Quick Answer
The answer is that the Nmap -sS flag is used to perform a SYN stealth scan, also known as a half-open scan. This technique works by sending a TCP SYN packet to the target port; if a SYN/ACK is received, the port is marked as open, and the scanner immediately sends an RST packet to abort the connection before the three-way handshake completes. By never fully establishing a TCP session, the scan avoids being logged by many application-layer services, making it stealthier than a full connect scan. On the Certified Ethical Hacker CEH exam, this flag tests your understanding of network reconnaissance and evasion tactics, often appearing in questions about stealth scanning or default scan behavior with root privileges. A common trap is confusing -sS with -sT (TCP connect scan), which completes the handshake and is more detectable. Remember the mnemonic: “SYN sent, SYN-ACK seen, send RST—stealth achieved.”
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.
What is the primary purpose of using the Nmap flag -sS?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Enable OS detection
Why wrong: OS detection uses -O.
Perform a SYN stealth scan
-sS is the SYN stealth scan flag.
Perform a UDP scan
Why wrong: UDP scan uses -sU.
Perform a TCP connect scan
Why wrong: TCP connect scan uses -sT.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Perform a SYN stealth scan
The Nmap flag -sS instructs Nmap to perform a SYN stealth scan, also known as a half-open scan. It sends a TCP SYN packet to the target port and, if a SYN/ACK is received, the port is considered open; the scanner then sends a RST to tear down the connection before the full TCP three-way handshake completes. This technique avoids completing the connection, making it less likely to be logged by the target's application layer and is the default scan type when run with root privileges.
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.
Enable OS detection
Why it's wrong here
OS detection uses -O.
Perform a SYN stealth scan
Why this is correct
-sS is the SYN stealth scan flag.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Perform a UDP scan
Why it's wrong here
UDP scan uses -sU.
Perform a TCP connect scan
Why it's wrong here
TCP connect scan uses -sT.
Common exam traps
EC-Council often tests the distinction between -sS (SYN stealth) and -sT (TCP connect), where candidates mistakenly think -sS completes the handshake or that -sT is the stealthier option.
Detailed technical explanation
The SYN stealth scan works by sending a TCP packet with the SYN flag set (as defined in RFC 793). If the target responds with a SYN/ACK, the port is open; if it responds with an RST, the port is closed; if no response or an ICMP unreachable error is received, the port is filtered. A subtle behavior is that on modern firewalls and intrusion detection systems (IDS), the pattern of SYN packets followed by RST packets can still be detected, so the 'stealth' aspect is relative and not guaranteed.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
What to study next
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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: Perform a SYN stealth scan — The Nmap flag -sS instructs Nmap to perform a SYN stealth scan, also known as a half-open scan. It sends a TCP SYN packet to the target port and, if a SYN/ACK is received, the port is considered open; the scanner then sends a RST to tear down the connection before the full TCP three-way handshake completes. This technique avoids completing the connection, making it less likely to be logged by the target's application layer and is the default scan type when run with root privileges.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 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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