Ubuntu Diskless Workstation and X-Terminal Howto – Part 3 X-Terminal Setup

At this point in the series we should have a working diskless workstation. Basically we have a full desktop running from an NFS share. This article will explore how to use the server to deliver a desktop, instead of our old laptop. In my case my server is much more powerful in terms of CPU and RAM than an old laptop, so the best linux experience will come from running applications on the server with the output showing on the laptop, which is the definition of an X-Terminal.

There are two steps to doing this – preparing the server and preparing the client.

There is also some tuning options that we can do with the terminal to make sure we get the best performance from the diskless that we can.

So, lets dive into the server preparation. Ubuntu 9.10 and up have a newer GDM that doesn’t have the gdmsetup configuration program available any more. So, we need to edit a file to turn on something called XDMCP. Our server is running Ubuntu 10.10, and officially XDMCP is not supported on this version, however it can be made to work. XDMCP is a method to allow an X server (in our case the diskless) to pull up a logon window from the server.

Basically a single file needs to be edited to allow xdmcp support. That file is /etc/gdm/custom.conf. A section called xdmcp is added to this file to enable the feature:


Once that section is added you can restart gdm with the following command:

sudo /etc/init.d/gdm restart

You can verify that the server is listening on UDP 177 with the following command:

netstat -uan

and look for the following entry in the command output:

udp 0 0*

One thing to note, this may not work properly on Ubuntu 10.10 if you have IPv6 enabled. You can disable IPv6 by adding ipv6.disable=1 to the kernel command line. This can be done by editing /etc/default/grub and modifying the line beginning with GRUB_CMDLINE_LINUX_DEFAULT=", adding the parameter to look something like this:

GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet splash"

Then run the following command on the server:

sudo update-grub2

Then reboot the server to disable IPv6. Unfortunately this will remove IPv6 support from the system, so if your relying on IPv6 – beware.

Ok – now lets move on the to the diskless. Lets make sure its booted up and showing the logon screen for Ubuntu. When the screen is up, drop to a command line by hitting Ctrl-Alt-F1. Logon to the system from the command line (hopefully you created a userid on your diskless when you built it).

Now lets bring down the gdm service so we can start X manually:

sudo /etc/init.d/gdm stop

Once gdm is stopped, we can try to use XDMCP to pull up the logon window from the server:

X -query "server IP address or hostname"

This should bring up a logon window from the server. If it does, then we can move on to the next step – if it doesn’t verify the server is configured properly as mentioned above. You can check the kernel command line on the server by running:

cat /proc/cmdline

You can try to login to the server if the window comes up. To continue, you will probably want to drop to the console again by hitting ctrl-alt-f2 and killing X with killall X. This should kill the X server.

In my normal setup I set Ubuntu to boot into console mode without GDM. This allows me to log on, and run a script that starts the X server and pulls the logon window from the server.

To set the diskless to boot to a text console, we need to modify the boot file used to PXE boot the diskless. In a previous article we showed you how to set up a PXE boot file for the diskless. Let’s pull that file up again and make the change we need. Basically the simple addition of a the kernel parameter “text” is needed. For example:

APPEND root=/dev/nfs rw initrd=initrd.img-2.6.35-24-generic.nfs nfsroot= ip=x.x.x.x:y.y.y.y:g.g.g.g: text acpi=force

The parameter is the second to the last one (before acpi=force). Adding this to your PXE boot file (or to a grub command line) will make ubuntu default to non-graphical boot. The next thing we need is a script to start X and use xdmcp to pull up the logon window from the server. Here is an example:

X -dpi 100 -ac -query

There are a couple of extra parameters in this script than what we used before. The first one forces the DPI to be 100, which will keep the desktop font size looking normal, and the -ac, which disables access control. Disabling access control still allows local X client programs such as a video player to run on the same desktop that is being pulled from the server. Running multimedia or OpenGL based software over the X protocol across the network is a less than optimal experience, so those should be run locally. More on that later. Lets save the command above in a script on the diskless called startterm. Make sure you make it executable and save it somewhere in your path.

The process will be to boot the diskless, logon as your user id, and run the startterm script, and you will be able to logon to your server.

Improving the Experience

We should have a diskless workstation set up as an X-Terminal. You should be able to use anything on the server you want – minus multimedia/sound apps. They will work but the video in mplayer, for example, will be less than adequate and there won’t be any sound unless you are near the server – because sound will still come from the server’s sound system!

Network sound is actually pretty easy to setup, but it requires some more tweaking on the diskless workstation – Enter Pulse audio system mode. Pulseaudio is the default sound server used on Ubuntu. Its actually pretty powerful, once you understand how to use it. Normally Pulseaudio is configured in per user mode, and is started when a user logs into gnome. Since we aren’t starting gnome on the diskless workstation (its running on the server) we need to reconfigure Pulseaudio to work in system mode, so its always running. Pulseaudio system mode is actually frowned upon in most scenarios, but this is one use case where it makes sense.

First we need to edit some configuration files to enable Pulseaudio system mode. They are located in the /etc/pulse directory. The first one is daemon.conf – edit it and look for the section that looks like this (its near the beginning):

; daemonize = no
; fail = yes
; allow-module-loading = yes
; allow-exit = yes
; use-pid-file = yes
; system-instance = no
; enable-shm = yes
; shm-size-bytes = 0 # setting this 0 will use the system-default, usually 64 MiB

And modify it to look like this:

daemonize = yes
; fail = yes
allow-module-loading = no
allow-exit = no
; use-pid-file = yes
system-instance = yes
enable-shm = no
; shm-size-bytes = 0 # setting this 0 will use the system-default, usually 64 MiB

The last file is /etc/pulse/system.pa – edit the file and add this to the bottom of the file:

### Enable TCP access to local sound devices
load-module module-native-protocol-tcp auth-anonymous=1
load-module module-zeroconf-publish

After these changes, you should reboot the diskless workstation to enable Pulseaudio in system mode, or you can run the following command to restart it:

sudo /etc/init.d/pulseaudio restart

If you restart the diskless, log on, and use the startterm script to log back into the server, you should here the ubuntu logon sound coming from your diskless speakers. If you don’t here are some things to check:

1. Log on locally with sudo /etc/init.d/gdm start instead of running startterm and make sure your audio system works.
2. Install padevchooser on the server, with sudo apt-get install padevchooser and run it from the diskless while logged into the server. It will place an icon in the menu bar. Right clicking on it should allow you select the sink that reflects your diskless – it will look like something pulse@

In our configuration we have enabled zeroconf so if the diskless is on the same subnet, it should just work out of the box, so to speak.

One more thing – you may need to add your diskless user that you logon to the system with to the pulse and pulse-access groups. This is important when running local applications that use Pulseaudio (see next section for details). Our user is term1 and this is how to add term1 to the groups needed:

usermod -a -G pulse,pulse-access term1

Local Applications

As I mentioned above, local applications can be handy for things that don’t run well over the X protocol across the network. Video is one of those applications. Luckily, its possible to create icons on the servers gui interface (maybe as part of the gnome ui) that actually start and run applications on the diskless workstation itself. The process is pretty simple. Essentially its an ssh session from the server to the diskless that starts an application with the display redirected to the diskless. Remember the -ac parameter in the startterm script? This is why its needed. -ac disables access control with allows this type of functionality to work properly.

Here is an example of a script that starts mythtv on the diskless:

/usr/bin/ssh term1@diskless1 /home/term1/bin/mythlocal

This script resides on the server – it starts and ssh to the diskless and then calls another script on the diskless call mythlocal. Here is what mythlocal looks like:

DISPLAY=localhost:0.0 mythfrontend -l /home/term1/mythfrontend.log -v important,general

Basically all this does is set the DISPLAY environment variable to the local X console on the diskless, and then start mythfrontend with options to log myth events to a log file. After a few seconds myth will start on the diskless and use the diskless video hardware to display Mythtv.

Since we are using Pulseaudio on the diskless it’s important to enable Pulseaudio output on Mythtv as well.

There is a caveat to making this work – the diskless must be configured to use key based ssh authentication, instead of password based. If you don’t do this, you won’t see the password prompt generated by the ssh session, and therefore Myth will never get started on the diskless.

Setting up ssh key based authenication is beyond the scope of this article – click here for a good place to start with this. I will attempt to get a future article together on this at a later date.

Tuning Performance

NFS isn’t nearly as efficient as local disk, so the less we write to the nfs based root file system, the better. One thing we can do is sent system log traffic to another form of storage. In our case, we will use tmpfs, which is temporary storage in the diskless computer’s memory. This means that system log files will not survive a reboot (unless you customize the system to dump them to the nfs root fs during reboot).

Essentially we are just going to redirect the /var/log /tmp and /var/tmp directories to tmpfs. This is done by adding the following to the /etc/fstab on the diskless:

tmpfs /var/log tmpfs defaults 0 0
tmpfs /tmp tmpfs defaults 0 0
tmpfs /var/tmp tmpfs defaults 0 0

After rebooting the diskless, all the system logger traffic will go to tmpfs and it should allow the diskless to be a little more responsive.

Well, I think that should about cover it. Comments and other suggestions are welcome as always.

Next up – Ubuntu vs Gentoo vs Arch Linux – I started out with RedHat 4 back in 1996, and moved to Gentoo in 2004, then to Ubuntu in 2008 – I have recently been playing around with Gentoo (again) and Arch Linux, and I may have found my next “favorite” linux distro – Arch Linux.

I also have tested debootstrap, which is a way of building a diskless filesystem on the server without installing ubuntu on the diskless and copying it across. I will see if I can get a quick article out on that process as well. Stay tuned!

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