ssh-forcecommand - Whitelist remote commands via ssh config

ssh-forcecommand is a trivial script to safely execute remote commands via
ssh.  It is especially aimed at automated remote commands (so, ssh keys not
secured via password), where a compromise of the remote system (-> private
key) could also compromise the local system.

To prevent this, you can put the forcecommand into the ssh config
(authorized_keys, to be precise), so the remote system can only execute a set
of statically defined commands.  This way, compromising the local system is
made much more difficult.


SETUP
-----

First, run "make install".  You will now have the script in
/usr/local/lib/ssh-forcecommand.

Next, for every publickey you want to restrict to the forcecommand, add the
following line to ~/.ssh/authorized_keys:

command="/usr/local/lib/ssh-forcecommand /etc/forcecommand/foo.cfg",no-agent-forwarding,no-port-forwarding,no-pty,no-X11-forwarding ssh-rsa yourfunkykey

command="..." sets the forcecommand, the other options disable potentially
dangerous stuff like port forwardig (Though that is not meant to be an
exhaustive list).

As you see, the forcecommand accepts exactly one argument, which is the config
defining the allowed commands.  This way, you can restrict different ssh keys
to different sets of commands.  For example configs, see the examples
directory.


USAGE
-----

Assume you have the following line in your forcecommand config:

home      = tar -C / -cf - home

Now, on the remote system, run this:

ssh user@yourhost home

On your system, this will translate to:

tar -C / -cf - home

The forcecommand is 100% static, variables or appending of stuff is not
supported.  No part of the original ssh command will be dynamically used in
the resulting command.  This makes ssh-forcecommand quite secure.