Automated SSL certificate renewal with Let’s Encrypt, typically through an ACME client like Certbot, is invaluable for maintaining HTTPS security. However, even with automated setups, unexpected issues can lead to renewal failures—and expired certificates. Let’s dive into the most common pitfalls and, crucially, how to prevent them.
Common Reasons for Auto-Renewal Failures
1. Misconfigured Cron Jobs or Systemd Timers
Certbot relies on cron jobs or systemd timers to handle automatic renewals. These can fail silently if:
- Cron jobs aren't properly set up or accidentally modified.
- Timers become disabled or misconfigured after system updates.
Solution: Periodically confirm scheduled tasks:
systemctl list-timers certbot.timer
# or
crontab -l | grep certbot
Note that some distributions, like Amazon Linux, ship without a cron daemon, which will silently cause cron-based automated renewals to fail.
You can install a cron daemon like cronie using the appropriate command for your distribution:
sudo yum install cronie
2. File System and Permission Issues
Certificate renewal requires Certbot to access certain directories. Common permission errors or restrictive file permissions can block this process.
Solution: Ensure proper permissions for Certbot directories:
sudo chmod -R 755 /etc/letsencrypt/
3. DNS and Domain Configuration Changes
When Let’s Encrypt tries to auto-renew your SSL certificate, it performs a validation check by making a request to your domain, verifying your server’s control over that domain. If your DNS or domain records have changed unexpectedly—such as pointing your domain to a different IP address, server, or hosting provider—the validation request might reach the wrong place or fail altogether.
Solution: Regularly audit DNS configurations and ensure domain records remain accurate and stable. After major DNS adjustments, run a Certbot renewal test:
sudo certbot renew --dry-run
to confirm that validation continues to succeed.
4. Firewall or Network Configuration Issues
Network misconfigurations, firewall rules, or server security groups blocking Let’s Encrypt’s validation requests will cause renewal attempts to fail. Let's Encrypt does not publish a fixed list of IP addresses for their validation servers because these addresses change frequently.
Solution: Instead of whitelisting IP addresses, configure your firewall and network to allow inbound HTTP traffic (typically ports 80 and 443) from any address. If stricter rules are needed, consider using DNS-based or HTTP-based validation methods and ensure these protocols are accessible during renewal attempts. Regularly test your renewal setup to confirm it functions as expected.
Catching Renewal Failures Before They Become Problems
Even the best-configured setups will experience occasional issues. Proactive monitoring and alerting are crucial:
Regularly Test Renewal Configuration
Conducting periodic dry runs can help you catch potential issues early:
sudo certbot renew --dry-run
If this test encounters problems, you'll have time to correct issues before real renewal attempts fail.
Implement External Monitoring
Always use specialized SSL monitoring tools such as CertNotifier to receive immediate alerts when renewal issues occur or your certificates are otherwise approaching expiration. Don't let automated renewals lull you into a false sense of security.