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The SecOps Group CNSP Exam - Topic 6 Question 11 Discussion

What is the response from a closed TCP port which is not behind a firewall?
C) A RST and an ACK packet
A) ICMP message showing Port Unreachable
B) A FIN and an ACK packet
D) A SYN and an ACK packet

The SecOps Group CNSP Exam - Topic 6 Question 11 Discussion

Actual exam question for The SecOps Group's CNSP exam
Question #: 11
Topic #: 6
[All CNSP Questions]

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

Show Suggested Answer Hide Answer
Suggested Answer: C

TCP uses a structured handshake, and its response to a connection attempt on a closed port follows a specific protocol when unobstructed by a firewall.

Why C is correct: A closed TCP port responds with a RST (Reset) and ACK (Acknowledgment) packet to terminate the connection attempt immediately. CNSP highlights this as a key scanning indicator.

Why other options are incorrect:

A: ICMP Port Unreachable is for UDP, not TCP.

B: FIN/ACK is for closing active connections, not rejecting new ones.

D: SYN/ACK indicates an open port during the TCP handshake.


by Marvel at Jul 08, 2025, 10:08 AM

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German
7 months ago
A RST and an ACK is not the right response here.
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Willie
7 months ago
Wait, are you sure? I thought it could be a RST packet too.
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Ligia
7 months ago
I always thought closed ports just ignored requests.
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Shawnna
7 months ago
Agreed, A is the correct answer!
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Sommer
8 months ago
It's definitely an ICMP message showing Port Unreachable.
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Portia
8 months ago
I’m pretty sure that a closed TCP port sends an ICMP message, but I might be mixing it up with how a firewall would respond.
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Earnestine
8 months ago
I feel like I saw something about a FIN and ACK in a similar question, but that doesn't seem right for a closed port.
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Melinda
8 months ago
I remember practicing a question about TCP responses, and I think a closed port sends a RST packet, but I can't recall if it includes an ACK.
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Cherrie
9 months ago
I think the response from a closed TCP port is an ICMP message, but I'm not entirely sure if it's specifically "Port Unreachable."
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Roosevelt
9 months ago
I'm pretty confident the answer is C. A closed TCP port that's not behind a firewall will send a RST and ACK packet to indicate the connection was reset.
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Buck
9 months ago
Okay, I've got this. If the port is closed and not behind a firewall, the correct response should be a RST and ACK packet, since the connection can't be established.
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Xuan
9 months ago
Hmm, I'm a bit unsure about this one. I know TCP has different packet types, but I'm not sure which one would be the response from a closed port. I'll have to think it through carefully.
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Marti
9 months ago
This seems like a straightforward networking question. I'll think through the TCP connection process to determine the correct response.
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