PSA: Mailman changes, From addresses no longer accurate
michael.meeks at collabora.com
Tue Feb 12 10:13:46 UTC 2019
Just to make everyone aware of some of this change. The punch-line is
simple: beware of re-written From: and Reply-To: headers - and check
that your mail is going to whom you think it is.
The behavior will differ depending on the sending domain so - somewhat
Just the messenger ;-)
---------- Forwarded message ---------
From: Daniel Stone <daniel at fooishbar.org>
Date: Mon, 11 Feb 2019 at 23:38
Subject: PSA: Mailman changes, From addresses no longer accurate
To: <freedesktop at lists.freedesktop.org>,
<sitewranglers at lists.freedesktop.org>
We have hit another step change in aggressive anti-spam techniques
from major mail providers. Over the past few days, we saw a huge spike
in the number of mails we were failing to deliver to GMail and
outlook.com in particular.
It looks like it is now no longer acceptable for us to break
DMARC/DKIM/SPF. These are DNS-based extensions to SMTP, which allow
domains to publish policies as to who should be allowed to send email
on their behalf. SPF provides source filtering, so e.g.
freedesktop.org could specify that no-one should accept mail with a
From: *@freedesktop.org unless it came from gabe.freedesktop.org.
Mailman completely breaks this: if I specified a filter only allowing
Google to send mail for @fooishbar.org, then anyone enforcing SPF
would reject receipt of this mail, as it would arrive from fd.o with
my From address.
DKIM allows domains to publish a public key in DNS, inserting a header
into mails sent from their domain cryptographically signing the value
of named headers. Mailman breaks this too: changing the Sender header
(such that bounces get processed by Mailman, rather than sending a
deluge of out-of-office and mailbox-over-quota messages to whoever
posts to the list) can break most DKIM signatures. Mailman adding the
unsubscribe footer also breaks this; we could make it not add the
footer, but in order to do so we'd have to convince ourselves that we
were not decreasing our GDPR compliance.
DMARC ties the two together, allowing domains to specify whether or
not DKIM/SPF should be mandatory, and if they fail, what action should
be taken. Despite some domains specifying a fail action of 'none'
(receiving MTA to send an advisory report to a named email address,
but still allow the email), it seems that popular services still
interpret 'none' as 'reject'.
As a result, Google in particular is dropping some number of our mails
on the floor. This does _not_ just apply to mails which fail
DMARC/DKIM/SPF: every mail we send that fails these filters (which is
a lot - e.g. Intel and NVIDIA both have SPF) decreases our sender
reputation with GMail and causes it to reject legitimate mails.
I've reached out to Google through a couple of channels to see what we
can do to increase our delivery rate to them. In the meantime, if your
mail is hosted by Google, or Outlook, and you think you're missing
mails - you probably are.
Mailman has also now been reconfigured such that if it spots a
potential DMARC violation, it rewrites the From address to be
@lists.freedesktop.org, keeping the original author in Reply-To. It
also strips DKIM headers. This seems to be about the best we can do,
unless and until the major mail service providers offer us some
alternate way to send mail. If you are replying privately to someone,
you should check very carefully that you are replying to them and not
to the list.
Unfortunately we don't have a good answer in the long run. The latest
published RFC at https://tools.ietf.org/html/rfc6377 suggests that
there are no good solutions. If anyone is blessed with an abundance of
time and familiar with the current email landscape, I would love to
talk to you and get your help to work on this. Unfortunately we don't
have the manpower to actually properly monitor email; it can often
take a collapse in successful-delivery rates for us to notice.
Ultimately, I suspect the best solution is to move most of our
discussions to dedicated fora like GitLab issues, or something like
Discourse. Fundamentally, the thing we're trying to do (send email to
thousands of people at a time using a fake From address) is ... kind
of the opposite of what the 2019 Internet wants us to do. Every few
months the major providers drop more of our mail as they become more
aggressive with spam, and every few months their userbase increases by
a non-trivial amount.
We've done a lot of work on our email infrastructure, and are doing
our best to be a responsible citizen within the constraint of having
to launder mail and forge identity on an industrial scale, but it's
coming to the point where it just may not be possible to run such a
service at such a scale anymore.
This is before even considering our other issues with Mailman 2.x: no
centralised identity management (mailing your passwords out every
month ... ?!), difficulty of GDPR compliance (editing archives
requires hand-editing every single HTML index, as there is no
non-destructive archive rebuild), the flat-out bugs (e.g. the mesa-dev
archives are usually missing half the messages), and the fact it's
been abandoned upstream in favour of Mailman 3.x, which is not
obviously better, nor is there a clear upgrade path to.
Of course we do not have any plans to stop providing email any time
soon, but it might be worth thinking about what you can do to reduce
your dependency on email lists. At the current rate of degradation, it
might be non-viable quicker than you'd think. Maybe this is unduly
gloomy, but the entire internet's direction of travel has been away
from services like Mailman, and its velocity is only increasing.
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