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.
Which THREE are built-in chains in the iptables filter table? (Choose three.)
POSTROUTING
Why wrong: POSTROUTING is in the nat and mangle tables, not filter.
INPUT
INPUT chain processes incoming packets destined for the local system.
OUTPUT
OUTPUT chain processes outgoing packets from the local system.
FORWARD
FORWARD chain processes packets routed through the system.
PREROUTING
Why wrong: PREROUTING is in the nat and mangle tables, not filter.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
INPUT
The filter table in iptables is used for packet filtering decisions based on IP addresses, ports, and protocols. Its built-in chains are INPUT (for packets destined for the local system), OUTPUT (for packets originating from the local system), and FORWARD (for packets routed through the system). These three chains allow you to control traffic at different points in the packet flow.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
POSTROUTING
Why it's wrong here
POSTROUTING is in the nat and mangle tables, not filter.
INPUT
Why this is correct
INPUT chain processes incoming packets destined for the local system.
Related concept
Read the scenario before looking for a memorised answer.
OUTPUT
Why this is correct
OUTPUT chain processes outgoing packets from the local system.
Related concept
Read the scenario before looking for a memorised answer.
FORWARD
Why this is correct
FORWARD chain processes packets routed through the system.
Related concept
Read the scenario before looking for a memorised answer.
PREROUTING
Why it's wrong here
PREROUTING is in the nat and mangle tables, not filter.
Common exam traps
The trap here is that candidates often confuse the filter table's chains with those of the nat table (PREROUTING, POSTROUTING) because all chains are used in packet traversal, but only INPUT, OUTPUT, and FORWARD belong to the filter table.
Detailed technical explanation
Under the hood, iptables uses netfilter hooks in the Linux kernel: NF_IP_LOCAL_IN (INPUT), NF_IP_LOCAL_OUT (OUTPUT), and NF_IP_FORWARD (FORWARD) correspond to the filter table chains. A common real-world scenario is using the FORWARD chain on a router to restrict traffic between internal and external networks, while INPUT protects the router itself. The filter table is the default table when no -t option is specified, which often leads to confusion about which chains are available.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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FAQ
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
The correct answer is: INPUT — The filter table in iptables is used for packet filtering decisions based on IP addresses, ports, and protocols. Its built-in chains are INPUT (for packets destined for the local system), OUTPUT (for packets originating from the local system), and FORWARD (for packets routed through the system). These three chains allow you to control traffic at different points in the packet flow.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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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