CEH Footprinting, Reconnaissance and Scanning Practice Question
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 TWO of the following are examples of passive OS fingerprinting techniques? (Select 2)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Analyzing the initial TTL value of received IP packets
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.
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.
✗
Performing a SYN scan on the target
Why it's wrong here
SYN scan is active reconnaissance.
✓
Analyzing the initial TTL value of received IP packets
Why this is correct
Different OSes use default TTL values; observing this helps identify the OS passively.
Related concept
Read the scenario before looking for a memorised answer.
✗
Sending a series of TCP packets with different flags and analyzing responses
Why it's wrong here
This is active fingerprinting (e.g., Nmap -O).
✓
Inspecting the TCP window size in SYN packets
Why this is correct
Different OSes use specific initial window sizes; this is passively observed.
Related concept
Read the scenario before looking for a memorised answer.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse passive fingerprinting (observing existing traffic) with active fingerprinting (sending crafted packets), leading them to select options like SYN scans or flag-based probes as passive techniques.
Detailed technical explanation
How to think about this question
Passive OS fingerprinting relies on subtle differences in how stacks implement RFC 793 and related standards. For example, the initial TTL value is set by the OS at connection setup and is not decremented by intermediate routers unless the packet is forwarded; analyzing it alongside the TCP window size (which can reveal stack-specific scaling factors) allows tools like p0f to identify OS families with high accuracy. In real-world scenarios, passive fingerprinting is stealthy because it leaves no logs on the target, making it ideal for initial reconnaissance in penetration testing.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Analyzing the initial TTL value of received IP packets — 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.
What should I do if I get this CEH 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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Question Discussion
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