As we all know, it is possible to use SSH not only for obtaining an interactive login session on a remote machine, but also for executing commands remotely. For instance, the following command will log on to myserver.example.com, execute “uname -a” and return to the local shell:
ssh myserver.example.com uname -a
(The local SSH client returns the exit code from the remote command, if you’re into this kind of detail.)
You might have some users (or scheduled automatisms) that you don’t want to be able to log on to that machine at all, but who should be permitted to execute only a given command. In order to achieve this, you can configure key-based authentication. Once this has been done, the key can be prefixed with a number of configuration options. Using one of these options, it is possible to enforce execution of a given command when this key is used for authentication.
In this example from ~/.ssh/authorized_keys, the user wants to look at the process list, so we set the command to “ps -ef”.
command="/bin/ps -ef"
Using this, when the user tries to log in, or tries to execute an arbitrary command, “/bin/ps -ef” is executed instead and the SSH session terminates.
In addition to enforcing a command, it is advisable to disable a number of advanced SSH features, such as TCP and X11 forwarding. Assignment of a pseudo terminal to the user’s SSH session may also be suppressed, by adding a number of additional configuration options next to the forced command:
no-port-forwarding,no-X11-forwarding,no-pty
Here’s what a full entry from ~/.ssh/authorized_keys might look like:
command="/bin/ps -ef",no-port-forwarding,no-X11-forwarding,no-pty ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAp0KMipajKK468mfihpZHqmrMk8w+PmzTnJrZUFYZZNmLkRk+icn+m71DdEHmza2cSf9WdiK7TGibGjZTE/Ez0IEhYRj5RM3dKkfYqitKTKlxVhXNda7az6VqAJ/jtaBXAMTjHeD82xlFoghLZOMkScTdWmu47FyVkv/IM1GjgX/I8s4307ds1M+sICyDUmgxUQyNF3UnAduPn1m8ux3V8/xAqPF+bRuFlB0fbiAEsSu4+AkvfX7ggriBONBR6eFexOvRTBWtriHsCybvd6tOpJHN8JYZLxCRYHOGX+sY+YGE4iIePKVf2H54kS5UlpC/fnWgaHbmu/XsGYjYrAFnVw== Test key
This is quite nice: We have successfully limited this user to requesting a process list.
This is called an SSH forced command.
So much for the introduction. 
Here’s what I’m really getting at today – What, if we want the user to not only execute a single command, but a number of commands, such as:
- Show process list (ps)
- Show virtual memory statistics (vmstat)
- Stop and start the print server (/etc/init.d/cupsys stop/start)
Following the approach described above, this would give us four key pairs, four entries in ~/.ssh/authorized_keys, and four entirely different invocations of SSH on the client side, each of them using a dedicated private key. In other words: An administrative nightmare.
This is where the environment variable $SSH_ORIGINAL_COMMAND comes in. (This nice facility was pointed out to me last week by G., who had read about it somewhere but wondered what it might be useful for.)
Until now, all we know is that with a forced command in place, the SSH server ignores the command requested by the user. This is not entirely true, though. The SSH server does in fact remember the command that was requested, stores it in $SSH_ORIGINAL_COMMAND and thus makes it available within the environment of the forced command.
With this in mind, it is possible to allow more flexibility inside forced commands, without the need to go crazy with countless key pairs. Instead, it is possible to just create a wrapper script that is called as the forced command from within ~/.ssh/authorized_keys and decides what to do, based on the content of $SSH_ORIGINAL_COMMAND:
#!/bin/sh
# Script: /usr/local/bin/wrapper.sh
case "$SSH_ORIGINAL_COMMAND" in
"ps")
ps -ef
;;
"vmstat")
vmstat 1 100
;;
"cups stop")
/etc/init.d/cupsys stop
;;
"cups start")
/etc/init.d/cupsys start
;;
*)
echo "Sorry. Only these commands are available to you:"
echo "ps, vmstat, cupsys stop, cupsys start"
exit 1
;;
esac
It is important to be aware of potential security issues here, such as the user escaping to a shell prompt from within one of the listed commands. Setting the “no-pty” option already makes this kind of attack somewhat difficult. In addition, some programs, such as “top”, for example, have special options to run them in a “secure” read-only mode. It is advisable to closely examine all programs that are called as SSH forced commands for well-meant “backdoors” and to find out about securing them.
It’s up to you to decide based on your own situation, whether you want to run this wrapper as the root user or if you prefer to use password-less “sudo” commands to raise privileges where needed.
If you encounter problems while debugging $SSH_ORIGINAL_COMMAND, please make absolutely sure that you are authenticating with the correct key. I found it helpful to unset SSH_AUTH_SOCK in the window where I do my testing, in order to prevent intervention from identies stored in the SSH agent.
