  Remote X Apps mini-HOWTO
  Vincent Zweije, zweije@xs4all.nl
  v, 19 November 1999

  This mini-HOWTO describes how to run remote X applications.  That is,
  how to have an X program display on a different computer than the one
  it's running on.  Or conversely: how to make an X program run on a
  different computer than the one you're sitting at.  The focus of this
  mini-HOWTO is on security.  This mini-HOWTO also contains information
  on running X applications locally, but with a different user-id.
  ______________________________________________________________________

  Table of Contents


  1. Introduction

  2. Related Reading

  3. The Scene

  4. A Little Theory

  5. Telling the Client

  6. Telling the Server

     6.1 Xhost
     6.2 Xauth
        6.2.1 Making the Cookie
        6.2.2 Transporting the Cookie
        6.2.3 Using the Cookie
     6.3 Ssh

  7. X Applications from Another User-id

     7.1 Different Users on the Same Host
     7.2 Client User Is Root

  8. Running a Remote Window Manager

  9. Troubleshooting



  ______________________________________________________________________

  1.  Introduction

  This mini-HOWTO is a guide how to do remote X applications.  It was
  written for several reasons.

  1. Many questions have appeared on usenet on how to run a remote X
     application.

  2. I see many, many hints of ``use xhost +hostname'' or even ``xhost
     +'' to allow X connections. This is ridiculously insecure, and
     there are better methods.

  3. I do not know of a simple document that describes the options you
     do have. Please inform me zweije@xs4all.nl if you know more.

  This document has been written with unix-like systems in mind. If
  either your local or remote operating system are of another flavour,
  you may find here how things work. However, you will have to translate
  examples yourself to apply to your own system(s).
  The most recent version of this document is always available on WWW at
  http://www.xs4all.nl/~zweije/xauth.html. It is also available as the
  Linux Remote X Apps mini-HOWTO at
  http://sunsite.unc.edu/LDP/HOWTO/mini/Remote-X-Apps. Linux
  (mini-)HOWTOs are available by http or ftp from sunsite.unc.edu.

  This is version 0.6.1. No guarantees, only good intentions. I'm open
  to suggestions, ideas, additions, useful pointers, (typo) corrections,
  etc... I want this to remain a simple readable document, though, in
  the best-meant HOWTO style. Flames to /dev/null.

  Contents last updated on 19 November 1999 by Vincent Zweije


  2.  Related Reading

  A related document on WWW is ``What to do when Tk says that your
  display is insecure'', http://ce-toolkit.crd.ge.com/tkxauth/. It was
  written by Kevin Kenny. It suggests a similar solution to X
  authentication to that in this document (xauth). However, Kevin aims
  more at using xdm to steer xauth for you.

  The X System Window System Vol. 8 X ``Window System Administrator's
  Guide'' from O'Reilly and Associates has also been brought to my
  attention as a good source of information. Unfortunately, I've not
  been able to check it out.

  Yet another document much like the one you're reading now, titled
  ``Securing X Windows'', is available at
  http://ciac.llnl.gov/ciac/documents/ciac2316.html.

  Also check out usenet newsgroups, such as comp.windows.x,
  comp.os.linux.x, and comp.os.linux.networking.


  3.  The Scene

  You're using two computers. You're using the X window system of the
  first to type to and look at. You're using the second to do some
  important graphical work. You want the second to show its output on
  the display of the first. The X window system makes this possible.

  Of course, you need a network connection for this. Preferably a fast
  one; the X protocol is a network hog. But with a little patience and
  suitable protocol compression, you can even run applications over a
  modem. For X protocol compression, you might want to check out dxpc
  http://ccwf.cc.utexas.edu/~zvonler/dxpc/ or LBX
  http://www.ultranet.com/~pauld/faqs/LBX-HOWTO.html
  <http://www.ultranet.com/~pauld/faqs/LBX-HOWTO.html> (also known as
  the LBX mini-HOWTO).

  You must do two things to achieve all this:


  1. Tell the local display (the server) to accept connections from the
     remote computer.

  2. Tell the remote application (the client) to direct its output to
     your local display.


  4.  A Little Theory

  The magic word is DISPLAY. In the X window system, a display consists
  (simplified) of a keyboard, a mouse and a screen. A display is managed
  by a server program, known as an X server. The server serves
  displaying capabilities to other programs that connect to it.

  A display is indicated with a name, for instance:


  ·  DISPLAY=light.uni.verse:0

  ·  DISPLAY=localhost:4

  ·  DISPLAY=:0

  The display consists of a hostname (such as light.uni.verse and
  localhost), a colon (:), and a sequence number (such as 0 and 4). The
  hostname of the display is the name of the computer where the X server
  runs. An omitted hostname means the local host. The sequence number is
  usually 0 -- it can be varied if there are multiple displays connected
  to one computer.

  If you ever come across a display indication with an extra .n attached
  to it, that's the screen number. A display can actually have multiple
  screens. Usually there's only one screen though, with number n=0, so
  that's the default.

  Other forms of DISPLAY exist, but the above will do for our purposes.

  For the technically curious:

  ·  hostname:D.S means screen S on display D of host hostname; the X
     server for this display is listening at TCP port 6000+D.

  ·  host/unix:D.S means screen S on display D of host host; the X
     server for this display is listening at UNIX domain socket
     /tmp/.X11-unix/XD (so it's only reachable from host).

  ·  :D.S is equivalent to host/unix:D.S, where host is the local
     hostname.


  5.  Telling the Client

  The client program (for instance, your graphics application) knows
  which display to connect to by inspecting the DISPLAY environment
  variable. This setting can be overridden, though, by giving the client
  the command line argument -display hostname:0 when it's started. Some
  examples may clarify things.

  Our computer is known to the outside as light, and we're in domain
  uni.verse. If we're running a normal X server, the display is known as
  light.uni.verse:0. We want to run the drawing program xfig on a remote
  computer, called dark.matt.er, and display its output here on light.

  Suppose you have already telnetted into the remote computer,
  dark.matt.er.

  If you have csh running on the remote computer:



       dark% setenv DISPLAY light.uni.verse:0
       dark% xfig &




  or alternatively:

       dark% xfig -display light.uni.verse:0 &




  If you have sh running on the remote computer:



       dark$ DISPLAY=light.uni.verse:0
       dark$ export DISPLAY
       dark$ xfig &




  or, alternatively:



       dark$ DISPLAY=light.uni.verse:0 xfig &




  or, of course, also:



       dark$ xfig -display light.uni.verse:0 &




  It seems that some versions of telnet automatically transport the
  DISPLAY variable to the remote host. If you have one of those, you're
  lucky, and you don't have to set it by hand. If not, most versions of
  telnet do transport the TERM environment variable; with some judicious
  hacking it is possible to piggyback the DISPLAY variable on to the
  TERM variable.

  The idea with piggybacking is that you do some scripting to achieve
  the following: before telnetting, attach the value of DISPLAY to TERM.
  Then telnet out.  At the remote end, in the applicable .*shrc file,
  read the value of DISPLAY from TERM.


  6.  Telling the Server

  The server will not accept connections from just anywhere. You don't
  want everyone to be able to display windows on your screen. Or read
  what you type -- remember that your keyboard is part of your display!

  Too few people seem to realise that allowing access to your display
  poses a security risk. Someone with access to your display can read
  and write your screens, read your keystrokes, and read your mouse
  actions.

  Most servers know two ways of authenticating connections to it: the
  host list mechanism (xhost) and the magic cookie mechanism (xauth).
  Then there is ssh, the secure shell, that can forward X connections.





  6.1.  Xhost

  Xhost allows access based on hostnames. The server maintains a list of
  hosts which are allowed to connect to it. It can also disable host
  checking entirely. Beware: this means no checks are done, so every
  host may connect!

  You can control the server's host list with the xhost program. To use
  this mechanism in the previous example, do:



       light$ xhost +dark.matt.er




  This allows all connections from host dark.matt.er. As soon as your X
  client has made its connection and displays a window, for safety,
  revoke permissions for more connections with:



       light$ xhost -dark.matt.er




  You can disable host checking with:



       light$ xhost +




  This disables host access checking and thus allows everyone to
  connect. You should never do this on a network on which you don't
  trust all users (such as Internet). You can re-enable host checking
  with:



       light$ xhost -




  xhost - by itself does not remove all hosts from the access list (that
  would be quite useless - you wouldn't be able to connect from
  anywhere, not even your local host).

  Xhost is a very insecure mechanism. It does not distinguish between
  different users on the remote host. Also, hostnames (addresses
  actually) can be spoofed. This is bad if you're on an untrusted
  network (for instance already with dialup PPP access to Internet).


  6.2.  Xauth

  Xauth allows access to anyone who knows the right secret. Such a
  secret is called an authorization record, or a magic cookie. This
  authorization scheme is formally called MIT-MAGIC-COOKIE-1.


  The cookies for different displays are stored together in
  ~/.Xauthority.  Your ~/.Xauthority must be inaccessible for
  group/other users.  The xauth program manages these cookies, hence the
  nickname xauth for the scheme.

  On starting a session, the server reads a cookie from the file that is
  indicated by the -auth argument. After that, the server only allows
  connections from clients that know the same cookie. When the cookie in
  ~/.Xauthority changes, the server will not pick up the change.

  Newer servers can generate cookies on the fly for clients that ask for
  it. Cookies are still kept inside the server though; they don't end up
  in ~/.Xauthority unless a client puts them there. According to David
  Wiggins:


       A further wrinkle was added in X11R6.3 that you may be
       interested in. Via the new SECURITY extension, the X server
       itself can generate and return new cookies on the fly. Fur­
       thermore, the cookies can be designated ``untrusted'' so
       that applications making connections with such cookies will
       be restricted in their operation. For example, they won't be
       able to steal keyboard/mouse input, or window contents, from
       other trusted clients. There is a new ``generate'' subcom­
       mand to xauth to make this facility at least possible to
       use, if not easy.


  Xauth has a clear security advantage over xhost. You can limit access
  to specific users on specific computers. It does not suffer from
  spoofed addresses as xhost does. And if you want to, you can still use
  xhost next to it to allow connections.


  6.2.1.  Making the Cookie

  If you want to use xauth, you must start the X server with the -auth
  authfile argument. If you use the startx script, that's the right
  place to do it. Create the authorization record as below in your
  startx script.

  Excerpt from /usr/X11R6/bin/startx:



       mcookie|sed -e 's/^/add :0 . /'|xauth -q
       xinit -- -auth "$HOME/.Xauthority"




  Mcookie is a tiny program in the util-linux package, primary site
  ftp://ftp.math.uio.no/pub/linux/.  Alternatively, you can use md5sum
  to massage some random data (from, for instance, /dev/urandom or ps
  -axl) into cookie format:



       dd if=/dev/urandom count=1|md5sum|sed -e 's/^/add :0 . /'|xauth -q
       xinit -- -auth "$HOME/.Xauthority"




  If you can't edit the startx script (because you aren't root), get
  your system administrator to set up startx properly, or let him set up
  xdm instead. If he can't or won't, you can make a ~/.xserverrc script.
  If you have this script, it is run by xinit instead of the real X
  server. Then you can start the real X server from this script with the
  proper arguments. To do so, have your ~/.xserverrc use the magic
  cookie line above to create a cookie and then exec the real X server:



       #!/bin/sh
       mcookie|sed -e 's/^/add :0 . /'|xauth -q
       exec /usr/X11R6/bin/X "$@" -auth "$HOME/.Xauthority"




  If you use xdm to manage your X sessions, you can use xauth easily.
  Define the DisplayManager.authDir resource in /etc/X11/xdm/xdm-config.
  Xdm will pass the -auth argument to the X server when it starts.  When
  you then log in under xdm, xdm puts the cookie in your ~/.Xauthority
  for you.  See xdm(1) for more information. For instance, my
  /etc/X11/xdm/xdm-config has the following line in it:



       DisplayManager.authDir: /var/lib/xdm





  6.2.2.  Transporting the Cookie

  Now that you have started your X session on the server host
  light.uni.verse and have your cookie in ~/.Xauthority, you will have
  to transfer the cookie to the client host, dark.matt.er.

  The easiest is when your home directories on light and dark are
  shared. The ~/.Xauthority files are the same, so the cookie is
  transported instantaneously.  However, there's a catch: when you put a
  cookie for :0 in ~/.Xauthority, dark will think it's a cookie for
  itself instead of for light.  You must use an explicit host name when
  you create the cookie; you can't leave it out.  You can install the
  same cookie for both :0 and light:0 with:



       #!/bin/sh
       cookie=`mcookie`
       xauth add :0 . $cookie
       xauth add "$HOST:0" . $cookie
       exec /usr/X11R6/bin/X "$@" -auth "$HOME/.Xauthority"




  If the home directories aren't shared, you can transport the cookie by
  means of rsh, the remote shell:



       light$ xauth nlist "${HOST}:0" | rsh dark.matt.er xauth nmerge -





  1. Extract the cookie from your local ~/.Xauthority (xauth nlist :0).

  2. Transfer it to dark.matt.er (| rsh dark.matt.er).

  3. Put it in the ~/.Xauthority there (xauth nmerge -).


  Notice the use of ${HOST}.  You need to transport the cookie that is
  explicitly associated with the local host.  A remote X application
  would interpret a display value of :0 as referring to the remote
  machine, which is not what you want!

  It's possible that rsh doesn't work for you. Besides that, rsh also
  has a security drawback (spoofed host names again, if I remember
  correctly). If you can't or don't want to use rsh, you can also
  transfer the cookie manually, like:



       light$ echo $DISPLAY
       :0
       light$ xauth list $DISPLAY
       light/unix:0 MIT-MAGIC-COOKIE-1 076aaecfd370fd2af6bb9f5550b26926
       light$ rlogin dark.matt.er
       Password:
       dark% setenv DISPLAY light.uni.verse:0
       dark% xauth add $DISPLAY . 076aaecfd370fd2af6bb9f5550b26926
       dark% xfig &
       [15332]
       dark% logout
       light$




  See also rsh(1) and xauth(1x) for more information.

  It may be possible to piggyback the cookie on the TERM or DISPLAY
  variable when you do a telnet to the remote host.  This would go the
  same way as piggybacking the DISPLAY variable on the TERM variable.
  See section 5: Telling the Client.  You're on own here from my point
  of view, but I'm interested if anyone can confirm or deny this.


  6.2.3.  Using the Cookie

  An X application on dark.matt.er, such as xfig above, will
  automatically look in ~/.Xauthority there for the cookie to
  authenticate itself with.


  There's a little wrinkle when using localhost:D.  X client
  applications may translate localhost:D into host/unix:D for the
  purpose of cookie retrieval.  Effectively, this means that a cookie
  for localhost:D in your ~/.Xauthority has no effect.


  6.3.  Ssh

  Authority records are transmitted with no encryption. If you're even
  worried someone might snoop on your connections, use ssh, the secure
  shell. It will do X forwarding over encrypted connections. And
  besides, it's great in other ways too. It's a good structural
  improvement to your system. Just visit http://www.cs.hut.fi/ssh/, the
  ssh home page.

  Who knows anything else on authentication schemes or encrypting X
  connections? Maybe kerberos?


  7.  X Applications from Another User-id

  Suppose you want to run a graphical configuration tool that requires
  root privileges.  However, your X session is running under your usual
  account.  It may seem strange at first, but the X server will not
  allow the tool to access your display.  How is this possible when root
  can normally do anything?  And how do you work around this problem?


  Let's generalise to the situation where you want to an X appliation
  under a user-id clientuser, but the X session was started by
  serveruser.  If you have read the section on cookies, it is clear why
  clientuser cannot access your display: ~clientuser/.Xauthority does
  not contain the right magic cookie for accessing the display.  The
  right cookie is found in ~serveruser/.Xauthority.


  7.1.  Different Users on the Same Host

  Of course, anything that works for remote X also works for X from a
  different user-id as well (particularly slogin localhost -l
  clientuser).  It's just that the client host and the server host
  happen to be the same.  However, when both hosts are the same, there
  are some shortcuts for transferring the magic cookie.


  We'll assume that you use su to switch user-ids.  Basically, what you
  have to do is write a script that will call su, but wraps the command
  that su executes with some code that does the necessary things for
  remote X.  These necessary things are setting the DISPLAY variable and
  transferring the magic cookie.


  Setting DISPLAY is relatively easy; it just means defining
  DISPLAY="$DISPLAY" before running the su command argument.  So you
  could just do:



       su - clientuser -c "env DISPLAY=$DISPLAY clientprogram &"





  This doesn't work yet, because we still have to transfer the cookie.
  We can retrieve the cookie using xauth list "$DISPLAY".  This command
  happens to list the cookie in a format that's suitable for feeding
  back to xauth; just what we need!  So we feed the obtained cookie back
  to xauth in the su command, set DISPLAY there, and run the command we
  want.



       su - clientuser -c "xauth add `xauth list $DISPLAY`; \
                           exec env DISPLAY=$DISPLAY clientprogram"






  You can write a script around this, parameterizing by clientuser and
  clientprogram.  Let's improve the script a little while we're at it,
  making it less readable but more robust.  It looks like this:



       #!/bin/sh
       if [ $# -lt 2 ]
       then echo "usage: `basename $0` clientuser command" >&2
            exit 2
       fi
       CLIENTUSER="$1"; shift
       exec su - "$CLIENTUSER" -c "xauth add `xauth list \"$DISPLAY\"`; \
                                   exec env DISPLAY='$DISPLAY' "'"$SHELL"'" -c '$*'"





  I think this is portable and works well enough in most circumstances.
  The only shortcoming I can think of right now is that, due to using su
  command argument ('$*').  If there's anything seriously wrong with it,
  please drop me an email.


  Call the script /usr/local/bin/xsu, and you can do:



       xsu clientuser 'command &'





  Easy, no?


  7.2.  Client User Is Root

  Obviously, anything that works for non-root client users is going to
  work for root as well.  However, with root you can make it even
  easier, because root can read anyone's ~/.Xauthority file.  There's no
  need to transfer the cookie.  All you have to do is set DISPLAY, and
  point XAUTHORITY to ~serveruser/.Xauthority.  So you can do:



       su - -c "exec env DISPLAY='$DISPLAY' \
                         XAUTHORITY='${XAUTHORITY-$HOME/.Xauthority}' \
                         command"





  Putting it into a script would give something like:









  #!/bin/sh
  if [ $# -lt 1 ]
  then echo "usage: `basename $0` command" >&2
       exit 2
  fi
  su - -c "exec env DISPLAY='$DISPLAY' \
                    XAUTHORITY='${XAUTHORITY-$HOME/.Xauthority}' \
                    "'"$SHELL"'" -c '$*'"





  Call the script /usr/local/bin/xroot, and you can do:



       xroot 'control-panel &'





  Even easier, no?


  8.  Running a Remote Window Manager

  A window manager (like twm, wmaker, or fvwm95) is an application like
  any other.  The normal procedure should work.


  Well, almost.  At most one window manager can be running on a display
  at any time.  If you are already running a local window manager, you
  cannot start the remote one (it will complain and exit).  You have to
  kill (or simply quit) the local one first.


  Unfortunately, many X session scripts end with an



       exec window-manager-of-choice




  and this means that when the (local) window manager exits, your
  session exits, and the X system (xdm or xinit) considers your session
  over and effectively logs you out.


  You have to jump through a few extra hoops, but it can be done and
  it's not too difficult.  Just play with your session script (normally
  ~/.xsession or ~/.xinitrc) to get it as you want it.


  Beware that a window manager often provides ways to run new programs,
  and that these will run on the local machine.  That is, local to where
  the window manager runs.  If you run a remote window manager, it will
  spawn remote applications, and this may not be what you want.  Of
  course, they still display on the display that is local to you.




  9.  Troubleshooting

  The first time you try to run a remote X application, it usually does
  not work. Here are a few common error messages, their probable causes,
  and solutions to help you on your way.



       xterm Xt error: Can't open display:




  There is no DISPLAY variable in the environment, and you didn't tell
  the application with the -display flag either. The application assumes
  the empty string, but that is syntactically invalid. To solve this, be
  sure that you set the DISPLAY variable correctly in the environment
  (with setenv or export depending on your shell).



       _X11TransSocketINETConnect: Can't connect: errno = 101
       xterm Xt error: Can't open display: love.dial.xs4all.nl:0




  Errno 101 is ``Network is unreachable''.  The application could not
  make a network connection to the server.  Check that you have the
  correct DISPLAY set, and that the server machine is reachable from
  your client (it should be, after all you're probably logged in to the
  server and telnetting to the client).



       _X11TransSocketINETConnect: Can't connect: errno = 111
       xterm Xt error: Can't open display: love.dial.xs4all.nl:0




  Errno 111 is ``Connection refused''.  The server machine you're trying
  to connect to is reachable, but the indicated server does not exist
  there. Check that you are using the right host name and the right
  display number.



       Xlib: connection to ":0.0" refused by server
       Xlib: Client is not authorized to connect to Server
       xterm Xt error: Can't open display: love.dial.xs4all.nl:0.0




  The client could make a connection to the server, but the server does
  not allow the client to use it (not authorized). Make sure that you
  have transported the correct magic cookie to the client, and that it
  has not expired (the server uses a new cookie when a new session
  starts).






