The answer is a default gateway failure, as the server’s inability to reach 203.0.113.1 prevents return traffic from reaching internet clients. Even though the web server listens on 0.0.0.0:443 and the firewall has no restrictive rules, outbound response packets must be routed through the default gateway to complete the TCP handshake; without a working gateway, those packets are dropped, causing intermittent unreachability. On the LFCS exam, this scenario tests your understanding of the routing table’s role in bidirectional communication—a common trap is focusing only on listening services or firewall rules while overlooking the gateway’s health. Remember, a server can receive incoming packets but still fail to reply if the default route is broken. Memory tip: “Listen doesn’t mean reply—check the gateway before you cry.”
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 systems administrator for a company that runs a critical web application on a Linux server. The server has two network interfaces: eth0 (public IP 203.0.113.10/24, gateway 203.0.113.1) and eth1 (private IP 10.0.0.10/24). The web server listens on port 443 (HTTPS) and must be accessible from the internet. The server also needs to connect to an internal database server at 10.0.0.50/24 on port 3306. Recently, users reported that the website is intermittently unreachable. You SSH into the server and run 'ss -tln' and see that the web server is listening on 0.0.0.0:443. You check the routing table with 'ip route show' and see: default via 203.0.113.1 dev eth0; 10.0.0.0/24 dev eth1 proto kernel scope link src 10.0.0.10. You also run 'iptables -L -n -v' and see: Chain INPUT (policy ACCEPT), with no rules. However, you notice that the server's default gateway is unreachable from the server itself when you run 'ping 203.0.113.1' (100% packet loss). What is the most likely cause of the intermittent unreachability?
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 default gateway is unreachable, preventing return traffic from the internet.
The correct answer is C. The server's default gateway (203.0.113.1) is unreachable, as confirmed by the 100% packet loss on ping. This means that although the web server is listening on 0.0.0.0:443 and the firewall allows traffic, any response packets from the server to internet clients must be routed through the default gateway. Without a working gateway, return traffic cannot reach the clients, causing intermittent unreachability from the internet.
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 web server is not listening on the public IP address.
Why it's wrong here
Listening on 0.0.0.0 means it listens on all interfaces, including the public one.
✗
A firewall rule is blocking incoming HTTPS traffic.
Why it's wrong here
The iptables output shows no rules, so no traffic is being blocked.
✓
The default gateway is unreachable, preventing return traffic from the internet.
Why this is correct
The gateway is down, so the server cannot send packets to the internet, making the web server unreachable.
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 private interface eth1 has a misconfigured IP address.
Why it's wrong here
The routing table shows a valid route on eth1, and the database connectivity is not mentioned as an issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on firewall rules or service binding, overlooking the fact that even with correct listening and permissive firewall, network-layer routing (specifically a working default gateway) is essential for bidirectional communication with the internet.
Trap categories for this question
Command / output trap
The iptables output shows no rules, so no traffic is being blocked.
Detailed technical explanation
How to think about this question
The default gateway is critical for routing packets to destinations outside the directly connected subnets. When a server sends a response to an internet client, the kernel performs a routing lookup and forwards the packet to the default gateway (203.0.113.1) via eth0. If the gateway is unreachable (e.g., due to a misconfigured router, ARP failure, or link issue), the packets are dropped, causing asymmetric routing failures. This is often diagnosed with 'ip route get' to verify the next hop or 'arping' to check Layer 2 reachability.
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 LFCS 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.
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 default gateway is unreachable, preventing return traffic from the internet. — The correct answer is C. The server's default gateway (203.0.113.1) is unreachable, as confirmed by the 100% packet loss on ping. This means that although the web server is listening on 0.0.0.0:443 and the firewall allows traffic, any response packets from the server to internet clients must be routed through the default gateway. Without a working gateway, return traffic cannot reach the clients, causing intermittent unreachability from the internet.
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 →
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. Based on the exhibit, a system administrator runs ping to 8.8.8.8 and gets 'Destination Host Unreachable'. What is the most likely cause?
medium
A.The IP address 192.168.10.15 is not configured on eth0.
B.A firewall is blocking ICMP traffic.
✓ C.The gateway 192.168.10.1 is not reachable from this host.
D.The default route is missing.
Why C: The 'Destination Host Unreachable' message indicates that the host has no route to the destination network or the next-hop gateway is not reachable. Since the ping target is 8.8.8.8 (a public IP), the host must forward traffic to its default gateway. If the gateway 192.168.10.1 is not reachable (e.g., due to a down interface, ARP failure, or missing neighbor entry), the kernel will immediately return this ICMP error without attempting to send the packet.
Last reviewed: Jun 11, 2026
Question Discussion
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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.
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