Understanding the Problem: Handling SSH Tunnel Ports on the SSH Server

The SSH (Secure Shell) protocol allows for secure remote access to a server. One of the powerful features offered by SSH is the creation of remote tunnels. These tunnels can be maintained as long as they are in use, which is generally beneficial. However, there are instances where you might want to close the tunnel after the command that created it has completed. This article explores how to manage such scenarios.

Problem Definition

Let's define the problem more concretely. You log into a remote machine to run a command and create a tunnel with the following command:

ssh -R 2200:localhost:22 sleep 30

During the execution of this command, another process might start using the tunnel:

ssh -p 2200 localhost

When the original command completes (in this case, after 30 seconds of sleep), the SSH session hangs, waiting for the tunnel to clear. This behavior can be problematic, especially if you wish to move on with other tasks or if the tunnel is not needed anymore.

Solution: Using SSH ControlMaster

To manage the situation where the SSH command completes before the tunnel is explicitly closed, you can utilize the SSH ControlMaster feature. This feature can kill the SSH session as soon as the command completes. Here's how you can implement it:

Step 1: Define Variables

remote_accountcontrol_file/tmp/ssh-control-$$.keystunnelsremote_commandecho control_file

Step 2: Start the Session Master and Tunnels

ssh -S $control_file $remote_account -M $tunnels -fN

Step 3: Run the Command

ssh -S $control_file $remote_account $remote_command

Step 4: Kill the Session Master

ssh -S $control_file -O exit $remote_account

Step 5: Remove the Control File

rm -f $control_file

Explanation: How Does It Work?

The SSH ControlMaster feature works by creating a master connection that can manage multiple slave connections. When the master connection is terminated, it instructs all slave connections to close, thus freeing up the ports and closing the tunnel.

In the examples above:

Step 1 defines the variables needed for the script to work. Step 2 starts the SSH session master and tunnels in the background (-fN). Step 3 runs the original command. Step 4 kills the SSH session master when the command completes. Step 5 removes the control file to avoid potential errors.

Conclusion

Using the SSH ControlMaster feature is a powerful way to manage SSH tunnels and ensure they are closed when the command that created them has completed. This approach helps to avoid hanging sessions and ensures clean and efficient use of resources.