200-201 Network Intrusion Analysis Practice Question
A network analyst finds a PCAP with a series of DNS queries for subdomains like "data12345.example.com" and "data67890.example.com" where the subdomain names appear to contain encoded base64 data. This pattern suggests:
⚠ Common exam trap
200-201 often tests the difference between DGA (random domains for C2 beaconing) and DNS tunnelling (encoded data in subdomains for exfiltration) — candidates see 'encoded subdomains' and wrongly pick DGA.
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
✓
DNS tunnelling for exfiltration
DNS tunnelling for exfiltration encodes stolen data into DNS query names (often base64) and sends it to an attacker-controlled authoritative DNS server. The pattern of many subdomains with encoded-looking labels under the same domain is a classic indicator of data being smuggled out via DNS, which is frequently allowed through firewalls.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Port scan via DNS
Why it's wrong here
A port scan probes TCP or UDP ports to map services; it does not encode payloads into DNS subdomain labels. The observed base64 strings indicate DNS tunnelling for data exfiltration or command retrieval. Port scanning via DNS would suit enumerating open ports, not transferring encoded data.
- ✗
Normal DNS resolution
Why it's wrong here
Normal DNS resolution uses human-readable, stable hostnames; base64-looking labels in rapid succession on one domain indicate covert data transfer, not routine lookups. It tempts because DNS traffic is ubiquitous, but this option fits ordinary browsing or service discovery, where subdomain labels are predictable and not encoded.
- ✗
DGA-based C2
Why it's wrong here
DGA domains are random-looking, but not necessarily carrying encoded data in subdomains.
- ✓
DNS tunnelling for exfiltration
Why this is correct
Repeated DNS queries carrying base64-encoded labels in subdomains indicate data encoded into DNS requests, a hallmark of DNS tunnelling used to exfiltrate data covertly through permitted DNS traffic, bypassing egress controls that block other channels.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.