The answer is `iptables -A INPUT -p tcp --dport 8080 -j ACCEPT`. This command is correct because the INPUT chain governs traffic destined for the local system itself, and the `-p tcp --dport 8080` flags precisely match incoming TCP packets whose destination port is 8080, while `-j ACCEPT` permits those packets through the firewall. On the Linux Foundation Certified System Administrator LFCS exam, this question tests your understanding of iptables chain directionality—a common trap is confusing the INPUT chain (for local traffic) with the FORWARD chain (for routed traffic). A frequent distractor might use `--sport` instead of `--dport`, so remember that `--dport` always refers to the destination port of the incoming connection. For a quick memory tip: think "INPUT for incoming, `--dport` for destination"—if you want to allow traffic into your server, you always pair the INPUT chain with the `--dport` flag.
LFCS Networking 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. 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 developer needs to temporarily allow incoming TCP connections on port 8080 for testing. Which iptables command adds a rule to the INPUT chain to accept this traffic?
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 8080 -j ACCEPT
Option B is correct because the INPUT chain processes traffic destined for the local system, and the `--dport 8080` flag matches incoming TCP packets with destination port 8080. The `-j ACCEPT` target allows these packets through, which is exactly what is needed to temporarily permit incoming TCP connections on port 8080 for testing.
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 OUTPUT -p tcp --sport 8080 -j ACCEPT
Why it's wrong here
This affects outgoing traffic, not incoming.
✓
iptables -A INPUT -p tcp --dport 8080 -j ACCEPT
Why this is correct
Appends a rule to INPUT chain to accept TCP on port 8080.
Related concept
Read the scenario before looking for a memorised answer.
✗
iptables -A FORWARD -p tcp --dport 8080 -j ACCEPT
Why it's wrong here
This affects forwarded traffic, not locally destined traffic.
✗
iptables -I INPUT 1 -p tcp --dport 8080 -j DROP
Why it's wrong here
This inserts a DROP rule at position 1, which would block traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the INPUT chain with the FORWARD chain, or mistakenly think that `--sport` (source port) is appropriate for incoming traffic, when `--dport` (destination port) is required for packets arriving at the local system.
Detailed technical explanation
How to think about this question
Under the hood, iptables uses netfilter hooks in the Linux kernel; the INPUT chain is traversed for packets whose destination IP matches a local interface. The `-p tcp` match implicitly uses the TCP protocol number 6, and `--dport` matches against the destination port field in the TCP header. In a real-world scenario, a developer might use this rule temporarily to test a web application running on port 8080, then remove it with `iptables -D INPUT 1` or by flushing the chain to restore security.
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 -p tcp --dport 8080 -j ACCEPT — Option B is correct because the INPUT chain processes traffic destined for the local system, and the `--dport 8080` flag matches incoming TCP packets with destination port 8080. The `-j ACCEPT` target allows these packets through, which is exactly what is needed to temporarily permit incoming TCP connections on port 8080 for testing.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
medium
A.iptables -A INPUT -d 192.168.2.0/24 -p tcp --dport 22 -j DROP
B.iptables -A OUTPUT -d 192.168.2.0/24 -p tcp --sport 22 -j DROP
C.iptables -A INPUT -s 192.168.2.0/24 -j DROP
✓ D.iptables -A INPUT -s 192.168.2.0/24 -p tcp --dport 22 -j DROP
Why D: 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.
Last reviewed: Jun 11, 2026
Question Discussion
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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.
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