On 2024-01-22 at 14:16:31 UTC-0500 (Mon, 22 Jan 2024 16:16:31 -0300)
Taco de Wolff via Postfix-users <tacodewo...@gmail.com>
is rumored to have said:
Regarding MTA-MTA connections, it seems I didn't fully understand it.
I was
surprised that port 25 (unencrypted) was used for all mail traffic,
but
surely (and hopefully) most connections upgrade with STARTTLS. This
happens
on port 25 though, unlike MUA-MTA connections which happen (regularly)
on
port 587 (and perhaps on port 25 as well?). I was under the impression
that
implicit TLS is (slightly) more secure since the TLS negotiation
happens
right at the start and not somewhere down along the connection.
Implicit TLS has been assigned to port 465, which is the port that the
original Netscape SSLv3 draft had proposed for 'smtps' analogous to
https. That assignment did not survive to the TLSv1 specification which
was derived from that draft. And yet, many MUAs adopted port 465 for
implicit TLS before it failed to make it to an RFC. More recently, the
fact that implicit TLS for the "submission" (MUA-MTA) protocol was de
facto broadly deployed and slightly more secure led to RFC8314.
The reason implicit TLS isn't useful for SMTP (MTA-MTA) use is that port
25 must always be backwards-compatible and so MUST start with a
plaintext server greeting, NOT a TLS handshake. Establishing a new
secure port would mean either every MTA trying to connect twice to sites
that have yet to upgrade or we'd have to finally switch to SRV records
for SMTPS, forcing every MTA to replace its whole DNS logic. Also, the
info disclosure of MTA-MTA STARTTLS vs implicit TLS is less meaningful
than it is for MUA-MTA, where it exposes end user info.
--
Bill Cole
b...@scconsult.com or billc...@apache.org
(AKA @grumpybozo and many *@billmail.scconsult.com addresses)
Not Currently Available For Hire
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