If the localAddr is empty, it will listen on a random port.

ExchangeInfo is used to exchange information between client and visitor.

MakeHole is used to make a NAT hole between client and visitor.

PreCheck is used to check if the proxy is ready for penetration.

Prepare is used to do some preparation work before penetration.

NatHoleTimeout seconds.

mode 0: simple detect mode, usually for both EasyNAT or HardNAT & EasyNAT(Public Network) a.