📖 ~1 min read
Table of contents
Symptom & Impact
After boot, network interfaces are down or missing routes, causing host isolation.
Environment & Reproduction
Ubuntu 18.04 cloud or VM instances with edited netplan YAML and predictable interface names.
Root Cause Analysis
YAML syntax, wrong NIC name (ens3 vs eth0), or invalid renderer setting prevents network bring-up.
Quick Triage
Run ip a, ip r, and sudo netplan –debug generate to identify parse or mapping errors quickly.
Step-by-Step Diagnosis
Check /etc/netplan/*.yaml indentation, match interface names from ip link, and inspect cloud-init generated config interaction.
Solution – Primary Fix
Correct YAML, set the right interface under ethernets, then run sudo netplan apply and confirm DHCP/static routes come up.
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Solution – Alternative Approaches
Use sudo netplan try for rollback safety or migrate to NetworkManager renderer for desktop-oriented systems.
Verification & Acceptance Criteria
Interface is UP, default route exists, DNS works, and connectivity survives reboot.
Rollback Plan
Revert YAML from backup or cloud-init defaults and apply configuration again.
Prevention & Hardening
Lint YAML before deployment and keep tested netplan templates for common hypervisor interface naming schemes.
Related Errors & Cross-Refs
Error in network definition, Unknown key, and Network is unreachable.
Related tutorial: View the step-by-step tutorial for Ubuntu 18.04 LTS.
View all Ubuntu 18.04 LTS tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Netplan examples, cloud-init network config docs, and ip-route manual pages.
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