Applies to SUSE Linux Enterprise Server 15 SP7

2 `sudo` basics #

Running certain commands requires root privileges. However, for security reasons and to avoid mistakes, it is not recommended to log in as root. A safer approach is to log in as a regular user, and then use sudo to run commands with elevated privileges.

On SUSE Linux Enterprise Server, sudo is configured to work similarly to su. However, sudo provides a flexible mechanism that allows users to run commands with privileges of any other user. This can be used to assign roles with specific privileges to certain users and groups. For example, it is possible to allow members of the group users to run a command with the privileges of user wilber. Access to the command can be further restricted by disallowing any command options. While su always requires the root password for authentication with PAM, sudo can be configured to authenticate with your own credentials. This means that the users do not need to share the root password, which improves security.

2.1 Basic `sudo` usage #

The following chapter provides an introduction to basic usage of sudo.

2.1.1 Running a single command #

As a regular user, you can run any command as root by adding sudo before it. This prompts you to provide the root password. If authenticated successfully, this runs the command as root:

> id -un1
tux
> sudo id -un
root's password:2
root
> id -un
tux3
> sudo id -un
4
root

1	The `id -un` command prints the login name of the current user.
2	The password is not shown during input, neither as clear text nor as masking characters.
3	Only commands that start with `sudo` run with elevated privileges.
4	The elevated privileges persist for a certain period of time, so you do not need to provide the `root` password again.

Tip: I/O redirection

When using sudo, I/O redirection does not work:

> sudo echo s > /proc/sysrq-trigger
bash: /proc/sysrq-trigger: Permission denied
> sudo cat < /proc/1/maps
bash: /proc/1/maps: Permission denied

In the example above, only the echo and cat commands run with elevated privileges. The redirection is done by the user's shell with user privileges. To perform redirection with elevated privileges, either start a shell as in Section 2.1.2, “Starting a shell” or use the dd utility:

echo s | sudo dd of=/proc/sysrq-trigger
sudo dd if=/proc/1/maps | cat

2.1.2 Starting a shell #

Using sudo every time to run a command with elevated privileges is not always practical. While you can use the sudo bash command, it is recommended to use one of the built-in mechanisms to start a shell:

sudo -s (<command>)

Starts a shell specified by the SHELL environment variable or the target user's default shell. If a command is specified, it is passed to the shell (with the -c option). Otherwise the shell runs in interactive mode.

tux:~ > sudo -s
root's password:
root:/home/tux # exit
tux:~ >

sudo -i (<command>)

Similar to -s, but starts the shell as a login shell. This means that the shell's start-up files (.profile etc.) are processed, and the current working directory is set to the target user's home directory.

tux:~ > sudo -i
root's password:
root:~ # exit
tux:~ >

Tip: Environment variables

By default, sudo does not propagate environment variables. This behavior can be changed using the env_reset option (see Useful flags and options).

2.2 Configuring `sudo` #

sudo provides a wide range on configurable options.

Note: Locked yourself out of sudo

If you accidentally locked yourself out of sudo, use su - and the root password to start a root shell. To fix the error, run visudo.

Warning: Example configurations are for demonstration purposes only

The example rules outlined below are purely for demonstration purposes. Use them to understand the general syntax of sudo configuration files. Do not use them in real-world setups, because they do not reflect the complexity of these environments.

2.2.1 `sudo` configuration best practices #

Before you start, here are a few ground rules for maintaining sudo configurations:

Always use visudo to edit sudo configuration files: Any changes to the sudo configuration should be done using the visudo command. visudo is a tailor-made tool that allows you to edit the sudo configuration files and runs basic syntax checks, making sure that the configuration remains intact and functional. A faulty sudo configuration can result in a user being locked out of their own system.
Always create custom configurations under /etc/sudoers.d/: Custom configurations must reside under /etc/sudoers.d/ to be pulled in by sudo. Settings in the custom configuration files take precedence over the ones in the default configuration in /etc/sudoers.
Always mind the order in which configurations are read: To make sure the custom configurations are read in the correct order, prefix them with numbers. Use leading zeroes to establish the order in which the files are read. For example, 01_myfirstconfig is parsed before 10_myotherconfig. If a directive has been set in a file that is read before another file that contains conflicting information, the last-read directive is applied.
Always use descriptive file names: Use file names that hint at what the configuration file does. This helps you keep track of what your sudo setup is supposed to do.

2.2.2 Create a user-specific configuration file #

Create a sudo configuration file that allows a normal user (tux) to use the useradd command with their own password instead of the root password.

Example 2.1: Create a user-specific configuration file #

As system administrator (root), create a custom configuration file that holds the new user-specific directives by starting visudo. Use both numbering and a descriptive name:
```
  # visudo -f /etc/sudoers.d/02_usermanagement
```

Create a rule that allows tux to execute the /usr/sbin/useradd binary in the entire environment that this sudo configuration is applied to:

  tux1 ALL2 = /usr/sbin/useradd3

Specify the user or group. List users by name or #UID, and groups by %GROUPNAME. Separate multiple items with commas. To negate entries, use !.

Specify one or several (separated by commas) hosts. Use (fully qualified) host names or IP addresses. Add ALL to enforce this setting globally across all hosts. Use ! for negations.

Specify one or several executables (separated by commas). When specifying them, make sure to mind the following rules:

/usr/sbin/useradd: Without any additional options added, this allows the execution of every possible useradd command.
/usr/sbin/useradd -c: If you explicitly specify an option, then that option is the only one that is allowed. Nothing else would be available to the user you specified above.
/usr/sbin/useradd "": This would just let the user invoke a mere useradd without any option at all.

In the example above, you would want to either allow all options and subcommands or limit them to a few for security reasons, but forbidding a user from specifying any option at all would be pointless in this context.

To let the user use their own password instead of the root password, add the following line:
```
Defaults:tux !targetpw
```
When active, this flag requires the user to enter the password of the target user, that is, root. This flag is enabled by default on any SUSE Linux Enterprise Server system. Negate it using ! to require the user to just enter their own password instead of the root password.
Save the configuration, leave the editor and open a second shell to test whether sudo honors your new configuration.

2.2.3 Create custom configurations by grouping items #

Modify the configuration from Example 2.1, “Create a user-specific configuration file” so that a group of named users can run the useradd command without the need for the root password. Also, add the usermod and userdel to the list of commands available to this group.

Example 2.2: Create custom configurations by grouping items #

To modify the example configuration, open it as system administrator with visudo:
```
# visudo /etc/sudoers.d/02_usermanagement
```
Add more users to the rule in a comma-separated list:
```
tux, wilber ALL = /usr/sbin/useradd
```
To allow the listed users to execute a list of commands, specify the commands as a comma-separated list:
```
tux, wilber ALL = /usr/sbin/useradd, /usr/sbin/usermod, /usr/sbin/userdel
```
To let the listed users use their own password instead of the root password, add the following line:
```
Defaults:tux, wilber !targetpw
```
When active, this flag requires the listed users to enter the password of the target user, that is, root. This flag is enabled by default on any SUSE Linux Enterprise Server system. Negate it using ! to require the listed users to just enter their own password instead of the root password.
Save the configuration, leave the editor and open a second shell to test whether sudo honors your new configuration.

2.2.4 Simplify configurations by applying aliases #

Use aliases to simplify your custom configuration from Example 2.2, “Create custom configurations by grouping items” even further. Grouping items helps to a certain extent, but using global aliases for users, commands and hosts is the most efficient way to keep a clean and lean sudo configuration.

Using aliases and groups instead of lists is a much better way to address changes in your setup. Should a user leave, just remove them from the global User_Alias declaration in your alias declaration file instead of scouring all the separate custom configuration files. The same procedure applies for any other type of alias (Host_Alias, Cmnd_Alias and Runas_Alias).

Example 2.3: Simplify configurations by applying aliases #

Create a new file to hold your global alias definitions:
```
# visudo /etc/sudoers.d/01_aliases
```
Add the following line to create the TEAMLEADERS alias:
```
User_Alias    TEAMLEADERS = tux, wilber
```

Add the following line to create the USERMANAGEMENT alias:

Cmnd_Alias    USERMANAGEMENT = /usr/sbin/useradd, /usr/sbin/usermod, /usr/sbin/userdel

Save your changes and exit visudo.
As system administrator, start visudo to edit the example configuration file:
```
# visudo -f /etc/sudoers.d/02_usermanagement
```
Delete the previous rule and replace it with the following rule that uses the aliases you have just defined above:
```
TEAMLEADERS ALL = USERMANAGEMENT
```
To let all the users defined by User_Alias use their own password instead of the root password, add the following line:
```
Defaults:TEAMLEADERS !targetpw
```
Save the configuration, leave the editor and open a second shell to test whether sudo honors your new configuration.

2.2.5 Basic sudoers configuration syntax #

The sudoers configuration files contain two types of options: strings and flags. While strings can contain any value, flags can be turned either ON or OFF. The most important syntax constructs for sudoers configuration files are as follows:

# Everything on a line after # is ignored 1
Defaults !insults # Disable the insults flag 2
Defaults env_keep += "DISPLAY HOME" # Add DISPLAY and HOME to env_keep
tux ALL = NOPASSWD: /usr/bin/frobnicate, PASSWD: /usr/bin/journalctl 3

1	There are two exceptions: `#include` and `#includedir` are regular commands.
2	Remove the `!` character to set the desired flag to ON.
3	See Section 2.2.6, “Basic sudoers rules”.

Useful flags and options #

targetpw

This flag controls whether the invoking user is required to enter the password of the target user (ON) (for example root) or the invoking user (OFF).

Defaults targetpw # Turn targetpw flag ON

rootpw

If set, sudo prompts for the root password. The default is OFF.

Defaults !rootpw # Turn rootpw flag OFF

env_reset

If set, sudo constructs a minimal environment with TERM, PATH, HOME, MAIL, SHELL, LOGNAME, USER, USERNAME, and SUDO_*. Additionally, variables listed in env_keep are imported from the calling environment. The default is ON.

Defaults env_reset # Turn env_reset flag ON

env_keep

List of environment variables to keep when the env_reset flag is ON.

# Set env_keep to contain EDITOR and PROMPT
Defaults env_keep = "EDITOR PROMPT"
Defaults env_keep += "JRE_HOME" # Add JRE_HOME
Defaults env_keep -= "JRE_HOME" # Remove JRE_HOME

env_delete

List of environment variables to remove when the env_reset flag is OFF.

# Set env_delete to contain EDITOR and PROMPT
Defaults env_delete = "EDITOR PROMPT"
Defaults env_delete += "JRE_HOME" # Add JRE_HOME
Defaults env_delete -= "JRE_HOME" # Remove JRE_HOME

The Defaults token can also be used to create aliases for a collection of users, hosts, and commands. Furthermore, it is possible to apply an option only to a specific set of users.

For detailed information about the sudoers configuration files, consult man 5 sudoers.

2.2.6 Basic sudoers rules #

Each rule follows the following scheme ([] marks optional parts):

#Who      Where         As whom      Tag                What
User_List Host_List = [(User_List)] [NOPASSWD:|PASSWD:] Cmnd_List

sudoers rule syntax #

User_List

One or several (separated by comma) identifiers: either a user name, a group in the format %GROUPNAME, or a user ID in the format #UID. Negation can be specified with the ! prefix.

Host_List

One or several (separated by comma) identifiers: either a (fully qualified) host name or an IP address. Negation can be specified with the ! prefix. ALL is a common choice for Host_List.

NOPASSWD:|PASSWD:

The user is not prompted for a password when running commands matching Cmd_List after NOPASSWD:.

PASSWD is the default. It only needs to be specified when both PASSWD and NOPASSWD are on the same line:

tux ALL = PASSWD: /usr/bin/foo, NOPASSWD: /usr/bin/bar

Cmnd_List

One or several (separated by comma) specifiers: a path to an executable, followed by an optional allowed argument.

/usr/bin/foo     # Anything allowed
/usr/bin/foo bar # Only "/usr/bin/foo bar" allowed
/usr/bin/foo ""  # No arguments allowed

ALL can be used as User_List, Host_List, and Cmnd_List.

A rule that allows tux to run all commands as root without entering a password:

tux ALL = NOPASSWD: ALL

A rule that allows tux to run systemctl restart apache2:

tux ALL = /usr/bin/systemctl restart apache2

A rule that allows tux to run wall as admin with no arguments:

tux ALL = (admin) /usr/bin/wall ""

Warning: Unsafe rules

Do not use rules like ALL ALL = ALL without Defaults targetpw. Otherwise anyone can run commands as root.

Important: Winbind and sudo

When specifying the group name in the sudoers file, make sure that you use the NetBIOS domain name instead of the realm, for example:

%DOMAIN\\GROUP_NAME ALL = (ALL) ALL

Keep in mind that when using winbindd, the format also depends on the winbind separator option in the smb.conf file. By default, it is \. If it is changed, for example, to +, then the account format in the sudoers file must be DOMAIN+GROUP_NAME.

2.3 Using `sudo` with X.Org applications #

Starting graphical applications with sudo normally results in the following error:

> sudo xterm
xterm: Xt error: Can't open display: %s
xterm: DISPLAY is not set

A simple workaround is to use xhost to temporarily allow the root user to access the local user's X session. This is done using the following command:

xhost si:localuser:root

The command below removes the granted access:

xhost -si:localuser:root

Warning: Potential security issue

Running graphical applications with root privileges has security implications. It is recommended to enable root access for a graphical application only as an exception. It is also recommended to revoke the granted root access as soon as the graphical application is closed.

2.4 More information #

The sudo --help command offers a brief overview of the available command line options, while the man sudoers command provides detailed information about sudoers and its configuration.