> In MX delivery without DNSSEC, if Eve injects an MX record: > gmail.com. IN MX 1 my-spy-agency.example.org.
> then using the hostname from DNS means that the client will happily go > talk to my-spy-agency.example.org, using that as the SNI, and validating > against that same domain, then present back an "all's good here boss" > status, saying that it's happily confirmed the identity of gmail.com in > validation. > The SNI identity should match the identity to be validated, else there's > no point doing any validation. Having clients willing to send SNI which > they're not willing to accept as a valid value for verification is > broken. Since the client can't a-priori know which is which (legitimate > gmail.com server or Other), when using hostnames from insecure DNS, the > client shouldn't send SNI unless and until it has an identity which it's > willing to validate. > If using DANE, or if using MTA-STS where the hostname from DNS has been > vetted against the whitelist from MTA-STS first, then everything changes > and SNI becomes important. AFAIK this does not happen in MTA-STS, that is, at no time is the MX hostname obtained from the DNS checked against the "mx" list from the MTA-STS policy. Rather, the DNS-ID of the certificate returned by the server is checked against the "mx" list from the MTA-STS policy. This means that the mx hostnames may not align with the certificates. If you believe otherwise, I'd appreciate a pointer to where in the specification it says that MX hostnames are supposed to be checked against the "mx" list. Ned _______________________________________________ mailop mailing list firstname.lastname@example.org https://chilli.nosignal.org/cgi-bin/mailman/listinfo/mailop