VNC Virtual Network Computing
VNC - Overview • Virtual Network Computing (VNC) • A graphical desktop sharing system • Uses the RFB protocol to remotely control another computer. • Uses a network • Transmits the keyboard and mouse events from one computer to another • Relays the graphical screen updates back
VNC - Overview • Platform-independent • VNC viewer resides any operating system • Can connect to a VNC server of any other operating system • Clients and servers available for almost all GUI operating systems and for Java • Multiple clients may connect to a VNC server at the same time • Popular uses for this technology include • Remote technical support • Accessing files on one's work computer from one's home computer
VNC - Overview • Originally developed at AT&T • The original VNC source code and many modern derivatives are • open source • under the GNU General Public License
History • Created at the Olivetti & Oracle Research Lab • Originally owned by Olivetti and Oracle Corporation • In 1999 AT&T acquired the lab • 2002 closed down the lab's research efforts • Following the closure of ORL in 2002 • several members of the development team formed RealVNC • Continue working on open source and commercial VNC software under that name • Several other versions of VNC have been developed from the original GPLed source code. • Forking has not led to compatibility problems • RFB protocol is designed to be extensible • VNC clients and servers negotiate their capabilities when handshaking • Make use of the most appropriate options supported at both ends
Etymology • 'Virtual Network Computer/Computing' • Originates from ORL's work on a thin client called the Videotile • Used the RFB protocol • Essentially an LCD display with • a pen input and • a fast ATM connection to the network • At the time a network computer was commonly used as a synonym for 'thin client' • VNC is essentially a software-only (i.e virtual) version of this network computer
Operation • A VNC system consists of a client, a server, and a communication protocol. • The VNC server • program on the machine that shares its screen • The VNC client (or viewer) • Program that watches and interacts with the server • The VNC protocol • Very simple, based on one graphic primitive: • "Put a rectangle of pixel data at the specified X,Y position".
Operation • Server sends small rectangles of the framebuffer to the client • In its simplest form, the VNC protocol can use a lot of bandwidth • Methods devised to reduce communication overhead • For example, various encodings • methods to determine the most efficient way to transfer these rectangles • VNC protocol allows the client and server to negotiate which encoding will be used
Operation • The simplest encoding is the raw encoding • Pixel data is sent in left-to-right scanline order • Once the original full screen has been transmitted • Only transfer rectangles that change • Supported by all clients and servers • This encoding works well: • If only a small portion of the screen changes from one frame to the next • Mouse pointer moving across a desktop • Text being written at the cursor • Bandwidth demands get very high if a lot of pixels change at the same time • Scrolling a window or viewing full-screen video
Operation • VNC by default uses TCP ports 5900 through 5906 • each port corresponding to a separate screen (:0 to :6). • A Java viewer is available in many implementations • Such as RealVNC allowing clients to interact through, among other things, a Java-enabled web browser • Ports 5800 through 5806 • Other ports can be used • Both client and server must be configured accordingly • Some operating systems only support a single VNC session at a time because the operating system supports only a single session at a time • E.g. Windows XP
Operation • Note that on some machines, the server does not necessarily have to have a physical display. • Xvnc is the Unix VNC server, which is based on a standard X server • Xvnc can be considered to be two servers in one; to applications it is an X server, and to remote VNC users it is a VNC server • Applications can display themselves on Xvnc as if it were a normal X display • Will appear on any connected VNC viewers rather than on a physical screen
Operation • The display that is served by VNC • Not necessarily the same display seen by a user on the server • Unix/Linux computers that support multiple simultaneous X11 sessions • VNC may be set to serve a particular existing X11 session • Start one of its own • Multiple VNC sessions can be run from the same computer. • On Microsoft Windows the VNC session served is always the current user session
Operation • VNC is commonly used as a cross-platform remote desktop system • Apple Remote Desktop for Mac OS X interoperates with VNC • Will connect to a Linux user's current desktop if it is served with x11vnc • As a separate X11 session if one is served with TightVNC • From Linux, TightVNC will connect to an OS X session served by Apple Remote Desktop if the VNC option is enabled, or to a VNC server running on Microsoft Windows
Security • By default, VNC is not a secure protocol. • Passwords are not sent in plain-text (as in telnet) but… • brute-force cracking could prove successful if both the encryption key and encoded password are sniffed from a network. • It is recommended that a password of at least 8 characters be used. • There is also an 8-character limit on some versions of VNC; • If a password is sent exceeding 8 characters • Excess characters are removed • Truncated string is compared to the password.
Security • VNC may be tunneled over an SSH or VPN connection • Adds 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 • Microsoft Windows clients • Macintosh clients many others
Security • UltraVNC supports the use of an open-source encryption plugin • Encrypts the entire VNC session • Including password authentication and data transfer • 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
The original • http://www.cl.cam.ac.uk/research/dtg/attarchive/vnc/index.html
RFB • RFB (“remote framebuffer”) is a simple protocol for remote access to graphical user interfaces • It works at the framebuffer level • Applicable to all windowing systems and applications • Including X11, Windows and Macintosh. • RFB is the protocol used in Virtual Network Computing (VNC) and its derivatives. • Although RFB started as a relatively simple protocol it has been enhanced with additional features (such as file transfers) and more sophisticated compression and security techniques as it has developed • To maintain seamless cross-compatibility between the many different VNC client and server implementations • Clients and servers negotiate a connection using the best RFB version • Use the most appropriate compression and security options they can both support.
History • RFB was originally developed at Olivetti Research Laboratory (ORL) • A remote display technology to be used by a simple thin client with ATM connectivity called a Videotile. • In order to keep the device as simple as possible, RFB was developed and used in preference to any of the existing remote display technologies. • RFB found a second, more enduring use when VNC was developed • VNC was released as open source software and the RFB specification published on the web. • Since then RFB has been a free protocol which anybody can use. • When ORL was closed in 2002 some of the key people behind VNC and RFB formed RealVNC Ltd in order to continue development of VNC and to maintain the RFB protocol. • The current RFB protocol is published on the RealVNC website.
Protocol versions • Published versions of the RFB protocol are shown below: • Developers are free to add additional encoding and security types • Must book unique identification numbers for these with the maintainers of the protocol so that the numbers do not clash. • Clashing type numbers would cause confusion when handshaking a connection and break cross-compatibility between implementations • The list of encoding and security types is maintained by RealVNC Ltd • Separate from the protocol specification • New types can be added without requiring the specification to be reissued