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.
Which TWO of the following Nmap flags are used for evasion of IDS/IPS? (Choose two.)
-sV
Why wrong: -sV is for version detection, not evasion.
-O
Why wrong: -O is for OS detection.
-D
Decoy scan uses multiple source IPs to confuse IDS.
-f
Fragment packets to evade signature detection.
-sT
Why wrong: -sT is a connect scan, not evasion.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
-D
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.
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.
-sV
Why it's wrong here
-sV is for version detection, not evasion.
-O
Why it's wrong here
-O is for OS detection.
-D
-f
Why this is correct
Fragment packets to evade signature detection.
Related concept
Read the scenario before looking for a memorised answer.
-sT
Why it's wrong here
-sT is a connect scan, not evasion.
Common exam traps
EC-Council often tests the misconception that -sV or -O are evasion techniques because they are 'stealthy' in some contexts, but the CEH exam specifically requires knowing that decoys (-D) and fragmentation (-f) are the standard Nmap evasion flags.
Detailed technical explanation
The -D flag works by sending packets from decoy IPs (up to 16) alongside the real source IP; the IDS/IPS must correlate all traffic to identify the true scanner, which is computationally expensive and often fails. The -f flag splits the TCP header (typically 20 bytes) into multiple 8-byte fragments, causing the IDS/IPS to reassemble them before inspection—many signature-based systems drop fragments or fail to reassemble correctly, allowing the scan to pass undetected. In real-world scenarios, combining -D with -f (e.g., nmap -D RND:10 -f 192.168.1.1) creates a layered evasion strategy that overwhelms both stateful inspection and pattern matching.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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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: -D — 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.
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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Last reviewed: Jun 30, 2026
This CEH practice question is part of Courseiva's free EC-Council certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CEH exam.
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