The correct answer combines setting PasswordAuthentication no in sshd_config with two complementary access controls: an iptables rule restricting SSH traffic to the 192.168.1.0/24 subnet and an AllowUsers directive matching that IP range. This layered approach is essential because disabling password authentication alone doesn’t block brute-force attempts from outside the subnet, while network-level filtering via iptables ensures only traffic from the allowed range reaches the SSH daemon. On the CompTIA Linux+ XK0-005 exam, this question tests your understanding of defense in depth—combining host-based firewall rules with SSH configuration directives to enforce both network and application-level restrictions. A common trap is thinking iptables alone suffices, but the exam expects you to recognize that AllowUsers provides an additional layer even if the firewall is misconfigured. Remember the mnemonic “FAD”: Firewall (iptables), AllowUsers, and Disable passwords—three steps that lock down SSH from the network to the service itself.
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.
An administrator needs to restrict SSH access to the server so that only a specific IP range (192.168.1.0/24) can connect, and password authentication is disabled. Which THREE steps must be taken?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a firewall rule: iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT; iptables -A INPUT -p tcp --dport 22 -j DROP
Option B is correct because it uses iptables to create a firewall rule that accepts SSH traffic (TCP port 22) only from the 192.168.1.0/24 subnet, followed by a catch-all rule that drops all other SSH traffic. This enforces network-level access control. Option D is correct because 'AllowUsers *@192.168.1.*' in sshd_config restricts SSH logins to users connecting from IP addresses matching that pattern, providing application-level filtering. Option E is correct because 'PasswordAuthentication no' disables password-based authentication, forcing the use of key-based or other non-password methods.
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.
✗
Set 'UsePAM yes' in sshd_config
Why it's wrong here
This enables PAM, not IP restriction.
✓
Add a firewall rule: iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT; iptables -A INPUT -p tcp --dport 22 -j DROP
Read the scenario before looking for a memorised answer.
✗
Set 'PermitRootLogin without-password' in sshd_config
Why it's wrong here
This only affects root login, not source IP restriction.
✓
Add 'AllowUsers *@192.168.1.*' to /etc/ssh/sshd_config
Why this is correct
This restricts SSH login to users originating from that subnet.
Related concept
Read the scenario before looking for a memorised answer.
✓
Set 'PasswordAuthentication no' in sshd_config
Why this is correct
Disables password-based authentication.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may think 'UsePAM yes' or 'PermitRootLogin without-password' are necessary for IP restriction or disabling passwords, when in fact they address unrelated authentication mechanisms and are not among the three required steps.
Detailed technical explanation
How to think about this question
The iptables rules in option B use a stateful firewall approach: the first rule accepts SSH from the allowed subnet, and the second rule drops all other SSH traffic, effectively creating a whitelist. The 'AllowUsers' directive in sshd_config uses pattern matching (globbing) to filter by client IP at the application layer, which can complement firewall rules. Disabling password authentication (PasswordAuthentication no) forces clients to use public key authentication, which is more secure and is a common requirement for compliance with standards like PCI-DSS.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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: Add a firewall rule: iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT; iptables -A INPUT -p tcp --dport 22 -j DROP — Option B is correct because it uses iptables to create a firewall rule that accepts SSH traffic (TCP port 22) only from the 192.168.1.0/24 subnet, followed by a catch-all rule that drops all other SSH traffic. This enforces network-level access control. Option D is correct because 'AllowUsers *@192.168.1.*' in sshd_config restricts SSH logins to users connecting from IP addresses matching that pattern, providing application-level filtering. Option E is correct because 'PasswordAuthentication no' disables password-based authentication, forcing the use of key-based or other non-password methods.
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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