📖 ~1 min read
Table of contents
Symptom & Impact
Host cannot resolve domains, breaking package installs, API calls, and service-to-service communication.
Environment & Reproduction
Appears after netplan edits, resolver overrides, VPN changes, or DHCP DNS drift.
Root Cause Analysis
Stub resolver points to invalid upstream nameservers or conflicting resolver configuration paths.
Quick Triage
Check local stub status and test direct queries against known upstream DNS servers.
Step-by-Step Diagnosis
Inspect resolved status, netplan YAML, and resolv.conf linkage for mismatch conditions.
Solution – Primary Fix
Correct netplan DNS entries, restart systemd-resolved, and revalidate resolver path consistency.
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Solution – Alternative Approaches
Temporarily set static resolv.conf for emergency recovery while permanent config is prepared.
Verification & Acceptance Criteria
Forward and reverse lookups succeed and dependent services restore normal connectivity.
Rollback Plan
Restore previous netplan and resolver settings from backup and reapply network configuration.
Prevention & Hardening
Version-control network configs and validate DNS health in post-change checks.
Related Errors & Cross-Refs
May appear with DHCP lease bugs, VPN resolver overrides, and split-horizon DNS design issues.
Related tutorial: View the step-by-step tutorial for Ubuntu 20.04 LTS.
View all Ubuntu 20.04 LTS tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Ubuntu netplan and systemd-resolved documentation for server DNS architecture.
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