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Linux Tutorial - The X Windowing System - Remote Access - XDMCP
  Remote Access ---- The Computer Itself  


As its name implies, the X Display Manager Control Protocol (XDMCP) is used to manage your display. One thing this means is that XDMCP is capable of provide an X server with a graphic login from a remote machine, and behave as if you had actually logged in locally to that machine.

In practice, this has many applications. For one, several years ago it was common to find X-terminals, which were little more than a monitor, keyboard and mouse. Like the serial terminals you had a remote connection to a server. Instead of a simple character-based session, X-terminals provided a graphical sessions. Although the use of X-terminals has declined in recent years, the same basic principle can be used with older PCs. As long as the PC is capable of running an X-server, it can run XDMCP. This is is useful in many cases where you cannot afford new computers for each user, but still want to give the access to new applications

Although my laptop is capable of running a full version of Linux (and it does), I take advantage of this same capability at home. Sometimes when my family is watching TV, I would like to work on things (for example this site) in their company. I could copy the site on to the laptop or telnet to my server and change the display on my web development environment (Quanta) to the laptop. However, when I am working on my laptop like that, all of the work is being done on the server. That would mean that for every application I have to redirect the display to the laptop. Instead, I use XDMCP to provide me a graphical login on the server and then the entire behaves as if I were logged in locally to the server.

In this situation, all that is really necessary on the laptop (or PC) is the X server. All of the other applications, be it my web development environment, KMail or anything else is running on the remote machine. Since everything is running on the server, the local machine can run with far fewer resources than if all the applications were running locally. (Note in this discussion I have been referring to the server as the machine on which the applications run. However, when talking about the X-Windowing System, the server is the machine with the display.)

As far as X is concerned, the X terminal will be running nothing but an X server. This X server will be configured to talk to a remote host using XDMCP (the X Display Manager Control Protocol). It will ask the remote host for an X session. The remote host will put up a login window on the X terminal, and after login it will run an X session with all bells and whistles, including the window manager, all using remote X to display on the X terminal.

You will probably notice that the remote host is acting like a server, though not an X server. The remote host is providing X sessions to X servers that ask for one. So, with respect to XDMCP, the remote host is actually a server, providing X sessions, also known as an XDMCP server. The X server is playing the role of an XDMCP client! Are you still with me?

The program that provides the XDMCP service on the XDMCP server is xdm. So, in order to get an X terminal up and running, you must configure two programs: X (the XDMCP client) on the X terminal, and xdm (the XDMCP server) on the remote host.

You must always remember that the X protocol (and the XDMCP protocol) are not encrypted. If you use remote X, everything that goes over the network can be sniffed by other hosts on the network. This is especially bad with remote X sessions, since the first thing that happens is logging in by giving a username and password. So, you must run remote X over a trusted network only!

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Copyright 2002-2009 by James Mohr. Licensed under modified GNU Free Documentation License (Portions of this material originally published by Prentice Hall, Pearson Education, Inc). See here for details. All rights reserved.

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