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
DNS tunnelling encodes stolen data within subdomain labels of queries sent to an attacker-controlled authoritative nameserver. Zeek's `service` field identifying DNS, combined with an abnormally long, high-entropy subdomain, satisfies the exfiltration indicator: legitimate DNS labels are short and structured, whereas tunnelled payloads appear random to evade signature matching.
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DNS amplification attack
Why it's wrong here
DNS amplification floods a victim with large responses to spoofed queries; the long random subdomain here indicates data exfiltration or DGA beaconing, not reflection. Amplification is tempting because it abuses DNS, and would be correct if logs showed high-volume responses to spoofed source addresses.
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DNS cache poisoning
Why it's wrong here
Cache poisoning corrupts a resolver's cached records via forged responses, leaving no long random subdomain in the query field. It is tempting because it targets DNS infrastructure, and would be correct if logs showed mismatched response records or unexpected authoritative answers for a cached name.
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Normal DNS resolution for a legitimate service
Why it's wrong here
Legitimate resolution uses predictable, registered domain labels, whereas the long random subdomain signals encoded exfiltration or algorithmically generated domains. Legitimate traffic is tempting because DNS resolution is routine, and would be correct if the query matched a known service with normal label entropy.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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