sudo, su, and you. oh, and root. and some tips...

sudo(8) executes commands as a different user on Unix systems, as allowed by the sudoers configuration file. Commands run via sudo are logged via syslog, providing an audit trail. While sudo may not work on your friends, I consider it essential to system administration.

• sudo homepage.
   
• Managing /etc/sudoers with CFEngine.

Alternatives

Consider also sudosh, or special logbash versions of the shell that log all commands. Never use the unsafe and unlogged sudo -s, sudo -i, and su commands. Between sudo and proper configuration management, logging in as root should be a very rare occasion.

List Commands

To see what commands can be run on a system, issue sudo -l. Depending on the sudoers configuration, this may prompt for the user’s password.

$ sudo -l
User admin may run the following commands on this host:
(ALL) NOPASSWD: ALL

If root is allowed to run sudo, one can inspect what commands another user may run:

$ sudo sudo -u someotheruser sudo -l
User someotheruser may run the following commands on this host:
(ALL) NOPASSWD: /usr/sbin/cleanup-logs

If administrators are allowed to sudo to any other user, this can be done directly via:

$ sudo -u someotheruser sudo -l
User someotheruser may run the following commands on this host:
(ALL) NOPASSWD: /usr/sbin/cleanup-logs

Configuration

The sudoers configuration file uses Extended Backus-Naur Form (EBNF), which is flexible but complex. For an overview, see the sudoers(5) documentation.

• Always use visudo(8).
The visudo command should be used to edit the sudoers data. Otherwise, errors or permissions problems may crop up randomly. If building a complex sudoers file using configuration management software, sanity check the resulting data with visudo -f tempsudoers -c before moving it into production use.
• Last entry wins
The last matching rule in sudoers wins; that is, if a NOPASSWD entry is followed by an entry that requires the implicit PASSWD, the user will be prompted to enter their password.
ALL ALL=(ALL) NOPASSWD: ALL ALL ALL=(ALL) ALL
$ sudo -l User admin may run the following commands on this host: (ALL) NOPASSWD: ALL (ALL) ALL $ sudo -k; sudo /bin/ls Password:
To avoid this problem, place NOPASSWD entries after any entries that require a password. The following requires passwords for all commands excepting xinetd service changes on a RedHat Linux system:
%wheel ALL=(ALL) ALL %wheel ALL=NOPASSWD: /sbin/service xinetd *

Disallow Shell Access

Use the following configuration to avoid needless use of unsafe and unlogged shells. Encourage users to avoid launching a root shell, and reserve a special logbash shell that logs all commands for the rare occasions a root shell is needed.

# specify full list of shells and login commands here
Cmnd_Alias SHELLS= /bin/sh, /bin/ksh, /bin/bash, /bin/zsh, \
/bin/csh, /bin/tcsh, \
/usr/bin/login, /usr/bin/su

%wheel ALL=(ALL) ALL, !SHELLS

If the configuration is correct, a user attempting to gain shell access will be properly rejected:

$ sudo -s
Sorry, user jdoe is not allowed to execute '/bin/zsh' as root on …
$ sudo -i
Sorry, user jdoe is not allowed to execute '/bin/sh' as root on …
$ sudo su
Sorry, user jdoe is not allowed to execute '/usr/bin/su' as root on …