Red Hat Bugzilla – Bug 867981
gdm doesn't start when connecting to localhost using gdm via XDMCP
Last modified: 2013-02-21 05:02:42 EST
Description of problem: When I enable XDMCP in gdm and then try to connect from the same machine via XDMCP, i.e. using localhost, the greeter keep spinning and never shows up. From greeter log: No protocol specified ** (gnome-session:16605): WARNING **: Cannot open display: When connecting from remote machine gdm starts normally. Version-Release number of selected component (if applicable): gdm-2.30.4-33.el6_2.1 gdm-2.30.4-33.el6_3.1 How reproducible: 100% Steps to Reproduce: 1. Enable XDMCP in gdm; edit /etc/gdm/custom.conf and add Enable=true under [xdmcp] 2. Restart gdm; # initctl restart prefdm 3. Login 4. Connect to localhost; $ Xephyr -query localhost :2 Actual results: gdm not starting up Expected results: gdm properly starts up Additional info: This is a regression as no such issue exist in gdm-2.30.4-33.el6_2
Created attachment 629584 [details] slave.log
Created attachment 629585 [details] slave.log.1
Seems to be a problem with access control. If I use Xephyr -ac it works, if I remove the -ac it fails. I'll investigate.
So the issue seems to be a problem with XCB. It actually forces FamilyLocal when DISPLAY=localhost:something (see xcb_auth.c:get_authptr) this fails in XauGetBestAuthByAddr() on the client side since the cookie is FamilyInternet. (We used to use FamilyWild which would match fine since it's wild) We'll probably either have to special case localhost here until xcb can be fixed or maybe add two entries.
Let's get the acks rolling for this, it's pretty bad. I'm sure we have customers who use XDMCP to the local machine.
This request was evaluated by Red Hat Product Management for inclusion in a Red Hat Enterprise Linux release. Product Management has requested further review of this request by Red Hat Engineering, for potential inclusion in a Red Hat Enterprise Linux release for currently deployed products. This request is not yet committed for inclusion in a release.
Created attachment 632272 [details] Work around xcb bug This updated xauth patch fixes the original problem from bug 860645 but also 1) adds two additional auth entries for local connections to work around the xcb bug 2) fixes an uninitialized variable access that causes XDMCP to occasionally fail during testing.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. http://rhn.redhat.com/errata/RHBA-2013-0381.html