On my local machine, I run:
ssh -X firstname.lastname@example.org
(For completeness, I have also tested all of the following using -Y with identical results).
As expected, this accesses remotemachine.com fine, and all appears well. If I then attempt to run xcalc however, I get:
connect /tmp/.X11-unix/X0: No such file or directory Error: Can't open display: localhost:10.0
$ ls -la /tmp/.X11-unix/ total 36 drwxrwxrwt 2 root root 4096 2012-11-23 09:29 . drwxrwxrwt 8 root root 32768 2012-11-29 08:22 .. srwxrwxrwx 1 root root 0 2012-11-23 09:29 X0
So not only does /tmp/.X11-unix/X0 exist, it has universal r/w/x permissions!
I've previously used x-forwarding without issue, though not in some time...
uname -a on the server for reference:
Linux machinename 2.6.32-25-generic #45-Ubuntu SMP Sat Oct 16 19:52:42 UTC 2010 x86_64 GNU/Linux
Been searching around on the web for a couple hours now without success. Other mentions of the same problem, but no solutions.