denied X11 forwarding

[Jiann-Ming Su]

This has to be a FAQ, but I can't find anything on it at the web site.
I've installed openssh on several
computers now and have yet to get X11 forwarding to work.  I get the
following when I log into
a computer with ssh:

Warning: Remote host denied X11 forwarding, perhaps xauth program could
not be run on the server side.

However:

$ which xauth
/usr/X11R6/bin/xauth

Please reply by email as I'm not on the developer's list.  Thanks for
any help.