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Remote X Apps mini-HOWTO
Vincent Zweije, zweije@xs4all.nl
v0.7.5, 8 December 2001
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, and
information on setting up a computer as an X terminal.
______________________________________________________________________
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
6.2.2.1 Shared Home Directories
6.2.2.2 By the Remote Shell,
6.2.2.3 Manually, by Telnet
6.2.2.4 Automating the Telnet Way
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. Setting Up an X Terminal
9.1 Once More, a Little Theory First
9.2 Configuring
9.3 Configuring
9.4 XDMCP Technically
10. 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 0.7.5. 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
released under version 1.1 of the GNU Free Documentation Licence.
Contents last updated on 8 December 2001 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 and confirmed as a good source of information. However, it
has not been revised since its original publication in 1992. As such
it only covers X11R4 and X11R5, anything specific to X11R6 will not be
covered.
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://www.vigor.nu/dxpc/ or LBX
http://www.paulandlesley.org/faqs/LBX-HOWTO.html
<http://www.paulandlesley.org/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:
<20> DISPLAY=light.uni.verse:0
<20> DISPLAY=localhost:4
<20> 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:
<20> 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.
<20> 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).
<20> :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.
Notice that some X servers (from XFree86) can be configured not to
listen on the usual TCP port with the -nolisten tcp argument. Notably
the default configuration of Debian GNU/Linux is to disable the X
server listening on the TCP port. If you wish to use remote X on a
Debian system, you should re-enable this by altering the way the X
server is started. Look at /etc/X11/xinit/xserverrc for a start.
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.
You can specify a different cookie file with the XAUTHORITY
environment variable, but you will rarely need this. If you're not
sure which cookie file your xauth is using, do an xauth -v, and it
will tell you.
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<75>
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<6F>
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. There are
several ways to do this.
6.2.2.1. Shared Home Directories
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 this little piece of sed
wizardry:
#!/bin/sh
mcookie|sed -e 's/^/add :0 . /' -e p -e "s/:/$HOST&/"|xauth -q
exec /usr/X11R6/bin/X "$@" -auth "$HOME/.Xauthority"
6.2.2.2. By the Remote Shell, rsh
If the home directories aren't shared, you can transport the cookie by
means of rsh, the remote shell:
light$ 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 ${HOST}. You need to transport the cookie that is
explicitly associated with the local host. A remote X application
would interpret a display value of :0 as referring to the remote
machine, which is not what you want!
6.2.2.3. Manually, by Telnet
It's possible that rsh doesn't work for you. Besides that, rsh also
has a security drawback (spoofed host names again, if I remember
correctly). If you can't or don't want to use rsh, you can also
transfer the cookie manually, like:
light$ 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
Using authority file /home/zweije/.Xauthority
xauth> add light.uni.verse:0 . 076aaecfd370fd2af6bb9f5550b26926
xauth> exit
Writing authority file /home/zweije/.Xauthority
dark% xfig &
[15332]
dark% logout
light$
See also rsh(1) and xauth(1x) for more information.
6.2.2.4. Automating the Telnet Way
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.
Notice, however, that environment variables can be observed by others
on some unices, and you won't be able to prevent the cookie in $TERM
from showing up if people are looking for it.
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 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.
If you think about it, it's only logical. The interpretation of
localhost depends entirely on the machine on which it's interpreted.
It would give a horrible mess when you have a shared home directory,
such as through NFS, with several hosts all interfering with each
other's cookies.
6.3. Ssh
Authority records are transmitted over the network with no encryption.
If you're even worried someone might snoop on your connections, use
ssh, the secure shell. It can do X forwarding over encrypted
connections.
To turn on X forwarding over ssh, use the command line switch -X or
write the following in your local ssh configuration file:
Host remote.host.name
ForwardX11 yes
The ssh server (sshd) at the remote end automatically sets DISPLAY to
point to its end of the X forwarding tunnel. The remote tunnel end
gets its own cookie; the remote ssh server generates it for you and
puts it in ~/.Xauthority there. So, X authorisation with ssh is fully
automatic.
By the way, ssh is great in other ways too. It's a good structural
improvement to your system. For more information, visit
http://www.ssh.org/, 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 the xauth add command; just what we need!
We shall want to pass the cookie through a pipe. Unfortunately, it
isn't easy to pass something through a pipe to the su command, because
su wants to read the password from its standard input. Fortunately
again, in a shell script we can joggle some file descriptors around,
and get it done.
So we 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="$1"
shift
# FD 4 becomes stdin too
exec 4>&0
xauth list "$DISPLAY" | sed -e 's/^/add /' | {
# FD 3 becomes xauth output
# FD 0 becomes stdin again
# FD 4 is closed
exec 3>&0 0>&4 4>&-
exec su - "$CLIENTUSER" -c \
"xauth -q <&3
exec env DISPLAY='$DISPLAY' "'"$SHELL"'" -c '$*' 3>&-"
}
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 ('$*'). If there's anything else seriously wrong
with it, please drop me an email.
Call the script /usr/local/bin/xsu, and you can do:
xsu clientuser 'command &'
Can't be much easier, unless you get rid of the password. Yes, there
are ways for that too (sudo), but this is not the place for that.
The tiny xsu script just mentioned has served as the basis for a more
extended script called sux which apparently has found its way as a
package into the Debian distribution.
7.2. Client User Is Root
Obviously, anything that works for non-root client users is going to
work for root as well. However, with root you can make it even
easier, because root can read anyone's ~/.Xauthority file. There's no
need to transfer the cookie. All you have to do is set DISPLAY, and
point XAUTHORITY to ~serveruser/.Xauthority. So you can do:
su - -c "exec env DISPLAY='$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 &'
Although, if you've set up xsu already, there's no real reason to do
this.
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. Setting Up an X Terminal
Make use of your old PC! Turn it into an extra work place! No need
for buying expensive new hardware! You've already got all it takes!
Seriously, you can set up an old PC as an X terminal. An X terminal
is a computer that basically runs nothing but an X server. You can
log in on it, and get an X session, with xterms, xbiff, xclock, every
other conceivable X client. However, all clients are running on a
remote host, and are using remote X to display their output on your
local X terminal. Even the window manager is running remotely.
An X terminal takes very few resources, compared to a full blown unix
machine. Over here I have an X terminal with a 486 CPU, 16M of RAM,
and 250M of disk space. Oh, and a network connection, of course. It
doesn't even have user home directories.
For some related reading, have a look at:
<20> The XDM and X Terminal mini-HOWTO
(http://www.ibiblio.org/pub/Linux/docs/HOWTO/mini/other-
formats/html_single/XDM-Xterm.html). This document is an extensive
description of what is possible with XDMCP and xdm, applied for
building X terminals. Definitely have a look at this.
<20> The XDMCP HOWTO
(http://www.ibiblio.org/pub/Linux/docs/HOWTO/other-
formats/html_single/XDMCP.html). This document describes the steps
necessary to set up xdm for use with remote X servers, such as from
X terminals. The setup of the X server in such a situation is
described less extensively.
<20> The Xterminal mini-HOWTO
(http://metalab.unc.edu/pub/Linux/docs/HOWTO/unmaintained/mini/Xterminal).
It is currently unmaintained, but it might contain some useful
information for you.
Contrasted to the above documents, this document (the Remote X Apps
mini-HOWTO) limits itself to a short description of XDMCP, but puts
more emphasis on the security issues involved.
9.1. Once More, a Little Theory First
As far as X is concerned, the X terminal will be running nothing but
an X server. This X server will be configured to talk to a remote
host using XDMCP (the X Display Manager Control Protocol). It will
ask the remote host for an X session. The remote host will put up a
login window on the X terminal, and after login it will run an X
session with all bells and whistles, including the window manager, all
using remote X to display on the X terminal.
You will probably notice that the remote host is acting like a server,
though not an X server. The remote host is providing X sessions to X
servers that ask for one. So, with respect to XDMCP, the remote host
is actually a server, providing X sessions, also known as an XDMCP
server. The X server is playing the role of an XDMCP client! Are you
still with me?
The program that provides the XDMCP service on the XDMCP server is
xdm. So, in order to get an X terminal up and running, you must
configure two programs: X (the XDMCP client) on the X terminal, and
xdm (the XDMCP server) on the remote host.
You must always remember that the X protocol (and the XDMCP protocol)
are not encrypted. If you use remote X, everything that goes over the
network can be sniffed by other hosts on the network. This is
especially bad with remote X sessions, since the first thing that
happens is logging in by giving a username and password. So, you must
run remote X over a trusted network only!
9.2. Configuring X as an XDMCP Client
If you want to set up a Linux machine as an X terminal, you need very
few resources. Basically, you need what it takes to get a bare bones
Linux machine running, plus an X server. Specifically, you do not
need the X clients and libraries. It can be useful to install some X
fonts, but you can also use a font server somewhere on the network.
There are a few ways for an X server to get an X session from an XDMCP
server. The simplest one is to go straight to a known XDMCP server
and ask for one. Alternatively, the X server can broadcast a request
for an XDMCP service and use the first XDMCP server that responds.
Lastly, the X server can go to an XDMCP server, ask it for a list of
hosts willing to provide a session, and let the user choose a session
host.
1. When you know the host that is going to provide you with sessions,
go straight to it. Run
X -query sessionhost
and, assuming xdm is running on sessionhost, you'll get a login
window, and after login, an X session.
2. When you don't really care on which host you're getting your
session, use the broadcast method. Run
X -broadcast
and, assuming xdm is running somewhere on the network, you'll get a
login window from the first (and hopefully quickest) xdm that
responds, and after login, an X session.
3. When you want to choose the host where you want to have your
session, ask an XDMCP server for a list. Run
X -indirect xdmcpserver
and, assuming xdm is configured right there, you'll be presented a
list of hosts to choose from. Choose one; you'll get the login window
for that host, and after login, the session you were looking for.
You may have noticed the absence of the -auth option. The X server
will use XDMCP to negotiate a magic cookie with the XDMCP server. The
XDMCP server will put the cookie in your remote ~/.Xauthority after
login.
After a session is over, the X server will loop and go back to the
original XDMCP server and ask for another session (or chooser list).
If you don't want that, you can use the -once option. Note: this
doesn't seem to work with the -indirect option due to the
implementation of the chooser.
When you have determined the way in which you want to run the X
server, you can also put it in a startup script, or even run it
straight from /etc/inittab. Please consult your own distribution's
documentation for how to modify your startup scripts or /etc/inittab.
Do not run an X server like this from the Xservers configuration file.
xdm expects to be able to connect to such servers, and may kill them
if it can't connect.
9.3. Configuring xdm as an XDMCP Server
The program that provides the XDMCP service (the session service) is
usually xdm. There are variants of this such as wdm or gdm on Linux,
but these basically work the same way. So, make sure xdm or variant
is installed on the host where you want to run your X sessions. If
you've got a local graphical login on the X session host, xdm is
already installed; most Linux distributions come that way these days.
In addition to xdm, you will need the programs that you wish to be
able to run in an X session. That is, all X clients like xterm, xfig,
xclock, window managers and all that. However, for an XDMCP server,
you do not have to install an X server; the X server will be running
on the X terminal instead.
From the X server story above, you can conclude that there are
basically two kinds of XDMCP service. There is the direct service,
consisting of letting an XDMCP client log in, and then providing it
with an X session. Alternatively, there is the indirect service, in
which an XDMCP client is provided with a list of hosts, providing a
direct service, to choose from.
All xdm services are configured in the access file, generally located
at /etc/X11/xdm/Xaccess or a similar location. This location is
actually defined in the general xdm configuration file
/etc/X11/xdm/xdm-config, through the accessFile resource. See your
xdm manual for the default location.
1.
If you want to allow xdm to provide connecting XDMCP clients with
an X session, whether by broadcast or not, you put the host name of
the XDMCP client (the X server, remember?) by itself on a line in
Xaccess. Actually, you can put a pattern on the line matching
multiple hosts. Here are some valid patterns:
xterm023.my.domain # xterm023.my.domain can get an X session
*.my.domain # any host in my.domain can get an X session
* # any host on Internet can get an X session (unsafe)
Whether you should want to provide any host in Internet with an X
session is arguable. Obviously, any service you provide is one more
possible hole in your server's security. On the other hand, the
server should be secure itself, and an XDMCP client asking for an X
session has to provide a valid authentication before the X session is
granted.
Furthermore, the X session uses a remote X connection, which is not
encrypted. The username/password pair for the login will be
transported on this connection. People out there could be sniffing
valid username/password combinations, just as with plain telnet
connections. This is even worse then having xauth magic cookies
sniffed.
Make your own decisions here, but I recommend not enabling this
service to the world unless you have a good reason.
2.
If you want to provide XDMCP clients (X -indirect xdmcpserver) with
a chooser list (a list of hosts to choose from to get an X
session), follow the client pattern with the keyword CHOOSER and
the list of hosts that that client may choose from. Instead of the
list of hosts to choose from, you can also specify BROADCAST; with
this, xdm broadcasts on the network to query for servers willing to
provide the session. Some valid examples:
xterm023.my.domain CHOOSER seshost1 seshost2
*.my.domain CHOOSER BROADCAST
* CHOOSER extseshost1 extseshost2
The first lets xterm023 choose between sessions on either seshost1 and
seshost2. The second lets any host in my.domain choose from any host
that is willing to provide an X session. The third lets any host out
there choose between a session on extseshost1 or etsseshost2.
It is probably not a good idea to do * CHOOSER BROADCAST. This will
allow hosts outside your network to get information about the hosts
inside your network. You probably don't want to pass out such
information. In fact, allowing a chooser to any outside host is
probably not useful anyway, since you should not be enabling arbitrary
direct connections either.
When you have reconfigured xdm, send it the HUP signal to make it re-
read its configuration files.
# kill -HUP pid-of-xdm
#
9.4. XDMCP Technically
Technically, as far as I can see, XDMCP is not entirely what you would
expect from the above description. xdm can redirect connecting X
servers to another place, and uses this trick to implement the
chooser. So, the choosing happens inside xdm, not in the X server,
although the chooser list is represented on the X server's display.
This is also why the X server's -once option does not combine with
-indirect.
10. 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.
Alternatively, it is possible that the X server was configured not to
listen to the usual TCP port. To find out if this is the case, see if
the X server is started with the -nolisten tcp argument, and if so,
remove it.
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).
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