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

2019-01-09 13:35:48

On Wed 09/Jan/2019 20:16:27 +0100 Ted Lemon wrote:

Port 26 requires new operational behavior, so it's not simple.   It requires
knowing that it's available, and trying it.   It requires making it 
 It requires deploying lots of new software.

I don't think software is a problem, because on my server (Courier-MTA) I could
do it without even changing a line of code.  Just duplicate 465 settings,
change port number and mandatory login.  For sending, I'd need to set special
routes, which is annoying; adding one line of code is handier.

And it requires allocating a reserved port, and reserved ports are scarce.

Yes.  And publish an RFC.

So that's not a good reason to do it: essentially we are doing a lot of new
work in order to accomplish something that we could already accomplish using
existing software and doing no new work.

Wouldn't the final scenario be better?


On Wed, Jan 9, 2019 at 2:11 PM Alessandro Vesely <vesely(_at_)tana(_dot_)it
<mailto:vesely(_at_)tana(_dot_)it>> wrote:

    On Tue 08/Jan/2019 19:43:25 +0100 Ted Lemon wrote:
    > On Jan 8, 2019, at 12:23 PM, valdis(_dot_)kletnieks(_at_)vt(_dot_)edu
    <mailto:valdis(_dot_)kletnieks(_at_)vt(_dot_)edu> wrote:
    >> Hint: If starttls is subject to a downgrade attack, what prevents the
    same attack
    >> against the same pair of hosts attempting smtps instead?
    > IOW, if the server is only listening on port 26, and the client is being
    > MITM'd, the attacker can listen on port 25 and then tunnel the client
    > connection to the server's port 26.  Only if the client knows that the 
    > supports TLS can you prevent a downgrade, and then STARTTLS works fine. 
    > you need some secure way of signaling this, e.g. DNSSEC, and if you have
    > then you don't need a second port allocation.

    Correct.  So doing port 26 wouldn't get us more security.  However, it 
    seem to get less security either.  I don't see it as a useless 
complication, it
    looks rather like a simplification to me.

    Valdis' citation about 60% of SMTP servers is also correct, but indeed it
    shouldn't be a problem.  I'd change the line about naming to:

       If a mail server support port 26 (smtps), then they MAY (was "should")
       name their MX server with "smtps-" prefix.

    Prefix should never be checked automatically.  Admins every now and then 
    at MX names, and if they start to see those prefixes, they may decide to
    activate their server test-port-26 option.  Gullible, eh?  However, 
    certainly more complicated and costly.  For one thing, you have to pay an 
extra <> certificate (unless you
    already afforded a wildcard).  For
    clients it's much much more work.

    *Port 26 is simple*.  Straightforward for servers that already implement 
    No-brainer for clients.  The only risk is connection timeout on a
    non-interactive job.  Does it hurt?


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