In Red Hat® Enterprise Linux® 4 and below the file /etc/X11/gdm/gdm.conf could be modified to enable xdmcp. With Red Hat Enterprise Linux 5, the location to enable xdmcp has changed.

There are two ways that xdmcp can be enabled:

• Modify the file /etc/gdm/custom.conf and under the [xdmcp] section, add this line:

  Enable=true
  

• Alternately, the tool gdmsetup can be used. This is a gui-based tool to modify gdm.

Once one of these two steps are done - simply run gdm-restart and then
login as desired user.

Red Hat’s customer service and support teams receive technical support questions from users all over the world. Red Hat technicians add the questions and answers to Red Hat Knowledgebase on a daily basis. Access to Red Hat Knowledgebase is free. Every month, Red Hat Magazine offers a preview into the Red Hat Knowledgebase by highlighting some of the most recent entries. The information provided in this article is for your information only. The origin of this information may be internal or external to Red Hat. While Red Hat attempts to verify the validity of this information before it is posted, Red Hat makes no express or implied claims to its validity.

Original link: http://www.redhatmagazine.com/2008/02/01/tips-and-tricks-how-do-i-enable-xdmcp-in-red-hat-enterprise-linux-5/