The SecOps Group CNSP Exam - Topic 15 Question 10 Discussion

Actual exam question for The SecOps Group's CNSP exam

Question #: 10
Topic #: 15

[All CNSP Questions]

What is the response from a closed UDP port which is not behind a firewall?

AICMP message showing Destination Unreachable

BA RST packet

CNo response

DNone of the above

Show Suggested Answer

Suggested Answer: A

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.

by Vanesa at Jul 05, 2025, 04:41 PM

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Jennifer

5 months ago

Wait, are you sure about that? Sounds a bit off.

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Lucille

5 months ago

I thought it would just be no response at all?

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Ivan

6 months ago

Agreed, ICMP is the way to go here!

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Kallie

6 months ago

RST packets are for TCP, not UDP!

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Hoa

6 months ago

Definitely an ICMP message showing Destination Unreachable.

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Rodolfo

6 months ago

I’m confused; I thought RST packets were more related to TCP, not UDP. So I’m not sure about B being the right choice here.

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Eladia

7 months ago

I practiced a similar question, and I remember that closed ports send ICMP messages, so I'm leaning towards A too.

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Nathan

7 months ago

I'm not entirely sure, but I feel like it could also be C, because closed UDP ports often just don't respond at all.

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Sabrina

7 months ago

I think the answer might be A, since I remember something about ICMP messages for unreachable destinations.

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Pearly

7 months ago

This is a tricky one. I'm not 100% sure, but I'm leaning towards B. A RST packet seems like the most logical response from a closed UDP port that's not firewalled.

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Esteban

7 months ago

Okay, let me think this through. If the port is closed and not behind a firewall, it should send back an ICMP message indicating the port is unreachable. So I think the answer is A.

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Azalee

8 months ago

Hmm, I'm a bit unsure about this one. I know UDP is connectionless, so I'm not sure if a RST packet is the right response. Maybe it's C, no response?

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Santos

8 months ago

I'm pretty sure the answer is B, a RST packet. That's the standard response from a closed UDP port that's not behind a firewall.

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Stephaine

10 months ago

That could be a possibility too, but I still think C) No response is more likely.

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Verdell

10 months ago

But wouldn't it make more sense for the response to be A) ICMP message showing Destination Unreachable?

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Elise

10 months ago

I agree with Stephaine, because if it's not behind a firewall, there's no need for any response.

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Stephaine

10 months ago

I think the response would be C) No response.

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Shenika

11 months ago

Wait, a closed UDP port that's not behind a firewall? Isn't that like a unicorn or something? I'm going with C, no response.

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Salena

11 months ago

Haha, I bet the right answer is D. 'None of the above' is always a good choice when you're not sure!

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Merlyn

11 months ago

This one's tricky, but I think the answer is B. The UDP protocol doesn't have the same handshaking as TCP, so a closed port should just send a RST packet.

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