Joachim Lindenberg via Postfix-users:
> Hello,
>
> are there any ideas or plans to implement SMTP Require TLS Option (RFC 8689)
> in postfix?
It is not on the calendar. Below is a preliminary analysis of the
implementation effort. Rumor has it that there was a preliminary
implementation for Exim based on an obsolete draft of REQUIRETLS.
> I am aware of that in order to really leverage that, one needs a
> MUA using it, plus a MTA supporting SMTP-DANE (RFC 7672) or MTA-STS
> (RFC 8461), but sure I may be missing something.
This is something that must be used selectively: all SMTP servers
in the forward path must support (and enforce) the REQUIRETLS option
in the MAIL FROM command. When such a message is bounced it must
also be protected with REQUIRETLS, unless the bounce message returns
only the message header.
Similarities/differences with SMTPUTF8:
- This requires Postfix changes for the propagation of the REQUIRETLS
option in the MAIL FROM command in the normal delivery path,
similar to SMTPUTF8.
- This requires changes to message stores (Dovecot, etc.) to announce
REQUIRETLS support in their SMTP or LMTP server, even if they do
nothing with it. To work around this, the Postfix SMTP/LMTP client
could have a 'fake REQUIRETLS' setting (do not send REQUIRETLS
and pretend that the message store SMTP/LMTP server supports it).
- This requires changes to SMTP-based content filters to pass
through the REQUIRETLS option, similar to SMTPUTF8. To work around
this, Postfix can't 'fake REQUIRETLS' in the after-filter SMTP
server because that would request REQUIRETLS for all mail.
- No need to propagate REQUIRETLS for header-only bounce messages,
unlike SMTPUTF8.
- Milters are not affected.
New mechanisms in Postfix would involve:
- A Postfix sendmail command-line option to request REQUIRETLS.
This is unlike SMTPUTF8 which can rely on autodetection.
- Propagating the REQUIRETLS option through a "pipe to command"
content filter, to the Postfix sendmail command at the other end
of that filter.
- Overriding server certificate verification (PKI, DANE, MTA-STS)
with a "TLS-Required: No" message header (ignored when REQUIRETLS
was requested).
- Returning email as undeliverable when all SMTP or LMTP server
certificates fail to verify.
Often when some end-to-end feature is added to SMTP, it is a royal
pain to make it work with a "pipe to command" content filter. I'd
like to get rid of that.
Wietse
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