The correct answer is a decoy scan, because it enables an attacker to mask their real IP address among a list of spoofed decoy IPs, causing intrusion detection systems (IDS) to flag multiple sources instead of the true attacker. In this scenario, the IDS alerts on the legitimate internal server 10.0.0.5, which is exactly how a decoy scan works—the attacker includes that server’s IP as a decoy to confuse detection and misdirect investigators. On the CEH exam, this question tests your understanding of Nmap decoy scan evade detection techniques, specifically how the `-D` option blends real and fake source addresses. A common trap is confusing a decoy scan with a zombie scan (used for idle scanning) or a simple spoofed scan; remember that decoys are simultaneous, not dependent on a third-party host’s IPID sequence. Memory tip: think “Decoy = Disguise with a crowd” — the attacker hides in a list of fake IPs, just like a decoy duck in a flock.
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.
A security analyst receives an alert from the IDS indicating a port scan originating from IP 10.0.0.5. Upon investigation, the analyst finds that 10.0.0.5 is a legitimate internal server. Which type of scan is the attacker likely using to evade detection?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Decoy scan
A decoy scan (option C) is the correct answer because it allows the attacker to blend their real IP address with multiple spoofed IP addresses, making it appear as though the scan originates from several hosts. In this scenario, the IDS alerts on IP 10.0.0.5, which is a legitimate internal server, indicating that the attacker is using that server's IP as a decoy to evade detection and misdirect the analyst's investigation.
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.
✗
SYN scan
Why it's wrong here
SYN scan does not spoof IP; the source IP is the attacker's real IP.
✗
Idle scan
Why it's wrong here
Idle scan uses a zombie host and spoofed IP, but the zombie's IP would appear, not necessarily a legitimate server unless chosen.
✓
Decoy scan
Why this is correct
Decoy scan includes multiple fake source IPs to hide the real attacker. The IDS may flag a decoy IP.
Related concept
Read the scenario before looking for a memorised answer.
✗
Fragmentation scan
Why it's wrong here
Fragmentation splits packets to evade detection, but does not spoof IP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse a decoy scan with an idle scan, mistakenly thinking that using a legitimate internal server as a decoy is the same as using a zombie host, but idle scans rely on IP ID side-channel analysis and do not involve spoofing the attacker's own traffic.
Detailed technical explanation
How to think about this question
In a decoy scan, the attacker sends packets from their real IP interleaved with packets from spoofed decoy IPs, often using tools like Nmap with the -D option. The IDS or firewall sees multiple IPs scanning the target, making it difficult to identify the true source. A subtle behavior is that the decoy IPs must be live (or at least routable) to avoid confusing the target's responses, and the attacker's real IP is typically placed in the middle of the decoy list to reduce suspicion.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: Decoy scan — A decoy scan (option C) is the correct answer because it allows the attacker to blend their real IP address with multiple spoofed IP addresses, making it appear as though the scan originates from several hosts. In this scenario, the IDS alerts on IP 10.0.0.5, which is a legitimate internal server, indicating that the attacker is using that server's IP as a decoy to evade detection and misdirect the analyst's investigation.
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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