LFCS Networking Practice Question
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?
⚠ Common exam trap
Watch out — candidates often 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.
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 iptables rules do not include a rule to allow established and related connections
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.
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 server's DNS resolver is not configured
Why it's wrong here
Ping uses IP address, not DNS.
- ✗
The SSH rule is misconfigured and accidentally blocks all traffic
Why it's wrong here
SSH works, so the SSH rule is fine.
- ✓
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.
- ✗
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.
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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.