This LFCS practice question tests your understanding of networking. 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 network administrator needs to block all incoming SSH traffic (port 22) from the 192.168.2.0/24 subnet. Which iptables command accomplishes this?
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 -s 192.168.2.0/24 -p tcp --dport 22 -j DROP
Option D is correct because it appends a rule to the INPUT chain that matches packets originating from the 192.168.2.0/24 subnet (-s 192.168.2.0/24) using TCP protocol with destination port 22 (--dport 22), and then drops them (-j DROP). This precisely blocks all incoming SSH traffic from that subnet while leaving other traffic unaffected.
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 -d 192.168.2.0/24 -p tcp --dport 22 -j DROP
Why it's wrong here
-d specifies destination, not source; this would drop traffic destined to that subnet.
✗
iptables -A OUTPUT -d 192.168.2.0/24 -p tcp --sport 22 -j DROP
Why it's wrong here
This affects outgoing SSH responses, not incoming SSH connections.
✗
iptables -A INPUT -s 192.168.2.0/24 -j DROP
Why it's wrong here
This drops all traffic from that subnet, not just SSH.
✓
iptables -A INPUT -s 192.168.2.0/24 -p tcp --dport 22 -j DROP
Why this is correct
This drops incoming TCP packets from the subnet to port 22.
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 often confuse the -s and -d flags, or mistakenly apply the rule to the OUTPUT chain, thinking they need to block outgoing responses rather than incoming connection attempts.
Detailed technical explanation
How to think about this question
The iptables INPUT chain processes packets destined for the local system, and the -s flag specifies the source IP range using CIDR notation. The -p tcp --dport 22 combination ensures only TCP segments with a destination port of 22 are matched, which corresponds to SSH traffic as defined by IANA port assignments. In a real-world scenario, using -j DROP instead of -j REJECT can cause connection timeouts for the client, which may be desirable to hide the existence of the SSH service.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: iptables -A INPUT -s 192.168.2.0/24 -p tcp --dport 22 -j DROP — Option D is correct because it appends a rule to the INPUT chain that matches packets originating from the 192.168.2.0/24 subnet (-s 192.168.2.0/24) using TCP protocol with destination port 22 (--dport 22), and then drops them (-j DROP). This precisely blocks all incoming SSH traffic from that subnet while leaving other traffic unaffected.
What should I do if I get this LFCS 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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This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.
Question Discussion
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