Quick Answer
The answer is SYN scan, also known as a half-open scan. This is correct because a SYN scan sends a TCP SYN packet to a target port but never completes the three-way handshake; if no SYN-ACK is returned, the port is considered filtered or the host is unresponsive, exactly matching the scenario where the attacker sees no SYN-ACK responses. On the Certified Ethical Hacker CEH exam, this question tests your ability to distinguish stealth scanning techniques from full-connection scans—a common trap is confusing SYN scan with TCP connect scan, which completes the handshake and logs a full connection. Remember that SYN scan is stealthier because it leaves the connection half-open, while TCP connect scan is noisy and easily logged. Memory tip: think “SYN = Stealth, Yet No-ACK.”
This CEH practice question tests your understanding of introduction to ethical hacking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 security analyst suspects that an attacker is scanning their network. They notice a large number of TCP SYN packets being sent to various ports on a single host, but no SYN-ACK responses are returned. Which type of scan is most likely being used?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
TCP connect scan
Why wrong: TCP connect scan completes the handshake, so SYN-ACK would be received from open ports.
UDP scan
Why wrong: UDP scans send UDP datagrams, not TCP SYN packets.
SYN scan
SYN scan sends SYN packets; lack of SYN-ACK indicates filtered/closed ports.
FIN scan
Why wrong: FIN scan sends FIN packets, not SYN.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
SYN scan
C is correct because a SYN scan (also known as a half-open scan) sends TCP SYN packets to target ports and does not complete the three-way handshake. If no SYN-ACK is returned, it indicates the port is filtered or the host is not responding, which matches the scenario where the attacker receives no SYN-ACK responses. This scan is stealthier than a full TCP connect scan because it never establishes a full connection.
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.
TCP connect scan
Why it's wrong here
TCP connect scan completes the handshake, so SYN-ACK would be received from open ports.
UDP scan
SYN scan
Why this is correct
SYN scan sends SYN packets; lack of SYN-ACK indicates filtered/closed ports.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
FIN scan
Why it's wrong here
FIN scan sends FIN packets, not SYN.
Common exam traps
The trap here is that candidates often confuse SYN scan with TCP connect scan, thinking that any TCP scan must complete the handshake, but the key distinction is that SYN scan never sends the final ACK, making it half-open and stealthier.
Detailed technical explanation
In a SYN scan, the attacker sends a SYN packet and if the port is open, the target responds with SYN-ACK; the attacker then sends a RST to tear down the connection, never completing the handshake. If no SYN-ACK is received, it could mean the port is filtered by a firewall, the host is down, or the packet was dropped. This scan is defined in RFC 793 and is commonly used in tools like Nmap with the -sS flag, but modern intrusion detection systems (IDS) can detect the pattern of multiple SYN packets without subsequent ACKs.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the CEH exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Introduction to Ethical Hacking — This question tests Introduction to Ethical Hacking — Read the scenario before looking for a memorised answer..
The correct answer is: SYN scan — C is correct because a SYN scan (also known as a half-open scan) sends TCP SYN packets to target ports and does not complete the three-way handshake. If no SYN-ACK is returned, it indicates the port is filtered or the host is not responding, which matches the scenario where the attacker receives no SYN-ACK responses. This scan is stealthier than a full TCP connect scan because it never establishes a full connection.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. During a penetration test, an ethical hacker needs to evade an IDS that detects port scans based on the number of packets per second. Which technique would be most effective to avoid detection?
hardWhy C: Option C is correct because slowing down the scan rate reduces the number of packets sent per second below the IDS threshold, allowing the scan to blend in with normal traffic. IDS systems like Snort use packet-per-second (pps) counters to detect port scans; by spacing out packets over a longer period, the scan avoids triggering these rate-based alerts.
Last reviewed: Jun 11, 2026
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