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Thin Client Computing, Part Two

Now that our servers are in place, we walk through configuring an x86 PC as a thin client workstation.

Welcome to the second in a two-part series on thin client computing. Last time I described setting up the necessary server protocols. This time I’ll cover configuring an x86 PC as a thin client computer. The minimal hardware requirements for thin clients can be an effective cost-saving measure, even though the servers used by the thin clients must be more powerful than an average desktop system would be.

To implement a thin client network, you must set up the servers (as described last time), decide what software you want to use on the clients, prepare a client configuration, and distribute the software to the clients. The software distribution task can be handled by a TFTP server, as described last month; or you can put the software on a CD-R or other local storage medium. Once you’ve done all of this, you can boot your thin client and, if all goes well, use it.

Thin Client Options

Broadly speaking, thin clients come in two forms: dedicated thin client hardware and general-purpose PCs that run thin client software. Dedicated thin client hardware can be handy if you’re building a network from scratch, but using PCs as thin clients has the advantage of enabling you to re-use outdated but still functional computers.

If you’re using a dedicated thin client, you should consult its documentation to learn how to configure and use it. You should be able to drop some files, provided by the hardware’s manufacturer, into the TFTP server’s files area and configure your DHCP server to point specific machines to particular files. You should then be able to boot your thin clients and get on with using them.

To use an ordinary PC as a thin client, you must have software for it. Linux can serve this role quite well, but the question is, which Linux? I recommend you turn to a dedicated thin client distribution, such as ThinStation or 2x PXES. These distributions include the Linux kernel, X, additional remote-access protocols, and a set of additional tools required to run a system as a thin client. Note that many Linux thin client distributions can access Windows servers as well as Linux (or other Unix) servers.

Requirements for a computer to operate as a thin client are minimal. A Pentium-class CPU will do the job well, but a 486 or even a 386 can serve in a pinch. The ThinStation developers recommend a minimum of 64MB of RAM. You do not need a hard disk.

You do need a keyboard, mouse, and monitor, of course. To function well with modern Linux software, the video card and monitor should be capable of 1024×768 resolution or better, although lower resolutions will work. A network card is an absolute necessity. If the network card supports network booting via the Preboot Execution Environment (PXE) protocol, the thin client need not have a floppy disk or CD-ROM drive, but one or both of these will be required if the network card doesn’t support PXE.

Configuring ThinStation

I’ll describe ThinStation in more detail, since it’s a ready-made package that’s relatively easy to configure. Several options for obtaining and configuring ThinStation exist in its Web site’s downloads section, including a couple of source packages, a pair of prebuilt images (for CD-R and network boots), and TS-O-Matic (for highly customized configurations).

Because they’re relatively easy to set up, I describe the two prebuilt images in more detail. You should download whichever one seems more suitable for your network. The NetBoot version requires a network card that’s supported by the network booting tools, as well as working DHCP and TFTP servers on your network. The NetBoot system might or might not require a working floppy or CD-ROM drive, depending on the client’s motherboard and network card BIOS support. I describe version 2.2 of ThinStation.

You should begin by downloading the relevant files (Thinstation-2.2-prebuilt-NetBoot.zip and Thinstation-2.2-prebuilt-LiveCD.zip), which you can uncompress with the Linux unzip utility. The result will be one directory tree for each zip file. How you proceed from here depends on which version you’re using.

Distributing ThinStation via CD-R

Once you’ve uncompressed the LiveCD image of ThinStation, you’ll need to prepare two disks: a CD-R and a floppy. The CD-R image is stored in the Cd subdirectory as thinstation.iso. You can use cdrecord under Linux to burn this file to a blank CD-R:

cdrecord dev=/dev/cdrom thinstation.iso

You may need to modify this command to suit your hardware; consult the cdrecord man pages for details.

The CD-R contains the ThinStation OS, including a Linux kernel, X server, and so on. This software does a remarkably good job of detecting and adapting to a wide variety of hardware; however, some configuration settings need to be set manually. For this task, you must prepare a floppy disk. Start with a FAT-formatted floppy (prepared on a Windows box or by using mkdosfs in Linux).

Mount it in Linux, copy the thinstation.conf.user file from the Floppy/thinstation.profile subdirectory of the LiveCD directory tree, and begin modifying the on-floppy version of this file. The file format is fairly obvious and the sample file does a good job of explaining the various options.

Use the NET_USE_DHCP option to enable or disable the use of DHCP. If you don’t use DHCP, you can set the IP address, and so on. Using DHCP is almost certainly the best approach unless your network lacks a DHCP server.

Much of the remaining configuration involves setting up various session types. ThinStation supports Windows Terminal Server, Citrix Server, X, VNC, Telnet, SSH, and AS400. Each has a sample configuration section with options that begin SESSION_#, where # is a session number.

You must number your sessions starting with 0. The original file supports just one session, a Windows Terminal Server session. To use a thin client with a Linux host, you’d comment out this configuration block, uncomment the X and/or VNC blocks, and make appropriate changes. For instance, to support one X and one VNC session, you might make changes so that your file includes the following lines:

SESSION_0_TITLE="Linux X sessions"
SESSION_0_TYPE=x
SESSION_0_X_SERVER=192.168.1.2
# Default is '-query'
SESSION_0_X_OPTIONS="-indirect"
# You should set also "SCREEN_X_FONT_SERVER", below

SESSION_1_TITLE="Linux via VNC"
SESSION_1_TYPE=vncviewer
SESSION_1_VNCVIEWER_SERVER=192.168.1.2:5901

This configuration tells ThinStation to connect to the server at 192.168.1.2 for both X and VNC. The use of the SESSION_0_X_OPTIONS="-indirect" line tells ThinStation to use an indirect XDMCP request. This results in a list of available XDMCP servers when you select this session type, so you can use any remote system that accepts XDMCP logins.

The default option of "-query" results in a direct login to just one X server, which may be preferable if you’ve only got one or two systems. The VNC IP address appends a port number (5901) to the end. This is handy if you define multiple VNC configurations on the server to support different screen resolutions or other features. Note that you can include more than one session of a single type. For instance, you might have three X sessions and two VNC sessions defined.

Further down in the file you’ll find some miscellaneous additional options. I recommend you pay particular attention to the following:

  • SCREEN_RESOLUTION Set the screen resolution in the obvious way using this option.
  • SCREEN_COLOR_DEPTH Set the screen’s color depth with this option.
  • SCREEN_HORIZSYNC and SCREEN_VERTREFRESH Set the monitor’s horizontal and vertical sync ranges with these options. You may need to consult the monitor’s manual to learn what appropriate values are. Note that very old monitors can be damaged if you set these values inappropriately.
  • MOUSE_ options Several options beginning with MOUSE_ default to values that are suitable for PS/2 mice. If the thin client has an RS-232 serial, USB, or other mouse, you may need to adjust these items.

Once you’ve made your changes, unmount the floppy disk. You can then walk the floppy and CD-R over to the computer you intend to use as a thin client and test them, as described shortly, in “Using ThinStation.”

Distributing ThinStation via TFTP

If you want to boot and distribute ThinStation via your network, you can do so with the help of the NetBoot image. Just as with the LiveCD version, you must attend to two key details: the ThinStation OS itself and a configuration file. Within the ThinStation-NetBoot directory tree, the TFtpdRoot subdirectory holds a file called thinstation.nbi (autoextract).exe. This is a Windows self-extracting archive that holds the ThinStation OS. You can extract this file on a Windows machine or use WINE under Linux to extract it.

The extracted file is called thinstation.nbi, and you should place it in your TFTP shared directory and ensure that your DHCP server is configured to point to this file, as described last month. If any of your computers have network cards that support direct network boots via PXE, you should also copy the thinstation.nbi.zpxe file to the same location, and point to this file using the filename line in dhcpd.conf.

If your network includes both PXE-enabled computers and those that don’t support PXE, configure DHCP to point to the thinstation.nbi.zpxe file; the disk-based boot tools described shortly are smart enough to drop the .zpxe filename extension.

In addition to the thinstation.nbi file, you’ll need to edit a ThinStation configuration file and place it in the TFTP files directory. The sample file is located in the TFtpdRoot subdirectory as thinstation.conf.network. Copy this file, without renaming it, to your TFTP server’s files directory. You can then edit the file to make the changes described for the thinstation.conf.user file for the LiveCD version of ThinStation; however, you don’t need to copy the file to a floppy disk, since clients will retrieve the file via TFTP.

If your thin clients have differing hardware capabilities, you can create custom configuration files for each client. The simplest way to do this for a small number of clients is to create a new configuration file for each computer. This file contains only the unique items for this computer — say, SCREEN_RESOLUTION if the computer has a higher- or lower-resolution screen than others.

Name the new file thinstation.conf-ID, where ID is the computer’s IP address or MAC address. For instance, thinstation.conf-192.168.1.27 holds the customizations for the thin client with an IP address of 192.168.1.27, or thinstation.conf-000C7696A373 holds customizations for the thin client with a network adapter MAC address of 00:0C:76:96:A3:73. A more sophisticated method of creating customizations, which may be preferable for networks with lots of systems, is detailed in the _HowTo-NetBoot.txt file that comes with the NetBoot distribution.

Once these files are all copied to your TFTP files directory, you may need to prepare a boot floppy or CD-R. These images are stored in the BootDisk subdirectory. You can prepare a boot floppy by placing a formatted floppy disk in the drive, entering the BootDisk subdirectory, and typing:

dd if=eb-net.dsk of=/dev/fd0

If your target thin client lacks a floppy disk but has a CD-ROM drive, you can use cdrecord to burn the eb-net.iso file to CD-R:

cdrecord dev=/dev/cdrom eb-net.iso

You may need to tweak one or both of these commands for your system.

If your thin client supports PXE network booting, you shouldn’t need either the boot floppy or boot CD-ROM, but you may need to enter your BIOS setup utility to configure the boot devices. Be sure that booting from the network is an option, and that it’s higher in priority than other valid boot devices (such as a hard disk, if that disk is bootable).

Some plug-in network cards have boot ROMs that support PXE booting. When using such cards, you may need to disable other valid boot devices in your main BIOS. Consult the network card’s documentation for details.

Using ThinStation

To begin using ThinStation, you should first ensure that your servers are all configured and running, as described last month. Boot your thin client using whatever boot media you’ve prepared. If all goes well, you’ll see a series of boot messages appear on the screen, followed by a selection screen that lists your session options. Select one to test it.

Check to be sure you can log in fully and run programs. When you log out, you may be returned to the session selection screen or to an XDMCP login screen. If you see the XDMCP login screen but want to try another session, press Ctrl+Alt+Backspace to kill X; this will return you to the session selection screen.

Unfortunately, things don’t always go according to plan, and thin client troubles can be tricky to debug. Some common symptoms and likely solutions include:

  • A failure to fully boot, with a repeated listing of network modules — ThinStation locks itself into an endless loop testing its network drivers when it can’t detect a network card. If this happens, you may need to swap in a new network card or use the TS-O-Matic or Main Distribution option to create a custom ThinStation distribution that includes support for your card. A similar symptom can occur if your boot file (as specified by the filename option in your DHCP configuration) doesn’t exist. Be sure the filename matches.
  • A blank screen when you select an X session — ThinStation sometimes fails if you select an X session too quickly after the session list appears. Reboot and pause for a few seconds before selecting the X session.
  • A corrupt or blank display when you select a session — This can happen when you’ve specified incorrect video timing (horizontal and vertical sync values) for your monitor. Review your documentation and try again.
  • An “X”-shaped cursor on a gray background after you select an X session — This problem can occur when your XDMCP server isn’t properly configured. Review that part of the configuration. You might find it necessary to change XDMCP servers; I find that GDM and XDM are easier to configure than KDM.

In addition to these issues, firewall configuration can cause problems. Your host computer must not block incoming access to any of the vital ports. Such blocks can manifest themselves as problems at various points along the way.

With any luck, you won’t run into serious problems. Once it’s running, a thin client can be used much like a conventional desktop system. The fact that it’s running remotely will cause a speed hit, particularly on display-intensive applications. For typical office productivity tools, though, thin clients can be very usable tools.

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