The xdm-config file provides a rich set of options, when it comes to defined scripts and other configuration files. In many cases, the defaults provided with your distribution should be fine, but for those of you who want more ...
The names of the startup scripts and configuration files used by XDM are determined by a series of statements in the top-level xdm-config file. This permits you to configure a different set of files for different X servers and X terminals, with different abilities.
For example, say you are using XDM to manage your local display, but also want it to accept queries from other X terminals on the network. It is possible to specify a different Xresources file for each of these cases, by using the following 2 lines in xdm-config:
DisplayManager._0.resources /etc/X11/xdm/Xres_0 DisplayManager*resources /etc/X11/xdm/Xresources
Similarly, if you wanted a particular resource file for a specific host, you would use an entry like the following:
Note that XDM configuration files use the terminology host_0, where you would normally use host:0, to designate 'display 0 on host'.
If you look over your default xdm-config file, you will probably find that it has been setup so that your local X server has different files to the remote ones anyway, as different things must be performed on startup and reset of the server. My Debian file has the following for local servers:
DisplayManager._0.resources: /etc/X11/xdm/Xresources_0 DisplayManager._0.setup: /etc/X11/xdm/Xsetup_0 DisplayManager._0.startup: /etc/X11/xdm/Xstartup_0 DisplayManager._0.reset: /etc/X11/xdm/Xreset_0
DisplayManager*resources: /etc/X11/xdm/Xresources DisplayManager*setup: /etc/X11/xdm/Xsetup DisplayManager*startup: /etc/X11/xdm/Xstartup DisplayManager*reset: /etc/X11/xdm/Xreset
This document has only briefly touched on the available X resources, but I should mention that it is possible to fully configure XDM via the Xresources file.
The following may all be changed if required:
Fonts, login prompt sizes
Window Titles, etc
There is a more detailed discussion of XDM resources, on Richard Kaszeta's web site (see Section 7)