Red Hat Knowledgebase

Release Found: Red Hat Enterprise Linux 3

Symptom: X Window System clients are unable to get a graphical login from the server.

Solution: Enable the X Display Manager Control Protocol (XDMCP) in GDM (the default X display manager).

Note: XDMCP is by no means a secure protocol. It also tends to be quite bandwidth intensive. You can make it secure and use less bandwidth by tunneling the connection over SSH with compression enabled. This article does not address these issues. This article also assumes that there is nothing network-wise blocking XDMCP, such as a firewall.

Assuming that you are using the default X Display Manager, GDM, open the file /etc/X11/gdm/gdm.conf as root. Scroll down to around line number 133. You should see something like this:

# XDMCP is the protocol that allows remote login.  If you want to log into
# gdm remotely (I'd never turn this on on open network, use ssh for such
# remote usage that).  You can then run X with -query <thishost> to log in,
# or -indirect <thishost> to run a chooser.  Look for the 'Terminal' server
# type at the bottom of this config file.
[xdmcp]
# Distributions: Ship with this off.  It is never a safe thing to leave
# out on the net.  Alternatively you can set up /etc/hosts.allow and
# /etc/hosts.deny to only allow say local access.
Enable=false

Change the line that says Enable=false to Enable=true and save the file.

Lastly, you must restart GDM for the changes to take effect. Assuming that you are already in runlevel 5, make sure you are logged out of your X Window System session. Go to a virtual terminal by pressing CTRL+ALT+F1. Login there as root, and execute the command killall gdm-binary.

You should now be able to establish a remote session using XDMCP to the system you just configured. You can test your setup locally by running the following command from a virtual terminal: X -query localhost :1