When 'ssh -X somehost' doesnt work...

So I went to try and remote run an xterm from a machine I had not turned on for a while…

~$ ssh -X me@somehost 
... 
~$ xterm

And unexpectedly received the following error message:

xterm Xt error: Can't open display: 
xterm:  DISPLAY is not set

After some scratching around I found the following error message in /var/log/auth:

sshd  error: Failed to allocate internet-domain X11 display socket.

After more had scratching and reading the sshd_config manpage, I remembered I often turn off ipv6 on test machines on my local network (yes, I know…) This doesn’t always play well with default configs of a lot of software, including ssh. Enabling the following setting in /etc/ssh/sshd_config resolved the problem:

AddressFamily inet