In UDP port scanning, what is the Open Response?

• A. Close Response: ICMP Port Unreachable (type 3, code 3)
• B. No response
• C. Open Response: A service responds with a UDP packet
• D. Filtered Response: Other ICMP unreachable errors (type 3, codes 0, 1, 2, 9, 10, 13)

Answer: (C)

In UDP port scanning, an Open Response means the port has a listening service and replies to your probe with its own UDP packet. The presence of that UDP response shows there is an active listener on that port, so the port is open. For example, a DNS server responding on UDP port 53 to your query indicates the port is open. In contrast, a reply of ICMP Port Unreachable (type 3, code 3) indicates the port is closed, since there’s no listener to handle the probe. If you get no response at all, the port could be open but unresponsive or it could be filtered by a firewall, which is why no response is ambiguous. If you see other ICMP unreachable codes (type 3 with codes 0, 1, 2, 9, 10, 13), that typically signals filtering rather than an open port.