- A
The scan uses fragmented packets and decoy IP addresses
Why wrong: Fragmentation and decoys are evasion techniques, not specific to idle scan.
- B
The scan uses FTP bounce to hide the true source IP
Why wrong: FTP bounce is a different technique using an FTP server to proxy connections.
- C
The scan sends SYN packets with the zombie host's IP address as the source and monitors IPID changes on the zombie to determine open ports
This correctly describes the idle scan technique: spoofing the zombie's IP and observing IPID increments to infer port states.
- D
The scan shows a SYN packet with the attacker's real IP but uses a zombie host to relay replies
Why wrong: In an idle scan, the attacker sends spoofed SYN packets to the target with the zombie's IP as source, then monitors the zombie's IPID to infer responses.
Quick Answer
The correct answer is the option describing SYN packets sent with the zombie host’s IP as the source and monitoring IPID changes on the zombie to determine open ports. This is accurate because the Nmap idle scan technique, also known as a zombie scan, exploits a predictable IPID sequence on a passive host to indirectly probe a target. By spoofing the zombie’s IP address in SYN packets sent to the target, the attacker observes whether the zombie’s IPID increments—if it does, the target port is open; if not, it is closed or filtered. On the CEH exam, this question tests your understanding of stealth scanning methods and the critical role of IPID behavior, often appearing as a scenario where an analyst must distinguish idle scans from other spoofed scans. A common trap is confusing idle scans with blind TCP scans, but the key differentiator is the reliance on a third-party zombie and IPID monitoring. Memory tip: “Idle = IPID zombie” to recall that the zombie’s IPID changes confirm the scan.
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.
A security analyst receives an alert about a scan originating from an IP address that appears to be using a 'sIdle scan' technique. Which of the following characteristics would confirm this?
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
The scan sends SYN packets with the zombie host's IP address as the source and monitors IPID changes on the zombie to determine open ports
Option C is correct because an idle scan (also known as a zombie scan) relies on sending SYN packets to the target with the spoofed source IP address of a zombie host, then monitoring the IPID sequence on that zombie to infer whether the target port is open, closed, or filtered. The key characteristic is the use of the zombie's IP as the source and the observation of IPID changes on the zombie, which is exactly what option C describes.
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.
- ✗
The scan uses fragmented packets and decoy IP addresses
Why it's wrong here
Fragmentation and decoys are evasion techniques, not specific to idle scan.
- ✗
The scan uses FTP bounce to hide the true source IP
Why it's wrong here
FTP bounce is a different technique using an FTP server to proxy connections.
- ✓
The scan sends SYN packets with the zombie host's IP address as the source and monitors IPID changes on the zombie to determine open ports
Why this is correct
This correctly describes the idle scan technique: spoofing the zombie's IP and observing IPID increments to infer port states.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The scan shows a SYN packet with the attacker's real IP but uses a zombie host to relay replies
Why it's wrong here
In an idle scan, the attacker sends spoofed SYN packets to the target with the zombie's IP as source, then monitors the zombie's IPID to infer responses.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the idle scan with other spoofing or relay techniques like FTP bounce or decoy scans, but the idle scan is uniquely defined by the use of a zombie host's IPID changes to infer port states, not by hiding the attacker's IP via relays or multiple decoys.
Detailed technical explanation
How to think about this question
The idle scan exploits the predictable IPID increment behavior of many operating systems (e.g., older Windows, Linux). The attacker sends a SYN-ACK to the zombie to obtain its current IPID, then sends a spoofed SYN to the target with the zombie's IP as source; if the target port is open, it sends a SYN-ACK to the zombie (which the zombie ignores), causing the zombie's IPID to increment by 1; the attacker then probes the zombie again to see if the IPID increased, confirming the port is open. This technique is stealthy because the target never sees the attacker's real IP, but it requires a zombie host with a globally reachable IP and predictable IPID behavior.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: The scan sends SYN packets with the zombie host's IP address as the source and monitors IPID changes on the zombie to determine open ports — Option C is correct because an idle scan (also known as a zombie scan) relies on sending SYN packets to the target with the spoofed source IP address of a zombie host, then monitoring the IPID sequence on that zombie to infer whether the target port is open, closed, or filtered. The key characteristic is the use of the zombie's IP as the source and the observation of IPID changes on the zombie, which is exactly what option C describes.
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.
About these practice questions
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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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