The correct answer is to configure egress filtering on the firewall to block outbound connections to known malicious domains. This control works by inspecting all outbound DNS queries at the network perimeter against a threat intelligence feed or blocklist, dropping any matches to command-and-control domains while allowing legitimate traffic to pass through unimpeded. For the Certified Ethical Hacker CEH exam, this scenario tests your understanding of IoT DNS C2 detection egress filtering as a practical defense against botnet-style exfiltration, often appearing in questions about network segmentation and perimeter security. A common trap is choosing DNS sinkholing alone, which can disrupt legitimate resolution; egress filtering is superior because it blocks only the malicious destination. Remember the mnemonic “Egress Ejects Evil” to recall that outbound filtering stops C2 traffic at the edge without breaking normal operations.
CEH Wireless, IoT and Cloud Security Practice Question
This CEH practice question tests your understanding of wireless, iot and cloud security. 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 discovers that an IoT device in a smart building is periodically sending small DNS queries to an external domain known for command-and-control activity. Which security control should be implemented to detect and block such traffic without disrupting legitimate operations?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure egress filtering on the firewall to block outbound connections to known malicious domains.
Option C is correct because egress filtering on the firewall can block outbound DNS queries to known malicious domains by using a blocklist or threat intelligence feed, preventing command-and-control (C2) communication without affecting legitimate traffic to other domains. This control operates at the network perimeter, inspecting DNS requests against a reputation database and dropping matches, which is the most effective way to stop C2 traffic while allowing normal operations.
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.
✗
Install a host-based firewall on the IoT device to restrict outbound traffic.
Why it's wrong here
IoT devices may not support host-based firewalls.
✗
Deploy an intrusion detection system (IDS) on the network to alert on suspicious DNS queries.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse detection (IDS) with prevention (firewall filtering), assuming an alert is sufficient to stop the threat, but CEH emphasizes that blocking at the perimeter is required for active defense against C2 traffic.
Detailed technical explanation
How to think about this question
Egress filtering for DNS typically uses a next-generation firewall (NGFW) or DNS sinkhole that inspects the query name (QNAME) against a threat intelligence feed (e.g., from AlienVault OTX or VirusTotal). When a match occurs, the firewall can drop the packet or respond with a spoofed NXDOMAIN, effectively breaking the C2 channel without affecting other DNS queries. In real-world smart buildings, IoT devices often use hardcoded DNS servers, so filtering at the firewall is the only reliable choke point.
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.
Wireless, IoT and Cloud Security — This question tests Wireless, IoT and Cloud Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure egress filtering on the firewall to block outbound connections to known malicious domains. — Option C is correct because egress filtering on the firewall can block outbound DNS queries to known malicious domains by using a blocklist or threat intelligence feed, preventing command-and-control (C2) communication without affecting legitimate traffic to other domains. This control operates at the network perimeter, inspecting DNS requests against a reputation database and dropping matches, which is the most effective way to stop C2 traffic while allowing normal operations.
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.
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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