John Levine:
> MTA-STS is a newish IETF spec that lets mail operators declare that
> all of their incoming mail servers support STARTTLS. (See RFC 8461.)
>
> The idea is to preclude mailstream hijacking. If a domain publishes
> MTA-STS, it says what what the names MX'es should be; Before they
> start sending mail, client systems check that STARTTLS works, and (if
> MTA-STS is in enforced mode) that the TLS certificate from the mail
> server is the right one. If not, the mail is presumably about to be
> hijacked by a middlebox and the sending system doesn't send it.
>
> For example, here's where you can find the MTA-STS for my iecc.com:
>
> https://mta-sts.iecc.com/.well-known/mta-sts.txt
>
> The big gorilla mail systems are implementing this now that they all
> have STARTTLS support.
>
> A detail a lot of people forget is that a mail server can have
> multiple names, just like a web server can. During the TLS startup,
> the client system sends SNI saying what name it expects, so the server
> can send the correct certificate. At least that's the plan -- in my
> experience a lot of mail client software doesn't send SNI, so MTA-STS
> verification fails on servers with multiple names since the server
> sends a default certificate that isn't the one the client expects.
>
> Looking at the mail logs for my servers, it's pretty clear that
> Postfix doesn't send SNI. I would also guess that if a Postfix MTA has
> multiple names, it doesn't have any way to select a certificate using
> SNI. This is not hard to fix; I added SNI support to the mailfront
> SMTP daemon in a couple of hours. It took longer to get all the
> certificates signed.
Postfix will send SNI when it is told (by policy) what servername
to use. It can be statically configured as smtp_tls_servername,
or dynamically in an smtp_tls_policy_map lookup result with the
servername attribute.
There are several MTA-STS plugins for Postfix that provide that
dynamic policy. It is not built into Postfix at this time, just
like DKIM and a lot of other protocols.
Wietse
> FYI, it's not just me. Real systems use multiple names, e.g., Tucows'
> large whitelabel mail service has a unique MX name for each hosted
> domain, like this:
>
> $ host tucows.com
> tucows.com mail is handled by 0 mx.tucows.com.cust.hostedemail.com.
> $ host tucows.net
> tucows.net mail is handled by 10 mx.tucows.net.cust.a.hostedemail.com.
> $ host opensrs.com
> opensrs.com mail is handled by 0 mx.opensrs.com.cust.a.hostedemail.com.
>
> If you're keeping score, Gmail gets SNI correct, Microsoft's Outlook.com
> doesn't, but I think I've found the right people to fix it.
>
> R's,
> John
>
