Quick Answer
The answer is capturing packets and analyzing TTL values, as this is a core passive OS fingerprinting technique. Unlike active methods that send probes to a target, passive fingerprinting relies on observing existing network traffic; by examining the Time to Live (TTL) value in captured packets, you can infer the operating system because different OSes set distinct default TTLs—Windows typically uses 128, Linux uses 64, and Cisco IOS uses 255. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of reconnaissance without generating detectable traffic, often appearing in questions that contrast passive versus active techniques. A common trap is confusing passive fingerprinting with active tools like Nmap’s OS detection, which sends crafted packets. For a quick memory tip, remember the phrase “Passive Peers, Active Pings”—passive techniques just watch the TTL values in normal traffic, while active ones send probes.
This CEH practice question tests your understanding of enumeration and system hacking. 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.
Which of the following is a passive OS fingerprinting technique?
Using nmap -O
Why wrong: nmap -O sends probes, making it active.
Performing a ping sweep
Why wrong: Ping sweep is active.
Sending SYN packets and analyzing responses
Why wrong: This is active fingerprinting.
Capturing packets and analyzing TTL values
TTL analysis is passive and does not generate traffic.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Capturing packets and analyzing TTL values
Passive OS fingerprinting involves observing network traffic without actively sending packets to the target. Option D is correct because capturing packets and analyzing TTL (Time to Live) values allows an attacker to infer the operating system, as different OSes use default TTL values (e.g., Windows uses 128, Linux uses 64, Cisco IOS uses 255). This technique does not generate any probe traffic, making it passive.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Using nmap -O
Why it's wrong here
nmap -O sends probes, making it active.
Performing a ping sweep
Why it's wrong here
Ping sweep is active.
Sending SYN packets and analyzing responses
Why it's wrong here
This is active fingerprinting.
Capturing packets and analyzing TTL values
Why this is correct
TTL analysis is passive and does not generate traffic.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates confuse 'passive' with 'stealthy' scanning, often picking nmap -O with stealth flags (e.g., -sS) as passive, but any active packet injection, regardless of stealth, constitutes active fingerprinting.
Detailed technical explanation
Passive fingerprinting relies on subtle differences in how OSes implement the IP and TCP stacks, such as initial TTL values, window size, DF flag, and TCP options (e.g., MSS, WS). For example, a captured packet with TTL=128 and a window size of 65535 strongly suggests a Windows system, while TTL=64 and a window size of 29200 indicates Linux. This technique is stealthy because it leaves no logs on the target, making it ideal for reconnaissance in red team operations or when avoiding detection is critical.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the CEH 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
Enumeration and System Hacking — This question tests Enumeration and System Hacking — Read the scenario before looking for a memorised answer..
The correct answer is: Capturing packets and analyzing TTL values — Passive OS fingerprinting involves observing network traffic without actively sending packets to the target. Option D is correct because capturing packets and analyzing TTL (Time to Live) values allows an attacker to infer the operating system, as different OSes use default TTL values (e.g., Windows uses 128, Linux uses 64, Cisco IOS uses 255). This technique does not generate any probe traffic, making it passive.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This CEH practice question is part of Courseiva's free EC-Council certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CEH exam.
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