🟡 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

Hostnames fail intermittently while raw IP connectivity works, disrupting apt and service discovery.

Environment & Reproduction

Seen with VPN clients, split DNS, or stale upstream resolver assignments via DHCP/netplan.

Root Cause Analysis

Inconsistent upstream DNS, stub resolver conflicts, or broken `/etc/resolv.conf` symlink to systemd-resolved stub.

Quick Triage

Validate resolver service health before changing firewall or routing rules.

Step-by-Step Diagnosis

Run `resolvectl status`, check `/etc/resolv.conf` target, and inspect `journalctl -u systemd-resolved -n 200`.

Illustrative mockup for ubuntu-26-04-lts — ubuntu2604_b01_p10_diag
Testing resolver path and stub listener behavior — Illustrative mockup — Progressive Robot

Solution – Primary Fix

Set reliable DNS in netplan, restart `systemd-resolved`, and ensure `/etc/resolv.conf` points to stub or desired managed file.

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Illustrative mockup for ubuntu-26-04-lts — ubuntu2604_b01_p10_fix
Repairing DNS server assignments and resolver state — Illustrative mockup — Progressive Robot

Solution – Alternative Approaches

Use local caching resolver or per-link DNS policies when split-horizon requirements exist.

Verification & Acceptance Criteria

`resolvectl query` resolves consistently and apt/network services no longer hit transient DNS failures.

Rollback Plan

Restore prior resolver config backup and restart resolved service.

Prevention & Hardening

Document DNS ownership between VPN, netplan, and NetworkManager; monitor resolver error rate.

`Temporary failure resolving`, timeout to upstream DNS, and VPN split DNS leaks.

Related tutorial: View the step-by-step tutorial for Ubuntu 26.04 LTS.

View all Ubuntu 26.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

`man resolvectl`, systemd-resolved docs, and Ubuntu DNS troubleshooting guides.

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