Establish a SSH tunnel to access a remote server behind firewall

• To start the tunnel (running background):

  ssh -L ${LOCAL_HOST}:${LOCAL_PORT}:${REMOTE_HOST}:${REMOTE_PORT} ${GATEWAY_USER}@${GATEWAY_HOST} -N -f -M -S ${SSH_CTRL_PATH}

  for example:

  ssh -L localhost:3389:192.168.20.10:3389 john@gateway-ssh-server.johns.org -N -f -M -S /tmp/ssh-tunnel.3389

  The command above starts the ssh tunnel (and keeps the ssh process running background.) After the tunnel is established, you can access the remote server port(192.168.20.10) via local port(localhost:3389).
• To stop the tunnel (running background):

  ssh -S ${SSH_CTRL_PATH} -O exit ${GATEWAY_USER}@${GATEWAY_HOST}

  for example:

  ssh -S /tmp/ssh-tunnel.3389 -O exit john@gateway-ssh-server.johns.org