Remote-X-Apps.pdf

Remote X Apps mini−HOWTO

Table of Contents

Remote X Apps mini−HOWTO. ........................................................................................................................1

Vincent Zweije, zweije@xs4all.nl. ........................................................................................................1

1. Introduction. .........................................................................................................................................1

2. Related Reading. ..................................................................................................................................1

3. The Scene. ............................................................................................................................................1

4. A Little Theory. ...................................................................................................................................1

5. Telling the Client. ................................................................................................................................1

6. Telling the Server. ................................................................................................................................1

7. X Applications from Another User−id. ...............................................................................................1

8. Running a Remote Window Manage. .................................................................................................1

9. Setting Up an X Terminal. ...................................................................................................................1

10. Troubleshooting. ................................................................................................................................2

1. Introduction. .........................................................................................................................................2

2. Related Reading. ..................................................................................................................................2

3. The Scene. ............................................................................................................................................3

4. A Little Theory. ...................................................................................................................................3

5. Telling the Client. ................................................................................................................................4

6. Telling the Server. ................................................................................................................................4

6.1 Xhost. .................................................................................................................................................5

6.2 Xauth. .................................................................................................................................................5

Making the Cookie. .....................................................................................................................6

Transporting the Cookie. ............................................................................................................7

Shared Home Directories. ...........................................................................................................7

By the Remote Shell, rsh. ...........................................................................................................7

Manually, by Telnet. ...................................................................................................................7

Automating the Telnet Way. .......................................................................................................8

Using the Cookie. .......................................................................................................................8

6.3 Ssh. .....................................................................................................................................................8

7. X Applications from Another User−id. ...............................................................................................9

7.1 Different Users on the Same Host .....................................................................................................9

7.2 Client User Is Root. .........................................................................................................................10

8. Running a Remote Window Manage. ...............................................................................................11

9. Setting Up an X Terminal. .................................................................................................................11

9.1 Once More, a Little Theory First. ....................................................................................................12

9.2 Configuring X as an XDMCP Client. ..............................................................................................12

9.3 Configuring xdm as an XDMCP Server. .........................................................................................13

9.4 XDMCP Technically .......................................................................................................................14

10. Troubleshooting. ..............................................................................................................................15

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.

1. Introduction

2. Related Reading

3. The Scene

4. A Little Theory

5. Telling the Client

6. Telling the Server

· 6.2 Xauth

6.1 Xhost

· 6.3 Ssh

7. X Applications from Another User−id

· 7.2 Client User Is Root

7.1 Different Users on the Same Host

8. Running a Remote Window Manager

9. Setting Up an X Terminal

· 9.2 Configuring X as an XDMCP Client

9.1 Once More, a Little Theory First

· 9.3 Configuring xdm as an XDMCP Server

· 9.4 XDMCP Technically

Remote X Apps mini−HOWTO

Remote X Apps mini−HOWTO

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 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 .

10. Troubleshooting

Remote X Apps mini−HOWTO

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=localhost:4

DISPLAY=light.uni.verse:0

· 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.

3. The Scene

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