Can I make SSH fail when a port forwarding fails?

Question

If I do a remote port forward, a la -R 3690:localhost:3690 when a binding already exists on the port on the remote host, I get this warning:

Warning: remote port forwarding failed for listen port 3690

Is there a way to have ssh fail (i.e. exit with a nonzero return code), rather than just emit a warning?

Answer 1

Run

ssh -o "ExitOnForwardFailure yes" ...

or put

ExitOnForwardFailure yes

into ~/.ssh/config. See man ssh_config for details.

Answer 2

I use bash script on the target host to make sure the forwarding was opened correctly. The SSH connection will run this and exit if there's a problem with the port forwarding, e.g.

client side script: ( this uses .ssh/config for port forwarding settings )

#!/bin/bash    

while true; do
    echo -n starting at : "
    date
    ssh user@server bin/sshloop.sh
    echo "got back, sleeping 17 "
    sleep 17
done 

server side script ( bin/sshloop.sh )

#!/bin/bash

while true; do 
  echo $(date)" : SSH Reverse 1090:80, 1232:22 From Server to Client"
  sleep 17
  if ! netstat -an | grep -q ":::1090 " ; then
     echo "1090 forward missing, bailing out"
     exit
  fi
done

Maybe even run the client side script under screen with -dmS