[Mailman-Users] Fwd: Re: Mailing list membership.

Stephen J. Turnbull turnbull.stephen.fw at u.tsukuba.ac.jp
Thu Mar 2 04:38:30 EST 2017

willi uebelherr writes:

 > Dear friends of Mailman,

Thanks for getting in touch!

 > in the IETF discussion list we have a discussion about the bounces, that 
 > are created based on the DMARC processing.
 > I know, from a discussion in this mailman list, that mailman follow 
 > strong the RFC 2821 (SMTP) and reject this DMARC nonsense.

RFC 2821 has little practical significance for DMARC.  DMARC
processing itself has very little to do with SMTP (except for the
indirect relation through SPF -- more about that below).  I have all
the respect in the world for Ted T'so, but he seems to be not very
familiar with DMARC, DKIM, and SPF.

Mailman can not and does not "reject" DMARC in any way (although some
Mailman developers are pretty unhappy about it ;-).  DMARC is a fact
of life for mailing lists now.

First, the good news:
I (as a representative of the GNU Mailman project, as well as for
personal interest) have been participating in the DMARC discussions as
well as in development of the new "Authenticated Received Chain" (ARC)
protocol.  ARC allows an intermediate site to cryptographically sign
its authentication results, which (we hope) will allow end receivers
to trust those for DMARC purposes.  However widespread implementation
is at least a year off (theoretically possible as the DMARC
Consortium, Google, Yahoo!, and AOL are all participating in ARC
discussions, but you know how the Internet moves).

Mailman 3 itself will be ARC-capable in limited ways in that time
frame, and we expect Postfix, Exim, and Sendmail to implement it as
well.  (The "big guys" use MTAs developed in-house.)

Now, the bad news.

 > The consequence should be, that the members in the list should change 
 > her mailbox servers to avoid this "bounce errors", based on DMARC 
 > processing, or not?

>From the list owner's point of view, "it would be nice if" posters
would post from DMARC "p=none" domains.  ("p=none" includes sites
which have no DMARC DNS record at all; it is the implicit default that
DMARC receivers must assume.)  However, in the great majority of
cases, users get very upset about being asked to use "p=none"
mailboxes if that means changing their workflows.  IETF mailing lists
*may* be an exception, but I suspect there are a lot of people using
Yahoo! and other DMARC-protected addresses in the IETF subscriber base.

 > For me, i don't like any form of work arounds. We need a clear
 > base. And this can only be the standard RFCs.

Unfortunately, there are no RFCs that help here.  DMARC is *intended*
to prevent "unwanted" indirect mail flows, and unfortunately the
heuristic chosen also will interdict mailing lists.  What the DMARC
RFC *should* say is "if your mailbox provider posts a DMARC policy
other than 'p=none', that is automatically a policy of prohibiting
posting to mailing lists".  And that is precisely why the DMARC RFC is
"Informative", rather than "Standards Track", as you would expect with
an RFC with so much promise for reducing phishing and other mail
abuse.  The DMARC Consortium really wanted to avoid any language that
would implicate DMARC users is negative effects of the standard, and
an informative RFC allowed the Consortium members to retain control.

The only RFC-conforming option that allows you to avoid triggering
DMARC backscatter is to turn off ALL message-modifying options:
subject tags and serial numbers, and body headers and footers.  (I
believe that is currently the complete list of Mailman features that
invalidate common DKIM signatures.)

Other than that, your choice is to turn your "p!=none" posters into
backscatter bombs, or use of one of the options that Mailman provides
to avoid triggering DMARC.  There are more, and more attractive,
options in the most recent Mailman 2 versions (at least 2.1.20), as
Jim Popovich mentioned.

I hope this helps.

Below some comments on technical aspects of the appended

 > 3) The IETF discussion list don't follow the DMARC processing. This 
 > means, it act only outside.

DMARC processing, by which I mean the protocols defined in RFC 7489,
is in no way done by mailing lists.  Anything that a mailing list or
its MTA does to handle DMARC is a "workaround".  (This will change
with ARC, but ARC will provide no guarantees -- use of ARC is entirely
optional for the receivers.)

 > I understand and agree absolutly, that the maillist server never change 
 > the From-line in the header.

Changing the From field in the RFC 2822 header is the most popular
workaround by far.  This is not a denial of your position, it's a
statement of current common practice.

 > The mailman maillist server use bounce-counters for every member and 
 > some limits for this bounce-counter. If the limit exceeds, and the 
 > admin-group do nothing, then the maillist server mailman disable the 
 > delivery. It is not an unsubscription.

Under some setting of the bounce processing in Mailman, it *can*
result in unsubscription.

 > The DMARC processing is defined in the DNS info. But we can ignore it, 
 > or not?

If you ignore it, your list(s) will be subject to adverse consequences
based on DMARC processing at poster and subscriber addresses.  That's
your choice, of course.

 > Based on that process, we can clean all this nonsense in our IETF
 > lists environment and work strong based on the RFC 2821, like
 > mailman do it.

As Jim Popovich mentioned, the IETF lists are handled by GNU Mailman.
According to the most recent message I received it is Mailman 2.1.18.

Ted T'so writes:

 > > RFC 2821, Simple Mail Transfer Protocol, section 3.10.2
 > >
 > >    "To expand a list, the recipient mailer replaces the
 > >    pseudo-mailbox address in the envelope with all of the expanded
 > >    addresses.  The return address in the envelope is changed so that all
 > >    error messages generated by the final deliveries will be returned to
 > >    a list administrator, not to the message originator, who generally
 > >    has no control over the contents of the list and will typically find
 > >    error messages annoying."
 > >
 > > This is the SMTP Envelope From field.  The FROM field is not changed,
 > > but the SMTP return address is changed, so that bounces go to the
 > > mailing list administrator as opposed to the person who sends mail to
 > > the mailing list.

This is true, but it is not part of the definition of mailing list,
nor is there any popular mailing list software left that doesn't give
you the option of fiddling with the FROM field (for several reasons
such as anonymization, as well as working around DMARC).

 > > Unfortunately, if you are using a system whose domain requests that
 > > all recipients enforce DMARC alignment, this specifically instructs
 > > recipients to bounce mail if the SMTP Envelope return address doesn't
 > > match the FROM field in the header.

This is a misunderstanding of the DMARC protocol.  I won't go into
details, but in practice the vast majority of originating domains use
DKIM as well as, or instead of, SPF.[1]  In the case of DKIM
verification, the authorizing domain is the one claimed in the
DKIM-Signature field, not the Envelope From.

 > > Hence mailing list systems that enforce DMARC, or request DMARC
 > > processing, are fundamentally incompatible with mailing lists as
 > > defined by section 3.10.2 of RFC 2821.

This is false.  In practice, a mailing list that does not alter the
From, Subject, Date, and Message-ID header fields and does not alter
the body is compatible with DMARC originators which provide DKIM
signatures.  That's pretty much all of them, as mentioned above.

As I mention above, Mailman can be configured that way, but it rarely
is.  A few list-owners run into legal issues where the list must
provide visible legal disclaimers or unsubscription instructions,
which typically occur in footers appended to the message body,
invalidating the DKIM signature.  I doubt such a legal requirement
applies to the IETF lists, however.

Hope this helps.


[1]  According to a source at DMARC Consortium, their analysis
shows that nearly 100% (I forget the exact figure) of p=reject
traffic is DKIM-signed.  Of course that's heavily weighted by Yahoo!
and AOL, the two biggest sources of p=reject traffic.

Associate Professor              Division of Policy and Planning Science
http://turnbull/sk.tsukuba.ac.jp/     Faculty of Systems and Information
Email: turnbull at sk.tsukuba.ac.jp                   University of Tsukuba
Tel: 029-853-5175                 Tennodai 1-1-1, Tsukuba 305-8573 JAPAN

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