Ubuntu 20.04 LTS – Common Problem 017 – Diagnosis and Fix

📖 ~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

Each sudo command warns ‘unable to resolve host’, adding latency and confusing operators.

Environment & Reproduction

Ubuntu 20.04 after hostname change where /etc/hostname and /etc/hosts are inconsistent.

Root Cause Analysis

Local hostname cannot be resolved via hosts file mapping for loopback entries.

Quick Triage

Run ‘hostnamectl’, view ‘/etc/hostname’, and inspect ‘/etc/hosts’ for matching short/FQDN entries.

Step-by-Step Diagnosis

Validate DNS vs local resolution behavior and ensure no malformed whitespace or duplicate host records.

Solution – Primary Fix

Align hostname files and add correct line like ‘127.0.1.1 ‘ then test sudo.

Solution – Alternative Approaches

If managed by cloud-init, update cloud config and regenerate host mappings consistently.

Verification & Acceptance Criteria

sudo runs without warning and hostname resolves correctly via ‘getent hosts’.

Rollback Plan

Restore previous /etc/hosts and /etc/hostname backups if naming conventions conflict with automation.

Prevention & Hardening

Change hostnames through standardized scripts that update all dependent files atomically.

Related Errors & Cross-Refs

cloud-init hostfile overwrite, DNS search domain confusion, and sudo execution delays.

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

hostnamectl docs, hosts file semantics, and Ubuntu network identity management notes.