The X Terminal HOWTO
How to connect an X terminal to a Linux PC
Version 1.0 BETA (July 1995)
Scot W. Stevenson <scot@catzen.gun.de>
This document gives a brief introduction to how to connect an X terminal
to a Linux PC. It assumes a basic knowledge of the X Window System,
TCP/IP addressing, and ethernet cards.
1.0 Introduction
This is the first version of the document and should be considered BETA.
It is more of a been-there, done-that text than a comprehensive treatment.
Discussions of access control mechanisms (e.g. xaccess, xhost,
MIT-COOKIEs), and the use of NFS, are not yet included.
Most X terminals now have a whole host of advanced features which allow
them to be more than mere X servers. For the most part, these features
will be ignored.
1.1 Changes from previous versions
(There are no previous versions, so everything was changed)
1.2 Disclaimers
Neither the author nor the distributors of this HOWTO are in any way
responsible for physical, financial, or moral damage incurred by
following the suggestions in this text. In short: "Yea though I talk...."
1.3 Copyright
The Linux Xterminal HOWTO is copyrighted (C) 1995 by Scot W. Stevenson.
Linux HOWTO documents may be reproduced and distributed in whole or in
part, in any medium physical or electronic, as long as this copyright
notice is retained on all copies. Commercial redistribution is allowed
and encouraged. The author, however, would like to be notified of any
such distributions.
All translations, derivative works, or aggregate works incorporating
any Linux HOWTO documents must be covered under this copyright notice.
In othe words, you may not produce a derivative work from a HOWTO and
impose additional restrictions on its distribution. Exceptions to these
rules may be granted under certain conditions.
In short, we wish to promote dissemination of this information through
as many channels as possible. However, we do wish to retain copyright
on the HOWTO documents, and would like to be notified of any plans to
redistribute the HOWTOs.
Should you have any questions, please contact Greg Hankins, the Linux
HOWTO coordinator, at gregh@sunsite.unc.edu. You may finger his address
for phone number and additional contact information.
1.3 New Versions and Feedback
New versions of this document can be found on sunsite.unc.edu in
the /pub/Linux/docs/HOWTO/ directory. If you do not have FTP
access, you can try to get Linux Help Files via Bill Riemers. Send
email to bcr@physics.purdue.edu with a subject of "help" for more
infomation and an index file.
Any additions to, corrections of, or comments about this document would
be most welcomed! Please send email to
scot@catzen.gun.de (Scot W. Stevenson)
I would especially like to hear from you if you already have experience
with linking an X terminal to a Linux machine, even if is only something
like "worked on this machine with this terminal."
On the board for the next versions are access control mechanisms and
the use of NFS filesystems for booting.
2.0 Background
This section provides some very basic information for those not familiar
with the X Window System and its terminal-ology. If you have at least
some experience with X and X terminals, you should be able to skip
this part with no ill effects.
2.1 What is X?
The X Window System, or just X (never X Windows), is "a portable,
network-transparent window system" as the man page has it. It provides
a graphic environment that cuts across operating systems, vendors, and
hardware types. When people talk about a window system in connection with
Unix, they almost always mean X.
The most important characteristic of X in our case is the strict
division between the programs that control the local hardware that the
user interfaces with (screen, keyboard, mouse, etc.), and those programs
the user actually wants to run (editor, spreadsheet, DOOM). This means
that the interface software, which is called the X server, can be
on a one machine, while the actual programs, or X clients, can be on
one or even more than one machine at a totally different location.
Note that the terms "server" and "client" are used in the reverse sense
of what they usually are.
Linux comes with a collection of X servers from the XFree86 project,
that is, servers for SVGA Video cards, as well as a whole host of X
clients such as xv, maze, and xterm. If you are new to X, you might
want to get some hands-on experience with X on the Linux machine before
attempting to link up an X terminal.
2.2 What is an X terminal?
An X terminal (referred to as XT from now on) is a specialized piece of
hard- and software which combine into an X server, that is, the part of
X that manages in- and output to and from the user. In the most
primitive case, only the X server program and communcication software are
included. Even the window manager comes from the host computer, to which
the XT is connected by ethernet (or in rare cases serial lines), using
TCP/IP.
The hardware of an XT will include at least a (large) screen, keyboard,
mouse, some RAM, and jacks for ethernet cables. Most XTs do not have a
hard disk, a floppy drive, or other such means of data transfer.
This means that the XT either has its operating system in ROM (rare),
or gets it from a host on the net that it is attached to.
To get to its operating system from the Linux computer at boot time,
the XT usually does something like this: It sends out a cry of help
through the net with its ethernet number as a name tag. A "real" computer
on the net compares this number with a list of them, and if a match
is found, sends the XT the IP number it has been assigned to (via the
bootpd daemon). This allows the XT to download its operating system and
other data it needs from the hard disk of the host computer (usually via
tftp). This is the procedure explained here in detail.
An XT is therefore actually a full-fledged computer, with its own IP
number, RAM, program, and independent hardware, albeit more like
an idiot savant. It's great at what it does best, managing X graphics,
but not much good for anything else.
2.3 Advantages and Disadvantages
Ideally, an XT is silent, swift, and deadly. Usually without fan, floppy
or hard disk, they make no noise at all, and with a few meters of
ethernet cable you can put your loudish computer in a different room and
have the silent XT on your desk. Since the XT is built for X and graphics,
it is faster than, say, an X server program unter MS Windows or DOS.
With the server on one machine and the client on another, the processor
doesn't have to handle both at once. Though this might not be noticable
in terms of speed (since the data now has to be moved over the ethernet),
it will reduce the CPU load and save memory on the Linux machine that
would otherwise be swallowed by the X server.
On the flip side, you will need an ethernet card, which usually means
giving up a slot and an IRQ. Depending on the manufacturer, the software
for the XT can take up around 20 MBytes of hard disk space on your Linux
machine. You can almost always delete a lot of unused stuff once you
figure out what is really needed. Most XTs will require the host machine
to have the bootpd and tftpd daemons installed and running - both are
potential security holes. You will probably want to have a further daemon,
xdm, running in the background. And yes, that big XT screen will take up
a lot of desk space.
2.4 What do I need?
Kind of you to ask! But more appropriately, what do you need?
First of all, you need an XT. If you have lots of money, and we do mean
lots, you can go out and buy one. Jim Morton <jim@applix.com> posts
a list of XTs and their prices to comp.windows.x on a regular basis.
Or fortune might smile on you. Since old XTs can't be used with DOS,
MS Windows, or OS/2, some firms solve the problem of what to do with
their old ones by just throwing them out.
On the Linux computer side, you will need an ethernet card. Though it
is in theory possible to run an XT via serial line and SLIP, this is
not recommended unless you have masochistic tendencies. Check out the
Ethernet-HOWTO maintained by Paul Gortmaker <Paul.Gortmaker@anu.edu.au>
on how to purchase and set up ethernet cards. SLIP und CSLIP are covered
in the same document, should you have no other choice. In this case, you
might also want to look at the Serial-HOWTO by Greg Hankins
<gregh@cc.gatech.edu> for information on how to get the best performance.
You will also need to have TCP/IP support compiled into the kernel,
as well as a IP number for at least your machine and the XT. The
Net-2-HOWTO by Terry Dawson <terryd@extro.ucc.su.oz.au> covers this.
Finally, you will need to have X installed on you Linux machine. In
theory you should be able to have just the X clients and programs
like xdm installed, without the servers package. But it is probably
not worth the effort to untangle the various parts. The XFree86 HOWTO
by Helmut Geyer <Helmut.Geyer@iwr.uni-heidelberg.de> will tell you how
to get X up and working.
3.0 Cables, Nets and Daemons
This section discusses the changes to hard- and software required to
get the XT connected to the Linux machine. Some conventions are that
the XT is called "whisper" (because it makes no sound), and the Linux
host machine "imlinux" (I am Linux). They are both part of the
"frog" domain in Germany (".de"). Their IP numbers are
192.168.13.1 for imlinux.frog.de (the Linux machine)
192.168.13.41 for whisper.frog.de (the XT linked to it).
Note that these are standalone IP numbers for systems not connected to
the greater Internet, and that to my knowledge there is no frog domain
in Germany. We will assume that there are no other machines on the net,
and that NFS is not installed.
[Bummer. If someone has used NFS in connection with his or her XT, I'd
love to hear from you.]
3.1 Physical connection
This should be as easy as plugging cables into both machines. Please note
that some XTs have two serial inputs that can only run at certain speeds
if both are in use. Check your XT manual for details. You will need the
ethernet number of the XT later on. It is displayed during boot of the
XT, even if no connection is yet established.
As soon as you have the wires in place, you should be able to test the
ethernet link. After booting the XT, it should start off by complaining
that its calls for a bootpd and/or tftpd are not being answered, and
then start a boot operating system (usually part of the XT ROM). This
should include a primitive ping command which will allow you to test
the ethernet from the XT to the Linux machine. Don't panic if it
doesn't work the other way around yet. The XT probably needs its full
operating system to be able to respond.
3.2 Configuring the Net
Your basic configuration needs should be covered by the Net2-HOWTO as
mentioned above. We'll assume here that you already have TCP/IP and
such up and running. Since the XT is considered to be just another
computer on the net, you will now have to make sure both Linux machine
and XT know each other's IP address and that of the net they are on.
3.2.1 Configuring the Linux Machine
Information on the XT will have to be included in at least these files:
/etc/hosts Add a line with the IP number of the XT, such as
# /etc/hosts line for Linux machine. lprhost and loghost
# are optional
192.168.13.1 imlinux imlinux.frog.de lprhost loghost
# The new line for the XT is next
192.168.13.41 whisper whisper.frog.de
/etc/ethers This file provides a list of ethernet numbers and the
corresponding host names. This does not seem to be needed
in all distributions and setups, but in case it is, you
will need to include the ethernet number of the
XT and its hostname. This would be something like
04:03:e8:cc:0d:24 imlinux
0f:03:11:31:45:f1 whisper
(Yes, the ethernet numbers are fakes)
[You might have to change further files if you are running programs like
named, routed, or gated. As I am not, I would be grateful if someone
could fill me in on which files, if any, must be modified.]
You will have to reboot your Linux machine to make sure all changes
take effect.
3.2.2 Configuring the X Terminal
Check your XT manual for which files will have to be edited. In my case,
there was a central configuration file which needed to have the
following entries changed:
ip_host_table 192.168.13.1 imlinux
ip_host_table 192.168.13.1 imlinux.frog.de
ip_host_table 192.168.13.41 whisper
ip_host_table 192.168.13.41 whisper.frog.de
file_access_1 TFTP
file_host_name_1 imlinux.frog.de
file_path_1 /usr/local/xterm/liveshere
display_access_table whisper
display_access_table imlinux
enable_access_control YES
xdmcp_server imlinux
broadcast_address 192.168.13.255
default_telnet_host imlinux
Note that the XT picks up its files via tftp from the directory
/usr/local/xterm/liveshere, and the terminal is able to do XDMCP
(important for the configuration of xdm).
There will also be other configurations files for things like fonts. You
should be able to use the fonts already installed with Linux. In my case,
the file for the fonts (font.tbl) looks something like this
/usr/lib/X11/fonts/75dpi
/usr/lib/X11/fonts/100dpi
...
/usr/local/xterm/misc
/usr/local/xterm/openlook
with a few more lines to the same effect. Later, when the XT boots off
the Linux machine, you should see a list of files it has successfully
loaded.
Another thing you will probably want to have on your XT is "backing
store," which means that those window parts covered by other windows
are not stored in the Linux' RAM, but in that of the XT. Check your
XT manual for further details.
3.3 bootpd
Bootpd is the daemon that hears the X terminal's cry for help
at boot time and responds by telling the XT who it is, and where
it can find the software it wants to download. For some odd reason
bootpd is not included in some current distributions, notably
Slackware 2.2.0.1. This means you will have to get it via FTP or
some other source. It should then be placed in /usr/sbin/ (not in
/etc, as the man page would have it) as in.bootpd. Add or uncomment
the following line in /etc/inetd.conf:
bootps dgram udp wait root /usr/sbin/tcpd /usr/sbin/in.bootpd
This will ensure that inetd can start bootpd if a boot request
is found.
The configuration file for bootpd is /etc/bootpd. The syntax is
explained in the man page. In our example, the /etc/bootpd file should
look something like this ("server" is now used in the classical sense
again):
# Sample /etc/bootpd file
# First, global entry for stuff every host uses
allhost:hd=/usr/local/xterm/liveshere:\ # Home directory XT software
:ds=192.168.13.1:\ # Domain name server (imlinx)
:sm=255.255.255.0:\ # Subnet mask
:gw=192.168.13.1:\ # Gateways
:ts=192.168.13.1:\ # Time Servers
:lp=192.168.13.1:\ # lpr Servers
:to=-7200: # Time Offset in seconds
# Next, individual entries of every single host. Futher XTs would
# have their own entry
whisper:ht=ethernet:\ # Type of hardware link
:ha=0f03113145f1:\ # Ethernet number of X terminal
:ip=192.168.13.41:\ # IP number of X terminal (whisper)
:tc=allhost:\ # Template for standard options as above
:bf=xtermOS: # Boot file name - the X terminal's OS
The name of the XT's operating system may not be included as part of the
hd (home directory) entry. In our example, the file that the XT grabs
as its operating system is /usr/local/xterm/liveshere/xtermOS, but the
hd entry is /usr/local/xterm/liveshere.
bootpd will write information to both /var/adm/syslog and
/var/adm/messages, which should look something like this after a
successful boot:
Jul 17 05:19:42 imlinux in.bootpd[110]: connect from 0.0.0.0
Jul 17 05:19:42 imlinux bootpd[110]: reading "/etc/bootptab"
Jul 17 05:19:42 imlinux bootpd[110]: read 2 entries from "/etc/bootptab"
Jul 17 05:19:43 imlinux bootpd[110]: request from hardware address
0F03113145F1 Type 1
Jul 17 05:19:43 imlinux bootpd[110]: found 192.168.13.41 whisper
After helping the XT to boot, bootpd will stick around for about
15 minutes, then remove itself if there is no further work to do.
3.4 tftpd
The Trivial File Transfer Program is used by the XT to load its
operating system from a Linux hard disk. It should be included in
every distribution and does not have a configuration file. You can
test tftp by just typing "tftp" from a command shell.
As with bootpd, you will have to include or uncomment the following line
in /etc/inetd.conf:
tftp dgram udp wait root /usr/sbin/tcpd /usr/sbin/in.tftpd
Note that tftp can only access files that have world read access set.
Note also that tftp is a potential security hole that you will have to
keep in mind, and that the version of tftp included in some Linux
packages does not include the -r (or -s) flags for more secure use.
tftp will also write to /var/adm/messages. If bootpd has successfully
done its stuff, the next lines should be something like
Jul 17 05:19:43 imlinux in.tftpd[111]: connect from whisper
Jul 17 05:19:58 imlinux in.tftpd[113]: connect from whisper
Jul 17 05:19:59 imlinux in.tftpd[115]: connect from whisper
Jul 17 05:20:00 imlinux in.tftpd[117]: connect from whisper
Jul 17 05:20:03 imlinux in.tftpd[125]: connect from whisper
Jul 17 05:20:05 imlinux in.tftpd[127]: connect from whisper
and so on for quite a while. These are the files that the XT is requesting
from its home directory on the Linux computer. You should see messages on
the XT's screen while this transfer takes place.
3.5 Testing the link
Once you have modified the files mentioned above, you should be able
to boot the XT. Depending on the manufacturer, more or less verbose
messages on the XT's screen will tell you what is happening. Look
carefully for messages about files that cannot be found.
If all is well, it should progress to the point where the XT can launch
its own version of X. This means a grey background and the X cursor.
If you already have xdm running on the Linux machine, it might even
put up the xdm login prompt, though things might go a little haywire
since some definitions are not in place yet. Be prepared to kill xdm as
root from the Linux machine as a last resort.
Most XTs have an inbuilt set of functions, like a telnet client, as
part of their boot operating system. You can test the link further by
telneting into the Linux computer.
At this point, depending on how access is set up, you might also
be able to start X programs on the XT by using the display option.
From the Linux computer, try something like
xclock -display whisper:0 &
This should produce the xlock on the XT. You can even start a window
manager such as fvwm this way.
4.0 X on the run
This section deals with setting up xdm so that a login prompt is
available on the XT, and will return when a user has logged out. The xdm
program is a display manager that is the (very) rough equivalent of the
login programs for normal terminals. It should be included in every
Linux X package.
4.1 xdm configuration
xdm's configuration files live in /usr/X11R6/lib/X11/xdm (/usr/X11R6 may
be a link to /usr/X11). The main configuration file is xdm-config. You
should find, among others, the following lines already in place:
DisplayManager._0.authorize: true
DisplayManager._0.setup: /usr/X11R6/lib/X11/xdm/Xsetup_0
DisplayManager._0.startup: /usr/X11R6/lib/X11/xdm/GiveConsole
DisplayManager._0.reset: /usr/X11R6/lib/X11/xdm/TakeConsole
These are the files that control the screen when X is run on the Linux
machine itself. For the XT, we add four lines of the same type:
DisplayManager.whisper_0.authorize: true
DisplayManager.whisper_0.setup: /usr/X11R6/lib/X11/xdm/Xsetup_whisper
DisplayManager.whisper_0.startup: /usr/X11R6/lib/X11/xdm/Xstartup
DisplayManager.whisper_0.reset: /usr/X11R6/lib/X11/xdm/Xreset
Note that whisper_0 is the xdm notation for whisper:0, just as _0 is :0.
Note also that GiveConsole has been replaced by Xstartup, which in my
case is a dummy file, and TakeConsole by Xreset, is also a dummy file.
The original files both control the ownership of the console when X is
run on the Linux machine, and there is no reason to fool around with
the Linux console just because the XT is running.
The setup files run programs before the login prompt is placed on the
screen. This is the place to, say, use xv or a similiar program to
have a picture in the background. You should be able to simply copy
the given Xsetup_0 file to Xsetup_whisper.
[Since this question comes up again and again: One way of putting a
picture in the root window is by placing the line
nice xv -root -quit -rmode 5 <picture_file> &
or such in the setup file. picture_file will then be displayed in
the root window under the xdm login prompt. Note that some XTs will
give an error message if the file is too large or too complex.]
Xaccess controls who can have access to the machine. You should be able
to leave the defaults as they are. Note hat Xaccess will let you greet
the user with a chooser, in case you have different machines on the net
that can service the XT.
Xresources controls the shape and size of the login prompt. You might
want to have different messages for the XT and Linux machine by
replacing the
DisplayManager*resources: /usr/X11R6/lib/X11/xdm/Xresources
with two lines such as
DisplayManager._0.resources: /usr/X11R6/lib/X11/xdm/Xres_0
DisplayManager.whisper_0.resources: /usr/X11R6/lib/X11/xdm/Xres_wh_0
where Xres_wh_0 is the name of the whisper resource file.
You should be able to leave the Xsession file unchanged, too.
Configuration of the Xservers file is slightly more tricky. Out of the
box, there is probably only one line uncommented (Slackware 2.2.0.1)
:0 local /usr/X11R6/bin/X
or something to that effect. This starts the Linux machine X server
when xdm is called. Commenting this line means that when xdm is called,
there will be no X started on the Linux machine. This is what you need
to do if you only want to have xdm managing the XT, but not the local
Linux X server. In this case, X can still be started with startx at any
time on the Linux machine with no ill effects found so far.
If your XT does not have XDMCP, you must also include a line for the
XT, such as
whisper:0 foreign
XDMCP is a standardized method that for example lets X terminals talk
to their hosts. If your terminal does have XDMCP, don't repeat do not
include that line here. This would make xdm think that there is an XT
out there that doesn't know XDMCP, while at the same time a terminal
with the same name is trying to get in, too. This can lead to all kinds
of ugly effects, such as two xdms fighting for control.
Note that you can use the xdm-config entries even if there is no line
in Xservers for the XT, that is, you can still customize the xdm
login prompt, etc, for a XT that uses XDMCP.
To make xdm start with every reboot, you can include a line such as
/usr/bin/X11/xdm
in /etc/rc.d/rc.local. Other people start xdm with the /etc/inittab
file. In any case, xdm should appear with the list of processes after
a reboot of the Linux machine.
4.2 Access questions
[BummerRank1. This is important, and we're working on it.]
To see if a user can access the XT screen from the Linux machine,
log in as non-root on the Linux machine and try a command like
xsetroot -solid white -display whisper:0 &
or
xterm -display whisper:0 &
Try this when somebody is logged in on the XT and when there is only the
xdm login to be seen. Depending on where you are, the ability to put
stuff on the XT screen from the console might more of a feature than a
bug.
5.0 Errors, Unknowns, and Thanks
5.1 Known problems
These are problems which have turned up, as well as interesting features
that might be considered problems. If you have come up with any
interesting features, or even solutions, please let me know.
talk - The interactive chat will work if the user at the XT starts the
session with a user at the Linux machine, but fails the other
way around. I'm sure I read a fix for this once, but have
forgotten it.
who - A user logged in via the XT does not appear in the output of the
who command, even if it is sent from the XT itself. This is
probably the reason why talk fails when initiated from the Linux
machine ("On an XT, nobody even knows you're a human").
xlock - Normal call of xlock will only produce a message to the effect
that the XT's screen can't be grabbed. The -remote option must
be included in the xlock call to allow terminal locking. Note
that some xlock modes are more resource hungry than others.
Qix seems to be more suited for XTs than others - check out the
FAQ by Art Mulder (below) for more details.
xv - Some XTs will not have enough video memory to be able to handle
large and/or complex colored background pictures. Try removing
any old pictures (with `xsetroot' or such) and refreshing the
screen before you move xv's window into the root.
5.2 Terminals tested
The procedures described in this text have so far only been seriously
tested on a Tektronix XP23 in connection with a 386DX-33MHz, 16 MByte
RAM running Linux 1.2.3 and the XFree86 Version 3.1.1 files from the
Slackware 2.2.0.1 distribution.
5.3 Futher reading
More information on X can be found on the net as:
David B. Lewis <dbl@ics.com> posts the detailed and extensive
Comp.windows.x Frequently Asked Questions (FAQ) to comp.windows.x,
news.answers, and comp.answers on a regular basis. This document also
contains entries on where to get more information on X.
Steve Kotsopoulos <steve@ecf.toronto.edu> posts the X on Intel-based Unix
Frequently Asked Questions (FAQ) list to the same groups.
Art Mulder <art@cs.ualberta.ca> maintains the Comp.windows.x: Getting
more performance out of X FAQ, which is als posted regularly to these
groups. It includes tips very useful for Linux under X, too.
5.4 Thanks
First thanks, as always, go out to Linus B. Torvalds
<torvalds@kruuna.helsinki.fi>. Futhermore to
Klaus ter Fehn <ktf@bc3.gun.de>, for making it possible and
Douglas K. Stevenson <duck@catzen.gun.de>, for making it passable.