This LFCS practice question tests your understanding of networking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
After configuring iptables rules on a Linux server, a junior administrator notices that incoming SSH connections from a specific IP address (192.168.1.100) are being blocked even though there is a rule to allow all traffic from that IP. The current rule set is: 1. -A INPUT -s 192.168.1.100 -j ACCEPT; 2. -A INPUT -p tcp --dport 22 -j DROP. What is the most likely reason for the block?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The DROP rule for port 22 appears before the ACCEPT rule for the IP.
Option C is correct because iptables processes rules in sequential order, and the first matching rule determines the packet's fate. In this rule set, the DROP rule for TCP port 22 is listed second, but since it is the first rule that matches SSH traffic from 192.168.1.100 (the ACCEPT rule matches the source IP but not the destination port, so it does not match SSH packets specifically), the DROP rule is applied before the ACCEPT rule can take effect. The ACCEPT rule for the IP address would only match non-SSH traffic from that IP, while SSH packets are dropped by the subsequent port-based rule.
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.
✗
The IP address is being matched by a conntrack rule instead.
Why it's wrong here
conntrack rules typically appear before new connection rules.
✗
The ACCEPT rule for the IP uses the wrong chain.
Why it's wrong here
Both rules are in the INPUT chain, which is appropriate for incoming connections.
✓
The DROP rule for port 22 appears before the ACCEPT rule for the IP.
Why this is correct
iptables processes rules sequentially; the first match stops processing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The SSH service is listening only on IPv6.
Why it's wrong here
IPv6 is separate from iptables rules unless using ip6tables.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume an ACCEPT rule for a source IP will override any subsequent DROP rules, but iptables processes rules in order and the first match wins, so the order of rules is critical.
Detailed technical explanation
How to think about this question
iptables uses a linear traversal of rules within a chain; the first rule that matches a packet's criteria (including protocol, port, source, etc.) applies its target (ACCEPT, DROP, etc.) and stops further processing. This means that a broad rule (e.g., matching only source IP) placed after a specific rule (e.g., matching source IP and destination port) will never be reached for packets that match the specific rule. In real-world scenarios, administrators often place more specific rules (like allow from a specific IP to a specific port) before general deny rules to avoid unintended blocks.
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: The DROP rule for port 22 appears before the ACCEPT rule for the IP. — Option C is correct because iptables processes rules in sequential order, and the first matching rule determines the packet's fate. In this rule set, the DROP rule for TCP port 22 is listed second, but since it is the first rule that matches SSH traffic from 192.168.1.100 (the ACCEPT rule matches the source IP but not the destination port, so it does not match SSH packets specifically), the DROP rule is applied before the ACCEPT rule can take effect. The ACCEPT rule for the IP address would only match non-SSH traffic from that IP, while SSH packets are dropped by the subsequent port-based rule.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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