Created attachment 145269 [details]
Wireshark network snoop of attempted connection

Attempting to connect from a recent cygwin-xfree X server to GDM 2.28 using:

XWin -query host

can fail because GDM attempts to add 'server interpreted' host entries for the
GDM user and root.  If these users do not exist on the remote system, the
connection fails.  The enties are the equivalent of executing these xhost
commands:

$ xhost +si:localuser:root
$ xhost +si:localuser:gdm

It does not make sense for GDM to add 'user' entries to connections from remote
systems.  Not all Unix systems will have the gdm user and cygwin systems have
neither user.

I've attached a Wireshark network snoop of an attempted connection.  It shows
the two X_ChangeHosts requests being rejected.

The GDM code is in gdm_slave_connect_to_x11_display() in gdm-slave.c:

                XHostAddress host_entries[2] = {
                        { FamilyServerInterpreted },
                        { FamilyServerInterpreted }
                };
                XServerInterpretedAddress si_entries[2];

                g_debug ("GdmSlave: Connected to display %s",
slave->priv->display_name);
                ret = TRUE;

                /* Give programs run by the slave and greeter access to the
display
                 * independent of current hostname
                 */
                si_entries[0].type = "localuser";
                si_entries[0].typelength = strlen ("localuser");
                si_entries[1].type = "localuser";
                si_entries[1].typelength = strlen ("localuser");

                si_entries[0].value = "root";
                si_entries[0].valuelength = strlen ("root");
                si_entries[1].value = GDM_USERNAME;
                si_entries[1].valuelength = strlen (GDM_USERNAME);

                host_entries[0].address = (char *) &si_entries[0];
                host_entries[0].length = sizeof (XServerInterpretedAddress);
                host_entries[1].address = (char *) &si_entries[1];
                host_entries[1].length = sizeof (XServerInterpretedAddress);

                XAddHosts (slave->priv->server_display, host_entries,
                           G_N_ELEMENTS (host_entries));