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What is the response from a closed UDP port which is not behind a firewall?
Suggested Answer: A
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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.
28 comments · newest first
JE
Jennifer
10 months ago
Wait, are you sure about that? Sounds a bit off.
LU
Lucille
10 months ago
I thought it would just be no response at all?
IV
Ivan
10 months ago
Agreed, ICMP is the way to go here!
KA
Kallie
10 months ago
RST packets are for TCP, not UDP!
HO
Hoa
11 months ago
Definitely an ICMP message showing Destination Unreachable.
RO
Rodolfo
11 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.
EL
Eladia
11 months ago
I practiced a similar question, and I remember that closed ports send ICMP messages, so I'm leaning towards A too.
NA
Nathan
11 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.
SA
Sabrina
12 months ago
I think the answer might be A, since I remember something about ICMP messages for unreachable destinations.
PE
Pearly
12 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.
ES
Esteban
12 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.
AZ
Azalee
1 year 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?
SA
Santos
1 year 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.
ST
Stephaine
1 year ago
That could be a possibility too, but I still think C) No response is more likely.
VE
Verdell
1 year ago
But wouldn't it make more sense for the response to be A) ICMP message showing Destination Unreachable?
EL
Elise
1 year ago
I agree with Stephaine, because if it's not behind a firewall, there's no need for any response.
ST
Stephaine
1 year ago
I think the response would be C) No response.
SH
Shenika
1 year 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.
SA
Salena
1 year ago
Haha, I bet the right answer is D. 'None of the above' is always a good choice when you're not sure!
ME
Merlyn
1 year 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.
LO
Lonna
1 year ago
So, the answer is C) No response.
PA
Paz
1 year ago
Actually, the correct response from a closed UDP port is no response.
JE
Jennifer
1 year ago
I think you're right, a closed UDP port should send a RST packet.
GO
Goldie
1 year ago
C, definitely. If the port is closed and not behind a firewall, there should be no response at all. Simple as that.
LA
Lachelle
1 year ago
Hmm, I'm pretty sure it's B. A closed UDP port should respond with a RST packet, right? The ICMP message is for a closed TCP port.
SH
Shantay
1 year ago
User 3: So the answer is B) A RST packet.
AL
Aleisha
1 year ago
User 2: That's correct. The ICMP message is for a closed TCP port.
FE
Felicia
1 year ago
User 1: A closed UDP port should respond with a RST packet.