Ubuntu 20.04 LTS DNS Resolution Fails Intermittently

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Hosts intermittently fail domain lookups, creating random application outages, package repository failures, and latency spikes in API calls using hostname targets.

Environment & Reproduction

Issue appears on Ubuntu 20.04 with mixed DHCP/static resolver settings, VPN clients, or stale netplan states where resolver order changes across interface events.

Root Cause Analysis

Conflicts between systemd-resolved, NetworkManager, and manually edited resolv.conf cause unstable upstream DNS selection and occasional unreachable resolver endpoints.

Quick Triage

Check current nameserver chain, test lookup against each upstream directly, and verify whether failures are local cache issues or external DNS service instability.

Step-by-Step Diagnosis

Review resolvectl status, inspect netplan YAML for DNS directives, and correlate failures with interface renewals and VPN connection logs.

Solution – Primary Fix

Standardize resolver ownership under systemd-resolved, define explicit upstream servers in netplan, apply changes, and flush caches before retesting name resolution paths.

Solution – Alternative Approaches

Deploy local caching resolvers, pin DNS over TLS endpoints, or configure split DNS policies by interface for hybrid private/public workloads.

Verification & Acceptance Criteria

Accept fix when repeated dig and application transactions resolve consistently during interface renewals, VPN toggles, and sustained workload traffic.

Rollback Plan

Revert to previous netplan profile, restore backed up resolver settings, and disable custom overrides if newly introduced resolver priorities break internal routing.

Prevention & Hardening

Apply configuration management, alert on resolver drift, and document DNS ownership boundaries to avoid manual edits that bypass the chosen network stack.

Related Errors & Cross-Refs

Related topics include apt mirror hostname failures, internal domain lookup mismatches, and VPN split-DNS conflicts addressed in linked networking tutorials.

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

See Ubuntu netplan guides, systemd-resolved man pages, and DNS operations best practices for resilient enterprise resolver architecture.