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XDM: The basic concept:
xdm is a "display manager", providing X login windows to users. The traditional use of xdm is to provide a graphical login on the local display on an X11 workstation, so that the user does not need to start up X "by hand".
However, xdm can also provide graphical X11 logins to remote machines, such as NCD Xterminals. The only requirement is that the remote machine speak X. To accomplish this, they use the X11 Display Manager Control Protocol, or XDMCP.
What we are describing here is configuring a cheap PC to act just like one of these Xterminals.
There are two distinct methods of using XDMCP, the "direct query", in which the X terminal directly requests a login window on a workstation, and the "indirect query", in which the X terminal contacts a "chooser server", which presents the X terminal with a menu of machines to log into. I'll discuss both of these in turn.
(If you don't care about the details, feel free to skip to the Setup section now.
Direct XDMCP Queries
The figure below shows a simple direct XDMCP setup, in which the machine xterminal is requesting a login window on xdmserver (for simplicity, we'll be using these same hostnames in all the direct XDMCP examples).
The process works as follows:
Indirect XDMCP Queries
In an indirect XDMCP query, the X terminal is configured to contact a specially configured machine, the "Chooser Server" (called chooserserver in this example), which is configured to present a list of machines (in this example, workstation1 and workstation2) to the X terminal from which the user can select which machine to log into.
The basic technique is shown in the figure below:
The process for an indirect XDMCP query is similar to a direct query:
This all becomes more clear when we do an example.