XDMCP is protocol that allows remote connections and provides you with a working Display Manager like KDE, GNome, et al. Its a remote connection protocol that is mostly enabled on Solaris and disabled on most Linux boxes. If you need to have a remote session where you need the X Display to open some GUI applications then you can have a connection with the remote server with XDMCP.
If you need to have a XDMCP session then you can use the following command to start a session:
Xephyr -name amit -title aka -query -wr :1 -screen 1024x768 -host-cursor &
For this session we are using Xephyr. To install the same you can do the following:
sudo yum install xorg-x11-server-Xephyr
You can use the above command with Xnest also :). Just replace Xephyr in both the commands with Xnest.
Also, this can be very useful to get a session on the current system with a different username also. For example, I normally have to work with 2 users on the same system, and hence open nested X-window for the second user like so:
Xephyr -name amit -title aka -wr :1 -screen 1024x768 -host-cursor & sudo su - username -c "export DISPLAY=:1 && selectwm" &
In the command above we are using selectwm to be started and you can install that with
sudo yum install selectwm
Description for selectwm :
selectwm is a simple but robust program that will let you pick a window manager (or other executable) to run at X startup, and optionally after a window manager exits. It uses the GTK+ toolkit, and includes options like a timer to start the default window manager, and modification of the window manager list from within selectwm.
To understand the options used in the commands above look at the respective man pages.