LPIC-2 Advanced Networking Configuration Practice Question
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?
⚠ Common exam trap
Watch out — candidates often 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.
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 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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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