The answer is to disable recursion on DNS servers for external queries. This is the most effective mitigation for DNS amplification DDoS because the attack exploits open resolvers that accept recursive queries from any source, allowing an attacker to send a small query with a spoofed victim IP and receive a large response, multiplying traffic. By restricting recursion to only trusted internal clients, the server no longer acts as an amplifier for external requests, directly cutting off the attack vector. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of DNS security controls and network defense; a common trap is confusing rate limiting or firewall rules as the primary fix, but those only reduce impact rather than eliminate the root cause. Remember the memory tip: “No recursion for outsiders stops amplification” — if recursion is off for strangers, the attack has no engine.
CEH Practice Question: Malware, Social Engineering and Network Attacks
This CEH practice question tests your understanding of malware, social engineering and network attacks. 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 team wants to mitigate a DNS amplification DDoS attack. Which of the following techniques would be MOST effective in preventing the attack from leveraging open DNS resolvers?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Disable recursion on DNS servers for external queries
DNS amplification relies on open resolvers that respond to queries from any source. Restricting recursive queries to trusted clients eliminates the amplification vector.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
Disable recursion on DNS servers for external queries
Why this is correct
Correct. Disabling recursion for external clients prevents the server from being used in amplification attacks.
Related concept
Static NAT maps one inside address to one outside address.
✗
Use anycast routing for DNS servers
Why it's wrong here
Anycast distributes load but does not prevent open resolver abuse.
✗
Implement rate limiting on DNS responses
Why it's wrong here
Rate limiting reduces impact but does not prevent the amplification; open resolvers still respond.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CEH NAT questions on configuration and troubleshooting.
Malware, Social Engineering and Network Attacks — This question tests Malware, Social Engineering and Network Attacks — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Disable recursion on DNS servers for external queries — DNS amplification relies on open resolvers that respond to queries from any source. Restricting recursive queries to trusted clients eliminates the amplification vector.
What should I do if I get this CEH question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related CEH NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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