Bash script to set up a temporary SSH tunnel

Question

On Cygwin, I want a Bash script to:

1. Create an SSH tunnel to a remote server.
2. Do some work locally that uses the tunnel.
3. Then shut down the tunnel.

The shutdown part has me perplexed.

Currently, I have a lame solution. In one shell I run the following to create a tunnel:

# Create the tunnel - this works! It runs forever, until the shell is quit.
ssh -nNT -L 50000:localhost:3306 [email protected]

Then, in another shell window, I do my work:

# Do some MySQL stuff over local port 50000 (which goes to remote port 3306)

Finally, when I am done, I close the first shell window to kill the tunnel.

I'd like to do this all in one script like:

# Create tunnel
# Do work
# Kill tunnel

How do I keep track of the tunnel process, so I know which one to kill?

Answer 1

You can do this cleanly with an ssh 'control socket'. To talk to an already-running SSH process and get it's pid, kill it etc. Use the 'control socket' (-M for master and -S for socket) as follows:

$ ssh -M -S my-ctrl-socket -fnNT -L 50000:localhost:3306 [email protected]
$ ssh -S my-ctrl-socket -O check [email protected]
Master running (pid=3517) 
$ ssh -S my-ctrl-socket -O exit [email protected]
Exit request sent. 

Note that my-ctrl-socket will be an actual file that is created.

I got this info from a very RTFM reply on the OpenSSH mailing list.

Answer 2

You can tell SSH to background itself with the -f option but you won't get the PID with $!. Also instead of having your script sleep an arbitrary amount of time before you use the tunnel, you can use -o ExitOnForwardFailure=yes with -f and SSH will wait for all remote port forwards to be successfully established before placing itself in the background. You can grep the output of ps to get the PID. For example you can use

...
ssh -Cfo ExitOnForwardFailure=yes -N -L 9999:localhost:5900 $REMOTE_HOST
PID=$(pgrep -f 'N -L 9999:')
[ "$PID" ] || exit 1
...

and be pretty sure you're getting the desired PID

Answer 3

• You can tell ssh to go into background with & and not create a shell on the other side (just open the tunnel) with a command line flag (I see you already did this with -N).
• Save the PID with PID=$!
• Do your stuff
• kill $PID

EDIT: Fixed $? to $! and added the &

Answer 4

I prefer to launch a new shell for separate tasks and I often use the following command combination:

  $ sudo bash; exit

or sometimes:

  $ : > sensitive-temporary-data.txt; bash; rm -f sensitive-temporary-data.txt; exit

These commands create a nested shell where I can do all my work; when I'm finished I hit CTRL-D and the parent shell cleans up and exits as well. You could easily throw bash; into your ssh tunnel script just before the kill part so that when you log out of the nested shell your tunnel will be closed:

#!/bin/bash
ssh -nNT ... &
PID=$!
bash
kill $PID

Answer 5

You could launch the ssh with a & a the end, to put it in the background and grab its id when doing. Then you just have to do a kill of that id when you're done.

Answer 6

https://github.com/aronpc/remina-ssh-tunnel

#!/usr/bin/env sh

scriptname="$(basename $0)"
actionname="$1"
tunnelname=$(echo "$2" | iconv -t ascii//TRANSLIT | sed -E 's/[^a-zA-Z0-9-]+/-/g' | sed -E 's/^-+|-+$//g' | tr A-Z a-z)
remotedata="$3"
tunnelssh="$4"

if [ $# -lt 4 ] 
 then
    echo "Usage: $scriptname start | stop LOCAL_PORT:RDP_IP:RDP_PORT SSH_NODE_IP"
    exit
fi

case "$actionname" in

start)

  echo "Starting tunnel to $tunnelssh"
  ssh -M -S ~/.ssh/sockets/$tunnelname.control -fnNT -L $remotedata $tunnelssh
  ssh -S ~/.ssh/sockets/$tunnelname.control -O check $tunnelssh
  ;;

stop)
  echo "Stopping tunnel to $tunnelssh"
  ssh -S ~/.ssh/sockets/$tunnelname.control -O exit $tunnelssh 
 ;;

*)
  echo "Did not understand your argument, please use start|stop"
  ;;

esac

usage example

Edit or create new remmina server connection

schema

~/.ssh/rdp-tunnel.sh ACTION TUNNELNAME LOCAL_PORT:REMOTE_SERVER:REMOTE_PORT TUNNEL_PROXY

name	description
ACTION	start|stop
TUNNELNAME	"string identify socket" slugify to create socket file into ~/.ssh/sockets/string-identify-socket.control
LOCAL_PORT	the door that will be exposed locally if we use the same port for two connections it will crash
REMOTE_SERVER	the ip of the server that you would access if you had it on the proxy server that will be used
REMOTE_PORT	the service port that runs on the server
TUNNEL_PROXY	the connection you are going to use as a proxy, it needs to be in your ~/.ssh/config preferably using the access keys

I use the combination (% g-% p) of the remmina group name and connection name to be my TUNNELNAME (this needs to be unique, it will see the socket name)

pre-command

~/.ssh/rdp-tunnel.sh start "%g-%p" 63394:192.168.8.176:3389 tunnel-name-1

post-command

~/.ssh/rdp-tunnel.sh stop "%g-%p" 63394:192.168.8.176:3389 tunnel-name-1

you can and should use this script to access anything, I use it constantly to access systems and services that do not have a public ip going through 1,2,3,4,5 or more ssh proxies

see more into :

1. ssh config
2. ssh mach
3. ssh jump hosts
4. sshuttle python ssh

Refs:

1. https://remmina.org/remmina-rdp-ssh-tunnel/
2. https://kgibran.wordpress.com/2019/03/13/remmina-rdp-ssh-tunnel-with-pre-and-post-scripts/
3. Bash script to set up a temporary SSH tunnel
4. https://gist.github.com/oneohthree/f528c7ae1e701ad990e6