312-50V13 · Question #89
What is a NULL scan?
The correct answer is A. A scan in which all flags are turned off. A NULL scan is a type of TCP port scan characterized by sending TCP packets with no control flags set in the header.
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Options
- AA scan in which all flags are turned off
- BA scan in which certain flags are off
- CA scan in which all flags are on
- DA scan in which the packet size is set to zero
- EA scan with an illegal packet size
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(27 responses)- A93% (25)
- D4% (1)
- E4% (1)
Why each option
A NULL scan is a type of TCP port scan characterized by sending TCP packets with no control flags set in the header.
A NULL scan sends a TCP segment where none of the control flags (SYN, ACK, RST, PSH, URG, FIN) are set in the TCP header, effectively turning all flags off.
Saying 'certain flags are off' is imprecise; a NULL scan specifically means *all* flags are off.
A scan with all flags on is typically referred to as an XMAS scan (FIN, PSH, URG flags set), not a NULL scan.
The packet size is not the defining characteristic of a NULL scan; it refers to the state of the TCP flags.
The concept of an 'illegal packet size' is unrelated to the definition of a NULL scan, which focuses on TCP flag settings.
Concept tested: TCP NULL scan definition
Source: https://nmap.org/book/man-port-scanning-techniques.html
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