Using X Window
Last revision August 6, 2004
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The XDMCP protocol for remote X servers
There is a special protocol called XDMCP (an acronym for X Display Manager Control Protocol) that allows X servers, such as X terminals or PCs or Macintosh computers running X server software, to emulate a directly attached Unix workstation console. The Unix computer must be running the xdm process (or equivalent), and the X terminal must be configured to use XDMCP to make connections. In this case, you will get a standard graphical login screen for the remote Unix computer, just as you would see if seated directly in front of the Unix console. This connection method creates an X session, and the remote Unix computer becomes the controlling computer for your display.
Your X terminal or PC/Mac X server software will usually provide two ways to make an XDMCP connection. First, it may allow you to simply type in the name of the remote computer on which you want to run your X session. Second, it usually provides an XDMCP Chooser window. For this Chooser, your X terminal or PC sends out a broadcast message on the local network asking all computers running the xdm server to respond. It then constructs a list of the responding computers and lets you select a computer from the list. Some X server software programs will sort the list alphabetically; other programs just list systems in the order in which they responded. This Chooser function only shows remote computers on your own local network (for example, the Earth Sciences network consisting of Geology Corner, Mitchell Earth Sciences, and Green Earth Sciences buildings).
After selecting the remote computer on which you want to run your X session, your X terminal or PC will contact that system and let it put up its normal graphical login display on your local display. You login with your normal account name and local password. NOTE: this login method passes your local password across the network in plain text, which makes it potentially vulnerable to capture by a hacker who has broken into your network.
After starting your X session via XDMCP, your normal X initialization files stored in your home directory on the remote computer will be read and executed (such as .xsession and .Xdefaults files). Thus, the standard windowing clients, including the window manager, that you would see on the workstation will also start up and display on your remote X terminal or PC.
XDMCP is the best way to configure a remote X terminal or PC when you want to run X client programs on the Unix system that expect to have the full environment of the workstation console. XDMCP also provides access controls to prevent random X clients from displaying on your screen. Of course, your X terminal or Mac/PC X Server must support the access control protocols to make this work -- most modern ones support the access control protocols listed on this page.