312-50V11 · Question #610
What are two things that are possible when scanning UDP ports? (Choose two)
The correct answer is B. An ICMP message will be returned E. Nothing. UDP is connectionless, so a closed port triggers an ICMP Port Unreachable response while an open port typically generates no response at all.
Question
What are two things that are possible when scanning UDP ports? (Choose two)
Options
- AA reset will be returned
- BAn ICMP message will be returned
- CThe four-way handshake will not be completed
- DAn RFC 1294 message will be returned
- ENothing
How the community answered
(33 responses)- A12% (4)
- B79% (26)
- C6% (2)
- D3% (1)
Why each option
UDP is connectionless, so a closed port triggers an ICMP Port Unreachable response while an open port typically generates no response at all.
A TCP RST (reset) packet is a TCP-layer mechanism for rejecting unexpected segments and is never sent in response to UDP traffic.
When a UDP datagram reaches a closed port, the target host responds with an ICMP Type 3 Code 3 (Port Unreachable) message as defined in RFC 792, allowing a scanner to infer the port is closed.
The four-way handshake (FIN/ACK sequence) is a TCP connection-termination procedure and has no applicability to the connectionless UDP protocol.
RFC 1294 defines multiprotocol interconnect over Frame Relay and has no relationship to UDP port scanning behavior or responses.
When a UDP datagram reaches an open port, the receiving application may silently accept it and send no reply, leaving the scanner with no response - this silence is interpreted as a potentially open port.
Concept tested: UDP port scanning behavior and ICMP unreachable responses
Source: https://www.rfc-editor.org/rfc/rfc792
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