Sudo

The sudo command provides a simple and secure way to configure privilege escalation, e.g. letting normal users to execute certain (or all) commands as root or some different user, possibly without giving a password.

When you want some people to perform certain administrative steps on your system without granting them total root access, using sudo is your best option. With sudo you can control who can do what. This guide offers you a small introduction to this wonderful tool.

This article is meant as a quick introduction. The package is a lot more powerful than what is described in this guide. It has special features for editing files as a different user (sudoedit), running from within a script (so it can background, read the password from standard in instead of the keyboard, ...), etc.

Please read the sudo and sudoers manual pages for more information.

Logging activity
One additional advantage of sudo is that it can log any attempt (successful or not) to run an application. This is very useful if you want to track who made that one fatal mistake that took you 10 hours to fix :)

Granting permissions
The package allows the system administrator to grant permission to other users to execute one or more applications they would normally have no right to. Unlike using the  bit on these applications sudo gives a more fine-grained control on who can execute a certain command and when.

With sudo you can make a clear list who can execute a certain application. If you would set the setuid bit, any user would be able to run this application (or any user of a certain group, depending on the permissions used). You can (and probably even should) require the user to provide a password when he wants to execute the application.

The sudo configuration is managed by the file. This file should never be edited through nano /etc/sudoers</tt> or vim /etc/sudoers</tt> or any other editor you might like. When you want to alter this file, you should use visudo</tt>.

This tool makes sure that no two system administrators are editing this file at the same time, preserves the permissions on the file and performs some syntax checking to make sure you make no fatal mistakes in the file.

Basic syntax
The most difficult part of sudo</tt> is the syntax. The basic syntax is like so:

This syntax tells sudo</tt> that the user, identified by  and logged in on the system   can execute any of the commands listed in   as the root user. A more real-life example might make this more clear: allow the user larry to execute emerge</tt> if he is logged in on localhost:

The user name can also be substituted with a group name - in this case you should start the group name with a  sign. For instance, to allow any one in the wheel</tt> group to execute emerge</tt>:

You can extend the line to allow for several commands (instead of making a single entry for each command). For instance, to allow the same user to not only run emerge</tt> but also ebuild</tt> and emerge-webrsync</tt> as root:

You can also specify a precise command and not only the tool itself. This is useful to restrict the use of a certain tool to a specified set of command options. The sudo</tt> tool allows shell-style wildcards (AKA meta or glob characters) to be used in path names as well as command-line arguments in the sudoers file. Note that these are not regular expressions.

Let us put this to the test:

The password that sudo</tt> requires is the user's own password. This is to make sure that no terminal that you accidentally left open to others is abused for malicious purposes.

You should know that sudo</tt> does not alter the  variable: any command you place after sudo</tt> is treated from your environment. If you want the user to run a tool in for instance he should provide the full path to sudo</tt>, like so:

Basic syntax with LDAP
The following USE flags are needed for the LDAP support.

When using sudo with LDAP, sudo will read configuration from LDAP Server as well. So you will need to edit two files.

{{FileBox|filename=/etc/ldap.conf.sudo|title=Please chmod 400 when done|1= host ldap.example.com port 389 base dc=example,dc=com uri ldap://ldap.example.com/ ldap_version 3 sudoers_base ou=SUDOers,dc=example,dc=com bind_policy soft }}
 * 1) See ldap.conf(5) and README.LDAP for details
 * 2) This file should only be readable by root
 * 1) supported directives: host, port, ssl, ldap_version
 * 2) uri, binddn, bindpw, sudoers_base, sudoers_debug
 * 3) tls_{checkpeer,cacertfile,cacertdir,randfile,ciphers,cert,key
 * 1) uri ldapi://%2fvar%2frun%2fopenldap%2fslapd.sock
 * 1) ssl start_tls
 * 1) sudoers_debug 2

Also you will need to add in the following LDAP Entry for Sudo.

The configuration on the sudoer on LDAP are similar to files with some different. Please read more about Sudo with LDAP on the link below.

alias (to make a good distinction between aliases and non-aliases it is recommended to use capital letters for aliases). As you might undoubtedly have guessed, the  alias is an alias to all possible settings.

A sample use of the  alias to allow any user to execute the <tt>shutdown</tt> command if he is logged on locally is:

Another example is to allow the user <tt>larry</tt> to execute the <tt>emerge</tt> command as root, regardless of where he is logged in from:

More interesting is to define a set of users who can run software administrative applications (such as <tt>emerge</tt> and <tt>ebuild</tt>) on the system and a group of administrators who can change the password of any user, except root!

Non-root execution
It is also possible to have a user run an application as a different, non-root user. This can be very interesting if you run applications as a different user (for instance <tt>apache</tt> for the web server) and want to allow certain users to perform administrative steps as that user (like killing zombie processes).

Inside you list the user(s) in between   and   before the command listing:

For instance, to allow <tt>larry</tt> to run the <tt>kill</tt> tool as the <tt>apache</tt> or <tt>gorg</tt> user:

With this set, the user can run <tt>sudo -u</tt> to select the user he wants to run the application as:

You can set an alias for the user to run an application as using the  directive. Its use is identical to the other  directives we have seen before.

Passwords and default settings
By default, <tt>sudo</tt> asks the user to identify himself using his own password. Once a password is entered, <tt>sudo</tt> remembers it for 5 minutes, allowing the user to focus on his tasks and not repeatedly re-entering his password.

Of course, this behavior can be changed: you can set the  directive in  to change the default behavior for a user.

For instance, to change the default 5 minutes to 0 (never remember):

A setting of  would remember the password indefinitely (until the system reboots).

A different setting would be to require the password of the user that the command should be run as and not the users' personal password. This is accomplished using. In the following example we also set the number of retries (how many times the user can re-enter a password before <tt>sudo</tt> fails) to  instead of the default 3:

Another interesting feature is to keep the  variable set so that you can execute graphical tools:

You can change dozens of default settings using the  directive. Fire up the <tt>sudoers</tt> manual page and search for.

If you however want to allow a user to run a certain set of commands without providing any password whatsoever, you need to start the commands with, like so:

Bash completion
Users that want bash completion with sudo need to run this once.

Zshell completion
Users that want zsh completion for sudo can set the following in and  respectively

With the above change, all commands in the, and  locations will be available to the shell for completion when the command is prefaced with 'sudo'.

Listing privileges
To inform yourself what your capabilities are, run <tt>sudo -l</tt> :

If you have any command in that does not require you to enter a password, it will not require a password to list the entries either. Otherwise you might be asked for your password if it isn't remembered.

Prolonging password timeout
By default, if a user has entered their password to authenticate their self to <tt>sudo</tt>, it is remembered for 5 minutes. If the user wants to prolong this period, he can run <tt>sudo -v</tt> to reset the time stamp so that it will take another 5 minutes before <tt>sudo</tt> asks for the password again.

The inverse is to kill the time stamp using <tt>sudo -k</tt>.