- A
Write custom Snort rules that monitor DNS query size, frequency, and domain name entropy, and use full packet capture to baseline typical DNS behavior.
DNS tunneling exhibits abnormal characteristics that can be detected with tailored rules and baselines.
- B
Block all DNS queries to external domains not on a whitelist, and log all blocked queries for review.
Why wrong: Whitelisting is too restrictive and would break normal business operations; also it doesn't detect existing exfiltration.
- C
Increase the Snort signature sensitivity for all DNS-related alerts to maximum.
Why wrong: This would generate excessive false positives from legitimate DNS traffic.
- D
Deploy NetFlow monitoring on the DNS server and look for traffic volume anomalies.
Why wrong: NetFlow provides metadata only; DNS tunneling can be detected via content analysis, which NetFlow lacks.
Quick Answer
The most effective approach is to write custom Snort rules that monitor DNS query size, frequency, and domain name entropy, and use full packet capture to baseline typical DNS behavior. This works because DNS tunneling hides data within legitimate DNS queries—often using oversized TXT records, unusually high query rates, or random-looking subdomain names—so signature-based detection alone misses it. By combining anomaly detection rules with a baseline of normal traffic from packet captures, you catch deviations without drowning in false positives. On the Cisco CyberOps Associate 200-201 exam, this question tests your understanding that DNS tunneling exploits protocol trust, not malware signatures; a common trap is relying solely on existing C2 signatures. Remember the mnemonic “SFE” for Size, Frequency, and Entropy—the three pillars of spotting a tunnel in the DNS stream.
200-201 Network Intrusion Analysis Practice Question
This 200-201 practice question tests your understanding of network intrusion analysis. 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.
You are a security analyst at a financial institution. The network consists of a traditional perimeter firewall, an internal IDS (Snort), and a separate network monitoring tool that captures full packet data. Recently, the bank experienced a breach where an attacker exfiltrated customer data via DNS tunneling. The attack went undetected for weeks. The CISO wants to improve detection of data exfiltration and has tasked you with proposing a new monitoring strategy. The current IDS has signatures for common malware C2 channels but no specific DNS tunneling rules. You have access to the full packet capture archive. Which approach would be most effective in detecting DNS tunneling while minimizing false positives?
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
Write custom Snort rules that monitor DNS query size, frequency, and domain name entropy, and use full packet capture to baseline typical DNS behavior.
Option A is correct because DNS tunneling exploits legitimate DNS protocol behavior by encoding data in query payloads, making it invisible to signature-based detection. By writing custom Snort rules that monitor query size (typically > 255 bytes for TXT records), frequency (abnormally high query rates per domain), and domain name entropy (random-looking subdomains), and using full packet capture to baseline normal DNS traffic, you can detect anomalies indicative of tunneling with high precision and low false positives.
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.
- ✓
Write custom Snort rules that monitor DNS query size, frequency, and domain name entropy, and use full packet capture to baseline typical DNS behavior.
Why this is correct
DNS tunneling exhibits abnormal characteristics that can be detected with tailored rules and baselines.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Block all DNS queries to external domains not on a whitelist, and log all blocked queries for review.
Why it's wrong here
Whitelisting is too restrictive and would break normal business operations; also it doesn't detect existing exfiltration.
- ✗
Increase the Snort signature sensitivity for all DNS-related alerts to maximum.
Why it's wrong here
This would generate excessive false positives from legitimate DNS traffic.
- ✗
Deploy NetFlow monitoring on the DNS server and look for traffic volume anomalies.
Why it's wrong here
NetFlow provides metadata only; DNS tunneling can be detected via content analysis, which NetFlow lacks.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between detection and prevention—candidates may incorrectly choose a blocking strategy (Option B) or a volume-based approach (Option D) instead of a detection method that leverages packet-level analysis and behavioral baselines.
Detailed technical explanation
How to think about this question
DNS tunneling often uses TXT or NULL record types to carry exfiltrated data, with base32/base64 encoding that increases query length and entropy. Full packet capture allows analysis of individual DNS query payloads, enabling detection of non-RFC-compliant lengths (e.g., TXT records exceeding 255 bytes) or high entropy in subdomain labels, which is a strong indicator of data encoding. In real-world scenarios, attackers may also use random subdomain labels to avoid simple domain blacklists, making entropy-based detection critical.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 200-201 question test?
Network Intrusion Analysis — This question tests Network Intrusion Analysis — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Write custom Snort rules that monitor DNS query size, frequency, and domain name entropy, and use full packet capture to baseline typical DNS behavior. — Option A is correct because DNS tunneling exploits legitimate DNS protocol behavior by encoding data in query payloads, making it invisible to signature-based detection. By writing custom Snort rules that monitor query size (typically > 255 bytes for TXT records), frequency (abnormally high query rates per domain), and domain name entropy (random-looking subdomains), and using full packet capture to baseline normal DNS traffic, you can detect anomalies indicative of tunneling with high precision and low false positives.
What should I do if I get this 200-201 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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Last reviewed: Jun 25, 2026
This 200-201 practice question is part of Courseiva's free Cisco 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 200-201 exam.
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