User-Agent:       Mozilla/5.0 (X11; Linux x86_64; rv:19.0) Gecko/20100101 Firefox/19.0

Yast2(graphical):Network_Settings:Global_Options:IPv6_Protocol_Settings
Contains a checkbox 'Enable IPV6'.

Un-setting the checkbox and rebooting resulted in X11 forwarding failing.

sshd emits a message to /var/log/messages upon an attempt to login with 'ssh -X':
   sshd[11290]: error: Failed to allocate internet-domain X11 display socket.

This is manually correctable by editing /etc/ssh/sshd_config, enabling option
'AddressFamily inet' instead of 'any' and restarting sshd.


Reproducible: Always

Steps to Reproduce:
1.Disable IPV6 in yast2 network settings
2.Reboot
3.use 'ssh -X' to login from a remote system and run xclock (or similar)
Actual Results:  
X11-forward fails : tested by attempting to run xclock
Client receives message saying 'Error: Can't open display:'
$DISPLAY is not present
/var/log/messages contains error message from sshd:
    sshd[11290]: error: Failed to allocate internet-domain X11 display socket.

Expected Results:  
xclock should run normally

Corrected by manually updating /etc/ssh/sshd_config to contain
'AddressFamily inet' and restarting sshd.