CEH Footprinting, Reconnaissance and Scanning Practice Question
Which of the following techniques is considered PASSIVE reconnaissance?
⚠ Common exam trap
EC-Council often tests the distinction between passive and active reconnaissance by making candidates think that any network-based query (like WHOIS) is active, when in fact WHOIS queries public third-party databases, not the target's own systems.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Performing a WHOIS lookup
WHOIS queries retrieve domain registration records from public databases like whois.iana.org or RDAP servers. Since the target server is never contacted directly — only public registries are queried — no packets are sent to the target's infrastructure, making it a purely passive reconnaissance technique.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Ping sweeping a subnet
Why it's wrong here
Ping sweeping involves sending Internet Control Message Protocol (ICMP) Echo Request packets directly to a range of IP addresses within a subnet. This direct network interaction solicits ICMP Echo Reply packets from live hosts, making it an active reconnaissance technique. The target network's firewalls or intrusion detection systems can easily log and detect these incoming ICMP probes, indicating an external scan.
- ✗
Banner grabbing with Telnet
Why it's wrong here
Banner grabbing via Telnet entails establishing a direct TCP connection to a specific port on the target system, such as port 80 for HTTP or port 21 for FTP. Upon connection, the server often responds with service version information, which is then captured. This explicit communication with a target service leaves connection logs and is therefore considered an active reconnaissance method, as it directly interacts with the target's application layer.
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Running a SYN scan with Nmap
Why it's wrong here
A SYN scan, commonly performed with tools like Nmap, initiates the TCP three-way handshake by sending a SYN packet to target ports. If the port is open, the target responds with a SYN-ACK packet, indicating direct interaction and port state discovery. This direct probing of network services generates traffic on the target system and is readily detectable by network monitoring tools, classifying it as active reconnaissance.
- ✓
Performing a WHOIS lookup
Why this is correct
Performing a WHOIS lookup involves querying publicly accessible databases maintained by domain registrars and Regional Internet Registries (RIRs) to retrieve information about domain names or IP address blocks. This process retrieves data from third-party servers, such as ICANN-accredited registrars, without ever sending a single packet to the target organization's network infrastructure. Consequently, the target remains completely unaware of the inquiry, making it a quintessential passive reconnaissance technique.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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