[Ach] BetterCrypto guide - POSTFIX configuration mistake / missing parameter

Sebastian sebix at sebix.at
Sat Oct 15 10:48:30 CEST 2016


On 10/14/2016 02:19 PM, Guillaume REMBERT wrote:
> So I would not qualify it as a bad/weird practice to use port 25 for
> sending mail,
But it is. The RFC states that 25 may be abused for submission because
of backwards compatibility. But we want to leave the past behind us, as
we do with broken crypto. Also note the last sentence from the RFC you

> A site MAY choose to use port 25 for message submission by designating some hosts to be MSAs and others to be MTAs.
A mixed setup with both MSA and MTA on Port 25 is not recommended/allowed.

> but as stated, then there should be reserved hosts for
> MTA and others for MSA (thus the problem of mixing configurations of TLS
> disappears).
> My references on allowed output ports might be too limited and
> it is true that for output TCP/25 port, in France, we have some FAI
> blocking it for botnets/SPAM fighting purposes.
Always depends on the provider.
> PS: I will discuss with OpenVAS team, so maybe they
> could decrease the security warning level of their TLS/SSL
> deprecated ciphers scan when it is linked to an SMTP/25 port?
See this thread:
Continued in October:
Seems they know about the issue and are planning to fix it. But it seems
it is planned to completely drop the check for old ciphers on
mailservers. The author of the linked mail is in now CC.
They have an invalid issuer for their certificate :/

> Le Fri, 14 Oct 2016 13:49:34 +0200,
> Gunnar Haslinger <gh.bettercrypto at hitco.at> a écrit :
>> Full-Quote of Guillaume's mail see below (mail was sent directly and
>> didn't go to the list). 
>> My Opinion about this: Yes, you have to use dedicated
>> submission-ports, that's how it is defined to work. Misusing port 25
>> is a wideseen configuration, but that's not how it was designed in
>> the RFC's. You say popular IT-Networks don't allow outgoing
>> connections to the dedicated submission-ports but allow outgoing
>> connections to port 25? That's weird. My personal experience when
>> traveling and using Public/Hotel/Airport/University/Company-WLANs is,
>> that port 25 is almost everywhere blocked (to prevent outgoing spam
>> from these LANs) but using submission-ports usually works fine. 
>> If you really have this problem feel free to configure your personal
>> client to use Port 25 or host an additional submission port on 443 to
>> go through these firewalls. 
>> Am 2016-10-14 13:34, schrieb Guillaume REMBERT:
>>> OK. I got it! This is driven by the master.cf config with -o
>>> smtpd_tls_security_level=encrypt.
>>> Thanks a lot for your feedbacks and for correcting me.
>>> One last question/remark to fully understand this topic and config.
>>> TLS is under the application layer SMTP. In my original setup,
>>> port 25 is used for both reception of Mail (MTA) and submission
>>> (MSA). How can be done the differenciation between a reception
>>> connexion and a submission connexion? It is not possible as TLS is
>>> done before any application exchange. So I need also to open a
>>> dedicated port reserved for submission as recommended in the doc -
>>> TCP/587?
>>> One problem that I see there is that most IT networks don't allow
>>> output traffic to port 587, thus it is not possible to directly send
>>> mail in most foreign corporate networks - example here-after of an
>>> access provided by a big european organisation:
>>> -     HTTP    TCP / 80
>>> -     HTTPS   TCP / 443
>>> -     SMTP*   TCP / 25
>>> -     POP3    TCP / 110
>>> -     POP3s   TCP / 995
>>> -     IMAP    TCP / 143
>>> -     IMAPs   TCP / 993
>>> -     IPSEC   UDP / 500
>>> -     IPSEC   UDP / 4500
>>> -     OpenVPN UDP / 1194
>>> In that case I would have to establish a VPN in order to send my
>>> mail.
>>> What would be your position related to this strong limitation?
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