The SecOps Group Exam CNSP Topic 15 Question 10 Discussion

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.