In UDP port scanning, what is the Filtered Response?

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

Answer: (D)

In UDP port scanning, a Filtered state means the probe was blocked by a firewall or filtering device, so no definitive open/closed result is returned. This is shown when an ICMP unreachable message arrives that isn’t the port-unreachable code—specifically ICMP Type 3 with codes 0, 1, 2, 9, 10, or 13. These codes indicate the traffic was administratively blocked or otherwise filtered along the path, not that the port itself is open or closed. The other options describe scenarios where there is a direct reply (open), a specific closed indication (port unreachable), or no reply at all (ambiguous open|filtered), whereas the listed ICMP codes are the signals used to classify a response as filtered.