May be of interest to some... ----- Forwarded message from sysadmin@gnu.org ----- From: sysadmin@gnu.org To: bug-mailutils@gnu.org Date: Thu, 24 Oct 2019 16:15:40 -0400 Subject: Fyi: this list, bug-mailutils, just had it's subject [tag] and footer removed List-Id: Bug reports and feature requests for GNU mailutils The Free Software Foundation has changed the GNU Mailman settings on this list. The short version is that any subject prefix or message footer has been removed, allowing us to turn off DMARC from munging. Any list administrator for this list is free to change these settings back, instructions are below. The change is to better deal with increased adoption of the DMARC email standard. The default Mailman settings were causing messages sent from users with strict DMARC policy domains like yahoo.com to be rejected when sent to list subscribers by Mailman. See the end of this email for a technical overview of DMARC and DKIM. There are two main 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, so we just turn those off. Many lists are already this way and there is no change for them. Instead of using the subject prefix to identify a list, subscribers should use the List-Id, To, and Cc headers. List footer information can also be be put in the welcome email to subscribers and the list information page by list administrators. We changed the default for new lists to send unmodified messages, and are now updating existing discussion lists to the new default. We emailed all list administrators and moderators and Savannah group admins to allow them to opt in to the alternate fix before we made this change. However, not all lists had a valid administrator contact. 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 turn on a Mailman list setting called dmarc_moderation_action: "Munge From". With this, if a strict DMARC sender sends to the list, we alter the headers of that message like so: A message sent to the list: From: Anne Example Person Is modified by Mailman and sent to subscribers as: From: Anne Example Person via Alist Reply-To: Anne Example Person 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 messages not to be sent to the expected recipients. 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 seems like a generally good security practice. Tools to manage patches, for example patchew, use the from field and are tripped up by from munging. 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". If no list administrators or moderators are around for this list, anyone should feel free to try to track them down or figure out who should become one and explain in detail by replying to sysadmin@gnu.org. Please be patient, this process may take several weeks. Please send any questions that should be public to mailman@gnu.org. For private ones, just reply to sysadmin@gnu.org. For the general announcement of these changes, please read https://lists.gnu.org/archive/html/savannah-hackers-public/2019-06/msg00018.... and https://lists.gnu.org/archive/html/savannah-hackers-public/2019-09/msg00016.... 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; rua=mailto:address@hidden;"; 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 ----- End forwarded message -----