200-201 Security Monitoring Practice Question
In a Zeek/Bro log, an analyst observes a connection with 'service' field set to 'dns' and 'query' field containing a long, random-looking subdomain. This could be indicative of which type of activity?
⚠ Common exam trap
Cisco often tests the distinction between DNS tunneling (exfiltration) and DNS amplification (DDoS), where candidates confuse the long query string of tunneling with the large response size of amplification, but the key is that amplification uses spoofed source IPs and small queries, not random subdomains.
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 tunneling for data exfiltration
A long, random-looking subdomain in a DNS query is a classic indicator of DNS tunneling, where an attacker encodes exfiltrated data into DNS queries to bypass network security controls. Zeek/Bro logs showing a 'service' of 'dns' with such queries suggest the client is using the DNS protocol to covertly transmit data to an external authoritative server, which decodes and reassembles the payload.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
DNS tunneling for data exfiltration
Why this is correct
Random subdomains can encode exfiltrated data.
- ✗
DNS amplification attack
Why it's wrong here
DNS amplification involves large response sizes, not random subdomains.
- ✗
DNS cache poisoning
Why it's wrong here
Cache poisoning involves corrupting DNS resolver cache, not random subdomains.
- ✗
Normal DNS resolution for a legitimate service
Why it's wrong here
Legitimate DNS queries typically have meaningful subdomains.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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