% VBoxManage list extpacks
Extension Packs: 1
Pack no. 0: VNC
Version: 4.2.6_OSE
Revision: 82870
Edition:
Description: VNC plugin module
VRDE Module: VBoxVNC
Usable: true
Why unusable:
% VBoxManage setproperty vrdeextpack VNC
% VBoxManage modifyvm test --vrdeproperty VNCPassword=somepass
% VBoxManage modifyvm test --vrdeauthlibrary null
% VBoxManage modifyvm test --vrdeport 1501
% VBoxHeadless -s test &