The answer is that port 80 is open on 10.0.0.1. This is correct because the hping3 SYN scan, using the -S flag, sends TCP SYN packets to the target port; when the port is open and listening, it responds with a SYN-ACK packet as part of the standard TCP three-way handshake. Receiving those SYN-ACK replies directly confirms the port is accepting connections, whereas a closed port would respond with a RST packet. On the Certified Ethical Hacker CEH exam, this scenario tests your understanding of TCP handshake mechanics and how tools like hping3 are used for stealthy port scanning—a core skill in the reconnaissance phase. A common trap is confusing a SYN-ACK response with a filtered port, but remember: only an open port completes the second step of the handshake. Memory tip: SYN sent, SYN-ACK received? The port is open and believed.
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.
During a security assessment, a tester runs hping3 with the command: hping3 -S -p 80 -c 5 10.0.0.1. The response shows that packets with the SYN flag set receive SYN-ACK replies. Which of the following conclusions is MOST accurate?
A SYN-ACK response to a SYN packet indicates an open TCP port.
B
A firewall is blocking the scan
Why wrong: If a firewall were blocking, there would be no response or an ICMP unreachable message. SYN-ACK indicates the packet reached the target.
C
Port 80 is closed on 10.0.0.1
Why wrong: A closed port would respond with an RST packet, not SYN-ACK.
D
The target is using a stateful firewall that allows the connection
Why wrong: Although a stateful firewall might allow the SYN packet, the SYN-ACK response directly indicates the port is open on the host, not just that the firewall allows traffic.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Port 80 is open on 10.0.0.1
The hping3 command with the -S flag sends TCP SYN packets to port 80 on 10.0.0.1. Receiving SYN-ACK replies indicates that the target responded with the second step of the TCP three-way handshake, which only occurs when the port is open and listening. This confirms that port 80 is open on the target host.
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.
✓
Port 80 is open on 10.0.0.1
Why this is correct
A SYN-ACK response to a SYN packet indicates an open TCP port.
Related concept
Read the scenario before looking for a memorised answer.
✗
A firewall is blocking the scan
Why it's wrong here
If a firewall were blocking, there would be no response or an ICMP unreachable message. SYN-ACK indicates the packet reached the target.
✗
Port 80 is closed on 10.0.0.1
Why it's wrong here
A closed port would respond with an RST packet, not SYN-ACK.
✗
The target is using a stateful firewall that allows the connection
Why it's wrong here
Although a stateful firewall might allow the SYN packet, the SYN-ACK response directly indicates the port is open on the host, not just that the firewall allows traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse the SYN-ACK response with firewall behavior or closed port responses, but the correct interpretation is that a SYN-ACK directly indicates an open port per TCP specification.
Detailed technical explanation
How to think about this question
The TCP three-way handshake requires a SYN, SYN-ACK, and ACK sequence. Receiving a SYN-ACK from a SYN probe confirms the target's TCP stack is willing to establish a connection on that port. In real-world scenarios, some stateless firewalls may allow SYN packets through but block subsequent ACK packets, but the SYN-ACK response itself is definitive proof of an open port.
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: Port 80 is open on 10.0.0.1 — The hping3 command with the -S flag sends TCP SYN packets to port 80 on 10.0.0.1. Receiving SYN-ACK replies indicates that the target responded with the second step of the TCP three-way handshake, which only occurs when the port is open and listening. This confirms that port 80 is open on the target host.
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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