CEH Footprinting, Reconnaissance and Scanning Practice Question
Which THREE of the following Nmap flags are commonly used for evasion techniques? (Select 3)
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
-f
Why this is correct
The -f flag instructs Nmap to fragment the probe packets into smaller pieces, typically 8 bytes or less, before sending them. This technique can bypass stateless firewalls or basic Intrusion Detection Systems (IDS) that only inspect the first fragment of a packet or fail to reassemble fragmented packets correctly. By splitting the TCP header across multiple fragments, it becomes harder for security devices to identify the true nature of the scan, making it a common evasion tactic.
- ✓
-D
Why this is correct
The -D flag enables a decoy scan, where Nmap sends scan probes from multiple spoofed source IP addresses in addition to the real scanner's IP. This makes it appear as if numerous hosts are scanning the target simultaneously, effectively obscuring the actual attacker's IP address within a flood of decoy traffic. This technique significantly complicates log analysis and traceback efforts for network defenders, making it a powerful evasion and attribution obfuscation method.
- ✓
--mtu
Why this is correct
The --mtu flag allows specifying a custom Maximum Transmission Unit (MTU) for outgoing Nmap packets, which must be a multiple of 8. If the specified MTU is smaller than the default packet size, Nmap will automatically fragment the packets to adhere to this limit. This method of fragmentation can bypass network devices that are configured to reassemble packets based on standard MTU values or that have specific rules for default-sized packets, providing another layer of evasion distinct from simple -f fragmentation.
- ✗
-O
Why it's wrong here
The -O flag is used for operating system detection, where Nmap sends a series of TCP and UDP probes to analyze the target's responses and infer its underlying OS. While crucial for reconnaissance, this flag does not inherently provide any evasion capabilities; instead, it actively generates traffic designed to elicit specific responses for identification purposes, potentially increasing the scan's detectability rather than hiding it.
- ✗
-sV
Why it's wrong here
The -sV flag enables service and version detection, instructing Nmap to query open ports to determine the application name and version running on them. This process involves sending specific application-layer probes, which are highly identifiable and often logged by security devices. Far from being an evasion technique, -sV is a verbose scanning method designed to gather detailed service information, making the scan more detectable.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.