Feature Request: socket-passing support in ssh client

[Chris Mitchell]

Hi all,

My apologies if what I'm looking for is already possible. If that is
the case, I would gratefully welcome pointers to where in the
documentation to find it and/or key words to search for.

What I hope to accomplish is on-demand activation of ssh port forwards.
I gather that the OpenSSH server's -i flag is essentially what I'm
looking for, in which something like inetd or systemd already has port
22 open and passes it to sshd at launch. As far as I understand things,
the OpenSSH *client* has no similar capability, which is unsurprising
given that it's not a server.  ;-)

For the use case where I want to use an SSH port forward to access a
remote service on a regular-ish (but not "always-on") basis, it doesn't
really make sense to try to hold that tunnel open all the time with
keepalives and timeouts and restarts and such. But setting it up
manually each time is suboptimal, especially for someone who uses
numerous such tunnels, eg to access a bunch of web-based management
interfaces behind a firewall. As far as I know, current capabilities
don't allow any reasonable on-demand automated solution.

It would be really cool if I could have a meta-server listening
on the local port right from boot, and then on access to that port it
would invoke something like:
/usr/bin/ssh -N -T -L localhost:12345:localhost:12345 user at server
...but with some equivalent to sshd's "-i" flag, so that the
ssh client would accept being passed local port 12345 instead of trying
to open it itself and erroring out because it's already in use.

In my particular use-case I would use a systemd socket unit for this,
but if I'm not mistaken xinetd could fulfill that role too, so what
I'm looking for is platform-independent and doesn't introduce any
systemd dependencies. Finally, it would be convenient if it could take
the local socket from the command line rather than requiring custom
entries in /etc/services, but that's a minor detail.

Does that make sense?

Cheers!
 -Chris