I use winpcap implemented TCP syn scan, I to construct data packets to send, wireshark show sends success, if the port is open, wireshark can see the returned response packet, I program can receive, but if the port is not open, returns an icmp port unreachable message, but is less than the icmp message, I am using filter rules "IP" or "icmp" are not receive, this is how one and the same, have the great spirit guide