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Subject: ssh -X (X11 forwarding) not working from 6.1 to 8.1
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DISPLAY is not getting set in a remote shell started by "ssh -X".

  $ echo $DISPLAY
  :0.0
  $ ssh -X [server] 'echo DISPLAY=%$DISPLAY%'
  DISPLAY=%%

How would I go about debugging this?

DISPLAY _is_ set correctly on the ssh client -- I am running in
an xterm there, and can successfully start another (local) one --
and the word DISPLAY is not present in any of the shell startup
files (.bash*, .cshrc, .login, .profile, .shrc) on either the
ssh client or the ssh server.

The ssh client is FreeBSD 6.1 and the ssh server is FreeBSD 8.1.

It does work correctly in the other direction (using the 8.1 system
as the ssh client and the 6.1 system as the ssh server), and I can
run X11 programs on the 8.1 system, displaying on the 6.1 system,
provided I set DISPLAY appropriately on the 8.1 system.  It's only
the ssh X11 forwarding that's broken, and only in one direction.