ssh forwarder

#!/bin/bash
#
# Usage: ${name} stop|start|status|help
#
# Manages an SSH tunnel between two servers. This script uses SSH sockets
# to multiplex a forwarded connection and allow interactive control of
# the tunnel

name="$(basename "${0}")"

LOCAL_PORT='4321'
TARGET_PORT='4242'
TARGET_HOST='172.30.0.196'
TARGET_USER='ap'
SSH_SOCKET="/var/run/${name}.sock"
SSH_KEY_FILE="${HOME}/.ssh/id_rsa"

ssh_target="${TARGET_USER}@${TARGET_HOST}"

running() {
    [ -S "${SSH_SOCKET}" ] && \
        ssh -S "${SSH_SOCKET}" -O check "${ssh_target}" > /dev/null 2>&1
}

usage() {
    echo "${0} start|stop|status|help"
}

case "${1}" in
    "start")
        if ! running; then
            [ -S "${SSH_SOCKET}" ] && rm -f "${SSH_SOCKET}"
            ssh -M -S "${SSH_SOCKET}" -fNT -L \*:${LOCAL_PORT}:localhost:${TARGET_PORT} \
                -o ServerAliveInterval=60 -o ExitOnForwardFailure=yes \
                -i "${SSH_KEY_FILE}" "${ssh_target}"
            [ $? -eq 0 ] && echo "${name} started"
        else
            echo "${name} is already running"
        fi
    ;;
    "stop")
        if running; then
            ssh -S "${SSH_SOCKET}" -O exit "${ssh_target}"
            [ $? -eq 0 ] && echo "${name} stopped successfully"
        fi
    ;;
    "status")
        if running; then
            ssh -S "${SSH_SOCKET}" -O check "${ssh_target}"
        else
            echo "${name} is stopped"
        fi
    ;;
    "help")
        usage
    ;;
    *)
        echo "Invalid Option: $1"
        usage
esac