Okay, I will talk about XDMCP today. First we will see what is xdmcp protocol.
There is a nice article on what is XDM is <a href="http://en.wikipedia.org/wiki/X_display_manager" target="_blank">here
In very simple words, this a protocol to get the X Windows login to a remote system. The client sends the request to the server and server completes the requests. For this the server allows the clients to connect to XDMCP port. There are various ways the XDMCP works most common being indirect, direct and query.
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