.subscribe_indication(
Name=unique_name,
Query="SELECT * FROM LMI_AccountInstanceCreationIndication WHERE
SOURCEINSTANCE ISA LMI_Account",
Destination="192.168.122.1:65500"
)
try:
while True:
sleep(60)
except KeyboardInterrupt:
sys.exit(0)
22.4.10. Example Usage
This section provides a number of examples for various CIM providers distributed with the OpenLMI
packages. All examples in this section use the following two variable definitions:
c = connect("host_name", "user_name", "password")
ns = c.root.cimv2
Replace host_name with the host name of the managed system,user_name with the name of user that
is allowed to connect to OpenPegasus CIMOM running on that system, and password with the user’s
password.
Using the OpenLMI Service Provider
The openlmi-service package installs a CIM provider for managing system services. The examples
below illustrate how to use this CIM provider to list available system services and how to start, stop,
enable, and disable them.
Example 22.41. Listing Available Services
To list all available services on the managed machine along with information regarding whether
the service has been started (TRUE) or stopped (FALSE) and the status string, use the following
code snippet:
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for service in ns.LMI_Service.instances():
print "%s:t%s" % (service.Name, service.Status)
To list only the services that are enabled by default, use this code snippet:
cls = ns.LMI_Service
for service in cls.instances():
if service.EnabledDefault == cls.EnabledDefaultValues.Enabled:
print service.Name
Note that the value of the EnabledDefault property is equal to2 for enabled services and3 for
disabled services.
To display information about the cups service, use the following:
cups = ns.LMI_Service.first_instance({"Name": "cups.service"})
cups.doc()
Example 22.42. Starting and Stopping Services
To start and stop the cups service and to see its current status, use the following code snippet:
cups.StartService()
print cups.Status
cups.StopService()
print cups.Status
Example 22.43. Enabling and Disabling Services
To enable and disable the cups service and to display itsEnabledDefault property, use the
following code snippet:
cups.TurnServiceOff()
print cups.EnabledDefault
cups.TurnServiceOn()
print cups.EnabledDefault
Using the OpenLMI Networking Provider
The openlmi-networking package installs a CIM provider for networking. The examples below
illustrate how to use this CIM provider to list IP addresses associated with a certain port number,
create a new connection, configure a static IP address, and activate a connection.
Example 22.44. Listing IP Addresses Associated with a Given Port Number
To list all IP addresses associated with the eth0 network interface, use the following code snippet:
device = ns.LMI_IPNetworkConnection.first_instance({'ElementName': 'eth0'})
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for endpoint in device.associators(AssocClass="LMI_NetworkSAPSAPDependency",
ResultClass="LMI_IPProtocolEndpoint"):
if endpoint.ProtocolIFType == ns.LMI_IPProtocolEndpoint.ProtocolIFTypeValues.IPv4:
print "IPv4: %s/%s" % (endpoint.IPv4Address, endpoint.SubnetMask)
elif endpoint.ProtocolIFType == ns.LMI_IPProtocolEndpoint.ProtocolIFTypeValues.IPv6:
print "IPv6: %s/%d" % (endpoint.IPv6Address, endpoint.IPv6SubnetPrefixLength)
This code snippet uses the LMI_IPProtocolEndpoint class associated with a given
LMI_IPNetworkConnection class.
To display the default gateway, use this code snippet:
for rsap in
device.associators(AssocClass="LMI_NetworkRemoteAccessAvailableToElement",
ResultClass="LMI_NetworkRemoteServiceAccessPoint"):
if rsap.AccessContext ==
ns.LMI_NetworkRemoteServiceAccessPoint.AccessContextValues.DefaultGateway:
print "Default Gateway: %s" % rsap.AccessInfo
The default gateway is represented by an LMI_NetworkRemoteServiceAccessPoint instance with
the AccessContext property equal toDefaultGateway.
To get a list of DNS servers, the object model needs to be traversed as follows:
1. Get the LMI_IPProtocolEndpoint instances associated with a given
LMI_IPNetworkConnection usingLMI_NetworkSAPSAPDependency.
2. Use the same association for the LMI_DNSProtocolEndpoint instances.
The LMI_NetworkRemoteServiceAccessPoint instances with theAccessContext property equal
to the DNS Server associated through LMI_NetworkRemoteAccessAvailableToElement have the
DNS server address in the AccessInfo property.
There can be more possible paths to get to the RemoteServiceAccessPath and entries can be
duplicated. The following code snippet uses the set() function to remove duplicate entries from
the list of DNS servers:
dnsservers = set()
for ipendpoint in device.associators(AssocClass="LMI_NetworkSAPSAPDependency",
ResultClass="LMI_IPProtocolEndpoint"):
for dnsedpoint in
ipendpoint.associators(AssocClass="LMI_NetworkSAPSAPDependency",
ResultClass="LMI_DNSProtocolEndpoint"):
for rsap in
dnsedpoint.associators(AssocClass="LMI_NetworkRemoteAccessAvailableToElement",
ResultClass="LMI_NetworkRemoteServiceAccessPoint"):
if rsap.AccessContext ==
ns.LMI_NetworkRemoteServiceAccessPoint.AccessContextValues.DNSServer:
dnsservers.add(rsap.AccessInfo)
print "DNS:", ", ".join(dnsservers)
Example 22.45. Creating a New Connection and Configuring a Static IP Address
To create a new setting with a static IPv4 and stateless IPv6 configuration for network interface
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To create a new setting with a static IPv4 and stateless IPv6 configuration for network interface
eth0, use the following code snippet:
capability = ns.LMI_IPNetworkConnectionCapabilities.first_instance({ 'ElementName':
'eth0' })
result = capability.LMI_CreateIPSetting(Caption='eth0 Static',
IPv4Type=capability.LMI_CreateIPSetting.IPv4TypeValues.Static,
IPv6Type=capability.LMI_CreateIPSetting.IPv6TypeValues.Stateless)
setting = result.rparams["SettingData"].to_instance()
for settingData in
setting.associators(AssocClass="LMI_OrderedIPAssignmentComponent"):
if setting.ProtocolIFType == ns.LMI_IPAssignmentSettingData.ProtocolIFTypeValues.IPv4:
# Set static IPv4 address
settingData.IPAddresses = ["192.168.1.100"]
settingData.SubnetMasks = ["255.255.0.0"]
settingData.GatewayAddresses = ["192.168.1.1"]
settingData.push()
This code snippet creates a new setting by calling the LMI_CreateIPSetting() method on the
instance of LMI_IPNetworkConnectionCapabilities, which is associated with
LMI_IPNetworkConnection throughLMI_IPNetworkConnectionElementCapabilities. It also uses
the push() method to modify the setting.
Example 22.46. Activating a Connection
To apply a setting to the network interface, call the ApplySettingToIPNetworkConnection()
method of the LMI_IPConfigurationService class. This method is asynchronous and returns a job.
The following code snippets illustrates how to call this method synchronously:
setting = ns.LMI_IPAssignmentSettingData.first_instance({ "Caption": "eth0 Static" })
port = ns.LMI_IPNetworkConnection.first_instance({ 'ElementName': 'ens8' })
service = ns.LMI_IPConfigurationService.first_instance()
service.SyncApplySettingToIPNetworkConnection(SettingData=setting,
IPNetworkConnection=port, Mode=32768)
The Mode parameter affects how the setting is applied. The most commonly used values of this
parameter are as follows:
1 — apply the setting now and make it auto-activated.
2 — make the setting auto-activated and do not apply it now.
4 — disconnect and disable auto-activation.
5 — do not change the setting state, only disable auto-activation.
32768 — apply the setting.
32769 — disconnect.
Using the OpenLMI Storage Provider
The openlmi-storage package installs a CIM provider for storage management. The examples below
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The openlmi-storage package installs a CIM provider for storage management. The examples below
illustrate how to use this CIM provider to create a volume group, create a logical volume, build a file
system, mount a file system, and list block devices known to the system.
In addition to the c and ns variables, these examples use the following variable definitions:
MEGABYTE = 1024*1024
storage_service = ns.LMI_StorageConfigurationService.first_instance()
filesystem_service = ns.LMI_FileSystemConfigurationService.first_instance()
Example 22.47. Creating a Volume Group
To create a new volume group located in /dev/myGroup/ that has three members and the default
extent size of 4 MB, use the following code snippet:
# Find the devices to add to the volume group
# (filtering the CIM_StorageExtent.instances()
# call would be faster, but this is easier to read):
sda1 = ns.CIM_StorageExtent.first_instance({"Name": "/dev/sda1"})
sdb1 = ns.CIM_StorageExtent.first_instance({"Name": "/dev/sdb1"})
sdc1 = ns.CIM_StorageExtent.first_instance({"Name": "/dev/sdc1"})
# Create a new volume group:
(ret, outparams, err) = storage_service.SyncCreateOrModifyVG(
ElementName="myGroup",
InExtents=[sda1, sdb1, sdc1])
vg = outparams['Pool'].to_instance()
print "VG", vg.PoolID,
"with extent size", vg.ExtentSize,
"and", vg.RemainingExtents, "free extents created."
Example 22.48. Creating a Logical Volume
To create two logical volumes with the size of 100 MB, use this code snippet:
# Find the volume group:
vg = ns.LMI_VGStoragePool.first_instance({"Name": "/dev/mapper/myGroup"})
# Create the first logical volume:
(ret, outparams, err) = storage_service.SyncCreateOrModifyLV(
ElementName="Vol1",
InPool=vg,
Size=100 * MEGABYTE)
lv = outparams['TheElement'].to_instance()
print "LV", lv.DeviceID,
"with", lv.BlockSize * lv.NumberOfBlocks,
"bytes created."
# Create the second logical volume:
(ret, outparams, err) = storage_service.SyncCreateOrModifyLV(
ElementName="Vol2",
InPool=vg,
Size=100 * MEGABYTE)
lv = outparams['TheElement'].to_instance()
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print "LV", lv.DeviceID,
"with", lv.BlockSize * lv.NumberOfBlocks,
"bytes created."
Example 22.49. Creating a File System
To create an ext3 file system on logical volumelv from Example 22.48, “Creating a Logical
Volume”, use the following code snippet:
(ret, outparams, err) = filesystem_service.SyncLMI_CreateFileSystem(
FileSystemType=filesystem_service.LMI_CreateFileSystem.FileSystemTypeValues.EXT3,
InExtents=[lv])
Example 22.50. Mounting a File System
To mount the file system created in Example 22.49, “Creating a File System”
, use the following
code snippet:
# Find the file system on the logical volume:
fs = lv.first_associator(ResultClass="LMI_LocalFileSystem")
mount_service = ns.LMI_MountConfigurationService.first_instance()
(rc, out, err) = mount_service.SyncCreateMount(
FileSystemType='ext3',
Mode=32768, # just mount
FileSystem=fs,
MountPoint='/mnt/test',
FileSystemSpec=lv.Name)
Example 22.51. Listing Block Devices
To list all block devices known to the system, use the following code snippet:
devices = ns.CIM_StorageExtent.instances()
for device in devices:
if lmi_isinstance(device, ns.CIM_Memory):
# Memory and CPU caches are StorageExtents too, do not print them
continue
print device.classname,
print device.DeviceID,
print device.Name,
print device.BlockSize*device.NumberOfBlocks
Using the OpenLMI Hardware Provider
The openlmi-hardware package installs a CIM provider for monitoring hardware. The examples below
illustrate how to use this CIM provider to retrieve information about CPU, memory modules, PCI
devices, and the manufacturer and model of the machine.
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Example 22.52. Viewing CPU Information
To display basic CPU information such as the CPU name, the number of processor cores, and the
number of hardware threads, use the following code snippet:
cpu = ns.LMI_Processor.first_instance()
cpu_cap = cpu.associators(ResultClass="LMI_ProcessorCapabilities")[0]
print cpu.Name
print cpu_cap.NumberOfProcessorCores
print cpu_cap.NumberOfHardwareThreads
Example 22.53. Viewing Memory Information
To display basic information about memory modules such as their individual sizes, use the
mem = ns.LMI_Memory.first_instance()
for i in mem.associators(ResultClass="LMI_PhysicalMemory"):
print i.Name
Example 22.54. Viewing Chassis Information
To display basic information about the machine such as its manufacturer or its model, use the
chassis = ns.LMI_Chassis.first_instance()
print chassis.Manufacturer
print chassis.Model
Example 22.55. Listing PCI Devices
To list all PCI devices known to the system, use the following code snippet:
for pci in ns.LMI_PCIDevice.instances():
print pci.Name
22.5. USING OPENLMI SCRIPTS
The LMIShell interpreter is built on top of Python modules that can be used to develop custom
management tools. The OpenLMI Scripts project provides a number of Python libraries for
interfacing with OpenLMI providers. In addition, it is distributed with lmi, an extensible utility that
can be used to interact with these libraries from the command line.
To install OpenLMI Scripts on your system, type the following at a shell prompt:
easy_install --user openlmi-scripts
This command installs the Python modules and the lmi utility in the~/.local/ directory. To extend the
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This command installs the Python modules and the lmi utility in the~/.local/ directory. To extend the
functionality of the lmi utility, install additional OpenLMI modules by using the following command:
easy_install --user package_name
For a complete list of available modules, see the Python website. For more information about
OpenLMI Scripts, see the official OpenLMI Scripts documentation.
For more information about OpenLMI and system management in general, see the resources listed
below.
lmishell(1) — The manual page for thelmishell client and interpreter provides detailed
information about its execution and usage.
Red Hat Enterprise Linux 7 Networking Guide— TheNetworking Guide for Red Hat
Enterprise Linux 7 documents relevant information regarding the configuration and
administration of network interfaces and network services on the system.
Red Hat Enterprise Linux 7 Storage Administration Guide— TheStorage Administration
Guide for Red Hat Enterprise Linux 7 provides instructions on how to manage storage devices
and file systems on the system.
Red Hat Enterprise Linux 7 Power Management Guide— ThePower Management Guidefor
Red Hat Enterprise Linux 7 explains how to manage power consumption of the system
effectively. It discusses different techniques that lower power consumption for both servers
and laptops, and explains how each technique affects the overall performance of the system.
Red Hat Enterprise Linux 7 Linux Domain Identity, Authentication, and Policy Guide
— The
Linux Domain Identity, Authentication, and Policy Guidefor Red Hat Enterprise Linux 7
covers all aspects of installing, configuring, and managing IPA domains, including both
servers and clients. The guide is intended for IT and systems administrators.
FreeIPA Documentation — TheFreeIPA Documentation serves as the primary user
documentation for using the FreeIPA Identity Management project.
OpenSSL Home Page— TheOpenSSL home page provides an overview of the OpenSSL
project.
Mozilla NSS Documentation — TheMozilla NSS Documentation serves as the primary user
documentation for using the Mozilla NSS project.
See Also
Chapter 4, Managing Users and Groups documents how to manage system users and groups
in the graphical user interface and on the command line.
Chapter 9, Yum describes how to use theYum package manager to search, install, update, and
uninstall packages on the command line.
Chapter 10, Managing Services with systemdprovides an introduction tosystemd and
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Chapter 10, Managing Services with systemdprovides an introduction tosystemd and
documents how to use the systemctl command to manage system services, configure
systemd targets, and execute power management commands.
Chapter 12,OpenSSH describes how to configure an SSH server and how to use thessh, scp,
and sftp client utilities to access it.
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Log files are files that contain messages about the system, including the kernel, services, and
applications running on it. There are different log files for different information. For example, there is
a default system log file, a log file just for security messages, and a log file for cron tasks.
Log files can be very useful when trying to troubleshoot a problem with the system such as trying to
load a kernel driver or when looking for unauthorized login attempts to the system. This chapter
discusses where to find log files, how to view log files, and what to look for in log files.
Some log files are controlled by a daemon called rsyslogd. Thersyslogd daemon is an enhanced
replacement for sysklogd, and provides extended filtering, encryption protected relaying of
messages, various configuration options, input and output modules, support for transportation via
the TCP or UDP protocols. Note thatrsyslog is compatible withsysklogd.
Log files can also be managed by the journald daemon – a component ofsystemd. Thejournald
daemon captures Syslog messages, kernel log messages, initial RAM disk and early boot messages as
well as messages written to standard output and standard error output of all services, indexes them
and makes this available to the user. The native journal file format, which is a structured and indexed
binary file, improves searching and provides faster operation, and it also stores meta data
information like time stamps or user IDs. Log files produced by journald are by default not persistent,
log files are stored only in memory or a small ring-buffer in the /run/log/journal/ directory. The
amount of logged data depends on free memory, when you reach the capacity limit, the oldest entries
are deleted. However, this setting can be altered – see Section 23.10.5, “Enabling Persistent Storage”.
For more information on Journal see Section 23.10, “Using the Journal”.
By default, these two logging tools coexist on your system. The journald daemon is the primary tool
for troubleshooting. It also provides additional data necessary for creating structured log messages.
Data acquired by journald is forwarded into the/run/systemd/journal/syslog socket that may be
used by rsyslogd to process the data further. However,rsyslog does the actual integration by default
via the imjournal input module, thus avoiding the aforementioned socket. You can also transfer data
in the opposite direction, from rsyslogd to journald with use ofomjournal module. SeeSection 23.7,
“Interaction of Rsyslog and Journal” for further information. The integration enables maintaining
text-based logs in a consistent format to ensure compatibility with possible applications or
configurations dependent on rsyslogd. Also, you can maintain rsyslog messages in a structured
format (see Section 23.8, “Structured Logging with Rsyslog”).
23.1. LOCATING LOG FILES
A list of log files maintained by rsyslogd can be found in the/etc/rsyslog.conf configuration file. Most
log files are located in the /var/log/ directory. Some applications such ashttpd and samba have a
directory within /var/log/ for their log files.
You may notice multiple files in the /var/log/ directory with numbers after them (for example,
cron-
20100906). These numbers represent a time stamp that has been added to a rotated log file. Log files
are rotated so their file sizes do not become too large. The logrotate package contains a cron task
that automatically rotates log files according to the /etc/logrotate.conf configuration file and the
configuration files in the /etc/logrotate.d/ directory.
23.2. BASIC CONFIGURATION OF RSYSLOG
The main configuration file for rsyslog is /etc/rsyslog.conf. Here, you can specifyglobal directives,
modules, and rules that consist offilter and action parts. Also, you can add comments in the form of
text following a hash sign (#).
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23.2.1. Filters
A rule is specified by a filter part, which selects a subset of syslog messages, and an
action part, which
specifies what to do with the selected messages. To define a rule in your /etc/rsyslog.conf
configuration file, define both, a filter and an action, on one line and separate them with one or more
spaces or tabs.
rsyslog offers various ways to filter syslog messages according to selected properties. The available
filtering methods can be divided into Facility/Priority-based, Property-based, and Expression-based
filters.
Facility/Priority-based filters
The most used and well-known way to filter syslog messages is to use the facility/priority-based
filters which filter syslog messages based on two conditions: facility and priority separated by a
dot. To create a selector, use the following syntax:
FACILITY.PRIORITY
where:
FACILITY specifies the subsystem that produces a specific syslog message. For example,
the mail subsystem handles all mail-related syslog messages.FACILITY can be
represented by one of the following keywords (or by a numerical code): kern (0), user (1),
mail (2), daemon (3),auth (4), syslog (5),lpr (6),news (7), cron (8),authpriv (9),ftp (10),
and local0 throughlocal7 (16 - 23).
PRIORITY specifies a priority of a syslog message.PRIORITY can be represented by one of
the following keywords (or by a number): debug (7), info (6),notice (5),warning (4), err
(3), crit (2), alert (1), andemerg (0).
The aforementioned syntax selects syslog messages with the defined or higher priority. By
preceding any priority keyword with an equal sign (=), you specify that only syslog
messages with the specified priority will be selected. All other priorities will be ignored.
Conversely, preceding a priority keyword with an exclamation mark (!) selects all syslog
messages except those with the defined priority.
In addition to the keywords specified above, you may also use an asterisk (*) to define all
facilities or priorities (depending on where you place the asterisk, before or after the
comma). Specifying the priority keyword none serves for facilities with no given priorities.
Both facility and priority conditions are case-insensitive.
To define multiple facilities and priorities, separate them with a comma (,). To define
multiple selectors on one line, separate them with a semi-colon (;). Note that each
selector in the selector field is capable of overwriting the preceding ones, which can
exclude some priorities from the pattern.
Example 23.1. Facility/Priority-based Filters
The following are a few examples of simple facility/priority-based filters that can be
specified in /etc/rsyslog.conf. To select all kernel syslog messages with any priority,
add the following text into the configuration file:
kern.*
To select all mail syslog messages with priority crit and higher, use this form:
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mail.crit
To select all cron syslog messages except those with the info or debug priority, set the
configuration in the following form:
cron.!info,!debug
Property-based filters
Property-based filters let you filter syslog messages by any property, such as timegenerated or
syslogtag. For more information on properties, seethe section called “Properties”. You can
compare each of the specified properties to a particular value using one of the compare-
operations listed in Table 23.1, “Property-based compare-operations”. Both property names and
compare operations are case-sensitive.
Property-based filter must start with a colon (:). To define the filter, use the following syntax:
:PROPERTY, [!]COMPARE_OPERATION, "STRING"
where:
The PROPERTY attribute specifies the desired property.
The optional exclamation point (!) negates the output of the compare-operation. Other
Boolean operators are currently not supported in property-based filters.
The COMPARE_OPERATION attribute specifies one of the compare-operations listed in
Table 23.1, “Property-based compare-operations”.
The STRING attribute specifies the value that the text provided by the property is
compared to. This value must be enclosed in quotation marks. To escape certain character
inside the string (for example a quotation mark (")), use the backslash character (
).
Table 23.1. Property-based compare-operations
Compare-operation Description
contains Checks whether the provided string matches
any part of the text provided by the property.
isequal Compares the provided string against all of the
text provided by the property. These two
values must be exactly equal to match.
startswith Checks whether the provided string is found
exactly at the beginning of the text provided
by the property.
regex Compares the provided POSIX BRE (Basic
Regular Expression) against the text provided
by the property.
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ereregex Compares the provided POSIX ERE (Extended
Regular Expression) regular expression against
the text provided by the property.
isempty Checks if the property is empty. The value is
discarded. This is especially useful when
working with normalized data, where some
fields may be populated based on
normalization result.
Compare-operation Description
Example 23.2. Property-based Filters
The following are a few examples of property-based filters that can be specified in
/etc/rsyslog.conf. To select syslog messages which contain the stringerror in their
message text, use:
:msg, contains, "error"
The following filter selects syslog messages received from the host name host1:
:hostname, isequal, "host1"
To select syslog messages which do not contain any mention of the words fatal and
error with any or no text between them (for example,fatal lib error), type:
:msg, !regex, "fatal .* error"
Expression-based filters
Expression-based filters select syslog messages according to defined arithmetic, Boolean or string
operations. Expression-based filters use rsyslog's own scripting language calledRainerScript to
build complex filters.
The basic syntax of expression-based filter looks as follows:
if EXPRESSION then ACTION else ACTION
where:
The EXPRESSION attribute represents an expression to be evaluated, for example:$msg
startswith 'DEVNAME' or $syslogfacility-text == 'local0'. You can specify more than one
expression in a single filter by using and and or operators.
Note that rsyslog supports case-insensitive comparisons in expression-based filters. You
can use contains_i or startswith_i compare-operations within theEXPRESSION attribute,
for example:
if $hostname startswith_i "<HOST_NAME>" then ACTION.
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The ACTION attribute represents an action to be performed if the expression returns the
value true. This can be a single action, or an arbitrary complex script enclosed in curly
braces.
Expression-based filters are indicated by the keyword if at the start of a new line. The
then
keyword separates the EXPRESSION from the ACTION. Optionally, you can employ the
else keyword to specify what action is to be performed in case the condition is not met.
With expression-based filters, you can nest the conditions by using a script enclosed in
curly braces as in Example 23.3, “Expression-based Filters”. The script allows you to use
facility/priority-based filters inside the expression. On the other hand,property-based
filters are not recommended here. RainerScript supports regular expressions with
specialized functions re_match() and re_extract().
Example 23.3. Expression-based Filters
The following expression contains two nested conditions. The log files created by a
program called prog1 are split into two files based on the presence of the "test" string in
the message.
if $programname == 'prog1' then {
action(type="omfile" file="/var/log/prog1.log")
if $msg contains 'test' then
action(type="omfile" file="/var/log/prog1test.log")
else
action(type="omfile" file="/var/log/prog1notest.log")
}
See the section called “Online Documentation”for more examples of various expression-based
filters. RainerScript is the basis for rsyslog's new configuration format, seeSection 23.3, “Using the
New Configuration Format”
23.2.2. Actions
Actions specify what is to be done with the messages filtered out by an already defined selector. The
following are some of the actions you can define in your rule:
Saving syslog messages to log files
The majority of actions specify to which log file a syslog message is saved. This is done by
specifying a file path after your already-defined selector:
FILTER PATH
where FILTER stands for user-specified selector andPATH is a path of a target file.
For instance, the following rule is comprised of a selector that selects all cron syslog messages and
an action that saves them into the /var/log/cron.log log file:
cron.* /var/log/cron.log
By default, the log file is synchronized every time a syslog message is generated. Use a dash mark
(-) as a prefix of the file path you specified to omit syncing:
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FILTER -PATH
Note that you might lose information if the system terminates right after a write attempt.
However, this setting can improve performance, especially if you run programs that produce very
verbose log messages.
Your specified file path can be either static or dynamic. Static files are represented by a fixed file
path as shown in the example above. Dynamic file paths can differ according to the received
message. Dynamic file paths are represented by a template and a question mark (?) prefix:
FILTER ?DynamicFile
where DynamicFile is a name of a predefined template that modifies output paths. You can use the
dash prefix (-) to disable syncing, also you can use multiple templates separated by a colon (
;). For
more information on templates, see the section called “Generating Dynamic File Names”
.
If the file you specified is an existing terminal or /dev/console device, syslog messages are sent to
standard output (using special terminal-handling) or your console (using special/dev/console-
handling) when using the X Window System, respectively.
Sending syslog messages over the network
rsyslog allows you to send and receive syslog messages over the network. This feature allows you
to administer syslog messages of multiple hosts on one machine. To forward syslog messages to a
remote machine, use the following syntax:
@(zNUMBER)HOST:PORT
where:
The at sign (@) indicates that the syslog messages are forwarded to a host using the
UDP
protocol. To use the TCP protocol, use two at signs with no space between them (
@@).
The optional zNUMBER setting enableszlib compression for syslog messages. The
NUMBER attribute specifies the level of compression (from 1 – lowest to 9 – maximum).
Compression gain is automatically checked by rsyslogd, messages are compressed only if
there is any compression gain and messages below 60 bytes are never compressed.
The HOST attribute specifies the host which receives the selected syslog messages.
The PORT attribute specifies the host machine’s port.
When specifying an IPv6 address as the host, enclose the address in square brackets (
[, ]).
Example 23.4. Sending syslog Messages over the Network
The following are some examples of actions that forward syslog messages over the
network (note that all actions are preceded with a selector that selects all messages
with any priority). To forward messages to 192.168.0.1 via the UDP protocol, type:
. @192.168.0.1
To forward messages to "example.com" using port 6514 and the TCP protocol, use:
. @@example.com:6514
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The following compresses messages with zlib (level 9 compression) and forwards them
to 2001:db8::1 using theUDP protocol
. @(z9)[2001:db8::1]
Output channels
Output channels are primarily used to specify the maximum size a log file can grow to. This is very
useful for log file rotation (for more information see Section 23.2.5, “Log Rotation”). An output
channel is basically a collection of information about the output action. Output channels are
defined by the $outchannel directive. To define an output channel in
/etc/rsyslog.conf, use the
following syntax:
$outchannel NAME, FILE_NAME, MAX_SIZE, ACTION
where:
The NAME attribute specifies the name of the output channel.
The FILE_NAME attribute specifies the name of the output file. Output channels can write
only into files, not pipes, terminal, or other kind of output.
The MAX_SIZE attribute represents the maximum size the specified file (in
FILE_NAME)
can grow to. This value is specified in bytes.
The ACTION attribute specifies the action that is taken when the maximum size, defined in
MAX_SIZE, is hit.
To use the defined output channel as an action inside a rule, type:
FILTER :omfile:$NAME
Example 23.5. Output channel log rotation
The following output shows a simple log rotation through the use of an output channel.
First, the output channel is defined via the $outchannel directive:
$outchannel log_rotation, /var/log/test_log.log, 104857600,
/home/joe/log_rotation_script
and then it is used in a rule that selects every syslog message with any priority and
executes the previously-defined output channel on the acquired syslog messages:
. :omfile:$log_rotation
Once the limit (in the example 100 MB) is hit, the /home/joe/log_rotation_script is
executed. This script can contain anything from moving the file into a different folder,
editing specific content out of it, or simply removing it.
Sending syslog messages to specific users
rsyslog can send syslog messages to specific users by specifying a user name of the user you want
to send the messages to (as in Example 23.7, “Specifying Multiple Actions”
). To specify more than
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one user, separate each user name with a comma (,). To send messages to every user that is
currently logged on, use an asterisk (*).
Executing a program
rsyslog lets you execute a program for selected syslog messages and uses thesystem() call to
execute the program in shell. To specify a program to be executed, prefix it with a caret character
(^). Consequently, specify a template that formats the received message and passes it to the
specified executable as a one line parameter (for more information on templates, see
Section 23.2.3, “Templates”).
FILTER ^EXECUTABLE; TEMPLATE
Here an output of the FILTER condition is processed by a program represented by
EXECUTABLE.
This program can be any valid executable. Replace TEMPLATE with the name of the formatting
template.
Example 23.6. Executing a Program
In the following example, any syslog message with any priority is selected, formatted with the
template template and passed as a parameter to thetest-program program, which is then
executed with the provided parameter:
. ^test-program;template
WARNING
When accepting messages from any host, and using the shell execute action,
you may be vulnerable to command injection. An attacker may try to inject and
execute commands in the program you specified to be executed in your
action. To avoid any possible security threats, thoroughly consider the use of
the shell execute action.
Storing syslog messages in a database
Selected syslog messages can be directly written into a database table using the database writer
action. The database writer uses the following syntax:
:PLUGIN:DB_HOST,DB_NAME,DB_USER,DB_PASSWORD;TEMPLATE
where:
The PLUGIN calls the specified plug-in that handles the database writing (for example,
the ommysql plug-in).
The DB_HOST attribute specifies the database host name.
The DB_NAME attribute specifies the name of the database.
The DB_USER attribute specifies the database user.

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The DB_PASSWORD attribute specifies the password used with the aforementioned
database user.
The TEMPLATE attribute specifies an optional use of a template that modifies the syslog
message. For more information on templates, see Section 23.2.3, “Templates”.
IMPORTANT
Currently, rsyslog provides support forMySQL and PostgreSQL databases
only. In order to use the MySQL and PostgreSQL database writer
functionality, install the rsyslog-mysql and rsyslog-pgsql packages,
respectively. Also, make sure you load the appropriate modules in your
/etc/rsyslog.conf configuration file:
module(load=”ommysql”) # Output module for MySQL support
module(load=”ompgsql”) # Output module for PostgreSQL support
For more information on rsyslog modules, seeSection 23.6, “Using Rsyslog
Modules”.
Alternatively, you may use a generic database interface provided by the
omlibdb module (supports: Firebird/Interbase, MS SQL, Sybase, SQLLite,
Ingres, Oracle, mSQL).
Discarding syslog messages
To discard your selected messages, use stop.
The discard action is mostly used to filter out messages before carrying on any further processing.
It can be effective if you want to omit some repeating messages that would otherwise fill the log
files. The results of discard action depend on where in the configuration file it is specified, for the
best results place these actions on top of the actions list. Please note that once a message has
been discarded there is no way to retrieve it in later configuration file lines.
For instance, the following rule discards all messages that matches the local5.* filter:
local5.* stop
In the following example, any cron syslog messages are discarded:
cron.* stop
NOTE
With versions prior to rsyslog 7, the tilde character (~) was used instead ofstop to
discard syslog messages.
Specifying Multiple Actions
For each selector, you are allowed to specify multiple actions. To specify multiple actions for one
selector, write each action on a separate line and precede it with an ampersand (&) character:
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FILTER ACTION
& ACTION
& ACTION
Specifying multiple actions improves the overall performance of the desired outcome since the
specified selector has to be evaluated only once.
Example 23.7. Specifying Multiple Actions
In the following example, all kernel syslog messages with the critical priority (crit) are sent to user
user1, processed by the templatetemp and passed on to thetest-program executable, and
forwarded to 192.168.0.1 via the UDP protocol.
kern.=crit user1
& ^test-program;temp
& @192.168.0.1
Any action can be followed by a template that formats the message. To specify a template, suffix an
action with a semicolon (;) and specify the name of the template. For more information on templates,
see Section 23.2.3, “Templates”.
WARNING
A template must be defined before it is used in an action, otherwise it is ignored.
In other words, template definitions should always precede rule definitions in
/etc/rsyslog.conf.
23.2.3. Templates
Any output that is generated by rsyslog can be modified and formatted according to your needs with
the use of templates. To create a template use the following syntax in/etc/rsyslog.conf:
template(name=”TEMPLATE_NAME” type=”string” string="text %PROPERTY% more text"
[option.OPTION="on"])
where:
template() is the directive introducing block defining a template.
The TEMPLATE_NAME mandatory argument is used to refer to the template. Note that
TEMPLATE_NAME should be unique.
The type mandatory argument can acquire one of these values: “list”, “subtree”, “string” or
“plugin”.
The string argument is the actual template text. Within this text, special characters, such as
n for newline or r for carriage return, can be used. Other characters, such as % or ", have to
be escaped if you want to use those characters literally. Within this text, special characters,

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such as n for new line orr for carriage return, can be used. Other characters, such as
% or ",
have to be escaped if you want to use those characters literally.
The text specified between two percent signs (%) specifies aproperty that allows you to
access specific contents of a syslog message. For more information on properties, see the
section called “Properties”.
The OPTION attribute specifies any options that modify the template functionality. The
currently supported template options are sql and stdsql, which are used for formatting the
text as an SQL query, or json which formats text to be suitable for JSON processing, and
casesensitive which sets case sensitiveness of property names.
NOTE
Note that the database writer checks whether the sql or stdsql options are
specified in the template. If they are not, the database writer does not perform
any action. This is to prevent any possible security threats, such as SQL
injection.
See section Storing syslog messages in a databaseinSection 23.2.2, “Actions”
for more information.
Generating Dynamic File Names
Templates can be used to generate dynamic file names. By specifying a property as a part of the file
path, a new file will be created for each unique property, which is a convenient way to classify syslog
messages.
For example, use the timegenerated property, which extracts a time stamp from the message, to
generate a unique file name for each syslog message:
template(name=”DynamicFile” type=”list”) {
constant(value=”/var/log/test_logs/”)
property(name=”timegenerated”)
constant(value”-test.log”)
}
Keep in mind that the $template directive only specifies the template. You must use it inside a rule for
it to take effect. In /etc/rsyslog.conf, use the question mark (
?) in an action definition to mark the
dynamic file name template:
. ?DynamicFile
Properties
Properties defined inside a template (between two percent signs (%)) enable access various contents
of a syslog message through the use of a property replacer. To define a property inside a template
(between the two quotation marks (“…”)), use the following syntax:
%PROPERTY_NAME:FROM_CHAR:TO_CHAR:OPTION%
where:
The PROPERTY_NAME attribute specifies the name of a property. A list of all available
properties and their detailed description can be found in the rsyslog.conf(5) manual page
under the section Available Properties.
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FROM_CHAR and TO_CHAR attributes denote a range of characters that the specified
property will act upon. Alternatively, regular expressions can be used to specify a range of
characters. To do so, set the letter R as theFROM_CHAR attribute and specify your desired
regular expression as the TO_CHAR attribute.
The OPTION attribute specifies any property options, such as thelowercase option to
convert the input to lowercase. A list of all available property options and their detailed
description can be found in the rsyslog.conf(5) manual page under the sectionProperty
Options.
The following are some examples of simple properties:
The following property obtains the whole message text of a syslog message:
%msg%
The following property obtains the first two characters of the message text of a syslog
message:
%msg:1:2%
The following property obtains the whole message text of a syslog message and drops its last
line feed character:
%msg:::drop-last-lf%
The following property obtains the first 10 characters of the time stamp that is generated
when the syslog message is received and formats it according to the RFC 3999 date
standard.
%timegenerated:1:10:date-rfc3339%
Template Examples
This section presents a few examples of rsyslog templates.
Example 23.8, “A verbose syslog message template”shows a template that formats a syslog message
so that it outputs the message’s severity, facility, the time stamp of when the message was received,
the host name, the message tag, the message text, and ends with a new line.
Example 23.8. A verbose syslog message template
template(name=”verbose” type=”list”) {
property(name="syslogseverity”)
property(name="syslogfacility”)
property(name="timegenerated”)
property(name="HOSTNAME”)
property(name="syslogtag”)
property(name="msg”)
constant(value=”n")
}
Example 23.9, “A wall message template”shows a template that resembles a traditional wall message
(a message that is send to every user that is logged in and has their mesg(1) permission set toyes).
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This template outputs the message text, along with a host name, message tag and a time stamp, on a
new line (using r and n) and rings the bell (using7).
Example 23.9. A wall message template
template(name=”wallmsg” type=”list”) {
constant(value="rn7Message from syslogd@”)
property(name="HOSTNAME”)
constant(value=” at ")
property(name="timegenerated”)
constant(value=" ...rn ”)
property(name="syslogtag”)
constant(value=” “)
property(name="msg”)
constant(value=”rn”)
}
Example 23.10, “A database formatted message template”shows a template that formats a syslog
message so that it can be used as a database query. Notice the use of the sql option at the end of the
template specified as the template option. It tells the database writer to format the message as an
MySQL SQL query.
Example 23.10. A database formatted message template
template(name="dbFormat" type="list" option.sql="on") {
constant(value="insert into SystemEvents (Message, Facility, FromHost, Priority,
DeviceReportedTime, ReceivedAt, InfoUnitID, SysLogTag)")
constant(value=" values ('")
property(name="msg")
constant(value="', ")
property(name="syslogfacility")
constant(value=", '")
property(name="hostname")
property(name="syslogpriority")
property(name="timereported" dateFormat="mysql")
constant(value="', '")
property(name="timegenerated" dateFormat="mysql")
property(name="iut")
property(name="syslogtag")
constant(value="')")
}
rsyslog also contains a set of predefined templates identified by the
RSYSLOG_ prefix. These are
reserved for the syslog’s use and it is advisable to not create a template using this prefix to avoid
conflicts. The following list shows these predefined templates along with their definitions.
RSYSLOG_DebugFormat
A special format used for troubleshooting property problems.
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template(name=”RSYSLOG_DebugFormat” type=”string” string="Debug line with all
properties:nFROMHOST: '%FROMHOST%', fromhost-ip: '%fromhost-ip%', HOSTNAME:
'%HOSTNAME%', PRI: %PRI%,nsyslogtag '%syslogtag%', programname:
'%programname%', APP-NAME: '%APP-NAME%', PROCID: '%PROCID%', MSGID:
'%MSGID%',nTIMESTAMP: '%TIMESTAMP%', STRUCTURED-DATA: '%STRUCTURED-
DATA%',nmsg: '%msg%'nescaped msg: '%msg:::drop-cc%'nrawmsg: '%rawmsg%'nn")
RSYSLOG_SyslogProtocol23Format
The format specified in IETF’s internet-draft ietf-syslog-protocol-23, which is assumed to become
the new syslog standard RFC.
template(name=”RSYSLOG_SyslogProtocol23Format” type=”string” string="%PRI%1
%TIMESTAMP:::date-rfc3339% %HOSTNAME% %APP-NAME% %PROCID% %MSGID%
%STRUCTURED-DATA% %msg%n ")
RSYSLOG_FileFormat
A modern-style logfile format similar to TraditionalFileFormat, but with high-precision time
stamps and time zone information.
template(name="RSYSLOG_FileFormat" type="list") {
property(name="timestamp" dateFormat="rfc3339")
constant(value=" ")
constant(value=" ")
property(name="msg" spifno1stsp="on" )
property(name="msg" droplastlf="on" )
constant(value="n")
}
RSYSLOG_TraditionalFileFormat
The older default log file format with low-precision time stamps.
template(name="RSYSLOG_TraditionalFileFormat" type="list") {
property(name="timestamp")
constant(value=" ")
constant(value=" ")
property(name="msg" droplastlf="on" )
constant(value="n")
}
RSYSLOG_ForwardFormat
A forwarding format with high-precision time stamps and time zone information.
template(name="ForwardFormat" type="list") {
constant(value="<")
property(name="pri")
constant(value=">")
property(name="timestamp" dateFormat="rfc3339")
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constant(value=" ")
constant(value=" ")
property(name="syslogtag" position.from="1" position.to="32")
}
RSYSLOG_TraditionalForwardFormat
The traditional forwarding format with low-precision time stamps.
template(name="TraditionalForwardFormat" type="list") {
constant(value="<")
property(name="pri")
constant(value=">")
property(name="timestamp")
constant(value=" ")
constant(value=" ")
property(name="syslogtag" position.from="1" position.to="32")
}
23.2.4. Global Directives
Global directives are configuration options that apply to the rsyslogd daemon. They usually specify a
value for a specific predefined variable that affects the behavior of the rsyslogd daemon or a rule
that follows. All of the global directives are enclosed in a global configuration block. The following is
an example of a global directive that specifies overriding local host name for log messages:
global(localHostname=”machineXY”)
The default size defined for this directive (10,000 messages) can be overridden by specifying a
different value (as shown in the example above).
You can define multiple directives in your /etc/rsyslog.conf configuration file. A directive affects the
behavior of all configuration options until another occurrence of that same directive is detected.
Global directives can be used to configure actions, queues and for debugging. A comprehensive list
of all available configuration directives can be found in the section called “Online Documentation”.
Currently, a new configuration format has been developed that replaces the $-based syntax (see
Section 23.3, “Using the New Configuration Format”
). However, classic global directives remain
supported as a legacy format.
23.2.5. Log Rotation
The following is a sample /etc/logrotate.conf configuration file:
# rotate log files weekly
weekly
# keep 4 weeks worth of backlogs
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rotate 4
# uncomment this if you want your log files compressed
compress
All of the lines in the sample configuration file define global options that apply to every log file. In our
example, log files are rotated weekly, rotated log files are kept for four weeks, and all rotated log
files are compressed by gzip into the.gz format. Any lines that begin with a hash sign (#) are
comments and are not processed.
You may define configuration options for a specific log file and place it under the global options.
However, it is advisable to create a separate configuration file for any specific log file in the
/etc/logrotate.d/ directory and define any configuration options there.
The following is an example of a configuration file placed in the /etc/logrotate.d/ directory:
/var/log/messages {
rotate 5
weekly
postrotate
/usr/bin/killall -HUP syslogd
endscript
}
The configuration options in this file are specific for the /var/log/messages log file only. The settings
specified here override the global settings where possible. Thus the rotated /var/log/messages log
file will be kept for five weeks instead of four weeks as was defined in the global options.
The following is a list of some of the directives you can specify in your logrotate configuration file:
weekly — Specifies the rotation of log files to be done weekly. Similar directives include:
daily
monthly
yearly
compress — Enables compression of rotated log files. Similar directives include:
nocompress
compresscmd — Specifies the command to be used for compressing.
uncompresscmd
compressext — Specifies what extension is to be used for compressing.
compressoptions — Specifies any options to be passed to the compression program
used.
delaycompress — Postpones the compression of log files to the next rotation of log files.
rotate INTEGER — Specifies the number of rotations a log file undergoes before it is removed
or mailed to a specific address. If the value 0 is specified, old log files are removed instead of
rotated.
mail ADDRESS — This option enables mailing of log files that have been rotated as many
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mail ADDRESS — This option enables mailing of log files that have been rotated as many
times as is defined by the rotate directive to the specified address. Similar directives include:
nomail
mailfirst — Specifies that the just-rotated log files are to be mailed, instead of the about-
to-expire log files.
maillast — Specifies that the about-to-expire log files are to be mailed, instead of the
just-rotated log files. This is the default option when mail is enabled.
For the full list of directives and various configuration options, see the logrotate(5) manual page.
23.2.6. Increasing the Limit of Open Files
Under certain circumstances the rsyslog exceeds the limit for a maximum number of open files.
Consequently, the rsyslog cannot open new files.
To increase the limit of open files in the rsyslog:
Create the /etc/systemd/system/rsylog.service.d/increase_nofile_limit.conf file with the following
content:
[Service]
LimitNOFILE=16384
23.3. USING THE NEW CONFIGURATION FORMAT
In rsyslog version 7, installed by default for Red Hat Enterprise Linux 7 in the
rsyslog package, a new
configuration syntax is introduced. This new configuration format aims to be more powerful, more
intuitive, and to prevent common mistakes by not permitting certain invalid constructs. The syntax
enhancement is enabled by the new configuration processor that relies on RainerScript. The legacy
format is still fully supported and it is used by default in the /etc/rsyslog.conf configuration file.
RainerScript is a scripting language designed for processing network events and configuring event
processors such as rsyslog. RainerScript was first used to define expression-based filters, see
Example 23.3, “Expression-based Filters”. The version of RainerScript in rsyslog version 7
implements the input() and ruleset() statements, which permit the/etc/rsyslog.conf configuration file
to be written in the new syntax. The new syntax differs mainly in that it is much more structured;
parameters are passed as arguments to statements, such as input, action, template, and module load.
The scope of options is limited by blocks. This enhances readability and reduces the number of bugs
caused by misconfiguration. There is also a significant performance gain. Some functionality is
exposed in both syntaxes, some only in the new one.
Compare the configuration written with legacy-style parameters:
$InputFileName /tmp/inputfile
$InputFileTag tag1:
$InputFileStateFile inputfile-state
$InputRunFileMonitor
and the same configuration with the use of the new format statement:
input(type="imfile" file="/tmp/inputfile" tag="tag1:" statefile="inputfile-state")
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This significantly reduces the number of parameters used in configuration, improves readability, and
also provides higher execution speed. For more information on RainerScript statements and
parameters see the section called “Online Documentation”.
23.3.1. Rulesets
Leaving special directives aside, rsyslog handles messages as defined byrules that consist of a filter
condition and an action to be performed if the condition is true. With a traditionally written
/etc/rsyslog.conf file, all rules are evaluated in order of appearance for every input message. This
process starts with the first rule and continues until all rules have been processed or until the
message is discarded by one of the rules.
However, rules can be grouped into sequences called rulesets. With rulesets, you can limit the effect
of certain rules only to selected inputs or enhance the performance of rsyslog by defining a distinct
set of actions bound to a specific input. In other words, filter conditions that will be inevitably
evaluated as false for certain types of messages can be skipped. The legacy ruleset definition in
/etc/rsyslog.conf can look as follows:
$RuleSet rulesetname
rule
rule2
The rule ends when another rule is defined, or the default ruleset is called as follows:
$RuleSet RSYSLOG_DefaultRuleset
With the new configuration format in rsyslog 7, the input() and ruleset() statements are reserved for
this operation. The new format ruleset definition in /etc/rsyslog.conf can look as follows:
ruleset(name="rulesetname") {
rule
rule2
call rulesetname2
…
}
Replace rulesetname with an identifier for your ruleset. The ruleset name cannot start with
RSYSLOG_ since this namespace is reserved for use byrsyslog. RSYSLOG_DefaultRuleset then
defines the default set of rules to be performed if the message has no other ruleset assigned. With
rule and rule2 you can define rules in filter-action format mentioned above. With thecall parameter,
you can nest rulesets by calling them from inside other ruleset blocks.
After creating a ruleset, you need to specify what input it will apply to:
input(type="input_type" port="port_num" ruleset="rulesetname");
Here you can identify an input message by input_type, which is an input module that gathered the
message, or by port_num – the port number. Other parameters such asfile or tag can be specified for
input(). Replacerulesetname with a name of the ruleset to be evaluated against the message. In case
an input message is not explicitly bound to a ruleset, the default ruleset is triggered.
You can also use the legacy format to define rulesets, for more information see the section called
“Online Documentation”.
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Example 23.11. Using rulesets
The following rulesets ensure different handling of remote messages coming from different ports.
Add the following into /etc/rsyslog.conf:
ruleset(name="remote-6514") {
action(type="omfile" file="/var/log/remote-6514")
}
ruleset(name="remote-601") {
cron.* action(type="omfile" file="/var/log/remote-601-cron")
mail.* action(type="omfile" file="/var/log/remote-601-mail")
}
input(type="imtcp" port="6514" ruleset="remote-6514");
input(type="imtcp" port="601" ruleset="remote-601");
Rulesets shown in the above example define log destinations for the remote input from two ports,
in case of port 601, messages are sorted according to the facility. Then, the TCP input is enabled
and bound to rulesets. Note that you must load the required modules (imtcp) for this
configuration to work.
23.3.2. Compatibility with sysklogd
The compatibility mode specified via the -c option exists inrsyslog version 5 but not in version 7. Also,
the sysklogd-style command-line options are deprecated and configuring rsyslog through these
command-line options should be avoided. However, you can use several templates and directives to
configure rsyslogd to emulate sysklogd-like behavior.
For more information on various rsyslogd options, see thersyslogd(8)manual page.
23.4. WORKING WITH QUEUES IN RSYSLOG
Queues are used to pass content, mostly syslog messages, between components of rsyslog. With
queues, rsyslog is capable of processing multiple messages simultaneously and to apply several
actions to a single message at once. The data flow inside rsyslog can be illustrated as follows:
Figure 23.1. Message Flow in Rsyslog
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Figure 23.1. Message Flow in Rsyslog
Whenever rsyslog receives a message, it passes this message to the preprocessor and then places it
into the main message queue. Messages wait there to be dequeued and passed to therule processor.
The rule processoris a parsing and filtering engine. Here, the rules defined in
/etc/rsyslog.conf are
applied. Based on these rules, the rule processor evaluates which actions are to be performed. Each
action has its own action queue. Messages are passed through this queue to the respective action
processor which creates the final output. Note that at this point, several actions can run
simultaneously on one message. For this purpose, a message is duplicated and passed to multiple
action processors.
Only one queue per action is possible. Depending on configuration, the messages can be sent right to
the action processor without action queuing. This is the behavior of direct queues (see below). In case
the output action fails, the action processor notifies the action queue, which then takes an
unprocessed element back and after some time interval, the action is attempted again.
To sum up, there are two positions where queues stand in rsyslog: either in front of the rule processor
as a single main message queueor in front of various types of output actions asaction queues.
Queues provide two main advantages that both lead to increased performance of message
processing:
they serve as buffers that decouple producers and consumers in the structure ofrsyslog
they allow for parallelization of actions performed on messages
Apart from this, queues can be configured with several directives to provide optimal performance for
your system. These configuration options are covered in the following sections.
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WARNING
If an output plug-in is unable to deliver a message, it is stored in the preceding
message queue. If the queue fills, the inputs block until it is no longer full. This will
prevent new messages from being logged via the blocked queue. In the absence
of separate action queues this can have severe consequences, such as preventing
SSH logging, which in turn can preventSSH access. Therefore it is advised to use
dedicated action queues for outputs which are forwarded over a network or to a
database.
23.4.1. Defining Queues
Based on where the messages are stored, there are several types of queues: direct, in-memory, disk,
and disk-assisted in-memory queues that are most widely used. You can choose one of these types
for the main message queue and also for action queues. Add the following into /etc/rsyslog.conf:
object(queue.type=”queue_type”)
By adding this you can apply the setting for:
main message queue: replace object withmain_queue
an action queue: replace object withaction
ruleset: replace object withruleset
Replace queue_type with one ofdirect, linkedlist or fixedarray (which are in-memory queues), or
disk.
The default setting for a main message queue is the FixedArray queue with a limit of 10,000
messages. Action queues are by default set as Direct queues.
Direct Queues
For many simple operations, such as when writing output to a local file, building a queue in front of an
action is not needed. To avoid queuing, use:
object(queue.type=”Direct”)
Replace object withmain_queue, action or ruleset to use this option to the main message queue, an
action queue or for the ruleset respectively. With direct queue, messages are passed directly and
immediately from the producer to the consumer.
Disk Queues
Disk queues store messages strictly on a hard drive, which makes them highly reliable but also the
slowest of all possible queuing modes. This mode can be used to prevent the loss of highly important
log data. However, disk queues are not recommended in most use cases. To set a disk queue, type the
following into /etc/rsyslog.conf:
object(queue.type=”Disk”)

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action queue or for the ruleset respectively. The default size of a queue can be modified with the
following configuration directive:
object(queue.size=”size”)
where size represents the specified size of disk queue part. The defined size limit is not restrictive,
rsyslog always writes one complete queue entry, even if it violates the size limit. Each part of a disk
queue matches with an individual file. The naming directive for these files looks as follows:
object(queue.filename=”name”)
This sets a name prefix for the file followed by a 7-digit number starting at one and incremented for
each file.
object(queue.maxfilesize=”size”)
Disk queues are written in parts, with a default size 1 MB. Specify size to use a different value.
In-memory Queues
With in-memory queue, the enqueued messages are held in memory which makes the process very
fast. The queued data is lost if the computer is power cycled or shut down. However, you can use the
action (queue.saveonshutdown=”on”) setting to save the data before shutdown. There are two
types of in-memory queues:
FixedArray queue — the default mode for the main message queue, with a limit of 10,000
elements. This type of queue uses a fixed, pre-allocated array that holds pointers to queue
elements. Due to these pointers, even if the queue is empty a certain amount of memory is
consumed. However, FixedArray offers the best run time performance and is optimal when
you expect a relatively low number of queued messages and high performance.
LinkedList queue — here, all structures are dynamically allocated in a linked list, thus the
memory is allocated only when needed. LinkedList queues handle occasional message bursts
very well.
In general, use LinkedList queues when in doubt. Compared to FixedArray, it consumes less memory
and lowers the processing overhead.
Use the following syntax to configure in-memory queues:
object(queue.type=”LinkedList”)
object(queue.type=”FixedArray”)
action queue or for the ruleset respectively.
Disk-Assisted In-memory Queues
Both disk and in-memory queues have their advantages and rsyslog lets you combine them indisk-
assisted in-memory queues. To do so, configure a normal in-memory queue and then add the
queue.filename=”file_name” directive to its block to define a file name for disk assistance. This
queue then becomes disk-assisted, which means it couples an in-memory queue with a disk queue to
work in tandem.
The disk queue is activated if the in-memory queue is full or needs to persist after shutdown. With a
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disk-assisted queue, you can set both disk-specific and in-memory specific configuration parameters.
This type of queue is probably the most commonly used, it is especially useful for potentially long-
running and unreliable actions.
To specify the functioning of a disk-assisted in-memory queue, use the so-called watermarks:
object(queue.highwatermark=”number”)
object(queue.lowwatermark=”number”)
action queue or for the ruleset respectively. Replace number with a number of enqueued messages.
When an in-memory queue reaches the number defined by the high watermark, it starts writing
messages to disk and continues until the in-memory queue size drops to the number defined with the
low watermark. Correctly set watermarks minimize unnecessary disk writes, but also leave memory
space for message bursts since writing to disk files is rather lengthy. Therefore, the high watermark
must be lower than the whole queue capacity set with queue.size. The difference between the high
watermark and the overall queue size is a spare memory buffer reserved for message bursts. On the
other hand, setting the high watermark too low will turn on disk assistance unnecessarily often.
Example 23.12. Reliable Forwarding of Log Messages to a Server
Rsyslog is often used to maintain a centralized logging system, where log messages are forwarded
to a server over the network. To avoid message loss when the server is not available, it is advisable
to configure an action queue for the forwarding action. This way, messages that failed to be sent
are stored locally until the server is reachable again. Note that such queues are not configurable
for connections using the UDP protocol.
Forwarding To a Single Server
Suppose the task is to forward log messages from the system to a server with host name
example.com, and to configure an action queue to buffer the messages in case of a server outage.
To do so, perform the following steps:
1. Use the following configuration in /etc/rsyslog.conf or create a file with the following
content in the /etc/rsyslog.d/ directory:
. action(type=”omfwd”
queue.type=”LinkedList”
queue.filename=”example_fwd”
action.resumeRetryCount="-1"
queue.saveonshutdown="on"
Target="example.com" Port="6514" Protocol="tcp")
Where:
queue.type enables a LinkedList in-memory queue,
queue.filename defines a disk storage, in this case the backup files are created in the
/var/lib/rsyslog/ directory with theexample_fwd prefix,
the action.resumeRetryCount= “-1” setting prevents rsyslog from dropping
messages when retrying to connect if server is not responding,
enabled queue.saveonshutdown saves in-memory data if rsyslog shuts down,
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the last line forwards all received messages to the logging server using reliable TCP
delivery, port specification is optional.
With the above configuration, rsyslog keeps messages in memory if the remote server
is not reachable. A file on disk is created only if rsyslog runs out of the configured
memory queue space or needs to shut down, which benefits the system performance.
Forwarding To Multiple Servers
The process of forwarding log messages to multiple servers is similar to the previous procedure:
1. Each destination server requires a separate forwarding rule, action queue specification,
and backup file on disk. For example, use the following configuration in /etc/rsyslog.conf
or create a file with the following content in the /etc/rsyslog.d/ directory:
queue.filename=”example_fwd1”
Target="example1.com" Protocol="tcp")
queue.filename=”example_fwd2”
Target="example2.com" Protocol="tcp")
23.4.2. Creating a New Directory for rsyslog Log Files
Rsyslog runs as the syslogd daemon and is managed by SELinux. Therefore all files to which rsyslog is
required to write to, must have the appropriate SELinux file context.
Creating a New Working Directory
1. If required to use a different directory to store working files, create a directory as follows:
~]# mkdir /rsyslog
2. Install utilities to manage SELinux policy:
~]# yum install policycoreutils-python
3. Set the SELinux directory context type to be the same as the /var/lib/rsyslog/ directory:
~]# semanage fcontext -a -t syslogd_var_lib_t /rsyslog
4. Apply the SELinux context:
~]# restorecon -R -v /rsyslog
restorecon reset /rsyslog context unconfined_u:object_r:default_t:s0-
>unconfined_u:object_r:syslogd_var_lib_t:s0
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5. If required, check the SELinux context as follows:
~]# ls -Zd /rsyslog
drwxr-xr-x. root root system_u:object_r:syslogd_var_lib_t:s0 /rsyslog
6. Create subdirectories as required. For example:
~]# mkdir /rsyslog/work/
The subdirectories will be created with the same SELinux context as the parent directory.
7. Add the following line in /etc/rsyslog.conf immediately before it is required to take effect:
global(workDirectory=”/rsyslog/work”)
This setting will remain in effect until the next WorkDirectory directive is encountered while
parsing the configuration files.
23.4.3. Managing Queues
All types of queues can be further configured to match your requirements. You can use several
directives to modify both action queues and the main message queue. Currently, there are more than
20 queue parameters available, see the section called “Online Documentation”. Some of these
settings are used commonly, others, such as worker thread management, provide closer control over
the queue behavior and are reserved for advanced users. With advanced settings, you can optimize
rsyslog's performance, schedule queuing, or modify the behavior of a queue on system shutdown.
Limiting Queue Size
You can limit the number of messages that queue can contain with the following setting:
object(queue.highwatermark=”number”)
action queue or for the ruleset respectively. Replace number with a number of enqueued messages.
You can set the queue size only as the number of messages, not as their actual memory size. The
default queue size is 10,000 messages for the main message queue and ruleset queues, and 1000 for
action queues.
Disk assisted queues are unlimited by default and cannot be restricted with this directive, but you can
reserve them physical disk space in bytes with the following settings:
object(queue.maxdiskspace=”number”)
Replace object withmain_queue, action or ruleset. When the size limit specified by number is hit,
messages are discarded until sufficient amount of space is freed by dequeued messages.
Discarding Messages
When a queue reaches a certain number of messages, you can discard less important messages in
order to save space in the queue for entries of higher priority. The threshold that launches the
discarding process can be set with the so-called discard mark:
object(queue.discardmark=”number”)
Replace object withMainMsg or withAction to use this option to the main message queue or for an
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Replace object withMainMsg or withAction to use this option to the main message queue or for an
action queue respectively. Here, number stands for a number of messages that have to be in the
queue to start the discarding process. To define which messages to discard, use:
object(queue.discardseverity=”number”)
Replace number with one of the following numbers for respective priorities:
7 (debug), 6 (info),5
(notice), 4 (warning),3 (err),2 (crit), 1 (alert), or0 (emerg). With this setting, both newly incoming and
already queued messages with lower than defined priority are erased from the queue immediately
after the discard mark is reached.
Using Timeframes
You can configure rsyslog to process queues during a specific time period. With this option you can,
for example, transfer some processing into off-peak hours. To define a time frame, use the following
syntax:
object(queue.dequeuetimebegin=”hour”)
object(queue.dequeuetimeend=”hour”)
With hour you can specify hours that bound your time frame. Use the 24-hour format without
minutes.
Configuring Worker Threads
A worker thread performs a specified action on the enqueued message. For example, in the main
message queue, a worker task is to apply filter logic to each incoming message and enqueue them to
the relevant action queues. When a message arrives, a worker thread is started automatically. When
the number of messages reaches a certain number, another worker thread is turned on. To specify
this number, use:
object(queue.workerthreadminimummessages=”number”)
Replace number with a number of messages that will trigger a supplemental worker thread. For
example, with number set to 100, a new worker thread is started when more than 100 messages arrive.
When more than 200 messages arrive, the third worker thread starts and so on. However, too many
working threads running in parallel becomes ineffective, so you can limit the maximum number of
them by using:
object(queue.workerthreads=”number”)
where number stands for a maximum number of working threads that can run in parallel. For the main
message queue, the default limit is 1 thread. Once a working thread has been started, it keeps running
until an inactivity timeout appears. To set the length of timeout, type:
object(queue.timeoutworkerthreadshutdown=”time”)
Replace time with the duration set in milliseconds. Specifies time without new messages after which
the worker thread will be closed. Default setting is one minute.
Batch Dequeuing
To increase performance, you can configure rsyslog to dequeue multiple messages at once. To set
the upper limit for such dequeueing, use:
$object(queue.DequeueBatchSize= ”number”)
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Replace number with the maximum number of messages that can be dequeued at once. Note that a
higher setting combined with a higher number of permitted working threads results in greater
memory consumption.
Terminating Queues
When terminating a queue that still contains messages, you can try to minimize the data loss by
specifying a time interval for worker threads to finish the queue processing:
object(queue.timeoutshutdown=”time”)
Specify time in milliseconds. If after that period there are still some enqueued messages, workers
finish the current data element and then terminate. Unprocessed messages are therefore lost.
Another time interval can be set for workers to finish the final element:
object(queue.timeoutactioncompletion=”time”)
In case this timeout expires, any remaining workers are shut down. To save data at shutdown, use:
object(queue.saveonshutdown=”on”)
If set, all queue elements are saved to disk before rsyslog terminates.
23.4.4. Using the New Syntax for rsyslog queues
In the new syntax available in rsyslog 7, queues are defined inside the action() object that can be used
both separately or inside a ruleset in /etc/rsyslog.conf. The format of an action queue is as follows:
action(type="action_type "queue.size="queue_size" queue.type="queue_type"
queue.filename="file_name"
Replace action_type with the name of the module that is to perform the action and replace
queue_size
with a maximum number of messages the queue can contain. For queue_type, choose disk or select
from one of the in-memory queues: direct, linkedlist or fixedarray. For file_name specify only a file
name, not a path. Note that if creating a new directory to hold log files, the SELinux context must be
set. See Section 23.4.2, “Creating a New Directory for rsyslog Log Files”for an example.
Example 23.13. Defining an Action Queue
To configure the output action with an asynchronous linked-list based action queue which can
hold a maximum of 10,000 messages, enter a command as follows:
action(type="omfile" queue.size="10000" queue.type="linkedlist" queue.filename="logfile")
The rsyslog 7 syntax for a direct action queues is as follows:
. action(type="omfile" file="/var/lib/rsyslog/log_file
)
The rsyslog 7 syntax for an action queue with multiple parameters can be written as follows:
. action(type="omfile"
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queue.filename="log_file"
queue.type="linkedlist"
queue.size="10000"
)
The default work directory, or the last work directory to be set, will be used. If required to use a
different work directory, add a line as follows before the action queue:
global(workDirectory="/directory")
Example 23.14. Forwarding To a Single Server Using the New Syntax
The following example is based on the procedure Forwarding To a Single Serverin order to show
the difference between the traditional sysntax and the rsyslog 7 syntax. The omfwd plug-in is
used to provide forwarding over UDP or TCP. The default isUDP. As the plug-in is built in it does
not have to be loaded.
Use the following configuration in /etc/rsyslog.conf or create a file with the following content in
the /etc/rsyslog.d/ directory:
. action(type="omfwd"
queue.type="linkedlist"
queue.filename="example_fwd"
queue.saveOnShutdown="on"
target="example.com" port="6514" protocol="tcp"
)
Where:
queue.type="linkedlist" enables a LinkedList in-memory queue,
queue.filename defines a disk storage. The backup files are created with the
example_fwd
prefix, in the working directory specified by the preceding global workDirectory directive,
the action.resumeRetryCount -1 setting prevents rsyslog from dropping messages when
retrying to connect if server is not responding,
enabled queue.saveOnShutdown="on" saves in-memory data if rsyslog shuts down,
the last line forwards all received messages to the logging server, port specification is
optional.
23.5. CONFIGURING RSYSLOG ON A LOGGING SERVER
The rsyslog service provides facilities both for running a logging server and for configuring individual
systems to send their log files to the logging server. See Example 23.12, “Reliable Forwarding of Log
Messages to a Server” for information on clientrsyslog configuration.
The rsyslog service must be installed on the system that you intend to use as a logging server and all
systems that will be configured to send logs to it. Rsyslog is installed by default in Red Hat Enterprise
Linux 7. If required, to ensure that it is, enter the following command as root:
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~]# yum install rsyslog
The default protocol and port for syslog traffic is UDP and 514, as listed in the/etc/services file.
However, rsyslog defaults to usingTCP on port 514. In the configuration file,/etc/rsyslog.conf, TCP is
indicated by @@.
Other ports are sometimes used in examples, however SELinux is only configured to allow sending
and receiving on the following ports by default:
~]# semanage port -l | grep syslog
syslog_tls_port_t tcp 6514, 10514
syslog_tls_port_t udp 6514, 10514
syslogd_port_t tcp 601, 20514
syslogd_port_t udp 514, 601, 20514
The semanage utility is provided as part of thepolicycoreutils-python package. If required, install
the package as follows:
~]# yum install policycoreutils-python
In addition, by default the SELinux type for rsyslog, rsyslogd_t, is configured to permit sending and
receiving to the remote shell (rsh) port with SELinux typersh_port_t, which defaults toTCP on port
514. Therefore it is not necessary to usesemanage to explicitly permitTCP on port 514. For example,
to check what SELinux is set to permit on port 514, enter a command as follows:
~]# semanage port -l | grep 514
output omitted
rsh_port_t tcp 514
syslogd_port_t tcp 6514, 601
syslogd_port_t udp 514, 6514, 601
For more information on SELinux, see Red Hat Enterprise Linux 7 SELinux User’s and Administrator’s
Guide.
Perform the steps in the following procedures on the system that you intend to use as your logging
server. All steps in these procedure must be made as the root user.
Configure SELinux to Permit rsyslog Traffic on a Port
If required to use a new port for rsyslog traffic, follow this procedure on the logging server and the
clients. For example, to send and receive TCP traffic on port10514, proceed with the following
sequence of commands:
1. Run the semanage port command with the following parameters:
~]# semanage port -a -t syslogd_port_t -p tcp 10514
2. Review the SELinux ports by entering the following command:
~]# semanage port -l | grep syslog
3. If the new port was already configured in /etc/rsyslog.conf, restart rsyslog now for the
change to take effect:
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~]# service rsyslog restart
4. Verify which ports rsyslog is now listening to:
~]# netstat -tnlp | grep rsyslog
tcp 0 0 0.0.0.0:10514 0.0.0.0:* LISTEN 2528/rsyslogd
tcp 0 0 :::10514 :::* LISTEN 2528/rsyslogd
See the semanage-port(8) manual page for more information on thesemanage port command.
Configuring firewalld
Configure firewalld to allow incomingrsyslog traffic. For example, to allowTCP traffic on port10514,
proceed as follows:
~]# firewall-cmd --zone=zone --add-port=10514/tcp
success
+ Where zone is the zone of the interface to use. Note that these changes will not persist after the
next system start. To make permanent changes to the firewall, repeat the commands adding the --
permanent option. For more information on opening and closing ports infirewalld, see the Red Hat
Enterprise Linux 7 Security Guide.
1. To verify the above settings, use a command as follows:
interfaces: eth0
sources:
ports: 10514/tcp
masquerade: no
forward-ports:
icmp-blocks:
rich rules:
Configuring rsyslog to Receive and Sort Remote Log Messages
1. Open the /etc/rsyslog.conf file in a text editor and proceed as follows:
a. Add these lines below the modules section but above the Provides UDP syslog
reception section:
# Define templates before the rules that use them
# Per-Host Templates for Remote Systems #
$template TmplAuthpriv,
"/var/log/remote/auth/%HOSTNAME%/%PROGRAMNAME:::secpath-replace%.log"
$template TmplMsg,
"/var/log/remote/msg/%HOSTNAME%/%PROGRAMNAME:::secpath-replace%.log"
b. Replace the default Provides TCP syslog reception section with the following:
# Provides TCP syslog reception
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$ModLoad imtcp
# Adding this ruleset to process remote messages
$RuleSet remote1
authpriv.* ?TmplAuthpriv
*.info;mail.none;authpriv.none;cron.none ?TmplMsg
$RuleSet RSYSLOG_DefaultRuleset #End the rule set by switching back to the
default rule set
$InputTCPServerBindRuleset remote1 #Define a new input and bind it to the
"remote1" rule set
$InputTCPServerRun 10514
Save the changes to the /etc/rsyslog.conf file.
2. The rsyslog service must be running on both the logging server and the systems attempting
to log to it.
a. Use the systemctl command to start thersyslog service.
~]# systemctl start rsyslog
b. To ensure the rsyslog service starts automatically in future, enter the following command
as root:
~]# systemctl enable rsyslog
Your log server is now configured to receive and store log files from the other systems in your
environment.
23.5.1. Using The New Template Syntax on a Logging Server
Rsyslog 7 has a number of different templates styles. The string template most closely resembles the
legacy format. Reproducing the templates from the example above using the string format would
look as follows:
template(name="TmplAuthpriv" type="string"
string="/var/log/remote/auth/%HOSTNAME%/%PROGRAMNAME:::secpath-replace%.log"
)
template(name="TmplMsg" type="string"
string="/var/log/remote/msg/%HOSTNAME%/%PROGRAMNAME:::secpath-replace%.log"
)
These templates can also be written in the list format as follows:
template(name="TmplAuthpriv" type="list") {
constant(value="/var/log/remote/auth/")
constant(value="/")
property(name="programname" SecurePath="replace")
constant(value=".log")
}
template(name="TmplMsg" type="list") {
constant(value="/var/log/remote/msg/")
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constant(value="/")
property(name="programname" SecurePath="replace")
constant(value=".log")
}
This template text format might be easier to read for those new to rsyslog and therefore can be
easier to adapt as requirements change.
To complete the change to the new syntax, we need to reproduce the module load command, add a
rule set, and then bind the rule set to the protocol, port, and ruleset:
module(load="imtcp")
ruleset(name="remote1"){
authpriv.* action(type="omfile" DynaFile="TmplAuthpriv")
*.info;mail.none;authpriv.none;cron.none action(type="omfile" DynaFile="TmplMsg")
}
input(type="imtcp" port="10514" ruleset="remote1")
23.6. USING RSYSLOG MODULES
Due to its modular design, rsyslog offers a variety ofmodules which provide additional functionality.
Note that modules can be written by third parties. Most modules provide additional inputs (see Input
Modules below) or outputs (seeOutput Modules below). Other modules provide special functionality
specific to each module. The modules may provide additional configuration directives that become
available after a module is loaded. To load a module, use the following syntax:
module(load=”MODULE”)
where MODULE represents your desired module. For example, if you want to load the Text File Input
Module (imfile) that enablesrsyslog to convert any standard text files into syslog messages, specify
the following line in the /etc/rsyslog.conf configuration file:
module(load=”imfile”)
rsyslog offers a number of modules which are split into the following main categories:
Input Modules — Input modules gather messages from various sources. The name of an input
module always starts with the im prefix, such asimfile and imjournal.
Output Modules — Output modules provide a facility to issue message to various targets such
as sending across a network, storing in a database, or encrypting. The name of an output
module always starts with the om prefix, such asomsnmp, omrelp, and so on.
Parser Modules — These modules are useful in creating custom parsing rules or to parse
malformed messages. With moderate knowledge of the C programming language, you can
create your own message parser. The name of a parser module always starts with the pm
prefix, such as pmrfc5424, pmrfc3164, and so on.
Message Modification Modules — Message modification modules change content of syslog
messages. Names of these modules start with the mm prefix. Message Modification Modules
such as mmanon, mmnormalize, or mmjsonparse are used for anonymization or
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normalization of messages.
String Generator Modules — String generator modules generate strings based on the
message content and strongly cooperate with the template feature provided by rsyslog. For
more information on templates, see Section 23.2.3, “Templates”. The name of a string
generator module always starts with the sm prefix, such assmfile or smtradfile.
Library Modules — Library modules provide functionality for other loadable modules. These
modules are loaded automatically by rsyslog when needed and cannot be configured by the
user.
A comprehensive list of all available modules and their detailed description can be found at
http://www.rsyslog.com/doc/rsyslog_conf_modules.html.
WARNING
Note that when rsyslog loads any modules, it provides them with access to some
of its functions and data. This poses a possible security threat. To minimize
security risks, use trustworthy modules only.
23.6.1. Importing Text Files
The Text File Input Module, abbreviated as imfile, enables rsyslog to convert any text file into a
stream of syslog messages. You can use imfile to import log messages from applications that create
their own text file logs. To load imfile, add the following into/etc/rsyslog.conf:
module(load=”imfile”
PollingInterval=”int”)
It is sufficient to load imfile once, even when importing multiple files. ThePollingInterval module
argument specifies how often rsyslog checks for changes in connected text files. The default interval
is 10 seconds, to change it, replace int with a time interval specified in seconds.
To identify the text files to import, use the following syntax in /etc/rsyslog.conf:
# File 1
input(type="imfile"
File="path_to_file"
Tag="tag:"
Severity="severity"
Facility="facility")
# File 2
input(type="imfile"
File="path_to_file2")
...
Settings required to specify an input text file:
replace path_to_file with a path to the text file.

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replace tag: with a tag name for this message.
Apart from the required directives, there are several other settings that can be applied on the text
input. Set the severity of imported messages by replacing severity with an appropriate keyword.
Replace facility with a keyword to define the subsystem that produced the message. The keywords
for severity and facility are the same as those used in facility/priority-based filters, see Section 23.2.1,
“Filters”.
Example 23.15. Importing Text Files
The Apache HTTP server creates log files in text format. To apply the processing capabilities of
rsyslog to apache error messages, first use theimfile module to import the messages. Add the
following into /etc/rsyslog.conf:
module(load=”imfile”)
input(type="imfile"
File="/var/log/httpd/error_log"
Tag="apache-error:")
23.6.2. Exporting Messages to a Database
Processing of log data can be faster and more convenient when performed in a database rather than
with text files. Based on the type of DBMS used, choose from various output modules such as
ommysql, ompgsql, omoracle, or ommongodb. As an alternative, use the genericomlibdbi output
module that relies on the libdbi library. Theomlibdbi module supports database systems
Firebird/Interbase, MS SQL, Sybase, SQLite, Ingres, Oracle, mSQL, MySQL, and PostgreSQL.
Example 23.16. Exporting Rsyslog Messages to a Database
To store the rsyslog messages in a MySQL database, add the following into /etc/rsyslog.conf:
module(load=”ommysql”)
. action(type”ommysql”
server=”database-server”
db=”database-name”
uid=”database-userid”
pwd=”database-password”
serverport=”1234”)
First, the output module is loaded, then the communication port is specified. Additional
information, such as name of the server and the database, and authentication data, is specified on
the last line of the above example.
23.6.3. Enabling Encrypted Transport
Confidentiality and integrity in network transmissions can be provided by either the TLS or GSSAPI
encryption protocol.
Transport Layer Security(TLS) is a cryptographic protocol designed to provide communication
security over the network. When using TLS, rsyslog messages are encrypted before sending, and
mutual authentication exists between the sender and receiver. For configuring TLS, see the section
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called “Configuring Encrypted Message Transfer with TLS”.
Generic Security Service API (GSSAPI) is an application programming interface for programs to
access security services. To use it in connection with rsyslog you must have a functioningKerberos
environment. For configuring GSSAPI, see the section called “Configuring Encrypted Message
Transfer with GSSAPI”.
Configuring Encrypted Message Transfer with TLS
To use encrypted transport through TLS, you need to configure both the server and the client.
1. Create public key, private key and certificate file, see Section 14.1.11, “Generating a New Key
and Certificate”.
2. On the server side, configure the following in the/etc/rsyslog.conf configuration file:
a. Set the gtls netstream driver as the default driver:
global(defaultnetstreamdriver="gtls")
b. Provide paths to certificate files:
global(defaultnetstreamdrivercafile="path_ca.pem"
defaultnetstreamdrivercertfile="path_cert.pem"
defaultnetstreamdriverkeyfile="path_key.pem")
You can merge all global directives into single block if you prefer a less cluttered
configuration file.
Replace:
path_ca.pem with a path to your public key
path_cert.pem with a path to the certificate file
path_key.pem with a path to the private key
c. Load the imtcp moduleand set driver options:
module(load=”imtcp”
StreamDriver.Mode=“number”
StreamDriver.AuthMode=”anon”)
d. Start a server:
input(type="imtcp" port="port″)
Replace:
number to specify the driver mode. To enable TCP-only mode, use
1
port with the port number at which to start a listener, for example
10514
The anon setting means that the client is not authenticated.
3. On the client side, configure the following in the/etc/rsyslog.conf configuration file:
a. Load the public key:
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global(defaultnetstreamdrivercafile="path_ca.pem")
Replace path_ca.pem with a path to the public key.
b. Set the gtls netstream driver as the default driver:
global(defaultnetstreamdriver="gtls")
c. Configure the driver and specify what action will be performed:
module(load=”imtcp”
streamdrivermode=”number”
streamdriverauthmode=”anon”)
input(type=”imtcp”
address=”server.net”
port=”port”)
Replace number, anon, and port with the same values as on the server.
On the last line in the above listing, an example action forwards messages from the server
to the specified TCP port.
Configuring Encrypted Message Transfer with GSSAPI
In rsyslog, interaction with GSSAPI is provided by theimgssapi module. To turn on the GSSAPI
transfer mode:
1. Put the following configuration in /etc/rsyslog.conf:
$ModLoad imgssapi
This directive loads the imgssapi module.
2. Specify the input as follows:
$InputGSSServerServiceName name
$InputGSSServerPermitPlainTCP on
$InputGSSServerMaxSessions number
$InputGSSServerRun port
Replace name with the name of the GSS server.
Replace number to set the maximum number of sessions supported. This number is not
limited by default.
Replace port with a selected port on which you want to start a GSS server.
The $InputGSSServerPermitPlainTCP on setting permits the server to receive also plain
TCP messages on the same port. This is off by default.
NOTE
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NOTE
The imgssapi module is initialized as soon as the configuration file reader encounters
the $InputGSSServerRun directive in the /etc/rsyslog.conf configuration file. The
supplementary options configured after $InputGSSServerRun are therefore ignored.
For configuration to take effect, all imgssapi configuration options must be placed
before $InputGSSServerRun.
Example 23.17. Using GSSAPI
The following configuration enables a GSS server on the port 1514 that also permits to receive
plain tcp syslog messages on the same port.
$ModLoad imgssapi
$InputGSSServerPermitPlainTCP on
$InputGSSServerRun 1514
23.6.4. Using RELP
Reliable Event Logging Protocol (RELP) is a networking protocol for data logging in computer
networks. It is designed to provide reliable delivery of event messages, which makes it useful in
environments where message loss is not acceptable.
Configuring RELP
To configure RELP, you need to configure both the server and the client using the /etc/rsyslog.conf
file.
1. To configure the client:
a. Load the required modules:
module(load="imuxsock")
module(load="omrelp")
b. Configure the TCP input as follows:
Replace port to start a listener at the required port.
c. Configure the transport settings:
action(type="omrelp" target="target_IP″ port="target_port″)
Replace target_IP and target_port with the IP address and port that identify the target
server.
2. To configure the server:
a. Configure loading the module:
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module(load="imrelp" ruleset="relp")
b. Configure the TCP input similarly to the client configuration:
input(type="imrelp" port="target_port″)
Replace target_port with the same value as on the clients.
c. Configure the rules and choose an action to be performed. In the following example,
log_path specifies the path for storing messages:
ruleset (name="relp") {
action(type="omfile" file="log_path")
}
Configuring RELP with TLS
To configure RELP with TLS, you need to configure authentication. Then, you need to configure both
the server and the client using the /etc/rsyslog.conf file.
1. Create public key, private key and certificate file. For instructions, see Section 14.1.11,
“Generating a New Key and Certificate”.
2. To configure the client:
a. Load the required modules:
module(load="omrelp")
b. Configure the TCP input as follows:
Replace port to start a listener at the required port.
c. Configure the transport settings:
action(type="omrelp" target="target_IP″ port="target_port″ tls="on"
tls.caCert="path_ca.pem"
tls.myCert="path_cert.pem"
tls.myPrivKey="path_key.pem"
tls.authmode="mode"
tls.permittedpeer=["peer_name"]
)
Replace:
target_IP and target_port with the IP address and port that identify the target server.
path_ca.pem, path_cert.pem, and path_key.pem with paths to the certification files
mode with the authentication mode for the transaction. Use either
"name" or
"fingerprint"
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peer_name with a certificate fingerprint of the permitted peer. If you specify this,
tls.permittedpeer restricts connection to the selected group of peers.
The tls="on" setting enables the TLS protocol.
3. To configure the server:
a. Configure loading the module:
module(load="imrelp" ruleset="relp")
b. Configure the TCP input similarly to the client configuration:
input(type="imrelp" port="target_port″ tls="on"
tls.caCert="path_ca.pem"
tls.myCert="path_cert.pem"
tls.myPrivKey="path_key.pem"
tls.authmode="name"
tls.permittedpeer=["peer_name","peer_name1","peer_name2"]
)
Replace the highlighted values with the same as on the client.
c. Configure the rules and choose an action to be performed. In the following example,
log_path specifies the path for storing messages:
ruleset (name="relp") {
action(type="omfile" file="log_path")
}
23.7. INTERACTION OF RSYSLOG AND JOURNAL
As mentioned above, Rsyslog and Journal, the two logging applications present on your system, have
several distinctive features that make them suitable for specific use cases. In many situations it is
useful to combine their capabilities, for example to create structured messages and store them in a
file database (see Section 23.8, “Structured Logging with Rsyslog”). A communication interface
needed for this cooperation is provided by input and output modules on the side of Rsyslog and by
the Journal's communication socket.
By default, rsyslogd uses theimjournal module as a default input mode for journal files. With this
module, you import not only the messages but also the structured data provided by journald. Also,
older data can be imported from journald (unless forbidden with theIgnorePreviousMessages
option). See Section 23.8.1, “Importing Data from Journal”for basic configuration ofimjournal.
As an alternative, configure rsyslogd to read from the socket provided byjournal as an output for
syslog-based applications. The path to the socket is /run/systemd/journal/syslog. Use this option
when you want to maintain plain rsyslog messages. Compared to imjournal the socket input currently
offers more features, such as ruleset binding or filtering. To import Journal data trough the socket,
use the following configuration in /etc/rsyslog.conf:
module(load="imuxsock"
SysSock.Use="on"
SysSock.Name="/run/systemd/journal/syslog")
You can also output messages from Rsyslog to Journal with theomjournal module. Configure the
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You can also output messages from Rsyslog to Journal with theomjournal module. Configure the
output in /etc/rsyslog.conf as follows:
module(load="omjournal")
action(type="omjournal")
For instance, the following configuration forwards all received messages on tcp port 10514 to the
Journal:
module(load="omjournal")
ruleset(name="remote") {
action(type="omjournal")
}
input(type="imtcp" port="10514" ruleset="remote")
23.8. STRUCTURED LOGGING WITH RSYSLOG
On systems that produce large amounts of log data, it can be convenient to maintain log messages in
a structured format. With structured messages, it is easier to search for particular information, to
produce statistics and to cope with changes and inconsistencies in message structure. Rsyslog uses
the JSON (JavaScript Object Notation) format to provide structure for log messages.
Compare the following unstructured log message:
Oct 25 10:20:37 localhost anacron[1395]: Jobs will be executed sequentially
with a structured one:
{"timestamp":"2013-10-25T10:20:37", "host":"localhost", "program":"anacron", "pid":"1395",
"msg":"Jobs will be executed sequentially"}
Searching structured data with use of key-value pairs is faster and more precise than searching text
files with regular expressions. The structure also lets you to search for the same entry in messages
produced by various applications. Also, JSON files can be stored in a document database such as
MongoDB, which provides additional performance and analysis capabilities. On the other hand, a
structured message requires more disk space than the unstructured one.
In rsyslog, log messages with meta data are pulled fromJournal with use of theimjournal module.
With the mmjsonparse module, you can parse data imported fromJournal and from other sources
and process them further, for example as a database output. For parsing to be successful,
mmjsonparse requires input messages to be structured in a way that is defined by the
Lumberjack
project.
The Lumberjack project aims to add structured logging torsyslog in a backward-compatible way. To
identify a structured message, Lumberjack specifies the@cee: string that prepends the actual JSON
structure. Also, Lumberjack defines the list of standard field names that should be used for entities in
the JSON string. For more information on Lumberjack, see the section called “Online
Documentation”.
The following is an example of a lumberjack-formatted message:
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@cee: {"pid":17055, "uid":1000, "gid":1000, "appname":"logger", "msg":"Message text."}
To build this structure inside Rsyslog, a template is used, seeSection 23.8.2, “Filtering Structured
Messages”. Applications and servers can employ thelibumberlog library to generate messages in the
lumberjack-compliant form. For more information on libumberlog, see the section called “Online
Documentation”.
23.8.1. Importing Data from Journal
The imjournal module isRsyslog's input module to natively read the journal files (seeSection 23.7,
“Interaction of Rsyslog and Journal”). Journal messages are then logged in text format as other
rsyslog messages. However, with further processing, it is possible to translate meta data provided by
Journal into a structured message.
To import data from Journal to Rsyslog, use the following configuration in/etc/rsyslog.conf:
module(load=”imjournal”
PersistStateInterval=”number_of_messages”
StateFile=”path”
ratelimit.interval=”seconds”
ratelimit.burst=”burst_number”
IgnorePreviousMessages=”off/on”)
With number_of_messages, you can specify how often the journal data must be saved. This
will happen each time the specified number of messages is reached.
Replace path with a path to the state file. This file tracks the journal entry that was the last
one processed.
With seconds, you set the length of the rate limit interval. The number of messages processed
during this interval cannot exceed the value specified in burst_number. The default setting is
20,000 messages per 600 seconds. Rsyslog discards messages that come after the
maximum burst within the time frame specified.
With IgnorePreviousMessages you can ignore messages that are currently in Journal and
import only new messages, which is used when there is no state file specified. The default
setting is off. Please note that if this setting is off and there is no state file, all messages in
the Journal are processed, even if they were already processed in a previous rsyslog session.
NOTE
You can use imjournal simultaneously withimuxsock module that is the traditional
system log input. However, to avoid message duplication, you must prevent imuxsock
from reading the Journal’s system socket. To do so, use the SysSock.Use directive:
module(load”imjournal”)
module(load”imuxsock”
SysSock.Use=”off”
Socket="/run/systemd/journal/syslog")
You can translate all data and meta data stored by Journal into structured messages. Some of these
meta data entries are listed in Example 23.19, “Verbose journalctl Output”, for a complete list of
journal fields see the systemd.journal-fields(7) manual page. For example, it is possible to focus on
kernel journal fields, that are used by messages originating in the kernel.
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23.8.2. Filtering Structured Messages
To create a lumberjack-formatted message that is required by rsyslog's parsing module, use the
following template:
template(name="CEETemplate" type="string" string="%TIMESTAMP% %HOSTNAME%
%syslogtag% @cee: %$!all-json%n")
This template prepends the @cee: string to the JSON string and can be applied, for example, when
creating an output file with omfile module. To access JSON field names, use the$! prefix. For
example, the following filter condition searches for messages with specific hostname and UID:
($!hostname == "hostname" && $!UID== "UID")
23.8.3. Parsing JSON
The mmjsonparse module is used for parsing structured messages.
These messages can come from Journal or from other input sources, and must be formatted in a way
defined by the Lumberjack project. These messages are identified by the presence of the
@cee:
string. Then, mmjsonparse checks if the JSON structure is valid and then the message is parsed.
To parse lumberjack-formatted JSON messages with mmjsonparse, use the following configuration
in the /etc/rsyslog.conf:
module(load”mmjsonparse”)
. :mmjsonparse:
In this example, the mmjsonparse module is loaded on the first line, then all messages are forwarded
to it. Currently, there are no configuration parameters available for mmjsonparse.
23.8.4. Storing Messages in the MongoDB
Rsyslog supports storing JSON logs in the MongoDB document database through the
ommongodb
output module.
To forward log messages into MongoDB, use the following syntax in the /etc/rsyslog.conf
(configuration parameters for ommongodb are available only in the new configuration format; see
Section 23.3, “Using the New Configuration Format”
):
module(load”ommongodb”)
. action(type="ommongodb" server="DB_server" serverport="port" db="DB_name"
collection="collection_name" uid="UID" pwd="password")
Replace DB_server with the name or address of the MongoDB server. Specify
port to select a
non-standard port from the MongoDB server. The default port value is0 and usually there is
no need to change this parameter.
With DB_name, you identify to which database on the MongoDB server you want to direct the
output. Replace collection_name with the name of a collection in this database. In MongoDB,
collection is a group of documents, the equivalent of an RDBMS table.
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You can set your login details by replacing UID and password.
You can shape the form of the final database output with use of templates. By default, rsyslog uses a
template based on standard lumberjack field names.
23.9. DEBUGGING RSYSLOG
To run rsyslogd in debugging mode, use the following command:
rsyslogd -dn
With this command, rsyslogd produces debugging information and prints it to the standard output.
The -n stands for "no fork". You can modify debugging with environmental variables, for example,
you can store the debug output in a log file. Before starting rsyslogd, type the following on the
command line:
export RSYSLOG_DEBUGLOG="path"
export RSYSLOG_DEBUG="Debug"
Replace path with a desired location for the file where the debugging information will be logged. For a
complete list of options available for the RSYSLOG_DEBUG variable, see the related section in the
rsyslogd(8) manual page.
To check if syntax used in the /etc/rsyslog.conf file is valid use:
rsyslogd -N 1
Where 1 represents level of verbosity of the output message. This is a forward compatibility option
because currently, only one level is provided. However, you must add this argument to run the
validation.
23.10. USING THE JOURNAL
The Journal is a component of systemd that is responsible for viewing and management of log files. It
can be used in parallel, or in place of a traditional syslog daemon, such as rsyslogd. The Journal was
developed to address problems connected with traditional logging. It is closely integrated with the
rest of the system, supports various logging technologies and access management for the log files.
Logging data is collected, stored, and processed by the Journal’s journald service. It creates and
maintains binary files called journals based on logging information that is received from the kernel,
from user processes, from standard output, and standard error output of system services or via its
native API. These journals are structured and indexed, which provides relatively fast seek times.
Journal entries can carry a unique identifier. The journald service collects numerous meta data fields
for each log message. The actual journal files are secured, and therefore cannot be manually edited.
To access the journal logs, use the journalctl tool. For a basic view of the logs type asroot:
journalctl
An output of this command is a list of all log files generated on the system including messages
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An output of this command is a list of all log files generated on the system including messages
generated by system components and by users. The structure of this output is similar to one used in
/var/log/messages/ but with certain improvements:
the priority of entries is marked visually. Lines of error priority and higher are highlighted
with red color and a bold font is used for lines with notice and warning priority
the time stamps are converted for the local time zone of your system
all logged data is shown, including rotated logs
the beginning of a boot is tagged with a special line
Example 23.18. Example Output of journalctl
The following is an example output provided by the journalctl tool. When called without
parameters, the listed entries begin with a time stamp, then the host name and application that
performed the operation is mentioned followed by the actual message. This example shows the
first three entries in the journal log:
# journalctl
-- Logs begin at Thu 2013-08-01 15:42:12 CEST, end at Thu 2013-08-01 15:48:48 CEST. --
Aug 01 15:42:12 localhost systemd-journal[54]: Allowing runtime journal files to grow to
49.7M.
Aug 01 15:42:12 localhost kernel: Initializing cgroup subsys cpuset
Aug 01 15:42:12 localhost kernel: Initializing cgroup subsys cpu
[...]
In many cases, only the latest entries in the journal log are relevant. The simplest way to reduce
journalctl output is to use the-n option that lists only the specified number of most recent log
entries:
journalctl -n Number
Replace Number with the number of lines to be shown. When no number is specified,
journalctl
displays the ten most recent entries.
The journalctl command allows controlling the form of the output with the following syntax:
journalctl -o form
Replace form with a keyword specifying a desired form of output. There are several options, such as
verbose, which returns full-structured entry items with all fields,
export, which creates a binary
stream suitable for backups and network transfer, and json, which formats entries as JSON data
structures. For the full list of keywords, see the journalctl(1) manual page.
Example 23.19. Verbose journalctl Output
To view full meta data about all entries, type:
# journalctl -o verbose
[...]
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Fri 2013-08-02 14:41:22 CEST
[s=e1021ca1b81e4fc688fad6a3ea21d35b;i=55c;b=78c81449c920439da57da7bd5c56a770;m=
27cc
_BOOT_ID=78c81449c920439da57da7bd5c56a770
PRIORITY=5
SYSLOG_FACILITY=3
_TRANSPORT=syslog
_MACHINE_ID=69d27b356a94476da859461d3a3bc6fd
_HOSTNAME=localhost.localdomain
_PID=562
_COMM=dbus-daemon
_EXE=/usr/bin/dbus-daemon
_CMDLINE=/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --
systemd-activation
_SYSTEMD_CGROUP=/system/dbus.service
_SYSTEMD_UNIT=dbus.service
SYSLOG_IDENTIFIER=dbus
SYSLOG_PID=562
_UID=81
_GID=81
_SELINUX_CONTEXT=system_u:system_r:system_dbusd_t:s0-s0:c0.c1023
MESSAGE=[system] Successfully activated service 'net.reactivated.Fprint'
_SOURCE_REALTIME_TIMESTAMP=1375447282839181
[...]
This example lists fields that identify a single log entry. These meta data can be used for message
filtering as shown in the section called “Advanced Filtering”. For a complete description of all
possible fields see the systemd.journal-fields(7) manual page.
23.10.2. Access Control
By default, Journal users withoutroot privileges can only see log files generated by them. The system
administrator can add selected users to the adm group, which grants them access to complete log
files. To do so, type as root:
usermod -a -G adm username
Here, replace username with a name of the user to be added to theadm group. This user then receives
the same output of the journalctl command as the root user. Note that access control only works
when persistent storage is enabled for Journal.
23.10.3. Using The Live View
When called without parameters, journalctl shows the full list of entries, starting with the oldest entry
collected. With the live view, you can supervise the log messages in real time as new entries are
continuously printed as they appear. To start journalctl in live view mode, type:
journalctl -f
This command returns a list of the ten most current log lines. The journalctl utility then stays running
and waits for new changes to show them immediately.
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23.10.4. Filtering Messages
The output of the journalctl command executed without parameters is often extensive, therefore you
can use various filtering methods to extract information to meet your needs.
Filtering by Priority
Log messages are often used to track erroneous behavior on the system. To view only entries with a
selected or higher priority, use the following syntax:
journalctl -p priority
Here, replace priority with one of the following keywords (or with a number):
debug (7), info (6),
notice (5),warning (4), err (3),crit (2), alert (1), andemerg (0).
Example 23.20. Filtering by Priority
To view only entries with error or higher priority, use:
journalctl -p err
Filtering by Time
To view log entries only from the current boot, type:
journalctl -b
If you reboot your system just occasionally, the -b will not significantly reduce the output of
journalctl. In such cases, time-based filtering is more helpful:
journalctl --since=value --until=value
With --since and --until, you can view only log messages created within a specified time range. You
can pass values to these options in form of date or time or both as shown in the following example.
Example 23.21. Filtering by Time and Priority
Filtering options can be combined to reduce the set of results according to specific requests. For
example, to view the warning or higher priority messages from a certain point in time, type:
journalctl -p warning --since="2013-3-16 23:59:59"
Advanced Filtering
Example 23.19, “Verbose journalctl Output”lists a set of fields that specify a log entry and can all be
used for filtering. For a complete description of meta data that systemd can store, see the
systemd.journal-fields(7) manual page. This meta data is collected for each log message, without
user intervention. Values are usually text-based, but can take binary and large values; fields can have
multiple values assigned though it is not very common.
To view a list of unique values that occur in a specified field, use the following syntax:
journalctl -F fieldname
519

Replace fieldname with a name of a field you are interested in.
To show only log entries that fit a specific condition, use the following syntax:
journalctl fieldname=value
Replace fieldname with a name of a field andvalue with a specific value contained in that field. As a
result, only lines that match this condition are returned.
NOTE
As the number of meta data fields stored by systemd is quite large, it is easy to forget
the exact name of the field of interest. When unsure, type:
journalctl
and press the Tab key two times. This shows a list of available field names.Tab
completion based on context works on field names, so you can type a distinctive set of
letters from a field name and then press Tab to complete the name automatically.
Similarly, you can list unique values from a field. Type:
journalctl fieldname=
and press Tab two times. This serves as an alternative tojournalctl-F fieldname.
You can specify multiple values for one field:
journalctl fieldname=value1 fieldname=value2 ...
Specifying two matches for the same field results in a logical OR combination of the matches. Entries
matching value1 or value2 are displayed.
Also, you can specify multiple field-value pairs to further reduce the output set:
journalctl fieldname1=value fieldname2=value ...
If two matches for different field names are specified, they will be combined with a logical AND.
Entries have to match both conditions to be shown.
With use of the + symbol, you can set a logicalOR combination of matches for multiple fields:
journalctl fieldname1

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