If you have setup your bridge and tap directly in the network interfaces file, you still have to chmod the /dev/net/tun folder to successfully utilize host networking. I use a quick startup script located conveniently in my home folder. sudo chmod 666 /dev/net/tun Place this line in a file and name it host.sh or something. Then open your preferences>>session configuration menu and set it to be run on startup. Now with correct network interfaces configuration your guest system will successfully recognize the tap.