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What is the response from a closed TCP port which is not behind a firewall?
Suggested Answer: C
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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.
13 comments · newest first
GE
German
10 months ago
A RST and an ACK is not the right response here.
WI
Willie
10 months ago
Wait, are you sure? I thought it could be a RST packet too.
LI
Ligia
10 months ago
I always thought closed ports just ignored requests.
SH
Shawnna
10 months ago
Agreed, A is the correct answer!
SO
Sommer
11 months ago
It's definitely an ICMP message showing Port Unreachable.
PO
Portia
11 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.
EA
Earnestine
11 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.
ME
Melinda
11 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.
CH
Cherrie
12 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."
RO
Roosevelt
12 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.
BU
Buck
12 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.
XU
Xuan
1 year 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.
MA
Marti
1 year ago
This seems like a straightforward networking question. I'll think through the TCP connection process to determine the correct response.