Recipe 5.19 Running root Commands via SSH

5.19.1 Problem

You want to grant root privileges to another user, but permit only certain commands to be run.

5.19.2 Solution

Share your root privileges via SSH [Recipe 5.18] and add forced commands to ~root/.ssh/authorized_keys.

5.19.3 Discussion

Using SSH forced commands, you can limit which programs a user may run as root. For example, this key entry:

 ~root/.ssh/authorized_keys:
command="/sbin/dump -0 /local/data" ssh-dss key... 

permits only the command /sbin/dump -0 /local/data to be run, on successful authentication.

Each key is limited to one forced command, but if you make the command a shell script, you can restrict users to a specific set of programs after authentication. Suppose you write a script /usr/local/bin/ssh-switch:

#!/bin/sh
case "$1" in
        backups)
                # Perform level zero backups
                /sbin/dump -0 /local/data
                ;;
        messages)
                # View log messages
                /bin/cat /var/log/messages
                ;;
        settime)
                # Set the system time via ntp
                /usr/sbin/ntpdate timeserver.example.com
                ;;
        *)
                # Refuse anything else
                echo 'Permission denied' 1>&2
                exit 1
                ;;
esac

and make it a forced command:

 ~root/.ssh/authorized_keys:
command="/usr/local/bin/ssh-switch $SSH_ORIGINAL_COMMAND" ssh-dss key... 

Then users can run selected commands as:

$ ssh -l root localhost backups                    Runs dump
$ ssh -l root localhost settime                    Runs ntpdate
$ ssh -l root localhost cat /etc/passwd            Not authorized: Permission denied

Take care that your forced commands use full paths and have no shell escapes, and do not let the user modify authorized_keys. Here's a bad idea:

 ~root/.ssh/authorized_keys: DON'T DO THIS!!!!
command="/usr/bin/less some_file" ssh-dss key... 

since less has a shell escape.

5.19.4 See Also

ssh(1), sshd(8), sshd_config(5).