π ~1 min read
Table of contents
Symptom & Impact
Remote operators cannot authenticate over SSH keys, interrupting administration and automation pipelines.
Environment & Reproduction
Observed after account migration, home directory restore, or SSH hardening change.
Root Cause Analysis
StrictModes rejects insecure file permissions, missing key entries, or unsupported key algorithm settings.
Quick Triage
Review sshd logs and confirm user shell/home path validity before editing daemon policy.
Step-by-Step Diagnosis
Validate ownership and mode on home directory, .ssh directory, and authorized_keys file.
Solution – Primary Fix
Repair permissions, restore expected key material, and reload sshd with compatible auth settings.
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Solution – Alternative Approaches
Use emergency console access, temporary password auth, or provisioning tool to redeploy keys.
Verification & Acceptance Criteria
SSH key login succeeds without fallback prompts and auth logs show Accepted publickey events.
Rollback Plan
Reinstate prior sshd_config and key files from backup if access remains disrupted.
Prevention & Hardening
Automate key distribution, enforce file mode checks, and monitor auth anomalies continuously.
Related Errors & Cross-Refs
Authentication refused: bad ownership or modes, invalid user, and key exchange mismatch errors.
Related tutorial: View the step-by-step tutorial for Ubuntu 16.04 LTS.
View all Ubuntu 16.04 LTS tutorials on the Tutorials Hub β
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
OpenSSH hardening references and Ubuntu 16.04 authentication troubleshooting resources.
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