CEH Footprinting, Reconnaissance and Scanning Practice Question
A penetration tester observes that an Nmap SYN scan shows all 1000 TCP ports as open. The tester suspects the target is using a security appliance that responds with SYN-ACK to all connection attempts, regardless of the actual port state. Which type of Nmap scan would be MOST effective in determining the true state of the ports?
⚠ Common exam trap
Candidates often assume a SYN scan (-sS) is always superior due to stealth, but they overlook that a security appliance can spoof SYN-ACKs, making the full handshake of -sT necessary to bypass the deception.
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
✓
TCP connect scan (-sT)
A TCP connect scan (-sT) completes the full three-way handshake, which forces the target to respond with a RST if the port is actually closed, even if a security appliance initially sends SYN-ACK to all ports. This distinguishes between ports that are truly open (where the handshake completes) and those that are falsely reported as open by the appliance (where the handshake fails or a RST is received).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
UDP scan (-sU)
Why it's wrong here
A UDP scan (-sU) targets connectionless UDP ports, sending UDP packets and waiting for ICMP port unreachable messages to infer closed ports, or application responses for open ones. This scan type is fundamentally different from TCP scans and does not involve the SYN/SYN-ACK handshake mechanism. Therefore, it cannot resolve issues related to spoofed TCP SYN-ACK responses observed during a TCP SYN scan, as it operates on a different protocol layer and port type entirely.
- ✗
Ping sweep (-sn)
Why it's wrong here
A ping sweep (-sn), or host discovery scan, is designed solely to determine if hosts are active on a network, typically using ICMP echo requests or other host discovery methods. It does not attempt to establish connections to specific ports or probe their state. Since it only checks for host availability and not the status of individual TCP ports, it provides no information to counteract or verify spoofed SYN-ACK responses during a port scan.
- ✓
TCP connect scan (-sT)
Why this is correct
The TCP connect scan (-sT) performs a full three-way handshake (SYN, SYN-ACK, ACK) with the target port, relying on the operating system's native `connect()` system call. If a port is genuinely open, the handshake completes. If a spoofed SYN-ACK is received, the scanner's subsequent ACK will likely be ignored by the true target, or the target will send a RST if the port is closed. This full interaction allows the scanner to observe the actual target's response to a completed connection attempt, thereby reliably determining the port's true state even if initial SYN-ACKs were manipulated.
- ✗
Idle scan (-sI)
Why it's wrong here
An Idle scan (-sI) is an advanced, stealthy technique that uses a "zombie" host's predictable IP ID sequence to infer port states on a target without sending packets directly from the scanner's IP. While it offers stealth by proxying the scan through an idle host, it does not inherently address situations where the target itself is actively spoofing SYN-ACKs in response to probes, whether those probes originate from the zombie or directly. The spoofing issue described implies the target is manipulating its own responses, which an indirect idle scan may not reliably circumvent.
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Related to this question
Learn chapter
Penetration Testing and Reporting
Key term
Active reconnaissance
Active reconnaissance is the process of directly interacting with a target system or network to gather information, often through scanning and probing.
Key term
Port Scanning Techniques
Port scanning techniques are methods used to probe a computer or network to discover which network ports are open and which services are running on those ports.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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