- A
iptables -P INPUT ACCEPT; iptables -A INPUT -p tcp --dport 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Why wrong: Default ACCEPT plus ACCEPT rules results in all traffic being allowed.
- B
iptables -P INPUT DROP; iptables -A INPUT -p tcp --dport 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Default DROP drops all incoming packets, then specific ACCEPT rules allow SSH and HTTPS.
- C
iptables -P FORWARD DROP; iptables -A INPUT -p tcp --dport 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Why wrong: Sets FORWARD chain policy, not INPUT; INPUT chain default remains ACCEPT, allowing all incoming traffic.
- D
iptables -P INPUT ACCEPT; iptables -A INPUT -p tcp --dport 22 -j DROP; iptables -A INPUT -p tcp --dport 443 -j DROP
Why wrong: Default ACCEPT would allow all traffic, then DROP rules only drop SSH and HTTPS, so all other traffic is allowed.
Quick Answer
The correct answer is the set of commands that first sets a default DROP policy on the INPUT chain and then adds ACCEPT rules for ports 22 and 443. This works because iptables processes rules in order, and the default policy acts as a catch-all for any traffic that does not match an explicit rule; by setting the default to DROP first, you ensure all incoming packets are denied unless they match the subsequent ACCEPT rules for SSH and HTTPS. On the CompTIA Linux+ XK0-005 exam, this scenario tests your understanding of iptables chain policy versus rule-based filtering—a common trap is to forget the default DROP or to mistakenly use the OUTPUT chain instead of INPUT. A reliable memory tip is “Default DROP, then open the locks”: always set your default policy to DROP before adding your allow rules, so no unwanted traffic slips through the gap.
XK0-005 Security Practice Question
This XK0-005 practice question tests your understanding of security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An administrator is configuring a Linux firewall to allow incoming SSH (port 22) and HTTPS (port 443) traffic while denying all other incoming traffic. Using iptables, which set of commands achieves this?
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 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Option B is correct because it first sets the default policy on the INPUT chain to DROP, which denies all incoming traffic by default. It then adds rules to explicitly ACCEPT incoming TCP traffic on ports 22 (SSH) and 443 (HTTPS), achieving the requirement of allowing only those two services while dropping everything else.
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.
- ✗
iptables -P INPUT ACCEPT; iptables -A INPUT -p tcp --dport 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Why it's wrong here
Default ACCEPT plus ACCEPT rules results in all traffic being allowed.
- ✓
iptables -P INPUT DROP; iptables -A INPUT -p tcp --dport 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
- ✗
iptables -P FORWARD DROP; iptables -A INPUT -p tcp --dport 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Why it's wrong here
Sets FORWARD chain policy, not INPUT; INPUT chain default remains ACCEPT, allowing all incoming traffic.
- ✗
iptables -P INPUT ACCEPT; iptables -A INPUT -p tcp --dport 22 -j DROP; iptables -A INPUT -p tcp --dport 443 -j DROP
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between the INPUT and FORWARD chains, and the trap here is that candidates mistakenly set the default policy on FORWARD instead of INPUT, thinking it controls incoming traffic to the local host.
Detailed technical explanation
How to think about this question
In iptables, the INPUT chain processes packets destined for the local system, while the FORWARD chain handles packets routed through the system. Setting the default policy to DROP on INPUT ensures that only traffic explicitly matched by subsequent rules is accepted; this is a common 'default deny' security posture. The order of rules matters: iptables processes rules sequentially, and once a packet matches a rule, the target (ACCEPT/DROP) is applied and no further rules in that chain are evaluated.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
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FAQ
Questions learners often ask
What does this XK0-005 question test?
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 22 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT — Option B is correct because it first sets the default policy on the INPUT chain to DROP, which denies all incoming traffic by default. It then adds rules to explicitly ACCEPT incoming TCP traffic on ports 22 (SSH) and 443 (HTTPS), achieving the requirement of allowing only those two services while dropping everything else.
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.
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 →
Same concept, more angles
1 more ways this is tested on XK0-005
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. 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?
hard- A.iptables -P INPUT DROP; iptables -A INPUT -p tcp -j ACCEPT
- B.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
- ✓ C.iptables -P INPUT DROP; iptables -A INPUT -p tcp --dport 80 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
- D.iptables -P INPUT ACCEPT; iptables -A INPUT -p tcp --dport 80 -j ACCEPT; iptables -A INPUT -p tcp --dport 443 -j ACCEPT
Why C: 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.
Last reviewed: Jun 30, 2026
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