Chip 1998 September/CHIP Eylül 1998.iso/Slackwar/docs/Tips-HOWTO

Text File | 1998-01-05 | 29KB | 735 lines

  The Linux Tips HOWTO
  Paul Anderson, [e-mail]
  v3.1, 26 December 1997

  This HOWTO contains those hard to find hints and tweekings that make
  Linux a bit nicer.

  1.  Introduction

  Welcome to the Linux Tips HOWTO, a list of neato tricks and
  optimizations that make Linux more fun.  All I have in here right now
  are tips off of the top of my head, and tips from the old Tips-
  HOWTO(Why take out decent tips, right?).  So send all your favorite
  hints and tips to me so I can put them in the next Tips-HOWTO.

  Paul Anderson Maintainer--Linux TIPS HOWTO

  [e-mail]

  2.  Short Tips

  2.1.  Handy Syslog Trick Paul Anderson, Tips-HOWTO maintainer

  Edit your /etc/syslog.conf, and put in the following line:

       # Dump everything on tty8
       *.*                                     /dev/tty8

  One caveat: REMEMBER TO USE TABS!  syslog doesn't like spaces...

  2.2.  Script to view those compressed HOWTOs. Didier Juges,
  [e-mail]

  From a newbie to another, here is a short script that eases looking
  for and viewing howto documents.  My howto's are in
  /usr/doc/faq/howto/ and are gzipped. The file names are XXX-HOWTO.gz,
  XXX being the subject.  I created the following script called "howto"
  in the /usr/local/sbin directory:

       ______________________________________________________________________
       #!/bin/sh
       if [ "$1" = "" ]; then
           ls /usr/doc/faq/howto | less
       else
           gunzip -c /usr/doc/faq/howto/$1-HOWTO.gz | less
       fi
       ______________________________________________________________________

  When called without argument, it displays a directory of the available
  howto's. Then when entered with the first part of the file name
  (before the hyphen) as an argument, it unzips (keeping the original
  intact) then displays the document.

  For instance, to view the Serial-HOWTO.gz document, enter:

  $ howto Serial

  2.3.  Is there enough free space??? Hans Zoebelein,
  [e-mail]

  Here comes a short script which will check from time to time that
  there is enough free space available on anything which shows up in
  mount (disks, cdrom, floppy...)

  If space runs out, a message is printed every X seconds to the screen
  and 1 mail message per filled device is fired up.

  ______________________________________________________________________
  #!/bin/sh

  #
  # $Id: check_hdspace,v 1.18 1996/12/11 22:33:29 root Exp root $
  #

  #
  # Since I got mysterious error messages during compile when
  # tmp files filled up my disks, I wrote this to get a warning
  # before disks are full.
  #
  # If this stuff saved your servers from exploding,
  # send praising email to [e-mail].
  # If your site burns down because of this, sorry but I
  # warned you: no comps.
  # If you really know how to handle sed, please forgive me :)
  #

  #
  # Shoot and forget: Put 'check_hdspace &' in rc.local.
  # Checks for free space on devices every $SLEEPTIME sec.
  # You even might check your floppies or tape drives. :)
  # If free space is below $MINFREE (kb), it will echo a warning
  # and send one mail for each triggering device to $MAIL_TO_ME.
  # If there is more free space than trigger limit again,
  # mail action is also armed again.
  #

  # TODO: Different $MINFREE for each device.
  # Free /*tmp dirs securely from old junk stuff if no more free space.

  DEVICES='/dev/sda2 /dev/sda8 /dev/sda9'         # device; your put disks here
  MINFREE=20480                                   # kb; below this do warning
  SLEEPTIME=10                                    # sec; sleep between checks
  MAIL_TO_ME='root@localhost'                     # fool; to whom mail warning

  # ------- no changes needed below this line (hopefully :) -------

  MINMB=0
  ISFREE=0
  MAILED=""
  let MINMB=$MINFREE/1024         # yep, we are strict :)

  while [ 1 ]; do
          DF="`/bin/df`"
                  for DEVICE in $DEVICES ; do
                  ISFREE=`echo $DF | sed s#.\*$DEVICE" "\*[0-9]\*""\*[0-9]\*" "\*## | sed s#" ".\*##`

                  if [ $ISFREE -le $MINFREE ] ; then
                          let ISMB=$ISFREE/1024
                          echo  "WARNING: $DEVICE only $ISMB mb free." >&2
                          #echo "more stuff here" >&2
                          echo -e "\a\a\a\a"

                          if [ -z  "`echo $MAILED | grep -w $DEVICE`" ] ; then
                                  echo "WARNING: $DEVICE only $ISMB mb free.      (Trigger is set to $MINMB mb)" \
                                  | mail -s "WARNING: $DEVICE only $ISMB mb free!" $MAIL_TO_ME
                                  MAILEDH="$MAILED $DEVICE"
                                  MAILED=$MAILEDH
                                  # put further action here like cleaning
                                  # up */tmp dirs...
                          fi
                          elif [ -n  "`echo $MAILED | grep -w $DEVICE`" ] ; then
                                  # Remove mailed marker if enough disk space
                                  # again. So we are ready for new mailing action.
                                  MAILEDH="`echo $MAILED  | sed s#$DEVICE##`"
                                  MAILED=$MAILEDH
                          fi

                  done
                  sleep $SLEEPTIME

  done
  ______________________________________________________________________

  2.4.  Util to clean up your logfiles. Paul Anderson, Tips-HOWTO Main¡
  tainer>

  If you're like me, you have a list with 250 subscribers, plus 100+
  messages per day coming in over UUCP.  Well, what's a hacker to do
  with these huge logs?  Install chklogs, that's what.  Chklogs is
  written by Emilio Grimaldo, [e-mail], and the current
  version 1.8 available from
  ftp.iaehv.nl:/pub/users/grimaldo/chklogs-1.8.tar.gz.  It's pretty self
  explanatory to install(you will, of course, check out the info in the
  doc subdirectory).  Once you've got it installed, add a crontab entry
  like this:

       # Run chklogs at 9:00PM daily.
       00 21 * * *       /usr/local/sbin/chklogs -m

  While you're at it, mention to the author how nice a peice of software
  this is:)

  2.5.  [e-mail] Handy Script to Clean Up Corefiles. Otto
  Hammersmith,

  Create a file called rmcores(the author calls it handle-cores) with
  the following in it:

  ______________________________________________________________________
  #!/bin/sh
  USAGE="$0 <directory> <message-file>"

  if [ $# != 2 ] ; then
          echo $USAGE
          exit
  fi

   echo Deleting...
  find $1 -name core -atime 7 -print -exec rm {} \;

  echo e-mailing
  for name in `find $1 -name core -exec ls -l {} \; | cut -c16-24`
  do
          echo $name
          cat $2 | mail $name
  done
  ______________________________________________________________________

  And have a cron job run it every so often.

  2.6.  Moving directories between filesystems. Alan Cox,
  [e-mail]

  Quick way to move an entire tree of files from one disk to another

       (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xvfp -)

   Change from cd /source/directory; tar....etc.  to prevent possibility
  of trashing directory in case of disaster.  Thanks to Jim Dennis,
  [e-mail], for letting me know. -Maint.

  2.7.  [e-mail] Finding out which directories are the
  largest. Mick Ghazey,

  Ever wondered which directories are the biggest on your computer?
  Here's how to find out.

       du -S | sort -n

  2.8.  The Linux Gazette

  Kudos go to John Fisk, creator of the Linux Gazette.  This is an
  excellent e-zine plus, it's FREE!!!  Now what more could you ask?
  Check it out at:

       http://www.ssc.com/lg

  BTW, It turns out that (1) LG is now out on a monthly basis, and (2)
  John Fisk no longer maintains it, the fellows at SSC do.

  2.9.  Ted Stern, [e-mail] Pointer to patch for GNU
  Make 3.70 to change VPATH behavior.

  I don't know if many people have this problem, but there is a
  "feature" of GNU make version 3.70 that I don't like. It is that VPATH
  acts funny if you give it an absolute pathname.  There is an extremely
  solid patch that fixes this, which you can get from Paul D. Smith
  <[e-mail]>.  He also posts the documentation and patch
  after every revision of GNU make on the newsgroup system I have access
  to.

  2.10.  How do I stop my system from fscking on each reboot? Dale Lutz,
  [e-mail]

  Q:  How do I stop e2fsck from checking my disk every time I boot up.

  A:  When you rebuild the kernel, the filesystem is marked as 'dirty'
  and so your disk will be checked with each boot.  The fix is to run:

  rdev -R /zImage 1

  This fixes the kernel so that it is no longer convinced that the
  filesystem is dirty.

  Note: If using lilo, then add read-only to your linux setup in your
  lilo config file (Usually /etc/lilo.conf)

  2.11.  How to avoid fscks caused by "device busy" at reboot time. Jon
  Tombs, [e-mail]

  If you often get device busy errors on shutdown that leave the
  filesystem in need of an fsck upon reboot, here is a simple fix:

  To /etc/rc.d/init.d/halt or /etc/rc.d/rc.0, add the line

       mount -o remount,ro /mount.dir

  for all your mounted filesystems except /, before the call to umount
  -a. This means if, for some reason, shutdown fails to kill all pro¡
  cesses and umount the disks they will still be clean on reboot. Saves
  a lot of time at reboot for me.

  2.12.  How to find the biggest files on your hard-drive.

  Simon Amor, [e-mail]

       ls -l | sort +4n

  Or, for those of you really scrunched for space this takes awhile but
  works great:

       cd /
       ls -lR | sort +4n

  2.13.  How to print pages with a margin for hole punching. Mike
  Dickey, [e-mail]

       ______________________________________________________________________
               #!/bin/sh
               # /usr/local/bin/print
               # a simple formatted printout, to enable someone to
               # 3-hole punch the output and put it in a binder

               cat $1 | pr -t -o 5 -w 85 | lpr
       ______________________________________________________________________

  2.14.  Raul Deluth Miller, [e-mail] A way to search
  through trees of files for a particular regular expression.

  I call this script 'forall'.  Use it like this:

       forall /usr/include grep -i ioctl
       forall /usr/man grep ioctl

  Here's forall:

       ______________________________________________________________________
       #!/bin/sh
       if [ 1 = `expr 2 \> $#` ]
       then
               echo Usage: $0 dir cmd [optargs]
               exit 1
       fi
       dir=$1
       shift
       find $dir -type f -print | xargs "$@"
       ______________________________________________________________________

  2.15.  Barry Tolnas, [e-mail] A script for cleaning
  up after programs that create autosave and backup files.

  Here is a simple two-liner which recursively descends a directory
  hierarchy removing emacs auto-save (#) and backup (~) files, .o files,
  and  TeX .log files. It also compresses .tex files and README files. I
  call it 'squeeze' on my system.

       ______________________________________________________________________
       #!/bin/sh
       #SQUEEZE removes unnecessary files and compresses .tex and README files
       #By Barry tolnas, [e-mail]
       #
       echo squeezing $PWD
       find  $PWD \( -name \*~ -or -name \*.o -or -name \*.log -or -name \*\#\) -exec
       rm -f {} \;
       find $PWD \( -name \*.tex -or -name \*README\* -or -name \*readme\* \) -exec gzip -9 {} \;
       ______________________________________________________________________

  2.16.  [e-mail] How to find out what process is eating the
  most memory. Simon Amor,

       ps -aux | sort +4n

  -OR-

       ps -aux | sort +5n

  2.17.  Rigging vi for C programming, Paul Anderson,Tips-HOWTO Main¡
  tainer

  I do a lot of C programming in my spare time, and I've taken the time
  to rig vi to be C friendly.  Here's my .exrc:

       ______________________________________________________________________
       set autoindent
       set shiftwidth=4
       set backspace=2
       set ruler
       ______________________________________________________________________

  What does this do?  autoindent causes vi to automatically indent each
  line following the first one indented, shiftwidth sets the distance of
  ^T to 4 spaces, backspace sets the backspace mode, and ruler makes it
  display the line number.  Remember, to go to a specific line number,
  say 20, use:

  ______________________________________________________________________
  vi +20 myfile.c
  ______________________________________________________________________

  2.18.  [e-mail] Why does sendmail hang for 5 minutes on
  startup with RedHat? Paul Anderson,

  This is a fairly common problem, almost to the point of being a FAQ.
  I don't know if RedHat has fixed this bug in their distribution, but
  you can repair it yourself.  If you look in your /etc/hosts file, you
  will find it looks something like:

       127.0.0.1               localhost       yourbox

  When sendmail starts, it does a lookup on your hostname(in this
  example, yourbox).  It then finds that the IP for yourbox is
  127.0.0.1, sendmail doesn't like this, so it does the lookup again.
  It continues with this for a while until it eventually gives up and
  exits.  Fixing the problem is extremely easy, edit your /etc/hosts
  file and change it to something like this:

       127.0.0.1               localhost
       10.56.142.1             yourbox

  2.19.  How do I configure RedHat for using color-ls? Paul Anderson,
  [e-mail]

  RedHat's distribution comes with color-ls, however why they don't
  configure it for colour use by default is beyond me.  Here's to fix
  it.

  First, type eval `DIRCOLORS`

  Next, alias ls='ls --color=yes'

  And put the 'alias.....' in your /etc/bashrc

  2.20.  [e-mail] How do I find which library in
  /usr/lib holds a certain function? Pawel Veselow,

  What if you're compiling and you've missed a library that needed
  linking in?  All gcc reports are function names...  Here's a simple
  command that'll find what you're looking for:

       for i in *; do echo $i:;nm $i|grep tgetnum 2>/dev/null;done

  Where tgetnum is the name of the function you're looking for.

  3.  Detailed Tips

  3.1.  Sharing swap partitions between Linux and Windows. Tony Acero,
  [e-mail]

  1. Format the partition as a dos partition, and create the Windows
     swap file on it, but don't run windows yet. (You want to keep the
     swap file completely empty for now, so that it compresses well).

  2. Boot linux and save the partition into a file.  For example if the
     partition was /dev/hda8:

       dd if=/dev/hda8 of=/etc/dosswap

  3. Compress the dosswap file; since it is virtually all 0's it will
     compress very well

       gzip -9 /etc/dosswap

  4. Add the following to the /etc/rc file to prepare and install the
     swap space under Linux:

     XXXXX is the number of blocks in the swap partition

       mkswap /dev/hda8 XXXXX
       swapon -av

  Make sure you add an entry for the swap partition in your /etc/fstab
  file

  5. If your init/reboot package supports /etc/brc or /sbin/brc add the
     following to /etc/brc, else do this by hand when you want to boot
     to dos|os/2 and you want to convert the swap partition back to the
     dos/windows version:

       swapoff -av
       zcat /etc/dosswap.gz | dd of=/dev/hda8 bs=1k count=100

  # Note that this only writes the first 100 blocks back to the parti¡
  tion. I've found empirically that this is sufficient

  >>  What are the pros and cons of doing this?

  Pros: you save a substantial amount of disk space.

  Cons: if step 5 is not automatic, you have to remember to do it by
  hand, and it slows the reboot process by a nanosecond :-)

  3.2.  Desperate Undelete. Michael Hamilton, [e-mail]

  Here's a trick I've had to use a few times.

  Desperate person's text file undelete.

  If you accidentally remove a text file, for example, some email, or
  the results of a late night programming session, all may not be lost.
  If the file ever made it to disk, ie it was around for more than 30
  seconds, its contents may still be in the disk partition.

  You can use the grep command to search the raw disk partition for the
  contents of file.

  For example, recently, I accidentally deleted a piece of email.  So I
  immediately ceased any activity that could modify that partition: in
  this case I just refrained from saving any files or doing any compiles
  etc.  On other occasions, I've actually gone to the trouble of bring
  the system down to single user mode, and unmounted the filesystem.

  I then used the egrep command on the disk partition:  in my case the
  email message was in /usr/local/home/michael/, so from the output from
  df, I could see this was in /dev/hdb5

         sputnik3:~ % df
           Filesystem         1024-blocks  Used Available Capacity Mounted on
           /dev/hda3              18621    9759     7901     55%   /
           /dev/hdb3             308852  258443    34458     88%   /usr
           /dev/hdb5             466896  407062    35720     92%   /usr/local

           sputnik3:~ % su
           Password:
           [michael@sputnik3 michael]# egrep -50 'ftp.+COL' /dev/hdb5 > /tmp/x

  Now I'm ultra careful when fooling around with disk partitions, so I
  paused to make sure I understood the command syntax BEFORE pressing
  return.  In this case the email contained the word 'ftp' followed by
  some text followed by the word 'COL'.  The message was about 20 lines
  long, so I used -50 to get all the lines around the phrase.  In the
  past I've used -3000 to make sure I got all the lines of some source
  code.  I directed the output from the egrep to a different disk parti¡
  tion - this prevented it from over writing the message I was looking
  for.

  I then used strings to help me inspect the output

          strings /tmp/x | less

  Sure enough the email was in there.

  This method can't be relied on, all, or some, of the disk space may
  have already been re-used.

  This trick is probably only useful on single user systems.  On multi-
  users systems with high disk activity, the space you free'ed up may
  have already been reused.  And most of use can't just rip the box out
  from under our users when ever we need to recover a file.

  On my home system this trick has come in handy on about three
  occasions in the past few years - usually when I accidentally trash
  some of the days work.  If what I'm working survives to a point where
  I feel I made significant progress, it get's backed up onto floppy, so
  I haven't needed this trick very often.

  3.3.  How to use the immutable flag. Jim Dennis, [e-mail]

  Use the Immutable Flag

  Right after you install and configure your system go through the /bin,
  /sbin/, /usr/bin, /usr/sbin and /usr/lib (and a few of the other usual
  suspects and make liberal use of the 'chattr +i command'.  Also add
  that to the the kernel files in root.  Now 'mkdir /etc/.dist/' copy
  everything from /etc/ on down (I do this in two steps using
  /tmp/etcdist.tar to avoid recursion) into that directory.  (Optionally
  you can just create /etc/.dist.tar.gz) -- and mark that as immutable.

  The reason for all of this is to limit the damage that you can do when
  logged in as root.  You won't overwrite files with a stray redirection
  operator, and you won't make the system unusable with a stray space in
  an 'rm -fr' command (you might still do alot of damage to your data --
  but your libs and bins will be safer.

  This also makes a variety of security and denial of service exploits
  either impossible or more difficult (since many of them rely on
  overwriting a file through the actions of some SUID program that
  *isn't providing an arbitrary shell command*).

  The only inconvenience of this is when building and doing your 'make
  install' on various sorts of system binaries.  On the other hand it
  also prevents the 'make install' from over-writing the files.  When
  you forget to read the Makefile and chattr -i the files that are to be
  overwritten (and the directories to which you want to add files) --
  the make fails, you just use the chattr command and rerun it.  You can
  also take that opportunity to move your old bin's, libs, or whatever
  into a .old/ directory or rename or tar them or whatever.

  3.4.  Jim Dennis, [e-mail] A suggestion for where to put new
  stuff.

  All new stuff starts under /usr/local! or /usr/local/`hostname`

  If your distribution is one that leaves /usr/local empty then just
  create your /usr/local/src, /usr/local/bin etc and use that.  If your
  distribution puts things in the /usr/local tree than you may want to
  'mkdir /usr/local/`hostname`' and give the 'wheel' group +w to it (I
  also make it SUID and SGID to insure that each member of the wheel
  group can only mess with their own files thereunder, and that all
  files created will belong to the 'wheel' group.

  Now discipline yourself to *ALWAYS! ALWAYS! ALWAYS!* put new packages
  under /usr/local/src/.from/$WHEREVER_I_GOT_IT/ (for the .tar or
  whatever files) and build them  under /usr/local/src (or
  .../$HOSTNAME/src).  Make sure that it installs under the local
  hierarchy.  If it *absolutely must* be installed back in /bin or
  /usr/bin or somewhere else -- put a symlink from the local heirarchy
  to each element that when anywhere else.

  The reason for this -- even though it's more work -- is that it helps
  isolate what has to be backed up and restored or reinstalled in the
  event of a full re-install from the distribution medio (usually CD
  these days).  By using a /usr/local/.from directory you also keep an
  informal log of where your sources are coming from -- which helps when
  you're looking for new updates -- and may be critical when monitoring
  the security announcement lists.

  One of my systems at home (the one I'm calling from) was put together
  before I adopted these policies for myself.  I still don't "know" all
  the ways that it differs from the stock "as installed" system.  This
  is despite the fact that I've done very little with my home system's
  configuration and I'm the *only* person who ever uses it.

  By contrast the systems I've set up at work (when I was thrust into
  the role of system administrator there) have all been configured this
  way -- have been administered by many contractors and other MIS
  people, and have had a large number of upgrades and package
  installations.  Nonetheless I have a very good idea which precise
  elements were put in *after* the initial installation and
  configuration.

  3.5.  Converting all files in a directory to lowercase. Justin Dossey,
  [e-mail]

  I noticed a few overly difficult or unnecessary procedures recommended
  in the 2c tips section of Issue 12.  Since there is more than one, I'm
  sending it to you:

       ______________________________________________________________________
       #!/bin/sh
                # lowerit
                # convert all file names in the current directory to lower case
                # only operates on plain files--does not change the name of directories
                # will ask for verification before overwriting an existing file
                for x in `ls`
                  do
                  if [ ! -f $x ]; then
                    continue
                    fi
                  lc=`echo $x  | tr '[A-Z]' '[a-z]'`
                  if [ $lc != $x ]; then
                    mv -i $x $lc
                  fi
                  done
       ______________________________________________________________________

  Wow.  That's a long script.  I wouldn't write a script to do that;
  instead, I would use this command:

       for i in * ; do [ -f $i ] && mv -i $i `echo $i | tr '[A-Z]' '[a-z]'`;
       done;

  on the command line.

  The contributor says he wrote the script how he did for
  understandability (see below).

  On the next tip, this one about adding and removing users, Geoff is
  doing fine until that last step.  Reboot?  Boy, I hope he doesn't
  reboot every time he removes a user.  All you have to do is the first
  two steps.  What sort of processes would that user have going, anyway?
  An irc bot?  Killing the processes with a simple

       kill -9 `ps -aux |grep ^<username> |tr -s " " |cut -d " " -f2`

  Example, username is foo

       kill -9 `ps -aux |grep ^foo |tr -s " " |cut -d " " -f2`

  That taken care of, let us move to the forgotten root password.

  The solution given in the Gazette is the most universal one, but not
  the easiest one.  With both LILO and loadlin, one may provide the boot
  parameter 'single' to boot directly into the default shell with no
  login or password prompt.  From there, one may change or remove any
  passwords before typing 'init 3' to start multiuser mode.  Number of
  reboots: 1 The other way Number of reboots: 2

  Justin Dossey

  3.6.  Jim Dennis, [e-mail] Some tips for new sysadmins.

  Create and maintain a /README.`hostname` and/or a
  /etc/README.`hostname` Or possibly /usr/local/etc/README.`hostname`
  -Maint.

  Absolutely, from *day one* of administering a system take notes in an
  online log file.  You might make Another way to do this is to write an
  su or a sudo script that does something like:

                       function exit \
                               { unset exit; exit; \
                                 cat ~/tmp/session.$(date +%y%m%d) \
                                 >> /README.$(hostname) && \
                                 vi /README.$(hostname)
                                 }
                       script -a ~/tmp/session.$(date +%y%m%d)
                       /bin/su.org -

  (use the typescript command to create a session log and create a
  function to automate appending and updating the log).

  I'll admit that I haven't implemented this automation of policy --
  I've just relied on self-discipline so far.  However I have been
  toying with the idea (even to the point of prototyping the scripts and
  shell functions as you see them).  One thing that holds me back on
  this is the 'script' command itself.  I think I'll have to grab the
  sources and add a couple of command line parameters (to pause/stop the
  script recording from the command line) before I commit to using
  this).

  My last suggestion (for this round):

  Root's path should consist of 'PATH= /bin'

  That's it.  Nothing else on root's path.  Everything root does is
  provided by a symlink from  /bin or by an alias or shell function, or
  is a script or binary in  /bin, or is typed out with an explicit path.

  This makes anyone running as root aware (sometimes painfully so) of
  how he or she is trusting binaries.  The wise admin of a multi-user
  host will periodically look through his or here  /bin and  /.*history
  files to look for patterns and loopholes.

  The really motivated admin will spot sequences that can be automated,
  places where sanity checks can be inserted, and tasks for which 'root'
  privileges should be temporarily eschewed (launching editors, MTA's
  and other large interactive programs with elaborate scripting features
  that *might* be embedded in transparent or data files -- like the
  infamous vi ./.exrc and emacs ./.emacs and the even more insidous
  $EXINIT and the embedded header/footer macros).  Naturally those sorts
  of commands can be run with something like:

                       cp $data $some_users_home/tmp
                       su -c $origcommand $whatever_switches
                       cp $some_users_home/tmp $data

  (...where the specifics depend on the command).

  Mostly these last sorts of precautions are overboard for the home or
  'single' user workstation -- but they are very good policy the admin
  of a multi-user -- particular a publicly exposed system (like the
  one's at netcom).

  3.7.  How to configure xdm's chooser for host selection. Arrigo Tri¡
  ulzi, [e-mail]

  1. Edit the file that launches xdm most likely /etc/rc/rc.6 or
     /etc/rc.local) so that it contains the following lines in the xdm
     startup section.

       /usr/bin/X11/xdm
       exec /usr/bin/X11/X -indirect hostname

  2. Edit /usr/lib/X11/xdm/Xservers and comment out the line which
     starts the server on the local machine (i.e. starting 0:)
  3. Reboot the machine and you're home and away.

  I add this because when I was, desperately, trying to set it up for my
  own subnet over here it took me about a week to suss out all the
  problems.

  Caveat: with old SLS (1.1.1) for some reason you can leave a -nodaemon
  after the xdm line -- this does NOT work for later releases.