Quick Answer
The answer is analyzing TTL and window size from captured packets, as this passive OS fingerprinting technique does not send any packets to the target. This method works by inspecting the Time-to-Live (TTL) and TCP window size values in intercepted traffic, which are operating system-specific defaults—for instance, Linux typically uses a TTL of 64 and a window size of 5840, while Windows uses TTL 128 and window size 65535. Since no packets are ever transmitted to the target, the technique remains entirely passive and undetectable, a key distinction from active fingerprinting. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of reconnaissance methods that avoid alerting the target; a common trap is confusing passive techniques with active ones like sending crafted probes. Remember the memory tip: “No packets, no problem—just watch the TTL and window to tell the OS tale.”
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 a passive OS fingerprinting technique that does NOT send any packets to the target?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "NOT"
Why it matters: Negative qualifier — you are looking for the one option that does NOT apply. Most options will be true; only one is false for this scenario.
Sending TCP SYN packets and analyzing responses
Why wrong: Sending packets is active fingerprinting.
Analyzing TTL and window size from captured packets
This is passive fingerprinting; it uses existing traffic.
Performing a NULL scan
Why wrong: NULL scan sends packets with no flags, which is active.
Using Nmap -O flag
Why wrong: -O sends probes and analyzes responses, making it active.
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 and window size from captured packets
Passive OS fingerprinting involves observing network traffic without sending any packets to the target. Analyzing the Time-to-Live (TTL) and TCP window size from captured packets is a classic passive technique because these values are OS-specific defaults (e.g., Linux often uses TTL 64 and window size 5840, while Windows uses TTL 128 and window size 65535). Since no packets are sent to the target, this method is entirely passive and undetectable.
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.
Sending TCP SYN packets and analyzing responses
Why it's wrong here
Sending packets is active fingerprinting.
Analyzing TTL and window size from captured packets
Why this is correct
This is passive fingerprinting; it uses existing traffic.
Clue confirmation
The clue word "NOT" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Performing a NULL scan
Why it's wrong here
NULL scan sends packets with no flags, which is active.
Using Nmap -O flag
Why it's wrong here
-O sends probes and analyzes responses, making it active.
Common exam traps
The trap here is that candidates often confuse passive fingerprinting with active techniques like banner grabbing or Nmap scans, assuming that any packet capture analysis is passive, but the key distinction is whether any packets are sent to the target.
Detailed technical explanation
Passive fingerprinting relies on observing subtle differences in how operating systems implement the TCP/IP stack. For example, the initial TTL value is set by the OS and decremented by each hop, but the initial value can be inferred if the hop count is known. The TCP window size is also OS-specific; Windows typically uses a multiple of 65535, while Linux uses 5840 or 29200 depending on the kernel version. Tools like p0f specialize in passive fingerprinting by analyzing these fields from captured packets without any interaction with the target.
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
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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 and window size from captured packets — Passive OS fingerprinting involves observing network traffic without sending any packets to the target. Analyzing the Time-to-Live (TTL) and TCP window size from captured packets is a classic passive technique because these values are OS-specific defaults (e.g., Linux often uses TTL 64 and window size 5840, while Windows uses TTL 128 and window size 65535). Since no packets are sent to the target, this method is entirely passive and undetectable.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "NOT". Negative qualifier — you are looking for the one option that does NOT apply. Most options will be true; only one is false for this scenario.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
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Variation 1. During a penetration test, the tester needs to identify the operating system of a remote host without sending any packets to it. Which technique should the tester use?
mediumWhy B: Passive OS fingerprinting (Option B) is correct because it analyzes captured network traffic—such as TCP/IP packet headers, TTL values, window sizes, and DF flags—to infer the remote host's operating system without sending any packets. This technique relies on subtle differences in how various OS stacks implement RFC 793, making it ideal for stealthy reconnaissance where no direct contact with the target is permitted.
Last reviewed: Jun 24, 2026
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