Application Level Gateway (ALG) objects providing protocol specific support for port forwarding. The first object returning non-null in a call to its connection method will be the one that handles that particular connection.
Configures how connections are forwarded. The first two elements in each sublist specify the protocol and port to listen on. If zero, then all ports are listened on. If a sublist has five elements, then the additional arguments specify the ip address to listen on, and the ip address and port to forward to. If a sublist only has two element, then the connection will be forwarded to the ip address and port given in the incoming ip packet.
TCP layer. Must implement the tcp interface.
UDP layer. Must implement the udp interface.