Search

Configuring basic system settings

download PDF
Red Hat Enterprise Linux 9

Set up the essential functions of your system and customize your system environment

Red Hat Customer Content Services

Abstract

Perform basic system administration tasks, configure the environment settings, register your system, and configure network access and system security. Administer users, groups, and file permissions. Use system roles for managing system configurations interface on multiple RHEL systems. Use systemd for efficient service management. Configure the Network Time Protocol (NTP) with chrony. Backup and restore your system by using ReaR.

Providing feedback on Red Hat documentation

We appreciate your feedback on our documentation. Let us know how we can improve it.

Submitting feedback through Jira (account required)

  1. Log in to the Jira website.
  2. Click Create in the top navigation bar
  3. Enter a descriptive title in the Summary field.
  4. Enter your suggestion for improvement in the Description field. Include links to the relevant parts of the documentation.
  5. Click Create at the bottom of the dialogue.

Chapter 1. Configuring and managing basic network access

This section describes only basic options on how to configure network settings in Red Hat Enterprise Linux.

1.1. Configuring the network and host name in the graphical installation mode

Follow the steps in this procedure to configure your network and host name.

Procedure

  1. From the Installation Summary window, click Network and Host Name.
  2. From the list in the left-hand pane, select an interface. The details are displayed in the right-hand pane.

    Note

    There are several types of network device naming standards used to identify network devices with persistent names, for example, em1 and wl3sp0. For information about these standards, see the Configuring and managing networking document.

  3. Toggle the ON/OFF switch to enable or disable the selected interface.

    Note

    The installation program automatically detects locally accessible interfaces, and you cannot add or remove them manually.

  4. Click + to add a virtual network interface, which can be either: Team (deprecated), Bond, Bridge, or VLAN.
  5. Click - to remove a virtual interface.
  6. Click Configure to change settings such as IP addresses, DNS servers, or routing configuration for an existing interface (both virtual and physical).
  7. Type a host name for your system in the Host Name field.

    Note
    • The host name can either be a fully qualified domain name (FQDN) in the format hostname.domainname, or a short host name without the domain. Many networks have a Dynamic Host Configuration Protocol (DHCP) service that automatically supplies connected systems with a domain name. To allow the DHCP service to assign the domain name to this system, specify only the short host name.
    • When using static IP and host name configuration, it depends on the planned system use case whether to use a short name or FQDN. Red Hat Identity Management configures FQDN during provisioning but some 3rd party software products may require short name. In either case, to ensure availability of both forms in all situations, add an entry for the host in /etc/hosts in the format IP FQDN short-alias.
    • The value localhost means that no specific static host name for the target system is configured, and the actual host name of the installed system is configured during the processing of the network configuration, for example, by NetworkManager using DHCP or DNS.
    • Host names can only contain alphanumeric characters and - or .. Host name should be equal to or less than 64 characters. Host names cannot start or end with - and .. To be compliant with DNS, each part of a FQDN should be equal to or less than 63 characters and the FQDN total length, including dots, should not exceed 255 characters.
  8. Click Apply to apply the host name to the installer environment.
  9. Alternatively, in the Network and Hostname window, you can choose the Wireless option. Click Select network in the right-hand pane to select your wifi connection, enter the password if required, and click Done.

1.2. Configuring an Ethernet connection by using nmcli

If you connect a host to the network over Ethernet, you can manage the connection’s settings on the command line by using the nmcli utility.

Prerequisites

  • A physical or virtual Ethernet Network Interface Controller (NIC) exists in the server’s configuration.

Procedure

  1. List the NetworkManager connection profiles:

    # nmcli connection show
    NAME                UUID                                  TYPE      DEVICE
    Wired connection 1  a5eb6490-cc20-3668-81f8-0314a27f3f75  ethernet  enp1s0

    By default, NetworkManager creates a profile for each NIC in the host. If you plan to connect this NIC only to a specific network, adapt the automatically-created profile. If you plan to connect this NIC to networks with different settings, create individual profiles for each network.

  2. If you want to create an additional connection profile, enter:

    # nmcli connection add con-name <connection-name> ifname <device-name> type ethernet

    Skip this step to modify an existing profile.

  3. Optional: Rename the connection profile:

    # nmcli connection modify "Wired connection 1" connection.id "Internal-LAN"

    On hosts with multiple profiles, a meaningful name makes it easier to identify the purpose of a profile.

  4. Display the current settings of the connection profile:

    # nmcli connection show Internal-LAN
    ...
    connection.interface-name:     enp1s0
    connection.autoconnect:        yes
    ipv4.method:                   auto
    ipv6.method:                   auto
    ...
  5. Configure the IPv4 settings:

    • To use DHCP, enter:

      # nmcli connection modify Internal-LAN ipv4.method auto

      Skip this step if ipv4.method is already set to auto (default).

    • To set a static IPv4 address, network mask, default gateway, DNS servers, and search domain, enter:

      # nmcli connection modify Internal-LAN ipv4.method manual ipv4.addresses 192.0.2.1/24 ipv4.gateway 192.0.2.254 ipv4.dns 192.0.2.200 ipv4.dns-search example.com
  6. Configure the IPv6 settings:

    • To use stateless address autoconfiguration (SLAAC), enter:

      # nmcli connection modify Internal-LAN ipv6.method auto

      Skip this step if ipv6.method is already set to auto (default).

    • To set a static IPv6 address, network mask, default gateway, DNS servers, and search domain, enter:

      # nmcli connection modify Internal-LAN ipv6.method manual ipv6.addresses 2001:db8:1::fffe/64 ipv6.gateway 2001:db8:1::fffe ipv6.dns 2001:db8:1::ffbb ipv6.dns-search example.com
  7. To customize other settings in the profile, use the following command:

    # nmcli connection modify <connection-name> <setting> <value>

    Enclose values with spaces or semicolons in quotes.

  8. Activate the profile:

    # nmcli connection up Internal-LAN

Verification

  1. Display the IP settings of the NIC:

    # ip address show enp1s0
    2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
        link/ether 52:54:00:17:b8:b6 brd ff:ff:ff:ff:ff:ff
        inet 192.0.2.1/24 brd 192.0.2.255 scope global noprefixroute enp1s0
           valid_lft forever preferred_lft forever
        inet6 2001:db8:1::fffe/64 scope global noprefixroute
           valid_lft forever preferred_lft forever
  2. Display the IPv4 default gateway:

    # ip route show default
    default via 192.0.2.254 dev enp1s0 proto static metric 102
  3. Display the IPv6 default gateway:

    # ip -6 route show default
    default via 2001:db8:1::ffee dev enp1s0 proto static metric 102 pref medium
  4. Display the DNS settings:

    # cat /etc/resolv.conf
    search example.com
    nameserver 192.0.2.200
    nameserver 2001:db8:1::ffbb

    If multiple connection profiles are active at the same time, the order of nameserver entries depend on the DNS priority values in these profile and the connection types.

  5. Use the ping utility to verify that this host can send packets to other hosts:

    # ping <host-name-or-IP-address>

Troubleshooting

  • Verify that the network cable is plugged-in to the host and a switch.
  • Check whether the link failure exists only on this host or also on other hosts connected to the same switch.
  • Verify that the network cable and the network interface are working as expected. Perform hardware diagnosis steps and replace defect cables and network interface cards.
  • If the configuration on the disk does not match the configuration on the device, starting or restarting NetworkManager creates an in-memory connection that reflects the configuration of the device. For further details and how to avoid this problem, see the NetworkManager duplicates a connection after restart of NetworkManager service solution.

Additional resources

  • nm-settings(5) man page

1.3. Configuring an Ethernet connection by using nmtui

If you connect a host to the network over Ethernet, you can manage the connection’s settings in a text-based user interface by using the nmtui application. Use nmtui to create new profiles and to update existing ones on a host without a graphical interface.

Note

In nmtui:

  • Navigate by using the cursor keys.
  • Press a button by selecting it and hitting Enter.
  • Select and clear checkboxes by using Space.

Prerequisites

  • A physical or virtual Ethernet Network Interface Controller (NIC) exists in the server’s configuration.

Procedure

  1. If you do not know the network device name you want to use in the connection, display the available devices:

    # nmcli device status
    DEVICE     TYPE      STATE                   CONNECTION
    enp1s0     ethernet  unavailable             --
    ...
  2. Start nmtui:

    # nmtui
  3. Select Edit a connection, and press Enter.
  4. Choose whether to add a new connection profile or to modify an existing one:

    • To create a new profile:

      1. Press Add.
      2. Select Ethernet from the list of network types, and press Enter.
    • To modify an existing profile, select the profile from the list, and press Enter.
  5. Optional: Update the name of the connection profile.

    On hosts with multiple profiles, a meaningful name makes it easier to identify the purpose of a profile.

  6. If you create a new connection profile, enter the network device name into the Device field.
  7. Depending on your environment, configure the IP address settings in the IPv4 configuration and IPv6 configuration areas accordingly. For this, press the button next to these areas, and select:

    • Disabled, if this connection does not require an IP address.
    • Automatic, if a DHCP server dynamically assigns an IP address to this NIC.
    • Manual, if the network requires static IP address settings. In this case, you must fill further fields:

      1. Press Show next to the protocol you want to configure to display additional fields.
      2. Press Add next to Addresses, and enter the IP address and the subnet mask in Classless Inter-Domain Routing (CIDR) format.

        If you do not specify a subnet mask, NetworkManager sets a /32 subnet mask for IPv4 addresses and /64 for IPv6 addresses.

      3. Enter the address of the default gateway.
      4. Press Add next to DNS servers, and enter the DNS server address.
      5. Press Add next to Search domains, and enter the DNS search domain.

    Figure 1.1. Example of an Ethernet connection with static IP address settings

    nmtui ethernet static IP
  8. Press OK to create and automatically activate the new connection.
  9. Press Back to return to the main menu.
  10. Select Quit, and press Enter to close the nmtui application.

Verification

  1. Display the IP settings of the NIC:

    # ip address show enp1s0
    2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
        link/ether 52:54:00:17:b8:b6 brd ff:ff:ff:ff:ff:ff
        inet 192.0.2.1/24 brd 192.0.2.255 scope global noprefixroute enp1s0
           valid_lft forever preferred_lft forever
        inet6 2001:db8:1::fffe/64 scope global noprefixroute
           valid_lft forever preferred_lft forever
  2. Display the IPv4 default gateway:

    # ip route show default
    default via 192.0.2.254 dev enp1s0 proto static metric 102
  3. Display the IPv6 default gateway:

    # ip -6 route show default
    default via 2001:db8:1::ffee dev enp1s0 proto static metric 102 pref medium
  4. Display the DNS settings:

    # cat /etc/resolv.conf
    search example.com
    nameserver 192.0.2.200
    nameserver 2001:db8:1::ffbb

    If multiple connection profiles are active at the same time, the order of nameserver entries depend on the DNS priority values in these profile and the connection types.

  5. Use the ping utility to verify that this host can send packets to other hosts:

    # ping <host-name-or-IP-address>

Troubleshooting

  • Verify that the network cable is plugged-in to the host and a switch.
  • Check whether the link failure exists only on this host or also on other hosts connected to the same switch.
  • Verify that the network cable and the network interface are working as expected. Perform hardware diagnosis steps and replace defect cables and network interface cards.
  • If the configuration on the disk does not match the configuration on the device, starting or restarting NetworkManager creates an in-memory connection that reflects the configuration of the device. For further details and how to avoid this problem, see the NetworkManager duplicates a connection after restart of NetworkManager service solution.

1.4. Managing networking in the RHEL web console

In the web console, the Networking menu enables you:

  • To display currently received and sent packets
  • To display the most important characteristics of available network interfaces
  • To display content of the networking logs.
  • To add various types of network interfaces (bond, team, bridge, VLAN)

Figure 1.2. Managing Networking in the RHEL web console

cs getting started networking new

1.5. Managing networking using RHEL system roles

You can configure the networking connections on multiple target machines using the network role.

The network role allows to configure the following types of interfaces:

  • Ethernet
  • Bridge
  • Bonded
  • VLAN
  • MacVLAN
  • InfiniBand

The required networking connections for each host are provided as a list within the network_connections variable.

Warning

The network role updates or creates all connection profiles on the target system exactly as specified in the network_connections variable. Therefore, the network role removes options from the specified profiles if the options are only present on the system but not in the network_connections variable.

The following example shows how to apply the network role to ensure that an Ethernet connection with the required parameters exists:

An example playbook applying the network role to set up an Ethernet connection with the required parameters

# SPDX-License-Identifier: BSD-3-Clause
---
- hosts: managed-node-01.example.com
  vars:
    network_connections:

      # Create one Ethernet profile and activate it.
      # The profile uses automatic IP addressing
      # and is tied to the interface by MAC address.
      - name: prod1
        state: up
        type: ethernet
        autoconnect: yes
        mac: "00:00:5e:00:53:00"
        mtu: 1450

  roles:
    - rhel-system-roles.network

1.6. Additional resources

Chapter 2. Registering the system and managing subscriptions

Subscriptions cover products installed on Red Hat Enterprise Linux, including the operating system itself.

You can use a subscription to Red Hat Content Delivery Network to track:

  • Registered systems
  • Products installed on your systems
  • Subscriptions attached to the installed products

2.1. Registering the system after the installation

Use the following procedure to register your system if you have not registered it during the installation process already.

Prerequisites

Procedure

  1. Register and automatically subscribe your system in one step:

    # subscription-manager register --username <username> --password <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:

    1. 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.

    2. 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 remote host configuration.

2.2. 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.

    cockpit subscription Health .

  3. In the Overview filed, click Register.

    cockpit subscription Overview

  4. In the Register system dialog box, select that you want to register using your account credentials.

    cockpit subscriptions account

  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, clear the Insights check box.
  8. Click the Register button.

At this point, your Red Hat Enterprise Linux Enterprise Linux system has been successfully registered.

2.3. Registering a system using Red Hat account on GNOME

Follow the steps in this procedure to enroll your system with your Red Hat account.

Prerequisites

Procedure

  1. Open the system menu, which is accessible from the upper-right screen corner, and click Settings.

    System menu

  2. Go to AboutSubscription.
  3. If you are not using the Red Hat server:

    1. In the Registration Server section, select Custom Address.
    2. Enter the server address in the URL field.
  4. In the Registration Type section, select Red Hat Account.
  5. In the Registration Details section:

    • 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.
  6. Click Register.

2.4. 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. Open the system menu, which is accessible from the upper-right screen corner, and click Settings.

    System menu

  2. Go to AboutSubscription.
  3. If you are not using the Red Hat server:

    1. In the Registration Server section, select Custom Address.
    2. Enter the server address in the URL field.
  4. In the Registration Type section, select Activation Keys.
  5. Under Registration Details:

    • Enter your activation keys in the Activation Keys field.

      Separate your keys by a comma (,).

    • Enter the name or ID of your organization in the Organization field.
  6. Click Register.

Chapter 3. 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.

3.1. 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:

    1. Open a new support case.
    2. Initiate a live chat with a Red Hat expert.
    3. Contact a Red Hat expert by making a call or sending an email.

3.2. 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

    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.

Chapter 4. Changing basic environment settings

Configuration of basic environment settings is a part of the installation process. The following sections guide you when you change them later. The basic configuration of the environment includes:

  • Date and time
  • System locales
  • Keyboard layout
  • Language

4.1. Configuring the date and time

Accurate timekeeping is important for several reasons. In Red Hat Enterprise Linux, timekeeping is ensured by the NTP protocol, which is implemented by a daemon running in user space. The user-space daemon updates the system clock running in the kernel. The system clock can keep time by using various clock sources.

Red Hat Enterprise Linux 9 and later versions use the chronyd daemon to implement NTP. chronyd is available from the chrony package. For more information, see Using the chrony suite to configure NTP.

4.1.1. Displaying the current date and time

To display the current date and time, use either of these steps.

Procedure

  1. Enter the date command:

    $ date
    Mon Mar 30 16:02:59 CEST 2020
  2. To see more details, use the timedatectl command:

    $ timedatectl
    Local time: Mon 2020-03-30 16:04:42 CEST
    Universal time: Mon 2020-03-30 14:04:42 UTC
      RTC time: Mon 2020-03-30 14:04:41
     Time zone: Europe/Prague (CEST, +0200)
    System clock synchronized: yes
    NTP service: active
    RTC in local TZ: no

Additional resources

4.2. Configuring the system locale

System-wide locale settings are stored in the /etc/locale.conf file that is read at early boot by the systemd daemon. Every service or user inherits the locale settings configured in /etc/locale.conf, unless individual programs or individual users override them.

Procedure

  • To list available system locale settings:

    $ localectl list-locales
    C.utf8
    aa_DJ
    aa_DJ.iso88591
    aa_DJ.utf8
    ...
  • To display the current status of the system locales settings:

    $ localectl status
  • To set or change the default system locale settings, use a localectl set-locale sub-command as the root user. For example:

    # localectl set-locale LANG=en_US

Additional resources

  • man localectl(1), man locale(7), and man locale.conf(5)

4.3. Configuring the keyboard layout

The keyboard layout settings control the layout used on the text console and graphical user interfaces.

Procedure

  • To list available keymaps:

    $ localectl list-keymaps
    ANSI-dvorak
    al
    al-plisi
    amiga-de
    amiga-us
    ...
  • To display the current status of keymaps settings:

    $ localectl status
    ...
    VC Keymap: us
    ...
  • To set or change the default system keymap. For example:

    # localectl set-keymap us

Additional resources

  • man localectl(1), man locale(7), and man locale.conf(5)

4.4. Changing the font size in text console mode

You can change the font size in the virtual console by using the setfont command.

  • Enter the setfont command with the name of the font, for example:

    # setfont /usr/lib/kbd/consolefonts/LatArCyrHeb-19.psfu.gz
Note

The setfont command searches for multiple hard-coded paths by default. Therefore, setfont does not require the full name and path to the font.

  • To double the size of the font horizontally and vertically, enter the setfont command with -d parameter:

    # setfont -d LatArCyrHeb-16
Note

The maximum font size that you can double is 16x16 pixel.

  • To preserve the selected font during the reboot of the system, use the FONT variable in the /etc/vconsole.conf file, for example:

    # cat /etc/vconsole.conf
    KEYMAP="us"
    FONT="eurlatgr"
  • You can find various fonts in the kbd-misc package, which is installed with the`kbd` package. For example, the font LatArCyrHeb has many variants:

    # rpm -ql kbd-misc | grep LatAr
    
    /usr/lib/kbd/consolefonts/LatArCyrHeb-08.psfu.gz
    /usr/lib/kbd/consolefonts/LatArCyrHeb-14.psfu.gz
    /usr/lib/kbd/consolefonts/LatArCyrHeb-16+.psfu.gz
    /usr/lib/kbd/consolefonts/LatArCyrHeb-16.psfu.gz
    /usr/lib/kbd/consolefonts/LatArCyrHeb-19.psfu.gz
Note

The maximum supported font size by the virtual console is 32 pixels. You can reduce the font readability problem by using smaller resolution for the console.

4.5. Additional resources

Chapter 5. 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.

5.1. 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

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.

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 the older 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

5.2. Configuring and starting an OpenSSH server

You can change the welcome banner and the IP address of your OpenSSH server. You can also switch to a slower dynamic network configuration.

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. If the sshd service is not already running, start it in the current session and set it to start automatically at boot time:

    # systemctl enable --now 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 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.

    # 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.

5.3. 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 KbdInteractiveAuthentication 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.

5.4. Generating SSH key pairs

You can log in to an OpenSSH server without providing a password by generating an SSH key pair on a local system and copying the generated public key to the OpenSSH server. The server must be configured to allow this option.

Prerequisites

  • You are logged in as the Linux user that will connect to the OpenSSH server. If you complete the following steps as root, only root can use the keys.

Procedure

  1. 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/<username>/.ssh/id_ecdsa):
    Enter passphrase (empty for no passphrase):
    Enter same passphrase again:
    Your identification has been saved in /home/<username>/.ssh/id_ecdsa.
    Your public key has been saved in /home/<username>/.ssh/id_ecdsa.pub.
    The key fingerprint is:
    SHA256:Q/x+qms4j7PCQ0qFd09iZEFHA+SqwBKRNaU72oZfaCI <username>@<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. Copy the public key to a remote machine:

    $ ssh-copy-id <username>@<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
    <username>@<ssh-server-example.com>'s password:
    ...
    Number of key(s) added: 1
    
    Now try logging into the machine, with: "ssh '<username>@<ssh-server-example.com>'" and check to make sure that only the key(s) you wanted were added.

    Replace <username>@<ssh-server-example.com> with your credentials.

    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 ssh-copy-id command with the -i option.

  3. Optional: To keep previously generated key pairs between reinstalls of the system, 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 <username>@<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.

5.5. Using SSH keys stored on a smart card

You can use RSA and ECDSA keys stored on a smart card on OpenSSH clients. Authentication by using a smart card replaces the default password authentication.

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:

    $ 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

5.6. Making OpenSSH more secure

You can tweak the system 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
Warning

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

  • 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
    Note

    In images with cloud-init enabled, the ssh-keygen units are automatically disabled. This is because the ssh-keygen template service can interfere with the cloud-init tool and cause problems with host key generation. To prevent these problems the etc/systemd/system/sshd-keygen@.service.d/disable-sshd-keygen-if-cloud-init-active.conf drop-in configuration file disables the ssh-keygen units if cloud-init is running.

  • 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
    Important

    The Ed25519 algorithm is not FIPS-140-compliant, and OpenSSH does not work with Ed25519 keys in FIPS mode.

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 therefore 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 --remove-port=22/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.

    Warning

    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/50-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.

    The OpenSSH server permits only connections that pass all Allow and Deny directives in /etc/ssh/sshd_config. For example, if the AllowUsers directive lists a user that is not part of a group listed in the AllowGroups directive, then the user cannot log in.

    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
    Warning

    If your system communicates on the internet, you might face interoperability problems due to the strict setting of the FUTURE policy.

You can also disable only specific ciphers for the SSH protocol through the system-wide cryptographic policies. See the Customizing system-wide cryptographic policies with subpolicies section in the Security hardening document for more information.

To opt out of the system-wide cryptographic policies for your OpenSSH server, specify the cryptographic policy in a drop-in configuration file located in the /etc/ssh/sshd_config.d/ directory, with a two-digit number prefix smaller than 50, so that it lexicographically precedes the 50-redhat.conf file, and with a .conf suffix, for example, 49-crypto-policy-override.conf.

See the sshd_config(5) man page for more information.

To opt out of system-wide cryptographic policies for your OpenSSH client, perform one of the following tasks:

  • For a given user, override the global ssh_config with a user-specific configuration in the ~/.ssh/config file.
  • For the entire system, specify the cryptographic policy in a drop-in configuration file located in the /etc/ssh/ssh_config.d/ directory, with a two-digit number prefix smaller than 50, so that it lexicographically precedes the 50-redhat.conf file, and with a .conf suffix, for example, 49-crypto-policy-override.conf.

Additional resources

5.7. Connecting to a remote server using an SSH jump host

You can connect 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.

  4. 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.example.com:75 joesec@remote1.example.com:220

Additional resources

  • ssh_config(5) and ssh(1) man pages.

5.8. 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:

    1. Connect to the remote host using SSH:

      $ ssh example.user1@198.51.100.1 hostname
    2. 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.

    $ 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.

5.9. Configuring secure communication with the ssh system roles

As an administrator, you can use the sshd system role to configure SSH servers and the ssh system role to configure SSH clients consistently on any number of RHEL systems at the same time using the Ansible Core package.

5.9.1. Variables of the sshd RHEL system role

In an sshd system role playbook, you can define the parameters for the SSH configuration file according to your preferences and limitations.

If you do not configure these variables, the system role produces an sshd_config file that matches the RHEL defaults.

In all cases, Booleans correctly render as yes and no in sshd configuration. You can define multi-line configuration items using lists. For example:

sshd_ListenAddress:
  - 0.0.0.0
  - '::'

renders as:

ListenAddress 0.0.0.0
ListenAddress ::

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.sshd/README.md file
  • /usr/share/doc/rhel-system-roles/sshd/ directory

5.9.2. Configuring OpenSSH servers by using the sshd RHEL system role

You can use the sshd RHEL system role to configure multiple SSH servers by running an Ansible playbook.

Note

You can use the sshd RHEL system role with other RHEL system roles that change SSH and SSHD configuration, for example the Identity Management RHEL system roles. To prevent the configuration from being overwritten, make sure that the sshd role uses namespaces (RHEL 8 and earlier versions) or a drop-in directory (RHEL 9).

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content:

    ---
    - name: SSH server configuration
      hosts: managed-node-01.example.com
      tasks:
        - name: Configure sshd to prevent root and password login except from particular subnet
          ansible.builtin.include_role:
            name: rhel-system-roles.sshd
          vars:
            sshd:
              PermitRootLogin: no
              PasswordAuthentication: no
              Match:
                - Condition: "Address 192.0.2.0/24"
                  PermitRootLogin: yes
                  PasswordAuthentication: yes

    The playbook configures the managed node as an SSH server configured so that:

    • password and root user login is disabled
    • password and root user login is enabled only from the subnet 192.0.2.0/24
  2. Validate the playbook syntax:

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook:

    $ ansible-playbook ~/playbook.yml

Verification

  1. Log in to the SSH server:

    $ ssh <username>@<ssh_server>
  2. Verify the contents of the sshd_config file on the SSH server:

    $ cat /etc/ssh/sshd_config.d/00-ansible_system_role.conf
    #
    # Ansible managed
    #
    PasswordAuthentication no
    PermitRootLogin no
    Match Address 192.0.2.0/24
      PasswordAuthentication yes
      PermitRootLogin yes
  3. Check that you can connect to the server as root from the 192.0.2.0/24 subnet:

    1. Determine your IP address:

      $ hostname -I
      192.0.2.1

      If the IP address is within the 192.0.2.1 - 192.0.2.254 range, you can connect to the server.

    2. Connect to the server as root:

      $ ssh root@<ssh_server>

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.sshd/README.md file
  • /usr/share/doc/rhel-system-roles/sshd/ directory

5.9.3. Variables of the ssh RHEL system role

In an ssh system role playbook, you can define the parameters for the client SSH configuration file according to your preferences and limitations.

If you do not configure these variables, the system role produces a global ssh_config file that matches the RHEL defaults.

In all cases, booleans correctly render as yes or no in ssh configuration. You can define multi-line configuration items using lists. For example:

LocalForward:
  - 22 localhost:2222
  - 403 localhost:4003

renders as:

LocalForward 22 localhost:2222
LocalForward 403 localhost:4003
Note

The configuration options are case sensitive.

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.ssh/README.md file
  • /usr/share/doc/rhel-system-roles/ssh/ directory

5.9.4. Configuring OpenSSH clients by using the ssh RHEL system role

You can use the ssh RHEL system role to configure multiple SSH clients by running an Ansible playbook.

Note

You can use the ssh RHEL system role with other system roles that change SSH and SSHD configuration, for example the Identity Management RHEL system roles. To prevent the configuration from being overwritten, make sure that the ssh role uses a drop-in directory (default in RHEL 8 and later).

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content:

    ---
    - name: SSH client configuration
      hosts: managed-node-01.example.com
      tasks:
        - name: "Configure ssh clients"
          ansible.builtin.include_role:
            name: rhel-system-roles.ssh
          vars:
            ssh_user: root
            ssh:
              Compression: true
              GSSAPIAuthentication: no
              ControlMaster: auto
              ControlPath: ~/.ssh/.cm%C
              Host:
                - Condition: example
                  Hostname: server.example.com
                  User: user1
            ssh_ForwardX11: no

    This playbook configures the root user’s SSH client preferences on the managed nodes with the following configurations:

    • Compression is enabled.
    • ControlMaster multiplexing is set to auto.
    • The example alias for connecting to the server.example.com host is user1.
    • The example host alias is created, which represents a connection to the server.example.com host the with the user1 user name.
    • X11 forwarding is disabled.
  2. Validate the playbook syntax:

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook:

    $ ansible-playbook ~/playbook.yml

Verification

  • Verify that the managed node has the correct configuration by displaying the SSH configuration file:

    # cat ~/root/.ssh/config
    # Ansible managed
    Compression yes
    ControlMaster auto
    ControlPath ~/.ssh/.cm%C
    ForwardX11 no
    GSSAPIAuthentication no
    Host example
      Hostname example.com
      User user1

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.ssh/README.md file
  • /usr/share/doc/rhel-system-roles/ssh/ directory

5.9.5. Using the sshd RHEL system role for non-exclusive configuration

Normally, applying the sshd system role overwrites the entire configuration. This may be problematic if you have previously adjusted the configuration, for example, with a different system role or playbook. To apply the sshd system role for only selected configuration options while keeping other options in place, you can use the non-exclusive configuration.

You can apply a non-exclusive configuration:

  • In RHEL 8 and earlier by using a configuration snippet.
  • In RHEL 9 and later by using files in a drop-in directory. The default configuration file is already placed in the drop-in directory as /etc/ssh/sshd_config.d/00-ansible_system_role.conf.

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content:

    • For managed nodes that run RHEL 8 or earlier:

      ---
      - name: Non-exclusive sshd configuration
        hosts: managed-node-01.example.com
        tasks:
          - name: <Configure SSHD to accept some useful environment variables>
            ansible.builtin.include_role:
              name: rhel-system-roles.sshd
            vars:
              sshd_config_namespace: <my-application>
              sshd:
                # Environment variables to accept
                AcceptEnv:
                  LANG
                  LS_COLORS
                  EDITOR
    • For managed nodes that run RHEL 9 or later:

      - name: Non-exclusive sshd configuration
        hosts: managed-node-01.example.com
        tasks:
          - name: <Configure sshd to accept some useful environment variables>
            ansible.builtin.include_role:
              name: rhel-system-roles.sshd
            vars:
              sshd_config_file: /etc/ssh/sshd_config.d/<42-my-application>.conf
              sshd:
                # Environment variables to accept
                AcceptEnv:
                  LANG
                  LS_COLORS
                  EDITOR

      In the sshd_config_file variable, define the .conf file into which the sshd system role writes the configuration options. Use a two-digit prefix, for example 42- to specify the order in which the configuration files will be applied.

  2. Validate the playbook syntax:

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook:

    $ ansible-playbook ~/playbook.yml

Verification

  • Verify the configuration on the SSH server:

    • For managed nodes that run RHEL 8 or earlier:

      # cat /etc/ssh/sshd_config.d/42-my-application.conf
      # Ansible managed
      #
      AcceptEnv LANG LS_COLORS EDITOR
    • For managed nodes that run RHEL 9 or later:

      # cat /etc/ssh/sshd_config
      ...
      # BEGIN sshd system role managed block: namespace <my-application>
      Match all
        AcceptEnv LANG LS_COLORS EDITOR
      # END sshd system role managed block: namespace <my-application>

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.sshd/README.md file
  • /usr/share/doc/rhel-system-roles/sshd/ directory

5.10. Additional resources

Chapter 6. Configuring basic 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

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

6.3. Additional resources

Chapter 7. Introduction to RHEL system roles

By using RHEL system roles, you can remotely manage the system configurations of multiple RHEL systems across major versions of RHEL.

Important terms and concepts

The following describes important terms and concepts in an Ansible environment:

Control node
A control node is the system from which you run Ansible commands and playbooks. Your control node can be an Ansible Automation Platform, Red Hat Satellite, or a RHEL 9, 8, or 7 host. For more information, see Preparing a control node on RHEL 9.
Managed node
Managed nodes are the servers and network devices that you manage with Ansible. Managed nodes are also sometimes called hosts. Ansible does not have to be installed on managed nodes. For more information, see Preparing a managed node.
Ansible playbook
In a playbook, you define the configuration you want to achieve on your managed nodes or a set of steps for the system on the managed node to perform. Playbooks are Ansible’s configuration, deployment, and orchestration language.
Inventory
In an inventory file, you list the managed nodes and specify information such as IP address for each managed node. In the inventory, you can also organize the managed nodes by creating and nesting groups for easier scaling. An inventory file is also sometimes called a hostfile.

Available roles on a Red Hat Enterprise Linux 9 control node

On a Red Hat Enterprise Linux 9 control node, the rhel-system-roles package provides the following roles:

Role nameRole descriptionChapter title

certificate

Certificate Issuance and Renewal

Requesting certificates by using RHEL system roles

cockpit

Web console

Installing and configuring web console with the cockpit RHEL system role

crypto_policies

System-wide cryptographic policies

Setting a custom cryptographic policy across systems

firewall

Firewalld

Configuring firewalld by using system roles

ha_cluster

HA Cluster

Configuring a high-availability cluster by using system roles

kdump

Kernel Dumps

Configuring kdump by using RHEL system roles

kernel_settings

Kernel Settings

Using Ansible roles to permanently configure kernel parameters

logging

Logging

Using the logging system role

metrics

Metrics (PCP)

Monitoring performance by using RHEL system roles

network

Networking

Using the network RHEL system role to manage InfiniBand connections

nbde_client

Network Bound Disk Encryption client

Using the nbde_client and nbde_server system roles

nbde_server

Network Bound Disk Encryption server

Using the nbde_client and nbde_server system roles

postfix

Postfix

Variables of the postfix role in system roles

postgresql

PostgreSQL

Installing and configuring PostgreSQL by using the postgresql RHEL system role

selinux

SELinux

Configuring SELinux by using system roles

ssh

SSH client

Configuring secure communication with the ssh system roles

sshd

SSH server

Configuring secure communication with the ssh system roles

storage

Storage

Managing local storage by using RHEL system roles

tlog

Terminal Session Recording

Configuring a system for session recording by using the tlog RHEL system role

timesync

Time Synchronization

Configuring time synchronization by using RHEL system roles

vpn

VPN

Configuring VPN connections with IPsec by using the vpn RHEL system role

Additional resources

Chapter 8. Troubleshooting problems by 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.

8.1. 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.

8.2. 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

8.3. 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 8.1. Inspecting the log files in the RHEL 9 web console

viewing logs

8.4. 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 8.1. Viewing system information
CommandDescription

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 8.2. Viewing information about specific services
CommandDescription

journalctl -b _SYSTEMD_UNIT=<name.service>

Filters log to show entries matching the systemd service.

journalctl -b _SYSTEMD_UNIT=<name.service> _PID=<number>

Combines matches. For example, this command shows logs for systemd-units that match <name.service> and the PID <number>.

journalctl -b _SYSTEMD_UNIT=<name.service> _PID=<number> + _SYSTEMD_UNIT=<name2.service>

The plus sign (+) separator combines two expressions in a logical OR. For example, this command shows all messages from the <name.service> service process with the PID plus all messages from the <name2.service> service (from any of its processes).

journalctl -b _SYSTEMD_UNIT=<name.service> _SYSTEMD_UNIT=<name2.service>

This command shows all entries matching either expression, referring to the same field. Here, this command shows logs matching a systemd-unit <name.service> or a systemd-unit <name2.service>.

Table 8.3. Viewing logs related to specific boots
CommandDescription

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=<name.service>

Shows information about the specified boot ID.

8.5. Additional resources

Chapter 9. Managing users and groups

Preventing unauthorized access to files and processes requires an accurate user and group management. If you do not manage accounts centrally or you require a user account or group only on a specific system, you can create them locally on this host.

9.1. 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.

9.1.1. 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.

9.1.2. 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.

    # 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.

9.1.3. 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.

9.2. 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.

9.2.1. Managing accounts and groups using command line tools

Use the following 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
  • 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)

9.2.2. 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.

9.2.3. Adding new accounts using the web console

You can add user accounts to the system and set administration rights to the accounts through the RHEL web console.

Prerequisites

Procedure

  1. Log in to the RHEL web console.
  2. Click Accounts.
  3. Click Create New Account.
  4. 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.

  5. In the Password/Confirm fields, enter the password and retype it for verification that your password is correct.

    The color bar below the fields shows you the security level of the entered password, which does not allow you to create a user with a weak password.

  6. Click Create to save the settings and close the dialog box.
  7. Select the newly created account.
  8. In the Groups drop-down menu, select the groups that you want to add to the new account.

    cockpit accounts new user

    Now you can see the new account in the Accounts settings and you can use its credentials to connect to the system.

9.3. 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.

9.3.1. Adding a new user from the command line

You can 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 9.1. 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

9.3.2. Adding a new group from the command line

You can use the groupadd utility to add a new group.

Prerequisites

  • 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 9.2. 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

9.3.3. 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.

    Example 9.3. 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

9.3.4. 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.

  4. Associate the user and group ownership of the directory with the group-name group:

    # chgrp 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

9.3.5. Removing a user on the command line

You can remove a user account using the command line. In addition to removing the user account, you can optionally remove the user data and metadata, such as their home directory and configuration files.

Prerequisites

  • You have root access.
  • The user currently exists.
  • Ensure that the user is logged out:

    # loginctl terminate-user user-name

Procedure

  • To only remove the user account, and not the user data:

    # userdel user-name
  • To remove the user, the data, and the metadata:

    1. Remove the user, their home directory, their mail spool, and their SELinux user mapping:

      # userdel --remove --selinux-user user-name
    2. Remove additional user metadata:

      # rm -rf /var/lib/AccountsService/users/user-name

      This directory stores information that the system needs about the user before the home directory is available. Depending on the system configuration, the home directory might not be available until the user authenticates at the login screen.

      Important

      If you do not remove this directory and you later recreate the same user, the recreated user will still use certain settings inherited from the removed user.

Additional resources

  • The userdel(8) man page.

9.4. 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

9.4.1. 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.

9.4.2. Adding new accounts using the web console

You can add user accounts to the system and set administration rights to the accounts through the RHEL web console.

Prerequisites

Procedure

  1. Log in to the RHEL web console.
  2. Click Accounts.
  3. Click Create New Account.
  4. 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.

  5. In the Password/Confirm fields, enter the password and retype it for verification that your password is correct.

    The color bar below the fields shows you the security level of the entered password, which does not allow you to create a user with a weak password.

  6. Click Create to save the settings and close the dialog box.
  7. Select the newly created account.
  8. In the Groups drop-down menu, select the groups that you want to add to the new account.

    cockpit accounts new user

    Now you can see the new account in the Accounts settings and you can use its credentials to connect to the system.

9.4.3. 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.
  3. Select the user account for which you want to enforce password expiration.
  4. Click edit on the Password line.

    cockpit edit password change

  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.

    The web console immediately shows the date of the future password change request on the Password line.

9.4.4. 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 9web 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.

    The RHEL web console terminates the sessions.

9.5. 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.

9.5.1. 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.

9.5.2. 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 9.4. 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 developer.

    Example 9.5. 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.

9.5.3. 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 9.6. 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

  • Verify that you changed the primary group of the user:

    $ groups sarah

    The output displays:

    sarah : sarah2

9.5.4. 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 9.7. 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

9.5.5. 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

  • 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 9.8. 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

9.5.6. 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

    If the user is currently a member of a group that you do not specify, the command removes the user from the group.

    Example 9.9. 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

9.6. 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.

9.6.1. Changing the root password as the root user

You can 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.

9.6.2. Changing or resetting the forgotten root password as a non-root user

You can 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.

9.6.3. 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.

    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:

    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

Chapter 10. 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.

10.1. User authorizations in sudoers

The /etc/sudoers file specifies which users can use the sudo command to execute other commands. The rules can apply to individual users and user groups. You can also define rules for groups of hosts, commands, and even users more easily by using aliases. Default aliases are defined in the first part of the /etc/sudoers file.

When a user enters a command with sudo for which the user does not have authorization, the system records a message that contains the string <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 uncommenting the corresponding line. The section with user authorizations 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 create new sudoers authorizations and modify existing authorizations by using the following format:

<username> <hostname.example.com>=(<run_as_user>:<run_as_group>) <path/to/command>

Where:

  • <username> is the user that enters the command, for example, user1. If the value starts with %, it defines a group, for example, %group1.
  • <hostname.example.com> is the name of the host on which the rule applies.
  • The section (<run_as_user>:<run_as_group>) defines the user or group as which the command is executed. If you omit this section, <username> can execute the command as root.
  • <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 apply the rule to all users, hosts, or commands by replacing any of these variables with ALL.

Warning

With overly permissive rules, such as ALL ALL=(ALL) ALL, all users can run all commands as all users on all hosts. This presents serious security risks.

You can specify the arguments negatively by using the ! operator. For example, !root specifies all users except root. Note that allowing specific users, groups, and commands is more secure than disallowing specific users, groups, and commands. This is because allow rules also block new unauthorized users or groups.

Warning

Avoid using negative rules for commands because users can overcome such rules by renaming commands with 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.

To preserve the rules during system updates and for easier fixing of errors, enter new rules by creating new files in the /etc/sudoers.d/ directory instead of entering rules directly to 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 (~).

Additional resources

  • sudo(8) and sudoers(5) man pages

10.2. Granting sudo access to a user

System administrators can allow non-root users to execute administrative commands by granting them sudo access. 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. If the user has authorization for the command, the command is executed as if they were root.

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

  • You have root access to the system.

Procedure

  1. As root, open the /etc/sudoers file.

    # visudo

    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 is not commented out with the number sign (#).
  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 in the administrative wheel group:

    # id <username>
    uid=5000(<username>) gid=5000(<username>) groups=5000(<username>),10(wheel)

Additional resources

  • sudo(8), visudo(8), and sudoers(5) man pages

10.3. Enabling unprivileged users to run certain commands

As an administrator, you can allow unprivileged users to enter certain commands on specific workstations by configuring a policy in the /etc/sudoers.d/ directory.

Prerequisites

  • You have root access to the system.

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/<filename>

    The file opens automatically.

  3. Add the following line to the /etc/sudoers.d/<filename> file:

    <username> <hostname.example.com> = (<run_as_user>:<run_as_group>) <path/to/command>
    • Replace <username> with the name of the user.
    • Replace <hostname.example.com> with the URL of the host.
    • Replace (<run_as_user>:<run_as_group>) with the user or group as which the command can be executed. If you omit this section, <username> can execute the command as root.
    • Replace <path/to/command> with 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.
    • To allow two and more commands on the same host on one line, you can list them separated by a comma followed by a space.

      For example, to allow user1 to execute the dnf and reboot commands on host1.example.com, enter user1 host1.example.com = /bin/dnf, /sbin/reboot.

  4. Optional: To receive email notifications every time the user attempts to use sudo privileges, add the following lines to the file:

    Defaults    mail_always
    Defaults    mailto="<email@example.com>"
  5. Save the changes, and exit the editor.

Verification

  1. To verify if a user can run a command with sudo privileges, switch the account:

    # su <username> -
  2. As the user, enter the command with the sudo command:

    $ sudo <command>
    [sudo] password for <username>:

    Enter the user’s sudo password.

  3. If the privileges are configured correctly, the system displays the list of commands and options. For example, with the dnf command, it shows the following output:

    ...
    usage: dnf [options] COMMAND
    ...

    If the system returns the error message <username> is not in the sudoers file. This incident will be reported, the file for <username> in /etc/sudoers.d/ does not exist.

    If the system returns the error message <username> is not allowed to run sudo on <host.example.com>, the configuration was not completed correctly. Ensure that you are logged in as root and that the configuration was performed correctly.

    If the system returns the error message Sorry, user <username> is not allowed to execute '<path/to/command>' as root on <host.example.com>., the command is not correctly defined in the rule for the user.

Additional resources

  • sudo(8), visudo(8), and sudoers(5) man pages

Chapter 11. Managing file system permissions

File system permissions control the ability of user and group accounts to read, modify, and execute the contents of the files and to enter directories. Set permissions carefully to protect your data against unauthorized access.

11.1. Managing file permissions

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).

11.1.1. 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

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 11.1. 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 11.2. 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-x indicates that the group has permissions to read the content of the directory, but not write - create new entries or delete files. The x permission means that you can also access the directory using the cd command.
  • --- 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.

  • . 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.

11.1.2. 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 both a standard user and 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 11.3. 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.

Users Groups Umask Example

11.1.3. 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 11.4. Default permissions for a directory

When a standard user or 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 11.5. Default permissions for a file

When a standard user or 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.

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.

11.1.4. 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 <level><operation><permission> file-name

    Replace <level> with the level of ownership you want to set the permissions for. Replace <operation> with one of the signs. Replace <permission> 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

    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 11.6. 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

11.1.5. 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.

11.2. 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).

11.2.1. 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.

11.2.2. 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 11.7. 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

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--

11.3. Managing the umask

You can use the umask utility to display, set, or change the current or default value of the umask.

11.3.1. 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.

11.3.2. 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 11.8. 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.

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.

11.3.3. 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

    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 <level><operation><permission>

    Replace <level> with the level of ownership you want to set the umask for. Replace <operation> with one of the signs. Replace <permission> 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.

11.3.4. 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.

11.3.5. 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
        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.

11.3.6. 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.

11.3.7. 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.

11.3.8. 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.

Chapter 12. Managing systemd

As a system administrator, you can manage critical aspects of your system with systemd. Serving as a system and service manager for Linux operating systems, systemd software suite provides tools and services for controlling, reporting, and system initialization. Key features of systemd include:

  • Parallel start of system services during boot
  • On-demand activation of daemons
  • Dependency-based service control logic

The basic object that systemd manages is a systemd unit, a representation of system resources and services. A systemd unit consists of a name, type and a configuration file that defines and manages a particular task. You can use unit files to configure system behavior. See the following examples of various systemd unit types:

Service
Controls and manages individual system services.
Target
Represents a group of units that define system states.
Device
Manages hardware devices and their availability.
Mount
Handles file system mounting.
Timer
Schedules tasks to run at specific intervals.
Note

To display all available unit types:

 # systemctl -t help

12.1. Systemd unit files locations

You can find the unit configuration files in one of the following directories:

Table 12.1. systemd unit files locations
DirectoryDescription

/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 using the systemctl enable command as well as unit files added for extending a service. This directory takes precedence over the directory with runtime unit files.

The default configuration of systemd is defined during the compilation and you can find the configuration in the /etc/systemd/system.conf file. By editing this file, you can modify the default configuration by overriding values for systemd units globally.

For example, to override the default value of the timeout limit, which is set to 90 seconds, use the DefaultTimeoutStartSec parameter to input the required value in seconds.

DefaultTimeoutStartSec=required value

12.2. Managing system services with systemctl

As a system administrator, you can manage system services by using the systemctl utility. You can perform various tasks, such as starting, stopping, restarting running services, enabling and disabling services to start at boot, listing available services, and displaying system services statuses.

12.2.1. Listing system services

You can list all currently loaded service units and display the status of all available service units.

Procedure

Use the systemctl command to perform any of the following tasks:

  • List all currently loaded service units:

    $ 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, or a 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'

    By default, the systemctl list-units command displays only active units. For each service unit file, the command provides an overview of the following parameters:

    UNIT
    The full name of the service unit
    LOAD
    The load state of the configuration file
    ACTIVE or SUB
    The current high-level and low-level unit file activation state
    DESCRIPTION
    A short description of the unit’s purpose and functionality
  • List all loaded units regardless of their state, by using the following command with the --all or -a command line option:

    $ systemctl list-units --type service --all
  • List the status (enabled or disabled) of all available service units:

    $ 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
    The full name of the service unit
    STATE
    The information whether the service unit is enabled or disabled to start automatically during boot

Additional resources

12.2.2. Displaying system service status

You can inspect any service unit to get detailed information and verify the state of the service, whether it is enabled to start during boot or currently running. You can also view services that are ordered to start after or before a particular service unit.

Procedure

Use the systemctl command to perform any of the following tasks:

  • Display detailed information about a service unit that corresponds to a system service:

    $ systemctl status <name>.service

    Replace <name> with the name of the service unit you want to inspect (for example, gdm).

    This command displays the following information:

    • The name of the selected service unit followed by a short description
    • One or more fields described in Available service unit information
    • The execution of the service unit: if the unit is executed by the root user
    • The most recent log entries

      Table 12.2. Available service unit information
      FieldDescription

      Loaded

      Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled to start during boot.

      Active

      Information whether the service unit is running followed by a time stamp.

      Main PID

      The process ID and the name of the corresponding system service.

      Status

      Additional information about the corresponding system service.

      Process

      Additional information about related processes.

      CGroup

      Additional information about related control groups (cgroups).

    Example 12.1. 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
               └─1047 /usr/bin/Xorg :0 -background none -verbose -auth /r...
    
    Oct 17 17:31:23 localhost systemd[1]: Started GNOME Display Manager.
  • Verify that a particular service unit is running:

    $ systemctl is-active <name>.service
  • Determine whether a particular service unit is enabled to start during boot:

    $ systemctl is-enabled <name>.service
    Note

    Both systemctl is-active and systemctl is-enabled commands return an exit status of 0 if the specified service unit is running or enabled.

  • Check what services systemd orders to start before the specified service unit

    # systemctl list-dependencies --after <name>.service

    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]
  • Check what services systemd orders to start after the specified service unit:

    # systemctl list-dependencies --before <name>.service

    For example, to view the list of services systemd orders 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

12.2.3. Starting a system service

You can start system service in the current session by using the start command.

Prerequisites

  • Root access

Procedure

  • Start a system service in the current session:

    # systemctl start <name>.service

    Replace <name> with the name of the service unit you want to start (for example, httpd.service).

    Note

    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 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.

12.2.4. Stopping a system service

If you want to stop a system service in the current session, use the stop command.

Prerequisites

  • Root access

Procedure

  • Stop a system service:

    # systemctl stop <name>.service

    Replace <name> with the name of the service unit you want to stop (for example, bluetooth).

12.2.5. Restarting a system service

You can restart system service in the current session using the restart command to perform the following actions:

  • 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.

Prerequisites

  • Root access

Procedure

  • Restart a system service:

    # systemctl restart <name>.service

    Replace <name> 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.

  • Optional: Restart a service unit only if the corresponding service is already running:

    # systemctl try-restart <name>.service
  • Optional: Reload the configuration without interrupting service execution:

    # systemctl reload <name>.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.

Additional resources

12.2.6. Enabling a system service to start at boot

You can enable a service to start automatically at boot, these changes apply with the next reboot.

Prerequisites

  • Root access
  • The service you want to enable must not be masked. If you have a masked service, unmask it first:

    # systemctl unmask <name>.service

Procedure

  • Enable a service to start at boot:

    # systemctl enable <name>.service

    Replace <name> with the name of the service unit you want to enable (for example, httpd).

  • Optional: You can also enable and start a service by using a single command:

    # systemctl enable --now <name>.service

Additional resources

12.2.7. Disabling a system service to start at boot

You can prevent a service unit from starting automatically at boot time. If you disable a service, it will not start at boot, but you can start it manually. You can also mask a service, so that it cannot be started manually. Masking is a way of disabling a service that makes the service permanently unusable until it is unmasked again.

Prerequisites

  • Root access

Procedure

  • Disable a service to start at boot:

    # systemctl disable <name>.service

    Replace <name> with the name of the service unit you want to disable (for example, bluetooth).

  • Optional: If you want to make a service permanently unusable, mask the service:

    # systemctl mask <name>.service

    This command replaces the /etc/systemd/system/name.service file with a symbolic link to /dev/null, rendering the actual unit file inaccessible to systemd.

Additional resources

12.3. Booting into a target system state

As a system administrator, you can control the boot process of your system, and define the state you want your system to boot into. This is called a systemd target, and it is a set of systemd units that your system starts to reach a certain level of functionality. While working with systemd targets, you can view the default target, select a target at runtime, change the default boot target, boot into emergency or rescue target.

12.3.1. Target unit files

Targets in systemd are groups of related units that act as synchronization points during the start of your system. Target unit files, which end with the .target file extension, represent the systemd targets. The purpose of target units is to group together various systemd units through a chain of dependencies.

Consider the following examples:

  • The graphical.target unit for starting 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.

You can set the following systemd targets as default or current targets:

Table 12.3. Common 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

Additional resources

  • systemd.special(7) man page
  • systemd.target(5) man page

12.3.2. Changing the default target to boot into

When a system starts, systemd activates the default.target symbolic link, which points to the true target unit. You can find the currently selected default target unit in the /etc/systemd/system/default.target file. Each target represents a certain level of functionality and is used for grouping other units. Additionally, target units serve as synchronization points during boot. You can change the default target your system boots into. When you set a default target unit, the current target remains unchanged until the next reboot.

Prerequisites

  • Root access

Procedure

  1. Determine the current default target unit systemd uses to start the system:

    # systemctl get-default
    graphical.target
  2. List the currently loaded targets:

    # systemctl list-units --type target
  3. Configure the system to use a different target unit by default:

    # systemctl set-default <name>.target

    Replace <name> with the name of the target unit you want to use by default.

    Example:
    # systemctl set-default multi-user.target
    Removed /etc/systemd/system/default.target
    Created symlink /etc/systemd/system/default.target -> /usr/lib/systemd/system/multi-user.target
  4. Verify the default target unit:

    # systemctl get-default
    multi-user.target
  5. Apply the changes by rebooting:

    # reboot

Additional resources

  • systemctl(1) man page
  • systemd.special(7) man page
  • bootup(7) man page

12.3.3. Changing the current target

On a running system, you can change the target unit in the current boot without reboot. If you switch to a different target, systemd starts all services and their dependencies that this target requires, and stops all services that the new target does not enable. Isolating a different target affects only the current boot.

Procedure

  1. Optional: Determine the current target:

    # systemctl get-default
    graphical.target
  2. Optional: Display the list of targets you can select:

    # systemctl list-units --type target
    Note

    You can only isolate targets that have the AllowIsolate=yes option set in the unit files.

  3. Change to a different target unit in the current boot:

    # systemctl isolate <name>.target

    Replace <name> with the name of the target unit you want to use in the current boot.

    Example:
    # systemctl isolate multi-user.target

    This command starts the target unit named multi-user and all dependent units, and immediately stops all other unit.

Additional resources

  • systemctl(1) man page

12.3.4. Booting to rescue mode

You can boot to the rescue mode that provides a single-user environment for troubleshooting or repair if the system cannot get to a later target, and the regular booting process fails. In rescue mode, the system attempts to mount all local file systems and start certain important system services, but it does not activate network interfaces.

Prerequisites

  • Root access

Procedure

  • To enter the rescue mode, change the current target in the current session:

    # systemctl rescue
    
    Broadcast message from root@localhost on pts/0 (Fri 2023-03-24 18:23:15 CEST):
    
    The system is going down to rescue mode NOW!
    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, enter the following command with the --no-wall command-line option:

    # systemctl --no-wall rescue

Troubleshooting steps

If your system is not able to enter the rescue mode, you can boot to emergency mode, which provides the most minimal environment possible. 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.

12.3.5. Troubleshooting the boot process

As a system administrator, you can select a non-default target at boot time to troubleshoot the boot process. Changing the target at boot time affects only a single boot. You can boot to emergency mode, which provides the most minimal environment possible.

Procedure

  1. Reboot the system, and interrupt the boot loader menu countdown by pressing any key except the Enter key, which would initiate a normal boot.
  2. Move the cursor to the kernel entry that you want to start.
  3. Press the E key to edit the current entry.
  4. Move to the end of the line that starts with linux and press Ctrl+E to jump to the end of the line:

    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
  5. To choose an alternate boot target, append the systemd.unit= parameter 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 systemd.unit=<name>.target

    Replace <name> with the name of the target unit you want to use. For example, systemd.unit=emergency.target

  6. Press Ctrl+X to boot with these settings.

12.4. Shutting down, suspending, and hibernating the system

As a system administrator, you can use different power management options to manage power consumption, perform a proper shutdown to ensure that all data is saved, or restart the system to apply changes and updates.

12.4.1. System shutdown

To shut down the system, you can either use the systemctl utility directly, or call this utility through the shutdown command.

Using the shutdown has the following advantages:

  • You can schedule a shutdown by using the time argument. This also gives users warning that a system shutdown has been scheduled.
  • You can cancel the shutdown.

12.4.2. Scheduling a system shutdown

As a system administrator, you can schedule a delayed shutdown to give users time to save their work and log off the system. Use the shutdown command to perform the following operations:

  • Shut down the system and power off the machine at a certain time
  • Shut down and halt the system without powering off the machine
  • Cancel a pending shutdown

Prerequisites

  • Root access

Procedure

Use the shutdown command to perform any of the following tasks:

  • Specify the time at which you want to shut down the system and power off the machine:

    # shutdown --poweroff hh:mm

    Where hh:mm is the time in the 24-hour time notation. To prevent new logins, the /run/nologin file is created 5 minutes before system shutdown.

    When you use the time argument, you can notify users logged in to the system of the planned shutdown by specifying an optional wall message, for example shutdown --poweroff 13:59 "Attention. The system will shut down at 13:59".

  • Shut down and halt the system after a delay, without powering off the machine:

    # shutdown --halt +m

    Where +m is the delay time in minutes. You can use the now keyword as an alias for +0.

  • Cancel a pending shutdown:

    # shutdown -c

Additional resources

12.4.3. Shutting down the system using the systemctl command

As a system administrator, you can shut down the system and power off the machine or shut down and halt the system without powering off the machine by using the systemctl command.

Prerequisites

  • Root access

Procedure

Use the systemctl command to perform any of the following tasks:

  • Shut down the system and power off the machine:

    # systemctl poweroff
  • Shut down and halt the system without powering off the machine:

    # 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.

12.4.4. Restarting the system

When you restart the system, systemd stops all running programs and services, the system shuts down, and then immediately starts again. Restarting the system can be helpful in the following situations:

  • After installing new software or updates
  • After making changes to system settings
  • When troubleshooting system issues

Prerequisites

  • Root access

Procedure

  • Restart the system:

    # systemctl reboot
Note

By default, when you use this command, systemd sends an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run this command with the --no-wall option.

12.4.5. Optimizing power consumption by suspending and hibernating the system

As a system administrator, you can manage power consumption, save energy on your systems, and preserve the current state of your system. To do so, apply one of the following modes:

Suspend
Suspending 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, the suspended system state is also vulnerable to power outages.
Hibernate
Hibernating 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.
Hybrid sleep
This combines elements of both hibernation and suspending. The system first saves the current state on the the hard disk drive, and enters a low-power state similar to suspending, which allows the system to resume more quickly. The benefit of hybrid sleep is that if the system loses power during the sleep state, it can still recover the previous state from the saved image on the hard disk, similar to hibernation.
Suspend-then-hibernate
This mode first suspends the system, which results in saving the current system state to RAM and putting the system in a low-power mode. The system hibernates if it remains suspended for a specific period of time that you can define in the HibernateDelaySec parameter. Hibernation saves the system state to the hard disk drive and shuts down the system completely. The suspend-then-hibernate mode provides the benefit of conserving battery power while you are still able to quickly resume work. Additionally, this mode ensures that your data is saved in case of a power failure.

Prerequisites

  • Root access

Procedure

Choose the appropriate method for power saving:

  • Suspend the system:

    # systemctl suspend
  • Hibernate the system:

    # systemctl hibernate
  • Hibernate and suspend the system:

    # systemctl hybrid-sleep
  • Suspend and then hibernate the system:

    # systemctl suspend-then-hibernate

12.4.6. 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 12.4. Overview of the systemctl power management commands
systemctl commandDescription

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.

12.4.7. Changing the power button behavior

When you press the power button on your computer, it suspends or shuts down the system by default. You can customize this behavior according to your preferences.

12.4.7.1. Changing the power button behavior in systemd

When you press the power button in a non-graphical systemd target, it shuts down the system by default. You can customize this behavior according to your preferences.

Prerequisites

  • Administrative access.

Procedure

  1. Open the /etc/systemd/logind.conf configuration file.
  2. Look for the line that says HandlePowerKey=poweroff.
  3. If the line starts with the # symbol, remove it to enable the setting.
  4. Replace poweroff with one of the following options:

    poweroff
    Shut down the computer.
    reboot
    Reboot the system.
    halt
    Initiate a system halt.
    kexec
    Initiate a kexec reboot.
    suspend
    Suspend the system.
    hibernate
    Initiate system hibernation.
    ignore
    Do nothing.

    For example, to reboot the system upon pressing the power button, use this setting:

    HandlePowerKey=reboot
  5. Save your changes and close the editor.

Next steps

12.4.7.2. Changing the power button behavior in GNOME

On the graphical login screen or in the graphical user session, pressing the power button suspends the machine by default. This happens both in cases when the user presses the power button physically or when pressing a virtual power button from a remote console. You can select a different power button behavior.

Prerequisites

Procedure

  1. Create a local database for system-wide settings in the /etc/dconf/db/local.d/01-power file. Enter the following content:

    [org/gnome/settings-daemon/plugins/power]
    power-button-action='suspend'

    Replace suspend with any of the following power button actions:

    nothing
    Does nothing .
    suspend
    Suspends the system.
    hibernate
    Hibernates the system.
    interactive

    Shows a pop-up query asking the user what to do.

    With interactive mode, the system powers off automatically after 60 seconds when pressing the power button. However, you can choose a different behavior from the pop-up query.

  2. Optional: Override the user’s setting, and prevent the user from changing it. Enter the following configuration in the /etc/dconf/db/local.d/locks/01-power file:

    /org/gnome/settings-daemon/plugins/power/power-button-action
  3. Update the system databases:

    # dconf update
  4. Log out and back in again for the system-wide settings to take effect.

Chapter 13. Configuring time synchronization

Accurate timekeeping in an IT environment is important. A consistent time across all network devices improves the traceability of log files and certain protocols rely on synchronized clocks. For example, Kerberos uses time stamps to prevent replay attacks.

13.1. Using the Chrony suite to configure NTP

Accurate timekeeping is important for several 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.

13.1.1. 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.

13.1.2. Using chronyc to control chronyd

You can control chronyd by using the chronyc command line utility.

Procedure

  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.

13.2. 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.

13.2.1. 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

13.2.2. 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:

    $  chronyc tracking
    Reference ID    : CB00710F (ntp-server.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

    You can specify the optional -v argument to print more verbose information. 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

13.2.3. 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 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.

13.2.4. Disabling a chrony dispatcher script

The chrony dispatcher script manages the online and offline state of the NTP servers. As a system administrator, you can disable the dispatcher script to keep chronyd polling the servers constantly.

If you enable NetworkManager on your system to manage networking configuration, the NetworkManager executes the chrony dispatcher script during interface reconfiguration, stop or start operations. However, if you configure certain interfaces or routes outside of NetworkManager, you can encounter the following situation:

  1. The dispatcher script might run when no route to the NTP servers exists, causing the NTP servers to switch to the offline state.
  2. If you establish the route later, the script does not run again by default, and the NTP servers remain in the offline state.

To ensure that chronyd can synchronize with your NTP servers, which have separately managed interfaces, disable the dispatcher script.

Prerequisites

  • You installed NetworkManager on your system and enabled it.
  • Root access

Procedure

  1. To disable the chrony dispatcher script, create a symlink to /dev/null:

    # ln -s /dev/null /etc/NetworkManager/dispatcher.d/20-chrony-onoffline
    Note

    After this change, the NetworkManager cannot execute the dispatcher script, and the NTP servers remain in the online state at all times.

13.2.5. 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 primary timeserver. The other computers are either direct clients of the server, or clients of clients. On the server, the drift file must be manually set with the average rate of drift of the system clock. If the server 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 a system in an isolated network.

Procedure

  1. On the system selected to be the server, 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 192.0.2.0/24

    Where 192.0.2.0/24 is the network or subnet address from which the clients are allowed to connect. For more details, see chrony.conf(7) man page

  2. On the systems selected to be direct clients of the server, using a text editor running as root, edit the /etc/chrony.conf as follows:

    server ntp1.example.net
    driftfile /var/lib/chrony/drift
    logdir /var/log/chrony
    log measurements statistics tracking
    keyfile /etc/chrony.keys
    commandkey 24
    local stratum 10
    initstepslew 20 ntp1.example.net
    allow 192.0.2.123

    Where 192.0.2.123 is the address of the server, and ntp1.example.net is the host name of the server. Clients with this configuration will resynchronize with the server if it restarts.

On the client systems which are not to be direct clients of the server, the /etc/chrony.conf file should be the same except that the local and allow directives should be omitted.

In an isolated network, you can also use the local directive that enables a local reference mode, which allows chronyd operating as an NTP server to appear synchronized to real time, even when it was never synchronized or the last update of the clock happened a long time ago.

To allow multiple servers in the network to use the same local configuration and to be synchronized to one another, without confusing clients that poll more than one server, use the orphan option of the local directive which enables the orphan mode. Each server needs to be configured to poll all other servers with local. This ensures that only the server with the smallest reference ID has the local reference active and other servers are synchronized to it. When the server fails, another one will take over.

13.2.6. 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:

    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

13.2.7. 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 or PTP client to synchronize the system clock.

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 13.1. An example playbook applying the timesync role for a single pool of servers

---
- 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.

13.2.8. Additional resources

13.3. 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

13.3.1. 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 13.2. Verifying support for hardware timestamping on a specific interface

# ethtool -T eth0

Output:

Timestamping parameters for eth0:
Capabilities:
        hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
        software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
        hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
        software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
        software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
        hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
        off                   (HWTSTAMP_TX_OFF)
        on                    (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
        none                  (HWTSTAMP_FILTER_NONE)
        all                   (HWTSTAMP_FILTER_ALL)
        ptpv1-l4-sync         (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
        ptpv1-l4-delay-req    (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
        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)

13.3.2. 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 13.3. Enabling hardware timestamping by using the hwtimestamp directive

hwtimestamp eth0
hwtimestamp eth1
hwtimestamp *

13.3.3. 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

13.3.4. 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

13.3.5. 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

13.3.6. 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 13.4. Log messages for interfaces with enabled hardware timestamping

chronyd[4081]: Enabled HW timestamping on eth0
chronyd[4081]: Enabled HW timestamping on eth1

When chronyd is configured as 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 13.5. Reporting the transmit, receive timestamping and interleaved mode for each NTP source

# chronyc ntpdata

Output:

Remote address  : 203.0.113.15 (CB00710F)
Remote port     : 123
Local address   : 203.0.113.74 (CB00714A)
Leap status     : Normal
Version         : 4
Mode            : Server
Stratum         : 1
Poll interval   : 0 (1 seconds)
Precision       : -24 (0.000000060 seconds)
Root delay      : 0.000015 seconds
Root dispersion : 0.000015 seconds
Reference ID    : 47505300 (GPS)
Reference time  : Wed May 03 13:47:45 2017
Offset          : -0.000000134 seconds
Peer delay      : 0.000005396 seconds
Peer dispersion : 0.000002329 seconds
Response time   : 0.000152073 seconds
Jitter asymmetry: +0.00
NTP tests       : 111 111 1111
Interleaved     : Yes
Authenticated   : No
TX timestamping : Hardware
RX timestamping : Hardware
Total TX        : 27
Total RX        : 27
Total valid RX  : 27

Example 13.6. 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

13.3.7. Configuring PTP-NTP bridge

If a highly accurate Precision Time Protocol (PTP) primary timeserver 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 client and a stratum-1 NTP server. Such a computer needs to have two or more network interfaces, and be close to the primary timeserver 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 13.7. Configuring chronyd to provide the system time using the other interface

bindaddress 203.0.113.74
hwtimestamp eth1
local stratum 1

13.4. 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.

Warning

NTS is not compatible with the FIPS and OSPP profile. When you enable the FIPS and OSPP profile, chronyd that is configured with NTS can abort with a fatal message. You can disable the OSPP profile and FIPS mode for chronyd service by adding the GNUTLS_FORCE_FIPS_MODE=0 to the /etc/sysconfig/chronyd file.

13.4.1. 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
    
    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

13.4.2. 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/<ntp-server.example.net>.key
    ntsservercert /etc/pki/tls/certs/<ntp-server.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//<ntp-server.example.net>.
  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
    
    ntp-server.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.

Chapter 14. 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.

14.1. 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

    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.

14.2. 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 supported only with the rear package version 2.6-17.el9 or later. Earlier versions are available as a Technology Preview feature only. 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 RAM disk (initrd) that can be used with the zIPL boot loader.

Prerequisites

  • 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://<nfsserver name>/<share path>
    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 boot loader, ReaR kernel, and initrd that were used to boot into the rescue environment. Ensure to keep a copy.

Legal Notice

Copyright © 2024 Red Hat, Inc.
The text of and illustrations in this document are licensed by Red Hat under a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). An explanation of CC-BY-SA is available at http://creativecommons.org/licenses/by-sa/3.0/. In accordance with CC-BY-SA, if you distribute this document or an adaptation of it, you must provide the URL for the original version.
Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent permitted by applicable law.
Red Hat, Red Hat Enterprise Linux, the Shadowman logo, the Red Hat logo, JBoss, OpenShift, Fedora, the Infinity logo, and RHCE are trademarks of Red Hat, Inc., registered in the United States and other countries.
Linux® is the registered trademark of Linus Torvalds in the United States and other countries.
Java® is a registered trademark of Oracle and/or its affiliates.
XFS® is a trademark of Silicon Graphics International Corp. or its subsidiaries in the United States and/or other countries.
MySQL® is a registered trademark of MySQL AB in the United States, the European Union and other countries.
Node.js® is an official trademark of Joyent. Red Hat is not formally related to or endorsed by the official Joyent Node.js open source or commercial project.
The OpenStack® Word Mark and OpenStack logo are either registered trademarks/service marks or trademarks/service marks of the OpenStack Foundation, in the United States and other countries and are used with the OpenStack Foundation's permission. We are not affiliated with, endorsed or sponsored by the OpenStack Foundation, or the OpenStack community.
All other trademarks are the property of their respective owners.
Red Hat logoGithubRedditYoutubeTwitter

Learn

Try, buy, & sell

Communities

About Red Hat Documentation

We help Red Hat users innovate and achieve their goals with our products and services with content they can trust.

Making open source more inclusive

Red Hat is committed to replacing problematic language in our code, documentation, and web properties. For more details, see the Red Hat Blog.

About Red Hat

We deliver hardened solutions that make it easier for enterprises to work across platforms and environments, from the core datacenter to the network edge.

© 2024 Red Hat, Inc.