X Window System is the display and networking protocol developed by MIT. The X is built with network in mind with the capability to run a (graphical) session on a remote computer. In it, an X Display Manager is used to start a session from a local system or from another computer. The request and the start of the session is handled by the XDMCP, which stands for “X Display Manager Control Protocol” and is a network protocol.

Environment
•Red Hat Enterprise Linux 5
•Red Hat Enterprise Linux 6

Resolution

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 and 6, the location to enable xdmcp has changed to /etc/gdm/custom.conf.

There are two ways that xdmcp can be enabled:

Method 1:

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

Restart GDM
# init 3
# init 5

Method 2 (RHEL5 only) :

The tool gdmsetup can be used. This is a gui-based tool to modify gdm.

1.Click on “Remote” tab, then style -> plain

2. Click on “Accessibility” tab, then select “Enable accessibille login”

3. Click on “Security” tab, then select “Allow remote system administrator login”

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

Gnome-session use UDP:177 port to listen XDMCP request and then create a TCP connection.

Run the command:

#netstat -nl | grep 177

NOTE
•Both the Methods are applicable for RHEL5
•Only Method 1 can be used in RHEL6. gdmsetup and gdm-restart are not available in RHEL6