LPIC-2 Advanced Networking Configuration Practice Question
Exhibit
Refer to the exhibit. # ip rule show 0: from all lookup local 32766: from all lookup main 32767: from all lookup default # ip route show table 100 default via 10.0.0.1 dev eth0 # ip route show table 200 default via 172.16.0.1 dev eth1 # ip rule add from 10.1.1.0/24 lookup 200 # ip rule add from 10.2.2.0/24 lookup 100
Traffic from the 10.1.1.0/24 subnet is seen leaving through eth1 as intended, but reply traffic from the internet never comes back. What is the most likely cause?
⚠ Common exam trap
Test-takers frequently assume the issue is a missing return route or a routing table misconfiguration, but the real culprit is the kernel's reverse path filtering, which silently drops packets that arrive on an interface that is not the best path back to the source.
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 kernel parameter net.ipv4.conf.all.rp_filter is set to 1.
When `net.ipv4.conf.all.rp_filter` is set to 1, the kernel performs strict reverse path filtering. This means the kernel checks whether the source address of incoming packets can be reached via the interface they arrived on. If reply traffic from the internet arrives on an interface (e.g., eth0) that does not have a route back to the original source subnet (10.1.1.0/24) through that same interface, the kernel drops the packet. This is the most likely cause because the outbound traffic leaves via eth1, but the reply comes back on a different interface, triggering the rp_filter check and causing the reply to be silently discarded.
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 kernel parameter net.ipv4.conf.all.rp_filter is set to 1.
Why this is correct
Strict reverse path filtering drops packets arriving on an interface that would not be used to reach the source, which commonly happens in asymmetric routing scenarios.
- ✗
The ip rule priority is not set, causing it to be overridden.
Why it's wrong here
Rules without explicit priority are assigned automatically and still apply.
- ✗
The default route in table 200 points to 172.16.0.1 which does not have a route back to 10.1.1.0/24.
Why it's wrong here
The remote gateway may have routes; but the typical issue with policy routing is reverse path filtering.
- ✗
The router does not have a route back to 10.1.1.0/24 via eth0.
Why it's wrong here
The router likely has a route; the issue is not lack of route but the reverse path check.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LPIC-2 practice question is part of Courseiva's free LPI 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 LPIC-2 exam.