Ssh – Can an OpenSSH server forward inbound traffic to another server

Unfortunately I didn't find your question earlier, but I've just got a really good answer from kamil-maciorowski:

https://unix.stackexchange.com/a/584505/251179

In summary, establish a master connection first, keep that in the background, then issue a second command with -O *ctl_cmd* set to forward to request/setup port forwarding:

ssh -fNMS /path/to/socket user@server

port="$(ssh -S /path/to/socket -O forward -R 0:localhost:22 placeholder)"

This will give you $port on your local machine, and a connection in the background.

You can then use $port locally; or use ssh again to run a command on the remote server, where you can use the same control socket.

As to the flags, these are:

• -f = Requests ssh to go to background.
• -N = Do not execute a remote command.
• -M = Places client into “master” mode, for connection sharing.
• -S = Location of a control socket for connection sharing.
• -O = Control an active connection multiplexing master process.

In my case I've added a bit more to keep checking the connection:

#!/bin/bash

#--------------------------------------------------
# Setup
#--------------------------------------------------

  set -u;

  tunnel_user="user";
  tunnel_host="1.1.1.1";

  local_port="22";
  local_name="my-name";

  path_key="$HOME/.ssh/tunnel_ed25519";

  path_lock="/tmp/tunnel.${tunnel_host}.pid"
  path_port="/tmp/tunnel.${tunnel_host}.port"
  path_log="/tmp/tunnel.${tunnel_host}.log"
  path_socket="/tmp/tunnel.${tunnel_host}.socket"

#--------------------------------------------------
# Key file
#--------------------------------------------------

  if [ ! -f "${path_key}" ]; then

    ssh-keygen -q -t ed25519 -f "${path_key}" -N "";

    /usr/local/bin/tunnel-client-key.sh
      # Sends the public key to a central server, also run via cron, so it can be added to ~/.ssh/authorized_keys
      # curl -s --form-string "pass=${pass}" --form-string "name=$(local_name)" -F "key=@${path_key}.pub" "https://example.com/key/";

  fi

#--------------------------------------------------
# Lock
#--------------------------------------------------

  if [ -e "${path_lock}" ]; then
    c=$(pgrep -F "${path_lock}" 2>/dev/null | wc -l);
      # MacOS 10.15.4 does not support "-c" to count processes, or the full "--pidfile" flag.
  else
    c=0;
  fi

  if [[ "${c}" -gt 0 ]]; then
    if tty -s; then
      echo "Already running";
    fi;
    exit;
  fi;

  echo "$$" > "${path_lock}";

#--------------------------------------------------
# Port forward
#--------------------------------------------------

  retry=0;

  while true; do

    #--------------------------------------------------
    # Log cleanup
    #--------------------------------------------------

      if [ ! -f "${path_log}" ]; then
        touch "${path_log}";
      fi

      tail -n 30 "${path_log}" > "${path_log}.tmp";

      mv "${path_log}.tmp" "${path_log}";

    #--------------------------------------------------
    # Exit old sockets
    #--------------------------------------------------

      if [ -S "${path_socket}" ]; then

        echo "$(date) : Exit" >> "${path_log}";

        ssh -S "${path_socket}" -O exit placeholder;

      fi

    #--------------------------------------------------
    # Lost lock
    #--------------------------------------------------

      if [ ! -e "${path_lock}" ] || ! grep -q "$$" "${path_lock}"; then

        echo "$(date) : Lost Lock" >> "${path_log}";

        exit;

      fi

    #--------------------------------------------------
    # Master connection
    #--------------------------------------------------

      echo "$(date) : Connect ${retry}" >> "${path_log}";

      ssh -o StrictHostKeyChecking=no -o ConnectTimeout=10 -o ServerAliveInterval=30 -o ExitOnForwardFailure=yes -fNTMS "${path_socket}" -i "${path_key}" "${tunnel_user}@${tunnel_host}" >> "${path_log}" 2>&1;

    #--------------------------------------------------
    # Setup and keep checking the port forwarding
    #--------------------------------------------------

      old_port=0;

      while ssh -S "${path_socket}" -O check placeholder 2>/dev/null; do

        new_port=$(ssh -S "${path_socket}" -O forward -R "0:localhost:${local_port}" placeholder 2>&1);

        if [[ "${new_port}" -gt 0 ]]; then

          retry=0;

          if [[ "${new_port}" -ne "${old_port}" ]]; then

            ssh -i "${path_key}" "${tunnel_user}@${tunnel_host}" "tunnel.port.sh '${new_port}' '${local_name}'" >> "${path_log}" 2>&1;
              # Tell remote server what the port is, and local_name.
              # Don't use socket, it used "-N"; if done, a lost connection keeps sshd running on the remote host, even with ClientAliveInterval/ClientAliveCountMax.

            echo "$(date) : ${new_port}" >> "${path_log}";

            echo "${new_port}" > "${path_port}";

            old_port="${new_port}";

            sleep 1;

          else

            sleep 300; # Looks good, check again in 5 minutes.

          fi

        else # Not a valid port number (0, empty string, number followed by an error message, etc?)

          ssh -S "${path_socket}" -O exit placeholder 2>/dev/null;

        fi

      done

    #--------------------------------------------------
    # Cleanup
    #--------------------------------------------------

      if [ ! -f "${path_port}" ]; then
        rm "${path_port}";
      fi

      echo "$(date) : Disconnected" >> "${path_log}";

    #--------------------------------------------------
    # Delay before next re-try
    #--------------------------------------------------

      retry=$((retry+1));

      if [[ $retry -gt 10 ]]; then
        sleep 180; # Too many connection failures, try again in 3 minutes
      else
        sleep 5;
      fi

  done

First, with your kind of setting, what would work, requiring anyway two ssh:

Host foo
    LocalForward 10022:bar:22

Host bar
    Hostname localhost
    Port 10022
    LocalForward 33389 rdp:3389

term1$ ssh foo # or use ssh -f -N -o ExitOnForwardFailure=yes foo for background task
term2$ ssh bar

What you'd really need wouldn't be RemoteCommand but ProxyJump, really simplifying your configuration, with goal reached only with:

ssh -L 33389:rdp:3389 -J foo bar

Or the equivalent (only) configuration:

Host bar
    ProxyJump foo
    LocalForward 33389 rdp:3389

With zero intermediate port needed.

Unfortunately ProxyJump is available only starting from openssh 7.3

But it's easily replaced with the ProxyCommand/-W combo you were using before.

ssh -L 33389:rdp:3389 -o 'ProxyCommand=ssh foo -W %h:%p' bar 

or as configuration file:

Host bar
    ProxyCommand ssh -W %h:%p foo
    LocalForward 33389 rdp:3389

There are still two ssh running: the hidden one is the one as ProxyCommand parameter still running on the local host doing the link using a pair of pipes with the main ssh, instead of an extra tcp port. Trying to involve the intermediate host in participating with the port forwarding is insecure or can clash (other users of foo can access the tunnel or use the same port), and prone to errors. Tunnel ends should always be kept on the client host if possible, where there's full control. Intermediate tunnels should either not exist or point to the next ssh entry point (that's the -W 's usage here).