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| Newer page: | version 3 | Last edited on Sunday, November 28, 2004 1:05:31 am | by AristotlePagaltzis | Revert |
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-
-
-
-Remote X Apps mini-HOWTO
-
-
-
-----
-
-!!!Remote X Apps mini-HOWTO
-
-!!
-Vincent Zweije,
-zweije@xs4all.nlv0.7.5, 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.''
-----
-
-
-
-
-!!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
-
-*6.3 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 X as an XDMCP Client
-
-*9.3 Configuring xdm as an XDMCP Server
-
-*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.
-
-
-# Many questions have appeared on usenet on how to run a remote
-X application.
-#
-
-# 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.
-#
-
-# 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 .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 (also
-known as the
-LBX mini-HOWTO).
-
-
-You must do two things to achieve all this:
-
-
-
-
-
-# Tell the local display (the server) to accept connections from
-the remote computer.
-
-#
-
-# 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:
-
-*
-
-* DISPLAY=localhost:4
-
-*
-
-* DISPLAY=:
-
-*
-
-
-
-The display consists of a hostname (such as light.uni.verse and
-localhost), a colon (:), and a sequence number (such as
-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=, 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:. 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:
-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:
-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. Furthermore, 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'' subcommand 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.
-
-
-
-
-!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
-
-
-
-
-
-
-!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.
-
-
-
-
-!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 "$HOME/.Xauthority"
-
-
-
-
-
-
-!By the Remote Shell, rsh
-
-
-If the home directories aren't shared, you can transport the cookie by
-means of rsh, the remote shell:
-
-
-
-
-
-light$ xauth nlist "${HOST}:" | rsh dark.matt.er xauth nmerge -
-
-
-
-
-
-
-
-# Extract the cookie from your local ~/.Xauthority
-(xauth nlist :).
-
-#
-
-# Transfer it to dark.matt.er (| rsh dark.matt.er).
-
-#
-
-# Put it in the ~/.Xauthority there (xauth nmerge -).
-
-#
-
-
-
-
-
-
-Notice the use of ${HOST}. You need to transport the cookie that is
-explicitly associated with the local host. A remote X application would
-interpret a display value of :0 as referring to the remote machine,
-which is not what you want!
-
-
-
-
-!Manually, by Telnet
-
-
-It's possible that rsh doesn't work for you. Besides that, rsh also has a
-security drawback (spoofed host names again, if I remember correctly). If
-you can't or don't want to use rsh, you can also transfer the cookie
-manually, like:
-
-
-
-
-
-light$ echo $DISPLAY
-:
-light$ xauth list $DISPLAY
-light/unix:0 MIT-MAGIC-COOKIE-1 076aaecfd370fd2af6bb9f5550b26926
-light$ rlogin dark.matt.er
-Password:
-dark% setenv DISPLAY light.uni.verse:
-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.
-
-
-
-
-!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.
-
-
-
-
-!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 $` clientuser command" >&2
-exit 2
-fi
-CLIENTUSER="$1"
-shift
-# FD 4 becomes stdin too
-exec 4>&
-xauth list "$DISPLAY" | sed -e 's/^/add /' | {
-# FD 3 becomes xauth output
-# FD 0 becomes stdin again
-# FD 4 is closed
-exec 3>&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
-'$*', single quotes in command will mess up quoting in the
-su command argument ('$*'). 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 $` command" >&2
-exit 2
-fi
-su - -c "exec env DISPLAY='$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:
-
-
-
-
-
-*
-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.
-
-*
-
-*
-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.
-
-*
-
-*
-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.
-
-
-
-
-
-#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.
-
-#
-
-#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.
-
-#
-
-#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.
-
-
-
-
-
-#
-
-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.
-
-
-
-
-#
-
-#
-
-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:
-
-
-
-
-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:
-
-
-
-
-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 ":." refused by server
-Xlib: Client is not authorized to connect to Server
-xterm Xt error: Can't open display: love.dial.xs4all.nl:.
-
-
-
-
-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)
.
-
-
-
-----
+Describe
[HowToRemoteXApps
] here.
