CEH Footprinting, Reconnaissance and Scanning Practice Question
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs the command: nmap -sS -p 80,443,8080 --script http-headers scanme.nmap.org. The output shows that port 80 is filtered. What does 'filtered' mean in this context?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall or IDS is preventing the probe from reaching the port
In Nmap, a 'filtered' port status indicates that the port is being blocked by a firewall, IDS, or other network filtering device, preventing the probe from reaching the target service. The -sS (SYN stealth scan) sends a SYN packet; if no response is received or an ICMP unreachable (type 3, code 13) is returned, Nmap marks the port as filtered. This does not mean the port is open or closed—it means the scan could not determine the state due to filtering.
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 port is open, but the service is not responding
Why it's wrong here
If the service does not respond, Nmap might still see a SYN-ACK, indicating open.
✗
The port is open and actively listening
Why it's wrong here
If open, the state would be 'open', not 'filtered'.
✗
The port is closed, but the target is responding with RST packets
Why it's wrong here
Closed ports typically respond with RST, resulting in a 'closed' state.
✓
A firewall or IDS is preventing the probe from reaching the port
Why this is correct
Filtered means the probe was dropped or blocked, likely by a filtering device.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse 'filtered' with 'closed' or 'open|filtered', forgetting that 'filtered' specifically indicates a firewall or IDS is interfering, not that the port is simply unresponsive or sending RSTs.
Detailed technical explanation
How to think about this question
Nmap's 'filtered' state often results from a firewall dropping the SYN packet silently or sending an ICMP unreachable (e.g., type 3 code 13, 'communication administratively prohibited'). In real-world scenarios, stateful firewalls may also send RST packets for filtered ports, but Nmap distinguishes this by analyzing timing and response patterns. The -sS scan relies on the TCP three-way handshake; if no SYN/ACK or RST is received within a timeout, the port is considered filtered.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CEH question in full detail.
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: A firewall or IDS is preventing the probe from reaching the port — In Nmap, a 'filtered' port status indicates that the port is being blocked by a firewall, IDS, or other network filtering device, preventing the probe from reaching the target service. The -sS (SYN stealth scan) sends a SYN packet; if no response is received or an ICMP unreachable (type 3, code 13) is returned, Nmap marks the port as filtered. This does not mean the port is open or closed—it means the scan could not determine the state due to filtering.
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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Question Discussion
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