The correct answer is to add an ACCEPT rule for SSH traffic from the 192.168.1.0/24 subnet first, followed by a DROP rule for all other SSH traffic. This works because iptables processes rules sequentially, and the first match wins; by placing the subnet-specific ACCEPT rule before the general DROP rule, you allow SSH from the trusted subnet while blocking everything else, even though the default INPUT policy is ACCEPT. On the CompTIA Linux+ XK0-005 exam, this scenario tests your understanding of rule ordering and the importance of explicit deny rules when the default policy is permissive—a common trap is forgetting the DROP rule, which would leave SSH open to all. To remember the sequence, think “allow first, then deny”: the ACCEPT for the subnet must come before the DROP for port 22, or the DROP will block everyone, including your allowed subnet.
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.
A Linux administrator needs to configure a firewall to allow incoming SSH connections only from the 192.168.1.0/24 subnet. The current iptables INPUT policy is ACCEPT. Which set of rules should be added?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
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 iptables processes rules in order, and the first matching rule determines the action. By placing the ACCEPT rule for the 192.168.1.0/24 subnet first, SSH traffic from that subnet is accepted. The subsequent DROP rule for port 22 then denies all other SSH traffic. This ensures that only the specified subnet can connect, while the default ACCEPT policy on the INPUT chain would otherwise allow all traffic if no rule matched.
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 -A INPUT -p tcp --dport 22 -j DROP; iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT
Why it's wrong here
Drop rule first blocks all SSH before accept 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
Why this is correct
Allows allowed subnet then drops others, correct order.
Related concept
Read the scenario before looking for a memorised answer.
✗
iptables -A INPUT -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT; iptables -A INPUT -p tcp --dport 22 -s 10.0.0.0/8 -j DROP
Why it's wrong here
Only drops from 10.0.0.0/8, other subnets still allowed.
Inserted drop rule still appears before accept, blocking all.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume the order of rules doesn't matter or that a DROP rule can be placed before an ACCEPT rule for the same port, not realizing that iptables stops processing on the first match, which would drop all traffic including from the allowed subnet.
Detailed technical explanation
How to think about this question
iptables uses a linear rule traversal; the first matching rule in a chain is applied, and subsequent rules are ignored for that packet. This makes rule order critical—ACCEPT rules for allowed sources must precede DROP rules for the same service. In real-world scenarios, administrators often combine this with stateful rules (e.g., -m state --state ESTABLISHED,RELATED) to allow return traffic, but for new inbound SSH connections, the order of ACCEPT then DROP is essential to avoid locking out legitimate access.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 -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 iptables processes rules in order, and the first matching rule determines the action. By placing the ACCEPT rule for the 192.168.1.0/24 subnet first, SSH traffic from that subnet is accepted. The subsequent DROP rule for port 22 then denies all other SSH traffic. This ensures that only the specified subnet can connect, while the default ACCEPT policy on the INPUT chain would otherwise allow all traffic if no rule matched.
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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