openssh bugs - Jul 2022 - [Bug 3456] New: provide a way to have forwardings killed, when the remote command/shell finishes

https://bugzilla.mindrot.org/show_bug.cgi?id=3456

            Bug ID: 3456
           Summary: provide a way to have forwardings killed, when the
                    remote command/shell finishes
           Product: Portable OpenSSH
           Version: v9.0p1
          Hardware: Other
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: calestyo at scientia.org

Hey.

When having a ssh connection, that sets up some forwardings (I tried
with LocalForward) and that executes a remote command (but I guess it's
the same when having not remote command but logging in
interactively)... the ssh connection is kept open as long as its
forwarding is still used by some connection, even when the remote
command/shell has finished.

AFAIU, that?s intentional behaviour and indeed it makes quite some
sense for many cases.

But I think there may be other use cases, when any forwardings shall be
killed once the remote command/shell has finished


I stumbled over this while doing the following:

I have a connection which sets up port forwardings for a VNC server and
the connection also executes a remote command which cleverly handles
starting that VNC server and also stopping it on Ctrl-C but e.g. not on
SIGHUP (i.e. accidental SSH connection loss).

That all works quite well, but in one case the SSH connection remains
open, even though the VNC server has already been stopped and the
remote command has finished.

This happens, when I actually connect to the VNC server with some
client, and quit the VNC server from within that X session.

It then stops the VNC server, my remote command also notices this after
a while, but netstat still shows me a connection from local to remote
which stays in CLOSE_WAIT forever.
Ctrl-C causes ssh to quit, also when I stop remmina (the VNC client I
use)

Guess there must be some issue with closing the connection when the
server is exited.



Anyway... from the SSH point of view it would be nice to be able to
forcefully kill any such forwarded connections, because it's already
clear that as soon as the remote command is finished, not connections
won't be possible anymore.


Cheers,
Chris.

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Darren Tucker <dtucker at dtucker.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dtucker at dtucker.net

--- Comment #1 from Darren Tucker <dtucker at dtucker.net> ---
I think you can do what you're describing on the command line,
something like:
$ ssh -L 1234:127.0.0.1:5900 yourserver 'vncserver; kill $PPID'

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--- Comment #2 from Christoph Anton Mitterer <calestyo at scientia.org>
---
I assume your idea here is that this kills the server side ssh process
and thereby also the forwarding, which isn't closed because of
whathever happens between remmina and vnc?

I tried it and while it works in principle it seems rather a hacky
workaround:
- the ssh client gives an error message as the connection is not
cleanly shut down
- exit status becomes 255 and not the one with which the remote command
exits

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |djm at mindrot.org

--- Comment #3 from Damien Miller <djm at mindrot.org> ---
You could also do this using the multiplexing control socket and -O
cancel

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--- Comment #4 from Christoph Anton Mitterer <calestyo at scientia.org>
---
> You could also do this using the multiplexing control socket and -O cancel
But AFAIU, this I'd need to do on the local side in an extra command.

So when I have some:
$ ssh foo.example.org

which sets up the forwarding and does via RemoteCommand start the
remote service... and that then "hangs" because the forwarding is kept
open even after the RemoteCommand has finished,... I would need to run
the cancel command from somewhere else, i.e. manually.

So that's merely "as good" as manually doing a Ctrl-C.

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--- Comment #5 from Damien Miller <djm at mindrot.org> ---
AFAIK this would work:

ssh -oControlMaster=auto -oControlPersist=1s \
    -oControlPath=/path -L[forward] user at host command ; \
ssh -oControlPath=/path -Oexit user at host

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--- Comment #6 from Christoph Anton Mitterer <calestyo at scientia.org>
---
Thanks Daimen... but still not particularly "user-friendly" in the
sense that one still has to write a special command (2x ssh with
special options) and cannot simply "ssh host" as usual.

Wouldn't it be possible to get a config option that simply decides what
should happen if there are still forwardings while the command has
finished, i.e. either keep them open or close them and thus exit?

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