LFCS Networking Practice Question
Which THREE conditions can cause an 'RTNETLINK answers: File exists' error when adding a static route?
⚠ Common exam trap
Candidates often assume the metric or gateway differentiates routes, but the Linux kernel treats the destination/netmask pair as the sole unique identifier for a route entry, so any duplicate prefix — even with different metrics or gateways — triggers the 'File exists' error.
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 route exists with a different metric but same destination and netmask.
The Linux kernel's routing table uses the destination and netmask as a unique key for route entries. When adding a route with 'ip route add', if a route already exists with the same destination and netmask but a different metric, the kernel returns 'RTNETLINK answers: File exists' because the metric is not part of the uniqueness check — only the destination prefix and netmask are considered. The metric is a per-route property that does not disambiguate routes in the FIB (Forwarding Information Base).
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 route exists with a different metric but same destination and netmask.
Why this is correct
Metric is not part of the uniqueness; still duplicates.
- ✗
The gateway is unreachable.
Why it's wrong here
Would give 'Network is unreachable' error.
- ✗
The interface is down.
Why it's wrong here
Would give 'Cannot assign requested address' or similar.
- ✓
The route exists with a different gateway but same destination and netmask.
Why this is correct
Still a duplicate route entry if the destination/netmask is the same.
- ✓
The route already exists with the same destination and netmask (exact duplicate).
Why this is correct
Adding exact duplicate route returns 'File exists'.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.