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Configuring an XDM server to provide a "chooser":

We've already set up a basic X Terminal. However, it can only talk to a single machine. If we would like it to be able to connect to a number of other machines, we'll have to have at least one machine in our network configured to provide a host "chooser" to our X terminals. In this discussion, the machine providing "chooser" xdm services is called "weber" (note that in this example "weber" is a Linux box, but it could be any xdm-enabled workstation).

The first step is to configure weber to provide the chooser to hosts that connect through an "indirect" XDM connection. This is controlled by the Xaccess file (located in /etc/X11/xdm on Debian machines, it may be located under /usr/lib/X11 or another location on other machines). Typically, the default Xaccess file on most systems is fairly well commented and includes a number of simple examples, so it's pretty easy to figure out.

Basically, you have to add a line to the file of the form

hostname CHOOSER host-a host-b
where hostname is the name of the host to provide the chooser to (it can be a wildcard such as "*" or "*.domain.name", the CHOOSER tells xdm to provide a chooser to these hosts, and the remainder of the line is a list of machine names to list in the chooser. If you use the special hostname BROADCAST, it will list all xdm-enabled machines on the local network.

So, if we want all machines to be given a chooser that allows them to select any machine on the local network, we'd make sure Xaccess has the line

* CHOOSER BROADCAST

However, in our system we have a number of machines in different subnets, so we can't rely on a broadcast to find them all. So we use

* CHOOSER machine list ...
instead.

Additionally, we can specify different lists for different machines. As mentioned previously we wanted to use one of the PCs as a graphical terminal for our headless SGI workstation (which runs xdm). So we have this machine, "console", be given a list of only the server machines:

console.me.umn.edu CHOOSER server1 server2 ...

The next step is to modify the X terminal to connect to the XDM server using an 'indirect' query. We first test it by logging into the X terminal PC, and starting X with

X -indirect weber
and we should then see the chooser come up:

standard
chooser
[click for a larger image]

Functional, but a little ugly, wouldn't you say?

So now that we know it works, we change our /etc/init.d/xterm script, replacing the "-query rayleigh" with "-indirect weber".

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