SSL certificate expiry causes sudden service outages and costly emergency fixes. This guide shows how to build an automated monitor using Python's ssl and socket modules with pyopenssl to check domain certificates. It connects to domains, parses the notAfter expiry timestamp, and calculates days remaining, replacing error-prone manual checks.

The script integrates with Slack API via an Incoming Webhook. It sends formatted alerts to your chosen channel when certificates approach a configurable EXPIRY_WARNING_THRESHOLD_DAYS. This proactive notification prevents missed deadlines, allowing teams to renew certificates before users encounter security warnings or downtime.

Automation is key. The guide details scheduling the script with cron on Linux for daily execution. While commercial monitoring tools exist, this DIY solution offers cost savings and full control over logic and integration. It's a practical approach for DevOps teams managing multiple services and domains.