The answer is to enable IP fragmentation with the -f flag. This technique works by splitting the TCP SYN packet into smaller fragments, which forces the IDS/IPS to attempt reassembly before inspection; many systems either drop fragments or fail to reconstruct the full header, allowing the fragmented probes to slip past the filter while Nmap still receives responses that reveal open ports. On the Certified Ethical Hacker CEH exam, this scenario tests your understanding of evasion tactics against network-based detection systems, often appearing in questions where a SYN scan is blocked but a fragmented scan succeeds. A common trap is confusing fragmentation with decoy scans (-D), but remember that fragmentation alters the packet structure itself rather than the source IP. Memory tip: think of the -f flag as "fragment to fool the filter"—smaller pieces can bypass the big picture.
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 security analyst observes that an Nmap SYN scan against a target network returns all ports as 'filtered'. The analyst suspects an IDS/IPS is dropping inbound SYN packets. Which Nmap technique would MOST likely bypass this detection while still identifying open ports?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable IP fragmentation with the -f flag
When an IDS/IPS drops inbound SYN packets, a standard SYN scan (-sS) is detected because the probe packets are easily recognized. Enabling IP fragmentation with the -f flag splits the TCP header across multiple fragments, making it harder for the IDS/IPS to reassemble and inspect the full packet, thus potentially bypassing the filter while still allowing Nmap to determine open ports based on responses.
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.
✓
Enable IP fragmentation with the -f flag
Why this is correct
Fragmentation can help evade detection by splitting the SYN packet across multiple fragments.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use the -sU flag for UDP scanning
Why it's wrong here
UDP scanning is for discovering UDP services, not for evading TCP SYN filters.
✗
Use the -sT flag for a TCP connect scan
Why it's wrong here
TCP connect scan completes the handshake, which is more likely to be logged and filtered.
✗
Increase scanning speed with -T5
Why it's wrong here
Faster scanning may trigger thresholds but does not evade filtering.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think increasing speed (-T5) or using a full connect scan (-sT) makes scanning stealthier, when in fact fragmentation (-f) is the classic evasion technique for bypassing packet filters and IDS/IPS that inspect full packets.
Detailed technical explanation
How to think about this question
IP fragmentation works by splitting the IP packet into smaller fragments, each with its own IP header. Nmap's -f flag sets the fragment size to 8 bytes, forcing the TCP header to be broken across multiple fragments. Many IDS/IPS systems struggle to reassemble fragments in real time or lack deep packet inspection for fragmented traffic, allowing the SYN probe to reach the target. However, some modern firewalls and IDS/IPS can reassemble fragments or drop all fragmented traffic, so this technique is not foolproof.
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: Enable IP fragmentation with the -f flag — When an IDS/IPS drops inbound SYN packets, a standard SYN scan (-sS) is detected because the probe packets are easily recognized. Enabling IP fragmentation with the -f flag splits the TCP header across multiple fragments, making it harder for the IDS/IPS to reassemble and inspect the full packet, thus potentially bypassing the filter while still allowing Nmap to determine open ports based on responses.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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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. During a vulnerability assessment, a security analyst receives an alert from the IDS that a scan with fragmented packets and spoofed source IPs is targeting the internal network. Which Nmap command MOST likely caused this alert?
medium
A.nmap -sS -O 192.168.1.1
B.nmap -sV -p 80 192.168.1.1
C.nmap -sU 192.168.1.1
✓ D.nmap -f -D 10.0.0.1,10.0.0.2 192.168.1.1
Why D: Option D is correct because the `-f` flag fragments the packets into smaller IP fragments, and the `-D` flag performs a decoy scan by spoofing source IPs. This combination causes the IDS to detect fragmented packets with spoofed source addresses, matching the alert description.
Last reviewed: Jun 24, 2026
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