🟡 Medium   ⏱ 5–30 min  Last verified: 19 May 2026

📖 ~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

Applications intermittently fail outbound connections because hostname resolution times out under moderate load.

Environment & Reproduction

Occurs on Ubuntu 18.04 using systemd-resolved with mixed DHCP and static DNS settings.

Root Cause Analysis

Conflicting resolver sources and stale upstream nameserver state create inconsistent cache and timeout behavior.

Quick Triage

Check resolved status, query test domains, and isolate whether failures affect one or multiple upstream nameservers.

Step-by-Step Diagnosis

Review resolvectl output, journal logs, and netplan definitions to identify resolver precedence conflicts.

Illustrative mockup for ubuntu-18-04-lts — dns_timeout_problem
Host lookup timeout in terminal during active network session — Illustrative mockup — Progressive Robot

Solution – Primary Fix

Set deterministic DNS servers, restart systemd-resolved, clear cache, and align netplan with intended resolver model.

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Illustrative mockup for ubuntu-18-04-lts — dns_resolved_fix
Resolved DNS configuration and stable name lookup — Illustrative mockup — Progressive Robot

Solution – Alternative Approaches

Use local caching resolver, disable conflicting resolver service, or apply dedicated DNS failover policy.

Verification & Acceptance Criteria

Repeated query loops complete without timeout and application dependency checks remain stable.

Rollback Plan

Restore previous resolved and netplan files if custom resolver path causes wider network regressions.

Prevention & Hardening

Document DNS ownership per interface and enforce one source of truth for resolver configuration.

Cross-reference apt repository lookup failures and container DNS isolation issues with linked tutorials.

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

See systemd-resolved manual, netplan examples, and Ubuntu DNS troubleshooting references.

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