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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 165">
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