The correct answer is that the Nmap command employs fragmentation, decoy scanning, and random source IP techniques to evade IDS/IPS. The `-f` flag fragments the TCP SYN probe into smaller packets, bypassing simple signature-based inspection, while the `-D RND:10` flag generates ten random decoy source IP addresses to obscure the real scanning host, effectively randomizing the source IP of each probe. On the Certified Ethical Hacker CEH exam, this tests your understanding of Nmap IDS evasion techniques as part of the scanning methodology domain, where examiners often set traps by mixing flags like `-f` (fragmentation) with `-D` (decoy) and `--data-length` (padding). A common mistake is confusing `-D RND:10` with simply using a fixed decoy IP; remember that `RND:` triggers random source IP generation, not just decoy spoofing. Memory tip: think “Frag, Decoy, Random” for the three pillars of evasion in this command.
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 penetration tester is attempting to evade an IDS/IPS while performing a port scan. They use the Nmap command: nmap -sS -f --data-length 20 -D RND:10 10.0.0.1. Which techniques are being employed to evade detection?
Idle scan, fragmentation, and MAC address spoofing
Why wrong: Idle scan uses a zombie host (-sI), not used here. MAC spoofing is not part of Nmap's evasion.
B
Packet timing manipulation, decoy, and avoiding DNS resolution
Why wrong: No timing manipulation is shown (e.g., -T). Avoiding DNS resolution is -n, not used.
C
Fragmentation, decoy, and using a random source IP
-f fragments IP packets, --data-length adds random data, and -D RND:10 generates random decoy IPs. The source IP is not random; decoys are additional spoofed sources.
D
Fragmentation, decoy, and source port spoofing
Why wrong: Source port spoofing is not used; -g can set source port, but not present.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Fragmentation, decoy, and using a random source IP
Option C is correct because the Nmap command `-sS -f --data-length 20 -D RND:10` employs three evasion techniques: fragmentation (the `-f` flag splits the TCP SYN packet into smaller fragments to bypass simple packet inspection), decoy scanning (`-D RND:10` generates 10 random decoy source IP addresses to obscure the real scanning host), and using a random source IP (the `RND:10` mechanism effectively randomizes the source IP of the probes, making it harder for the IDS/IPS to attribute the scan to a single attacker).
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, fragmentation, and MAC address spoofing
Why it's wrong here
Idle scan uses a zombie host (-sI), not used here. MAC spoofing is not part of Nmap's evasion.
✗
Packet timing manipulation, decoy, and avoiding DNS resolution
Why it's wrong here
No timing manipulation is shown (e.g., -T). Avoiding DNS resolution is -n, not used.
✓
Fragmentation, decoy, and using a random source IP
Why this is correct
-f fragments IP packets, --data-length adds random data, and -D RND:10 generates random decoy IPs. The source IP is not random; decoys are additional spoofed sources.
Related concept
Read the scenario before looking for a memorised answer.
✗
Fragmentation, decoy, and source port spoofing
Why it's wrong here
Source port spoofing is not used; -g can set source port, but not present.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the `-D RND:10` decoy option with source IP spoofing or idle scanning, but the command does not include the `-sI` flag for idle scanning or `--source-port` for port spoofing, and the `RND:10` specifically randomizes decoy IPs, not the source IP of the attacker's machine.
Trap categories for this question
Command / output trap
No timing manipulation is shown (e.g., -T). Avoiding DNS resolution is -n, not used.
Detailed technical explanation
How to think about this question
The `-f` flag in Nmap splits the TCP header across multiple IP fragments (typically 8-byte fragment sizes), which can evade IDS/IPS that only reassemble full packets or inspect only the first fragment. The `-D RND:10` option generates 10 random decoy IPs, but the real source IP is still sent among them; the IDS/IPS sees traffic from multiple IPs, making it difficult to isolate the true scanner. In practice, modern IDS/IPS can reassemble fragments and correlate decoy traffic, but these techniques remain effective against simpler or misconfigured systems.
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: Fragmentation, decoy, and using a random source IP — Option C is correct because the Nmap command `-sS -f --data-length 20 -D RND:10` employs three evasion techniques: fragmentation (the `-f` flag splits the TCP SYN packet into smaller fragments to bypass simple packet inspection), decoy scanning (`-D RND:10` generates 10 random decoy source IP addresses to obscure the real scanning host), and using a random source IP (the `RND:10` mechanism effectively randomizes the source IP of the probes, making it harder for the IDS/IPS to attribute the scan to a single attacker).
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 TWO of the following Nmap flags can be used to bypass firewall restrictions? (Select 2)
medium
A.-P0 (disable ping)
✓ B.-f (fragment packets)
C.-T4 (aggressive timing)
D.-sS (SYN scan)
✓ E.-D (decoy scan)
Why B: Option B is correct because the -f flag fragments packets into smaller 8-byte chunks, which can evade simple firewall rules that inspect packet headers for known signatures or block oversized packets. Option E is correct because the -D flag performs a decoy scan by spoofing multiple source IP addresses, making it difficult for a firewall to identify the true scanning host and block it.
Variation 2. Which TWO of the following Nmap flags are used for evasion of IDS/IPS? (Choose two.)
medium
A.-sV
B.-O
✓ C.-D
✓ D.-f
E.-sT
Why C: Option C (-D) is correct because the Nmap decoy scan flag allows you to spoof multiple source IP addresses, making it difficult for IDS/IPS to distinguish the real scanning host from decoys. Option D (-f) is correct because fragmenting packets (e.g., using -f to split TCP headers into 8-byte fragments) evades signature-based detection by bypassing pattern-matching rules that expect complete packet headers.
Last reviewed: Jun 24, 2026
Question Discussion
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