Quick Answer
The answer is -sS, the Nmap flag for a SYN scan. This is correct because a SYN scan sends a TCP SYN packet to the target port and, upon receiving a SYN-ACK response, immediately terminates the connection by sending a RST packet instead of completing the three-way handshake with an ACK. This stealthy TCP scan avoids establishing a full connection, making it far less likely to be logged by the target’s application layer, which is precisely why it is the go-to technique for reconnaissance in the Certified Ethical Hacker CEH exam. On the exam, this concept tests your understanding of TCP stateful inspection and evasion; a common trap is confusing -sS with -sT (a full connect scan) or -sA (an ACK scan). Remember the memory tip: “SYN sends, then RST ends” – the half-open scan never shakes the third hand.
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. 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 penetration tester wants to perform a stealthy TCP scan that does not complete the three-way handshake. Which Nmap flag should be used?
-sU
Why wrong: -sU is for UDP scans.
-sS
-sS is the SYN stealth scan that doesn't complete the handshake.
-sV
Why wrong: -sV is for version detection, not scanning technique.
-sT
Why wrong: -sT performs a full TCP connect scan, completing the handshake.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
-sS
Option B (-sS) is correct because it performs a SYN scan, which sends a TCP SYN packet and waits for a SYN-ACK response without completing the three-way handshake (i.e., it sends a RST instead of an ACK). This makes the scan stealthy as it avoids establishing a full TCP connection, reducing the chance of being logged by the target.
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.
-sU
Why it's wrong here
-sU is for UDP scans.
-sS
Why this is correct
-sS is the SYN stealth scan that doesn't complete the handshake.
Related concept
Read the scenario before looking for a memorised answer.
-sV
Why it's wrong here
-sV is for version detection, not scanning technique.
-sT
Why it's wrong here
-sT performs a full TCP connect scan, completing the handshake.
Common exam traps
The trap here is that candidates often confuse -sS (SYN scan) with -sT (TCP connect scan), mistakenly thinking that -sT is stealthy because it uses TCP, but -sT actually completes the full handshake and is easily logged, while -sS is the true stealthy option.
Detailed technical explanation
In a SYN scan, Nmap sends a TCP SYN packet to the target port; if a SYN-ACK is received, the port is considered open, and Nmap immediately sends a RST to tear down the half-open connection, never completing the handshake. This technique exploits the TCP three-way handshake mechanism (RFC 793) and requires raw socket privileges (often root/administrator) to craft the packets. In real-world scenarios, some firewalls and intrusion detection systems (IDS) can detect SYN scans by monitoring for multiple SYN packets without corresponding ACKs, but it remains less conspicuous than a full connect scan.
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: -sS — Option B (-sS) is correct because it performs a SYN scan, which sends a TCP SYN packet and waits for a SYN-ACK response without completing the three-way handshake (i.e., it sends a RST instead of an ACK). This makes the scan stealthy as it avoids establishing a full TCP connection, reducing the chance of being logged by the target.
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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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, you need to identify all live hosts on a target network without being detected by intrusion detection systems. Which Nmap flag would BEST achieve this?
easyWhy B: The -sn flag (ping sweep) sends ICMP echo requests, TCP SYN to port 443, TCP ACK to port 80, and ICMP timestamp requests by default to determine if a host is alive. This is the best choice for stealthy host discovery because it does not complete a full TCP handshake or send application-layer probes, minimizing the chance of triggering IDS signatures that look for port scans or OS fingerprinting.
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Last reviewed: Jun 24, 2026
This CEH practice question is part of Courseiva's free EC-Council certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CEH exam.
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