This isn't intended to be an introduction to VNC - have a look at the Getting
Started page first.
This list is being updated regularly, so, particularly as you are reading
a mirrored copy, it's worth checking the
original from time to time. The archives
of the mailing list are also available, and
they can be a good source of help - try searching them here.
Historical Note: With the release of 3.3.2 we have moved
some of the old questions into the Old FAQ page
because they should not apply to recent versions. As with most software,
it's worth checking to make sure you've got the lastest version.
VNC Frequently Asked Questions
Where can I get VNC?
The latest versions of VNC and this documentation are
always available from the AT&T Laboratories Cambridge web site at http://www.uk.research.att.com/vnc.
In addition, various add-ons, extras, and ports to other platforms can
be found on the contribs page.
I thought this was something to do with ORL? What's the Olivetti/Oracle
In January 1999, AT&T
acquired ORL, the Olivetti Research Laboratory founded 12 years earlier,
and recently jointly funded by Oracle,
to create AT&T Laboratories Cambridge.
I started the X server using vncserver, but it dies with a message "Could
not open default font 'fixed'. "
The 'fixed' font is needed for the server to start
- if it can't find it, you need to specify the correct font path for your
machine in the 'vncserver' script. If you're not sure what the path
should be, type 'xset q' from within a normal X session. One of the
things reported is the font path used by your current X server, which is
generally the right thing to use for Xvnc. On some platforms you
may need to use a colon as a separator in the font path instead of a comma.
If you're on a recent version of Linux but still using VNC version 3.3.1,
you may have compressed fonts which VNC doesn't understand. Either upgrade
or see the Old FAQ.
The VNC server can also get upset if you have directories on
your font path which don't actually exist on your system. Make sure
you remove those. Also note that older versions of the Xvnc server, by
default, act as if they had a resolution of 100dpi. Some RedHat
installations, for example, only install 75dpi fonts, so you may need
to install the missing font RPMs from your distribution or use the
-dpi option to Xvnc, or update to the latest version.
I get errors like "failed to bind listener" and "Failed to establish
all listening sockets" in the log file.
This is probably due to the permissions on /tmp/.X11-unix.
You may well see this if you update to Solaris 2.7 or Redhat 6.0, for
example. See the section below entitled "Why can I only run
vncserver/Xvnc as root?".
Vncserver seems to be dying quietly without putting any messages in
the log file.
Check that the Xvnc process really has died. If so, then check
that your VNC font path (set by uncommenting lines in the vncserver
script) only includes directories which actually exist. The XFree86
code in older versions of Xvnc seems to have a problem which causes
the server to die quietly if non-existent directories are
searched. Upgrading to 3.3.3 or later should reduce these
I connected to my Unix VNC server and I just see a grey desktop with
After the vncserver script has started the Xvnc server,
it then runs your ~/.vnc/xstartup script. By default this will try
to start the twm window manager, but if twm isn't on your path, or if you
prefer something else, you can edit xstartup. The log file may also
give you clues about what is happening.
The log file is showing an error message from xrdb / Xlib.
By default, the first thing your xstartup script does
is to run xrdb to load your resources. So if the Xvnc server has
not started for any reason, the xrdb is often the first thing to notice
it and print an error. (Though if you're getting a 'command not found'
message, then xrdb is probably not on your path - you need to find where
it is on your system and add it.)
If you get something like 'connection refused' or 'Can't
connect: errno = 111', the Xvnc server probably isn't there. So you
should check whether the Xvnc process is actually running, and whether
there is anything earlier in the log file indicating why it might have
died. By far the most common reason for the server not starting is
that it can't find the 'fixed' font (see above). Other possibilities are
that the server has quietly crashed, or that it is taking a very long time
to start up. The vncserver script has a 3-second delay before running
xstartup, but in extreme cases this may not be enough. Lastly, the
DISPLAY variable used by vncserver is based on the results given by the
'uname - n' command. If your applications cannot resolve this to the right
IP address, perhaps because of funny settings in /etc/hosts, then they
won't be able to connect.
If you get a 'Client is not authorized to connect to
Server' or similar message, there's something wrong with the X authority
setup - perhaps xauth is not on your path? You could try using xhost to
bypass this temporarily, but we wouldn't recomend this as a long-term solution.
There should be some indication in the log file if xauth has failed.
The Java client doesn't work in my browser.
Several Java implementations have bugs which upset the VNC
applet. Try pressing Reload. We've seen this problem on Netscape
Navigator 3 and Internet Explorer 5. Note, too, that the Java applet
often won't work if you are accessing the server via a web proxy. Make
a direct connection if you can.
My viewer failed to connect to my server!
VNC relies on a correctly-configured and operational
TCP/IP network, so please make 100% sure that your TCP/IP setup is right
before you start asking questions on the mailing list. Here are some
things you should check before assuming it's a VNC problem; consult your
local expert if you don't know how to check them:
Can you ping the server machine from the client?
Is the VNC server definitely running on the server machine?
The server listens on port 5900+displaynumber. Can you telnet to this port
from the client machine?
Have you specified the address correctly to the viewer? If you're using a display number other than zero for the server, (usually the case on Unix machines), did you remember to specify it?
Is the server name known to the DNS? Try using an explicit IP address
instead of a name (eg. 123.456.78.9:0 instead of snoopy:0).
Do you have any firewalls or proxies in the way that could be blocking
If using the java client, did you remember to specify the correct port
as part of the URL? (eg. http://snoopy:5800)?
Can you try running either the server, or the client, or both, on different
machines on your network to find whether the problem is at one end or the
Can you try running the software on a different architecture? eg.,
if you are having problems viewing a PC from another PC, can you
try connecting from a Unix machine?
Running the programs
My X VNC server is working, but I don't see my normal environment. How can I change the Window manager etc?
The window manager is started by the ~/.vnc/xstartup script. We use
twm, as this is available on almost all Unix platforms. Edit the
script if you'd rather replace it with something else. On many platforms
you can, as an alternative, just make xstartup a link to whatever script
normally starts your X environment.
If you want to be more sophisticated, you can specify the -name option
to vncserver, and then take different actions in the xstartup script based
on the name given. For example:
case "$VNCDESKTOP" in
xterm -geometry 40x10+40+40 -ls -title "$VNCDESKTOP Desktop" &
Why can I only run vncserver/Xvnc as root?
The most likely reason for this is that Xvnc can't
create the unix domain socket (the path for this unix domain socket is
usually /tmp/.X11-unix/Xn). Try making sure that users can write
to this directory by making it world-writable, i.e. chmod 01777 /tmp/.X11-unix
An alternative is to set the Xvnc binary to have the same
permissions as your normal X server, but this may be more of a security
Can I remote the normal X display of my workstation (display :0) in
the same way as the Windows server does?
It would be possible to add VNC support to a standard
X server, so that it operated this way, but we have no plans to do so.
It shouldn't be too difficult, if you have the full source code for your
X server. Xvnc is basically XFree86 with the hardware-dependent bits
taken out and VNC put in their place, so it doesn't have drivers for any
graphics cards. You could have a look at the Xvnc source code to see which
bits have been added to the standard XFree86, and make the equivalent changes
on your X server.
We tend to run all our X sessions as VNC sessions and only use the
local X server to run the viewer. It's very fast when on the same
machine as the server! If you feel that it's overkill to run two X servers
on the same machine, you might consider Ganesh Varadarajan's svgalib-based
viewer, available from the contribs page.
What X Visual does Xvnc use?
By default, vncserver will start Xvnc with the same
depth as the current X display, if there is one, or 8 bits deep if there
isn't. We've tried to steer clear of colour maps as much as possible
and normally use "true colour", even when there are only 8 bits per pixel.
Unfortunately some X applications don't cope too well
with an 8 bit TrueColor visual. You can make Xvnc use the more
normal PseudoColor visual by giving a "-cc 3" option to vncserver.
Can I cut and paste between the viewer and the server?
VNC supports copying and pasting of ASCII text in both
directions, provided the viewer and server allow it. When the clipboard
changes on the machine running the viewer, the changes are copied to the
server and vice versa. Some notable exceptions:
X has more than one method of using the clipboard and different applications
do it different ways. Emacs and xterm should just work. If you find that
your X application doesn't work via VNC, you can generally use the
program to copy the clipboard between the different X methods. VNC
uses Cut_Buffer0, so if you select text in Unix Netscape, for example,
you may need to click 'Copy PRIMARY to 0' before it is accessible at the
other end of the VNC link. You can use X resources to make the button labels more meaningful. For example, here's a script:
exec xcutsel \
-xrm '*quit.borderWidth:0' \
-xrm '*quit.height: 1' \
-xrm '*quit.label:' \
-xrm '*sel-cut.label: Clipboard: out of netscape' \
-xrm '*cut-sel.label: Clipboard: into netscape' \
-xrm '*font: -*-helvetica-*-r-*-*-*-*-*-*-*-*-*-*'
Java applets running in the browser cannot access the clipboard of the
machine on which they are running, so the Java viewer has a clipboard button.
This pops up a window displaying the contents of the remote clipboard,
which should allow you to manipulate it locally.
The code to do copying from the X server to
the viewer didn't make it in the original version 3.3.2. It's back
in 3.3.2R2 and later, so upgrade if you haven't already!
There's a memory leak in Xvnc!
This is fixed in versions 3.3.2r3 and later. If you're using an older
distribution you can find a patch for it here.
Why doesn't Ctrl-Alt-Del work? Why can't I unlock my NT workstation remotely? Why can't I stop the screensaver remotely?
Make sure you are running a recent version of VNC,
that you are running it as a service. From some platforms you
will not be able to type Ctrl-Alt-Del directly, because it will be caught
by the local machine. The Windows viewer, for example, has an option on
its menu to send a Ctrl-Alt-Del to the remote host. In some situations,
you will find that something like Ctrl-Alt-Backspace or Ctrl-Alt- may work instead. Screensavers sometimes use a different resolution
and so can disconnect you when they stop or start - see the next question.
When I connect using VNC and then log into my Windows
machine, I get disconnected and have to reconnect!
Sometimes logging in will involve a change in screen
resolution, if the user's display settings are different from the
defaults. If this happens, the server will disconnect you and you will
need to reconnect to get the new screen size. Just occasionally on NT,
the mode seems to change resolution temporarily as you log in, and if
WinVNC happens to see this you can also be disconnected, even if the
final resolution changes.
If the user has set a different
display number in their personal WinVNC properties dialog you will
also be disconnected.
The dead keys don't work on my keyboard
It's quite a challenge working out how to do international
keyboard support across different platforms. We'll sort out dead
keys, but if you'd like to work out how to do them, we'd be
The keyboard doesn't work / keys do strange things!
There is one common problem which can cause this.
If a modifier key, such as Shift, Ctrl or Alt, is pressed, and the viewer
window then loses focus or dies, the 'key release' message never gets to
the viewer and hence never gets to the remote server. The remote
machine will then think that M is Ctrl-M etc. We have done various
things to reduce the chance of this happening; the viewers release various
modifiers automatically when they lose focus, for example, but it can still
occur and can be confusing when it does. The solution is easy: simply
press and release the modifier key which is stuck. If you don't know
which it is, then try them one at a time.
How do I make VNC go faster?
We find VNC to be perfectly acceptable as our normal method of
accessing Unix desktops on a daily basis. This is over a 10 Mbit/s
ethernet on reasonably modern machines, using the X or Win32 viewer.
Because Windows gives us fewer hints about what it's doing, and
because we don't have the source code for Windows in the same way that
we do for X, the WinVNC server has to work harder to find out what's
changed, and so a really fast machine should make a big speed
difference. For more information about how the Windows server works,
see the WinVNC documentation. But if you've
been disappointed by the speed of the Windows server, don't give up.
We're improving it gradually, but it'll be a while before it's as fast
as on Unix.
There are several things that can slow any VNC session
down, however, and you may like to consider these if you find it too slow:
Unusually 'busy' desktops. The VNC protocol is very efficient at
rendering areas of a single colour, such as you generally find on window
title bars, scrollbars, backgrounds of pages etc. But if, for example,
you have pretty 24-bit photographs of your girlfriend as your screen background,
or dithered title-bars on your windows, you may pay a price for the aesthetics.
colourful or patterned desktop background will probably slow down VNC more
than any other single factor. We have some suggestions on
up the twm window manager, some of which will also apply to other environments.
Hi-colour desktops. Don't use 24-bit colour if you can use
16 or 8 equally well. Remember, on Unix you can run multiple servers, so
I have a big 16-bit desktop for normal work and a small 8-bit one for when
I log in from home. The server can send out a wide range of pixel
formats, and some viewers will allow you to request a specific format for
that session. On the Windows viewer, for example, if you click Options...
when making the connection, you can request only 8-bit pixels from the
server - useful if the network gets slow. If you are using a modem,
I recommend changing the shortcut in the Start menu to include the /8bit
option - this will then be the default. Similarly, if you regularly
connect to a remote WinVNC server, consider whether you could run happily
at lower resolution. A 1280x1024 screen has more then 4 times as many pixels
as a 640x480 one, and if all you are doing is checking a printer queue
you probably don't need them all! Note, though, that on WinVNC, 16-bit
colour is usually the best to use. See below.
Elderly graphics cards or drivers may make quite a difference; this is
a graphics-intensive application! On Windows the graphics system
on the server will affect the speed as well as the one on the viewer.
Some applications are not very economical about redrawing their display.
Early versions of Unix Netscape, for example, tended to draw everything
twice when scrolling, which did nothing to help the smoothness under VNC.
X11Amp flashes its display very fast when in 'pause' mode.
Some Java Virtual Machines are particularly fast at reading from the network
and particularly slow at drawing to the screen, or vice versa. With
the Java viewer it is worth experimenting with the encodings available
from the Options menu, as we sometimes find big differences in speed.
If you are connecting to WinVNC, don't change the default settings in the
Properties box unless you need to.
All the standard clients can do local rectangle copying, and this generally
means that dragging a full window on X is much faster than dragging its
outline. If your window manager allows this, it may speed things
up. Roman Mitnitski also reports that when he increased the
mouse dragging threshold (-t option of the server) the performance was
Generally, with WinVNC, use 16-bit colour (65536 colours) on the server
if you can. 16-bit is almost always the best depth to use, because:
256-colour screens have to be palette-converted before they can be transmitted
to truecolour clients. Only if the client is 256-colour palette- based
will you see any performance increase. Even if the client is 256 colour
truecolour, it'll have to convert via a 32-bit truecolour palette!
24-bit screens have to be specially munged via 32-bit since VNC's internal
colour-handling routines don't work with 24-bit directly.
24 and 32-bit screens have to have each pixel looked up in three tables
to get the converted value.
Graphics cards claiming to do 24-bit often actually do 32-bit with munging
- this in many cases makes 24-bit slower just for general use than 32-bit!
Finally, 16-bit involves no palette processing and a single lookup in a
cached src_format to dest_format table to convert the pixels.
On slow links, you may also want to use some software to compress the data
between the two ends. SSH is good for this. See the section on security
Will VNC work through a firewall?
It depends on your firewall, and whether you want to
access a server inside your firewall from elsewhere, or a server outside
your firewall from inside.
Generally firewalls are designed to prevent incoming
connections except to certain well-known machines and ports. If you
can configure these to include your VNC server, then you will be able to
access it from anywhere in the world. There is a good argument to
be made for the fact that VNC is less of a security risk than X, so if
your site doesn't allow X in or out it may still allow VNC.
Many modern firewalls will allow outgoing connections
initiated from inside, so you can often access servers on outside machines.
It is straightforward, for example, to recompile the viewer source to include
SOCKS support, or to make other special arrangements. See the contribs
It's a pity that Java within a browser doesn't automatically
use SOCKS if the browser is configured to use it. There's probably
Java SOCKS support out there somewhere...
If your internet access is through a router which does
Network Address Translation, you may be able to configure the router to
redirect particular incoming ports to particular machines. So you could
run WinVNC with a display number of 0 on machine snoopy, and with display
1 on machine woodstock, then set your router to send port 5900 to snoopy
and 5901 to woodstock. See below for information on the other port
numbers used by VNC.
Which TCP/IP ports does VNC use?
A VNC server listens on two ports. The exact port numbers
depend on the VNC display number, because a single machine may run multiple
servers. The most important one is 59xx, where xx is the display number.
The VNC protocol itself runs over this port. So for most PC servers, the
port will be 5900, because they use display 0 by default.
In addition, VNC servers normally have a small and
very restricted web server built in, which allows you to connect a browser
to them and use the Java viewer. This runs on port 58xx. Note
that this is the HTTP port used for downloading pages and applets, but
once the applet is running it uses 59xx for VNC just like any other viewer.
The servers can be changed to listen on other ports
if, for any reason, these are not suitable for you. See the server's
for more details. Most of the viewers, if given a display number
larger than 99, will interpret it as a direct port number and will not
add 5900. See also the next question.
If you are running a viewer in 'listening' mode, where
it accepts connections initiated by the server, it will listen for incoming
VNC on port 5500.
Can I run VNC over a port normally used for a standard service? (eg. port 21, or port 80)
In rare circumstances, people may want to do this, perhaps because they have a firewall which only allows connections to certain ports. This can be done, at least for the Windows and Unix servers (see their documentation), but the following points need to be borne in mind:
On some systems (eg. most forms of Unix), ordinary users are not
allowed to run servers on ports below 1024.
You obviously can't run a VNC server on a port that's already
being used for other things.
Many VNC servers use two ports: one for the VNC server, and one
for the HTTP server that provides the Java applet (see previous
question). If you plan to use the Java viewer, you may want to change
both. Not all servers will allow this at present.
You need to tell the viewer the right display number. Normally,
display numbers come between 0 and 99. If you specify any number
smaller than 99, the viewers add 5900 to get the port number. If you
specify a larger number, the viewers take it as a port number
directly. So how do you use port numbers lower than 99? You have to
specify a negative display number! For example, to connect to a
server running on port 80 on machine 'snoopy': vncviewer snoopy:-5820
because -5820 + 5900 = 80. This may not work with all viewers, but Unix and Windows seem to be fine.
How secure is VNC?
Access to your VNC desktop generally allows access
to your whole environment, so security is obviously important. VNC uses
a challenge-response password scheme to make the initial connection: the
server sends a random series of bytes, which are encrypted using the password
typed in, and then returned to the server, which checks them against the
'right' answer. After that the data is unencrypted and could, in theory,
be watched by other malicious users, though it's a bit harder to snoop
a VNC session than, say, a telnet, rlogin, or X session. Since VNC
runs over a simple single TCP/IP socket, it is easy to add support for
SSL or some other encryption scheme if this is important to you, or to
tunnel it through something like SSH.
SSH allows you to redirect remote TCP/IP ports so that all
traffic is strongly encrypted, and this can be combined with VNC.
SSH can also compress the encrypted data - this can
be very useful if using VNC over slow links. See the 'Using SSH with VNC' page.
While we're on the subject of security, you should
also be aware that only the first 8 characters of VNC passwords are significant.
This is because the 'getpass' call used in the Unix server to read a password
has this restriction, and the other platforms have been made compatible
Ray Jones <firstname.lastname@example.org>
built a version of VNC which uses SSLeay public key encryption, and Wolfram
has built Xvnc with the TCP Wrapper library, allowing you more control
over which hosts are allowed to connect. See the contribs
page for details.
Are you going to make it more secure?
We do hope eventually to add better security to VNC, but
there's also a good argument for not doing so. If security is
a concern, it can be better to use a single system such as SSH or
FreeS/WAN to encrypt all your traffic, rather than relying on the
individual packages to do the right thing. Then, if you decide in a
year's time that one system is too easily crackable, you can replace
it yourself and all of your communications will benefit. It may also
be easier to fit in with corporate security systems this way.
Could you do file transfer (e.g. by drag & drop) between the two ends?
It's certainly possible, but we aren't going to do it. File transfer
seems simple, but is actually rather complicated to do in a cross-
platform way. Some examples: How do you map filenames between
platforms with wildly different naming conventions? How do you cope
with running out of disk space? What about permissions? How do you
do it on platforms which don't have a concept of drag and drop? Can you interrupt the transfer if it's taking too long? Can you restart it if it gets interrupted?
Since there are a very large number of perfectly good systems out
there for transferring files over IP, we aren't planning to
incorporate it in VNC. It would probably more than double the size of
the code, and would introduce all sorts of issues that we, quite
frankly, aren't interested in! So use the standard file transfer built
into Windows, or use FTP, or netcat, or the web, or rcp, or ssh, or...
Are you planning support for AIX, EPOC,HP-UX, SGI, Win 3.1,
or my favourite platform ?
We have provided VNC on all the platforms we use here, and
it's difficult to provide binaries for anything we don't have, and it
takes a while to get up to speed on new platforms. Information about
third-party ports of VNC to a large number of other platforms can be
found on the 'contribs' page, so check
there first. Remember that a viewer is available for any platform
which runs Java, though the speed may vary quite a bit. But for many
platforms it should not be difficult to compile at least the
viewer. If anyone tailors the sources for a particular platform we
will happily either incorporate the changes in the main source
distributions or make the patches available from our site.
Would things work better if you compressed the stream?
VNC incorporates really quite efficient compression
in the sense that we generally send a tiny fraction of the raw data, probably
something like 1/20 on average. The details are in the protocol spec if
anyone's interested. On a couple of test screen dumps we found that
the Hextile encoding was more efficient than GIF! I don't know whether
this is true in general.
But we haven't done more general encoding after that;
we've tended to the view that (a) it might introduce too much latency and
(b) most modems compress pretty well anyway. We are planning
some zlib-compression experiments in the near future to see how this affects
things. Because different bits of the screen can be sent using different
encodings, the server could, in theory, detect that one bit would be most
efficiently sent as JPEG, while another would be better hextiled.
The question is always how much work it's worth doing
at the server to find this out. To some degree you can control this
already, because the viewers allow you to specify your preferred encoding.
Under X, if your viewer and server are on the same machine the viewer will
use the raw encoding by default, otherwise it will use hextile. You
may find that by selecting different encodings on the command line you
get better performance.
See also the suggestions above about using ssh, which also
Have you thought about caching bits of the screen at the viewer end?
Yes, that could also be good. You could have an off-screen
cache in the viewer and the server could copy things from there to the
screen. Management of this would add a certain amount of complexity,
Since there is already a CopyRect primitive in VNC,
an alternative approach would be to copy updates from another part of the
screen if they already exist there, rather than resending them. Again,
to make the server find out efficiently when this is worth doing would
be an interesting challenge, and volunteers for the project are welcome!
Can I use VNC over a modem without using TCP/IP?
Not at present. VNC could run over other transports such as
RS232, firewire, USB, modems, IrDA etc, in fact, anything which gives
a reliable 2-way connection. At present we just use TCP/IP, because
it's convenient, ubiquitous, and easy to route. This means that you
can use VNC over anything which supports TCP/IP, so using it over a
modem is just the same as any other network, once you have Dial-Up
Networking set up. If you need to communicate directly between two
machines without going via the internet/intranet, then set up a remote
access server on one and dial in from the other.
Does VNC have any Y2K (Year 2000) bugs?
The WinVNC server and Windows viewer have been tested
on a PC with its date running through the 2000 boundary without any problems,
so unless the underlying OS or BIOS has difficulties, VNC on a PC should
be fine. The VNC part of the X-based Unix VNC server only uses dates
when writing the log files; the logfile entries are timestamped with a
two-digit year, but the format is easy to change if required and the entries
are not intended to be machine-readable. The developers of the XFree86
server on which Xvnc is based state that there are no Y2K problems (see
http://www.xfree86.org/FAQ/). We therefore issue the standard disclaimer:
we believe the VNC code, in its entirety, to be free from Year 2000 problems,
subject to the other components of the systems on which it is running.
Any other tips?
Several people have indicated that they have to use
Windows occasionally but prefer to use Unix most of the time, and so want
to access a PC under the desk from the Unix box.
Here's a suggestion: all other things being equal,
I recommend using the Windows box to view the Unix machine rather than
the other way around. This is chiefly because Windows generally works
better as a client than as a server, and also because PC graphics cards
are often better than those in Unix workstations. Remember, you can
create a VNC session of any size and pixel depth you like.
If you're very anti-Windows you can make your VNC desktop
the same size as the local screen and set the taskbar to 'Auto hide' and just
pretend you're on an X terminal, but pop up the Start menu when you have
to use PowerPoint.... The Windows viewer also now has a proper
'full-screen mode', so you don't even need to bother with auto-hide.
You misspelled 'organization' on the download page!
No we didn't. We spell it like that in the UK.
Actually, we spell it both ways, but the 's' spelling is more common, despite
what the OED says! Now, as for 'misspelt'...
Compiling the source
I'm having trouble compiling VNC on my platform...
Have you checked the contribs
page? Several people have provided hints on how to build VNC on other
platforms. If yours is not listed there, you might at least get some
For comments, feedback, etc, please see the 'Keeping
in touch' page. Copyright 1999 - AT&T Laboratories Cambridge