🟡 Medium   ⏱ 5–30 min  Last verified: 20 May 2026
Affected versions: CentOS Stream 9

📖 ~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

Hosts on CentOS Stream 9 show chronyc tracking stratum 16 after firewall change.

Environment & Reproduction

Hosts behind firewalls or in air-gapped segments.

cat /etc/os-release
uname -r
rpm -qa | grep

Root Cause Analysis

Egress UDP 123 blocked or all configured NTP peers offline.

Quick Triage

Confirm package, network, SELinux and firewall state.

chronyc sources -v
chronyc tracking
ss -ulpn | grep :123

Step-by-Step Diagnosis

Drill into time/chrony state and recent journal entries to isolate the failure.

journalctl -u chronyd -n 50
firewall-cmd --list-all
Illustrative mockup for centos-stream-9 — cs9-b02-p045-diag
Diagnosis console for chronyd reports stratum 16 unreachable on CentOS Stream 9 — Illustrative mockup — Progressive Robot

Solution – Primary Fix

Apply the standard remediation for time/chrony on CentOS Stream 9.

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sudo firewall-cmd --permanent --add-service=ntp
sudo firewall-cmd --reload
sudo systemctl restart chronyd
Illustrative mockup for centos-stream-9 — cs9-b02-p045-fix
Remediation output for chronyd reports stratum 16 unreachable on CentOS Stream 9 — Illustrative mockup — Progressive Robot

Solution – Alternative Approaches

Configure internal NTP appliance and pin servers in chrony.conf.

Verification & Acceptance Criteria

Validate the fix with positive functional checks before closing.

chronyc tracking
timedatectl

Rollback Plan

Revert to the previous known-good configuration if the fix regresses.

cp /root/backup/chrony.conf /etc/chrony.conf
systemctl restart chronyd

Prevention & Hardening

Codify the fix in configuration management and add monitoring.

Monitor offset via Prometheus node exporter chrony module.

Kerberos clock skew and TLS clock errors.

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

chrony documentation.

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