openvpn has had an option for such behavior (over udp obviously) for a while. the option is called tls-auth, it requires you to go through and generate an aditi

openvpn has had an option for such behavior (over udp obviously) for a while. the option is called tls-auth, it requires you to go through and generate an aditional key which has to exist on all clients and server. Last i remember is that even if you scan the server, it is completely quiet unless the right signature is received as well for each frame.