hardMultiple Choice
200-201 A security analyst at a financial institution Practice Question
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?
⚠ Common exam trap
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.
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.
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.
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
Custom Snort rules inspecting DNS query size, frequency and domain entropy target the tunnelling mechanism directly, while full packet capture baselines normal DNS to tune thresholds. This satisfies the requirement to detect DNS exfiltration with minimal false positives, since existing signatures cover only common malware C2 channels.
- ✗
Block all DNS queries to external domains not on a whitelist, and log all blocked queries for review.
Why it's wrong here
Whitelisting DNS blocks resolution rather than detecting tunnelling, and the exfiltration already occurred, so it produces no visibility into encoded queries; the archive must be analysed for anomalous query length and entropy. It tempts as preventive DNS control, which suits hardening, not the detection strategy requested.
- ✗
Increase the Snort signature sensitivity for all DNS-related alerts to maximum.
Why it's wrong here
Raising sensitivity on existing DNS signatures cannot detect tunnelling, because the IDS has no DNS tunnelling rules at all; it merely amplifies alerts on benign DNS, worsening false positives. It tempts because tuning sensitivity is a familiar IDS adjustment, but it addresses alert volume, not the missing detection logic.
- ✗
Deploy NetFlow monitoring on the DNS server and look for traffic volume anomalies.
Why it's wrong here
NetFlow records flow metadata only, so it cannot expose the query names, lengths or entropy that distinguish tunnelled DNS from normal resolution; full packet capture analysis is required. It tempts because volume anomalies suit flood detection, but DNS tunnelling is low-volume and encoded, defeating flow-based thresholds.
Visual reference
Go deeper
Related to this question
About these practice questions
Courseiva writes every 200-201 question from scratch — 968 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.