Multiseat Linux on Kubuntu Maverick Meerkat 10.10
Using KDM

For years I've had two machines in my Tech room: my main desktop and an auxilliary machine for the kids to use. Keeping two machines up to date with both software and hardware was a pain. So the last time the aux machine went down, I decided that I would abandon that path and seek to use my primary machine in a multiseat configuration. I had been interested in multiseat configs ever since reading this article on the subject several years ago. So I set off on my journey to get such a system working at home. This document is designed to collect the wisdom gained along the way so that others that want similar setups can accompish their objectives. Note that critical information I encountered will be marked in green font.

Hardware Setup

Nothing too particularly special. My hardware is a Fry's white box special:

The final software stack

An outline of the final software stack is as follows:

The setup of each item is described below.

Xorg X server

The key piece of wisdom learned: It is not possible to drive a single video card with more than one Xorg server! So forget any tutorial that discusses configuring more than one card if you only have one card. The basic idea is to set up multiple monitors and screens then using a layout to stack the screens together into one large virtual display. My xorg.conf setup is shown below:

The second critical item: configure the base X server not to use any input devices. By default the hardware X server will attach any keyboards and mice that it finds. To prevent it from doing that add the following line to the Server Layout:

Option "AutoAddDevices" "off"

Also for the default core input devices, both mouse and keyboard, set the device line to "/dev/null". Note this makes the X server somewhat inconvenient because it cannot be killed from a keyboard. I found that working on this setup remotely using SSH quite helpful.

Xephyr configuration

Virtually every search for multiseat linux leads you to the C3SL multiseat site. While this site is a great wiki resource, absolutely none of the software downloads work anymore. Note that the mdm manager they discuss is in fact in the Ubuntu repositories, but I didn't get any joy in working with them.

Since in this setup I am limited to a single real X server, my solution required the use of nested X servers. These servers are a client application to the real X server, and a server to clients that run on top of them. Two popular solutions are Xnest and Xephyr. Xephyr has a couple of advantages:

The second item is important because to have a multiseat setup, not only are multiple keyboards and mice required, but a way to direct specific keyboards and mice to specific servers are required too. evdev facilitates this activity.

I spent a significant amount of time working with the Xephyr that is currently in the Meerkat repositories. It comes up. Applications can be run in them. But frustration set in because: The Xephyr in the Meerkat respositories DOES NOT HAVE EVDEV SUPPORT! It doesn't work. Period. After quite a bit of digging I found this bug report on the issue. The takeaway is The Xephyr version 2:1.7.6-2ubuntu7 in Lucid Lynx does have evdev support!. A simple repository add, update, and download later, and a working keyboard and mouse in Xephyr is going.

Xephyr has a second issue. The author and maintainer has refused to add a geometry specifier for windows placement for Xephyr. So it is not possible to position each Xephyr server in its proper place on the virtual screen. Fortunately a simple workaround exists. Xephyr does has a -parent option to overlay the Xephyr screen on top of an existing window. The Two Desktops Section of this Multiseat using One Card Tutorial describes the process. xmessage is used as the carrier window. Multiple links are used so that each xmessage can be identified and its window id extracted for the parent option of Xephyr.

Configuring KDM

Since I run Kubuntu, KDM is my logon system. KDM will happily start and support multiple X servers with some changes in the configuration file: (/etc/kde4/kdm/kdmrc). My first efforts were focused on having KDM start the Xephyr servers directly. If this route is taken the crucial item is that: KDM will not run a shell script for the ServerCmd. This led to lots of frustration. A tool that helped quite a bit was this shell script compiler that converts a script into a C program which can be exec'ed.

In my last go around, KDM refused to connect to my locally started X servers. I could not gather enough information from the log files to figure out why it was timing out. Finally I tried configuring KDM to drive the Xephyr servers as remote displays and Voila! they came up instanteanously. This requires that the base hardware X server and Xephyr nested servers be up and running before KDM is started. A small price to pay for working service.

I've done a lot of fiddling with the kdmrc. However, I believe that KDM service can be started on multiple Xephyr displays using a single change in the file. Change the StaticServers line so that is reads:

StaticServers=localhost:[display_for_seat1],localhost:[display_for_seat2],...

where display_for_seatX is the display number for the seat in question. In my setup I arbitrarily used display numbers 5 and 6. Any values greater than :0 should be fine. Because of the localhost specification, KDM will not try to fire up a server for the screens.

Startup Scripts to get everything running

I started out with custom scripts for each seat. Changes in keyboards and mice drove the realization that a generic config file based script would be more effective. The result is the following script called doseat:

Some quick notes:

Challenges and leftovers

Here is the short list of what's left to be done:

Coda

I hope this document helps someone in their setup. There's a lot of info floating around, but much of it is out of date, or incorrect. Feel free to send any questions or suggestions for making this document better to byronjeff@mail.clayton.edu.