hardMultiple ChoiceObjective-mapped
XK0-006 Practice Question: A Linux administrator is configuring a firewall…
A Linux administrator is configuring a firewall using iptables to allow incoming HTTP and HTTPS traffic but block all other incoming traffic. Which set of rules should be applied?
⚠ Common exam trap
A common mistake is choosing option B, which sets a default ACCEPT policy and then adds a final DROP rule. While this technically blocks all unmatched traffic because the final DROP matches before the default policy, the default ACCEPT policy is considered insecure. The recommended approach is to set a default DROP (deny-all) and explicitly allow only required services, as in option C.
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
✓
iptables -P INPUT DROP; iptables -A INPUT -p tcp --dport 80 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
It sets the default policy for the INPUT chain to DROP, which blocks all incoming traffic by default, and then explicitly adds rules to ACCEPT TCP traffic on ports 80 (HTTP) and 443 (HTTPS). This implements a whitelist approach: only the specified services are allowed, and all other incoming packets are dropped by the default policy. The order is critical — the ACCEPT rules must be evaluated before the default DROP policy takes effect for unmatched traffic.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
iptables -P INPUT DROP; iptables -A INPUT -p tcp -j ACCEPT
Why it's wrong here
Allows all TCP traffic.
- ✗
iptables -P INPUT ACCEPT; iptables -A INPUT -p tcp --dport 80 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT; iptables -A INPUT -j DROP
Why it's wrong here
Default ACCEPT then DROP rule is redundant and order matters.
- ✓
iptables -P INPUT DROP; iptables -A INPUT -p tcp --dport 80 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Why this is correct
Default DROP blocks all; allow only HTTP/HTTPS.
- ✗
iptables -P INPUT ACCEPT; iptables -A INPUT -p tcp --dport 80 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Why it's wrong here
Default policy ACCEPT allows all traffic.
Go deeper
Related to this question
Learn chapter
Linux Fundamentals and History
Key term
Linux
Linux is an open-source operating system that manages computer hardware and software, widely used in servers, desktops, and embedded systems.
Key term
Policy
A policy is a set of rules or guidelines that defines how an organization manages, secures, and operates its IT systems and services.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This XK0-006 practice question is part of Courseiva's free CompTIA 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 XK0-006 exam.