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 THREE of the following are valid Nmap flags that can be used to evade detection by an IDS? (Select exactly 3.)
-O
Why wrong: OS detection is a feature, not evasion.
-D
Decoy scan hides the real source among multiple IPs.
-f
Fragments packets to evade signature-based detection.
-sS
Why wrong: SYN scan is a scan type, not an evasion technique.
--data-length
Adds random data to packets to confuse IDS.
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 B (-D) is correct because the decoy scan flag allows you to spoof multiple source IP addresses, making it difficult for an IDS to distinguish the real scanning host from the decoys. This technique floods the target with scan traffic from many IPs, obscuring the true origin and evading detection.
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.
-O
Why it's wrong here
OS detection is a feature, not evasion.
-D
Why this is correct
Decoy scan hides the real source among multiple IPs.
Related concept
Read the scenario before looking for a memorised answer.
-f
Why this is correct
Fragments packets to evade signature-based detection.
Related concept
Read the scenario before looking for a memorised answer.
-sS
Why it's wrong here
SYN scan is a scan type, not an evasion technique.
--data-length
Why this is correct
Adds random data to packets to confuse IDS.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
EC-Council often tests the misconception that -sS is an evasion technique, but it is actually a stealth scan that reduces connection logging, not IDS evasion; the real evasion flags are those that alter packet structure or source identity.
Detailed technical explanation
The -f flag fragments IP packets into smaller 8-byte fragments, forcing the IDS to reassemble them, which can cause evasion if the IDS lacks proper reassembly logic. The --data-length flag appends random data to packets, altering their size and making them appear less like standard scan probes, which can bypass signature-based detection. In practice, combining these flags with decoys is common in penetration testing to avoid triggering alerts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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 B (-D) is correct because the decoy scan flag allows you to spoof multiple source IP addresses, making it difficult for an IDS to distinguish the real scanning host from the decoys. This technique floods the target with scan traffic from many IPs, obscuring the true origin and evading detection.
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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