I just have a look at the auth file Using authority file /tmp/.gdm1TUMLT xauth> list #ffff##:20 MIT-MAGIC-COOKIE-1 be4aaa905e5cf980cfe10715b9e1258b -- XDMCP not working in ipv6, gdm should be compiled with --enable-ipv6=yes https://launchpad.net/bugs/75254