Publish CAA (DNS TXT) records to improve security

[ianeiloart]

Certification Authority Authorization (CAA), specified in RFC 6844 in 2013, is a proposal to improve the strength of the PKI ecosystem with a new control to restrict which CAs can issue certificates for a particular domain name. It prevents bad people obtaining certificates from rogue or sloppy certification authorities.

It's a simple DNS text record to say, for example:
example.org. CAA 128 issue "letsencrypt.org"

At minimum, we could publish this record for a domain that's protected by a LetsEncrypt certificate.

https://blog.qualys.com/ssllabs/2017/03/13/caa-mandated-by-cabrowser-forum