Theory & Logic

Bash SSH Tunnel Wrapper

I recently became the caretaker of a Chef server that sits behind a firewall, inaccessible to all but the nodes that require access. Even though it’s locked down, I still need to be able to administer it. My tool of choice is knife, the command-line utility used to interact with Chef.

I have SSH access to the box, so to use knife I only have to create an SSH tunnel:

ssh -T -f -N -L 4000:localhost:4000 dryan@chef.example.com

This works, but I don’t really want to manually manage SSH tunnels. What I really want is a wrapper that 1) creates the tunnel 2) runs my command, and 3) closes the tunnel. So I wrote a bit of Bash to allow me to do just that:

function create_chef_tunnel() {
  cmd='ps -eo pid,args | egrep "[s]sh -T -f -N -L 4000:localhost" | cut -c1-6'
  if [ $? -eq 0 ]
  then
    echo "SSH tunnel exists"
    return
  else
    ssh -T -f -N -L 4000:localhost:4000 dryan@chef.example.com
  fi

}

function close_chef_tunnel() {
  cmd='ps -eo pid,args | egrep "[s]sh -T -f -N -L 4000:localhost" | cut -c1-6'
  pid=$(eval $cmd)
  kill -9 $pid
}

function chef_tunnel_wrapper() {
  create_chef_tunnel
  "$@"
  close_chef_tunnel
}

alias knife="chef_tunnel_wrapper knife"

Surely, the above snippet could be made more re-usable. One could modify it to support specifying ports, hosts, usernames, etc, but I didn’t need it for anything else just yet. I’ll leave that as an exercise to the reader :)