Diskless Linux X Terminals
The XFree86 executable normally is found in /usr/X11R6/bin, a subdirectory of /usr. We don't need to provide the X server in the RAM disk then, but can take it from the NFS mount. Although the modular XFree86 server itself has not been hardware-specific since about version 4.0, its configuration file definitely is. If you are managing several X terminals with different video hardware, it is impossible to use the same XF86Config file for all of them. Therefore, we prefer not to keep it in the RAM disk root filesystem, where it usually would be found in /etc/X11/XF86Config. Instead, we can use a per-terminal configuration file stored in the NFS /usr directory. Ultimately, the BusyBox init process is configured to respawn a shell script continuously containing the single line:
/usr/X11R6/bin/XFree86 \ -xf86config /usr/X11R6/configs/iphex -query \ server
where iphex is the client's IP address in hexadecimal (a naming convention borrowed from PXELINUX) and server is the server's IP address in dotted-decimal. With a few clever awk-on-/proc/cmdline tricks, we can entirely avoid hard coding any hostnames or IP addresses into the RAM disk image.
A basic XFree86 configuration file can be created by running XFree86 -configure on the terminal. In general, this correctly identifies the video hardware, and the resulting configuration file loads the appropriate XFree86 modules. It is worth mentioning, however, that the default pointer device, /dev/mouse, generally doesn't exist on a system using the device filesystem. For example, the PS/2 mouse is found at /dev/misc/psaux instead.
The part that makes the X terminal an X terminal instead of a Linux box with a graphical display is the -query server part of the XFree86 command line above. This causes the XFree86 server on the terminal to run an XDMCP (X Display Manager Control Protocol) session to try to get the server to manage its display. However, not every server is going to agree to do so.
First, and most obviously, the server must be listening for incoming XDMCP connections. XDMCP is normally on UDP port 177, and most display managers (xdm, gdm, kdm) can be configured to listen for XDMCP requests. Although most distributions are configured to run a display manager on bootup, most do not listen for incoming XDMCP requests due to security considerations. For example, the classic X display manager, xdm, usually is distributed with the line:
in its configuration file (commonly /etc/X11/xdm/xdm-config). This would have to be commented out in order for xdm to accept XDMCP requests. In addition, xdm can be configured to restrict itself to connections on a per-host or per-subnet basis using the configuration file /etc/X11/xdm/Xaccess (don't be confused by /etc/X11/xdm/Xservers, which is largely a historical relic). For example, to restrict xdm to terminals in the 192.168.1.0/24 subnet, add a line containing only 192.168.1.0/24 to the end of /etc/X11/xdm/Xaccess.
In addition, it can be convenient if the server also provides fonts to the terminals, by way of the X font server process xfs. Once again, although most distributions run a font server process, it usually is configured not to listen for incoming requests. For example, the configuration file for xfs, /etc/X11/fs/config, generally contains the line no-listen = tcp. If this is commented out, the Files section of the terminal's XF86Config file (stored in /usr/X11R6/configs/iphex on the server) can contain only one FontPath instead of the usual half-dozen, as shown in Listing 3 (where a server IP of 192.168.1.1 is assumed).
Listing 3. Terminal XF86Config Fragment
Section "Files" RgbPath "/usr/X11R6/lib/X11/rgb" ModulePath "/usr/X11R6/lib/modules" FontPath "tcp/192.168.1.1:7100" EndSection
Finally, the server must be configured to NFS export its /usr filesystem read-only to the terminal, as this is where the terminal gets the XFree86 server.
A number of security considerations should be kept in mind when running X terminals. First, it should be fairly obvious that the changes made to the xdm and xfs configurations are undoing things that were done to increase the security of the server. Furthermore, the setup described in this article does not encrypt any traffic. Every keystroke on the terminal goes over the network unencrypted. The only reasonably safe way to run with X terminals is to put them all on a private LAN that is used only by X terminals and that does not route to the Internet. The terminals and one interface on the server should be the only ones on the terminal LAN.
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