Add option to add a forwarding of a range of ports

[jorrit]

No description provided.

[rkr87]

Need to amend as below;

        pflist.append(
        proto=snmp.IPProtocol[args.protocol],
        ext_port_start=args.external_port,
        ext_port_end=args.external_port + args.number_of_ports - 1,
        local_port_start=args.internal_port,
        local_port_end=args.internal_port + args.number_of_ports - 1,
        local_addr=args.internal_ip)

otherwise it will open 1 additional port for every request.

[jorrit]

That off by one error again! Thanks @rkr87 !