Generate certificates for a private server not visible from everywhere

@phphil,

There is another authentication method called DNS-01 in which you add specified TXT records to your DNS zone.

The manual mode in Certbot can help you with this, but if you do it manually, it won't be compatible with automated renewal.

The clients written in bash have historically had better support for this method than Certbot does, and so far continue to (including sometimes supporting DNS provider APIs to update the DNS records for you).