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200-201 Practice Question: A Snort rule is configured: alert udp $HOME_NET…
A Snort rule is configured: alert udp $HOME_NET 123 -> $EXTERNAL_NET any (msg:'NTP DDoS'; content:'|17 00 03 2a|'; depth:4;). What does this rule detect?
⚠ Common exam trap
Cisco often tests the distinction between the protocol and port used in the attack (NTP on UDP 123) versus other amplification vectors (DNS on UDP 53, SNMP on UDP 161), so the trap here is confusing the NTP amplification attack with DNS amplification because both use reflection, but the rule's port and content bytes uniquely identify NTP.
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
✓
NTP amplification attack
The rule triggers on UDP traffic from port 123 (NTP) on the internal network to any port on an external network, with a payload starting with the bytes `17 00 03 2a`. These bytes correspond to the NTP control message header for a `MON_GETLIST` request (opcode 0x17, sequence 0x00, status 0x03, association ID 0x002a), which is the classic pattern used in NTP amplification attacks. This detects an outgoing NTP query that attempts to exploit the `monlist` command to reflect a large volume of traffic toward a victim, making D correct.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
SNMP community string
Why it's wrong here
Incorrect. SNMP uses different ports and payloads.
- ✗
NTP server misconfiguration
Why it's wrong here
Incorrect. Misconfiguration would not necessarily have this specific payload.
- ✗
DNS amplification attack
Why it's wrong here
Incorrect. DNS uses port 53, not 123.
- ✓
NTP amplification attack
Why this is correct
Correct. The rule detects NTP monlist requests used in DDoS amplification.
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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.