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I'm pretty lost on this. From the man page:

 -f      Requests ssh to go to background just before command execution.

After starting SSH with the -f option, I have a working tunnel. But after I finish using it I don't know how to further interaction with it. For example, I cannot close the SSH tunnel when I finish with it.

The usual methods I know don't work. For example, jobs returns nothing. The ~ command is not recognized (and I don't know exactly how to use it, anyway).

However, pgrep tells me that the SSH tunnel is still running (after I have closed the terminal, etc.). How do I interact with it? How do I close it?

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I found the solution here: http://www.g-loaded.eu/2006/11/24/auto-closing-ssh-tunnels/

The best way – Tunnels that auto-close

As it has been mentioned previously, instead of using the -f -N switch combination, we can just use -f alone, but also execute a command on the remote machine. But, which command should be executed, since we only need to initialize a tunnel?

This is when sleep can be the most useful command of all! In this particular situation, sleep has two advantages:

  • it does nothing, so no resources are consumed
  • the user can specify for how long it will be executed

How these help in auto-closing the ssh tunnel is explained below.

We start the ssh session in the background, while executing the sleep command for 10 seconds on the remote machine. The number of seconds is not crucial. At the same time, we execute vncviewer exactly as before:

[me@local]$ ssh -f -L 25901:127.0.0.1:5901 me@remote.example.org sleep 10; \
          vncviewer 127.0.0.1:25901:1

In this case, the ssh client is instructed to fork the ssh session to the background (-f), create the tunnel (-L 25901:127.0.0.1:5901) and execute the sleep command on the remote server for 10 seconds (sleep 10).

The difference between this method and the previous one (-N switch), basically, is that in this case the ssh client’s primary goal is not to create the tunnel, but rather to execute the sleep command for 10 seconds. The creation of the tunnel is some kind of side-effect, a secondary goal. If vncviewer was not used, the ssh client would exit after the 10 sec period, as it would have no more jobs to do, destroying the tunnel at the same time.

During the execution of the sleep command, if another process, vncviewer in this case, starts using that tunnel and keeps it occupied beyond the 10 sec period, then, even if the ssh client finishes its remote job (execution of sleep), it cannot exit because another process occupies the tunnel. In other words, the ssh client cannot destroy the tunnel because it would have to kill vncviewer as well. When vncviewer stops using the tunnel, then the ssh client exits too, as it has already accomplished its goal.

This way, no ssh processes are left running in the background.

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A short remark/correction: As far as I know you have to specify vncviewer 127.0.0.1::25091 as you are using the port syntax and not the display syntax. –  Christian Wolf Sep 26 '13 at 10:07
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To kill the tunnel, use ps -C ssh or ps | grep ssh or any other variant to determine which ssh process is running your tunnel. Then kill it.

Alternatively, you can look for the process by determining which one has this port open:

netstat -lnpt | awk '$4 ~ /:1234$/ {sub(/\/.*/, "", $7); print $7}'

If you want to kill all ssh clients running on your machine (as your user), pkill ssh will do it.

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