Quick Answer
The answer is WHOIS lookup and Shodan search. Both are considered passive reconnaissance techniques because they gather information from publicly accessible sources without directly interacting with the target’s own systems or network infrastructure. WHOIS queries public registration databases like ARIN or RIPE under RFC 3912 to retrieve domain ownership and name server details, while Shodan indexes internet-connected devices and services from external scans, neither of which sends packets to the target’s servers. On the Certified Ethical Hacker CEH exam, this question tests your ability to distinguish passive from active methods—a common trap is confusing DNS enumeration or banner grabbing as passive when they actually involve direct contact with the target. A useful memory tip: if you can collect the data without the target knowing you exist, it’s passive; think “look up, don’t touch.”
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.
A security analyst wants to perform passive reconnaissance on a target domain. Which TWO of the following methods are considered passive? (Choose 2)
WHOIS lookup
Queries public WHOIS databases without contacting the target.
Shodan search
Shodan indexes Internet-facing devices; querying it is passive.
Telnet banner grab
Why wrong: Connecting to a service via Telnet is active.
Ping sweep
Why wrong: Ping sweep sends ICMP packets.
Nmap SYN scan
Why wrong: SYN scan sends packets to the target.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
WHOIS lookup
WHOIS lookup is passive because it queries public registration databases (e.g., ARIN, RIPE) via the WHOIS protocol (RFC 3912) to retrieve domain ownership, registrar, and name server information without sending any packets to the target's own infrastructure. This data is publicly available and does not interact with the target's servers or network, making it a classic passive reconnaissance technique.
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.
WHOIS lookup
Why this is correct
Queries public WHOIS databases without contacting the target.
Related concept
Read the scenario before looking for a memorised answer.
Shodan search
Why this is correct
Shodan indexes Internet-facing devices; querying it is passive.
Related concept
Read the scenario before looking for a memorised answer.
Telnet banner grab
Why it's wrong here
Connecting to a service via Telnet is active.
Ping sweep
Why it's wrong here
Ping sweep sends ICMP packets.
Nmap SYN scan
Why it's wrong here
SYN scan sends packets to the target.
Common exam traps
EC-Council often tests the distinction between passive and active reconnaissance by including tools like Shodan (which is passive) alongside active scanning tools like Nmap, leading candidates to mistakenly classify Shodan as active because it involves a search engine rather than direct network interaction.
Detailed technical explanation
Passive reconnaissance relies on publicly available information from third-party sources like WHOIS databases, DNS records (e.g., via nslookup or dig), search engines, and Shodan's indexed data. Shodan crawls the internet and stores banners from services, so querying Shodan is passive because it retrieves historical data without contacting the target. In contrast, active methods like SYN scans use raw sockets to send packets and can trigger firewall logs or IDS alerts, which is why they are avoided during stealthy initial reconnaissance.
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: WHOIS lookup — WHOIS lookup is passive because it queries public registration databases (e.g., ARIN, RIPE) via the WHOIS protocol (RFC 3912) to retrieve domain ownership, registrar, and name server information without sending any packets to the target's own infrastructure. This data is publicly available and does not interact with the target's servers or network, making it a classic passive reconnaissance technique.
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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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A security analyst performs a passive reconnaissance of a target domain using public resources. Which of the following techniques would be considered passive reconnaissance?
mediumWhy C: Passive reconnaissance involves gathering information without directly interacting with the target's systems, thus avoiding any network traffic that could be detected. A WHOIS lookup queries a public registry database (e.g., whois.arin.net) over port 43 or via a web interface, retrieving domain registration details without sending any packets to the target's own infrastructure. This makes it a purely passive technique.
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Last reviewed: Jun 30, 2026
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