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Re: [ietf-smtp] SMTP Over TLS on Port 26 - Implicit TLS Proposal

2019-01-07 02:58:31
On Mon, Jan 07, 2019 at 01:55:27PM +0530, Viruthagiri Thirumavalavan wrote:
Hello Everyone,


I have proposal for SMTPS. Already gathered some feedback from UTA working
group and improved my draft.

My proposal is a very simple document. So, please go though it and give
me feedback if you can.

Here is my abstract.

SMTP is still suffering from downgrade attacks like STRIPTLS. While we have
"Opportunistic TLS", we still don't have "Implicit TLS" in the SMTP. Don't
get me wrong. We do have "Implicit TLS" for "SMTP Submission" on port 465.
But we don't have a secure port 25 alternative. i.e. The real SMTPS

465/tcp was already SMTPS, not Implicit TLS.

Both MTA-STS and MTA-DANE tries to fix the STARTTLS downgrade issue.
However the implementation is not simple. The former requires a HTTPS
server and the latter requires DNSSEC.


This document propose a new port 26, an "Implicit TLS" alternative for port
25 and recommends the MX server to signal the port via a prefix.

e.g. should be prefixed like

Where "smtps-" says "We prefer if you connect to our SMTPS in port 26. But
we also accept mails in port 25. And our port 25 supports Opportunistic
TLS. So if STARTTLS command not found in the EHLO response or certificate
is invalid, then drop the connection".

Wouldn't it be simpler to let implementations try 465/tcp first and fall
back to 25/tcp, or use SRV records ?

Gilles Chehade                                                 @poolpOrg                 tip me:

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