- A
The router is dropping TCP packets due to ACLs.
Why wrong: Ping works, so basic routing is fine; ACLs would likely block ICMP too.
- B
The legacy server's firewall is not allowing SSH; the rule might be misconfigured.
Why wrong: The firewall rule is checked and seems correct.
- C
The app server's firewall (ufw) is blocking incoming SSH responses.
Since SSH is a TCP connection, the app server sends SYN, and the legacy server replies with SYN-ACK. If the app server's ufw does not allow related/established connections or has a rule that blocks new incoming connections, the SYN-ACK will be dropped, causing a timeout. This is a common misconfiguration.
- D
The legacy server's SSH service is not listening on the correct interface.
Why wrong: The stem says it listens on 0.0.0.0:22.
LFCS Networking Practice Question
This LFCS practice question tests your understanding of networking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
You are a senior Linux administrator for a large data center. A junior admin reports that a newly deployed application server (192.168.100.50/24, default gateway 192.168.100.1) cannot communicate with a legacy server (192.168.200.50/24, default gateway 192.168.200.1). The two subnets are connected via a router (192.168.100.1 and 192.168.200.1). From the app server, you can ping the legacy server's IP successfully. However, when you try to establish an SSH session from the app server to the legacy server, it times out. You check the legacy server's firewall (ufw) and find that it allows SSH (port 22) from the entire 192.168.0.0/16 range. You also confirm that the SSH daemon is running and listening on 0.0.0.0:22. What is the most likely cause?
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
The app server's firewall (ufw) is blocking incoming SSH responses.
The app server can ping the legacy server successfully, which confirms that ICMP traffic (Layer 3) passes through the router and that the legacy server's firewall allows ICMP. However, SSH (TCP port 22) fails because the app server's own firewall (ufw) is blocking the incoming SSH response packets (SYN-ACK) from the legacy server. Since the SSH client initiates the connection from the app server, the response packets must be allowed by the app server's firewall; if ufw on the app server blocks established or related incoming traffic, the TCP handshake cannot complete, resulting in a timeout.
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 router is dropping TCP packets due to ACLs.
Why it's wrong here
Ping works, so basic routing is fine; ACLs would likely block ICMP too.
- ✗
The legacy server's firewall is not allowing SSH; the rule might be misconfigured.
Why it's wrong here
The firewall rule is checked and seems correct.
- ✓
The app server's firewall (ufw) is blocking incoming SSH responses.
Why this is correct
Since SSH is a TCP connection, the app server sends SYN, and the legacy server replies with SYN-ACK. If the app server's ufw does not allow related/established connections or has a rule that blocks new incoming connections, the SYN-ACK will be dropped, causing a timeout. This is a common misconfiguration.
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 legacy server's SSH service is not listening on the correct interface.
Why it's wrong here
The stem says it listens on 0.0.0.0:22.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the problem must be on the target server (firewall or SSH service) because the symptom is a timeout, but the ping success proves Layer 3 connectivity, shifting the issue to the client-side firewall blocking the TCP handshake response.
Detailed technical explanation
How to think about this question
When a client initiates an outbound TCP connection, the client's firewall must allow the outgoing SYN packet and also allow the incoming SYN-ACK (part of the connection tracking state). In ufw, the default policy may be to deny incoming traffic unless explicitly allowed; if the app server's ufw does not have a rule to allow SSH responses (e.g., 'ufw allow out 22' or proper stateful tracking), the SYN-ACK from the legacy server is dropped, causing a TCP handshake timeout. This is a common pitfall where administrators focus on the target server's firewall but forget that the client's firewall also filters inbound traffic, even for responses to outbound 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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this LFCS question test?
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The app server's firewall (ufw) is blocking incoming SSH responses. — The app server can ping the legacy server successfully, which confirms that ICMP traffic (Layer 3) passes through the router and that the legacy server's firewall allows ICMP. However, SSH (TCP port 22) fails because the app server's own firewall (ufw) is blocking the incoming SSH response packets (SYN-ACK) from the legacy server. Since the SSH client initiates the connection from the app server, the response packets must be allowed by the app server's firewall; if ufw on the app server blocks established or related incoming traffic, the TCP handshake cannot complete, resulting in a timeout.
What should I do if I get this LFCS 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 →
Last reviewed: Jun 25, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation 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 LFCS exam.
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