Quick Answer
The answer is active OS fingerprinting, the technique that involves sending crafted packets to a target to elicit responses that reveal the operating system. This method works by dispatching specially designed probes—such as TCP SYN, FIN, or ICMP packets—and analyzing how the target’s TCP/IP stack replies, since different operating systems implement subtle variations in fields like initial TTL values, window sizes, and TCP option ordering. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of network reconnaissance and the distinction between active and passive fingerprinting; a common trap is confusing it with passive fingerprinting, which merely sniffs traffic without sending any packets. To remember it, think of the Nmap `-O` flag as “O” for “Offensive” or “Outgoing” packets—you have to actively send something to get the OS answer.
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 techniques involves sending crafted packets to a target to elicit responses that reveal the operating system?
Passive OS fingerprinting
Why wrong: Passive fingerprinting observes traffic without sending packets.
Banner grabbing
Why wrong: Banner grabbing retrieves service banners, not necessarily OS identification.
DNS enumeration
Why wrong: DNS enumeration gathers DNS records, not OS information.
Active OS fingerprinting
Active fingerprinting sends probes to infer the OS.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Active OS fingerprinting
Active OS fingerprinting involves sending crafted packets (e.g., TCP SYN, FIN, or ICMP probes) to a target and analyzing the responses to deduce the operating system. Unlike passive methods, it actively interacts with the target, leveraging subtle differences in how various OSes implement TCP/IP stack behaviors (e.g., initial TTL values, window sizes, or TCP option ordering). This technique is commonly used by tools like Nmap with its `-O` flag.
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.
Passive OS fingerprinting
Why it's wrong here
Passive fingerprinting observes traffic without sending packets.
Banner grabbing
Why it's wrong here
Banner grabbing retrieves service banners, not necessarily OS identification.
DNS enumeration
Why it's wrong here
DNS enumeration gathers DNS records, not OS information.
Active OS fingerprinting
Why this is correct
Active fingerprinting sends probes to infer the OS.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often confuse 'active OS fingerprinting' with 'banner grabbing' because both involve sending packets, but banner grabbing targets application-layer banners (e.g., HTTP, FTP) and not the OS-level TCP/IP stack responses that fingerprinting analyzes.
Detailed technical explanation
Active OS fingerprinting exploits RFC non-compliance and implementation quirks in TCP/IP stacks; for example, sending a TCP SYN packet to a closed port may elicit an RST with a specific window size or IP ID pattern unique to Windows vs. Linux. Tools like Nmap use a database of over 2,000 signatures, comparing responses to probes like T2 (TCP NULL) or T3 (TCP FIN with no flags) to narrow down the OS. In real-world pentests, this helps tailor exploits to the target's OS, such as selecting a Linux-specific kernel exploit over a Windows one.
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: Active OS fingerprinting — Active OS fingerprinting involves sending crafted packets (e.g., TCP SYN, FIN, or ICMP probes) to a target and analyzing the responses to deduce the operating system. Unlike passive methods, it actively interacts with the target, leveraging subtle differences in how various OSes implement TCP/IP stack behaviors (e.g., initial TTL values, window sizes, or TCP option ordering). This technique is commonly used by tools like Nmap with its `-O` flag.
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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