Teach the master server about game servers with IPv4 and IPv6 addresses.

On receiving ‘getInfo’, a game server may store the challenge sent by master
server, along with whether it was received over UDPv4 or UDPv6.

Later, in response to a ‘getInfo’ received over the OTHER protocol (UDPv6 or
UDPv4) from the same master server, it may send that saved response, using
the key name ‘challenge2’.

The master server will look up the challenge string and, if valid, make a
link between the two IP addresses.