Quick Answer
The answer is analyzing TTL values from captured packets, as this is a textbook example of passive OS fingerprinting. This technique works because different operating systems set distinct default initial Time to Live values—Windows typically uses 128, Linux uses 64, and Cisco IOS uses 255—so by examining the TTL in a passively captured packet, you can infer the source OS without ever sending a probe to the target. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of reconnaissance methods that avoid active detection; a common trap is confusing passive fingerprinting with active techniques like Nmap scans that send crafted packets. Remember that passive fingerprinting relies solely on sniffing existing traffic, making it stealthy. For a quick memory tip: think “TTL tells the OS tale”—if you see a TTL near 64, think Linux; near 128, think Windows; near 255, think Cisco.
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. 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 an example of passive OS fingerprinting?
Performing a TCP SYN scan
Why wrong: SYN scan is active.
Nmap -O scan
Why wrong: -O sends probes, making it active.
Sending ICMP echo requests
Why wrong: ICMP echo requests are active probes.
Analyzing TTL values from captured packets
TTL can indicate OS type passively.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Analyzing TTL values from captured packets
Passive OS fingerprinting involves analyzing captured network traffic without sending any packets to the target. Examining TTL (Time to Live) values from captured packets is a classic passive technique because different operating systems use default initial TTL values (e.g., Windows uses 128, Linux uses 64, Cisco IOS uses 255), and by observing the TTL in a received packet, you can infer the OS without actively probing the host.
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.
Performing a TCP SYN scan
Why it's wrong here
SYN scan is active.
Nmap -O scan
Why it's wrong here
-O sends probes, making it active.
Sending ICMP echo requests
Why it's wrong here
ICMP echo requests are active probes.
Analyzing TTL values from captured packets
Why this is correct
TTL can indicate OS type passively.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
EC-Council often tests the distinction between active and passive techniques, and the trap here is that candidates confuse 'analyzing captured data' (passive) with 'sending probes and analyzing responses' (active), leading them to pick options like Nmap -O scan or TCP SYN scan.
Detailed technical explanation
Passive fingerprinting relies on subtle differences in how OS stacks implement TCP/IP, such as initial TTL, window size, DF flag, and TCP options like MSS and SACK. For example, a captured packet with TTL 128 and a window size of 65535 strongly suggests a Windows system, while TTL 64 with a window of 5840 points to a Linux kernel. This technique is stealthy because it never generates traffic to the target, making it ideal for initial reconnaissance in penetration testing.
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
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
The correct answer is: Analyzing TTL values from captured packets — Passive OS fingerprinting involves analyzing captured network traffic without sending any packets to the target. Examining TTL (Time to Live) values from captured packets is a classic passive technique because different operating systems use default initial TTL values (e.g., Windows uses 128, Linux uses 64, Cisco IOS uses 255), and by observing the TTL in a received packet, you can infer the OS without actively probing the host.
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.
About these practice questions
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which TWO of the following are examples of passive OS fingerprinting techniques? (Select 2)
mediumWhy B: Option B is correct because passive OS fingerprinting involves observing network traffic without sending any packets to the target. Analyzing the initial TTL value of received IP packets is a classic passive technique: different operating systems set specific default TTL values (e.g., Windows uses 128, Linux uses 64, Cisco IOS uses 255), allowing an attacker to infer the OS without direct interaction.
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Last reviewed: Jun 30, 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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