Access to Unix Systems using X Windows

The methods you can use to login to CUS and Thor depend on what type of terminal you are using; specifically, whether you are running an X-server program such as eXceed, XWin etc. If you are not running an X-server, then you need a text-based login method as described in "Access to Unix systems from text terminals"

If you are using an X server on your own machine, please see "X Server Configuration" and ensure that your system is configured correctly; many X servers are supplied with a configuration which is inherently insecure.

X Servers

On the PWF, eXceed is provided for logging onto CUS and Thor. See Leaflet M521: "An Introduction to Vista-eXceed - a DIY course".

For personal Linux boxes, the simplest way to log on using XFree86 is to use X -query hostname on the command line, or to configure xdm to automatically start the X server.

Running X-clients on other machines

There are two things you need to do to display X-clients from other machines on one display.

Firstly, you must give the machine on which you wish to run the X-client the 'key' to the X-server. That will allow the client to prove that it has the authority to connect to the X-server. Note that you should not use the xhost command to communicate this authority; the xauth command should be used instead, as described below. Copying the key across is done using xauth:
```
xauth extract - $DISPLAY | rsh otherhost.cam.ac.uk xauth merge -
```
Secondly, you need to tell the X-client which X-server to display on. This is done by setting DISPLAY on the remote machine. The following are equivalent:
```
rsh otherhost.cam.ac.uk DISPLAY=$DISPLAY Mosaic
xon otherhost.cam.ac.uk Mosaic
```

This can go wrong if DISPLAY is set to just ":0", in which case you should prepend the hostname of the X-server. If the PATH on the remote end is not set correctly, try replacing the second "xauth" with "/usr/bin/X11/xauth". If you get the error "permission denied", you need to setup your .rhosts file (see "Configuring .rhosts").

If you happen to be using a Linux machine, life is much easier: use the Computing Service's replacement for xon: xcmd. It contains both xauth and xon in one program, sets the DISPLAY correctly and gives better help when errors happen. See Access to Thor from Personal Linux Boxes or view the xcmd source.

xhost

It's worth putting in a section about xhost, to explain why it is such a bad thing.

xhost +hostname allows access to all users on that host, and
xhost + (without a hostname) allows access to all users throughout the entire Internet.

This is a Bad Thing. Giving other users access means they can read what you're writing (eg. so they can steal Thesis or see what your favourite Web pages are) and they can read your keyboard (so you can't type passwords safely).

Last updated by Peter Benie <pjb1008@cam.ac.uk>
unix-support@ucs.cam.ac.uk
[Back to index]