Developer Learns ACME Protocol Basics After Debugging HTTPS Certificate Failures

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A developer encountered repeated HTTPS certificate issuance failures while deploying an application using Caddy as a reverse proxy and Let's Encrypt for SSL certificates. Despite correct DNS records, open firewall ports, and proper Caddy configuration, certificates consistently failed to issue. The root cause turned out to be a networking problem that prevented Let's Encrypt from reaching the server during ACME domain validation — not a misconfiguration in Caddy or TLS itself. The ACME protocol requires Let's Encrypt to actively validate domain ownership by contacting the applicant's server, a detail the developer had initially overlooked. Reading ACME logs and following a structured checklist — verifying DNS, checking external port reachability, and confirming challenge responses — ultimately resolved the issue and highlighted that certificate failures often stem from underlying network problems.

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