- A
The IP address is being matched by a conntrack rule instead.
Why wrong: A conntrack rule is not described; the issue is about rule order.
- B
The ACCEPT rule for the IP uses the wrong chain.
Why wrong: The ACCEPT rule is in the INPUT chain, which is correct for incoming traffic.
- C
The DROP rule for port 22 appears before the ACCEPT rule for the IP.
Correct. The block occurs because the DROP rule is evaluated before the ACCEPT rule.
- D
The SSH service is listening only on IPv6.
Why wrong: There is no evidence of IPv6-only listening; the issue is with iptables rule order.
Question 258 of 513
LFCS iptables rule order Practice Question
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. A key principle to apply: iptables rule order. 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 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
✓
The DROP rule for port 22 appears before the ACCEPT rule for the IP.
The rule set as listed would allow SSH from 192.168.1.100 because the ACCEPT rule appears first and matches all traffic from that IP, including SSH. The fact that connections are being blocked indicates that the actual rule order in the iptables chain is different: the DROP rule for port 22 must appear before the ACCEPT rule. Since iptables uses first-match-wins, SSH packets from that IP match the DROP rule first and are dropped. Therefore, the most likely reason is that the DROP rule appears before the ACCEPT rule in the live chain.
Key principle: iptables rule order
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
A conntrack rule is not described; the issue is about rule order.
- ✗
The ACCEPT rule for the IP uses the wrong chain.
Why it's wrong here
The ACCEPT rule is in the INPUT chain, which is correct for incoming traffic.
- ✓
The DROP rule for port 22 appears before the ACCEPT rule for the IP.
Why this is correct
Correct. The block occurs because the DROP rule is evaluated before the ACCEPT rule.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
iptables rule order
- ✗
The SSH service is listening only on IPv6.
Why it's wrong here
There is no evidence of IPv6-only listening; the issue is with iptables rule order.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that the numbered list in the STEM might not reflect the actual order of rules in the iptables chain. Candidates assume the ACCEPT rule is first, but in practice, the DROP rule could have been inserted earlier, causing the block.
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
- iptables rule order
- First match wins
- Rule insertion (iptables -I vs -A)
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
iptables rule order
Real-world example
How this comes up in practice
A practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. iptables rule order Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review iptables rule order, then practise related LFCS questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this LFCS question test?
Networking — This question tests Networking — iptables rule order.
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. — The rule set as listed would allow SSH from 192.168.1.100 because the ACCEPT rule appears first and matches all traffic from that IP, including SSH. The fact that connections are being blocked indicates that the actual rule order in the iptables chain is different: the DROP rule for port 22 must appear before the ACCEPT rule. Since iptables uses first-match-wins, SSH packets from that IP match the DROP rule first and are dropped. Therefore, the most likely reason is that the DROP rule appears before the ACCEPT rule in the live chain.
What should I do if I get this LFCS question wrong?
Review iptables rule order, then practise related LFCS questions on the same topic to reinforce the concept.
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?
iptables rule order
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Last reviewed: Jun 25, 2026
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