== On Fedora 8 and later or Centos 5 and later ==
In a X11 session, run gdmsetup.  Go to page 2 (remote) and tick "Same as Local".  Close gdmsetup.  Open it again to make sure it saved, Sometimes it needs to be done twice.

== To set up Clients ==
On Thin clients, you connect this using a X11 or xdmcp configuration.

On Backup computers that are rsync'd from the main computer, Go to gdmsetup, page 3 (Security).  Click on Configure X server.  Modify VT0.  Change type to "Terminal".  In the Options, add "-query <IP.Address.of.Server>".  Restart the whole computer for it to take effect.

To undo this, and make it a normal server:  edit /etc/gdm/custom.conf .  Delete the whole line with your -query option above.  This  has to be done when converting backup computer to main.