This XK0-005 practice question tests your understanding of security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
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?
Option C is correct because 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.
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.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the default policy with explicit rules, thinking that setting a default ACCEPT and then adding a DROP rule at the end will block all other traffic, but the default policy is evaluated only after all rules are checked, so a default ACCEPT will allow unmatched traffic regardless of a final DROP rule.
Detailed technical explanation
How to think about this question
Under the hood, iptables processes rules in order within a chain; the first matching rule determines the action (ACCEPT, DROP, etc.). The default policy (-P) applies only to packets that do not match any rule in the chain. In a whitelist configuration, setting the default to DROP ensures that any unlisted traffic is implicitly denied, which is a security best practice (principle of least privilege). A subtle behavior: if the ACCEPT rules for ports 80 and 443 are placed after a rule that matches all TCP traffic (as in Option A), the earlier rule would accept all TCP, making the port-specific rules irrelevant. In real-world scenarios, administrators often also add stateful rules (e.g., -m state --state ESTABLISHED,RELATED -j ACCEPT) before the port-specific rules to allow return traffic for outbound connections.
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 XK0-005 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this XK0-005 question in full detail.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: iptables -P INPUT DROP; iptables -A INPUT -p tcp --dport 80 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT — Option C is correct because 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.
What should I do if I get this XK0-005 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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