The answer is that the FORWARD chain in the filter table must have an explicit ACCEPT rule. While the MASQUERADE rule in the POSTROUTING chain correctly handles source NAT for outbound traffic, it does not automatically permit packet forwarding through the kernel. Without a rule like `iptables -A FORWARD -s 192.168.1.0/24 -d 10.0.0.0/24 -j ACCEPT`, the default DROP policy on the FORWARD chain silently discards the packets, making the MASQUERADE rule appear non-functional. On the LPIC-2 exam, this scenario tests your understanding that NAT and packet filtering operate on separate iptables tables—nat and filter respectively—and that a missing FORWARD rule is a classic trap when troubleshooting why MASQUERADE fails. A common memory tip is "NAT hides, FORWARD decides": MASQUERADE only rewrites the source address, but the FORWARD chain must grant permission for the packet to travel between interfaces.
LPIC-2 Advanced Networking Configuration Practice Question
This LPIC-2 practice question tests your understanding of advanced networking configuration. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 server with IP 10.0.0.1 needs to forward packets from network 192.168.1.0/24 to 10.0.0.0/24. The administrator runs: 'iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 10.0.0.0/24 -j MASQUERADE'. However, traffic from 192.168.1.0/24 cannot reach 10.0.0.0/24. What is the most likely missing configuration?
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.
A DNAT rule is also needed to translate the destination address.
Why wrong: DNAT is not needed; the source is being masqueraded, but the destination is the actual server.
B
A route must be added on the 10.0.0.0/24 network pointing back to 10.0.0.1.
Why wrong: The server is the gateway, so return traffic will go through it; no extra route needed if the server is the default gateway for 10.0.0.0/24.
C
The FORWARD chain in the filter table must have a rule to allow traffic.
By default, the FORWARD chain policy is DROP; need to allow forwarding.
D
The IP address 10.0.0.1 is not configured on the external interface.
Why wrong: The server has IP 10.0.0.1, which is the source address for masqueraded traffic.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The FORWARD chain in the filter table must have a rule to allow traffic.
The MASQUERADE rule in the POSTROUTING chain handles source NAT, but it does not automatically allow forwarding of packets. By default, the FORWARD chain in the filter table has a policy of DROP or lacks an explicit ACCEPT rule. Without a rule like `iptables -A FORWARD -s 192.168.1.0/24 -d 10.0.0.0/24 -j ACCEPT`, the kernel's netfilter will drop the forwarded packets, preventing traffic from reaching the destination.
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.
✗
A DNAT rule is also needed to translate the destination address.
Why it's wrong here
DNAT is not needed; the source is being masqueraded, but the destination is the actual server.
✗
A route must be added on the 10.0.0.0/24 network pointing back to 10.0.0.1.
Why it's wrong here
The server is the gateway, so return traffic will go through it; no extra route needed if the server is the default gateway for 10.0.0.0/24.
✓
The FORWARD chain in the filter table must have a rule to allow traffic.
Why this is correct
By default, the FORWARD chain policy is DROP; need to allow forwarding.
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 IP address 10.0.0.1 is not configured on the external interface.
Why it's wrong here
The server has IP 10.0.0.1, which is the source address for masqueraded traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a MASQUERADE rule alone enables forwarding, but they overlook the separate requirement to allow traffic in the FORWARD chain of the filter table, which is a distinct step in iptables configuration.
Detailed technical explanation
How to think about this question
Under the hood, iptables uses the FORWARD chain to control packets that traverse the server (i.e., not destined for the local host). Even with NAT rules, the filter table's FORWARD chain must explicitly permit the flow; otherwise, the kernel's netfilter hooks will drop the packet before it reaches the POSTROUTING chain. In a real-world scenario, administrators often forget to set the `net.ipv4.ip_forward` sysctl parameter to 1, which is also required for forwarding, but the question specifically asks about the most likely missing configuration among the options, and the FORWARD rule is a common oversight.
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.
Advanced Networking Configuration — This question tests Advanced Networking Configuration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The FORWARD chain in the filter table must have a rule to allow traffic. — The MASQUERADE rule in the POSTROUTING chain handles source NAT, but it does not automatically allow forwarding of packets. By default, the FORWARD chain in the filter table has a policy of DROP or lacks an explicit ACCEPT rule. Without a rule like `iptables -A FORWARD -s 192.168.1.0/24 -d 10.0.0.0/24 -j ACCEPT`, the kernel's netfilter will drop the forwarded packets, preventing traffic from reaching the destination.
What should I do if I get this LPIC-2 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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