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The X Terminal HOWTO

How to connect an X terminal to a Linux PC
Version 1.0 BETA (July 1995)
Scot W. Stevenson <[e-mail]>


    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 [e-mail]. 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 [e-mail] 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

                 [e-mail] (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 <[e-mail]> 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 <[e-mail]>
    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
    <[e-mail]> 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 <[e-mail]> 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 <[e-mail]> 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 <[e-mail]> 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 <[e-mail]> posts the X on Intel-based Unix
    Frequently Asked Questions (FAQ) list to the same groups.

    Art Mulder <[e-mail]> 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
    <[e-mail]>. Futhermore to

    Klaus ter Fehn <[e-mail]>, for making it possible and
    Douglas K. Stevenson <[e-mail]>, for making it passable.