200-201 Network Intrusion Analysis Practice Question
An analyst reviewing network alerts notices a rule triggered for 'ET SCAN NMAP -sU scan' based on traffic to a Linux server. The packet capture shows multiple UDP packets to various ports, and for closed ports, the server responds with ICMP Destination Unreachable (Port Unreachable). Which type of scan is being performed, and how should the analyst classify this alert?
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
✓
UDP scan; true positive
UDP scans send UDP packets; closed ports respond with ICMP Port Unreachable. This matches the alert signature, indicating a true positive for a UDP scan.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
TCP SYN scan; true positive
Why it's wrong here
TCP SYN scans send TCP segments and expect SYN-ACK or RST responses; the capture instead shows UDP datagrams answered by ICMP Port Unreachable, so no TCP handshake occurs. A SYN scan would be correct if half-open TCP connections to ports were captured, which is not the case in this traffic.
- ✓
UDP scan; true positive
Why this is correct
Nmap's UDP scan sends zero-byte UDP datagrams to target ports; closed ports return ICMP port unreachable, matching the capture. The rule signature and traffic genuinely reflect scanning activity, so the analyst classifies it as a true positive rather than a false positive.
- ✗
UDP scan; false positive
Why it's wrong here
The traffic genuinely is a UDP scan, so labelling the alert a false positive misclassifies it; the rule correctly detected scanning activity against the Linux server. A false positive would apply if benign traffic matched the signature, but here the UDP probes and ICMP Port Unreachable replies confirm actual reconnaissance.
- ✗
TCP connect scan; true negative
Why it's wrong here
The capture shows UDP probes with ICMP Port Unreachable replies, which is characteristic of a UDP scan, not a TCP connect scan; TCP connect scans complete full three-way handshakes on TCP ports. A TCP connect scan would be the right classification when full TCP sessions to ports are observed, but no TCP traffic appears here.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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