easyMultiple Choice
200-201 Practice Question: During network intrusion analysis, an analyst…
During network intrusion analysis, an analyst observes a TCP connection with the SYN flag set but no subsequent ACK. This pattern is indicative of:
⚠ Common exam trap
Cisco often tests the distinction between a normal three-way handshake and an incomplete handshake pattern, where candidates mistakenly think any SYN packet indicates a legitimate connection attempt rather than recognizing the missing ACK as the hallmark of a SYN flood.
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
✓
SYN flood attack
A SYN flood attack is a type of denial-of-service (DoS) attack where the attacker sends a high volume of TCP SYN packets to a target server but never completes the three-way handshake by sending the final ACK. This leaves the server with half-open connections, consuming resources and potentially exhausting the connection backlog, which prevents legitimate clients from establishing connections.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
SYN flood attack
Why this is correct
A SYN flood exploits the TCP three-way handshake by sending numerous SYN packets without completing the ACK, exhausting the server's half-open connection backlog. The stem's single SYN-without-ACK pattern directly matches this mechanism, confirming the attack type observed during intrusion analysis.
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DNS resolution
Why it's wrong here
DNS resolution is a UDP query-response exchange on port 53, carrying no TCP SYN flag at all. The observed SYN without ACK is a TCP-layer event, so it cannot indicate name resolution; DNS would be the answer if the capture showed query and response packets on port 53.
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Normal three-way handshake
Why it's wrong here
A completed three-way handshake requires SYN, then SYN-ACK from the responder, then a final ACK. Observing SYN alone with no ACK means the handshake never completed, so this cannot be normal traffic; a half-open connection instead suggests scanning or a SYN flood.
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ICMP echo request
Why it's wrong here
ICMP echo request is a layer-3 message with its own type and code fields, not a TCP segment, so it carries no SYN flag. The observation is a TCP header flag, which ICMP cannot produce; ICMP would be relevant if the capture showed echo request and reply packets.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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