Department of Computer Science and Technology

Frequently asked questions

Does the lab have a ...

Video Conference Facility

See the Videoconferencing facilities page.

Git server

Subversion repository service

There are two ways to get a local Subversion repository accessible by collaborators:

Filespace issues

Can I have more quota?

If the request is for a modest increase then the answer will almost certainly be yes. You should make sure you have tidied up things like unpurged deleted mail messages and Firefox cache before you ask.

It may be the case that material does not need to be in backed up filespace — we can offer bulk space on "bigdisc". Although we formally class this as global scratch space, it's actually pretty resilient, being double raid protected in the filer and with the usual snapshots. You may be asked to consider this instead.

There is also local scratch space available on most Unix machines: use sudo cl-mkscratchdir to create a filespace in /local/scratch. Local scratch space is not backed up at all, and hence any deleted files are lost. You should take care only to use this space for data that you can re-create quite trivially.

For more information on creating and extending scratch space, see Wiki.

How can I keep long-running jobs going?

See our Wiki pages regarding Ticket-Granting Tickets.

I'm leaving the lab, can you keep my filespace/email address/web pages ?

These things are kept for a while by default anyway, we don't delete filespaces on the day that you leave. Your filespace will eventually be archived, but we can forward email indefinitely and set up WWW forwarding if given sufficient information. See Leaving the Computer Laboratory for more information.

I've just accidentally removed a file. Can I get it back?

Probably. See Retrieving lost data for more information.

Login Account

I can't change my password!

If you get the error message "Token manipulation error" when you try to reset your password from a Linux machine then the most probable cause is that you are not giving a new password of sufficient complexity. Check the rules for password complexity below. If you still are unable to set a password then try to do so on a Windows workstation which should give a more helpful error message.

Password complexity

User passwords must meet the following requirements:

  • The password is at least eight characters long.
  • The password contains characters from at least three of the following five categories:
    • Uppercase letters
    • Lowercase letters
    • Base 10 digits
    • Non-alphanumeric characters
    • Unicode characters which are categorized as alphabetic but are neither upper nor lower case (use of these is not recommended)
  • The password does not contain three or more sequential characters from the user's account name.

The precise definition of what counts as a letter is documented in detail by Microsoft. Characters outside the ASCII printable set permitted but are best avoided because of the difficulty of typing them on many keyboards.

We recommend that you choose a longer password rather than just using the minimum length necessary. You are unlikely to exceed the maximum permitted length, but some older systems may not accept more than about 28 characters. Around 12 characters is a good compromise between security and convenience.

We do not enforce a regular password change interval, believing that this does more harm than good. However it is important that you change your password if you have any reason to believe that it may have been compromised.


Do I need to install an antivirus on my computer?

In short: yes. You can find more information, as well as download links, on the University Information Services webpage.

Can I change the default power plan on Windows?

Machines running Microsoft 7 and newer have their power saving option controlled by group policy. The main issue with the power saving mode is if you start a task that cannot be suspended (eg something that requires a network connection to be kept alive), in such cases you may wish to consider setting an alternative power policy which does not sleep. If you do this then please try to use such a setting only when you need it or if you want to use this as your standard setting please try to rember to sleep your machine when you know it is not needed for some time such as at the end of the working day.

Microsoft Imagine membership

Microsoft makes selected operating systems and development tools freely available to academic staff and students of the Computer Laboratory, under the Microsoft Imagine for Academic Institutions programme. You will receive the account details needed by e-mail in the Michaelmas term after arrival. This email is send from the hosting institution so may be classified as spam.

Please ensure you keep a record of all licenses issued to you, as they cannot be retrieved after you leave the department.

This scheme does not cover Microsoft Office and is not available to students from other departments or faculties.

The network and connections

Can I attach my own (or a visitors) machine to the Department's network?

Possibly, but only if you are a member of staff or a research student and only subject to certain rules. For more information, see The CL network.

I can't connect to a host address or, what does strict checking mean ?

This question is usually in response to a message from ssh like this:

No RSA host key is known for "host address" and you have requested strict checking.
Host key verification failed.

and you are not offered the chance to accept the key on the basis of the fingerprint.

The purpose of the strict checking is to make sure that the host really is the expected one, and that there isn't a man-in-the-middle compromise. You need some way to make sure that the public key is that of the ACTUAL machine. If you can get the system administrator at that address to mail you the fingerprint or key in such a way that you trust the information, use that.

Otherwise, the best bet is probably to login using your user key if it is already installed (you may be able to get the system manager to do this for you) using

	slogin -o StrictHostKeyChecking=ask $host

and making sure that it really is the machine you expect. Something like

ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key

should confirm the fingerprint you just received.

In general, if you have a problem connecting to a host using ssh and need to ask us for assistance, please try using ssh -v hostname, which causes ssh to print debugging messages about its progress. You should then cut & paste the result into your email message asking us for help.

For more information, see SSH access to the CL systems.

Does the Lab have Microsoft Office available for download?

Anyone with an University of Cambridge account can install Microsoft Office for free. For more details, see the University Information Services webpage.

Lab-managed Linux

Can I have piece of software X installed on my lab-managed Linux box?

Probably. See Software installation under Unix. (If you are using an FC3 system see bundles and yum instead.)

Where is java/javac?

Machines usually have java installed when they are set up, and have the version of the jre installed that is appropriate at the time. Thus we have a mixture of versions on different machines, and sometimes more than one version. Whatever the version it will be placed in a subdirectory of /usr/java, so that is the first place to look. The RPMs we install from do not always place links in /usr/bin, so you may have to add the /usr/java/some-version-of-the-jre/bin directory to your PATH.

You may not have the java compiler installed by default (if you do it will also be in some subdirectory of /usr/java), in which case you may be able to install it yourself. See Can I have piece of software X installed on my lab-managed linux box ? and look for the jdk bundle.

Where is MATLAB?

The University obtained in 2018 a MATLAB/Simulink site licence 666637 that covers all toolboxes and can be used without connectivity to a licence server. (This supersedes the previous departmental licence 136605.)

The current MATLAB for Linux version is installed for everyone under /usr/groups/matlab/current on the filer. Some Linux machines have a local command cl-matlab installed (package: cl-matlab) to access the current version from the default path.

After hibernation I can't unlock my desktop

If after your lab-managed Linux hibernated overnight you cannot unlock the screensaver, this is usually caused by a combination of

  • your X11 authority file ($XAUTHORITY or $HOME/.Xauthority) is stored on a fileserver that can only be accessed with a valid Kerberos ticket;
  • your screensaver invokes a separate "greeter" program to ask you for your password (this program will need to read your X11 authority file);
  • your Kerberos ticket has expired (typically after 10 hours) as the utility that renews it automatically was inactive during hibernation.

The solution is to configure the X11 startup sequence such that $XAUTHORITY points to an X11 authority file in a directory on a local RAM disk, not a fileserver:

  • If you use the LightDM display manager on Ubuntu 16.04/18.04, the following line should fix this:
    # echo -e '[LightDM]\nuser-authority-in-system-dir=true' \

(This will also enable you to run an X11 client as root.)

Why am I told Firefox is running, when its not?

Every time you click on the shortcut to launch Mozilla Firefox, you get a pop-up message box saying:

"Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system."

The and websites have solutions to this problem for Linux, Windows and Mac. Select the solution which best suits your operating system.

The cause seems to be a .parentlock file which has remained locked after a crash, preventing further use of the application. You need to remove this file (which resides in the .mozilla folder in your home folder) to get back up and working again.

Web server

How can I put files on the Lab web server?

For your personal web space, see Personal web pages, but you should remember that when you leave the lab, this must either be removed or redirected to another private web space elsewhere. For anything that should remain in the lab after you have gone, you will need a directory elsewhere in the server. Most research groups already have a suitable area (under /anfs/www/html/{research,events,projects}) and a group webmaster looking after it, so talk to them first. If you have something which does not seem to fit in any existing structure, contact pagemaster for advice.

How do I run CGI, PHP or similar scripts on the Lab web server?

Unfortunately you cannot do this on the main lab server. The lab web server is a mission critical server and we cannot allow user-written code to run on it. Scripts may introduce security holes, or simply consume excessive resources. However, there is a separate server that only has access to a small part of the departmental file server and where CGI and PHP scripts can be executed.

Can I run my own web server?

A web server is just a program, and any machine can run one, though it may not be possible to bind to port 80. Such a server will be available inside the department but will not be accessible from the outside. If you have a genuine need to make a private web server publicly accessible, you should discuss your requirements with webmaster well in advance. Any externally open server is a potential security risk, and moreover any web server becomes part of the public face of the department. These considerations mean that we need to take some care over what is exposed to the outside world.