The answer is a decoy scan. This is correct because the `-D RND:10` flag in Nmap generates ten random, spoofed source IP addresses, and the real scan packet from the tester is interleaved among them, making the true IP invisible in the IDS logs while multiple fake SYN packets appear. On the Certified Ethical Hacker CEH exam, this technique tests your understanding of how to obscure an attacker’s origin during reconnaissance, often appearing in questions about evading IDS or firewall logging. A common trap is confusing decoy scans with idle scans—remember that decoy scans use live, spoofed IPs, not a zombie host. For a quick memory tip, think “Decoy = Disguise with Dummies,” where the `-D` flag directly creates the fake sources to hide your real address.
CEH Footprinting, Reconnaissance and Scanning Practice Question
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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, a tester uses Nmap with the command: nmap -sS -D RND:10 192.168.1.100. After the scan, the IDS logs show multiple SYN packets from different source IPs hitting the target. However, the tester's true IP is not among them. Which of the following techniques is being used?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Decoy scan
The command `nmap -sS -D RND:10` performs a SYN stealth scan with 10 randomly generated decoy IP addresses. The IDS logs show multiple SYN packets from different source IPs, but the tester's true IP is not among them because Nmap sends the decoy packets with spoofed source addresses while the real scan packet is interleaved among them. This is the definition of a decoy scan, which aims to obscure the attacker's true origin by blending it with fake sources.
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.
✗
Idle scan
Why it's wrong here
Idle scan (-sI) uses a zombie host to bounce packets, not multiple decoys. The -D flag is for decoy.
✗
SYN flood
Why it's wrong here
SYN flood is a DoS attack, not a scanning technique. The command is a scan, not a flood.
✓
Decoy scan
Why this is correct
The -D flag specifies decoy IPs. RND:10 generates random decoys. This hides the real source by making it appear from multiple IPs.
Related concept
Read the scenario before looking for a memorised answer.
✗
Fragmentation attack
Why it's wrong here
Fragmentation is done with -f flag, splitting packets into smaller fragments to evade IDS. Decoy is a different technique.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is confusing decoy scans with idle scans, as both involve spoofed IPs, but idle scans require a zombie host and IPID manipulation, while decoy scans simply flood the target with fake sources to hide the real one.
Trap categories for this question
Command / output trap
SYN flood is a DoS attack, not a scanning technique. The command is a scan, not a flood.
Detailed technical explanation
How to think about this question
In a decoy scan, Nmap generates random IPs using the `RND:10` argument, which creates 10 decoy addresses, and the real scan packet is sent alongside them with a random delay to avoid pattern detection. The target sees SYN packets from multiple IPs, but the tester's actual IP is hidden because Nmap does not include it in the decoy list unless explicitly specified with `-D` and a real IP. This technique is effective against simple IDS that trigger on single-source scans, but advanced IDS can correlate response behavior or use statistical analysis to identify the true scanner.
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 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.
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 — The command `nmap -sS -D RND:10` performs a SYN stealth scan with 10 randomly generated decoy IP addresses. The IDS logs show multiple SYN packets from different source IPs, but the tester's true IP is not among them because Nmap sends the decoy packets with spoofed source addresses while the real scan packet is interleaved among them. This is the definition of a decoy scan, which aims to obscure the attacker's true origin by blending it with fake sources.
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.
About these practice questions
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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. Which THREE of the following Nmap flags are commonly used for evasion techniques? (Select 3)
hard
✓ A.-f
✓ B.-D
✓ C.--mtu
D.-O
E.-sV
Why A: The -f flag fragments the probe packets into smaller 8-byte fragments (or less, depending on the MTU). This helps evade simple packet-filtering firewalls and intrusion detection systems that do not reassemble fragments before applying rules, as the fragmented headers may bypass signature-based detection.
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Question Discussion
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