312-50V13 · Question #90
What is the proper response for a NULL scan if the port is open?
The correct answer is F. No response. When a NULL scan packet is sent to an open port, the expected behavior, according to the TCP specification, is for the port to provide no response.
Question
Options
- ASYN
- BACK
- CFIN
- DPSH
- ERST
- FNo response
How the community answered
(52 responses)- B2% (1)
- D8% (4)
- E4% (2)
- F87% (45)
Why each option
When a NULL scan packet is sent to an open port, the expected behavior, according to the TCP specification, is for the port to provide no response.
A SYN flag is used to initiate a connection, not as a response to a NULL scan on an open port.
An ACK flag is used to acknowledge received data or connection establishment, which doesn't occur with a NULL scan on an open port.
A FIN flag is used to terminate a connection, not as a response to a NULL scan on an open port.
A PSH flag is used to instruct the receiver to deliver data immediately, not as a response to a NULL scan on an open port.
An RST (reset) flag is typically sent in response to a NULL scan when the port is *closed*, indicating an immediate connection termination, not for an open port.
According to RFC 793 (the TCP specification), if an incoming TCP segment does not have the RST or SYN flags set, and the destination port is open, the segment should be dropped without a response. Therefore, a NULL scan (no flags set) to an open port yields no reply.
Concept tested: TCP NULL scan response (open port)
Source: https://nmap.org/book/man-port-scanning-techniques.html
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