CentOS Stream 9 – Common Problem 097 – 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

sshd refuses keys after policy hardening on CentOS Stream 9 disrupts services and slows incident response until the root cause is resolved.

Environment & Reproduction

Crypto-policies update to FUTURE breaks legacy ssh-rsa keys for service automation.

sshd -T | grep -i pubkey
update-crypto-policies --show
grep -i rsa /etc/ssh/sshd_config

Root Cause Analysis

Misalignment between auth configuration and CentOS Stream 9 defaults causes the failure path described above.

Quick Triage

Confirm package state, service status, and recent changes before deeper diagnostics.

systemctl status
rpm -qa | grep -i 
journalctl -p err -b --no-pager | tail -100

Step-by-Step Diagnosis

Capture detailed logs, configuration deltas, and runtime state to isolate the failing component.

ssh -vvv user@host
journalctl -u sshd -n 100
update-crypto-policies --show

Solution – Primary Fix

Apply the targeted configuration change and restart the relevant services to restore expected behavior.

sudo update-crypto-policies --set DEFAULT
sudo sshd -t && sudo systemctl restart sshd
sudo ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub

Solution – Alternative Approaches

Re-key automation accounts to ed25519 and retire ssh-rsa fully.

Verification & Acceptance Criteria

Validate the fix with deterministic checks and ensure no regressions in dependent services.

ssh user@host true
ssh-keygen -lf ~/.ssh/id_.pub

Rollback Plan

Revert configuration and restart services to return to the previous known-good state.

update-crypto-policies --set FUTURE
systemctl restart sshd

Prevention & Hardening

Inventory key types and run crypto-policies in a staged rollout.

Related Errors & Cross-Refs

Related: crypto-policies, ssh-rsa, ed25519; see also adjacent topics in the CentOS Stream 9 common problems series.

Related tutorial: View the step-by-step tutorial for centos-stream-9.

View all centos-stream-9 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

CentOS Stream documentation, Red Hat upstream guides, and CentOS Stream 9 release notes covering this subsystem.