The answer is the TCP SYN scan (-sS) and the Idle scan (-sI). These two Nmap scan types are most effective for evading a stateful firewall that only allows established connections because they avoid completing the full TCP three-way handshake. A stateful firewall tracks connection state by monitoring the SYN, SYN-ACK, and ACK sequence; a SYN scan sends only the initial SYN packet and never responds to the SYN-ACK, so the firewall never sees a fully established session, often allowing the probe to slip through as an incomplete initiation. The Idle scan takes this further by bouncing the probe off a zombie host, making the scan appear to come from an unsuspecting third party, which can bypass stateful inspection entirely. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of firewall evasion techniques versus simple port scanning—a common trap is assuming a full connect scan (-sT) works, but that completes the handshake and is logged as an established connection. Memory tip: “SYN sends half, Idle hides the trail.”
CEH Footprinting, Reconnaissance and Scanning Practice Question
This CEH practice question tests your understanding of footprinting, reconnaissance and scanning. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Which TWO of the following Nmap scan types are MOST effective for evading a stateful firewall that only allows established connections? (Select 2)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
TCP SYN scan (-sS)
A TCP SYN scan (-sS) sends a SYN packet to initiate a connection without completing the three-way handshake. A stateful firewall that only allows established connections typically permits incoming SYN packets if they are part of an outbound-initiated session, but a standalone SYN packet from an external source is often blocked unless the firewall is configured to allow it. However, if the firewall is tracking connection state, a SYN scan can still be effective if the firewall is configured to allow new connections to specific ports, but in the context of evading a firewall that only allows established connections, the SYN scan is less likely to be blocked than a full connect scan because it does not complete the handshake and thus may not be logged as an established session.
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.
✓
TCP SYN scan (-sS)
Why this is correct
Uses half-open connections; may bypass some stateful filters that only inspect full connections.
Related concept
Read the scenario before looking for a memorised answer.
✗
TCP connect scan (-sT)
Why it's wrong here
Completes the handshake; likely logged and may be blocked by stateful rules.
✓
Idle scan (-sI)
Why this is correct
Spoofs the zombie's IP; the firewall sees the zombie initiating connections, evading detection.
Related concept
Read the scenario before looking for a memorised answer.
✗
Ping sweep (-sn)
Why it's wrong here
Only checks host availability; not a port scan and may still be detected.
✗
UDP scan (-sU)
Why it's wrong here
UDP is connectionless; stateful firewalls may still filter based on other criteria.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a TCP SYN scan is always stealthy, but in modern stateful firewalls, even a SYN packet can be logged and blocked if the firewall is configured to deny all inbound new connections; the key is that the question specifies a firewall that 'only allows established connections,' which means it permits traffic matching an existing session, and the idle scan exploits this by using a zombie that already has an established session with the target.
Detailed technical explanation
How to think about this question
An idle scan (-sI) leverages a zombie host with a predictable IP ID sequence to indirectly probe target ports, allowing the attacker to remain anonymous and bypass stateful firewalls because the scan appears to originate from the zombie, which may have an established connection with the target. The TCP SYN scan (-sS) sends a raw SYN packet and, upon receiving a SYN/ACK, responds with a RST instead of completing the handshake, so no connection state is created in the firewall's table; this is why it is often called a 'half-open' scan and is effective against simple stateful firewalls that only track fully established connections.
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.
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: TCP SYN scan (-sS) — A TCP SYN scan (-sS) sends a SYN packet to initiate a connection without completing the three-way handshake. A stateful firewall that only allows established connections typically permits incoming SYN packets if they are part of an outbound-initiated session, but a standalone SYN packet from an external source is often blocked unless the firewall is configured to allow it. However, if the firewall is tracking connection state, a SYN scan can still be effective if the firewall is configured to allow new connections to specific ports, but in the context of evading a firewall that only allows established connections, the SYN scan is less likely to be blocked than a full connect scan because it does not complete the handshake and thus may not be logged as an established session.
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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