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[nmh-workers] FSF is changing Mailman list settings unless you opt out (

From: Ken Hornstein
Subject: [nmh-workers] FSF is changing Mailman list settings unless you opt out (fwd)
Date: Thu, 26 Sep 2019 12:36:41 -0400


I received this email, and I wanted to pass it along.  The executive
summary is: in the near future subject lines to nmh-workers will no
longer be prefixed with "[nmh-workers]" and there won't be a footer
at the end of the message anyone saying that this is the nmh-workers
mailing list.  You can read the details in the message for the complete
technical reasons why this is happening; the other option is to do
what is called "Munge From" and I personally think this is 100x worse
(I am on a Yahoo mailing list where this is done and I hate it).  If
people think we should switch to "Munge From" for this mailing list,
then please make your case here.

--- Begin Message --- Subject: FSF is changing Mailman list settings unless you opt out Date: Thu, 26 Sep 2019 12:25:17 -0400
The Free Software Foundation is making changes to our GNU Mailman

You are being notified because you are listed as a Savannah
administrator ( https://savannah.gnu.org/ or
https://savannah.nongnu.org/ ) for the group(s) that have the following
mailing list(s):


We plan to change the list settings in about one week unless you or
another savannah group administrator or someone who's email is listed in
the Mailman list administration as list owner opts out by replying to
address@hidden and asking to opt out for those lists.

The reason for the change is that messages sent from users with strict
DMARC policy domains like yahoo.com were being rejected when sent to
list subscribers by Mailman. DMARC adoption is growing. See the end of
this email for a technical overview of DMARC and DKIM. There are two
ways to fix the issue by changing Mailman list settings.

The first option, and the preferable way for discussion lists, is what
we call the "unmodified message fix." There are Mailman list settings
which modify the messages by adding a subject prefix (e.g. [list-name])
or a footer. Modifying the message breaks DKIM message signatures and
thus DMARC. Following this option, we will turn those settings off. Many
lists are already this way. Instead of using the subject prefix to
identify a list, subscribers should use the "List-Id" header, To, and
Cc.  List footer information can also be be put in the welcome email to
subscribers and the list information page by list administrators.

The second option is for lists which want or need to continue to modify
the message, for example with subject prefix or footer settings.  In
this case, we set dmarc_moderation_action: "Munge From". With this
setting, if a strict DMARC sender sends to the list, Mailman alters the
headers of that message like so:

A message sent to the list:

To: alist@listdomain
From: Anne Example Person <address@hidden>

Is modified and sent to subscribers as:

To: alist@listdomain
From: Anne Example Person via Alist <alist@listdomain>
Reply-To: Anne Example Person <address@hidden>

Without going into all of the details, here's a few points about why we
concluded the unmodified message fix is better for discussion
lists. Email clients don't all treat munged messages the same way as
unmunged, and humans read these headers so it can confuse people,
causing problems like replies not sent to the expected recipients and
tripping up tools that read emailed patches. GNU Mailman has an option
to do "Munge From" always, but does not recommend using it[1]. While
we're not bound by what others do, it's worth noting that other very
large free software communities like Debian GNU/Linux have adopted the
unmodified message fix[2]. The unmodified messages fix avoids breaking
DKIM cryptographic signatures, which show the message was authorized by
the signing domain and that seems like a generally good thing for

There are a few very rare messages where a DKIM signature is bad and we
still do from munging through Exim: 1. messages from misconfigured email
servers. 2. If a strict DMARC sender's message is modified by list
content filtering settings (attachment removal, or plain-text conversion
for html-only email).

We applied the munge from fix recently as it is initially less
disruptive, but now are switching discussion lists to unmodified
messages unless they opt out. We will notify the list directly when we
make the prefix & footer change.

For any Mailman list administrator who wants to change or look over the
relevant settings: The dmarc_moderation_action setting is under "Privacy
Options" subsection "Sender Filters". The only options that should be
selected are "Accept" or "Munge From", along with corresponding changes
to the subject_prefix option under "General Options", and msg_footer is
under "Non-digest options".

Please send any questions that should be public to address@hidden. For
private ones, just reply to address@hidden.

For the general announcement of these changes and more details, see

A short DMARC technical overview:

DMARC policy is a DNS txt record at a _dmarc subdomain. For example:

$ host -t txt _dmarc.yahoo.com
_dmarc.yahoo.com descriptive text "v=DMARC1; p=reject; pct=100;

The only important thing there for our purpose is p=reject. p=reject
means that conforming mail servers that receive mail with a from header
of *@yahoo.com will reject that email unless it was either 1. sent from
Yahoo's email servers, or 2. its DKIM signature is verified. A DKIM
signature[5] is a public key cryptographic signature of the email body
and some headers included in the message header "DKIM-Signature". A
verified DKIM signature means that email body and signed headers have
not been modified.

Comprehensive resources about DMARC tend to downplay or ignore its
problems, but some that have helped me are Wikipedia[6], the Mailman
wiki[1], dmarc.org wiki[7], and the DMARC rfc[8].

[1]: https://wiki.list.org/DEV/DMARC
[2]: https://lists.debian.org/debian-devel-announce/2015/08/msg00003.html
[5]: https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail
[6]: https://en.wikipedia.org/wiki/DMARC
[7]: https://dmarc.org/wiki/FAQ#senders
[8]: https://tools.ietf.org/html/rfc7489

Ian Kelling | Senior Systems Administrator, Free Software Foundation
GPG Key: B125 F60B 7B28 7FF6 A2B7  DF8F 170A F0E2 9542 95DF
https://fsf.org | https://gnu.org

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