How to Run Remote X Applications

How to Run Remote X Applications

Post by Vincent Zweij » Wed, 26 Feb 1997 04:00:00

                       How to Run Remote X Applications


   This is (supposed to be) a guide how to do remote X applications. I
   have written this text for several reasons.

    1. Many questions have appeared on how to run a remote X application
       on usenet.
    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 an simple document that describes the options you
       _do_ have. Please [1]inform me if you know more.

   Version 0.1. 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.

   This document is available on [2]WWW and posted irregularly to

    Contents last updated on 24 February 1997 by [4]Vincent Zweije.


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 [5]dxpc
   or lbx.

     _NOTE:_ Who has a pointer on lbx?

    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.

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.

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

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, and display its output here on light.

   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 &

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

   Most servers know two ways of authenticating connections to it: the
   xhost mechanism and the magic cookie mechanism.


   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

   This allows all connections from host When you want to
   revoke the permission, do:

     light$ xhost

   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 an insecure network. You can
   re-enable host checking with:

     light$ xhost -

     _WARNING:_ 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).


   Xauth allows access to anyone who knows the right "secret". Such a
   secret is called an authorization record, or a magic cookie. The
   cookies for different displays are stored together in ~/.Xauthority.
   Your ~/.Xauthority must be inaccessible for group/other users.

   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. If you don't have urandom, use some other means of
   generating random data. ps -axl will probably do.

   Excerpt from /usr/X11R6/bin/startx:

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

   This is a stripped down version of my startx. See startx(1x),
   xinit(1x), xauth(1x), md5sum(1) to customise your startx script.

     _NOTE:_ If xauth complains about an illegal add line, your version
     of md5sum may be appending a dash (-) to the checksum to indicate
     that it was computed over its standard input. If you can't convince
     md5sum to forget about the dash, you can strip it with sed. Change
     the argument to the sed command as below. In fact, this ought to
     work with non-dash-appending md5sums as well, but it's less
     understandable. _(Thanks Jeffrey)_

     ... 's/^\([0-9a-f]*\).*$/add :0 . \1/' ...

   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, and
   all will be well. 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

   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, You can do
   this with the following command:

     light$ xauth nlist :0 | rsh xauth nmerge -

    This says:
    1. Extract the authorization record from your local ~/.Xauthority
       (xauth nlist :0).
    2. Transfer it to (| rsh
    3. Put it in the ~/.Xauthority there (xauth nmerge -).

   It's possible that rsh doesn't work for you this way. You can also
   transfer the cookie manually, like:

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

   See xauth(1x) for more information.

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

   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.


   <PLUG CLASS="shameless">
   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 the [6]ssh home page.

     _NOTE:_ Who knows anything else on authentication schemes or
     encrypting X connections?




1. cannot run X-application from remote Unix


I am new to Linux and X-windows, and would appreciate it if somebody
could tell me
how to run a X-application from a remote Unix.

I have included the remote Unix host in the /etc/host.allow file.
However, when I run
X-application in remote Unix (through telnet), I got the error messages,

"Xlib: connection to 'my host:0.0' refused by server"
"Xlib: client is not authorized to connect to Server"
"Cannot open display 'my host:0.0' "
where 'my host' is my Linux IP address.

Thank you very much



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