- A
iptables -P INPUT DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
This correctly implements the policy.
- B
iptables -P INPUT DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Why wrong: Allows all traffic on eth1 from anywhere, not just internal, violating policy.
- C
iptables -P INPUT DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Why wrong: Allows SSH on eth1 but not on eth0; however, it also allows all traffic on eth0 except HTTPS? Actually it allows only HTTPS on eth0, but SSH is not allowed on eth0, so internal users cannot SSH from internal network via eth0, but they can via eth1. However, the policy requires SSH only from internal network, but it doesn't specify interface; typically SSH should be allowed on any internal interface. This option blocks SSH on eth0, but internal users might need to SSH via eth0 if they are on internal network? The policy says 'SSH access should be allowed only from the internal network (192.168.1.0/24)'. It doesn't specify interface, so SSH should be allowed from that subnet on any interface. Option A allows SSH on eth1 only, but internal users could also come through eth0 if they have internal IP? Actually if internal network is 192.168.1.0/24, it's likely on eth1, so it's fine. However, option C blocks SSH on eth0, which might be unnecessary but not violating policy if internal users only use eth1. But option C also does not allow SSH on eth0, which could be a problem if internal users try to SSH to the public IP from internal network. Typically, you would allow SSH on both interfaces from internal subnet. Option A is more complete. Option C is not the best because it does not allow SSH on eth0 from internal, which might be needed. So A is correct.
- D
iptables -P INPUT DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Why wrong: Allows SSH from anywhere, violating policy.
XK0-005 Security Practice Question
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.
You are a systems administrator for a small company. The company uses a Linux server running Ubuntu 22.04 LTS that hosts a web application and a PostgreSQL database. The server has two network interfaces: eth0 (public IP) and eth1 (private IP). The web application listens on port 443 (HTTPS) on eth0, and the PostgreSQL database listens on port 5432 on eth1. The company security policy requires that only the web application should be accessible from the internet; all other ports must be blocked on the public interface. Additionally, SSH access should be allowed only from the internal network (192.168.1.0/24). The current iptables rules are as follows:
-P INPUT ACCEPT -P FORWARD ACCEPT -P OUTPUT ACCEPT
There are no other rules. You need to implement the security policy using iptables. Which of the following sets of commands will achieve the required security policy?
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 -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Option A is correct because it sets the default policy to DROP, allows established/related connections, permits HTTPS (port 443) on eth0, restricts SSH (port 22) to the internal network (192.168.1.0/24) on eth1, and then allows all traffic on eth1. This ensures that only the web application is accessible from the internet, SSH is limited to the internal network, and all internal traffic on eth1 is permitted, including PostgreSQL on port 5432.
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 DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Why this is correct
This correctly implements the policy.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
iptables -P INPUT DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Why it's wrong here
Allows all traffic on eth1 from anywhere, not just internal, violating policy.
- ✗
iptables -P INPUT DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Why it's wrong here
Allows SSH on eth1 but not on eth0; however, it also allows all traffic on eth0 except HTTPS? Actually it allows only HTTPS on eth0, but SSH is not allowed on eth0, so internal users cannot SSH from internal network via eth0, but they can via eth1. However, the policy requires SSH only from internal network, but it doesn't specify interface; typically SSH should be allowed on any internal interface. This option blocks SSH on eth0, but internal users might need to SSH via eth0 if they are on internal network? The policy says 'SSH access should be allowed only from the internal network (192.168.1.0/24)'. It doesn't specify interface, so SSH should be allowed from that subnet on any interface. Option A allows SSH on eth1 only, but internal users could also come through eth0 if they have internal IP? Actually if internal network is 192.168.1.0/24, it's likely on eth1, so it's fine. However, option C blocks SSH on eth0, which might be unnecessary but not violating policy if internal users only use eth1. But option C also does not allow SSH on eth0, which could be a problem if internal users try to SSH to the public IP from internal network. Typically, you would allow SSH on both interfaces from internal subnet. Option A is more complete. Option C is not the best because it does not allow SSH on eth0 from internal, which might be needed. So A is correct.
- ✗
iptables -P INPUT DROP iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -p tcp --dport 22 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT
Why it's wrong here
Allows SSH from anywhere, violating policy.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often forget to allow all traffic on the internal interface (eth1) after setting a default DROP policy, mistakenly thinking that only specific ports need to be opened, which would block essential internal services like PostgreSQL.
Detailed technical explanation
How to think about this question
The iptables rule `-m state --state ESTABLISHED,RELATED -j ACCEPT` is crucial for allowing return traffic for outbound connections and related ICMP errors, without which legitimate responses would be dropped. The order of rules matters: iptables processes rules sequentially, so the catch-all `-i eth1 -j ACCEPT` must come after specific restrictions to avoid bypassing them. In a real-world scenario, failing to allow all traffic on the private interface could break internal communications like database replication or monitoring.
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 -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -i eth1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 443 -j ACCEPT iptables -A INPUT -i eth1 -j ACCEPT — Option A is correct because it sets the default policy to DROP, allows established/related connections, permits HTTPS (port 443) on eth0, restricts SSH (port 22) to the internal network (192.168.1.0/24) on eth1, and then allows all traffic on eth1. This ensures that only the web application is accessible from the internet, SSH is limited to the internal network, and all internal traffic on eth1 is permitted, including PostgreSQL on port 5432.
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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Last reviewed: Jun 11, 2026
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