I have an additional answer to Cygwin/X FAQ 6.1, "X11Forwarding does not
work with OpenSSH under Cygwin"
--- begin ------------
A6:
If the *remote* machine is a Windows machine using Cygwin OpenSSH,
make sure the Cygwin xauth package is installed on the *remote*
machine. The OpenSSH server needs xauth to do X11 Forwarding.
--- end ------------
For a while I was confounded by this:
% export DISPLAY=:0
% ssh -Y -f remote-windows-host printenv DISPLAY
Warning: No xauth data; using fake authentication data for X11
forwarding.
*** DISPLAY not printed here !!! ***
% ssh -Y -f remote-unix-host printenv DISPLAY
Warning: No xauth data; using fake authentication data for X11
forwarding.
localhost:21.0 *** DISPLAY printed as expected ***
I finally noticed a message about xauth in the output of
ssh -vvv -Y -f remote-windows-host printenv DISPLAY
This was hard enough to diagnose that I think it deserves the FAQ
entry above.