Letsencrypt/dehydrated/DNS-01について、ここに記述してください。

https://www.aaflalo.me/2017/02/lets-encrypt-with-dehydrated-dns-01/

Records

When generating a certificate for your main domain, you only need to set the TXT record with the given let’s encrypt challenge. However, if you want to generate a certificate for a subdomain, you need to set 2 TXT records: one for your main domain and one for your sub-domain since you still need to prove let’s encrypt that you have ownership to the main domain.

Lexicon https://github.com/AnalogJ/lexicon (DNS apiをもつDNSを操作する道具)

Lexicon is a great python script that takes care of DNS management for a lot of providers like Namecheap, CloudFlare, DigitalOcean, etc … the list is huge, and the software is easy to uses.