Configuring basic system settings
Set up the essential functions of your system and customize your system environment
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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
- From the Installation Summary window, click .
From the list in the left-hand pane, select an interface. The details are displayed in the right-hand pane.
NoteThere are several types of network device naming standards used to identify network devices with persistent names, for example,
em1
andwl3sp0
. For information about these standards, see the Configuring and managing networking document.Toggle the
switch to enable or disable the selected interface.NoteThe installation program automatically detects locally accessible interfaces, and you cannot add or remove them manually.
- Click to add a virtual network interface, which can be either: Team (deprecated), Bond, Bridge, or VLAN.
- Click to remove a virtual interface.
- Click to change settings such as IP addresses, DNS servers, or routing configuration for an existing interface (both virtual and physical).
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 formatIP 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.
-
The host name can either be a fully qualified domain name (FQDN) in the format
- Click to apply the host name to the installer environment.
- Alternatively, in the Network and Hostname window, you can choose the Wireless option. Click in the right-hand pane to select your wifi connection, enter the password if required, and click .
Additional resources
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
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.
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.
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.
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 ...
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 toauto
(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
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 toauto
(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
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.
Activate the profile:
# nmcli connection up Internal-LAN
Verification
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
Display the IPv4 default gateway:
# ip route show default default via 192.0.2.254 dev enp1s0 proto static metric 102
Display the IPv6 default gateway:
# ip -6 route show default default via 2001:db8:1::ffee dev enp1s0 proto static metric 102 pref medium
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.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.
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
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 -- ...
Start
nmtui
:# nmtui
- Select Edit a connection, and press Enter.
Choose whether to add a new connection profile or to modify an existing one:
To create a new profile:
- Press Add.
- 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.
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.
- If you create a new connection profile, enter the network device name into the Device field.
Depending on your environment, configure the IP address settings in the
IPv4 configuration
andIPv6 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:
- Press Show next to the protocol you want to configure to display additional fields.
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.- Enter the address of the default gateway.
- Press Add next to DNS servers, and enter the DNS server address.
- 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
- Press OK to create and automatically activate the new connection.
- Press Back to return to the main menu.
-
Select Quit, and press Enter to close the
nmtui
application.
Verification
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
Display the IPv4 default gateway:
# ip route show default default via 192.0.2.254 dev enp1s0 proto static metric 102
Display the IPv6 default gateway:
# ip -6 route show default default via 2001:db8:1::ffee dev enp1s0 proto static metric 102 pref medium
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.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
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](https://access.redhat.com/webassets/avalon/d/Red_Hat_Enterprise_Linux-9-Configuring_basic_system_settings-en-US/images/fcaed98a3b20fc8983dfbe20a92fdc0b/cs_getting_started_networking_new.png)
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.
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
Additional resources
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
- A valid user account in the Red Hat Customer Portal.
- See the Create a Red Hat Login page.
- An active subscription for the RHEL system.
- For more information about the installation process, see Interactively installing RHEL from installation media.
Procedure
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:
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.
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
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
You can register a newly installed Red Hat Enterprise Linux with your account credentials in the RHEL web console.
Prerequisites
A valid user account on the Red Hat Customer Portal.
See the Create a Red Hat Login page.
- An active subscription for your RHEL system.
You have installed the RHEL 9 web console.
For instructions, see Installing and enabling the web console.
Procedure
Log in to the RHEL 9 web console.
For details, see Logging in to the web console.
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.
- In the Overview field, click .
In the Register system dialog box, select Account to register by using your account credentials.
- Enter your username.
- Enter your password.
Optional: Enter your organization’s name or ID.
If your account belongs to more than one organization on the Red Hat Customer Portal, you must 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.
- Click .
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
A valid account on Red Hat customer portal.
See the Create a Red Hat Login page for new user registration.
Procedure
Open the system menu, which is accessible from the upper-right screen corner, and click Settings.
- Go to → .
If you are not using the Red Hat server:
- In the Registration Server section, select Custom Address.
- Enter the server address in the URL field.
- In the Registration Type section, select Red Hat Account.
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.
- Click .
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
Open the system menu, which is accessible from the upper-right screen corner, and click Settings.
- Go to → .
If you are not using the Red Hat server:
- In the Registration Server section, select Custom Address.
- Enter the server address in the URL field.
- In the Registration Type section, select Activation Keys.
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.
- Click .
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
Access Red Hat support:
- Open a new support case.
- Initiate a live chat with a Red Hat expert.
- Contact a Red Hat expert by making a call or sending an email.
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
Install the
sos
package:# dnf install sos
NoteThe default minimal installation of Red Hat Enterprise Linux does not include the
sos
package, which provides thesosreport
command.Generate a report:
# sosreport
Attach the report to your support case.
See the How can I attach a file to a Red Hat support case? Red Hat Knowledgebase article for more information.
Note that when attaching the report, you are prompted to enter the number of the relevant support case.
Additional resources
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
Enter the
date
command:$ date Mon Mar 30 16:02:59 CEST 2020
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
-
date(1)
andtimedatectl(1)
man pages on your system
4.2. Configuring time settings by using the web console
You can set a time zone and synchronize the system time with a Network Time Protocol (NTP) server in the RHEL web console.
Prerequisites
You have installed the RHEL 9 web console.
For instructions, see Installing and enabling the web console.
Procedure
Log in to the RHEL 9 web console.
For details, see Logging in to the web console.
Click the current system time in Overview.
- Click System time.
- In the Change System Time dialog box, change the time zone if necessary.
In the Set Time drop-down menu, select one of the following:
- Manually
- Use this option if you need to set the time manually, without an NTP server.
- Automatically using NTP server
- This is a default option, which synchronizes time automatically with the preset NTP servers.
- Automatically using specific NTP servers
- Use this option only if you need to synchronize the system with a specific NTP server. Specify the DNS name or the IP address of the server.
Click Change.
Verification
- Check the system time displayed in the System tab.
Additional resources
4.3. 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 theroot
user. For example:# localectl set-locale LANG=en_US
The GNOME Terminal does not support non-UTF8 system locales. For more information, see The gnome-terminal application fails to start when the system locale is set to non-UTF8 knowledge base solution.
Additional resources
-
man localectl(1)
,man locale(7)
, andman locale.conf(5)
4.4. 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)
, andman locale.conf(5)
4.5. 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
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
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 fontLatArCyrHeb
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
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.
4.6. 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 from collecting unencrypted passwords from the connection.
5.1. Generating SSH key pairs
You can log in to an OpenSSH server without entering a password by generating an SSH key pair on a local system and copying the generated public key to the OpenSSH server. Each user who wants to create a key must run this procedure.
To preserve previously generated key pairs after you reinstall the system, back up the ~/.ssh/
directory before you create new keys. After reinstalling, copy it back to your home directory. You can do this for all users on your system, including root
.
Prerequisites
- You are logged in as a user who wants to connect to the OpenSSH server by using keys.
- The OpenSSH server is configured to allow key-based authentication.
Procedure
Generate an ECDSA key pair:
$ 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): <password> Enter same passphrase again: <password> 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
ssh-keygen
command without any parameter or an Ed25519 key pair by entering thessh-keygen -t ed25519
command. Note that the Ed25519 algorithm is not FIPS-140-compliant, and OpenSSH does not work with Ed25519 keys in FIPS mode.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 byssh-agent
, use thessh-copy-id
command with the-i
option.
Verification
Log in to the OpenSSH server by using the key file:
$ ssh -o PreferredAuthentications=publickey <username>@<ssh-server-example.com>
Additional resources
-
ssh-keygen(1)
andssh-copy-id(1)
man pages on your system
5.2. Setting key-based authentication as the only method on an OpenSSH server
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. You can already connect to the OpenSSH server by using a key.
See the Generating SSH key pairs section for details.
Procedure
Open the
/etc/ssh/sshd_config
configuration in a text editor, for example:# vi /etc/ssh/sshd_config
Change the
PasswordAuthentication
option tono
:PasswordAuthentication no
-
On a system other than a new default installation, check that the
PubkeyAuthentication
parameter is either not set or set toyes
. Set the
KbdInteractiveAuthentication
directive tono
.Note that the corresponding entry is commented out in the configuration file and the default value is
yes
.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
- If you are connected remotely, not using console or out-of-band access, test the key-based login process before disabling password authentication.
Reload the
sshd
daemon to apply the changes:# systemctl reload sshd
Additional resources
-
sshd_config(5)
andsetsebool(8)
man pages on your system
5.3. Caching your SSH credentials by 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 for a login session. If the agent is running and your keys are unlocked, you can log in to SSH servers by using these keys but without having to enter the key’s password again. The private key and the passphrase remain secure.
Prerequisites
- You have a remote host with the 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.
See the Generating SSH key pairs section for details.
Procedure
Add the command for automatically starting
ssh-agent
in your session to the~/.bashrc
file:Open
~/.bashrc
in a text editor of your choice, for example:$ vi ~/.bashrc
Add the following line to the file:
eval $(ssh-agent)
- Save the changes, and quit the editor.
Add the following line to the
~/.ssh/config
file:AddKeysToAgent yes
With this option and
ssh-agent
started in your session, the agent prompts for a password only for the first time when you connect to a host.
Verification
Log in to a host which uses the corresponding public key of the cached private key in the agent, for example:
$ ssh <example.user>@<ssh-server@example.com>
Note that you did not have to enter the passphrase.
5.4. Authenticating by SSH keys stored on a smart card
You can create and store ECDSA and RSA keys on a smart card and authenticate by the smart card on an OpenSSH client. Smart-card authentication replaces the default password authentication.
Prerequisites
-
On the client side, the
opensc
package is installed and thepcscd
service is running.
Procedure
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
Transfer the public key to the remote server. Use the
ssh-copy-id
command with thekeys.pub
file created in the previous step:$ ssh-copy-id -f -i keys.pub <username@ssh-server-example.com>
Connect to <ssh-server-example.com> by using the ECDSA key. 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" <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-example.com] $
Because OpenSSH uses the
p11-kit-proxy
wrapper and the OpenSC PKCS #11 module is registered to thep11-kit
tool, you can simplify the previous command:$ ssh -i "pkcs11:id=%01" <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-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: <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-example.com] $
Optional: 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 <ssh-server-example.com> Enter PIN for 'SSH key': [ssh-server-example.com] $
The
ssh
client utility now automatically uses this URI and the key from the smart card.
Additional resources
-
p11-kit(8)
,opensc.conf(5)
,pcscd(8)
,ssh(1)
, andssh-keygen(1)
man pages on your system
5.5. Additional resources
-
sshd(8)
,ssh(1)
,scp(1)
,sftp(1)
,ssh-keygen(1)
,ssh-copy-id(1)
,ssh_config(5)
,sshd_config(5)
,update-crypto-policies(8)
, andcrypto-policies(7)
man pages. - Configuring SELinux for applications and services with non-standard configurations
- Controlling network traffic using firewalld
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 theroot
user, and start thefirewalld
service and enable to start it automatically after the system restarts:# systemctl enable --now firewalld
Verification
Check that
firewalld
is running and enabled:$ systemctl status firewalld ● firewalld.service - firewalld - dynamic firewall daemon Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled) Active: active (running) ...
Additional resources
- Using and configuring firewalld
-
man firewalld(1)
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 name | Role description | Chapter title |
---|---|---|
| Certificate Issuance and Renewal | Requesting certificates by using RHEL system roles |
| Web console | Installing and configuring web console with the cockpit RHEL system role |
| System-wide cryptographic policies | Setting a custom cryptographic policy across systems |
| Firewalld | Configuring firewalld by using system roles |
| HA Cluster | Configuring a high-availability cluster by using system roles |
| Kernel Dumps | Configuring kdump by using RHEL system roles |
| Kernel Settings | Using Ansible roles to permanently configure kernel parameters |
| Logging | Using the logging system role |
| Metrics (PCP) | Monitoring performance by using RHEL system roles |
| Networking | Using the network RHEL system role to manage InfiniBand connections |
| Network Bound Disk Encryption client | Using the nbde_client and nbde_server system roles |
| Network Bound Disk Encryption server | Using the nbde_client and nbde_server system roles |
| Postfix | Variables of the postfix role in system roles |
| PostgreSQL | Installing and configuring PostgreSQL by using the postgresql RHEL system role |
| SELinux | Configuring SELinux by using system roles |
| SSH client | Configuring secure communication with the ssh system roles |
| SSH server | Configuring secure communication with the ssh system roles |
| Storage | Managing local storage by using RHEL system roles |
| Terminal Session Recording | Configuring a system for session recording by using the tlog RHEL system role |
| Time Synchronization | Configuring time synchronization by using RHEL system roles |
| VPN | Configuring VPN connections with IPsec by using the vpn RHEL system role |
Additional resources
- Automating system administration by using RHEL system roles
- Red Hat Enterprise Linux (RHEL) system roles
-
/usr/share/ansible/roles/rhel-system-roles.<role_name>/README.md
file -
/usr/share/doc/rhel-system-roles/<role_name>/
directory
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
- allsyslog
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. 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
...
Command | Description |
---|---|
| Shows all collected journal entries. |
|
Shows logs related to a specific file. For example, the |
| Shows logs for the current boot. |
| Shows kernel logs for the current boot. |
Command | Description |
---|---|
|
Filters log to show entries matching the |
|
Combines matches. For example, this command shows logs for |
|
The plus sign (+) separator combines two expressions in a logical OR. For example, this command shows all messages from the |
|
This command shows all entries matching either expression, referring to the same field. Here, this command shows logs matching a systemd-unit |
Command | Description |
---|---|
| 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. |
| Shows information about the specified boot ID. |
8.4. Reviewing logs in the web console
Learn how to access, review and filter logs in the RHEL web console.
8.4.1. Reviewing logs in the web console
The RHEL 9 web console Logs section is a UI for the journalctl
utility. You can access system logs in the web console interface.
Prerequisites
You have installed the RHEL 9 web console.
For instructions, see Installing and enabling the web console.
Procedure
Log in to the RHEL 9 web console.
For details, see Logging in to the web console.
Click Logs.
- Open log entry details by clicking on your selected log entry in the list.
You can use the
button to pause new log entries from appearing. Once you resume new log entries, the web console will load all log entries that were reported after you used the button.You can filter the logs by time, priority or identifier. For more information, see Filtering logs in the web console.
8.4.2. Filtering logs in the web console
You can filter log entries in the web console.
Prerequisites
You have installed the RHEL 9 web console.
For instructions, see Installing and enabling the web console.
Procedure
Log in to the RHEL 9 web console.
For details, see Logging in to the web console.
- Click Logs.
By default, web console shows the latest log entries. To filter by a specific time range, click the Time drop-down menu and choose a preferred option.
Error and above severity logs list is shown by default. To filter by different priority, click the Error and above drop-down menu and choose a preferred priority.
By default, web console shows logs for all identifiers. To filter logs for a particular identifier, click the All drop-down menu and select an identifier.
- To open a log entry, click on a selected log.
8.4.3. Text search options for filtering logs in the web console
The text search option functionality provides a big variety of options for filtering logs. If you decide to filter logs by using the text search, you can use the predefined options that are defined in the three drop-down menus, or you can type the whole search yourself.
Drop-down menus
There are three drop-down menus that you can use to specify the main parameters of your search:
- Time: This drop-down menu contains predefined searches for different time ranges of your search.
-
Priority: This drop-down menu provides options for different priority levels. It corresponds to the
journalctl --priority
option. The default priority value is Error and above. It is set every time you do not specify any other priority. -
Identifier: In this drop-down menu, you can select an identifier that you want to filter. Corresponds to the
journalctl --identifier
option.
Quantifiers
There are six quantifiers that you can use to specify your search. They are covered in the Options for filtering logs table.
Log fields
If you want to search for a specific log field, it is possible to specify the field together with its content.
Free-form text search in logs messages
You can filter any text string of your choice in the logs messages. The string can also be in the form of a regular expressions.
Advanced logs filtering I
Filter all log messages identified by 'systemd' that happened since October 22, 2020 midnight and journal field 'JOB_TYPE' is either 'start' or 'restart.
-
Type
identifier:systemd since:2020-10-22 JOB_TYPE=start,restart
to search field. Check the results.
Advanced logs filtering II
Filter all log messages that come from 'cockpit.service' systemd unit that happened in the boot before last and the message body contains either "error" or "fail".
-
Type
service:cockpit boot:-1 error|fail
to the search field. Check the results.
8.4.4. Using a text search box to filter logs in the web console
You can filter logs according to different parameters by using the text search box in the web console. The search combines usage of the filtering drop-down menus, quantifiers, log fields, and free-form string search.
Prerequisites
You have installed the RHEL 9 web console.
For instructions, see Installing and enabling the web console.
Procedure
Log in to the RHEL web console.
For details, see Logging in to the web console.
- Click Logs.
Use the drop-down menus to specify the three main quantifiers - time range, priority, and identifier(s) - you want to filter.
The Priority quantifier always has to have a value. If you do not specify it, it automatically filters the Error and above priority. Notice that the options you set reflect in the text search box.
Specify the log field you want to filter.
You can add several log fields.
- You can use a free-form string to search for anything else. The search box also accepts regular expressions.
8.4.5. Options for logs filtering
There are several journalctl
options, which you can use for filtering logs in the web console, that may be useful. Some of these are already covered as part of the drop-down menus in the web console interface.
Option name | Usage | Notes |
---|---|---|
| Filter output by message priorities. Takes a single numeric or textual log level. The log levels are the usual syslog log levels. If a single log level is specified, all messages with this log level or a lower (therefore more important) log level are shown. | Covered in the Priority drop-down menu. |
| Show messages for the specified syslog identifier SYSLOG_IDENTIFIER. Can be specified multiple times. | Covered in the Identifier drop-down menu. |
| Shows only the most recent journal entries, and continuously prints new entries as they are appended to the journal. | Not covered in a drop-down. |
|
Show messages for the specified |
Is not covered in a drop-down. Corresponds to the |
| Show messages from a specific boot. A positive integer will look up the boots starting from the beginning of the journal, and an equal-or-less-than zero integer will look up boots starting from the end of the journal. Therefore, 1 means the first boot found in the journal in chronological order, 2 the second and so on; while -0 is the last boot, -1 the boot before last, and so on. | Covered only as Current boot or Previous boot in the Time drop-down menu. Other options need to be written manually. |
| Start showing entries on or newer than the specified date, or on or older than the specified date, respectively. Date specifications should be of the format "2012-10-30 18:17:16". If the time part is omitted, "00:00:00" is assumed. If only the seconds component is omitted, ":00" is assumed. If the date component is omitted, the current day is assumed. Alternatively the strings "yesterday", "today", "tomorrow" are understood, which refer to 00:00:00 of the day before the current day, the current day, or the day after the current day, respectively. "now" refers to the current time. Finally, relative times may be specified, prefixed with "-" or "+", referring to times before or after the current time, respectively. | Not covered in a drop-down. |
8.5. Additional resources
-
journalctl(1)
man page on your system - Configuring a remote logging solution
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:
-
Open the
/etc/login.defs
file in an editor of your choice. Find the lines that define the minimum value for automatic UID selection.
# Min/max values for automatic uid selection in useradd # UID_MIN 1000
Modify the
UID_MIN
value to start at 5000.# Min/max values for automatic uid selection in useradd # UID_MIN 5000
Find the lines that define the minimum value for automatic GID selection.
# Min/max values for automatic gid selection in groupadd # GID_MIN 1000
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.
NoteThe 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.
WarningDo 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.
WarningSystem 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)
, andman usermod(8)
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 ID5000
, use:# useradd -u 5000 sarah
Verification
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 ID5000
, use:# groupadd -g 5000 sysadmins
Verification
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 groupsystem-administrators
, use:# usermod --append -G system-administrators sysadmin
Verification
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
Create a directory:
# mkdir directory-name
Replace directory-name with the name of the directory.
Create a group:
# groupadd group-name
Replace group-name with the name of the group.
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.
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.
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 thedirectory-name
directory. Newly created files retain the group ownership ofgroup-name
group.
Verification
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:
Remove the user, their home directory, their mail spool, and their SELinux user mapping:
# userdel --remove --selinux-user user-name
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.
ImportantIf 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 provides a graphical interface for adding, editing, and removing system user accounts.
You can also set password expiration and terminate user sessions in the web console.
9.4.1. Adding new accounts by 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
You have installed the RHEL 9 web console.
For instructions, see Installing and enabling the web console.
Procedure
Log in to the RHEL 9 web console.
For details, see Logging in to the web console.
- Click .
- Click .
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.
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.
- Click to save the settings and close the dialog box.
- Select the newly created account.
In the Groups drop-down menu, select the groups that you want to add to the new account.
Now you can see the new account in the Accounts settings and you can use its credentials to connect to the system.
9.4.2. 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.
Prerequisites
You have installed the RHEL 9 web console.
For instructions, see Installing and enabling the web console.
Procedure
- Log in to the RHEL 9 web console.
- Click .
- Select the user account for which you want to enforce password expiration.
Click Password line.
on the- 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.
Click
.The web console immediately shows the date of the future password change request on the Password line.
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 groupsarah
and is a member of supplementary groupswheel
anddeveloper
.Example 9.5. Listing of groups for user marc:
$ groups marc
The output displays:
marc : marc
User
marc
has only a primary groupmarc
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:
-
root
access - 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.
NoteWhen 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 groupsarah1
, and you want to change the primary group of the user tosarah2
, use:# usermod -g sarah2 sarah
Verification
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 groupsystem-administrators
, use:# usermod --append -G system-administrators sysadmin
Verification
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 groupswheel
anddevelopers
, and you want to remove that user from the groupdevelopers
, use:# gpasswd -d sarah developers
Verification
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.
ImportantIf 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 groupsarah2
, and belongs to the supplementary groupwheel
, and you want the user to belong to three more supplementary groupsdeveloper
,sysadmin
, andsecurity
, use:# usermod -G wheel,developer,sysadmin,security sarah
Verification
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 thewheel
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
Reboot the system and, on the GRUB 2 boot screen, press the
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 $tuned_initrd
Go to the end of the line that starts with linux.
linux ($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
to jump to the end of the line.Add
rd.break
to the end of the line that starts withlinux
.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
Press
to start the system with the changed parameters.The
switch_root
prompt appears.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.Enter the
chroot
environment:chroot /sysroot
The
sh-4.4#
prompt appears.Reset the
root
password:passwd
Follow the instructions displayed by the command line to finalize the change of the
root
password.Enable the SELinux relabeling process on the next system boot:
touch /.autorelabel
Exit the
chroot
environment:exit
Exit the
switch_root
prompt:exit
- 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
-
To verify that the
root
password is successfully changed, log in as a normal user and open the Terminal. Run the interactive shell as root:
$ su
-
Enter your new
root
password. 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
.
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.
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)
andsudoers(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 thesudo
command. -
The command is executed in the shell of the user, not in the root shell. However, there are some exceptions such as when full
sudo
privileges are granted to any user. In such cases, users can switch to and run the commands in root shell. For example: -
sudo -i
-
sudo su -
Prerequisites
- You have root access to the system.
Procedure
As root, open the
/etc/sudoers
file.# visudo
The
/etc/sudoers
file defines the policies applied by thesudo
command.In the
/etc/sudoers
file, find the lines that grantsudo
access to users in the administrativewheel
group.## Allows people in group wheel to run all commands %wheel ALL=(ALL) ALL
-
Make sure the line that starts with
%wheel
is not commented out with the number sign (#
). - Save any changes, and exit the editor.
Add users you want to grant
sudo
access to into the administrativewheel
group.# usermod --append -G wheel <username>
Replace
<username>
with the name of the user.
Verification
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)
, andsudoers(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. This is more secure than granting full sudo
access to a user or giving someone the root password for the following reasons:
- More granular control over privileged actions. You can allow a user to perform certain actions on specific hosts instead of giving them full administrative access.
-
Better logging. When a user performs an action through
sudo
, the action is logged with their user name and not just root. -
Transparent control. You can set email notifications for every time the user attempts to use
sudo
privileges.
Prerequisites
- You have root access to the system.
Procedure
As root, create a new
sudoers.d
directory under/etc/
:# mkdir -p /etc/sudoers.d/
Create a new file in the
/etc/sudoers.d
directory:# visudo -f /etc/sudoers.d/<filename>
The file opens automatically.
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 thednf
andreboot
commands onhost1.example.com
, enteruser1 host1.example.com = /bin/dnf, /sbin/reboot
.
-
Replace
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>"
- Save the changes, and exit the editor.
Verification
To verify if a user can run a command with
sudo
privileges, switch the account:# su <username> -
As the user, enter the command with the
sudo
command:$ sudo <command> [sudo] password for
<username>
:Enter the user’s
sudo
password.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)
, andsudoers(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. Thex
permission means that you can also access the directory using thecd
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.
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](https://access.redhat.com/webassets/avalon/d/Red_Hat_Enterprise_Linux-9-Configuring_basic_system_settings-en-US/images/1c3738d5acf20b7a7fff36ee1acada1b/Users_Groups-Umask_Example.png)
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.
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 thechmod a=rwx my-script.sh
command.See Base file permissions for more details.
Verification
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: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
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.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
fromdrwxrwx---
todrwxrwxr-x
, use:Display the current permissions for
my-directory
:$ ls -dl my-directory drwxrwx---. 2 username username 4096 Feb 24 18:12 my-directory
Add the read and execute (
r-x
) access for all users (a
):$ chmod o+rx my-directory
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 therw-
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
To verify that the user
andrew
has therw-
permission, the usersusan
has the---
permission, and other users have ther--
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
NoteWhen displaying the umask in octal mode, you may notice it displayed as a four digit number (
0002
or0022
). The first digit of the umask represents a special bit (sticky bit, SGID bit, or SUID bit). If the first digit is set to0
, 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 $0
command returnsbash
, you are executing the command in a non-login shell.$ 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 onesNoteAny 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 tou=rwx,g=rwx,o=rwx
, useumask -S a=rwx
.See User file-creation mode for more details.
NoteThe 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.
NoteThe 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
-
As
root
, open the/etc/bashrc
file in the editor. Modify the following sections to set a new default
bash
umask:if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then umask 002 else umask 022 fi
Replace the default octal value of the umask (
002
) with another octal value. See User file-creation mode mask for more details.- Save the changes and exit the editor.
11.3.6. Changing the default umask for the login shell
You can change the default bash
umask for the root
user by modifying the /etc/login.defs
file.
Prerequisites
-
root
access
Procedure
-
As
root
, open the/etc/login.defs
file in the editor. Modify the following sections to set a new default
bash
umask:# Default initial "umask" value used by login(1) on non-PAM enabled systems. # Default "umask" value for pam_umask(8) on PAM enabled systems. # UMASK is also used by useradd(8) and newusers(8) to set the mode for new # home directories if HOME_MODE is not set. # 022 is the default value, but 027, or even 077, could be considered # for increased privacy. There is no One True Answer here: each sysadmin # must make up their mind. UMASK 022
Replace the default octal value of the umask (
022
) with another octal value. See User file-creation mode mask for more details.- Save the changes and exit the editor.
11.3.7. Changing the default umask for a specific user
You can change the default umask for a specific user by modifying the .bashrc
for that user.
Procedure
Append the line that specifies the octal value of the umask into the
.bashrc
file for the particular user.$ 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
-
As
root
, open the/etc/login.defs
file in the editor. 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.- If HOME_MODE is set, save the changes and exit the editor.
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.- 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.
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:
Directory | Description |
---|---|
|
|
|
|
|
|
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
orSUB
- 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 Field Description Loaded
Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled 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
NoteBoth
systemctl is-active
andsystemctl is-enabled
commands return an exit status of0
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 aftergdm
, 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
).NoteIn
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 thesendmail
service,systemd
first automatically stopspostfix
, because these two services are conflicting and cannot run on the same port.
Additional resources
-
systemctl(1)
man page - Enabling a system service to start at boot
- Displaying system service status
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
).
Additional resources
-
systemctl(1)
man page - Disabling a system service to start at boot
- Displaying system service status
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
).NoteIf 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
NoteSystem services that do not support this feature, ignore this command. To restart such services, use the
reload-or-restart
andreload-or-try-restart
commands instead.
Additional resources
-
systemctl
man page - Displaying system service status
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
-
systemctl (1)
man page - Displaying system service status
- Starting a system service
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 tosystemd
.
Additional resources
-
systemctl (1)
man page - Displaying system service status
- Stopping a system service
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 themulti-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 namedbasic.target
.
You can set the following systemd
targets as default or current 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
Determine the current default target unit
systemd
uses to start the system:# systemctl get-default graphical.target
List the currently loaded targets:
# systemctl list-units --type target
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
Verify the default target unit:
# systemctl get-default multi-user.target
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
Optional: Determine the current target:
# systemctl get-default graphical.target
Optional: Display the list of targets you can select:
# systemctl list-units --type target
NoteYou can only isolate targets that have the
AllowIsolate=yes
option set in the unit files.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!
NoteThis 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
- Reboot the system, and interrupt the boot loader menu countdown by pressing any key except the Enter key, which would initiate a normal boot.
- Move the cursor to the kernel entry that you want to start.
- Press the E key to edit the current entry.
Move to the end of the line that starts with
linux
and press Ctrl+E to jump to the end of the line:linux ($root)/vmlinuz-5.14.0-70.22.1.e19_0.x86_64 root=/dev/mapper/rhel-root ro crash\ kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhgb quiet
To choose an alternate boot target, append the
systemd.unit=
parameter to the end of the line that starts withlinux
:linux ($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
- 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.
Additional resources
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 thenow
keyword as an alias for+0
.Cancel a pending shutdown:
# shutdown -c
Additional resources
-
shutdown(8)
manual page - Shutting down the system using the systemctl command
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
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
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.
systemctl command | Description |
---|---|
| Halts the system. |
| Powers off the system. |
| Restarts the system. |
| Suspends the system. |
| Hibernates the system. |
| 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
-
Open the
/etc/systemd/logind.conf
configuration file. -
Look for the line that says
HandlePowerKey=poweroff
. -
If the line starts with the
#
symbol, remove it to enable the setting. 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
- Save your changes and close the editor.
Next steps
- If you use the graphical session, also configure the power button in GNOME. See Section 12.4.7.2, “Changing the power button behavior in GNOME”.
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
-
You have configured the power button behavior in
systemd
. See Section 12.4.7.1, “Changing the power button behavior in systemd”.
Procedure
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.
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
Update the system databases:
# dconf update
- 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
To make changes to the local instance of
chronyd
using the command line utility chronyc in interactive mode, enter the following command asroot
:# 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>
-
To list all of the commands, type
help
. Alternatively, the utility can also be invoked in non-interactive command mode if called together with a command as follows:
chronyc command
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
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
.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
To start
chronyd
, issue the following command asroot
:# systemctl start chronyd
To ensure
chronyd
starts automatically at system start, issue the following command asroot
:# systemctl enable chronyd
To stop
chronyd
, issue the following command asroot
:# systemctl stop chronyd
To prevent
chronyd
from starting automatically at system start, issue the following command asroot
:# 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
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
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.The
sourcestats
command displays information about the drift rate and offset estimation process for each of the sources currently being examined bychronyd
. 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
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:
- The dispatcher script might run when no route to the NTP servers exists, causing the NTP servers to switch to the offline state.
- 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
To disable the
chrony
dispatcher script, create a symlink to/dev/null
:# ln -s /dev/null /etc/NetworkManager/dispatcher.d/20-chrony-onoffline
NoteAfter 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
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, seechrony.conf(7)
man pageOn 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, andntp1.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
orchrony
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
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 ::
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
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
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.
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.
Additional resources
13.2.8. Additional resources
-
chronyc(1)
man page -
chronyd(8)
man page - Frequently Asked Questions
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.
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:
Specify the server with the
nts
option in addition to the recommendediburst
option.For example: server time.example.com iburst nts server nts.netnod.se iburst nts server ptbtime1.ptb.de iburst nts
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
To disable synchronization with Network Time Protocol (NTP) servers provided by
DHCP
, comment out or remove the following line inchrony.conf
, if it is present:sourcedir /run/chrony-dhcp
- Save your changes.
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
, andKLen
should have non-zero values. If the value is zero, check the system log for error messages fromchronyd
.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
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
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>.
-
Ensure the
ntsdumpdir /var/lib/chrony
directive is present in thechrony.conf
. Restart the
chronyd
service:# systemctl restart chronyd
ImportantIf the server has a firewall, it needs to allow both the
UDP 123
andTCP 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
andAuthenticated 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:
- Install ReaR.
- Modify ReaR configuration file, to add backup method details.
- Create rescue system.
- 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
Install the ReaR utility by running the following command:
# dnf install rear
Modify the ReaR configuration file in an editor of your choice, for example:
# vi /etc/rear/local.conf
Add the backup setting details to
/etc/rear/local.conf
. For example, in the case of theNETFS
backup method, add the following lines:BACKUP=NETFS BACKUP_URL=backup.location
Replace backup.location by the URL of your backup location.
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
To make the backups incremental, meaning that only the changed files are backed up on each run, add the following line:
BACKUP_TYPE=incremental
Create a rescue system:
# rear mkrescue
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
andrear 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 and is fully supported since RHEL 9.2. 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.
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
-
To install ReaR, run the
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:
-
To configure the
IPL
output method, addOUTPUT=IPL
. To configure the backup method and destination, add
BACKUP
andBACKUP_URL
variables. For example:BACKUP=NETFS BACKUP_URL=nfs://<nfsserver name>/<share path>
ImportantThe local backup storage is currently not supported on the 64-bit IBM Z architecture.
-
Optional: You can also configure the
OUTPUT_URL
variable to save the kernel andinitrd
files. By default, theOUTPUT_URL
is aligned withBACKUP_URL
. To perform backup and rescue image creation:
# rear mkbackup
-
This creates the kernel and initrd files at the location specified by the
BACKUP_URL
orOUTPUT_URL
(if set) variable, and a backup using the specified backup method. -
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, andinitrd
. For more information, see Using a Prepared DASD. -
When the rescue kernel and
initrd
get booted, it starts the ReaR rescue environment. Proceed with system recovery.
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.
Additional resources