x11vnc is a Virtual Network Computing (VNC) server program. It allows remote access from a remote client to a computer hosting an X Window session and the x11vnc software, continuously polling the X server's frame buffer for changes. This allows the user to control their X11 desktop (KDE, GNOME, Xfce, etc.) from a remote computer either on the user's own network, or from over the Internet as if the user were sitting in front of it. x11vnc can also poll non-X11 frame buffer devices, such as webcams or TV tuner cards, iPAQ, Neuros OSD, the Linux console, and the Mac OS X graphics display. x11vnc is part of the LibVNCServer project and is free software available under the GNU General Public License. x11vnc was written by Karl Runge.

x11vnc does not create an extra display (or X desktop) for remote control. Instead, it uses the existing X11 display shown on the monitor of a Unix-like computer in real time, unlike other Linux alternatives such as TightVNC Server. However, it is possible to use Xvnc or Xvfb to create a 'virtual' extra display, and have x11vnc connect to it, enabling X-11 access to headless servers.

x11vnc has security features that allows the user to set an access password or to use Unix usernames and passwords. It also has options for connection via a secure SSL link. An SSL Java VNC viewer applet is provided that enables secure connections from a web browser. The VeNCrypt SSL/TLS VNC security type is also supported.

Many of the UltraVNC extensions to VNC are supported by x11vnc, including file transfer.

x11vnc keeps a copy of the X server's frame buffer in RAM. The X11 programming interface XShmGetImage is used to retrieve the frame buffer pixel data. x11vnc compares the X server's frame buffer against its copy to see which pixel regions have changed (and hence need to be sent to the VNC viewers.) Reading pixel data from the physical frame buffer can be much slower than writing to it (because graphics devices are not optimized for reading) and so a sequential pixel by pixel check would often be too slow.

To improve the situation, x11vnc reads in full rows of pixels separated by 32 pixels vertically. Once it gets to the bottom of the screen it starts again near the top with a slightly different offset. After 32 passes like this it has covered the entire screen. This method enables x11vnc to detect changes on the screen roughly 32 times more quickly than a sequential check would (unless the changes are very small, say only 1 pixel tall.) If the X11 DAMAGE extension is present, x11vnc uses it to provide hints where to focus its polling, thereby finding changes even more quickly and also lowering the system load.

When x11vnc receives user input events (keystrokes, pointer motion, and pointer button clicks) from a VNC viewer, it must inject them synthetically into the X server. The X11 programming interfaces XTestFakeKeyEvent, XTestFakeMotionEvent, and XTestFakeButtonEvent of the XTEST extension are used to achieve this.

For non-X11 managed devices (such as the Mac OS X graphics display) different programming interfaces must be used. x11vnc also provides an interface where the user can supply their own input injection program.