ACME/Validationについて、ここに記述してください。

https://www.eff.org/ja/deeplinks/2018/02/technical-deep-dive-securing-automation-acme-dns-challenge-validation

A Technical Deep Dive: Securing the Automation of ACME DNS Challenge Validation By Joona Hoikkala February 26, 2018

What's A Wildcard Certificate?

How Does the DNS Challenge Work?

opensslコマンド