Quick Answer
The answer is an NTP amplification attack. This Snort rule detects an outgoing NTP control message from your internal network to an external server, specifically the `monlist` (MON_GETLIST) request, identified by the hex bytes `17 00 03 2a` at the start of the payload. The `monlist` command is the classic vector for NTP amplification because it causes the server to reply with a much larger response than the query, flooding a spoofed victim’s IP. On the Cisco CyberOps Associate 200-201 exam, this tests your ability to interpret Snort rule syntax and recognize attack signatures—a common trap is confusing the direction: the rule alerts on the *outbound* trigger, not the inbound flood. For memory, think of the bytes as “17” for the opcode (like a “lucky” number for attackers) and “monlist” as “monster list” because it generates a monstrously amplified reply.
This 200-201 practice question tests your understanding of network intrusion analysis. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
SNMP community string
Why wrong: Incorrect. SNMP uses different ports and payloads.
NTP server misconfiguration
Why wrong: Incorrect. Misconfiguration would not necessarily have this specific payload.
DNS amplification attack
Why wrong: Incorrect. DNS uses port 53, not 123.
NTP amplification attack
Correct. The rule detects NTP monlist requests used in DDoS amplification.
Answer choices
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
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.
Detailed technical explanation
The NTP `monlist` command (opcode 17) returns a list of the last 600 clients that interacted with the NTP server, and when spoofed, the response can be up to 206 times larger than the request, enabling a powerful amplification vector. The rule uses `depth:4` to check only the first 4 bytes of the payload, which is sufficient to match the NTP control header without needing to inspect the full packet. In real-world scenarios, attackers often spoof the source IP to the victim's address, so this rule on an egress filter can detect compromised internal hosts being used to launch the attack.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Network Intrusion Analysis — This question tests Network Intrusion Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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