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| University of Cambridge > Engineering Department > computing help |
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Note: This page is long because it deals with many scenarios. For any particular user/situation the procedure isn't too tricky, though depending on your machine you may initially need to install and configure some free software When making our machines available to users beyond our walls we need to balance ease of use against security considerations. Several options exist. Your choice of method to use depends on what you want to access the machines for, how far away you are, and the type of machine you have. You may well have to install extra software on your machine. In particular you're likely to need programs that are "ssh-enabled" - "ssh" being short for "secure shell", a mechanism that provides the extra security needed nowadays. Once you've installed and configured the software it's possible to use our machine as easily remotely as locally for most purposes, though speed may be an issue for graphics-intensive programs. Note that the range of methods may change at short notice in response to security alerts. |
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Most of our machines are hidden from the outside world. If you want to access the Teaching System from outside the University you're currently restricted to getting in using gate - our "gateway" machine. Note that gate doesn't have many programs installed, so you might want to use slogin to access a linux server from gate.
If you want to access the Teaching System from within CUED you can use
Somehow you'll have to connect your machine to the internet. You can use a standard phone line (see Accessing CUED and the Internet via modem for options) or broadband if your machine isn't already networked.
If you're local, and you have problems with commercial broadband or domestic networks, see Robin Walker's Cable Modem Troubleshooting Tips.
You can use the Departmental license server to run MATLAB or Abaqus on your computer in college or in a private (non-college) residence. See
 | If you want to run programs like Octave or would like to practice C++ on your own PC you can get a set-up that's much like the one in the DPO by getting the Multidisciplinary Design Project (MDP) disk |
We have thousands of files on the WWW. Most of them are visible to everyone, but a few resources are protected. They're protected in one of 2 ways
The programs mentioned below are those most commonly used by students from college rooms, etc. They can be customised to make remote login a matter of pressing a button or two (rather than typing the command lines mentioned below), but you might first need to configure your set-up.
There are several ways to remotely access our machines. The situation's further complicated by the range of user machines and by various security issues. If the instructions below don't work for you, read the Troubleshooting and Technical Information sections. The latter presents some of the issues you'll need to be aware of if something goes wrong.
Gdk-ERROR **: BadWindow (invalid Window parameter)
serial 1260 error_code 3 request_code 38 minor_code 0
Gdk-ERROR **: BadAccess (attempt to access private resource denied)
serial 1261 error_code 10 request_code 102 minor_code 0
then upgrade to OSX10.5 if you can.
slogin abc123@gate.eng.cam.ac.uk
slogin ts-access
vncserverIt will reply with something like
New 'X' desktop is scimitar:4 Starting applications specified in /users/s/abc123/.vnc/xstartup Log file is /users/s/abc123/.vnc/scimitar:4.log
ssh -N -f -L 5904:scimitar.eng.cam.ac.uk:5904 abc123@gate.eng.cam.ac.uk(it's 5904 because the desktop is number 4)
Use Xwin32 (30 day free license), XMing (free and optionally includes an enhanced PuTTY Link SSH client and a portable PuTTY replacement package), or VNC for graphical access.
If you want to want to login to the Teaching System from within CUED, you can use the Xwin version that that CUED's purchased and follow these instructions
slogin abc123@gate.eng.cam.ac.uk(replace abc123 by your CRS ID)
slogin ts-access
vncserverIf you're doing for the first time you'll be asked to choose a VNC password. It will reply with something like
New 'X' desktop is scimitar:3 Starting applications specified in /users/s/abc123/.vnc/xstartup Log file is /users/s/abc123/.vnc/scimitar:3.log
ssh -N -f -L 5903:ts-access.eng.cam.ac.uk:5903 abc123@gate.eng.cam.ac.uk
(it's 5903 because the desktop is number 3)
vncviewer localhost:3
(it's :3 because the desktop is number 3). A desktop should appear
vncserver -kill :3
(it's :3 because the desktop is number 3).
Lab sessions are often started by clicking on
the icon in the taskbar. If this isn't available you can type start from a terminal window instead - you'll be shown a list of options.
Failure or unreliabilty could be due to problems on your machine, on CUED machines, or anywhere between. Problems aren't always easy to diagnose, so if you report a problem to help, tell us the operating system you're using, the method of access you're using, which CUED machines you're using, when you had the problem, and what programs you were running.
See net news for reports of University network trouble.
Access to CUED from certain colleges might be prone to problems because they use NAT-based firewalls. If you're trying to connect using ssh from Jesus for example, you may find that it helps to use
ssh -o 'ServerAliveInterval 60' -X ...or with newer vesions of ssh
ssh -o 'ServerAliveInterval 60' -Y ...
New 'X' desktop is harrier:4
X windows programs (like xclock and the programs on the Teaching System, etc) don't put graphics directly onto the screen. They need to have another program - an X server - running. Programs like xclock tell the X server what to do using commands that are part of the X protocol. These commands might be just a few bytes long, almost as simple as "draw a line from (10,10) to 30,10)". The X server interprets these commands, using the current settings of line-thickness, color, etc to produce graphics. Note that xclock and the X server needn't be on the same machine.
This approach is flexible network-wise, and remote working is simple, but the X server's quite a complicated program. The protocol messages might be compact for some actions, but bulky for others.
An alternative strategy is used by VNC, which essentially keeps copying screendumps over from a virtual screen on the remote machine to the screen you're sat at. It does this efficiently, so that only modified areas of the screen are copied over. An extra program needs to run on the distant machine and on the local machine, but these needn't be too complex. Because VNC only deals with the end product - the graphics - it doesn't care how the graphics are produced. Some kinds of graphics will produce more network traffic with X11 than with VNC, and some less.
VPN (Virtual Private Network) provides secure access over public telecommunications infrastructure. When you have successfully created a VPN connection to the department you should then be able to connect to internal local resources as if your machine were on the departmental network.
Some of our machines are only available from within CUED. Some are only available within the cam.ac.uk domain. You need to be aware of these restrictions when trying to work remotely. Note also that gate is a general name for more than one machine - if you access gate twice you may be using 2 different machines.
Machines communicate by sending messages to numbered input 'sockets' (called 'ports'). VNC communicates on "TCP ports 5900-5906". These are often blocked by firewalls (firewalls are security programs that might be on your machine or might be run by the college). The trick is to use "ssh tunnelling". ssh encrypts information and uses port 22, which is rarely blocked, to "tunnel" through the firewall. This means that if you can use ssh to get into a machine, you can use VNC. The encryption involves an overhead. The less that's transmitted, the less that needs coding and decoding.
Both X11 and VNC approaches should only be used with encryption.
Do you want the remote machine's output to take over your whole screen, or should the remote screen be a window on the local desktop? Sometimes you have no choice. Both options have their problems.
There may be situations where ssh/slogin has doubts about whether the machine claiming to be (say) gate.eng.cam.ac.uk really is that machine. The warning message might mention .ssh/known_hosts:line-number for example. There's usually an innocent reason for this. Removing the line in question should solve the problem but if you want to check on authenticity look at our Certificates and keys for secure connections page.
See also Wikipedia's Comparison of remote desktop software
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