XDM is a very configurable utility and this document will only just 'scratch the surface' of what may be achieved. This document aims to provide enough information to configure your X terminals and application servers to connect to each other. The reader is referred to Section 7 for further information on the topics discussed here.
A note on security: X (in its default configuration) and XDMCP are not particularly secure. I am assuming that you are running X on a 'trusted' network and that security is not an issue. For details of how to tighten up your X connections (and more details about using the networking capabilities of X) please refer to the 'Running Remote X Applications' Howto document, which is also part of the LDP (see Section 7).
This would be a machine with no local disks, that would perform its boot up from an EPROM (or similar) and utilises a network connection to a server. It would obtain its network configuration, operating system, system configuration and all applications from the server. Once booted however, this would be the same as a 'dumb X terminal' (see below). Typically this configuration would use a combination of the following network protocols in order to boot: BOOTP, DHCP, TFTP, etc. Refer to Section 7 for some references that detail how to build diskless X terminals.
This would be a machine that boots from its local disk into an operating system, and starts the 'X server' program and nothing more. Somehow, a login prompt would be provided on the machine, to enable a user to login to an 'application server' somewhere on the network.
This would be similar to a dumb X terminal, but would provide the option of logging on to the local machine itself, hence would be quite capable of becoming a standalone workstation (i.e. no network connectivity) if required. Most distributions can be configured 'out of the box' as a stand-alone X Workstation, with a graphical login prompt.
In the context of this document, I use the term 'application server' to describe a machine that will provide the applications (X clients) that our X terminal will want to run. This can include everything from editors and browsers, through to the actual 'Window Manager' itself.
This is the program that manages the display of a machine with a physical console (display, keyboard, mouse, etc). It can be thought of as a combined graphics card, keyboard and mouse 'driver'. This will provide these facilities as a service to X clients (hence the term 'server'). Please refer to the X User Howto in Section 7 for more details.
This is an application that requires the use of an X server to access input (keyboard and mouse) and output (display). An X client cannot produce output without the services of the X server. The X server could be running locally (on the same machine, as is the case with an X workstation) or elsewhere on the network (as is the case with an X terminal connecting to an Application Server).
From the above descriptions, an X Workstation could be thought of as consisting of a dumb X terminal and application server running on the same machine.
This document will be looking at the architecture of the various options listed above and will describe the role that XDM can play in configuring them.
There are 2 main ways that XDM can interact with an X Server:
The query from the X server can take one of 3 forms:
There are several other options, but these will not be described here - refer to the XDM and XDMCP documentation in Section 7 for more details.