Quick Answer
The answer is closed. This classification is correct because when Nmap sends a TCP SYN packet to a port and receives a TCP RST packet in response, it confirms the port is reachable but no service is listening, per RFC 793 which mandates that a RST be sent when a SYN arrives on a closed port. On the Certified Ethical Hacker CEH exam, this tests your understanding of the TCP three-way handshake and Nmap’s port state classification, often appearing in questions that distinguish closed ports from filtered or open ports. A common trap is confusing the RST response with a firewall block; remember that a RST means the host actively rejected the connection, not that it dropped the packet silently. For a memory tip, think “SYN to closed gets RST—no service, just a reset.”
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 sends a TCP SYN packet to a port and receives a TCP RST packet in response. According to Nmap's port state classification, what is the state of this port?
Filtered
Why wrong: Filtered means no response or ICMP unreachable.
Unfiltered
Why wrong: Unfiltered means the port is accessible but state unknown; Nmap uses this for ACK scans.
Closed
A closed port responds with RST.
Open
Why wrong: An open port would respond with SYN/ACK.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Closed
When Nmap sends a TCP SYN packet to a port and receives a TCP RST packet in response, it indicates that the port is reachable but no service is listening on it. Per RFC 793, a RST is sent when a SYN arrives on a closed port, so Nmap classifies this port as 'closed'.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Filtered
Why it's wrong here
Filtered means no response or ICMP unreachable.
Unfiltered
Why it's wrong here
Unfiltered means the port is accessible but state unknown; Nmap uses this for ACK scans.
Closed
Why this is correct
A closed port responds with RST.
Related concept
Read the scenario before looking for a memorised answer.
Open
Why it's wrong here
An open port would respond with SYN/ACK.
Common exam traps
EC-Council often tests the misconception that a RST response means the port is 'filtered' or 'open', but the correct interpretation per Nmap's classification is that a RST directly indicates a 'closed' port.
Detailed technical explanation
In a TCP SYN scan (-sS), Nmap sends a SYN packet and waits for a response. A RST packet is generated by the target's TCP stack when no application is bound to the port, as per RFC 793 Section 3.4. This behavior is consistent even if the target is behind a stateful firewall that permits outbound RSTs, though some firewalls may spoof RSTs to confuse scanners. Understanding this distinction is critical for accurate port state mapping.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
Footprinting, Reconnaissance and Scanning — This question tests Footprinting, Reconnaissance and Scanning — Read the scenario before looking for a memorised answer..
The correct answer is: Closed — When Nmap sends a TCP SYN packet to a port and receives a TCP RST packet in response, it indicates that the port is reachable but no service is listening on it. Per RFC 793, a RST is sent when a SYN arrives on a closed port, so Nmap classifies this port as 'closed'.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 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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