The correct answer is that the `-f` flag splits the TCP header into multiple packets to bypass simple packet filters. This occurs because the `-f` flag in Nmap enables IP packet fragmentation, typically breaking the packet into 8-byte fragments, which forces the TCP header to be divided across several smaller packets. As a result, signature-based intrusion detection systems (IDS) that do not perform full TCP reassembly see out-of-order or incomplete headers, allowing the tester to evade detection. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of evasion techniques against network defenses, often appearing as a scenario where a tester must bypass a stateless firewall or basic IDS. A common trap is confusing `-f` with the `--mtu` flag, which sets a custom fragment size—remember that `-f` uses a fixed 8-byte default. Memory tip: think “fragments for filters” to recall that `-f` breaks packets to fool simple packet filters.
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 penetration tester is trying to evade an IDS that detects out-of-order TCP packets. The tester uses Nmap with the -f flag. What is the PRIMARY effect of this flag?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Splits the TCP header into multiple packets to bypass simple packet filters
The `-f` flag in Nmap fragments the IP packet into smaller pieces, typically 8-byte fragments, which splits the TCP header across multiple packets. This fragmentation causes the IDS to see out-of-order or incomplete TCP headers, making it harder for signature-based detection to reassemble and match malicious patterns, thus bypassing simple packet filters that do not perform full TCP reassembly.
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.
✓
Splits the TCP header into multiple packets to bypass simple packet filters
Why this is correct
Fragmenting packets can evade IDS that don't reassemble fragments before inspection.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Slows down the scan to avoid triggering rate-based alarms
Why it's wrong here
Slowing is achieved with --scan-delay or -T options, not -f.
✗
Sends packets from multiple decoy IP addresses to obscure the source
Why it's wrong here
Decoy scan uses -D flag, not -f.
✗
Spoofs the source IP address to hide the attacker's identity
Why it's wrong here
Spoofing is done with -S or decoy, not -f.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the `-f` flag with decoy or spoofing techniques, but the primary effect is fragmentation to evade packet filters and simple IDS, not to hide the source IP or slow the scan.
Detailed technical explanation
How to think about this question
When Nmap fragments a TCP SYN packet with `-f`, it splits the IP payload into fragments of 8 bytes (or multiples thereof with `--mtu`), forcing the IDS to buffer and reassemble fragments before inspection. Many older or stateless IDS lack proper IP reassembly and drop or ignore fragmented packets, allowing the scan to proceed undetected. In real-world scenarios, modern IDS/IPS like Snort or Suricata can reassemble fragments using preprocessor modules (e.g., `frag3`), but misconfigured or resource-constrained systems may still be evaded.
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 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.
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: Splits the TCP header into multiple packets to bypass simple packet filters — The `-f` flag in Nmap fragments the IP packet into smaller pieces, typically 8-byte fragments, which splits the TCP header across multiple packets. This fragmentation causes the IDS to see out-of-order or incomplete TCP headers, making it harder for signature-based detection to reassemble and match malicious patterns, thus bypassing simple packet filters that do not perform full TCP reassembly.
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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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