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Sunday, March 7, 2010

Setting Up a Thin Client Network with LTSP

I've become interested in thin client software as a potential way to use a cheap laptop as a mobile GUI for applications running on a more powerful desktop or server. Yesterday, I tried out one called the Linux Terminal Server Project (LTSP). I found out it's distributed with Edubuntu as a way for colleges and universities to set up thin client labs. I just wanted to see if I could get it working, and I was pleasantly surprised by how easy it was. My steps:

Install LTSP server

With VirtualBox, I created a VM with 512M RAM and a 5G dynamically sized hard drive. Following the Ubuntu Community Documentation, I downloaded the Ubuntu 9.10 alternate amd64 ISO to install the LTSP server on it. It was pretty simple: choose "Install an LTSP server" from the Modes menu, then "Install Ubuntu".

Realize I was supposed to have two network adapters

One thing I could've done differently to make things simpler later on: add a second network adapter to the VM. LTSP server wants two NICs: one for communicating with the outside world and one for communicating with the network where the clients are. There's supposed to be a way to configure it with just one NIC, but I belatedly added the second one and configured it myself rather than puzzling that out. The first adapter had the VirtualBox default setting of "NAT", which lets it communicate out through the host interface. When I created the second one, I set it to "Internal Network" so that it could only communicate with other VMs, where my client would be.

Make LTSP DHCP work with new adapter

LTSP server runs a DHCP server on the client-facing network interface. This is the first part of how LTSP clients start up: they get an IP address and some other instructions from the DHCP server on the LTSP server. Because I had only the one NIC at first, the DHCP server wouldn't start, either, and I had to figure out right network settings to make DHCP run on the second, "Internal" adapter.

Create LTSP client VM

After the DHCP server was working, I created a second VM with 512M RAM and no hard drive. I set its sole network adapter to "Internal Network" so that it would be able to see the second adapter of the LTSP server, where the DHCP server was running. Then I set it to boot only from the network and started it up. It almost immediately got a response from the DHCP server, but it failed with this error:

PXE-T01: File not found
PXE-E3B: TFTP Error - File not found.

Fix bad path in LTSP DHCP configuration

This one took me some research to figure out, but it turns out that it's just a simple path problem. In the DHCP configuration of the LTSP server, there's a line that gives a path to a file called "pxeconfig.0", which is the initial file that has to be retrieved for a network boot to work. The server tells the client via DHCP where that file lives, and the client uses TFTP, "trivial file transfer protocol", to retrieve it. For some reason, the path to the file was wrong when I installed it. I don't remember what it was set to at first, and I don't have the correct value in front of me. It was something like /var/lib/tftpboot/ltsp/amd64/pxelinux.0. Also, something does some "chroot"ing somewhere, making the correct path for requesting the file /ltsp/amd64/pxelinux.0 (I think). As soon as I got that right and restarted the DHCP server, I was good to go.

Create a user with a strong enough password to suit LTSP

One last thing: after fixing the path to pxelinux.0, the diskless VM was able to get all its required files and boot to a login screen, but I couldn't log in to the client with the same credentials I was using on the server. I had created a user with a trivial password because this was just a test VM, and I read something somewhere about LTSP having different password requirements than Ubuntu, so I made another user with a stronger password, and then I could log in. That's it. Now I can start the client VM, which boots from the network, and get to a Gnome desktop. Everything I run displays on the client but runs and stores files on the server. Pretty neat.