312-50V9 · Question #523
Which Type of scan sends a packets with no flags set? Select the Answer
The correct answer is B. Null Scan. A Null scan is a TCP stealth scanning technique that sends packets with no TCP flags set, exploiting RFC 793 behavior to infer port states.
Question
Which Type of scan sends a packets with no flags set? Select the Answer
Options
- AOpen Scan
- BNull Scan
- CXmas Scan
- DHalf-Open Scan
How the community answered
(33 responses)- A6% (2)
- B91% (30)
- D3% (1)
Why each option
A Null scan is a TCP stealth scanning technique that sends packets with no TCP flags set, exploiting RFC 793 behavior to infer port states.
Open Scan (also called a full connect or TCP connect scan) completes the full three-way handshake using SYN, SYN-ACK, and ACK flags.
A Null scan sends TCP packets with none of the standard flags (SYN, ACK, FIN, RST, PSH, URG) set in the header. According to RFC 793, closed ports should respond with a RST packet, while open ports drop the packet and send no response. This behavior allows attackers to infer open ports while potentially evading some firewalls and intrusion detection systems that only monitor flagged packets.
Xmas Scan sets the FIN, PSH, and URG flags simultaneously, giving the packet an 'all lit up' appearance - the opposite of a Null scan.
Half-Open Scan (SYN scan) sends a packet with only the SYN flag set and does not complete the three-way handshake.
Concept tested: TCP Null scan - no flags set behavior
Source: https://nmap.org/book/scan-methods-null-fin-xmas-scan.html
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