Introduction
In today's tech ecosystem, automation is king. But what happens when something doesn't work as planned? This is what happened with a Caddy certificate expiration caused by a failure in systemd-resolved. This article explores how an incorrect DNS configuration can lead to the failure of automatic certificate renewals.
The Background
It all started on a Sunday evening when alerts flagged that the Matrix server certificate had expired. The certificate was supposed to be automatically renewed by Caddy using a Cloudflare DNS-01 challenge for Let's Encrypt certificates. However, something didn't go as planned.
Investigating the Cause
The first step was to check the Docker containers. All services were operational. The issue clearly lay with the TLS certificate renewal. Upon examining the Caddy logs, it was found that the renewal was failing due to a DNS problem.
The Root Cause: systemd-resolved
This is where systemd-resolved comes into play. This service is responsible for DNS resolution and, in this case, it was selectively failing for certain queries. This malfunction led to a SERVFAIL error on a specific DNS zone, preventing the certificate renewal.
Solution and Prevention
The solution was to reconfigure systemd-resolved to ensure all necessary DNS queries were properly resolved. This involved checking DNS configurations and ensuring the resolution path was valid. Additionally, actively monitoring certificate renewals is crucial to avoid such surprises.
Conclusion
This incident highlights the importance of understanding critical dependencies in an automated system. A faulty configuration can have significant impacts, such as interrupting the federation of Matrix servers. Teams need to be vigilant and ready to intervene quickly.
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