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.
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?
Why wrong: Banner grabbing requires connecting to a service.
B
Passive OS fingerprinting using captured packets
Passive fingerprinting analyzes packet headers from existing traffic, such as via p0f.
C
Querying Shodan for the target IP
Why wrong: Shodan provides information from previous scans, but the tester is not sending packets itself; however, the technique described is passive fingerprinting.
D
Active OS fingerprinting with Nmap -O
Why wrong: Nmap -O sends active probes to the target.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Passive OS fingerprinting using captured packets
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.
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.
✗
Banner grabbing with Telnet
Why it's wrong here
Banner grabbing requires connecting to a service.
✓
Passive OS fingerprinting using captured packets
Why this is correct
Passive fingerprinting analyzes packet headers from existing traffic, such as via p0f.
Related concept
Read the scenario before looking for a memorised answer.
✗
Querying Shodan for the target IP
Why it's wrong here
Shodan provides information from previous scans, but the tester is not sending packets itself; however, the technique described is passive fingerprinting.
✗
Active OS fingerprinting with Nmap -O
Why it's wrong here
Nmap -O sends active probes to the target.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'passive OS fingerprinting' with 'banner grabbing' or 'Shodan queries,' assuming any non-intrusive method qualifies, but the key constraint is 'without sending any packets,' which eliminates all options except passive analysis of already-captured traffic.
Detailed technical explanation
How to think about this question
Passive fingerprinting works by examining fields like the initial TTL (e.g., 64 for Linux, 128 for Windows), TCP window size (e.g., 65535 for Linux, 8192 for older Windows), and the IP ID behavior (e.g., incremental vs. random). In a real-world scenario, a tester might capture traffic from a public Wi-Fi hotspot to identify a server's OS without alerting intrusion detection systems, using tools like p0f or Satori. A subtle behavior is that some modern OSes randomize certain fields, requiring analysis of multiple packets to avoid misidentification.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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: Passive OS fingerprinting using captured packets — 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.
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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