CEH Footprinting, Reconnaissance and Scanning Practice Question
A penetration tester runs the following Nmap command: nmap -sU -sS -p 53,161,162,500 10.0.0.1 and receives no responses for UDP scans but standard results for TCP. The tester suspects the target is dropping all UDP packets. Which Nmap option could help increase the likelihood of UDP responses by fragmenting the probe?
⚠ Common exam trap
Candidates often confuse -f (fragmentation) with -T4 (timing) or -Pn (no ping), assuming any option that makes the scan 'faster' or 'more aggressive' will also bypass packet drops, when in fact fragmentation is the specific technique to alter packet structure.
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 option fragments the probe packets into smaller IP fragments. When a target drops unfragmented UDP packets, fragmenting the probes can sometimes bypass simple packet filters or IDS/IPS that drop larger or complete UDP datagrams, increasing the chance that the target will process and respond to the fragments.
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 (fragment packets) Nmap option instructs Nmap to split the IP header of the probe packets into several smaller IP packets. This technique, known as IP fragmentation, can bypass simple stateless firewalls or intrusion detection systems (IDS) that only inspect the first fragment of a packet or are configured to drop packets exceeding a certain size. By breaking the packet into 8-byte chunks, it makes reassembly more complex for network security devices, potentially allowing the scan to proceed undetected.
- ✗
-T4
Why it's wrong here
The -T4 option sets the timing template to "Aggressive," which significantly speeds up the Nmap scan by reducing timeouts and increasing the number of probes sent in parallel. While this can make a scan faster and potentially more noticeable on the network, it does not involve any form of packet fragmentation. Its primary purpose is to optimize scan efficiency against responsive targets, not to evade network defenses through packet manipulation.
- ✗
--reason
Why it's wrong here
The --reason Nmap option instructs Nmap to display the specific reason for a port's state, such as "syn-ack" for an open port or "no-response" for a filtered port. This provides valuable debugging information and insight into how Nmap determined the port status. However, this option is purely for output verbosity and diagnostic purposes; it has no impact on how Nmap constructs or fragments its outgoing probe packets.
- ✗
-Pn
Why it's wrong here
The -Pn (No Ping) option tells Nmap to skip the host discovery phase, treating all target hosts as online and proceeding directly to port scanning. This is particularly useful when scanning hosts that block ICMP echo requests or other host discovery probes, ensuring that the scan attempts to reach all specified targets. While crucial for certain network environments, -Pn solely affects the initial host reachability check and has no mechanism for fragmenting network packets.
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JA
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.