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

[Alessandro Vesely <[e-mail]>]

I like Viruthagiri's proposal.

On Mon 07/Jan/2019 13:33:39 +0100 John C Klensin wrote:
>>...
>> A simple TXT record saying "This domain's MTAs support
>> STARTTLS (and, possibly, optionally, this is the certificate
>> fingerprint)" would seem useful and not need anything else,
>> and would protect against STARTTLS downgrade for any sender
>> willing to support it.
>> 
>> Obviously it would be vulnerable to DNS pollution, but so
>> would the original proposal.
> 
> Please see at least one of Burt Hubert's "DNS Camel" pieces
> and/or RFC 8324, and think about not only pollution/cache
> poisoning but about the observation that DNS zone managers and
> email administrators are often in separate departments with less
> effective communication than one might like, before going down
> that path.

Well, yes, tweaking host names in MX record would have been a nice hack.  As it
seems such a hack won't work well, however, it's better to save the camel a
useless burden altogether.  Perhaps, the probability of finding an smtps
service on port 26 can be advertised by some other means.

As for port 26, nmap reports[*] a seldom used, unofficial "rsftp" there:

# Fields : Service name, portnum/protocol, open-frequency, optional comments
#
# [...]

telnet	23/tcp	0.221265
telnet	23/udp	0.006211
priv-mail	24/tcp	0.001154	# any private mail system
priv-mail	24/udp	0.000329	# any private mail system
smtp	25/tcp	0.131314	# Simple Mail Transfer
smtp	25/udp	0.001285	# Simple Mail Transfer
rsftp	26/tcp	0.007991	# RSFTP

Would 24 be better?

Rsftp seems to be a buggy ftp-like program.[†]  Google didn't help me to find
much more.  There are some projects on github called rsftp[‡], but none of them
is that one.

Best
Ale

-- 
[*] https://svn.nmap.org/nmap/nmap-services
[†] http://codegrazer.com/blog/rsftp-to-command-injection.html
[‡] https://github.com/tuzhao/rsftp
    https://github.com/cyroxx/rsftp
    https://github.com/Sage-Bionetworks/Rsftp
    https://github.com/rsWinAutomationSupport/rsFTPAdministration (related)