Quick Answer
The answer is the SYN scan (-sS). This Nmap scan type is the most likely cause of a large number of incomplete TCP connections in the SYN_RECV state because it deliberately aborts the three-way handshake: it sends a SYN packet, and upon receiving a SYN/ACK from the server, it immediately replies with a RST instead of completing the handshake with an ACK. This leaves the server waiting for the final ACK, creating a half-open connection that remains in the SYN_RECV queue. On the Certified Ethical Hacker CEH exam, this question tests your understanding of stealth scanning techniques and their network footprint—a common trap is confusing SYN scan with a full connect scan (-sT), which completes the handshake and leaves no half-open connections. A useful memory tip: think “SYN sent, SYN/ACK received, RST sent—connection left half-open, hence SYN_RECV.”
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 notices a large number of incomplete TCP connections (SYN_RECV) on a server. Which Nmap scan type is the MOST likely cause of this symptom?
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.
SYN scan (-sS)
SYN scan sends SYN and stops after receiving SYN-ACK, leaving the connection half-open (SYN_RECV).
Ping sweep (-sn)
Why wrong: Ping sweep only sends ICMP or TCP pings, not SYN packets to ports.
TCP Connect scan (-sT)
Why wrong: TCP Connect scan completes the handshake, resulting in ESTABLISHED connections, not SYN_RECV.
UDP scan (-sU)
Why wrong: UDP scan uses UDP packets, not TCP, so it does not cause SYN_RECV states.
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 (-sS)
A SYN scan (-sS) sends a SYN packet to initiate a TCP handshake and, upon receiving a SYN/ACK, sends a RST to tear down the connection before it completes. This leaves the server with half-open connections in the SYN_RECV state because the three-way handshake is never finished. The large number of incomplete connections directly matches the behavior of a SYN scan.
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.
SYN scan (-sS)
Why this is correct
SYN scan sends SYN and stops after receiving SYN-ACK, leaving the connection half-open (SYN_RECV).
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.
Ping sweep (-sn)
TCP Connect scan (-sT)
Why it's wrong here
TCP Connect scan completes the handshake, resulting in ESTABLISHED connections, not SYN_RECV.
UDP scan (-sU)
Common exam traps
The trap here is that candidates confuse SYN scan with TCP Connect scan, assuming both complete the handshake, but SYN scan deliberately avoids the final ACK to remain stealthy, leaving the connection half-open in SYN_RECV.
Detailed technical explanation
Under the hood, the SYN_RECV state indicates the server has received a SYN, sent a SYN/ACK, and is waiting for the final ACK to complete the handshake. In a SYN scan, the client sends a RST instead of the ACK, so the server's TCP stack keeps the connection in SYN_RECV until a timeout (typically 60-120 seconds per RFC 793). In real-world scenarios, an attacker might use a SYN flood to exhaust the server's backlog queue, causing denial of service, which is why this scan type is often detected by intrusion detection systems.
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
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
The correct answer is: SYN scan (-sS) — A SYN scan (-sS) sends a SYN packet to initiate a TCP handshake and, upon receiving a SYN/ACK, sends a RST to tear down the connection before it completes. This leaves the server with half-open connections in the SYN_RECV state because the three-way handshake is never finished. The large number of incomplete connections directly matches the behavior of a SYN scan.
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, a security analyst observes that Nmap SYN scans to a target server are not returning any results, but TCP connect scans succeed. The server is running an IDS. Which evasion technique is the analyst MOST likely encountering?
hardWhy A: The IDS is configured to drop packets with only the SYN flag set, which is the hallmark of a SYN scan. This evasion technique forces the attacker to use a full TCP connect scan (which completes the three-way handshake) to bypass the IDS detection. The IDS drops the initial SYN packet, preventing the scan from receiving any response, while a full connect scan is allowed because it mimics legitimate traffic.
Last reviewed: Jun 24, 2026
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