Ubuntu 16.04 LTS Common Problem 024 – Definitive Diagnosis, Recovery, and Prevention

📖 ~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 prompts for password or denies login even when valid private key is supplied, breaking automation and remote administration.

Environment & Reproduction

Ubuntu 16.04 OpenSSH servers after home directory permission changes or edited sshd_config.

Root Cause Analysis

StrictModes checks fail due to insecure permissions/ownership, wrong AuthorizedKeysFile path, or disabled PubkeyAuthentication.

Quick Triage

Use ssh -vvv from client and inspect server logs with journalctl -u ssh or /var/log/auth.log.

Step-by-Step Diagnosis

Validate ownership and modes on home, .ssh, and authorized_keys, then test effective sshd configuration with sshd -T.

Solution – Primary Fix

Set secure permissions (700 .ssh, 600 authorized_keys), correct sshd_config auth settings, and restart ssh service safely.

Solution – Alternative Approaches

Use centralized SSH certificate authentication or configuration management to enforce canonical authorized_keys placement.

Verification & Acceptance Criteria

Key-based login succeeds without password and sshd logs show accepted publickey for expected user/key fingerprint.

Rollback Plan

Restore previous sshd_config and authorized_keys backups; keep console session open while reverting to avoid lockout.

Prevention & Hardening

Automate file permission checks, enable fail2ban/rate limits, and audit SSH config drift regularly.

Related Errors & Cross-Refs

Related to UFW SSH blocks, DNS host mismatch warnings, and PAM/account policy denials.

Related tutorial: View the step-by-step tutorial for Ubuntu 16.04 LTS.

View all Ubuntu 16.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

sshd_config(5), ssh(1), and secure remote access hardening baselines.