Red Hat Enterprise Linux 9.3 64bit for ARM
Sep 30, 2024
--password
--auto-
attach
Registering to: subscription.rhsm.redhat.com:443/subscription
The system has been registered with ID: 37to907c-ece6-49ea-9174-20b87ajk9ee7
The registered system name is: client1.idm.example.com
Installed Product Current Status:
Product Name: Red Hat Enterprise Linux for x86_64
Status: Subscribed
The command prompts you to enter your Red Hat Customer Portal user name and password.
If the registration process fails, you can register your system with a specific pool. For guidance
on how to do it, proceed with the following steps:
a. 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.
27Red Hat Enterprise Linux 9 Configuring basic system settings
b. 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
NOTE
To register the system with Red Hat Insights, you can use the rhc connect utility. See
Setting up Red Hat connector .
Additional resources
Understanding autoattaching subscriptions on the Customer Portal
Understanding the manual registration and subscription on the Customer Portal
4.2. REGISTERING SUBSCRIPTIONS WITH CREDENTIALS IN THE WEB
CONSOLE
Use the following steps to register a newly installed Red Hat Enterprise Linux with account credentials
using the RHEL web console.
Prerequisites
A valid user account on the Red Hat Customer Portal.
See the Create a Red Hat Login page.
Active subscription for your RHEL system.
Procedure
1. Log in to the RHEL web console. For details, see Logging in to the web console .
2. In the Health filed in the Overview page, click the Not registered warning, or click
Subscriptions in the main menu to move to page with your subscription information.
.
3. In the Overview filed, click Register.
28CHAPTER 4. REGISTERING THE SYSTEM AND MANAGING SUBSCRIPTIONS
4. In the Register system dialog box, select that you want to register using your account
credentials.
5. Enter your username.
6. Enter your password.
7. Optionally, enter your organization’s name or ID.
If your account belongs to more than one organization on the Red Hat Customer Portal, you
have to add the organization name or organization ID. To get the org ID, go to your Red Hat
contact point.
If you do not want to connect your system to Red Hat Insights, uncheck the Insights check
box.
8. Click the Register button.
At this point, your Red Hat Enterprise Linux system has been successfully registered.
4.3. REGISTERING A SYSTEM USING RED HAT ACCOUNT ON GNOME
Follow the steps in this procedure to enroll your system with your Red Hat account.
29Red Hat Enterprise Linux 9 Configuring basic system settings
Prerequisites
A valid account on Red Hat customer portal.
See the Create a Red Hat Login page for new user registration.
Procedure
1. Go to the system menu, which is accessible from the top-right screen corner and click the
Settings icon.
2. In the Details → About section, click Register.
3. Select Registration Server.
4. If you are not using the Red Hat server, enter the server address in the URL field.
5. In the Registration Type menu, select Red Hat Account.
6. Under Registration Details:
Enter your Red Hat account user name in the Login field.
Enter your Red Hat account password in the Password field.
Enter the name of your organization in the Organization field.
7. Click Register.
4.4. REGISTERING A SYSTEM USING AN ACTIVATION KEY ON GNOME
Follow the steps in this procedure to register your system with an activation key. You can get the
activation key from your organization administrator.
Prerequisites
Activation key or keys.
See the Activation Keys page for creating new activation keys.
Procedure
1. Go to the system menu, which is accessible from the top-right screen corner and click the
Settings icon.
2. In the Details → About section, click Register.
3. Select Registration Server.
4. Enter URL to the customized server, if you are not using the Red Hat server.
5. In the Registration Type menu, select Activation Keys.
6. Under Registration Details:
Enter Activation Keys.
Separate multiple keys by a comma (,).
30CHAPTER 4. REGISTERING THE SYSTEM AND MANAGING SUBSCRIPTIONS
Enter the name or ID of your organization in the Organization field.
7. Click Register
31Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 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.
This section provides information on how to ensure that a service is enabled or disabled at boot time. It
also explains how to manage the services through the web console.
5.1. ENABLING OR DISABLING THE SERVICES
You can determine which services are enabled or disabled at boot time already during the installation
process. You can also enable or disable a service on an installed operating system.
This section describes the steps for enabling or disabling those services on an already installed operating
system:
Prerequisites
You must have root access to the system.
Procedure
1. To enable a service, use the enable option:
# systemctl enable service_name
Replace service_name with the service you want to enable.
You can also enable and start a service in a single command:
# systemctl enable --now service_name
2. To disable a service, use the disable option:
# systemctl disable service_name
Replace service_name with the service you want to disable.
WARNING
You cannot enable a service that has been previously masked. You have to unmask
it first:
# systemctl unmask service_name
5.2. MANAGING SERVICES IN THE RHEL WEB CONSOLE
32CHAPTER 5. MAKING SYSTEMD SERVICES START AT BOOT TIME
This section describes how you can also enable or disable a service using the web console. You can
manage systemd targets, services, sockets, timers, and paths. You can also check the service status,
start or stop services, enable or disable them.
Prerequisites
You must have root access to the system.
Procedure
1. Open localhost:9090 in a web browser of your preference.
2. Log in to the web console with your root credentials on the system.
3. To display the web console panel, click the Host icon, which is in the upper-left corner of the
window.
4. On the menu, click Services.
You can manage systemd targets, services, sockets, timers, and paths.
5. For example, to manage the service NFS client services:
a. Click Targets.
b. Select the service NFS client services.
c. To enable or disable the service, click the Toogle button.
d. To stop the service, click the button and choose the option Stop.
33Red Hat Enterprise Linux 9 Configuring basic system settings
34CHAPTER 6. CONFIGURING SYSTEM SECURITY
CHAPTER 6. CONFIGURING SYSTEM SECURITY
Computer security is the protection of computer systems and their hardware, software, information, and
services from theft, damage, disruption, and misdirection. Ensuring computer security is an essential
task, in particular in enterprises that process sensitive data and handle business transactions.
This section covers only the basic security features that you can configure after installation of the
operating system.
6.1. ENABLING THE FIREWALLD SERVICE
A firewall is a network security system that monitors and controls incoming and outgoing network traffic
according to configured security rules. A firewall typically establishes a barrier between a trusted secure
internal network and another outside network.
The firewalld service, which provides a firewall in Red Hat Enterprise Linux, is automatically enabled
during installation.
To enable the firewalld service, follow this procedure.
Procedure
Display the current status of firewalld:
$ systemctl status firewalld
● firewalld.service - firewalld - dynamic firewall daemon
Loaded: loaded (/usr/lib/systemd/system/firewalld.service; disabled; vendor preset:
enabled)
Active: inactive (dead)
...
If firewalld is not enabled and running, switch to the root user, and start the firewalld service
and enable to start it automatically after the system restarts:
# systemctl enable --now firewalld
Verification steps
Check that firewalld is running and enabled:
$ systemctl status firewalld
● firewalld.service - firewalld - dynamic firewall daemon
Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset:
enabled)
Active: active (running)
...
Additional resources
Using and configuring firewalld
man firewalld(1)
35Red Hat Enterprise Linux 9 Configuring basic system settings
6.2. MANAGING BASIC SELINUX SETTINGS
Security-Enhanced Linux (SELinux) is an additional layer of system security that determines which
processes can access which files, directories, and ports. These permissions are defined in SELinux
policies. A policy is a set of rules that guide the SELinux security engine.
SELinux has two possible states:
Disabled
Enabled
When SELinux is enabled, it runs in one of the following modes:
Enabled
Enforcing
Permissive
In enforcing mode, SELinux enforces the loaded policies. SELinux denies access based on SELinux
policy rules and enables only the interactions that are explicitly allowed. Enforcing mode is the safest
SELinux mode and is the default mode after installation.
In permissive mode, SELinux does not enforce the loaded policies. SELinux does not deny access, but
reports actions that break the rules to the /var/log/audit/audit.log log. Permissive mode is the default
mode during installation. Permissive mode is also useful in some specific cases, for example when
troubleshooting problems.
Additional resources
Using SELinux
6.3. ENSURING THE REQUIRED STATE OF SELINUX
By default, SELinux operates in enforcing mode. However, in specific scenarios, you can set SELinux to
permissive mode or even disable it.
IMPORTANT
Red Hat recommends to keep your system in enforcing mode. For debugging purposes,
you can set SELinux to permissive mode.
Follow this procedure to change the state and mode of SELinux on your system.
Procedure
1. Display the current SELinux mode:
$ getenforce
2. To temporarily set SELinux:
a. To Enforcing mode:
36CHAPTER 6. CONFIGURING SYSTEM SECURITY
# setenforce Enforcing
b. To Permissive mode:
# setenforce Permissive
NOTE
After reboot, SELinux mode is set to the value specified in the
/etc/selinux/config configuration file.
3. To set SELinux mode to persist across reboots, 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.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=enforcing
...
WARNING
Disabling SELinux reduces your system security. Avoid disabling SELinux
using the SELINUX=disabled option in the /etc/selinux/config file
because this can result in memory leaks and race conditions causing kernel
panics. Instead, disable SELinux by adding the selinux=0 parameter to the
kernel command line. For more information, see Changing SELinux modes
at boot time.
Additional resources
Changing SELinux states and modes
6.4. ADDITIONAL RESOURCES
Generating SSH key pairs
Setting an OpenSSH server for key-based authentication
Security hardening
Using SELinux
Securing networks
37Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 7. GETTING STARTED WITH MANAGING USER
ACCOUNTS
Red Hat Enterprise Linux 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.
The following are the different types of user accounts:
Normal user accounts:
Normal accounts are created for users of a particular system. Such accounts can be added,
removed, and modified during normal system administration.
System user accounts:
System user 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. For
assigning IDs, see the /etc/login.defs file.
Group:
A group is an entity which ties together multiple user accounts for a common purpose, such as
granting access to particular files.
7.1. MANAGING ACCOUNTS AND GROUPS USING COMMAND LINE
TOOLS
This section describes basic command-line tools to manage user accounts and groups.
To display user and group IDs:
$ id
uid=1000(example.user) gid=1000(example.user) groups=1000(example.user),10(wheel)
context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
To create a new user account:
# useradd example.user
38CHAPTER 7. GETTING STARTED WITH MANAGING USER ACCOUNTS
To assign a new password to a user account belonging to example.user:
# passwd example.user
To add a user to a group:
# usermod -a -G example.group example.user
Additional resources
man useradd(8), man passwd(1), and man usermod(8)
7.2. SYSTEM USER ACCOUNTS MANAGED IN THE WEB CONSOLE
With user accounts displayed in the RHEL web console you can:
Authenticate users when accessing the system.
Set the access rights to the system.
The RHEL web console displays all user accounts located in the system. Therefore, you can see at least
one user account just after the first login to the web console.
After logging into the RHEL web console, you can perform the following operations:
Create new users accounts.
Change their parameters.
Lock accounts.
Terminate user sessions.
7.3. ADDING NEW ACCOUNTS USING THE WEB CONSOLE
Use the following steps for adding user accounts to the system and setting administration rights to the
accounts through the RHEL web console.
Prerequisites
The RHEL web console must be installed and accessible. For details, see Installing the web
console.
Procedure
1. Log in to the RHEL web console.
2. Click Accounts.
3. Click Create New Account.
1. In the Full Name field, enter the full name of the user.
The RHEL web console automatically suggests a user name from the full name and fills it in the
39Red Hat Enterprise Linux 9 Configuring basic system settings
The RHEL web console automatically suggests a user name from the full name and fills it in the
User Name field. If you do not want to use the original naming convention consisting of the first
letter of the first name and the whole surname, update the suggestion.
2. In the Password/Confirm fields, enter the password and retype it for verification that your
password is correct.
The color bar placed below the fields shows you security level of the entered password, which
does not allow you to create a user with a weak password.
1. Click Create to save the settings and close the dialog box.
2. Select the newly created account.
3. Select Server Administrator in the Roles item.
Now you can see the new account in the Accounts settings and you can use the credentials to
connect to the system.
40CHAPTER 8. DUMPING A CRASHED KERNEL FOR LATER ANALYSIS
CHAPTER 8. DUMPING A CRASHED KERNEL FOR LATER
ANALYSIS
To analyze why a system crashed, you can use the kdump service to save the contents of the system’s
memory for later analysis. This section provides a brief introduction to kdump, and information about
configuring kdump using the RHEL web console or using the corresponding RHEL system role.
8.1. WHAT IS KDUMP
kdump is a service which provides a crash dumping mechanism. The service enables you to save the
contents of the system memory for analysis. kdump uses the kexec system call to boot into the second
kernel (a capture kernel) without rebooting; and then captures the contents of the crashed kernel’s
memory (a crash dump or a vmcore) and saves it into a file. The second kernel resides in a reserved part
of the system memory.
IMPORTANT
A kernel crash dump can be the only information available in the event of a system failure
(a critical bug). Therefore, operational kdump is important in mission-critical
environments. Red Hat advise that system administrators regularly update and test
kexec-tools in your normal kernel update cycle. This is especially important when new
kernel features are implemented.
You can enable kdump for all installed kernels on a machine or only for specified kernels. This is useful
when there are multiple kernels used on a machine, some of which are stable enough that there is no
concern that they could crash.
When kdump is installed, a default /etc/kdump.conf file is created. The file includes the default
minimum kdump configuration. You can edit this file to customize the kdump configuration, but it is not
required.
8.2. CONFIGURING KDUMP MEMORY USAGE AND TARGET LOCATION
IN WEB CONSOLE
The procedure below shows you how to use the Kernel Dump tab in the RHEL web console interface to
configure the amount of memory that is reserved for the kdump kernel. The procedure also describes
how to specify the target location of the vmcore dump file and how to test your configuration.
Procedure
1. Open the Kernel Dump tab and start the kdump service.
2. Configure the kdump memory usage using the command line.
3. Click the link next to the Crash dump location option.
41Red Hat Enterprise Linux 9 Configuring basic system settings
4. Select the Local Filesystem option from the drop-down and specify the directory you want to
save the dump in.
Alternatively, select the Remote over SSH option from the drop-down to send the vmcore
to a remote machine using the SSH protocol.
Fill the Server, ssh key, and Directory fields with the remote machine address, ssh key
location, and a target directory.
Another choice is to select the Remote over NFS option from the drop-down and fill the
Mount field to send the vmcore to a remote machine using the NFS protocol.
NOTE
Tick the Compression check box to reduce the size of the vmcore file.
5. Test your configuration by crashing the kernel.
42CHAPTER 8. DUMPING A CRASHED KERNEL FOR LATER ANALYSIS
a. Click Test configuration.
b. In the Test kdump settings field, click Crash system.
WARNING
This step disrupts execution of the kernel and results in a system crash
and loss of data.
Additional resources
Supported kdump targets
Using secure communications between two systems with OpenSSH
8.3. KDUMP USING RHEL SYSTEM ROLES
RHEL System Roles is a collection of Ansible roles and modules that provide a consistent configuration
interface to remotely manage multiple RHEL systems. The kdump role enables you to set basic kernel
dump parameters on multiple systems.
WARNING
The kdump role replaces the kdump configuration of the managed hosts entirely
by replacing the /etc/kdump.conf file. Additionally, if the kdump role is applied, all
previous kdump settings are also replaced, even if they are not specified by the role
variables, by replacing the /etc/sysconfig/kdump file.
The following example playbook shows how to apply the kdump system role to set the location of the
43Red Hat Enterprise Linux 9 Configuring basic system settings
The following example playbook shows how to apply the kdump system role to set the location of the
crash dump files:
---
- hosts: kdump-test
vars:
kdump_path: /var/crash
roles:
- rhel-system-roles.kdump
For a detailed reference on kdump role variables, install the rhel-system-roles package, and see the
README.md or README.html files in the /usr/share/doc/rhel-system-roles/kdump directory.
Additional resources
Introduction to RHEL System Roles
8.4. ADDITIONAL RESOURCES
Installing kdump
Configuring kdump on the command line
Configuring kdump in the web console
44CHAPTER 9. RECOVERING AND RESTORING A SYSTEM
CHAPTER 9. RECOVERING AND RESTORING A SYSTEM
To recover and restore a system using an existing backup, Red Hat Enterprise Linux provides the Relax-
and-Recover (ReaR) utility.
You can use the utility as a disaster recovery solution and also for system migration.
The utility enables you to perform the following tasks:
Produce a bootable image and restore the system from an existing backup, using the image.
Replicate the original storage layout.
Restore user and system files.
Restore the system to a different hardware.
Additionally, for disaster recovery, you can also integrate certain backup software with ReaR.
Setting up ReaR involves the following high-level steps:
1. Install ReaR.
2. Modify ReaR configuration file, to add backup method details.
3. Create rescue system.
4. Generate backup files.
9.1. SETTING UP REAR
Use the following steps to install the package for using the Relax-and-Recover (ReaR) utility, create a
rescue system, configure and generate a backup.
Prerequisites
Necessary configurations as per the backup restore plan are ready.
Note that you can use the NETFS backup method, a fully-integrated and built-in method with
ReaR.
Procedure
1. Install the ReaR utility by running the following command:
# dnf install rear
2. Modify the ReaR configuration file in an editor of your choice, for example:
# vi /etc/rear/local.conf
3. Add the backup setting details to /etc/rear/local.conf. For example, in the case of the NETFS
backup method, add the following lines:
BACKUP=NETFS
BACKUP_URL=backup.location
45Red Hat Enterprise Linux 9 Configuring basic system settings
Replace backup.location by the URL of your backup location.
4. To configure ReaR to keep the previous backup archive when the new one is created, also add
the following line to the configuration file:
NETFS_KEEP_OLD_BACKUP_COPY=y
5. To make the backups incremental, meaning that only the changed files are backed up on each
run, add the following line:
BACKUP_TYPE=incremental
6. Create a rescue system:
# rear mkrescue
7. Take a backup as per the restore plan. For example, in the case of the NETFS backup method,
run the following command:
# rear mkbackuponly
Alternatively, you can create the rescue system and the backup in a single step by running the
following command:
# rear mkbackup
This command combines the functionality of the rear mkrescue and rear mkbackuponly
commands.
9.2. USING A REAR RESCUE IMAGE ON THE 64-BIT IBM Z
ARCHITECTURE
Basic Relax and Recover (ReaR) functionality is now available on the 64-bit IBM Z architecture as a
Technology Preview. You can create a ReaR rescue image on IBM Z only in the z/VM environment.
Backing up and recovering logical partitions (LPARs) has not been tested.
IMPORTANT
ReaR on the 64-bit IBM Z architecture is a Technology Preview feature only. Technology
Preview features are not supported with Red Hat production service level agreements
(SLAs) and might not be functionally complete. Red Hat does not recommend using
them in production. These features provide early access to upcoming product features,
enabling customers to test functionality and provide feedback during the development
process. For more information about the support scope of Red Hat Technology Preview
features, see https://access.redhat.com/support/offerings/techpreview.
The only output method currently available is Initial Program Load (IPL). IPL produces a kernel and an
initial ramdisk (initrd) that can be used with the zIPL bootloader.
Prerequisites
46CHAPTER 9. RECOVERING AND RESTORING A SYSTEM
ReaR is installed.
To install ReaR, run the dnf install rear command
Procedure
Add the following variables to the /etc/rear/local.conf to configure ReaR for producing a rescue image
on the 64-bit IBM Z architecture:
1. To configure the IPL output method, add OUTPUT=IPL.
2. To configure the backup method and destination, add BACKUP and BACKUP_URL variables.
For example:
BACKUP=NETFS
BACKUP_URL=nfs:///
IMPORTANT
The local backup storage is currently not supported on the 64-bit IBM Z
architecture.
3. Optionally, you can also configure the OUTPUT_URL variable to save the kernel and initrd files.
By default, the OUTPUT_URL is aligned with BACKUP_URL.
4. To perform backup and rescue image creation:
rear mkbackup
5. This creates the kernel and initrd files at the location specified by the BACKUP_URL or
OUTPUT_URL (if set) variable, and a backup using the specified backup method.
6. To recover the system, use the ReaR kernel and initrd files created in step 3, and boot from a
Direct Attached Storage Device (DASD) or a Fibre Channel Protocol (FCP)-attached SCSI
device prepared with the zipl boot loader, kernel, and initrd. For more information, see Using a
Prepared DASD.
7. When the rescue kernel and initrd get booted, it starts the ReaR rescue environment. Proceed
with system recovery.
WARNING
Currently, the rescue process reformats all the DASDs (Direct Attached Storage
Devices) connected to the system. Do not attempt a system recovery if there is any
valuable data present on the system storage devices. This also includes the device
prepared with the zipl bootloader, ReaR kernel, and initrd that were used to boot
into the rescue environment. Ensure to keep a copy.
Additional resources
47Red Hat Enterprise Linux 9 Configuring basic system settings
Installing under z/VM
Using a Prepared DASD
48CHAPTER 10. TROUBLESHOOTING PROBLEMS USING LOG FILES
CHAPTER 10. TROUBLESHOOTING PROBLEMS USING LOG
FILES
Log files contain messages about the system, including the kernel, services, and applications running on
it. These contain information that helps troubleshoot issues or monitor system functions. The logging
system in Red Hat Enterprise Linux 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.
10.1. SERVICES HANDLING SYSLOG MESSAGES
The following two services handle syslog messages:
The systemd-journald daemon
The Rsyslog service
The systemd-journald daemon collects messages from various sources and forwards them to Rsyslog
for further processing. The systemd-journald daemon collects messages from the following sources:
Kernel
Early stages of the boot process
Standard and error output of daemons as they start up and run
Syslog
The Rsyslog service sorts the syslog messages by type and priority and writes them to the files in the
/var/log directory. The /var/log directory persistently stores the log messages.
10.2. SUBDIRECTORIES STORING SYSLOG MESSAGES
The following subdirectories under the /var/log directory store syslog messages.
/var/log/messages - all syslog messages except the following
/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
10.3. INSPECTING LOG FILES USING THE WEB CONSOLE
Follow the steps in this procedure to inspect the log files using the RHEL web console.
Procedure
1. Log into the RHEL web console. For details see Logging in to the web console .
2. Click Logs.
Figure 10.1. Inspecting the log files in the RHEL 9 web console
49Red Hat Enterprise Linux 9 Configuring basic system settings
Figure 10.1. Inspecting the log files in the RHEL 9 web console
10.4. VIEWING LOGS USING THE COMMAND LINE
The Journal is a component of systemd that helps to view and manage log files. It addresses problems
connected with traditional logging, closely integrated with the rest of the system, and supports various
logging technologies and access management for the log files.
You can use the journalctl command to view messages in the system journal using the command line,
for example:
$ journalctl -b | grep kvm
May 15 11:31:41 localhost.localdomain kernel: kvm-clock: Using msrs 4b564d01 and 4b564d00
May 15 11:31:41 localhost.localdomain kernel: kvm-clock: cpu 0, msr 76401001, primary cpu clock
...
Table 10.1. Viewing system information
Command Description
journalctl Shows all collected journal entries.
journalctl FILEPATH Shows logs related to a specific file. For example, the
journalctl /dev/sda command displays logs related
to the /dev/sda file system.
journalctl -b Shows logs for the current boot.
journalctl -k -b -1 Shows kernel logs for the current boot.
Table 10.2. Viewing information on specific services
Command Description
journalctl -b _SYSTEMD_UNIT=foo Filters log to see ones matching the "foo" systemd
service.
journalctl -b _SYSTEMD_UNIT=foo Combines matches. For example, this command
_PID=number shows logs for systemd-units that match foo and
the PID number.
50CHAPTER 10. TROUBLESHOOTING PROBLEMS USING LOG FILES
Command Description
journalctl -b _SYSTEMD_UNIT=foo The separator “+” combines two expressions in a
_PID=number + _SYSTEMD_UNIT=foo1 logical OR. For example, this command shows all
messages from the foo service process with the PID
plus all messages from the foo1 service (from any of
its processes).
journalctl -b _SYSTEMD_UNIT=foo This command shows all entries matching either
_SYSTEMD_UNIT=foo1 expression, referring to the same field. Here, this
command shows logs matching a systemd-unit foo
or a systemd-unit foo1.
Table 10.3. Viewing logs related to specific boots
Command Description
journalctl --list-boots Shows a tabular list of boot numbers, their IDs, and
the timestamps of the first and last message
pertaining to the boot. You can use the ID in the next
command to view detailed information.
journalctl --boot=ID _SYSTEMD_UNIT=foo Shows information about the specified boot ID.
10.5. ADDITIONAL RESOURCES
man journalctl(1)
Configuring a remote logging solution
51Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 11. ACCESSING THE RED HAT SUPPORT
This section describes how to effectively troubleshoot your problems using Red Hat support and
sosreport.
To obtain support from Red Hat, use the Red Hat Customer Portal , which provides access to everything
available with your subscription.
11.1. OBTAINING RED HAT SUPPORT THROUGH RED HAT CUSTOMER
PORTAL
The following section describes how to use the Red Hat Customer Portal to get help.
Prerequisites
A valid user account on the Red Hat Customer Portal. See Create a Red Hat Login .
An active subscription for the RHEL system.
Procedure
1. Access Red Hat support:
a. Open a new support case.
b. Initiate a live chat with a Red Hat expert.
c. Contact a Red Hat expert by making a call or sending an email.
11.2. TROUBLESHOOTING PROBLEMS USING SOSREPORT
The sosreport command collects configuration details, system information and diagnostic information
from a Red Hat Enterprise Linux system.
The following section describes how to use the sosreport command to produce reports for your support
cases.
Prerequisites
A valid user account on the Red Hat Customer Portal. See Create a Red Hat Login .
An active subscription for the RHEL system.
A support-case number.
Procedure
1. Install the sos package:
# dnf install sos
NOTE
52CHAPTER 11. ACCESSING THE RED HAT SUPPORT
NOTE
The default minimal installation of Red Hat Enterprise Linux does not include the
sos package, which provides the sosreport command.
2. Generate a report:
# sosreport
3. Attach the report to your support case.
See the How can I attach a file to a Red Hat support case? Red Hat Knowledgebase article for
more information.
Note that when attaching the report, you are prompted to enter the number of the relevant
support case.
Additional resources
What is an sosreport and how to create one in Red Hat Enterprise Linux?
53Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 12. 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.
Starting with Red Hat Enterprise Linux 7, systemd replaced 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 the following table:
Table 12.1. systemd unit files locations
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.
The units encapsulate information about:
System services
Listening sockets
Other objects that are relevant to the init system
The default configuration of systemd is defined during the compilation and it can be found in the
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=pass:_required value_
12.1. SYSTEMD UNIT TYPES
For a complete list of available systemd unit types, see the following table:
Table 12.2. Available systemd unit types
Unit Type File Extension Description
Service unit .service A system service.
54CHAPTER 12. INTRODUCTION TO SYSTEMD
Unit Type File Extension Description
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.
Socket unit .socket An inter-process communication
socket.
Swap unit .swap A swap device or a swap file.
Timer unit .timer A systemd timer.
12.2. SYSTEMD MAIN FEATURES
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
55Red Hat Enterprise Linux 9 Configuring basic system settings
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.
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.
12.3. 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 Red Hat
Enterprise Linux 6 system that used SysV init:
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 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 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.
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.
56CHAPTER 12. INTRODUCTION TO SYSTEMD
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 Changing the timeout limit .
57Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 13. MANAGING SYSTEM SERVICES WITH
SYSTEMCTL
The systemctl utility helps managing system services. You can use the systemctl utility to perform
different tasks related to different services such as starting, stopping, restarting, enabling, and disabling
services, listing services, and displaying system services statuses.
This section describes how to manage system services with the systemctl utility.
13.1. SERVICE UNIT MANAGEMENT WITH SYSTEMCTL
The service units help to control the state of services and daemons in your system.
Service units end with the .service file extension, for example nfs-server.service. However, while using
service file names in commands, you can omit the file extension. The systemctl utility assumes the
argument is a service unit. For example, to stop the nfs-server.service, enter the following command:
# systemctl stop nfs-server
Additionally, some service units have alias names. Aliases can be shorter than units, and you can use
them instead of the actual unit names.
To find all aliases that can be used for a particular unit, use:
# systemctl show nfs-server.service -p Names
Additional resources
Comparison of a service utility with systemctl
Listing system services
Displaying system service status
Starting a system service
Restarting a system service
Enabling a system service
Disabling a system service
Stopping a system service
13.2. COMPARISON OF A SERVICE UTILITY WITH SYSTEMCTL
This section shows a comparison between a service utility and the usage of systemctl command.
Table 13.1. Comparison of the service utility with systemctl
58CHAPTER 13. MANAGING SYSTEM SERVICES WITH SYSTEMCTL
service systemctl Description
service start systemctl start Starts a service.
.service
service stop systemctl stop Stops a service.
.service
service restart systemctl restart Restarts a service.
.service
service condrestart systemctl try-restart Restarts a service only if it is
.service running.
service reload systemctl reload Reloads configuration.
.service
service status systemctl status Checks if a service is running.
.service
systemctl is-active
.service
service --status-all systemctl list-units --type Displays the status of all services.
service --all
13.3. LISTING SYSTEM SERVICES
You can list all currently loaded service units and the status of all available service units.
Procedure
To list all currently loaded service units, enter:
$ 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
abrtd.service loaded active running ABRT Automated Bug Reporting Tool
----
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''
59Red Hat Enterprise Linux 9 Configuring basic system settings
By default, the systemctl list-units command displays only active units. For each service unit
file, the command displays:
UNIT: its full name
LOAD: information whether the unit file has been loaded
ACTIVE\ SUB: its high-level and low-level unit file activation state
DESCRIPTION: a short description
To list all loaded units regardless of their state, enter the following command with the --all or
-a command line option:
$ systemctl list-units --type service --all
To list the status (enabled / disabled) of all available service units, enter:
$ systemctl list-unit-files --type service
UNIT FILE STATE
abrt-ccpp.service enabled
abrt-oops.service enabled
abrtd.service enabled
...
wpa_supplicant.service disabled
ypbind.service disabled
208 unit files listed.
For each service unit, this command displays:
UNIT FILE: its full name
STATE: information whether the service unit is enabled or disabled
Additional resources
Displaying system service status
13.4. DISPLAYING SYSTEM SERVICE STATUS
You can inspect any service unit to get its detailed information and verify the state of the service
whether it is enabled or running. You can also view services that are ordered to start after or before a
particular service unit.
Procedure
To display detailed information about a service unit that corresponds to a system service, enter:
$ systemctl status .service
Replace 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,
60CHAPTER 13. MANAGING SYSTEM SERVICES WITH SYSTEMCTL
This command displays the name of the selected service unit followed by its short description,
one or more fields described in Available service unit information , if it is executed by the root
user, and the most recent log entries.
Table 13.2. 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.
Status Additional information about the corresponding
system service.
Process Additional information about related processes.
CGroup Additional information about related Control
Groups (cgroups).
Example 13.1. 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.
To only verify that a particular service unit is running, enter:
$ systemctl is-active .service
To determine whether a particular service unit is enabled, enter:
$ systemctl is-enabled .service
NOTE
61Red Hat Enterprise Linux 9 Configuring basic system settings
NOTE
Both systemctl is-active and systemctl is-enabled return an exit status of 0 if
the specified service unit is running or enabled.
To determine what services are ordered to start before the specified service unit, enter:
# systemctl list-dependencies --after .service
Replace with the name of the service in the command. For example, to view the list of
services ordered to start before gdm, enter:
# systemctl list-dependencies --after gdm.service
gdm.service
├─dbus.socket
├─getty@tty1.service
├─livesys.service
├─plymouth-quit.service
├─system.slice
├─systemd-journald.socket
├─systemd-user-sessions.service
└─basic.target
[output truncated]
To determine what services are ordered to start after the specified service unit, enter:
# systemctl list-dependencies --before .service
Replace with the name of the service in the command. For example, to view the list of
services ordered to start after gdm, enter:
# 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
Additional resources
Listing system services
13.5. POSITIVE AND NEGATIVE SERVICE DEPENDENCIES
In systemd, positive and negative dependencies between services exist. Starting a 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, without
62CHAPTER 13. MANAGING SYSTEM SERVICES WITH SYSTEMCTL
When you attempt to start a new service, systemd resolves all dependencies automatically, without
explicit notification to the user. This means that 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 attempt to start the sendmail service,
systemd first automatically stops postfix, because these two services are conflicting and cannot run on
the same port.
Additional resources
Starting a system service
13.6. STARTING A SYSTEM SERVICE
You can start system service in the current session using the start command. You must have a root
access as starting a service may affect the state of the operating system.
Procedure
To start a selected service unit corresponding to a system service, type the following command
as root:
# systemctl start .service
Replace with the name of the service unit you want to start (for example,
httpd.service).
Example 13.2. Starting httpd.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, enter the following command as root:
# systemctl start httpd.service
Additional resources
Positive and negative service dependencies
Enabling a system service
Displaying system service status
13.7. STOPPING A SYSTEM SERVICE
You can stop system service in the current session using the stop command. You must have a root
access as stopping a service may affect the state of the operating system.
Procedure
To stop the service unit corresponding to a system service, enter the following command as
root:
63Red Hat Enterprise Linux 9 Configuring basic system settings
# systemctl stop .service
Replace with the name of the service unit you want to stop (for example, bluetooth).
Example 13.3. Stopping bluetoothd.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, enter the following
command as root:
# systemctl stop bluetooth.service
Additional resources
Disabling a system service
Displaying system service status
13.8. RESTARTING A SYSTEM SERVICE
You can restart system service in the current session using the restart command. You must have a root
access as restarting a service may affect the state of the operating system.
This procedure describes how to:
Stop the selected service unit in the current session and immediately start it again
Restart a service unit only if the corresponding service is already running
Reload configuration of a system service without interrupting its execution
Procedure
To restart a service unit corresponding to a system service, enter the following command as
root:
# systemctl restart .service
Replace with the name of the service unit you want to restart (for example, httpd).
NOTE
If the selected service unit is not running, this command starts it too.
Alternatively, to restart a service unit only if the corresponding service is already running,
enter the following command as root:
# systemctl try-restart .service
To reload the configuration without interrupting service execution, enter the following
command as root:
64CHAPTER 13. MANAGING SYSTEM SERVICES WITH SYSTEMCTL
# systemctl reload .service
NOTE
System services that do not support this feature, ignore this command. To
restart such services, use the reload-or-restart and reload-or-try-restart
commands instead.
Example 13.4. Reloading httpd.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, enter the
following as root:
# systemctl reload httpd.service
Additional resources
Displaying system service status
13.9. ENABLING A SYSTEM SERVICE
You can configure service to start automatically at the system booting time. The enable 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 sub-
directories. However, it does not rewrite links that already exist.
Procedure
To configure a service unit that corresponds to a system service to be automatically started at
boot time, enter the following command as root:
# systemctl enable .service
Replace with the name of the service unit you want to enable (for example, httpd).
If you want to ensure that the symbolic links are re-created, enter the following command as
root:
# systemctl reenable .service
This command disables the selected service unit and immediately enables it again.
Example 13.5. Enabling httpd.service
To configure the Apache HTTP Server to start automatically at boot time, enter the
following command as root:
65Red Hat Enterprise Linux 9 Configuring basic system settings
# systemctl enable httpd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to
/usr/lib/systemd/system/httpd.service.
Additional resources
Displaying system service status
Starting a system service
13.10. DISABLING A SYSTEM SERVICE
You can prevent a service unit from starting automatically at boot time. The disable 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 sub-
directories.
Procedure
To configure a service unit that corresponds to a system service not to start automatically at
boot time, enter the following command as root:
# systemctl disable .service
Replace with the name of the service unit you want to disable (for example, bluetooth).
Example 13.6. Disabling bluetoothd.service
The service unit for the bluetoothd daemon is named bluetooth.service. To prevent this
service unit from starting at boot time, enter the following command as a root:
# systemctl disable bluetooth.service
Removed symlink /etc/systemd/system/bluetooth.target.wants/bluetooth.service.
Removed symlink /etc/systemd/system/dbus-org.bluez.service.
To mask any service unit and prevent it from being started manually or by another service,
enter the following command as root:
# systemctl mask .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, enter:
# systemctl unmask .service
Additional resources
Displaying system service status
66CHAPTER 13. MANAGING SYSTEM SERVICES WITH SYSTEMCTL
Stopping a system service
67Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 14. WORKING WITH SYSTEMD TARGETS
systemd targets are represented by target units. Target units file ends 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.
This section includes procedures to implement while working with systemd targets.
14.1. DIFFERENCE BETWEEN SYSV RUNLEVELS AND SYSTEMD
TARGETS
The previous versions of Red Hat Enterprise Linux were distributed with SysV init or Upstart, and
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. Starting with Red Hat Enterprise Linux 7,
the concept of runlevels has been replaced with systemd targets.
Red Hat Enterprise Linux 7 was distributed with a number of predefined targets that are more or less
similar to the standard set of runlevels from the previous releases. For compatibility reasons, it also
provides aliases for these targets that directly maps to the SysV runlevels.
The following table provides a complete list of SysV runlevels and their corresponding systemd targets:
Table 14.1. Comparison of SysV runlevels with systemd targets
Runlevel Target Units Description
0 runlevel0.target, Shut down and power off the
poweroff.target system.
1 runlevel1.target, Set up a rescue shell.
rescue.target
2 runlevel2.target, multi- Set up a non-graphical multi-user
user.target system.
3 runlevel3.target, multi- Set up a non-graphical multi-user
user.target system.
4 runlevel4.target, multi- Set up a non-graphical multi-user
user.target system.
5 runlevel5.target, Set up a graphical multi-user
graphical.target system.
6 runlevel6.target, Shut down and reboot the system.
reboot.target
68CHAPTER 14. WORKING WITH SYSTEMD TARGETS
The following table compares the SysV init commands with systemctl. Use the systemctl utility to view,
change, or configure systemd targets:
IMPORTANT
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 14.2. Comparison of SysV init commands with systemctl
Old Command New Command Description
runlevel systemctl list-units --type Lists currently loaded target units.
target
telinit runlevel systemctl isolate Changes the current target.
name.target
Additional resources
man sysv init
man upstart init
man systemctl
14.2. VIEWING THE DEFAULT TARGET
The default target unit is represented by the /etc/systemd/system/default.target file.
Procedure
To determine which target unit is used by default:
$ systemctl get-default
graphical.target
To determine the default target using the symbolic link:
$ ls -l /usr/lib/systemd/system/default.target
= Viewing the target units
By default, the systemctl list-units command displays only active units.
Procedure
To list all loaded units regardless of their state:
$ systemctl list-units --type target --all
To list all currently loaded target units:
69Red Hat Enterprise Linux 9 Configuring basic system settings
$ 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.
14.2.1. Changing the default target
The default target unit is represented by the /etc/systemd/system/default.target file. The following
procedure describes how to change the default target by using the systemctl command:
Procedure
1. To determine the default target unit:
# systemctl get-default
2. To configure the system to use a different target unit by default:
# 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
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. Replace multi-user with the name of the target unit you want to use by default.
Table 14.3. Common targets for set-default command
basic unit target covering basic boot-up
70CHAPTER 14. WORKING WITH SYSTEMD TARGETS
rescue unit target that pulls in the base system and
spawns a rescue shell
multi-user unit target for setting up a multi-user system
graphical unit target for setting up a graphical login screen
emergency unit target that starts an emergency shell on the
main console
sysinit unit target that pulls in the services required for
system initialization
3. Reboot
# reboot
Additional resources
For more details about target units see systemd.special manpage
bootup man page
14.2.2. Changing the default target using symbolic link
The following procedure describes how to change the default target by creating a symbolic link to the
target.
Procedure
1. Determine the default target unit:
# ls -l /etc/systemd/system/default.target
Note that in certain cases, the /etc/systemd/system/default.target link might not exist, and
systemd looks for the default target unit in /usr. In such cases, determine the default target unit
using the following command:
# ls -l /usr/lib/systemd/system/default.target
2. Delete the /etc/systemd/system/default.target link:
# rm /etc/systemd/system/default.target
3. Create a symbolic link:
# ln -sf /usr/lib/systemd/system/graphical.target /etc/systemd/system/default.target
4. Reboot the system:
71Red Hat Enterprise Linux 9 Configuring basic system settings
# reboot
Verification steps
Verify the newly created default.target:
$ systemctl get-default
multi-user.target
14.2.3. Changing the current target
This procedure explains how to change the target unit in the current session using the systemctl
command.
Procedure
To change to a different target unit in the current session:
# systemctl isolate multi-user.target
This command starts the target unit named multi-user and all dependent units, and immediately
stops all others.
Replace multi-user with the name of the target unit you want to use by default.
Verification steps
Verify the newly created default.target:
$ systemctl get-default
multi-user.target
= Booting 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.
Procedure
To change the current target and enter rescue mode in the current session:
# 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!
NOTE
72CHAPTER 14. WORKING WITH SYSTEMD TARGETS
NOTE
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 a message, run the following command with
the --no-wall command-line option: # systemctl --no-wall rescue
14.2.3.1. Booting 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.
Procedure
To change the current target and enter emergency mode:
# systemctl emergency
NOTE
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 the following command with
the --no-wall command-line option: # systemctl --no-wall emergency
73Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 15. SHUTTING DOWN, SUSPENDING, AND
HIBERNATING THE SYSTEM
This section contains instructions about shutting down, suspending, or hibernating your operating
system.
15.1. SYSTEM SHUTDOWN
To shut down the system, you can either use the systemctl utility directly, or call this utility through the
shutdown command.
The advantage of using the shutdown command is:
The support for time argument
This is particularly useful for scheduled maintenance. Also, users have more time to react to the
warning that a system shutdown has been scheduled.
The option to cancel the shutdown
Additional resources
Shutting down the system using the shutdown command
Shutting down the system using the systemctl command
Overview of the power management commands with systemctl
15.2. SHUTTING DOWN THE SYSTEM USING THE SHUTDOWN
COMMAND
By following this procedure, you can use the shutdown command to perform various operations. You
can either shut down the system and power off the machine at a certain time, or shut down and halt the
system without powering off the machine, or cancel a pending shutdown.
Prerequisites
Switch to the root user
Procedure
To shut down the system and power off the machine at a certain time, use the command in the
following format:
shutdown --poweroff hh:mm
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 wall message can be appended to the command.
Alternatively, to shut down and halt the system after a delay, without powering off the machine,
use:
74CHAPTER 15. SHUTTING DOWN, SUSPENDING, AND HIBERNATING THE SYSTEM
shutdown --halt +m
Where +m is the delay time in minutes. The now keyword is an alias for +0.
To cancel a pending shutdown, use:
shutdown -c
Additional resources
shutdown(8) manual page
Shutting down the system using the systemctl command
15.3. SHUTTING DOWN THE SYSTEM USING THE SYSTEMCTL
COMMAND
By following this procedure, you can use the systemctl command to perform various operations. You
can either shut down the system and power off the machine, or shut down and halt the system without
powering off the machine.
Prerequisites
Switch to the root user
Procedure
To shut down the system and power off the machine, use the command in the following format:
systemctl poweroff
Alternatively, to shut down and halt the system without powering off the machine, use:
systemctl halt
NOTE
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.
Additional resources
Shutting down the system using the shutdown command
15.4. RESTARTING THE SYSTEM
You can restart the system by following this procedure.
Prerequisites
75Red Hat Enterprise Linux 9 Configuring basic system settings
Switch to the root user
Procedure
To restart the system, run the following command:
systemctl reboot
NOTE
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.
15.5. SUSPENDING THE SYSTEM
You can suspend the system by following this procedure.
Prerequisites
Switch to the root user.
Procedure
To suspend the system, run the following command:
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 from hibernation. However, note that the suspended
system state is also vulnerable to power outages.
Additional resources
Hibernating the system
15.6. HIBERNATING THE SYSTEM
By following this procedure, you can either hibernate the system, or hibernate and suspend the system.
Prerequisites
Switch to the root user.
Procedure
To hibernate the system, run the following command:
76CHAPTER 15. SHUTTING DOWN, SUSPENDING, AND HIBERNATING THE SYSTEM
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 maintain electrical power to the RAM module. However, as a consequence, restoring the
system from hibernation is significantly slower than restoring it from suspend mode.
Alternatively, to hibernate and suspend the system, run the following command:
systemctl hybrid-sleep
Additional resources
Suspending the system
15.7. OVERVIEW OF THE POWER MANAGEMENT COMMANDS WITH
SYSTEMCTL
You can use the following list of the systemctl commands to control the power management of your
system.
Table 15.1. Overview of the systemctl power management commands
systemctl command Description
systemctl halt Halts the system.
systemctl poweroff Powers off the system.
systemctl reboot Restarts the system.
systemctl suspend Suspends the system.
systemctl hibernate Hibernates the system.
systemctl hybrid-sleep Hibernates and suspends the system.
77Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
This chapter includes the description of systemd unit files. The following sections show you how to:
Create custom unit files
Convert SysV init scripts to unit files
Modify existing unit files
Work with instantiated units
16.1. INTRODUCTION TO 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. 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. For a
complete list of unit types, see systemd unit files
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 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 or at
runtime based on [Unit] options. It is also possible to create these directories and symbolic links
manually. For more details on [Install] and [Unit] options, see the tables below.
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 Working with instantiated units .
16.2. UNIT FILE STRUCTURE
Unit files typically consist of three sections:
The [Unit] section — 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 Important [Unit] section
options.
The [Unit type] section — 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.
78CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
The [Install] section — contains information about unit installation used by systemctl enable
and disable commands. For a list of options for the [Install] section, see Important [Install]
section options.
Additional resources
Important [Unit] section options
Important [Service] section options
Important [Install] section options
16.3. IMPORTANT [UNIT] SECTION OPTIONS
The following tables lists important options of the [Unit] section.
Table 16.1. Important [Unit] section options
Option [a] 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.
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] section, see the systemd.unit(5) manual page.
[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.
79Red Hat Enterprise Linux 9 Configuring basic system settings
16.4. IMPORTANT [SERVICE] SECTION OPTIONS
The following tables lists important options of the [Service] section.
Table 16.2. Important [Service] section options
Option [a] Description
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 with ExecStart
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 to simple, but the
process exits before starting consequent units.
* dbus – This type is similar to simple, but
consequent units are started only after the main
process gains a D-Bus name.
* notify – This type is similar to simple, 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.
80CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
Option [a] Description
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.
[a] For a complete list of options configurable in the [Service] section, see the systemd.service(5) manual page.
16.5. IMPORTANT [INSTALL] SECTION OPTIONS
The following tables lists important options of the [Install] section.
Table 16.3. Important [Install] section options
Option [a] 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
Working with instantiated units.
[a] For a complete list of options configurable in the [Install] section, see the systemd.unit(5) manual page.
16.6. 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 Creating a custom unit file by using the second instance
of the sshd service
On the other hand, if you intend just to modify or extend the behavior of an existing unit, use the
81Red Hat Enterprise Linux 9 Configuring basic system settings
On the other hand, if you intend just to modify or extend the behavior of an existing unit, use the
instructions from Modifying existing unit files .
Procedure
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 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
[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
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.
82CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
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. Note, that on systems with
a large number of units this can take a long time, as the state of each unit
has to be serialized and subsequently deserialized during the reload.
16.7. CREATING A CUSTOM UNIT FILE BY USING THE 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.
Procedure
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
4. Alter the sshd-second.service created in the previous step as follows:
a. Modify the Description option:
Description=OpenSSH server second instance daemon
83Red Hat Enterprise Linux 9 Configuring basic system settings
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]
WantedBy=multi-user.target
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
7. Verify if the sshd-second.service is running by using the systemctl status command.
8. 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.
16.8. 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
84CHAPTER 16. WORKING WITH SYSTEMD 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 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 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.
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 $remote_fs
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# Short-Description: start and stop postfix
# Description: Postfix is a Mail Transport Agent, which is the program that moves mail from one
machine to another.
### END INIT INFO
In the above example, only lines starting with # chkconfig and # description are mandatory, so you
might not find the rest in different init files. The text enclosed between the BEGIN INIT INFO and END
INIT INFO lines is called Linux Standard Base (LSB) header. If specified, LSB headers contain
directives defining the service description, dependencies, and default runlevels. What follows is an
overview of analytic tasks aiming to collect the data needed for a new unit file. The postfix init script is
used as an example.
16.9. FINDING THE SYSTEMD SERVICE DESCRIPTION
You can find descriptive information about the script on the line starting with #description. Use this
description together with the service name in the Description option in the [Unit] section of the unit file.
The LSB header might contain similar data on the #Short-Description and #Description lines.
16.10. FINDING THE SYSTEMD SERVICE DEPENDENCIES
The LSB header might contain several directives that form dependencies between services. Most of
them are translatable to systemd unit options, see the following table:
Table 16.4. Dependency options from the LSB header
85Red Hat Enterprise Linux 9 Configuring basic system settings
LSB Option Description Unit File Equivalent
Provides Specifies the boot facility name –
of the service, that can be
referenced in other init scripts
(with the "$" prefix). This is no
longer needed as unit files refer to
other units by their file names.
Required-Start Contains boot facility names of After, Before
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.
Should-Start Constitutes weaker dependencies After, Before
than Required-Start. Failed
Should-Start dependencies do
not affect the service startup.
Required-Stop, Should-Stop Constitute negative Conflicts
dependencies.
16.11. FINDING DEFAULT TARGETS OF THE SERVICE
The line starting with #chkconfig contains three numerical values. The most important is the first
number that represents the default runlevels in which the service is started. 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. Note that the graphical.target depends on multiuser.target, therefore it
is not necessary to specify both. 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.
16.12. 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,
86CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
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
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. The /usr/sbin/postfix
together with supporting scripts are executed on service start. 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.
87Red Hat Enterprise Linux 9 Configuring basic system settings
16.13. 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.
Procedure
1. 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 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 Overriding the default unit
configuration for details.
2. To return to the default configuration of the unit, delete custom-created configuration files in
/etc/systemd/system/.
3. To apply changes to unit files without rebooting the system, execute:
systemctl daemon-reload
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, which must be executed under the root user:
init q
4. If the modified unit file belongs to a running service, this service must be restarted to accept
new settings:
systemctl restart name.service
IMPORTANT
88CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
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: Extending the default unit configuration
and 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
16.14. EXTENDING THE DEFAULT UNIT CONFIGURATION
This section describes how to extend the default unit file with additional configuration options.
Procedure
1. 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.
2. 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 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
89Red Hat Enterprise Linux 9 Configuring basic system settings
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.
3. To apply changes made to the unit, execute as root:
systemctl daemon-reload
systemctl restart name.service
Example 16.1. Extending the httpd.service configuration
To modify the httpd.service unit so that a custom shell script is automatically executed when starting
the Apache service, perform the following steps.
1. 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
2. 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
3. To apply the unit changes, execute:
# systemctl daemon-reload
# 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 Monitoring overridden units ,such units are always marked
as [EXTENDED], even though in sum, certain options are actually overridden.
16.15. OVERRIDING THE DEFAULT UNIT CONFIGURATION
This section describes how to override the default unit configuration.
Procedure
1. 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
90CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
Where name stands for the name of the service unit you wish to modify. The above syntax
applies to all unit types.
2. Open the copied file with a text editor, and make the desired changes. To apply the unit
changes, execute as root:
systemctl daemon-reload
systemctl restart name.service
16.16. 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:
Procedure
1. Copy the httpd unit file to the /etc/systemd/system/ directory:
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]
WantedBy=multi-user.target
…
3. Reload the systemd daemon:
systemctl daemon-reload
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.
16.17. MONITORING OVERRIDDEN UNITS
91Red Hat Enterprise Linux 9 Configuring basic system settings
This section describes how to display an overview of overridden or modified unit files.
Procedure
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]
WantedBy=multi-user.target
[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.
16.18. 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
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
92CHAPTER 16. WORKING WITH SYSTEMD UNIT FILES
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.
For example, the getty@.service template contains the following directives:
[Unit]
Description=Getty on %I
…
[Service]
ExecStart=-/sbin/agetty --noclear %I $TERM
…
When the getty@ttyA.service and getty@ttyB.service are instantiated from the above template,
Description= is resolved as Getty on ttyA and Getty on ttyB.
16.19. IMPORTANT UNIT SPECIFIERS
Wildcard characters, called unit specifiers, can be used in any unit configuration file. Unit specifiers
substitute certain unit parameters and are interpreted at runtime. The following table lists unit specifiers
that are particularly useful for template units.
Table 16.5. 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.
93Red Hat Enterprise Linux 9 Configuring basic system settings
Unit Specifier Meaning Description
%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.
16.20. ADDITIONAL RESOURCES
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
Is there any useful information about writing unit files?
How to set limits for services in RHEL 7 and systemd
94CHAPTER 17. OPTIMIZING SYSTEMD TO SHORTEN THE BOOT TIME
CHAPTER 17. OPTIMIZING SYSTEMD TO SHORTEN THE BOOT
TIME
There is a list of systemd unit files that are enabled by default. System services that are defined by these
unit files are automatically run at boot, which influences the boot time.
This section describes:
The tools to examine system boot performance.
The purpose of systemd units enabled by default, and circumstances under which you can safely
disable such systemd units in order to shorten the boot time.
17.1. EXAMINING SYSTEM BOOT PERFORMANCE
To examine system boot performance, you can use the systemd-analyze command. This command has
many options available. However, this section covers only the selected ones that may be important for
systemd tuning in order to shorten the boot time.
For a complete list and detailed description of all options, see the systemd-analyze man page.
Prerequisites
Before starting to examine systemd in order to tune the boot time, you may want to list all
enabled services:
Procedure
$ systemctl list-unit-files --state=enabled
Analyzing overall boot time
Procedure
For the overall information about the time that the last successful boot took, use:
$ systemd-analyze
Analyzing unit initialization time
Procedure
For the information about the initialization time of each systemd unit, use:
$ systemd-analyze blame
The output lists the units in descending order according to the time they took to initialize during the last
successful boot.
Identifying critical units
Procedure
95Red Hat Enterprise Linux 9 Configuring basic system settings
To identify the units that took most time to initialize at the last successful boot, use:
$ systemd-analyze critical-chain
The output highlights the units that critically slow down the boot with the red color.
Figure 17.1. The output of the systemd-analyze critical-chain command
17.2. A GUIDE TO SELECTING SERVICES THAT CAN BE SAFELY
DISABLED
If you find the boot time of your system long, you can shorten it by disabling some of the services
enabled on boot by default.
To list such services, run:
$ systemctl list-unit-files --state=enabled
To disable a service, run:
# systemctl disable service_name
However, certain services must stay enabled in order that your operating system is safe and functions in
the way you need.
You can use the table below as a guide to selecting the services that you can safely disable. The table
lists all services enabled by default on a minimal installation of Red Hat Enterprise Linux, and for each
service it states whether this service can be safely disabled.
The table also provides more information about the circumstances under which the service can be
disabled, or the reason why you should not disable the service.
Table 17.1. Services enabled by default on a minimal installation of RHEL
96CHAPTER 17. OPTIMIZING SYSTEMD TO SHORTEN THE BOOT TIME
Service Can it be disabled? More information
name
auditd.servic yes Disable auditd.service only if you do not need audit messages
e from the kernel. Be aware that if you disable auditd.service, the
/var/log/audit/audit.log file is not produced. Consequently, you
are not able to retroactively review some commonly-reviewed
actions or events, such as user logins, service starts or password
changes. Also note that auditd has two parts: a kernel part, and a
service itself. By using the systemctl disable auditd command,
you only disable the service, but not the kernel part. To disable
system auditing in its entirety, set audit=0 on kernel command
line.
autovt@.servi no This service runs only when it is really needed, so it does not need
ce to be disabled.
crond.service yes Be aware that no items from crontab will run if you disable
crond.service.
dbus- yes A symlink to firewalld.service
org.fedorapr
oject.Firewall
D1.service
dbus- yes A symlink to NetworkManager.service
org.freedeskt
op.NetworkM
anager.servic
e
dbus- yes A symlink to NetworkManager-dispatcher.service
org.freedeskt
op.nm-
dispatcher.se
rvice
firewalld.servi yes Disable firewalld.service only if you do not need firewall.
ce
getty@.servic no This service runs only when it is really needed, so it does not need
e to be disabled.
import- yes Disable import-state.service only if you do not need to boot
state.service from a network storage.
irqbalance.se yes Disable irqbalance.service only if you have just one CPU. Do not
rvice disable irqbalance.service on systems with multiple CPUs.
97Red Hat Enterprise Linux 9 Configuring basic system settings
Service Can it be disabled? More information
name
kdump.servic yes Disable kdump.service only if you do not need reports from
e kernel crashes.
loadmodules. yes This service is not started unless the /etc/rc.modules or
service /etc/sysconfig/modules directory exists, which means that it is
not started on a minimal RHEL installation.
lvm2- yes Disable lvm2-monitor.service only if you do not use Logical
monitor.servi Volume Manager (LVM).
ce
microcode.se no Do not be disable the service because it provides updates of the
rvice microcode software in CPU.
NetworkMan yes Disable NetworkManager-dispatcher.service only if you do
ager- not need notifications on network configuration changes (for
dispatcher.se example in static networks).
rvice
NetworkMan yes Disable NetworkManager-wait-online.service only if you do
ager-wait- not need working network connection available right after the
online.service boot. If the service is enabled, the system does not finish the boot
before the network connection is working. This may prolong the
boot time significantly.
NetworkMan yes Disable NetworkManager.service only if you do not need
ager.service connection to a network.
nis- yes Disable nis-domainname.service only if you do not use
domainname. Network Information Service (NIS).
service
rhsmcertd.se no
rvice
rngd.service yes Disable rngd.service only if you do not need a lot of entropy on
your system, or you do not have any sort of hardware generator.
Note that the service is necessary in environments that require a
lot of good entropy, such as systems used for generation of X.509
certificates (for example the FreeIPA server).
rsyslog.servic yes Disable rsyslog.service only if you do not need persistent logs,
e or you set systemd-journald to persistent mode.
98CHAPTER 17. OPTIMIZING SYSTEMD TO SHORTEN THE BOOT TIME
Service Can it be disabled? More information
name
selinux- yes Disable selinux-autorelabel-mark.service only if you do not
autorelabel- use SELinux.
mark.service
sshd.service yes Disable sshd.service only if you do not need remote logins by
OpenSSH server.
sssd.service yes Disable sssd.service only if there are no users who log in the
system over the network (for example by using LDAP or Kerberos).
Red Hat recommends to disable all sssd-* units if you disable
sssd.service.
syslog.servic yes An alias for rsyslog.service
e
tuned.service yes Disable tuned.service only if you do need to use performance
tuning.
lvm2- yes Disable lvm2-lvmpolld.socket only if you do not use Logical
lvmpolld.sock Volume Manager (LVM).
et
dnf- yes Disable dnf-makecache.timer only if you do not need your
makecache.ti package metadata to be updated automatically.
mer
unbound- yes Disable unbound-anchor.timer only if you do not need daily
anchor.timer update of the root trust anchor for DNS Security Extensions
(DNSSEC). This root trust anchor is used by Unbound resolver and
resolver library for DNSSEC validation.
To find more information about a service, you can run one of the following commands:
$ systemctl cat
$ systemctl help
The systemctl cat command provides the content of the service file located under
/usr/lib/systemd/system/, as well as all applicable overrides. The applicable overrides include
unit file overrides from the /etc/systemd/system/ file or drop-in files from a corresponding
unit.type.d directory.
For more information on drop-in files, see the systemd.unit man page.
The systemctl help command shows the man page of the particular service.
99Red Hat Enterprise Linux 9 Configuring basic system settings
17.3. ADDITIONAL RESOURCES
systemctl(1) man page
systemd(1) man page
systemd-delta(1) man page
systemd.directives(7) man page
systemd.unit(5) man page
systemd.service(5) man page
systemd.target(5) man page
systemd.kill(5) man page
systemd Home Page
100CHAPTER 18. INTRODUCTION TO MANAGING USER AND GROUP ACCOUNTS
CHAPTER 18. INTRODUCTION TO MANAGING USER AND
GROUP ACCOUNTS
The control of users and groups is a core element of Red Hat Enterprise Linux (RHEL) system
administration. Each RHEL user has distinct login credentials and can be assigned to various groups to
customize their system privileges.
18.1. INTRODUCTION TO USERS AND GROUPS
A user who creates a file is the owner of that file and the group owner of that file. The file is assigned
separate read, write, and execute permissions for the owner, the group, and those outside that group.
The file owner can be changed only by the root user. Access permissions to the file can be changed by
both the root user and the file owner. A regular user can change group ownership of a file they own to a
group of which they are a member of.
Each user is associated with a unique numerical identification number called user ID (UID). Each group is
associated with a group ID (GID). Users within a group share the same permissions to read, write, and
execute files owned by that group.
18.2. CONFIGURING RESERVED USER AND GROUP IDS
RHEL reserves user and group IDs below 1000 for system users and groups. You can find the reserved
user and group IDs in the setup package. To view reserved user and group IDs, use:
cat /usr/share/doc/setup*/uidgid
It is recommended to assign IDs to the new users and groups starting at 5000, as the reserved range
can increase in the future.
To make the IDs assigned to new users start at 5000 by default, modify the UID_MIN and GID_MIN
parameters in the /etc/login.defs file.
Procedure
To modify and make the IDs assigned to new users start at 5000 by default:
1. Open the /etc/login.defs file in an editor of your choice.
2. Find the lines that define the minimum value for automatic UID selection.
# Min/max values for automatic uid selection in useradd
#
UID_MIN 1000
3. Modify the UID_MIN value to start at 5000.
# Min/max values for automatic uid selection in useradd
#
UID_MIN 5000
4. Find the lines that define the minimum value for automatic GID selection.
101Red Hat Enterprise Linux 9 Configuring basic system settings
# Min/max values for automatic gid selection in groupadd
#
GID_MIN 1000
5. Modify the GID_MIN value to start at 5000.
# Min/max values for automatic gid selection in groupadd
#
GID_MIN 5000
The dynamically assigned UIDs and GIDs for the regular users now start at 5000.
NOTE
The UID’s and GID’s of users and groups created before you changed the
UID_MIN and GID_MIN values do not change.
This will allow new user’s group to have same 5000+ ID as UID and GID.
WARNING
Do not raise IDs reserved by the system above 1000 by changing
SYS_UID_MAX to avoid conflict with systems that retain the 1000 limit.
18.3. USER PRIVATE GROUPS
RHEL uses the user private group (UPG) system configuration, which makes UNIX groups easier to
manage. A user private group is created whenever a new user is added to the system. The user private
group has the same name as the user for which it was created and that user is the only member of the
user private group.
UPGs simplify the collaboration on a project between multiple users. In addition, UPG system
configuration makes it safe to set default permissions for a newly created file or directory, as it allows
both the user, and the group this user is a part of, to make modifications to the file or directory.
A list of all groups is stored in the /etc/group configuration file.
102CHAPTER 19. MANAGING USER ACCOUNTS IN THE WEB CONSOLE
CHAPTER 19. MANAGING USER ACCOUNTS IN THE WEB
CONSOLE
The RHEL web console offers a graphical interface that enables you to execute a wide range of
administrative tasks without accessing your terminal directly. For example, you can add, edit or remove
system user accounts.
After reading this section, you will know:
From where the existing accounts come from.
How to add new accounts.
How to set password expiration.
How and when to terminate user sessions.
Prerequisites
Set up the RHEL web console. For details, see Getting started using the RHEL web console .
Log in to the RHEL web console with an account that has administrator permissions assigned.
For details, see Logging in to the RHEL web console .
19.1. SYSTEM USER ACCOUNTS MANAGED IN THE WEB CONSOLE
With user accounts displayed in the RHEL web console you can:
Authenticate users when accessing the system.
Set the access rights to the system.
The RHEL web console displays all user accounts located in the system. Therefore, you can see at least
one user account just after the first login to the web console.
After logging into the RHEL web console, you can perform the following operations:
Create new users accounts.
Change their parameters.
Lock accounts.
Terminate user sessions.
19.2. ADDING NEW ACCOUNTS USING THE WEB CONSOLE
Use the following steps for adding user accounts to the system and setting administration rights to the
accounts through the RHEL web console.
Prerequisites
The RHEL web console must be installed and accessible. For details, see Installing the web
console.
103Red Hat Enterprise Linux 9 Configuring basic system settings
Procedure
1. Log in to the RHEL web console.
2. Click Accounts.
3. Click Create New Account.
1. In the Full Name field, enter the full name of the user.
The RHEL web console automatically suggests a user name from the full name and fills it in the
User Name field. If you do not want to use the original naming convention consisting of the first
letter of the first name and the whole surname, update the suggestion.
2. In the Password/Confirm fields, enter the password and retype it for verification that your
password is correct.
The color bar placed below the fields shows you security level of the entered password, which
does not allow you to create a user with a weak password.
1. Click Create to save the settings and close the dialog box.
2. Select the newly created account.
3. Select Server Administrator in the Roles item.
Now you can see the new account in the Accounts settings and you can use the credentials to
connect to the system.
19.3. ENFORCING PASSWORD EXPIRATION IN THE WEB CONSOLE
By default, user accounts have set passwords to never expire. You can set system passwords to expire
after a defined number of days. When the password expires, the next login attempt will prompt for a
password change.
Procedure
1. Log in to the RHEL 9 web console.
2. Click Accounts.
104CHAPTER 19. MANAGING USER ACCOUNTS IN THE WEB CONSOLE
3. Select the user account for which to enforce password expiration.
4. In the user account settings, click the second edit.
5. In the Password Expiration dialog box, select Require password change every … days and
enter a positive whole number representing the number of days after which the password
expires.
6. Click Change.
Verification steps
To verify that the password expiration is set, open the account settings.
The RHEL 9 web console displays a link with the date of expiration.
19.4. TERMINATING USER SESSIONS IN THE WEB CONSOLE
A user creates user sessions when logging into the system. Terminating user sessions means to log the
user out from the system. It can be helpful if you need to perform administrative tasks sensitive to
configuration changes, for example, system upgrades.
In each user account in the RHEL 9 web console, you can terminate all sessions for the account except
for the web console session you are currently using. This prevents you from loosing access to your
system.
Procedure
1. Log in to the RHEL 9 web console.
2. Click Accounts.
3. Click the user account for which you want to terminate the session.
4. Click Terminate Session.
If the Terminate Session button is inactive, the user is not logged in to the system.
105Red Hat Enterprise Linux 9 Configuring basic system settings
The RHEL web console terminates the sessions.
106CHAPTER 20. MANAGING USERS FROM THE COMMAND LINE
CHAPTER 20. MANAGING USERS FROM THE COMMAND LINE
You can manage users and groups using the command-line interface (CLI). This enables you to add,
remove, and modify users and user groups in Red Hat Enterprise Linux environment.
20.1. ADDING A NEW USER FROM THE COMMAND LINE
This section describes how to use the useradd utility to add a new user.
Prerequisites
Root access
Procedure
To add a new user, use:
# useradd options username
Replace options with the command-line options for the useradd command, and replace
username with the name of the user.
Example 20.1. Adding a new user
To add the user sarah with user ID 5000, use:
+
# useradd -u 5000 sarah
Verification steps
To verify the new user is added, use the id utility.
# id sarah
The output returns:
uid=5000(sarah) gid=5000(sarah) groups=5000(sarah)
Additional resources
useradd man page
20.2. ADDING A NEW GROUP FROM THE COMMAND LINE
This section describes how to use the groupadd utility to add a new group.
Prerequisites
107Red Hat Enterprise Linux 9 Configuring basic system settings
Root access
Procedure
To add a new group, use:
# groupadd options group-name
Replace options with the command-line options for the groupadd command, and replace
group-name with the name of the group.
Example 20.2. Adding a new group
To add the group sysadmins with group ID 5000, use:
+
# groupadd -g 5000 sysadmins
Verification steps
To verify the new group is added, use the tail utility.
# tail /etc/group
The output returns:
sysadmins:x:5000:
Additional resources
groupadd man page
20.3. ADDING A USER TO A SUPPLEMENTARY GROUP FROM THE
COMMAND LINE
You can add a user to a supplementary group to manage permissions or enable access to certain files or
devices.
Prerequisites
root access
Procedure
To add a group to the supplementary groups of the user, use:
# usermod --append -G group-name username
Replace group-name with the name of the group, and replace username with the name of the
user.
108CHAPTER 20. MANAGING USERS FROM THE COMMAND LINE
Example 20.3. Adding a user to a supplementary group
To add the user sysadmin to the group system-administrators, use:
# usermod --append -G system-administrators sysadmin
Verification steps
To verify the new groups is added to the supplementary groups of the user sysadmin, use:
# groups sysadmin
The output displays:
sysadmin : sysadmin system-administrators
20.4. CREATING A GROUP DIRECTORY
Under the UPG system configuration, you can apply the set-group identification permission (setgid bit)
to a directory. The setgid bit makes managing group projects that share a directory simpler. When you
apply the setgid bit to a directory, files created within that directory are automatically assigned to a
group that owns the directory. Any user that has the permission to write and execute within this group
can now create, modify, and delete files in the directory.
The following section describes how to create group directories.
Prerequisites
Root access
Procedure
1. Create a directory:
# mkdir directory-name
Replace directory-name with the name of the directory.
2. Create a group:
# groupadd group-name
Replace group-name with the name of the group.
3. Add users to the group:
# usermod --append -G group-name username
Replace group-name with the name of the group, and replace username with the name of the
user.
109Red Hat Enterprise Linux 9 Configuring basic system settings
4. Associate the user and group ownership of the directory with the group-name group:
# chown :group-name directory-name
Replace group-name with the name of the group, and replace directory-name with the name of
the directory.
5. Set the write permissions to allow the users to create and modify files and directories and set
the setgid bit to make this permission be applied within the directory-name directory:
# chmod g+rwxs directory-name
Replace directory-name with the name of the directory.
Now all members of the group-name group can create and edit files in the directory-name
directory. Newly created files retain the group ownership of group-name group.
Verification steps
To verify the correctness of set permissions, use:
# ls -ld directory-name
Replace directory-name with the name of the directory.
The output returns:
drwxrwsr-x. 2 root group-name 6 Nov 25 08:45 directory-name
110CHAPTER 21. EDITING USER GROUPS USING THE COMMAND LINE
CHAPTER 21. EDITING USER GROUPS USING THE COMMAND
LINE
A user belongs to a certain set of groups that allow a logical collection of users with a similar access to
files and folders. You can edit the primary and supplementary user groups from the command line to
change the user’s permissions.
21.1. PRIMARY AND SUPPLEMENTARY USER GROUPS
A group is an entity which ties together multiple user accounts for a common purpose, such as granting
access to particular files.
On Linux, user groups can act as primary or supplementary. Primary and supplementary groups have the
following properties:
Primary group
Every user has just one primary group at all times.
You can change the user’s primary group.
Supplementary groups
You can add an existing user to an existing supplementary group to manage users with the
same security and access privileges within the group.
Users can be members of zero or multiple supplementary groups.
21.2. LISTING THE PRIMARY AND SUPPLEMENTARY GROUPS OF A
USER
You can list the groups of users to see which primary and supplementary groups they belong to.
Procedure
Display the names of the primary and any supplementary group of a user:
$ groups user-name
Replace user-name with the name of the user. If you do not provide a user name, the command
displays the group membership for the current user. The first group is the primary group
followed by the optional supplementary groups.
Example 21.1. Listing of groups for user sarah:
$ groups sarah
The output displays:
sarah : sarah wheel developer
User sarah has a primary group sarah and is a member of supplementary groups wheel and
111Red Hat Enterprise Linux 9 Configuring basic system settings
User sarah has a primary group sarah and is a member of supplementary groups wheel and
developer.
Example 21.2. Listing of groups for user marc:
$ groups marc
The output displays:
marc : marc
User marc has only a primary group marc and no supplementary groups.
21.3. CHANGING THE PRIMARY GROUP OF A USER
You can change the primary group of an existing user to a new group.
Prerequisites:
1. root access
2. The new group must exist
Procedure
Change the primary group of a user:
# usermod -g group-name user-name
Replace group-name with the name of the new primary group, and replace user-name with the
name of the user.
NOTE
When you change a user’s primary group, the command also automatically
changes the group ownership of all files in the user’s home directory to the new
primary group. You must fix the group ownership of files outside of the user’s
home directory manually.
Example 21.3. Example of changing the primary group of a user:
If the user sarah belongs to the primary group sarah1, and you want to change the primary
group of the user to sarah2, use:
# usermod -g sarah2 sarah
Verification steps
112CHAPTER 21. EDITING USER GROUPS USING THE COMMAND LINE
Verify that you changed the primary group of the user:
$ groups sarah
The output displays:
sarah : sarah2
21.4. ADDING A USER TO A SUPPLEMENTARY GROUP FROM THE
COMMAND LINE
You can add a user to a supplementary group to manage permissions or enable access to certain files or
devices.
Prerequisites
root access
Procedure
To add a group to the supplementary groups of the user, use:
# usermod --append -G group-name username
Replace group-name with the name of the group, and replace username with the name of the
user.
Example 21.4. Adding a user to a supplementary group
To add the user sysadmin to the group system-administrators, use:
# usermod --append -G system-administrators sysadmin
Verification steps
To verify the new groups is added to the supplementary groups of the user sysadmin, use:
# groups sysadmin
The output displays:
sysadmin : sysadmin system-administrators
21.5. REMOVING A USER FROM A SUPPLEMENTARY GROUP
You can remove an existing user from a supplementary group to limit their permissions or access to files
and devices.
Prerequisites
113Red Hat Enterprise Linux 9 Configuring basic system settings
root access
Procedure
Remove a user from a supplementary group:
# gpasswd -d user-name group-name
Replace user-name with the name of the user, and replace group-name with the name of the
supplementary group.
Example 21.5. Removing user from a supplementary group
If the user sarah has a primary group sarah2, and belongs to the secondary groups wheel
and developers, and you want to remove that user from the group developers, use:
# gpasswd -d sarah developers
Verification steps
Verify that you removed the user sarah from the secondary group developers:
$ groups sarah
The output displays:
sarah : sarah2 wheel
21.6. CHANGING ALL OF THE SUPPLEMENTARY GROUPS OF A USER
You can overwrite the list of supplementary groups that you want the user to remain a member of.
Prerequisites
root access
The supplementary groups must exist
Procedure
Overwrite a list of user’s supplementary groups:
# usermod -G group-names username
Replace group-names with the name of one or more supplementary groups. To add the user to
several supplementary groups at once, separate the group names using commas and no
intervening spaces. For example: wheel,developer.
Replace user-name with the name of the user.
IMPORTANT
114CHAPTER 21. EDITING USER GROUPS USING THE COMMAND LINE
IMPORTANT
If the user is currently a member of a group that you do not specify, the
command removes the user from the group.
Example 21.6. Changing the list of supplementary groups of a user
If the user sarah has a primary group sarah2, and belongs to the supplementary group
wheel, and you want the user to belong to three more supplementary groups developer,
sysadmin, and security, use:
# usermod -G wheel,developer,sysadmin,security sarah
Verification steps
Verify that you set the list of the supplementary groups correct:
# groups sarah
The output displays:
sarah : sarah2 wheel developer sysadmin security
115Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 22. MANAGING SUDO ACCESS
System administrators can grant sudo access to allow non-root users to execute administrative
commands that are normally reserved for the root user. As a result, non-root users can execute such
commands without logging in to the root user account.
22.1. USER AUTHORIZATIONS IN SUDOERS
The /etc/sudoers file specifies which users can run which commands using the sudo command. The
rules can apply to individual users and user groups. You can also use aliases to simplify defining rules for
groups of hosts, commands, and even users. Default aliases are defined in the first part of the
/etc/sudoers file.
When a user tries to use sudo privileges to run a command that is not allowed in the /etc/sudoers file,
the system records a message containing username : user NOT in sudoers to the journal log.
The default /etc/sudoers file provides information and examples of authorizations. You can activate a
specific example rule by removing the # comment character from the beginning of the line. The
authorizations section relevant for user is marked with the following introduction:
## Next comes the main part: which users can run what software on
## which machines (the sudoers file can be shared between multiple
## systems).
You can use the following format to create new sudoers authorizations and to modify existing
authorizations:
username hostname=path/to/command
Where:
username is the name of the user or group, for example, user1 or %group1.
hostname is the name of the host on which the rule applies.
path/to/command is the complete absolute path to the command. You can also limit the user to
only performing a command with specific options and arguments by adding those options after
the command path. If you do not specify any options, the user can use the command with all
options.
You can replace any of these variables with ALL to apply the rule to all users, hosts, or commands.
WARNING
With overly permissive rules, such as ALL ALL=(ALL) ALL, all users are able to run
all commands as all users on all hosts. This can lead to security risks.
You can specify the arguments negatively using the ! operator. For example, use !root to specify all
users except the root user. Note that using the allowlists to allow specific users, groups, and commands,
116CHAPTER 22. MANAGING SUDO ACCESS
is more secure than using the blocklists to disallowing specific users, groups, and commands. By using
the allowlists you also block new unauthorized users or groups.
WARNING
Avoid using negative rules for commands because users can overcome such rules by
renaming commands using the alias command.
The system reads the /etc/sudoers file from beginning to end. Therefore, if the file contains multiple
entries for a user, the entries are applied in order. In case of conflicting values, the system uses the last
match, even if it is not the most specific match.
The preferred way of adding new rules to sudoers is by creating new files in the /etc/sudoers.d/
directory instead of entering rules directly to the /etc/sudoers file. This is because the contents of this
directory are preserved during system updates. In addition, it is easier to fix any errors in the separate
files than in the /etc/sudoers file. The system reads the files in the /etc/sudoers.d directory when it
reaches the following line in the /etc/sudoers file:
#includedir /etc/sudoers.d
Note that the number sign # at the beginning of this line is part of the syntax and does not mean the line
is a comment. The names of files in that directory must not contain a period . and must not end with a
tilde ~.
22.2. GRANTING SUDO ACCESS TO A USER
System administrators can grant sudo access to allow non-root users to execute administrative
commands. The sudo command provides users with administrative access without using the password
of the root user.
When users need to perform an administrative command, they can precede that command with sudo.
The command is then executed as if they were the root user.
Be aware of the following limitations:
Only users listed in the /etc/sudoers configuration file can use the sudo command.
The command is executed in the shell of the user, not in the root shell.
Prerequisites
root access
Procedure
1. As root, open the /etc/sudoers file.
# visudo
117Red Hat Enterprise Linux 9 Configuring basic system settings
The /etc/sudoers file defines the policies applied by the sudo command.
2. In the /etc/sudoers file, find the lines that grant sudo access to users in the administrative
wheel group.
## Allows people in group wheel to run all commands
%wheel ALL=(ALL) ALL
3. Make sure the line that starts with %wheel does not have the # comment character before it.
4. Save any changes, and exit the editor.
5. Add users you want to grant sudo access to into the administrative wheel group.
# usermod --append -G wheel username
Replace username with the name of the user.
Verification steps
Verify that the user is added to the administrative wheel group:
# id username
uid=5000(username) gid=5000(_username) groups=5000(username),10(wheel)
22.3. ENABLING UNPRIVILEGED USERS TO RUN CERTAIN
COMMANDS
You can configure a policy that allows unprivileged user to run certain command on a specific
workstation. To configure this policy, you need to create and edit file in the sudoers.d directory.
Prerequisites
root access
Procedure
1. As root, create a new sudoers.d directory under /etc/:
# mkdir -p /etc/sudoers.d/
2. Create a new file in the /etc/sudoers.d directory:
# visudo -f /etc/sudoers.d/file-name
Replace file-name with the name of the file you want to create. The file will open automatically.
3. Add the following line to the newly created file:
username hostname = /path/to/the/command
Replace username with the name of the user. Replace hostname with the name of the host.
118CHAPTER 22. MANAGING SUDO ACCESS
Replace username with the name of the user. Replace hostname with the name of the host.
Replace /path/to/the/command with the absolute path to the command (for example,
/usr/bin/dnf).
4. Save any changes, and exit the editor.
Example 22.1. Enabling an unprivileged user to install programs with dnf
To enable the user sarah to install programs on the localhost.localdomain workstation
using the dnf utilities with sudo privileges, use:
1. As root, create a new sudoers.d directory under /etc/:
# mkdir -p /etc/sudoers.d/
2. Create a new file in the /etc/sudoers.d directory:
# visudo -f /etc/sudoers.d/sarah
The file will open automatically.
3. Add the following line to the /etc/sudoers.d/sarah file:
sarah localhost.localdomain = /usr/bin/dnf
Ensure that the two command paths are separated by a , comma followed by a space.
4. Optional: To receive email notifications every time the user sarah attempts to use sudo
privileges, add the following lines to the file:
Defaults mail_always
Defaults mailto="email@domain.com"
5. To verify if the user sarah can run the dnf command with sudo privileges, switch the
account:
# su sarah -
6. Enter the sudo dnf command:
$ sudo dnf
[sudo] password for sarah:
Enter the sudo password for the user sarah.
7. The system displays the list of dnf commands and options:
...
usage: dnf [options] COMMAND
...
If you receive the sarah is not in the sudoers file. This incident will be reported.
message, the configuration was not completed correctly. Ensure that you are executing
this procedure as root and that you followed the steps thoroughly.
119Red Hat Enterprise Linux 9 Configuring basic system settings
22.4. ADDITIONAL RESOURCES
The sudo(8) man page
The visudo(8) man page
120CHAPTER 23. CHANGING AND RESETTING THE ROOT PASSWORD
CHAPTER 23. CHANGING AND RESETTING THE ROOT
PASSWORD
If the existing root password is no longer satisfactory or is forgotten, you can change or reset it both as
the root user and a non-root user.
23.1. CHANGING THE ROOT PASSWORD AS THE ROOT USER
This section describes how to use the passwd command to change the root password as the root user.
Prerequisites
Root access
Procedure
To change the root password, use:
# passwd
You are prompted to enter your current password before you can change it.
23.2. CHANGING OR RESETTING THE FORGOTTEN ROOT PASSWORD
AS A NON-ROOT USER
This section describes how to use the passwd command to change or reset the forgotten root
password as a non-root user.
Prerequisites
You are able to log in as a non-root user.
You are a member of the administrative wheel group.
Procedure
To change or reset the root password as a non-root user that belongs to the wheel group, use:
$ sudo passwd root
You are prompted to enter your current non-root password before you can change the root
password.
23.3. RESETTING THE ROOT PASSWORD ON BOOT
If you are unable to log in as a non-root user or do not belong to the administrative wheel group, you can
reset the root password on boot by switching into a specialized chroot jail environment.
Procedure
1. Reboot the system and, on the GRUB 2 boot screen, press the e key to interrupt the boot
process.
121Red Hat Enterprise Linux 9 Configuring basic system settings
The kernel boot parameters appear.
load_video
set gfx_payload=keep
insmod gzio
linux ($root)/vmlinuz-5.14.0-70.22.1.e19_0.x86_64 root=/dev/mapper/rhel-root ro crash\
kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhgb quiet
initrd ($root)/initramfs-5.14.0-70.22.1.e19_0.x86_64.img $tuned_initrd
2. Go to the end of the line that starts with linux.
linux ($root)/vmlinuz-5.14.0-70.22.1.e19_0.x86_64 root=/dev/mapper/rhel-root ro crash\
kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhgb quiet
Press Ctrl+e to jump to the end of the line.
3. Add rd.break to the end of the line that starts with linux.
linux ($root)/vmlinuz-5.14.0-70.22.1.e19_0.x86_64 root=/dev/mapper/rhel-root ro crash\
kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhgb quiet rd.break
4. Press Ctrl+x to start the system with the changed parameters.
The switch_root prompt appears.
5. Remount the file system as writable:
mount -o remount,rw /sysroot
The file system is mounted as read-only in the /sysroot directory. Remounting the file system
as writable allows you to change the password.
6. Enter the chroot environment:
chroot /sysroot
The sh-4.4# prompt appears.
7. Reset the root password:
passwd
Follow the instructions displayed by the command line to finalize the change of the root
password.
8. Enable the SELinux relabeling process on the next system boot:
touch /.autorelabel
9. Exit the chroot environment:
exit
10. Exit the switch_root prompt:
122CHAPTER 23. CHANGING AND RESETTING THE ROOT PASSWORD
exit
11. Wait until the SELinux relabeling process is finished. Note that relabeling a large disk might take
a long time. The system reboots automatically when the process is complete.
Verification steps
1. To verify that the root password is successfully changed, log in as a normal user and open the
Terminal.
2. Run the interactive shell as root:
$ su
3. Enter your new root password.
4. Print the user name associated with the current effective user ID:
whoami
The output returns:
root
123Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 24. MANAGING FILE PERMISSIONS
File permissions control the ability of user and group accounts to view, modify, access, and execute the
contents of the files and directories.
Every file or directory has three levels of ownership:
User owner (u).
Group owner (g).
Others (o).
Each level of ownership can be assigned the following permissions:
Read (r).
Write (w).
Execute (x).
Note that the execute permission for a file allows you to execute that file. The execute permission for a
directory allows you to access the contents of the directory, but not execute it.
When a new file or directory is created, the default set of permissions are automatically assigned to it.
The default permissions for a file or directory are based on two factors:
Base permission.
The user file-creation mode mask (umask).
24.1. BASE FILE PERMISSIONS
Whenever a new file or directory is created, a base permission is automatically assigned to it. Base
permissions for a file or directory can be expressed in symbolic or octal values.
Permission Symbolic value Octal value
No permission --- 0
Execute --x 1
Write -w- 2
Write and execute -wx 3
Read r-- 4
Read and execute r-x 5
Read and write rw- 6
124CHAPTER 24. MANAGING FILE PERMISSIONS
Read, write, execute rwx 7
The base permission for a directory is 777 (drwxrwxrwx), which grants everyone the permissions to
read, write, and execute. This means that the directory owner, the group, and others can list the
contents of the directory, create, delete, and edit items within the directory, and descend into it.
Note that individual files within a directory can have their own permission that might prevent you from
editing them, despite having unrestricted access to the directory.
The base permission for a file is 666 (-rw-rw-rw-), which grants everyone the permissions to read and
write. This means that the file owner, the group, and others can read and edit the file.
Example 24.1. Permissions for a file
If a file has the following permissions:
$ ls -l
-rwxrw----. 1 sysadmins sysadmins 2 Mar 2 08:43 file
- indicates it is a file.
rwx indicates that the file owner has permissions to read, write, and execute the file.
rw- indicates that the group has permissions to read and write, but not execute the file.
--- indicates that other users have no permission to read, write, or execute the file.
. indicates that the SELinux security context is set for the file.
Example 24.2. Permissions for a directory
If a directory has the following permissions:
$ ls -dl directory
drwxr-----. 1 sysadmins sysadmins 2 Mar 2 08:43 directory
d indicates it is a directory.
rwx indicates that the directory owner has the permissions to read, write, and access the
contents of the directory.
As a directory owner, you can list the items (files, subdirectories) within the directory, access
the content of those items, and modify them.
r-- indicates that the group has permissions to read, but not write or access the contents of
the directory.
As a member of the group that owns the directory, you can list the items within the directory.
You cannot access information about the items within the directory or modify them.
--- indicates that other users have no permission to read, write, or access the contents of the
directory.
As someone who is not a user owner, or as group owner of the directory, you cannot list the
items within the directory, access information about those items, or modify them.
125Red Hat Enterprise Linux 9 Configuring basic system settings
. indicates that the SELinux security context is set for the directory.
NOTE
The base permission that is automatically assigned to a file or directory is not the default
permission the file or directory ends up with. When you create a file or directory, the base
permission is altered by the umask. The combination of the base permission and the
umask creates the default permission for files and directories.
24.2. USER FILE-CREATION MODE MASK
The user file-creation mode mask (umask) is variable that controls how file permissions are set for newly
created files and directories. The umask automatically removes permissions from the base permission
value to increase the overall security of a linux system. The umask can be expressed in symbolic or octal
values.
Permission Symbolic value Octal value
Read, write, and execute rwx 0
Read and write rw- 1
Read and execute r-x 2
Read r-- 3
Write and execute -wx 4
Write -w- 5
Execute --x 6
No permissions --- 7
The default umask for a standard user is 0002. The default umask for a root user is 0022.
The first digit of the umask represents special permissions (sticky bit, ). The last three digits of the
umask represent the permissions that are removed from the user owner ( u), group owner (g), and
others (o) respectively.
Example 24.3. Applying the umask when creating a file
The following example illustrates how the umask with an octal value of 0137 is applied to the file with
the base permission of 777, to create the file with the default permission of 640.
126CHAPTER 24. MANAGING FILE PERMISSIONS
24.3. DEFAULT FILE PERMISSIONS
The default permissions are set automatically for all newly created files and directories. The value of the
default permissions is determined by applying the umask to the base permission.
Example 24.4. Default permissions for a directory created by a standard user
When a standard user creates a new directory, the umask is set to 002 (rwxrwxr-x), and the base
permissions for a directory are set to 777 (rwxrwxrwx). This brings the default permissions to 775
(drwxrwxr-x).
Symbolic value Octal value
Base permission rwxrwxrwx 777
Umask rwxrwxr-x 002
Default permission rwxrwxr-x 775
This means that the directory owner and the group can list the contents of the directory, create, delete,
and edit items within the directory, and descend into it. Other users can only list the contents of the
directory and descend into it.
Example 24.5. Default permissions for a file created by a standard user
When a standard user creates a new file, the umask is set to 002 (rwxrwxr-x), and the base
permissions for a file are set to 666 (rw-rw-rw-). This brings the default permissions to 664 (-rw-rw-r-
-).
127Red Hat Enterprise Linux 9 Configuring basic system settings
Symbolic value Octal value
Base permission rw-rw-rw- 666
Umask rwxrwxr-x 002
Default permission rw-rw-r-- 664
This means that the file owner and the group can read and edit the file, while other users can only
read the file.
Example 24.6. Default permissions for a directory created by the root user
When a root user creates a new directory, the umask is set to 022 (rwxr-xr-x), and the base
permissions for a directory are set to 777 (rwxrwxrwx). This brings the default permissions to 755
(rwxr-xr-x).
Symbolic value Octal value
Base permission rwxrwxrwx 777
Umask rwxr-xr-x 022
Default permission rwxr-xr-x 755
This means that the directory owner can list the contents of the directory, create, delete, and edit
items within the directory, and descend into it. The group and others can only list the contents of the
directory and descend into it.
Example 24.7. Default permissions for a file created by the root user
When a root user creates a new file, the umask is set to 022 (rwxr-xr-x), and the base permissions
for a file are set to 666 (rw-rw-rw-). This brings the default permissions to 644 (-rw-r— r--).
Symbolic value Octal value
Base permission rw-rw-rw- 666
Umask rwxr-xr-x 022
Default permission rw-r—r-- 644
This means that the file owner can read and edit the file, while the group and others can only read
the file.
128CHAPTER 24. MANAGING FILE PERMISSIONS
NOTE
For security reasons, regular files cannot have execute permissions by default, even if the
umask is set to 000 (rwxrwxrwx). However, directories can be created with execute
permissions.
24.4. CHANGING FILE PERMISSIONS USING SYMBOLIC VALUES
You can use the chmod utility with symbolic values (a combination letters and signs) to change file
permissions for a file or directory.
You can assign the following permissions:
Read (r)
Write (w)
Execute (x)
Permissions can be assigned to the following levels of ownership:
User owner (u)
Group owner (g)
Other (o)
All (a)
To add or remove permissions you can use the following signs:
+ to add the permissions on top of the existing permissions
- to remove the permissions from the existing permission
= to remove the existing permissions and explicitly define the new ones
Procedure
To change the permissions for a file or directory, use:
$ chmod file-name
Replace with the level of ownership you want to set the permissions for. Replace
with one of the signs. Replace with the permissions you want to
assign. Replace file-name with the name of the file or directory. For example, to grant everyone
the permissions to read, write, and execute (rwx) my-script.sh, use the chmod a=rwx my-
script.sh command.
See Base file permissions for more details.
Verification steps
To see the permissions for a particular file, use:
$ ls -l file-name
129Red Hat Enterprise Linux 9 Configuring basic system settings
Replace file-name with the name of the file.
To see the permissions for a particular directory, use:
$ ls -dl directory-name
Replace directory-name with the name of the directory.
To see the permissions for all the files within a particular directory, use:
$ ls -l directory-name
Replace directory-name with the name of the directory.
Example 24.8. Changing permissions for files and directories
To change file permissions for my-file.txt from -rw-rw-r-- to -rw------, use:
1. Display the current permissions for my-file.txt:
$ ls -l my-file.txt
-rw-rw-r--. 1 username username 0 Feb 24 17:56 my-file.txt
2. Remove the permissions to read, write, and execute (rwx) the file from group owner ( g)
and others (o):
$ chmod go= my-file.txt
Note that any permission that is not specified after the equals sign (=) is automatically
prohibited.
3. Verify that the permissions for my-file.txt were set correctly:
$ ls -l my-file.txt
-rw-------. 1 username username 0 Feb 24 17:56 my-file.txt
To change file permissions for my-directory from drwxrwx--- to drwxrwxr-x, use:
1. Display the current permissions for my-directory:
$ ls -dl my-directory
drwxrwx---. 2 username username 4096 Feb 24 18:12 my-directory
2. Add the read and execute (r-x) access for all users ( a):
$ chmod o+rx my-directory
3. Verify that the permissions for my-directory and its content were set correctly:
$ ls -dl my-directory
drwxrwxr-x. 2 username username 4096 Feb 24 18:12 my-directory
130CHAPTER 24. MANAGING FILE PERMISSIONS
24.5. CHANGING FILE PERMISSIONS USING OCTAL VALUES
You can use the chmod utility with octal values (numbers) to change file permissions for a file or
directory.
Procedure
To change the file permissions for an existing file or directory, use:
$ chmod octal_value file-name
Replace file-name with the name of the file or directory. Replace octal_value with an octal value.
See Base file permissions for more details.
131Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 25. MANAGING THE UMASK
You can use the umask utility to display, set, or change the current or default value of the umask.
25.1. DISPLAYING THE CURRENT VALUE OF THE UMASK
You can use the umask utility to display the current value of the umask in symbolic or octal mode.
Procedure
To display the current value of the umask in symbolic mode, use:
$ umask -S
To display the current value of the umask in the octal mode, use:
$ umask
NOTE
When displaying the umask in octal mode, you may notice it displayed as a four
digit number (0002 or 0022). The first digit of the umask represents a special bit
(sticky bit, SGID bit, or SUID bit). If the first digit is set to 0, the special bit is not
set.
25.2. DISPLAYING THE DEFAULT BASH UMASK
There are a number of shells you can use, such as bash, ksh, zsh and tcsh. Those shells can behave as
login or non-login shells. You can invoke the login shell by opening a native or a GUI terminal.
To determine whether you are executing a command in a login or a non-login shell, use the echo $0
command.
Example 25.1. Determining if you are working in a login or a non-login bash shell
If the output of the echo $0 command returns bash, you are executing the command in a
non-login shell.
$ echo $0
bash
The default umask for the non-login shell is set in the /etc/bashrc configuration file.
If the output of the echo $0 command returns -bash, you are executing the command in a
login shell.
# echo $0
-bash
The default umask for the login shell is set in the /etc/login.defs configuration file.
132CHAPTER 25. MANAGING THE UMASK
Procedure
To display the default bash umask for the non-login shell, use:
$ grep umask /etc/bashrc
The output returns:
# By default, we want umask to get set. This sets it for non-login shell.
umask 002
umask 022
To display the default bash umask for the login shell, use:
grep "UMASK" /etc/login.defs
The output returns:
# UMASK is also used by useradd(8) and newusers(8) to set the mode for new
UMASK 022
# If HOME_MODE is not set, the value of UMASK is used to create the mode.
25.3. SETTING THE UMASK USING SYMBOLIC VALUES
You can use the umask utility with symbolic values (a combination letters and signs) to set the umask
for the current shell session
You can assign the following permissions:
Read (r)
Write (w)
Execute (x)
Permissions can be assigned to the following levels of ownership:
User owner (u)
Group owner (g)
Other (o)
All (a)
To add or remove permissions you can use the following signs:
+ to add the permissions on top of the existing permissions
- to remove the permissions from the existing permission
= to remove the existing permissions and explicitly define the new ones
NOTE
133Red Hat Enterprise Linux 9 Configuring basic system settings
NOTE
Any permission that is not specified after the equals sign (=) is automatically
prohibited.
Procedure
To set the umask for the current shell session, use:
$ umask -S
Replace with the level of ownership you want to set the umask for. Replace
with one of the signs. Replace with the permissions you want to
assign. For example, to set the umask to u=rwx,g=rwx,o=rwx, use umask -S a=rwx.
See User file-creation mode for more details.
NOTE
The umask is only valid for the current shell session.
25.4. SETTING THE UMASK USING OCTAL VALUES
You can use the umask utility with octal values (numbers) to set the umask for the current shell session.
Procedure
To set the umask for the current shell session, use:
$ umask octal_value
Replace octal_value with an octal value. See User file-creation mode mask for more details.
NOTE
The umask is only valid for the current shell session.
25.5. CHANGING THE DEFAULT UMASK FOR THE NON-LOGIN SHELL
You can change the default bash umask for standard users by modifying the /etc/bashrc file.
Prerequisites
root access
Procedure
1. As root, open the /etc/bashrc file in the editor.
2. Modify the following sections to set a new default bash umask:
if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then
umask 002
134CHAPTER 25. MANAGING THE UMASK
else
umask 022
fi
Replace the default octal value of the umask (002) with another octal value. See User file-
creation mode mask for more details.
3. Save the changes and exit the editor.
25.6. CHANGING THE DEFAULT UMASK FOR THE LOGIN SHELL
You can change the default bash umask for the root user by modifying the /etc/login.defs file.
Prerequisites
root access
Procedure
1. As root, open the /etc/login.defs file in the editor.
2. Modify the following sections to set a new default bash umask:
# Default initial "umask" value used by login(1) on non-PAM enabled systems.
# Default "umask" value for pam_umask(8) on PAM enabled systems.
# UMASK is also used by useradd(8) and newusers(8) to set the mode for new
# home directories if HOME_MODE is not set.
# 022 is the default value, but 027, or even 077, could be considered
# for increased privacy. There is no One True Answer here: each sysadmin
# must make up their mind.
UMASK 022
Replace the default octal value of the umask (022) with another octal value. See User file-
creation mode mask for more details.
3. Save the changes and exit the editor.
25.7. CHANGING THE DEFAULT UMASK FOR A SPECIFIC USER
You can change the default umask for a specific user by modifying the .bashrc for that user.
Procedure
Append the line that specifies the octal value of the umask into the .bashrc file for the
particular user.
$ echo ''umask octal_value'' >> /home/username/.bashrc
Replace octal_value with an octal value and replace username with the name of the user. See
User file-creation mode mask for more details.
25.8. SETTING DEFAULT PERMISSIONS FOR NEWLY CREATED HOME
135Red Hat Enterprise Linux 9 Configuring basic system settings
25.8. SETTING DEFAULT PERMISSIONS FOR NEWLY CREATED HOME
DIRECTORIES
You can change the permission modes for home directories of newly created users by modifying the
/etc/login.defs file.
Procedure
1. As root, open the /etc/login.defs file in the editor.
2. Modify the following section to set a new default HOME_MODE:
# HOME_MODE is used by useradd(8) and newusers(8) to set the mode for new
# home directories.
# If HOME_MODE is not set, the value of UMASK is used to create the mode.
HOME_MODE 0700
Replace the default octal value (0700) with another octal value. The selected mode will be used
to create the permissions for the home directory.
3. If HOME_MODE is set, save the changes and exit the editor.
4. If HOME_MODE is not set, modify the UMASK to set the mode for the newly created home
directories:
# Default initial "umask" value used by login(1) on non-PAM enabled systems.
# Default "umask" value for pam_umask(8) on PAM enabled systems.
# UMASK is also used by useradd(8) and newusers(8) to set the mode for new
# home directories if HOME_MODE is not set.
# 022 is the default value, but 027, or even 077, could be considered
# for increased privacy. There is no One True Answer here: each sysadmin
# must make up their mind.
UMASK 022
Replace the default octal value (022) with another octal value. See User file-creation mode
mask for more details.
5. Save the changes and exit the editor.
136CHAPTER 26. MANAGING THE ACCESS CONTROL LIST
CHAPTER 26. MANAGING THE ACCESS CONTROL LIST
Each file and directory can only have one user owner and one group owner at a time. If you want to grant
a user permissions to access specific files or directories that belong to a different user or group while
keeping other files and directories private, you can utilize Linux Access Control Lists (ACLs).
26.1. DISPLAYING THE CURRENT ACCESS CONTROL LIST
You can use the getfacl utility to display the current ACL.
Procedure
To display the current ACL for a particular file or directory, use:
$ getfacl file-name
Replace file-name with the name of the file or directory.
26.2. SETTING THE ACCESS CONTROL LIST
You can use the setfacl utility to set the ACL for a file or directory.
Prerequisites
root access.
Procedure
To set the ACL for a file or directory, use:
# setfacl -m u:username:symbolic_value file-name
Replace username with the name of the user, symbolic_value with a symbolic value, and file-name with
the name of the file or directory. For more information see the setfacl man page.
Example 26.1. Modifying permissions for a group project
The following example describes how to modify permissions for the group-project file owned by the
root user that belongs to the root group so that this file is:
Not executable by anyone.
The user andrew has the rw- permissions.
The user susan has the --- permissions.
Other users have the r-- permissions.
Procedure
# setfacl -m u:andrew:rw- group-project
# setfacl -m u:susan:--- group-project
137Red Hat Enterprise Linux 9 Configuring basic system settings
Verification steps
To verify that the user andrew has the rw- permission, the user susan has the ---
permission, and other users have the r-- permission, use:
$ getfacl group-project
The output returns:
# file: group-project
# owner: root
# group: root
user:andrew:rw-
user:susan:---
group::r--
mask::rw-
other::r--
138CHAPTER 27. USING THE CHRONY SUITE TO CONFIGURE NTP
CHAPTER 27. USING THE CHRONY SUITE TO CONFIGURE
NTP
Accurate timekeeping 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.
Starting with Red Hat Enterprise Linux 8, the NTP protocol is implemented by the chronyd daemon,
available from the repositories in the chrony package.
The following sections describe how to use the chrony suite to configure NTP.
27.1. INTRODUCTION TO 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 of chronyd, a daemon that runs in user space, and chronyc, a command line program
which can be used to monitor the performance of chronyd and to change various operating parameters
when it is running.
The chrony daemon, chronyd, can be monitored and controlled by the command line utility chronyc.
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.
27.2. USING CHRONYC TO CONTROL CHRONYD
This section describes how to control chronyd using the chronyc command line utility.
Procedure
1. To make changes to the local instance of chronyd using the command line utility chronyc in
139Red Hat Enterprise Linux 9 Configuring basic system settings
1. To make changes to the local instance of chronyd using the command line utility chronyc in
interactive mode, enter the following command as root:
# chronyc
chronyc must run as root if some of the restricted commands are to be used.
The chronyc command prompt will be displayed as follows:
chronyc>
2. To list all of the commands, type help.
3. Alternatively, 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.
140CHAPTER 28. USING CHRONY
CHAPTER 28. USING CHRONY
The following sections describe how to install, start, and stop chronyd, and how to check if chrony is
synchronized. Sections also describe how to manually adjust System Clock.
28.1. MANAGING CHRONY
The following procedure describes how to install, start, stop, and check the status of chronyd.
Procedure
1. The chrony suite is installed by default on Red Hat Enterprise Linux. To ensure that it is, run the
following command as root:
# dnf install chrony
The default location for the chrony daemon is /usr/sbin/chronyd. The command line utility will
be installed to /usr/bin/chronyc.
2. To check the status of chronyd, issue the following command:
$ systemctl status chronyd
chronyd.service - NTP client/server
Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled)
Active: active (running) since Wed 2013-06-12 22:23:16 CEST; 11h ago
3. To start chronyd, issue the following command as root:
# systemctl start chronyd
To ensure chronyd starts automatically at system start, issue the following command as root:
# systemctl enable chronyd
4. To stop chronyd, issue the following command as root:
# systemctl stop chronyd
To prevent chronyd from starting automatically at system start, issue the following command
as root:
# systemctl disable chronyd
28.2. CHECKING IF CHRONY IS SYNCHRONIZED
The following procedure describes how to check if chrony is synchronized with the use of the tracking,
sources, and sourcestats commands.
Procedure
1. To check chrony tracking, issue the following command:
141Red Hat Enterprise Linux 9 Configuring basic system settings
$ 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
Skew : 0.129 ppm
Root delay : 0.013639022 seconds
Root dispersion : 0.001100737 seconds
Update interval : 64.2 seconds
Leap status : Normal
2. The sources command displays information about the current time sources that chronyd is
accessing. To check chrony sources, issue the following command:
$ 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 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.
3. The sourcestats command displays information about the drift rate and offset estimation
process for each of the sources currently being examined by chronyd. To check chrony source
statistics, issue the following command:
$ chronyc sourcestats
210 Number of sources = 1
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 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.
Additional resources
chronyc(1) man page
28.3. MANUALLY ADJUSTING THE SYSTEM CLOCK
The following procedure describes how to manually adjust the System Clock.
Procedure
1. To step the system clock immediately, bypassing any adjustments in progress by slewing, issue
142CHAPTER 28. USING CHRONY
1. To step the system clock immediately, bypassing any adjustments in progress by slewing, issue
the following command as root:
# 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.
28.4. 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.
The following procedure describes how to set up chrony for asystem in an isolated network.
Procedure
1. On the system selected to be the master, using a text editor running as root, edit
/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.
2. On the systems selected to be direct clients of the master, using a text editor running as root,
edit the /etc/chrony.conf as follows:
server master
driftfile /var/lib/chrony/drift
logdir /var/log/chrony
log measurements statistics tracking
keyfile /etc/chrony.keys
commandkey 24
local stratum 10
initstepslew 20 master
allow 192.0.2.123
Where 192.0.2.123 is the address of the master, and master is the host name of the master.
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
143Red Hat Enterprise Linux 9 Configuring basic system settings
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.
28.5. CONFIGURING REMOTE MONITORING ACCESS
chronyc can access chronyd 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 when chronyc 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
smoothing
sources
sourcestats
tracking
waitsync
The set of hosts from which chronyd accepts these commands can be configured with the cmdallow
directive in the configuration file of chronyd, or the cmdallow command in chronyc. 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 a Not authorised error, even if it is from localhost.
The following procedure describes how to access chronyd remotely with chronyc.
Procedure
1. Allow access from both IPv4 and IPv6 addresses by adding the following to the
/etc/chrony.conf file:
144CHAPTER 28. USING CHRONY
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
Optionally, you can open port 323 permanently using the --permanent option:
# firewall-cmd --permanent --zone=public --add-port=323/udp
4. If you opened port 323 permanently, reload the firewall configuration:
firewall-cmd --reload
Additional resources
chrony.conf(5) man page
28.6. MANAGING TIME SYNCHRONIZATION USING RHEL SYSTEM
ROLES
You can manage time synchronization on multiple target machines using the timesync role. The
timesync role installs and configures an NTP or PTP implementation to operate as an NTP client or PTP
slave in order to synchronize the system clock with NTP servers or grandmasters in PTP domains.
WARNING
The timesync role replaces the configuration of the given or detected provider
service on the managed host. Previous settings are lost, even if they are not
specified in the role variables. The only preserved setting is the choice of provider if
the timesync_ntp_provider variable is not defined.
The following example shows how to apply the timesync role in a situation with just one pool of servers.
Example 28.1. An example playbook applying the timesync role for a single pool of servers
145Red Hat Enterprise Linux 9 Configuring basic system settings
---
- hosts: timesync-test
vars:
timesync_ntp_servers:
- hostname: 2.rhel.pool.ntp.org
pool: yes
iburst: yes
roles:
- rhel-system-roles.timesync
For a detailed reference on timesync role variables, install the rhel-system-roles package, and see the
README.md or README.html files in the /usr/share/doc/rhel-system-roles/timesync directory.
Additional resources
Getting started with RHEL System Roles
28.7. ADDITIONAL RESOURCES
chronyc(1) man page
chronyd(8) man page
Frequently Asked Questions
146CHAPTER 29. CHRONY WITH HW TIMESTAMPING
CHAPTER 29. CHRONY WITH HW 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 and NTP 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.
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 with PTP support, and
prefer NTP on networks that do not have such switches and routers.
The following sections describe how to:
Verify support for hardware timestamping
Enable hardware timestamping
Configure client polling interval
Enable interleaved mode
Configure server for large number of clients
Verify hardware timestamping
Configure PTP-NTP bridge
29.1. VERIFYING SUPPORT FOR HARDWARE TIMESTAMPING
To verify that hardware timestamping with NTP is supported by an interface, use the ethtool -T
command. An interface can be used for hardware timestamping with NTP if ethtool lists the
SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE capabilities
and also the HWTSTAMP_FILTER_ALL filter mode.
Example 29.1. 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:
147Red Hat Enterprise Linux 9 Configuring basic system settings
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)
ptpv2-l4-sync (HWTSTAMP_FILTER_PTP_V2_L4_SYNC)
ptpv2-l4-delay-req (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ)
ptpv2-l2-sync (HWTSTAMP_FILTER_PTP_V2_L2_SYNC)
ptpv2-l2-delay-req (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ)
ptpv2-event (HWTSTAMP_FILTER_PTP_V2_EVENT)
ptpv2-sync (HWTSTAMP_FILTER_PTP_V2_SYNC)
ptpv2-delay-req (HWTSTAMP_FILTER_PTP_V2_DELAY_REQ)
29.2. 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 ptp4l from the linuxptp package, is using hardware timestamping on an interface.
Multiple hwtimestamp directives are allowed in the chrony configuration file.
Example 29.2. Enabling hardware timestamping by using the hwtimestamp directive
hwtimestamp eth0
hwtimestamp eth1
hwtimestamp *
29.3. 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 local NTP server using one second polling
interval:
server ntp.local minpoll 0 maxpoll 0
29.4. ENABLING INTERLEAVED MODE
NTP servers that are not hardware NTP appliances, but rather general purpose computers running a
software NTP implementation, like chrony, 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 the xleave option to
the server directive in /etc/chrony.conf:
server ntp.local minpoll 0 maxpoll 0 xleave
148CHAPTER 29. CHRONY WITH HW TIMESTAMPING
29.5. 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
29.6. 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 29.3. 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 an NTP client or peer, you can have the transmit and receive
timestamping modes and the interleaved mode reported for each NTP source by the chronyc ntpdata
command:
Example 29.4. 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
149Red Hat Enterprise Linux 9 Configuring basic system settings
TX timestamping : Hardware
RX timestamping : Hardware
Total TX : 27
Total RX : 27
Total valid RX : 27
Example 29.5. Reporting the stability of NTP measurements
# 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:
210 Number of sources = 1
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
ntp.local 12 7 11 +0.000 0.019 +0ns 49ns
29.7. 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.
Configure chronyd to provide the system time using the other interface:
Example 29.6. Configuring chronyd to provide the system time using the other interface
bindaddress 203.0.113.74
hwtimestamp eth1
local stratum 1
150CHAPTER 30. OVERVIEW OF NETWORK TIME SECURITY (NTS) IN CHRONY
CHAPTER 30. OVERVIEW OF NETWORK TIME SECURITY
(NTS) IN CHRONY
Network Time Security (NTS) is an authentication mechanism for Network Time Protocol (NTP),
designed to scale substantial clients. It verifies that the packets received from the server machines are
unaltered while moving to the client machine. Network Time Security (NTS) includes a Key
Establishment (NTS-KE) protocol that automatically creates the encryption keys used between the
server and its clients.
30.1. ENABLING NETWORK TIME SECURITY (NTS) IN THE CLIENT
CONFIGURATION FILE
By default, Network Time Security (NTS) is not enabled. You can enable NTS in the /etc/chrony.conf.
For that, perform the following steps:
Prerequisites
Server with the NTS support
Procedure
In the client configuration file:
1. Specify the server with the nts option in addition to the recommended iburst option.
For example:
server time.example.com iburst nts
server nts.netnod.se iburst nts
server ptbtime1.ptb.de iburst nts
2. To avoid repeating the Network Time Security-Key Establishment (NTS-KE) session during
system boot, add the following line to chrony.conf, if it is not present:
ntsdumpdir /var/lib/chrony
3. To disable synchronization with Network Time Protocol (NTP) servers provided by DHCP,
comment out or remove the following line in chrony.conf, if it is present:
sourcedir /run/chrony-dhcp
4. Save your changes.
5. Restart the chronyd service:
systemctl restart chronyd
Verification
Verify if the NTS keys were successfully established:
# chronyc -N authdata
151Red Hat Enterprise Linux 9 Configuring basic system settings
Name/IP address Mode KeyID Type KLen Last Atmp NAK Cook CLen
================================================================
time.example.com NTS 1 15 256 33m 0 0 8 100
nts.sth1.ntp.se NTS 1 15 256 33m 0 0 8 100
nts.sth2.ntp.se NTS 1 15 256 33m 0 0 8 100
The KeyID, Type, and KLen should have non-zero values. If the value is zero, check the system
log for error messages from chronyd.
Verify the client is making NTP measurements:
# chronyc -N sources
MS Name/IP address Stratum Poll Reach LastRx Last sample
=========================================================
time.example.com 3 6 377 45 +355us[ +375us] +/- 11ms
nts.sth1.ntp.se 1 6 377 44 +237us[ +237us] +/- 23ms
nts.sth2.ntp.se 1 6 377 44 -170us[ -170us] +/- 22ms
The Reach column should have a non-zero value; ideally 377. If the value rarely gets 377 or
never gets to 377, it indicates that NTP requests or responses are getting lost in the network.
Additional resources
chrony.conf(5) man page
30.2. ENABLING NETWORK TIME SECURITY (NTS) ON THE SERVER
If you run your own Network Time Protocol (NTP) server, you can enable the server Network Time
Security (NTS) support to facilitate its clients to synchronize securely.
If the NTP server is a client of other servers, that is, it is not a Stratum 1 server, it should use NTS or
symmetric key for its synchronization.
Prerequisites
Server private key in PEM format
Server certificate with required intermediate certificates in PEM format
Procedure
1. Specify the private key and the certificate file in chrony.conf
For example:
ntsserverkey /etc/pki/tls/private/foo.example.net.key
ntsservercert /etc/pki/tls/certs/foo.example.net.crt
2. Ensure that both the key and certificate files are readable by the chrony system user, by setting
the group ownership.
For example:
chown :chrony /etc/pki/tls/*/foo.example.net.*
152CHAPTER 30. OVERVIEW OF NETWORK TIME SECURITY (NTS) IN CHRONY
3. Ensure the ntsdumpdir /var/lib/chrony directive is present in the chrony.conf.
4. Restart the chronyd service:
systemctl restart chronyd
IMPORTANT
If the server has a firewall, it needs to allow both the UDP 123 and TCP 4460
ports for NTP and Network Time Security-Key Establishment (NTS-KE).
Verification
Perform a quick test from a client machine with the following command:
$ chronyd -Q -t 3 ''server
foo.example.net iburst nts maxsamples 1''
2021-09-15T13:45:26Z chronyd version 4.1 starting (+CMDMON +NTP +REFCLOCK +RTC
+PRIVDROP +SCFILTER +SIGND +ASYNCDNS +NTS +SECHASH +IPV6 +DEBUG)
2021-09-15T13:45:26Z Disabled control of system clock
2021-09-15T13:45:28Z System clock wrong by 0.002205 seconds (ignored)
2021-09-15T13:45:28Z chronyd exiting
The System clock wrong message indicates the NTP server is accepting NTS-KE connections
and responding with NTS-protected NTP messages.
Verify the NTS-KE connections and authenticated NTP packets observed on the server:
# chronyc serverstats
NTP packets received : 7
NTP packets dropped : 0
Command packets received : 22
Command packets dropped : 0
Client log records dropped : 0
NTS-KE connections accepted: 1
NTS-KE connections dropped : 0
Authenticated NTP packets: 7
If the value of the NTS-KE connections accepted and Authenticated NTP packets field is a
non-zero value, it means that at least one client was able to connect to the NTS-KE port and
send an authenticated NTP request.
153Red Hat Enterprise Linux 9 Configuring basic system settings
CHAPTER 31. USING SECURE COMMUNICATIONS BETWEEN
TWO SYSTEMS WITH OPENSSH
SSH (Secure Shell) is a protocol which provides 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, which prevents
intruders to collect unencrypted passwords from the connection.
Red Hat Enterprise Linux includes the basic OpenSSH packages: the general openssh package, the
openssh-server package and the openssh-clients package. Note that the OpenSSH packages require
the OpenSSL package openssl-libs, which installs several important cryptographic libraries that enable
OpenSSH to provide encrypted communications.
31.1. SSH AND OPENSSH
SSH (Secure Shell) is a program for logging into a remote machine and executing commands on that
machine. The SSH protocol provides secure encrypted communications between two untrusted hosts
over an insecure network. You can also forward X11 connections and arbitrary TCP/IP ports over the
secure channel.
The SSH protocol mitigates security threats, such as interception of communication between two
systems and impersonation of a particular host, when you use it for remote shell login or file copying.
This is because the SSH client and server use digital signatures to verify their identities. Additionally, all
communication between the client and server systems is encrypted.
A host key authenticates hosts in the SSH protocol. Host keys are cryptographic keys that are
generated automatically when OpenSSH is first installed, or when the host boots for the first time.
OpenSSH is an implementation of the SSH protocol supported by Linux, UNIX, and similar operating
systems. It includes the core files necessary for both the OpenSSH client and server. The OpenSSH
suite consists of the following user-space tools:
ssh is a remote login program (SSH client).
sshd is an OpenSSH SSH daemon.
scp is a secure remote file copy program.
sftp is a secure file transfer program.
ssh-agent is an authentication agent for caching private keys.
ssh-add adds private key identities to ssh-agent.
ssh-keygen generates, manages, and converts authentication keys for ssh.
ssh-copy-id is a script that adds local public keys to the authorized_keys file on a remote SSH
server.
ssh-keyscan gathers SSH public host keys.
NOTE
154CHAPTER 31. USING SECURE COMMUNICATIONS BETWEEN TWO SYSTEMS WITH OPENSSH
NOTE
In RHEL 9, the Secure copy protocol (SCP) is replaced with the SSH File Transfer
Protocol (SFTP) by default. This is because SCP has already caused security issues, for
example CVE-2020-15778.
If SFTP is unavailable or incompatible in your scenario, you can use the -O option to force
use of the original SCP/RCP protocol.
For additional information, see the OpenSSH SCP protocol deprecation in Red Hat
Enterprise Linux 9 article.
Two versions of SSH currently exist: version 1, and the newer version 2. The OpenSSH suite in RHEL
supports only SSH version 2. It has an enhanced key-exchange algorithm that is not vulnerable to
exploits known in version 1.
OpenSSH, as one of core cryptographic subsystems of RHEL, uses system-wide crypto policies. This
ensures that weak cipher suites and cryptographic algorithms are disabled in the default configuration.
To modify the policy, the administrator must either use the update-crypto-policies command to adjust
the settings or manually opt out of the system-wide crypto policies.
The OpenSSH suite uses two sets of configuration files: one for client programs (that is, ssh, scp, and
sftp), and another for the server (the sshd daemon).
System-wide SSH configuration information is stored in the /etc/ssh/ directory. User-specific SSH
configuration information is stored in ~/.ssh/ in the user’s home directory. For a detailed list of
OpenSSH configuration files, see the FILES section in the sshd(8) man page.
Additional resources
Man pages listed by using the man -k ssh command
Using system-wide cryptographic policies
31.2. CONFIGURING AND STARTING AN OPENSSH SERVER
Use the following procedure for a basic configuration that might be required for your environment and
for starting an OpenSSH server. Note that after the default RHEL installation, the sshd daemon is
already started and server host keys are automatically created.
Prerequisites
The openssh-server package is installed.
Procedure
1. Start the sshd daemon in the current session and set it to start automatically at boot time:
# systemctl start sshd
# systemctl enable sshd
2. To specify different addresses than the default 0.0.0.0 (IPv4) or :: (IPv6) for the
ListenAddress directive in the /etc/ssh/sshd_config configuration file and to use a slower
dynamic network configuration, add the dependency on the network-online.target target unit
155Red Hat Enterprise Linux 9 Configuring basic system settings
to the sshd.service unit file. To achieve this, create the
/etc/systemd/system/sshd.service.d/local.conf file with the following content:
[Unit]
Wants=network-online.target
After=network-online.target
3. Review if OpenSSH server settings in the /etc/ssh/sshd_config configuration file meet the
requirements of your scenario.
4. Optionally, change the welcome message that your OpenSSH server displays before a client
authenticates by editing the /etc/issue file, for example:
Welcome to ssh-server.example.com
Warning: By accessing this server, you agree to the referenced terms and conditions.
Ensure that the Banner option is not commented out in /etc/ssh/sshd_config and its value
contains /etc/issue:
# less /etc/ssh/sshd_config | grep Banner
Banner /etc/issue
Note that to change the message displayed after a successful login you have to edit the
/etc/motd file on the server. See the pam_motd man page for more information.
5. Reload the systemd configuration and restart sshd to apply the changes:
# systemctl daemon-reload
# systemctl restart sshd
Verification
1. Check that the sshd daemon is running:
# systemctl status sshd
● sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2019-11-18 14:59:58 CET; 6min ago
Docs: man:sshd(8)
man:sshd_config(5)
Main PID: 1149 (sshd)
Tasks: 1 (limit: 11491)
Memory: 1.9M
CGroup: /system.slice/sshd.service
└─1149 /usr/sbin/sshd -D -oCiphers=aes128-ctr,aes256-ctr,aes128-cbc,aes256-cbc -
oMACs=hmac-sha2-256,>
Nov 18 14:59:58 ssh-server-example.com systemd[1]: Starting OpenSSH server daemon...
Nov 18 14:59:58 ssh-server-example.com sshd[1149]: Server listening on 0.0.0.0 port 22.
Nov 18 14:59:58 ssh-server-example.com sshd[1149]: Server listening on :: port 22.
Nov 18 14:59:58 ssh-server-example.com systemd[1]: Started OpenSSH server daemon.
2. Connect to the SSH server with an SSH client.
156CHAPTER 31. USING SECURE COMMUNICATIONS BETWEEN TWO SYSTEMS WITH OPENSSH
# ssh user@ssh-server-example.com
ECDSA key fingerprint is SHA256:dXbaS0RG/UzlTTku8GtXSz0S1++lPegSy31v3L/FAEc.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added ''ssh-server-example.com'' (ECDSA) to the list of known hosts.
user@ssh-server-example.com''s password:
Additional resources
sshd(8) and sshd_config(5) man pages.
31.3. SETTING AN OPENSSH SERVER FOR KEY-BASED
AUTHENTICATION
To improve system security, enforce key-based authentication by disabling password authentication on
your OpenSSH server.
Prerequisites
The openssh-server package is installed.
The sshd daemon is running on the server.
Procedure
1. Open the /etc/ssh/sshd_config configuration in a text editor, for example:
# vi /etc/ssh/sshd_config
2. Change the PasswordAuthentication option to no:
PasswordAuthentication no
On a system other than a new default installation, check that PubkeyAuthentication no has not
been set and the ChallengeResponseAuthentication directive is set to no. If you are
connected remotely, not using console or out-of-band access, test the key-based login process
before disabling password authentication.
3. To use key-based authentication with NFS-mounted home directories, enable the
use_nfs_home_dirs SELinux boolean:
# setsebool -P use_nfs_home_dirs 1
4. Reload the sshd daemon to apply the changes:
# systemctl reload sshd
Additional resources
sshd(8), sshd_config(5), and setsebool(8) man pages.
157Red Hat Enterprise Linux 9 Configuring basic system settings
31.4. GENERATING SSH KEY PAIRS
Use this procedure to generate an SSH key pair on a local system and to copy the generated public key
to an OpenSSH server. If the server is configured accordingly, you can log in to the OpenSSH server
without providing any password.
IMPORTANT
If you complete the following steps as root, only root is able to use the keys.
Procedure
1. To generate an ECDSA key pair for version 2 of the SSH protocol:
$ ssh-keygen -t ecdsa
Generating public/private ecdsa key pair.
Enter file in which to save the key (/home/joesec/.ssh/id_ecdsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/joesec/.ssh/id_ecdsa.
Your public key has been saved in /home/joesec/.ssh/id_ecdsa.pub.
The key fingerprint is:
SHA256:Q/x+qms4j7PCQ0qFd09iZEFHA+SqwBKRNaU72oZfaCI
joesec@localhost.example.com
The key''s randomart image is:
+---[ECDSA 256]---+
|.oo..o=++ |
|.. o .oo . |
|. .. o. o |
|....o.+... |
|o.oo.o +S . |
|.=.+. .o |
|E.*+. . . . |
|.=..+ +.. o |
| . oo*+o. |
+----[SHA256]-----+
You can also generate an RSA key pair by using the -t rsa option with the ssh-keygen
command or an Ed25519 key pair by entering the ssh-keygen -t ed25519 command.
2. To copy the public key to a remote machine:
$ ssh-copy-id joesec@ssh-server-example.com
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are
already installed
joesec@ssh-server-example.com''s password:
...
Number of key(s) added: 1
Now try logging into the machine, with: "ssh ''joesec@ssh-server-example.com''" and check to
make sure that only the key(s) you wanted were added.
If you do not use the ssh-agent program in your session, the previous command copies the
most recently modified ~/.ssh/id*.pub public key if it is not yet installed. To specify another
public-key file or to prioritize keys in files over keys cached in memory by ssh-agent, use the
158CHAPTER 31. USING SECURE COMMUNICATIONS BETWEEN TWO SYSTEMS WITH OPENSSH
ssh-copy-id command with the -i option.
NOTE
If you reinstall your system and want to keep previously generated key pairs, back up the
~/.ssh/ directory. After reinstalling, copy it back to your home directory. You can do this
for all users on your system, including root.
Verification
1. Log in to the OpenSSH server without providing any password:
$ ssh joesec@ssh-server-example.com
Welcome message.
...
Last login: Mon Nov 18 18:28:42 2019 from ::1
Additional resources
ssh-keygen(1) and ssh-copy-id(1) man pages.
31.5. USING SSH KEYS STORED ON A SMART CARD
Red Hat Enterprise Linux enables you to use RSA and ECDSA keys stored on a smart card on OpenSSH
clients. Use this procedure to enable authentication using a smart card instead of using a password.
Prerequisites
On the client side, the opensc package is installed and the pcscd service is running.
Procedure
1. List all keys provided by the OpenSC PKCS #11 module including their PKCS #11 URIs and save
the output to the keys.pub file:
$ ssh-keygen -D pkcs11: > keys.pub
$ ssh-keygen -D pkcs11:
ssh-rsa AAAAB3NzaC1yc2E...KKZMzcQZzx
pkcs11:id=%02;object=SIGN%20pubkey;token=SSH%20key;manufacturer=piv_II?module-
path=/usr/lib64/pkcs11/opensc-pkcs11.so
ecdsa-sha2-nistp256 AAA...J0hkYnnsM=
pkcs11:id=%01;object=PIV%20AUTH%20pubkey;token=SSH%20key;manufacturer=piv_II?
module-path=/usr/lib64/pkcs11/opensc-pkcs11.so
2. To enable authentication using a smart card on a remote server (example.com), transfer the
public key to the remote server. Use the ssh-copy-id command with keys.pub created in the
previous step:
$ ssh-copy-id -f -i keys.pub username@example.com
3. To connect to example.com using the ECDSA key from the output of the ssh-keygen -D
command in step 1, you can use just a subset of the URI, which uniquely references your key, for
example:
159Red Hat Enterprise Linux 9 Configuring basic system settings
$ ssh -i "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so" example.com
Enter PIN for ''SSH key'':
[example.com] $
4. You can use the same URI string in the ~/.ssh/config file to make the configuration permanent:
$ cat ~/.ssh/config
IdentityFile "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so"
$ ssh example.com
Enter PIN for ''SSH key'':
[example.com] $
Because OpenSSH uses the p11-kit-proxy wrapper and the OpenSC PKCS #11 module is
registered to PKCS#11 Kit, you can simplify the previous commands:
$ ssh -i "pkcs11:id=%01" example.com
Enter PIN for ''SSH key'':
[example.com] $
If you skip the id= part of a PKCS #11 URI, OpenSSH loads all keys that are available in the proxy module.
This can reduce the amount of typing required:
$ ssh -i pkcs11: example.com
Enter PIN for ''SSH key'':
[example.com] $
Additional resources
Fedora 28: Better smart card support in OpenSSH
p11-kit(8), opensc.conf(5), pcscd(8), ssh(1), and ssh-keygen(1) man pages
31.6. MAKING OPENSSH MORE SECURE
The following tips help you to increase security when using OpenSSH. Note that changes in the
/etc/ssh/sshd_config OpenSSH configuration file require reloading the sshd daemon to take effect:
# systemctl reload sshd
IMPORTANT
The majority of security hardening configuration changes reduce compatibility with
clients that do not support up-to-date algorithms or cipher suites.
Disabling insecure connection protocols
To make SSH truly effective, prevent the use of insecure connection protocols that are replaced
by the OpenSSH suite. Otherwise, a user’s password might be protected using SSH for one
session only to be captured later when logging in using Telnet. For this reason, consider
disabling insecure protocols, such as telnet, rsh, rlogin, and ftp.
Enabling key-based authentication and disabling password-based authentication
160CHAPTER 31. USING SECURE COMMUNICATIONS BETWEEN TWO SYSTEMS WITH OPENSSH
Disabling passwords for authentication and allowing only key pairs reduces the attack surface
and it also might save users’ time. On clients, generate key pairs using the ssh-keygen tool and
use the ssh-copy-id utility to copy public keys from clients on the OpenSSH server. To disable
password-based authentication on your OpenSSH server, edit /etc/ssh/sshd_config and
change the PasswordAuthentication option to no:
PasswordAuthentication no
Key types
Although the ssh-keygen command generates a pair of RSA keys by default, you can instruct it
to generate ECDSA or Ed25519 keys by using the -t option. The ECDSA (Elliptic Curve Digital
Signature Algorithm) offers better performance than RSA at the equivalent symmetric key
strength. It also generates shorter keys. The Ed25519 public-key algorithm is an implementation
of twisted Edwards curves that is more secure and also faster than RSA, DSA, and ECDSA.
OpenSSH creates RSA, ECDSA, and Ed25519 server host keys automatically if they are missing.
To configure the host key creation in RHEL, use the sshd-keygen@.service instantiated
service. For example, to disable the automatic creation of the RSA key type:
# systemctl mask sshd-keygen@rsa.service
To exclude particular key types for SSH connections, comment out the relevant lines in
/etc/ssh/sshd_config, and reload the sshd service. For example, to allow only Ed25519 host
keys:
# HostKey /etc/ssh/ssh_host_rsa_key
# HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
Non-default port
By default, the sshd daemon listens on TCP port 22. Changing the port reduces the exposure
of the system to attacks based on automated network scanning and thus increase security
through obscurity. You can specify the port using the Port directive in the
/etc/ssh/sshd_config configuration file.
You also have to update the default SELinux policy to allow the use of a non-default port. To do
so, use the semanage tool from the policycoreutils-python-utils package:
# semanage port -a -t ssh_port_t -p tcp port_number
Furthermore, update firewalld configuration:
# firewall-cmd --add-port port_number/tcp
# firewall-cmd --runtime-to-permanent
In the previous commands, replace port_number with the new port number specified using the
Port directive.
Root login
PermitRootLogin is set to prohibit-password by default. This enforces the use of key-based
authentication instead of the use of passwords for logging in as root and reduces risks by
preventing brute-force attacks.
161Red Hat Enterprise Linux 9 Configuring basic system settings
CAUTION
Enabling logging in as the root user is not a secure practice because the administrator cannot
audit which users run which privileged commands. For using administrative commands, log in
and use sudo instead.
Using the X Security extension
The X server in Red Hat Enterprise Linux clients does not provide the X Security extension.
Therefore, clients cannot request another security layer when connecting to untrusted SSH
servers with X11 forwarding. Most applications are not able to run with this extension enabled
anyway.
By default, the ForwardX11Trusted option in the /etc/ssh/ssh_config.d/05-redhat.conf file is
set to yes, and there is no difference between the ssh -X remote_machine (untrusted host)
and ssh -Y remote_machine (trusted host) command.
If your scenario does not require the X11 forwarding feature at all, set the X11Forwarding
directive in the /etc/ssh/sshd_config configuration file to no.
Restricting access to specific users, groups, or domains
The AllowUsers and AllowGroups directives in the /etc/ssh/sshd_config configuration file
server enable you to permit only certain users, domains, or groups to connect to your OpenSSH
server. You can combine AllowUsers and AllowGroups to restrict access more precisely, for
example:
AllowUsers *@192.168.1.*,*@10.0.0.*,!*@192.168.1.2
AllowGroups example-group
The previous configuration lines accept connections from all users from systems in 192.168.1.*
and 10.0.0.* subnets except from the system with the 192.168.1.2 address. All users must be in
the example-group group. The OpenSSH server denies all other connections.
Note that using allowlists (directives starting with Allow) is more secure than using blocklists
(options starting with Deny) because allowlists block also new unauthorized users or groups.
Changing system-wide cryptographic policies
OpenSSH uses RHEL system-wide cryptographic policies, and the default system-wide
cryptographic policy level offers secure settings for current threat models. To make your
cryptographic settings more strict, change the current policy level:
# update-crypto-policies --set FUTURE
Setting system policy to FUTURE
To opt-out of the system-wide crypto policies for your OpenSSH server, uncomment the line
with the CRYPTO_POLICY= variable in the /etc/sysconfig/sshd file. After this change, values
that you specify in the Ciphers, MACs, KexAlgoritms, and GSSAPIKexAlgorithms sections in
the /etc/ssh/sshd_config file are not overridden. Note that this task requires deep expertise in
configuring cryptographic options.
See Using system-wide cryptographic policies in the Security hardening title for more
information.
Additional resources
162CHAPTER 31. USING SECURE COMMUNICATIONS BETWEEN TWO SYSTEMS WITH OPENSSH
sshd_config(5), ssh-keygen(1), crypto-policies(7), and update-crypto-policies(8) man pages.
31.7. CONNECTING TO A REMOTE SERVER USING AN SSH JUMP
HOST
Use this procedure for connecting your local system to a remote server through an intermediary server,
also called jump host.
Prerequisites
A jump host accepts SSH connections from your local system.
A remote server accepts SSH connections only from the jump host.
Procedure
1. Define the jump host by editing the ~/.ssh/config file on your local system, for example:
Host jump-server1
HostName jump1.example.com
The Host parameter defines a name or alias for the host you can use in ssh commands. The
value can match the real host name, but can also be any string.
The HostName parameter sets the actual host name or IP address of the jump host.
2. Add the remote server jump configuration with the ProxyJump directive to ~/.ssh/config file
on your local system, for example:
Host remote-server
HostName remote1.example.com
ProxyJump jump-server1
3. Use your local system to connect to the remote server through the jump server:
$ ssh remote-server
The previous command is equivalent to the ssh -J jump-server1 remote-server command if
you omit the configuration steps 1 and 2.
NOTE
163Red Hat Enterprise Linux 9 Configuring basic system settings
NOTE
You can specify more jump servers and you can also skip adding host definitions to the
configurations file when you provide their complete host names, for example:
$ ssh -J jump1.example.com,jump2.example.com,jump3.example.com
remote1.example.com
Change the host name-only notation in the previous command if the user names or SSH
ports on the jump servers differ from the names and ports on the remote server, for
example:
$ ssh -J
johndoe@jump1.example.com:75,johndoe@jump2.example.com:75,johndoe@jump3.e
xample.com:75 joesec@remote1.example.com:220
Additional resources
ssh_config(5) and ssh(1) man pages.
31.8. CONNECTING TO REMOTE MACHINES WITH SSH KEYS USING
SSH-AGENT
To avoid entering a passphrase each time you initiate an SSH connection, you can use the ssh-agent
utility to cache the private SSH key. The private key and the passphrase remain secure.
Prerequisites
You have a remote host with SSH daemon running and reachable through the network.
You know the IP address or hostname and credentials to log in to the remote host.
You have generated an SSH key pair with a passphrase and transferred the public key to the
remote machine.
For more information, see Generating SSH key pairs .
Procedure
1. Optional: Verify you can use the key to authenticate to the remote host:
a. Connect to the remote host using SSH:
$ ssh example.user1@198.51.100.1 hostname
b. Enter the passphrase you set while creating the key to grant access to the private key.
$ ssh example.user1@198.51.100.1 hostname
host.example.com
2. Start the ssh-agent.
164CHAPTER 31. USING SECURE COMMUNICATIONS BETWEEN TWO SYSTEMS WITH OPENSSH
$ eval $(ssh-agent)
Agent pid 20062
3. Add the key to ssh-agent.
$ ssh-add ~/.ssh/id_rsa
Enter passphrase for ~/.ssh/id_rsa:
Identity added: ~/.ssh/id_rsa (example.user0@198.51.100.12)
Verification
Optional: Log in to the host machine using SSH.
$ ssh example.user1@198.51.100.1
Last login: Mon Sep 14 12:56:37 2020
Note that you did not have to enter the passphrase.
31.9. ADDITIONAL RESOURCES
sshd(8), ssh(1), scp(1), sftp(1), ssh-keygen(1), ssh-copy-id(1), ssh_config(5),
sshd_config(5), update-crypto-policies(8), and crypto-policies(7) man pages.
OpenSSH Home Page
Configuring SELinux for applications and services with non-standard configurations
Controlling network traffic using firewalld
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