Gdm cannot start sessions on hosts running without IPv6 support (eg. ipv6.disable=1 on Linux). While gdm is able to show the login prompt, once a user attempts to log in, gdm fails to perform the login while issuing lots of messages:
/usr/libexec/gdm-x-session: _XSERVTransOpen: transport open failed for inet6/XXXXXX:44992 /usr/libexec/gdm-x-session: _XSERVTransMakeAllCOTSServerListeners: failed to open listener for inet6
This patch adds a new configuration file option in the [security] section, AllowIpv6, which defaults to True. If the user sets this to False, then gdm will pass options down to the X server to prevent IPv6 socket creation. This patch shouldn't affect existing users as it defaults to the current code behavior.