Quick Answer
The answer is passive OS fingerprinting and DNS reverse lookup. These two techniques are the most appropriate for stealth host discovery because they are entirely passive, meaning they never send packets directly to the target hosts. A DNS reverse lookup queries the DNS server for PTR records associated with IP addresses, revealing live hosts without generating network traffic that could trigger an IDS or firewall. Passive OS fingerprinting, meanwhile, analyzes packet headers already present in network traffic—such as TTL values and window sizes—to determine the operating system of a host without actively probing it. On the CEH exam, this question tests your understanding of the reconnaissance phase, where the goal is to gather intelligence without detection. A common trap is confusing active techniques like ping sweeps or port scans with passive ones; remember that if you are not sending a packet directly to the target, it is passive. Memory tip: “DNS and OS, both passive—no packets, no fuss.”
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. 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.
A penetration tester is conducting reconnaissance and wants to identify live hosts in a range without being detected. Which TWO techniques would be MOST appropriate? (Choose two.)
Ping sweep
Why wrong: Ping sweep sends ICMP packets and can be detected.
ARP scan
Why wrong: ARP scans are local and can be noisy.
DNS query for reverse lookup
DNS queries are generally allowed and less suspicious.
Passive OS fingerprinting
Passive fingerprinting sniffs traffic without sending packets.
TCP SYN scan
Why wrong: SYN scan sends packets and may trigger IDS.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
DNS query for reverse lookup
A DNS query for reverse lookup (PTR record) is a passive reconnaissance technique that queries DNS servers for hostnames associated with IP addresses. It does not send packets directly to the target hosts, making it difficult for intrusion detection systems (IDS) or firewalls to detect the probing activity. This aligns with the goal of identifying live hosts without generating network traffic that would alert defensive measures.
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.
Ping sweep
Why it's wrong here
Ping sweep sends ICMP packets and can be detected.
ARP scan
Why it's wrong here
ARP scans are local and can be noisy.
DNS query for reverse lookup
Why this is correct
DNS queries are generally allowed and less suspicious.
Related concept
Read the scenario before looking for a memorised answer.
Passive OS fingerprinting
Why this is correct
Passive fingerprinting sniffs traffic without sending packets.
Related concept
Read the scenario before looking for a memorised answer.
TCP SYN scan
Why it's wrong here
SYN scan sends packets and may trigger IDS.
Common exam traps
The trap here is that candidates often confuse passive OS fingerprinting (which analyzes network traffic patterns without sending probes) with active fingerprinting techniques, or they assume that any DNS query is inherently passive, overlooking that DNS queries themselves generate traffic that may be logged by DNS servers.
Detailed technical explanation
Reverse DNS lookups leverage PTR records stored in the in-addr.arpa domain for IPv4 (or ip6.arpa for IPv6). A single DNS query can resolve an IP to a hostname, and by iterating through a range, a tester can infer live hosts without direct interaction. However, many networks do not maintain consistent PTR records, and rate-limiting or DNS query logging on authoritative servers can still reveal reconnaissance activity.
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: DNS query for reverse lookup — A DNS query for reverse lookup (PTR record) is a passive reconnaissance technique that queries DNS servers for hostnames associated with IP addresses. It does not send packets directly to the target hosts, making it difficult for intrusion detection systems (IDS) or firewalls to detect the probing activity. This aligns with the goal of identifying live hosts without generating network traffic that would alert defensive measures.
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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