- A
SYN scan
Why wrong: A SYN scan sends packets with only the SYN flag set to initiate connections.
- B
Xmas tree scan
An Xmas tree scan sets the FIN, PSH, and URG flags. It is a stealthy technique used to probe open/closed ports.
- C
Null scan
Why wrong: A Null scan sends a packet with no flags set, which is different from the FIN, PSH, URG combination.
- D
ACK scan
Why wrong: An ACK scan sends packets with only the ACK flag set, used to map firewall rules, not to set FIN, PSH, and URG.
Question 165 of 520
N10-009 Network Security Practice Question
This N10-009 practice question tests your understanding of network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A security analyst notices a large number of incoming TCP packets to a server with the FIN, PSH, and URG flags set. This pattern is characteristic of which type of network scan?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Xmas tree scan
An Xmas tree scan sends TCP packets with the FIN, PSH, and URG flags set (the packet 'lights up' like a Christmas tree). This is a stealth scan technique used to probe open or closed ports based on RFC 793 behavior: closed ports should respond with an RST packet, while open ports may ignore the packet (or respond differently depending on the OS). The pattern described—FIN, PSH, and URG all set—is the definitive signature of an Xmas tree scan.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
SYN scan
Why it's wrong here
A SYN scan sends packets with only the SYN flag set to initiate connections.
When this WOULD be correct
A SYN scan would be correct if the question described a scan that sends packets with only the SYN flag set to initiate a TCP handshake and determine open ports based on SYN-ACK responses.
- ✓
Xmas tree scan
Why this is correct
An Xmas tree scan sets the FIN, PSH, and URG flags. It is a stealthy technique used to probe open/closed ports.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Null scan
Why it's wrong here
A Null scan sends a packet with no flags set, which is different from the FIN, PSH, URG combination.
- ✗
ACK scan
Why it's wrong here
An ACK scan sends packets with only the ACK flag set, used to map firewall rules, not to set FIN, PSH, and URG.
When this WOULD be correct
An ACK scan would be correct if the question described a scan that sends packets with only the ACK flag set to map firewall rules, as it can determine whether a port is filtered or unfiltered based on RST responses.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓Xmas tree scanCorrect answer▾
Why this is correct
An Xmas tree scan sets the FIN, PSH, and URG flags. It is a stealthy technique used to probe open/closed ports.
✗SYN scanWrong answer — click to see why▾
Why this is wrong here
A SYN scan sends packets with only the SYN flag set, not the FIN, PSH, and URG flags. The described pattern of FIN, PSH, and URG flags is characteristic of an Xmas tree scan.
★ When this WOULD be the correct answer
A SYN scan would be correct if the question described a scan that sends packets with only the SYN flag set to initiate a TCP handshake and determine open ports based on SYN-ACK responses.
Why candidates choose this
Candidates often confuse different scan types because SYN scans are very common and well-known, leading them to incorrectly associate any unusual flag pattern with a SYN scan.
✗Null scanWrong answer — click to see why▾
Why this is wrong here
A null scan sends packets with no flags set (all flags off), whereas the question describes packets with FIN, PSH, and URG flags set, which is characteristic of an Xmas tree scan.
★ When this WOULD be the correct answer
A null scan would be the correct answer for a question describing a scan that sends TCP packets with no flags set (all flags zero), used to probe firewall rules or identify open ports based on RFC 793 behavior.
Why candidates choose this
Candidates may confuse null scans with other stealth scans like Xmas tree scans, as both are types of TCP flag scans used to evade detection, but they differ in which flags are set.
✗ACK scanWrong answer — click to see why▾
Why this is wrong here
ACK scan sends packets with only the ACK flag set, not FIN, PSH, and URG. The question describes a scan with multiple flags set, which is characteristic of an Xmas tree scan.
★ When this WOULD be the correct answer
An ACK scan would be correct if the question described a scan that sends packets with only the ACK flag set to map firewall rules, as it can determine whether a port is filtered or unfiltered based on RST responses.
Why candidates choose this
Candidates may confuse ACK scan with other scan types because they know ACK is used in TCP, and the mention of TCP packets might lead them to think of ACK without considering the specific flag combination.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between Xmas tree, Null, and SYN scans by focusing on the specific flag combinations, so the trap here is confusing the FIN/PSH/URG set (Xmas tree) with a Null scan (no flags) or a SYN scan (only SYN).
Detailed technical explanation
How to think about this question
Under the hood, the Xmas tree scan exploits RFC 793's requirement that a closed port must respond with an RST to any incoming segment not containing an RST, regardless of flags. However, many modern operating systems (e.g., Windows, some Linux kernels) do not comply strictly with this RFC, making the scan unreliable against those targets. In real-world penetration testing, this scan is often used to evade simple packet filters that only block SYN packets, but it is easily detected by modern intrusion detection systems (IDS) due to the unusual flag combination.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Xmas tree scan — An Xmas tree scan sends TCP packets with the FIN, PSH, and URG flags set (the packet 'lights up' like a Christmas tree). This is a stealth scan technique used to probe open or closed ports based on RFC 793 behavior: closed ports should respond with an RST packet, while open ports may ignore the packet (or respond differently depending on the OS). The pattern described—FIN, PSH, and URG all set—is the definitive signature of an Xmas tree scan.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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