UDP is a connectionless protocol, and its behavior when a packet reaches a port depends on whether the port is open or closed. Without a firewall altering the response, the standard protocol applies.
Why A is correct:When a UDP packet is sent to a closed port, the host typically responds with an ICMP Type 3 (Destination Unreachable), Code 3 (Port Unreachable) message, indicating no service is listening. CNSP notes this as a key indicator in port scanning.
Why other options are incorrect:
B:RST packets are TCP-specific, not used in UDP.
C:No response occurs for open UDP ports unless an application replies, not closed ports.
D:A is correct, so "none of the above" is invalid.
References:CNSP "Network Scanning Techniques" (Section on UDP Responses) confirms ICMP Port Unreachable as the standard response for closed UDP ports.
Contribute your Thoughts:
Chosen Answer:
This is a voting comment (?). You can switch to a simple comment. It is better to Upvote an existing comment if you don't have anything to add.
Submit