Sudo on suse linux

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 have 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

The id -un command prints the login name of the current user.

The password is not shown during input, neither as clear text nor as masking characters.

Only commands that start with sudo run with elevated privileges.

The elevated privileges persist for a certain period of time, so you do not need to provide the root password again.

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:

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:~ > 

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:~ > 

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.

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 .

2.2.1 Editing the Configuration Files #

The main policy configuration file for sudo is /etc/sudoers . As it is possible to lock yourself out of the system if the file is malformed, it is strongly recommended to use visudo for editing. It prevents editing conflicts and checks for syntax errors before saving the modifications.

You can use another editor instead of vi by setting the EDITOR environment variable, for example:

sudo EDITOR=/usr/bin/nano visudo

Keep in mind that the /etc/sudoers file is supplied by the system packages, and modifications done directly in the file may break updates. Therefore, it is recommended to put custom configuration into files in the /etc/sudoers.d/ directory. Use the following command to create or edit a file:

sudo visudo -f /etc/sudoers.d/NAME

The command bellow opens the file using a different editor (in this case, nano ):

sudo EDITOR=/usr/bin/nano visudo -f /etc/sudoers.d/NAME

The #includedir directive in /etc/sudoers ignores files that end with the ~ (tilde) character or contain the . (dot) character.

For more information on the visudo command, run man 8 visudo .

2.2.2 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

There are two exceptions: #include and #includedir are regular commands.

Remove the ! character to set the desired flag to ON.

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

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

Defaults !rootpw # Turn rootpw flag OFF

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

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

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 /etc/sudoers configuration file, consult man 5 sudoers .

2.2.3 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

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.

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 .

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

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:

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 ""

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

When specifying the group name in the sudoers file, make sure that you use the 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 sudoers file must be DOMAIN+GROUP_NAME .

2.3 sudo Use Cases #

While the default configuration works for standard usage scenarios, you can customize the default configuration to meet your specific needs.

2.3.1 Using sudo without root password #

By design, members of the group wheel can run all commands with sudo as root. The following procedure explains how to add a user account to the wheel group.

If the previous command returned no result, install the system-group-wheel package that creates the wheel group:

> sudo zypper install system-group-wheel

## Uncomment to allow members of group wheel to execute any command # %wheel ALL=(ALL) ALL ## Same thing without a password # %wheel ALL=(ALL) NOPASSWD: ALL

ALL ALL=(ALL) ALL # WARNING! Only use this together with 'Defaults targetpw'!

tux:~ > groups users wheel tux:~ > sudo id -un tux's password: root wilber:~ > groups users wilber:~ > sudo id -un wilber is not in the sudoers file. This incident will be reported.

2.3.2 Using sudo with X.Org Applications #

Starting graphical applications with sudo usually 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:

The command below removes the granted access:

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 Further 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.

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