- A
The target host has a host-based firewall that drops SYN packets but allows full connections.
Why wrong: Host-based firewalls typically treat SYN packets the same as any other TCP packet; they don't distinguish half-open vs. full connection.
- B
A network firewall is performing stateful inspection and dropping unsolicited SYN packets while allowing established connections.
Stateful firewalls track connections; a SYN scan sends unsolicited SYN packets that may be dropped, while a connect scan completes the handshake and appears as a normal connection.
- C
The target host has ICMP filtering enabled, preventing Nmap from identifying it as online.
Why wrong: ICMP filtering would affect ping sweeps, not TCP scans; Nmap uses TCP probes to determine host status regardless of ICMP.
- D
Nmap's SYN scan requires root privileges, and the tester ran it without sudo.
Why wrong: If the tester ran SYN scan without root, Nmap would fall back to connect scan automatically, not return no open ports.
Quick Answer
The answer is a network firewall performing stateful inspection that drops unsolicited SYN packets while allowing established connections. This discrepancy occurs because a SYN scan sends only a raw SYN packet and waits for a SYN-ACK to indicate an open port, but a stateful firewall sees this as an unsolicited new connection attempt and silently drops the packet, making the port appear closed or filtered. In contrast, a TCP connect scan completes the full three-way handshake (SYN, SYN-ACK, ACK), which the firewall recognizes as a legitimate established session and permits through, revealing port 443 as open. On the CEH exam, this scenario tests your understanding of how stateful firewalls differ from stateless ones and how scan types interact with packet filtering. A common trap is assuming both scans should yield identical results, but the key is that SYN scans never finish the handshake, so they evade application-layer logging but also trigger stateful drops. Memory tip: think “SYN is stealthy but gets stonewalled by stateful walls; Connect completes the dance, so the firewall gives it a chance.”
CEH Scanning Networks and Enumeration Practice Question
This CEH practice question tests your understanding of scanning networks and enumeration. 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 penetration tester discovers that an Nmap SYN scan against a target host returns no open ports, but a TCP connect scan reveals port 443 open. Which of the following is the most likely reason for this discrepancy?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
A network firewall is performing stateful inspection and dropping unsolicited SYN packets while allowing established connections.
The discrepancy occurs because a network firewall is performing stateful inspection. It drops unsolicited SYN packets (the first step of a SYN scan) but allows full TCP connections (SYN, SYN-ACK, ACK) to complete, as the connect scan establishes a full three-way handshake. This behavior is typical of stateful firewalls that track connection state and only permit packets belonging to established sessions.
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 target host has a host-based firewall that drops SYN packets but allows full connections.
Why it's wrong here
Host-based firewalls typically treat SYN packets the same as any other TCP packet; they don't distinguish half-open vs. full connection.
- ✓
A network firewall is performing stateful inspection and dropping unsolicited SYN packets while allowing established connections.
Why this is correct
Stateful firewalls track connections; a SYN scan sends unsolicited SYN packets that may be dropped, while a connect scan completes the handshake and appears as a normal connection.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The target host has ICMP filtering enabled, preventing Nmap from identifying it as online.
- ✗
Nmap's SYN scan requires root privileges, and the tester ran it without sudo.
Why it's wrong here
If the tester ran SYN scan without root, Nmap would fall back to connect scan automatically, not return no open ports.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a host-based firewall is the cause, but stateful network firewalls specifically differentiate between incomplete SYN probes and full TCP handshakes, making option B the correct answer.
Detailed technical explanation
How to think about this question
Stateful firewalls maintain a connection table and enforce rules based on TCP state flags. A SYN scan sends only a SYN packet and expects a SYN-ACK; the firewall sees this as an unsolicited SYN and drops it. A connect scan uses the OS's TCP stack to complete the three-way handshake (SYN, SYN-ACK, ACK), which the firewall recognizes as a legitimate connection initiation, allowing the SYN-ACK to return. This is why Nmap's -sS (SYN scan) and -sT (connect scan) yield different results in such environments.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this CEH question test?
Scanning Networks and Enumeration — This question tests Scanning Networks and Enumeration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A network firewall is performing stateful inspection and dropping unsolicited SYN packets while allowing established connections. — The discrepancy occurs because a network firewall is performing stateful inspection. It drops unsolicited SYN packets (the first step of a SYN scan) but allows full TCP connections (SYN, SYN-ACK, ACK) to complete, as the connect scan establishes a full three-way handshake. This behavior is typical of stateful firewalls that track connection state and only permit packets belonging to established sessions.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on CEH
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. During a penetration test, you discover that an internal web server responds to ICMP echo requests but does not respond to TCP SYN scans on port 80. However, when you browse to the server's IP using a browser, the web page loads successfully. What is the most likely reason for this behavior?
medium- ✓ A.A stateful firewall is blocking inbound SYN packets to port 80 but allowing responses to outbound connections.
- B.The web server is running on a non-standard port that you did not scan.
- C.The server's TCP/IP stack is misconfigured and does not respond to SYN scans.
- D.A web application firewall is blocking the SYN scan traffic.
Why A: A stateful firewall tracks the state of network connections. When you browse to the server, your browser initiates an outbound TCP connection, and the firewall allows the return SYN-ACK packets as part of the established session. However, a standalone TCP SYN scan sends unsolicited SYN packets to port 80, which the firewall sees as a new inbound connection attempt and blocks, preventing the server from responding. The server's ICMP echo reply is allowed because ICMP is stateless and not typically filtered by the same rules.
Last reviewed: Jun 11, 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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