Question 1hardmultiple choice
Full question →- A
-sS
Why wrong: -sS is a SYN scan.
- B
-sI
-sI specifies an idle scan using a zombie host.
- C
-sA
Why wrong: -sA is an ACK scan.
- D
-sT
Why wrong: -sT is a TCP connect scan.
Question 62 of 1,010
Quick Answer
The answer is the `-sI` flag. This is correct because the Nmap idle scan, also known as a zombie scan, works by spoofing the IP address of a third-party host with a predictable IP ID sequence; by sending packets to the target that appear to come from the zombie, and then checking the zombie’s IP ID counter for an increment, you can infer whether the target’s ports are open, closed, or filtered without exposing your own IP. On the Certified Ethical Hacker CEH exam, this technique tests your understanding of stealth scanning and network evasion, often appearing in questions about avoiding detection or bypassing firewall logs. A common trap is confusing the idle scan with a decoy scan (`-D`), but remember that the idle scan requires a live zombie host with a globally reachable and predictable IP ID, while decoys simply send packets from multiple spoofed IPs. Memory tip: think “Idle” for “IP ID” – the zombie sits idle while you watch its counter change.
CEH Footprinting, Reconnaissance and Scanning Practice Question
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.
An attacker uses an idle scan with Nmap to probe a target. This technique relies on a third-party host with a predictable IP ID sequence to infer port states. Which Nmap flag enables an idle scan?
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
✓
-sI
The idle scan (-sI) leverages a third-party host (zombie) with a predictable IP ID sequence to indirectly probe a target's ports. By spoofing the zombie's IP and observing changes in its IP ID counter, Nmap can infer whether a port is open, closed, or filtered without sending packets directly from the attacker's IP.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
-sS
Why it's wrong here
-sS is a SYN scan.
- ✓
-sI
Why this is correct
-sI specifies an idle scan using a zombie host.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
-sA
Why it's wrong here
-sA is an ACK scan.
- ✗
-sT
Why it's wrong here
-sT is a TCP connect scan.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse -sI with -sS (SYN scan) because both are stealthy, but -sI specifically requires a zombie host and IP ID manipulation, not just half-open connections.
Detailed technical explanation
How to think about this question
The idle scan works by sending a SYN+ACK packet to the zombie's IP to obtain its current IP ID, then spoofing a SYN packet to the target with the zombie's source IP. If the target port is open, it sends a SYN+ACK to the zombie, causing the zombie to increment its IP ID; a subsequent probe to the zombie reveals the change, indicating an open port. This technique is stealthy but requires a zombie with a global IP ID increment (e.g., many older Windows or Linux systems) and fails if the zombie's IP ID is randomized (e.g., modern Linux kernels with RFC 1948).
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
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FAQ
Questions learners often ask
What does this CEH question test?
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: -sI — The idle scan (-sI) leverages a third-party host (zombie) with a predictable IP ID sequence to indirectly probe a target's ports. By spoofing the zombie's IP and observing changes in its IP ID counter, Nmap can infer whether a port is open, closed, or filtered without sending packets directly from the attacker's IP.
What should I do if I get this CEH question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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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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