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Table of contents
Symptom & Impact
SSH access fails on RHEL 9 after changes to sshd_config or authentication policy.
Environment & Reproduction
Connections are refused, immediately closed, or produce authentication method mismatch errors.
Root Cause Analysis
A strict hardening setting conflicts with key setup, PAM rules, firewalld policy, or allowed users/groups.
Quick Triage
Keep an active console session open while testing SSH changes to avoid remote lockout.
Step-by-Step Diagnosis
Run ‘sudo sshd -t’ to detect syntax errors and unsupported directives before reloading the service.
Solution – Primary Fix
Use ‘sudo journalctl -u sshd –since -30m –no-pager’ to identify rejected auth method or account policy.
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Solution – Alternative Approaches
Confirm SSH service is allowed: ‘sudo firewall-cmd –list-services –zone=$(firewall-cmd –get-default-zone)’.
Verification & Acceptance Criteria
If using a custom SSH port, ensure it is labeled with ‘ssh_port_t’ using semanage.
Rollback Plan
Review PAM includes and SSSD availability when central identity providers are in the login chain.
Prevention & Hardening
Apply verified config with ‘sudo systemctl reload sshd’ and keep key-based fallback accounts available.
Related Errors & Cross-Refs
Use ‘ssh -vvv user@host’ from a client to correlate protocol-level errors with server-side journal entries.
Related tutorial: View the step-by-step tutorial for rhel-9.
View all rhel-9 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Use staged hardening templates and run config tests plus firewalld/SELinux checks in CI before rollout.
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