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In the environment I work in, we use tunnels to SSH to various servers. For example, I'll 'ssh -p XXXXX username@localhost' to reach the server.

If the port was always the same, I could do this, and I'd be done:

Host somehost
  User bryan
  Hostname localhost
  Port 12345
  ProxyCommand ssh -p 2218 [email protected] -W %h:%p

However, the port used can and will change if the tunnel goes down and comes back up. This isn't something I have the ability to change - it's built into the infrastructure. So, I wrote a program to find the current port. But I don't know how to either:

a) Run that program and use the output for the %p variable; or b) Run a cron job on first.server.com to write out a text file with the port in it, or set an environment variable, or something.

In effect, I want to do this. Is it possible?

Host somehost
  User bryan
  Hostname localhost
  Port `sh get_port_for_somehost.sh`
  ProxyCommand ssh -p 2218 [email protected] -W %h:%p

The only thing I can think of right now is to run a program on my laptop which rewrites my .ssh/config after going and querying what the ports currently are, but I'd prefer not to do that.

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  • 1
    There may be something I don't understand about your setup. Why are you specifying a Port directive and then using %p in the ProxyCommand, if specifying Port is a problem? Why not use ssh -W %h:$(get_port_for %h) … as the ProxyCommand?
    – Gilles 'SO- stop being evil'
    Oct 26, 2017 at 21:34
  • @Gilles, I'm just starting to use .ssh/config, so I think that's just a case of my misunderstanding what the best use case is. I will try out what you suggest. Thanks!
    – Bryan
    Oct 27, 2017 at 21:21

1 Answer 1

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1

It is not possible to write a script in the configuration file to pull a variable for port number.

But you can write a bash function to get the port for you and place it into the correct place. For example place the following to the ~/.bashrc:

function ssh-dynamic() {
  PORT=`sh get_port_for_somehost.sh`
  exec ssh -p "$PORT" somehost "$@"
}

where the other configuration may stay in the ~/.ssh/config.

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3
  • Just to verify, this needs to be on the initiating system (i.e., my laptop), correct? Or could I do this on the proxy server via ProxyCommand or another method?
    – Bryan
    Oct 26, 2017 at 20:25
  • This needs to be on the initiating system. The other hosts using ProxyCommand are used merely for dumb io redirection. I didn't test the function, but you should be able to get the idea from it.
    – Jakuje
    Oct 26, 2017 at 20:27
  • While this appears to be the only option as long as using the -p option is required, there is absolutely no point in using that option, because it will only be interpreted by the ProxyCommand—which can be trivially modified to get it directly.
    – Jan Hudec
    Jan 4, 2019 at 11:34

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