5.19. Running root Commands via SSH

Problem

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

Solution

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

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 ...

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