CEH Footprinting, Reconnaissance and Scanning Practice Question
What is the primary purpose of using the Nmap flag -sS?
⚠ Common exam trap
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.
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
✓
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Enable OS detection
Why it's wrong here
The -sS flag is specifically designed for performing a SYN stealth scan, not for operating system detection. Nmap utilizes the -O flag to enable its sophisticated OS detection capabilities, which involve sending a series of TCP and UDP packets to analyze responses, identify unique characteristics, and fingerprint the target's operating system. This process is distinct from port scanning methods and requires a different Nmap option.
- ✓
Perform a SYN stealth scan
Why this is correct
The -sS flag instructs Nmap to perform a SYN stealth scan, also known as a half-open scan. This method sends a SYN packet to the target port; if a SYN/ACK is received, Nmap knows the port is open but does not complete the three-way handshake by sending an ACK. This technique often bypasses non-stateful firewalls and avoids full connection logs on the target system, making it less detectable than a full TCP connect scan.
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Perform a UDP scan
Why it's wrong here
The -sS flag is exclusively used for TCP-based SYN stealth scanning and does not perform UDP port scanning. To scan for open UDP ports, Nmap requires the -sU flag, which sends UDP packets to target ports and infers port status based on whether an ICMP port unreachable error is received or if an application-layer response is obtained. TCP and UDP scanning are fundamentally different processes operating on different transport layer protocols.
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Perform a TCP connect scan
Why it's wrong here
While both are TCP scans, the -sS flag performs a SYN stealth scan, which is distinct from a full TCP connect scan. A TCP connect scan, initiated with the -sT flag, completes the full three-way TCP handshake (SYN, SYN/ACK, ACK) for every open port. This method is more easily logged by target systems and firewalls because it establishes a complete connection, making it significantly less stealthy than the half-open SYN scan.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.