Quick Answer
The answer is the -f (fragment) option, along with -D (decoy) and --mtu (custom MTU), as three commonly used Nmap options to evade firewall detection. The -f flag works by splitting IP packets into smaller 8-byte fragments, which can bypass stateless packet-filtering firewalls that only inspect the first fragment and allow subsequent fragments through unchecked. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of how attackers exploit fragmentation to slip past simple intrusion detection systems, often appearing in scenario-based questions where you must choose evasion techniques for a restrictive network. A common trap is confusing -f with --mtu, which sets a custom fragment size instead of the default 8 bytes. For a quick memory tip, remember “Frag Decoy MTU” as the three key evasion tools—think of a decoy sending fragmented packets with a custom size to confuse the firewall.
This CEH practice question tests your understanding of scanning networks and enumeration. 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 Nmap options are commonly used to evade firewall detection during a scan? (Choose three.)
-f (fragment packets)
Fragmentation splits packet headers to evade simple packet filters.
-T0 (paranoid timing)
Slow timing evades rate-based detection.
-D (decoy)
Decoy option spoofs multiple source IPs to confuse firewalls.
-sT (TCP connect scan)
Why wrong: TCP connect scan completes the handshake and is more likely logged.
-sV (version detection)
Why wrong: Version detection sends additional probes, increasing detectability.
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 (fragment packets)
Option A is correct because the -f flag fragments the IP packets into smaller pieces (typically 8-byte fragments) to bypass simple packet-filtering firewalls and intrusion detection systems that do not reassemble fragmented packets before applying filtering rules. This technique exploits the fact that many stateless firewalls only inspect the first fragment and allow subsequent fragments through unchecked, enabling the scan to reach the target.
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 (fragment packets)
Why this is correct
Fragmentation splits packet headers to evade simple packet filters.
Related concept
Read the scenario before looking for a memorised answer.
-T0 (paranoid timing)
Why this is correct
Slow timing evades rate-based detection.
Related concept
Read the scenario before looking for a memorised answer.
-D (decoy)
Why this is correct
Decoy option spoofs multiple source IPs to confuse firewalls.
Related concept
Read the scenario before looking for a memorised answer.
-sT (TCP connect scan)
Why it's wrong here
TCP connect scan completes the handshake and is more likely logged.
-sV (version detection)
Why it's wrong here
Version detection sends additional probes, increasing detectability.
Common exam traps
The trap here is that candidates often mistake -sT (TCP connect scan) as a stealthy option because it is the default scan type, but it actually completes the full handshake and is highly detectable, whereas the correct evasion options focus on fragmentation, timing, and decoys.
Detailed technical explanation
Fragmentation with -f works by splitting the TCP header across multiple IP fragments, causing the firewall to see only the first fragment with the header and allowing subsequent fragments to pass without inspection. The -T0 (paranoid timing) option introduces a 5-minute delay between probes, effectively evading rate-based detection mechanisms by making the scan appear as normal background noise. The -D (decoy) option generates multiple source IP addresses in the scan, confusing firewall logs and making it difficult for the target to identify the true scanning host.
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
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FAQ
Scanning Networks and Enumeration — This question tests Scanning Networks and Enumeration — Read the scenario before looking for a memorised answer..
The correct answer is: -f (fragment packets) — Option A is correct because the -f flag fragments the IP packets into smaller pieces (typically 8-byte fragments) to bypass simple packet-filtering firewalls and intrusion detection systems that do not reassemble fragmented packets before applying filtering rules. This technique exploits the fact that many stateless firewalls only inspect the first fragment and allow subsequent fragments through unchecked, enabling the scan to reach the target.
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 11, 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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