In re-reading the spec, I noticed that the spec, as it's written now, effectively forbids DANE authentication, which may or may not be what you wanted.
I am thinking about a scenario in which I outsource my email to a third-party. Let's assume that I don't have DNSSEC on my domain name user.com but that the provider does on their provider.com. Consequently, their SMTP servers (mx.provider.com) might conceivably have self-signed certificates that rely on DANE for validation. MTA-STS is still useful in this case because it prevents MITM from manipulating the MX servers that are allowed to receive email for user.com. I've read the DANE language in the spec that says that MTA-STS doesn't override DANE. That's fine. But should a MTA-STS-aware client fail a sever with a non-public certificate that's DANE-validated? I think the answer is probably yes, but perhaps that should be explicitly stated in the spec. This is also an area where the current hostname matching might interfere with DANE, because DANE is validated for the specific hostname that you're connecting to, and all other hostnames in the certificate are ignored. -- Ivan
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