Ubuntu 18.04 LTS – Common Problem 020 – 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

SSH clients show WARNING REMOTE HOST IDENTIFICATION HAS CHANGED and block connections.

Environment & Reproduction

Ubuntu 18.04 servers rebuilt, reimaged, or rotated host keys while clients retain old known_hosts records.

Root Cause Analysis

Server host key fingerprint changed legitimately or due to potential man-in-the-middle risk.

Quick Triage

Validate new fingerprint out-of-band from console access before modifying client trust records.

Step-by-Step Diagnosis

Compare ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub output with client warning details.

Solution – Primary Fix

After verification, remove stale key with ssh-keygen -R and reconnect to store the new fingerprint.

Solution – Alternative Approaches

Publish SSH host certificates or centralized known_hosts management to reduce manual trust drift.

Verification & Acceptance Criteria

SSH connection succeeds without mismatch warning and fingerprint matches approved inventory.

Rollback Plan

If fingerprint cannot be validated, block access and revert to previous host snapshot for investigation.

Prevention & Hardening

Document host key rotations and automate distribution of trusted fingerprints.

Related Errors & Cross-Refs

Offending key in known_hosts, possible DNS spoofing detected, and host key verification failed.

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

OpenSSH manuals, ssh-keygen usage, and operational key management practices.