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 systems administrator for a company that runs a critical application on a Linux server with two network interfaces: eth0 (public IP 203.0.113.10/24, gateway 203.0.113.1) and eth1 (private IP 10.0.1.10/24, no gateway). The server must be accessible via SSH (port 22) from the internet, but only from a specific management subnet 198.51.100.0/24. Additionally, the server should be able to access the internet for package updates, but no other inbound traffic from the internet is allowed. The local firewall is iptables. After implementing rules, you find that the server cannot reach the internet (e.g., ping 8.8.8.8 fails), but SSH from the management subnet works. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The iptables rules do not include a rule to allow established and related connections
Option C is correct because iptables is stateful: by default, the INPUT chain processes only the first packet of a connection. Without a rule allowing established and related connections (e.g., `-m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT`), return traffic from the server's outbound internet requests (e.g., ping to 8.8.8.8) is blocked by the INPUT chain, causing the failure. SSH from the management subnet works because the initial SYN packet is allowed, but the server's outbound traffic fails because the response packets are not recognized as part of an allowed flow.
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 iptables rules do not include a rule to allow established and related connections
Why this is correct
Without a state tracking rule, return traffic for outbound connections is blocked, breaking internet access.
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 default policy on the INPUT chain is DROP
Why it's wrong here
If the policy were DROP without any rules, SSH would not work either.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a default DROP policy on INPUT is the root cause, but they overlook that stateful filtering requires an explicit rule for return traffic, which is a classic LFCS and iptables nuance.
Detailed technical explanation
How to think about this question
iptables uses connection tracking via the `conntrack` module to maintain state for TCP, UDP, and ICMP flows. When a server sends an outbound packet, the conntrack system creates an entry; the return packet must be matched by a rule allowing `ESTABLISHED` or `RELATED` state, or it will be dropped by the INPUT chain. A common real-world mistake is to allow only `NEW` inbound connections (e.g., SSH) without also allowing `ESTABLISHED,RELATED` traffic, which breaks all outbound-initiated communication like HTTP or ICMP echo replies.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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 iptables rules do not include a rule to allow established and related connections — Option C is correct because iptables is stateful: by default, the INPUT chain processes only the first packet of a connection. Without a rule allowing established and related connections (e.g., `-m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT`), return traffic from the server's outbound internet requests (e.g., ping to 8.8.8.8) is blocked by the INPUT chain, causing the failure. SSH from the management subnet works because the initial SYN packet is allowed, but the server's outbound traffic fails because the response packets are not recognized as part of an allowed flow.
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.
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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.
Question Discussion
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