Why a Working Web Server Becomes Unreachable: Gateway Failure
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?
Quick Answer
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.”
⚠ Common exam trap
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.
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 default gateway is unreachable, preventing return traffic from the internet.
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.
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.
- ✗
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.
Go deeper
Related to this question
About these practice questions
This LFCS question is part of Courseiva's 507-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way this is tested on LFCS
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.