History
VNC was created at the Olivetti & Oracle Research Lab, which was then owned by Olivetti and Oracle Corporation. In 1999 AT&T acquired the lab, and in 2002 closed down the research part of the lab.
The name originates from a thin client ATM Network Computer called the Videotile, which was essentially an LCD with a pen input and a fast ATM connection to the network. VNC is essentially a software-only version of this 'ATM Network Computer'.
Developers that worked on VNC while still at the AT&T Research Lab:
How it works
VNC has two parts, a client and a server. The server is the program
on the machine that shares its screen, and the client (or viewer) is
the program that watches and interacts with the server.
VNC is a very simple protocol, based on one graphic primitive: "Put
a rectangle of pixel data at a given x, y position". That is, the
server sends small rectangles of the framebuffer to the client. This in its simplest form uses a lot of bandwidth, so various methods are used to reduce it. For example, there are various encodings
- methods to determine the most efficient way to transfer these
rectangles. The VNC protocol allows the client and server to negotiate
which encoding will be used. The simplest encoding, which is supported
by all clients and servers, is the raw encoding where pixel data is sent in left-to-right scanline
order, and after initial setup, then only transfers rectangles that
change. Because of that, this encoding works very well if only a small
portion of the screen changes from one frame to the next (like a mouse
pointer moving across a desktop, or text being written at the cursor),
but bandwidth demands get very high if a lot of pixels change. (Full
screen video is the most radical example of this.)
VNC by default uses ports
5900 to 5906, each representing the corresponding X screen (ports 6000
to 6006, for screens :0 to :6). A Java viewer is available in
many implementations such as RealVNC on ports 5800 to 5806, following
the same pattern. These ports can be changed.
Most Windows computers can only use a single port because Windows
lacks the multisession features of unix based servers. The default
display number for Windows based computers is 0 which maps to TCP port
5900.
It is possible to run more than one display on Windows terminal
servers by running VNC once from each terminal session. To do this,
connect to the terminal server using a standard terminal services
client, then launch a new instance of the VNC server on a different
display than any other running instances on that same server. Repeat
this many times and you get many different instances of VNC running on
a single Windows server with different desktops.
Security in VNC
By default, VNC is not a secure protocol. While passwords are not sent in plain-text (as in telnet), brute-force cracking could prove successful if both the encryption
key and encoded password are sniffed from a network. For this reason it
is recommended that a password of at least 8 characters is used.
However, VNC may be tunnelled over an SSH or VPN
connection which would add an extra security layer with stronger
encryption. SSH clients are available for all major platforms (and many
smaller platforms as well); SSH tunnels can be created from UNIX
clients, Windows clients, Macintosh clients (including OS X and System
7 and up) - and many others.
UltraVNC
supports the use of an open-source encryption plugin which encrypts the
entire VNC session including password authentication and data transfer.
It also allows authentication to be performed based on NTLM and Active Directory user accounts.
RealVNC offers high-strength encryption as part of its commercial package.
Workspot released AES encryption patches for VNC.
As with any server software, if the port used by this service is explicitly blocked by a firewall
it is practically impossible to break into the service. In other words
a network administrator has to enable firewalling for 5901 on the local
router in order to prevent access from outside the LAN.
On July 31, 2005, Tridia announced that they were discontinuing
development of their free product Tridia VNC and suggested users
instead pay for their commercial remote administration software iTvity,
claiming that software based on the VNC protocol is unsuitable for
deployment in a business environment due to design deficiencies in VNC
itself.
VNC Loop
A VNC Loop (shortening of VNC Loopback) is a simple fractal pattern, depicting exact self-similarity, that develops after one or more computers VNC into itself or into each other in a logically organized ring topology.
Depending upon the implementation of VNC being used or upon its
specific configuration, such an attempt to initiate a VNC Loop using
one or two computers may not be allowed, although a workaround to this
is to use more than two computers. Some other implementations,
especially on relatively slower computers, may inadvertently cause one
or more of the connected computers to freeze as a VNC Loop develops and
until it persists. One potential way to break a VNC Loop on a
completely unresponsive computer is to physically disconnect its
network cable. During a VNC Loop, control of the mouse cursor may at
times seem erratic, but this is a direct and predictable effect of the
loop.
In a stable VNC Loop, the traversal of screen data feedback between the computers can exhibit crude short-term memory. Changes made on one computer, for example activating and then deactivating the Start button in Microsoft Windows, take time to propagate down the VNC Loop.
Further reading
- Tristan Richardson, Quentin Stafford-Fraser, Kenneth R. Wood & Andy Hopper, "Virtual Network Computing", IEEE Internet Computing, Vol.2 No.1, Jan/Feb 1998 pp33-38 PDF format
See also
External links
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