VNC LED state Pseudo-encoding


This document describes the Pseudo-encoding of LED state for RFB which is the protocol used in VNC as reference link below:

When accessing a guest by console through VNC, there might be mismatch between the lock keys notification LED on the computer running the VNC client session and the current status of the lock keys on the guest machine.

To solve this problem it attempts to add LED state Pseudo-encoding extension to VNC protocol to deal with setting LED state.


This Pseudo-encoding requested by client declares to server that it supports LED state extensions to the protocol.

The Pseudo-encoding number for LED state defined as:

Number Name
-261 ‘LED state Pseudo-encoding’

LED state Pseudo-encoding

The LED state Pseudo-encoding describes the encoding of LED state which consists of 3 bits, from left to right each bit represents the Caps, Num, and Scroll lock key respectively. ‘1’ indicates that the LED should be on and ‘0’ should be off.

Some example encodings for it as following:

Code Description
100 CapsLock is on, NumLock and ScrollLock are off
010 NumLock is on, CapsLock and ScrollLock are off
111 CapsLock, NumLock and ScrollLock are on