sshd needs shorter client timeout for ltsp sessions

[Oliver Grawert]

If a ltsp session is closed on the client, the session is kept alive for ~30
seconds on the server due to the default TCP timeout value, so a quick relogin
attempt will fail. Setting the ClientAliveInterval/ClientAliveCountMax to a
shorter value (10 sec should suffice) with sshd -o "..." on ltsp server startup
should solve this.

[Colin Watson (cjwatson)]

I think this is the wrong answer; if the LTSP session is persisting even after
client logout then that's the problem (and will possibly have other bad
side-effects because things are exiting uncleanly). We need to figure out why
that's happening.

[Oliver Grawert (ogra)]

i tried to track the issue a bit more, it seems the DISPLAY variable isnt
cleaned up fast enough in the environment after logout.
i can see in the ldm.log that the ssh connection was established correctly on
the second login attempt (see attached ldm.log), but was closed from the server.
as the attached xsession-errors file from the server shows, x-session-manager
exits with "cannot open display:". to make sure its not caused by any leftovers
from the former session i ran slay for the user between the two login attempts.

[Oliver Grawert (ogra)]

Created an attachment (id=3816)
ldm.log

[Oliver Grawert (ogra)]

Created an attachment (id=3817)
.xsession-errors

[Oliver Grawert (ogra)]

hmm, actually DISPLAY should be overwritten, so i suspect it still has something
to do with the internal tcp connection sshd uses for X forwarding...

[Oliver Grawert (ogra)]

the behavior is identical to bug #25528

This bug has been marked as a duplicate of bug 25528.