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 Nmap flags are commonly used for evasion techniques? (Select 3)
-f
Fragmenting packets can help evade simple IDS/firewall rules.
-D
Decoy scan uses multiple source IPs to hide the real scanner.
--mtu
Setting a custom MTU can cause fragmentation in a different way, helping evasion.
-O
Why wrong: -O is for OS fingerprinting, not evasion.
-sV
Why wrong: -sV is for version detection, 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
-f
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.
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.
-f
-D
Why this is correct
Decoy scan uses multiple source IPs to hide the real scanner.
Related concept
Read the scenario before looking for a memorised answer.
--mtu
Why this is correct
Setting a custom MTU can cause fragmentation in a different way, helping evasion.
Related concept
Read the scenario before looking for a memorised answer.
-O
Why it's wrong here
-O is for OS fingerprinting, not evasion.
-sV
Why it's wrong here
-sV is for version detection, not evasion.
Common exam traps
EC-Council often tests the distinction between scanning techniques (like -O and -sV) and evasion techniques (like -f, -D, --mtu), so candidates mistakenly select -O or -sV because they are common Nmap flags, even though they serve reconnaissance, not evasion.
Detailed technical explanation
Fragmentation exploits the fact that many firewalls only inspect the first fragment of a packet, allowing subsequent fragments to pass unchecked. The -D flag (decoy scan) sends spoofed source IPs alongside the real one, confusing logs and making it harder to identify the true scanning host. The --mtu flag sets a custom Maximum Transmission Unit size, which can force fragmentation at specific boundaries to evade signature-based detection that expects standard packet sizes.
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: -f — 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.
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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