ZDKIMFILTER
"z" DKIM filter for Courier-MTA using the OpenDKIM Project's library

current--> v3.5	Tue 01 Jun 2021 12:06:14	[gpg signature]	[announce]	[rel.notes]
v3.4	Thu 20 May 2021 17:05:53	[gpg signature]	[announce]	[rel.notes]
v3.3	Fri 02 Apr 2021 19:04:26	[gpg signature]	[announce]	[rel.notes]
v3.2	Thu 25 Feb 2021 11:02:02	[gpg signature]	[announce]	[rel.notes]
v3.1	Tue 02 Feb 2021 11:02:43	[gpg signature]	[announce]	[rel.notes]
v3.0	Sat 28 Nov 2020 13:11:53	[gpg signature]	[announce]	[rel.notes]

zdkimfilter source repository can also be accessed using Subversion or a web browser. In that case, check READMEsvn before building.
For example, to get the latest sources:

svn checkout http://www.tana.it/svn/zdkimfilter/trunk zdkimfilter

Gentoo: Daniel Black and Hanno Böck added this to gentoo linux.

Debian: Viktor Szépe created the setup, and the debian subdirectory is included in the tarball since version 1.6. That way, tarball users can build a Debian package instead of a local installation obtained by ./configure. An experimental Debian binary package built on AMD (Ryzen) by dpkg-buildpackage -b -uc -us can be found here.

Basic installation: Toby Heywood described his own twist on DKIM + Courier-MTA on CentOS7, based on a previous post by Patrick Contreras. In addition, older tips that may be still worth considering are available in the OLD page Install and Set Up sections.

Complete installation requires a database. The examples included in the distribution are based on MySQL and explained in the DB page. This work can be customized at will, which is why they're called examples. An utility to browse the database is still missing; maybe next version will feature something to tweak per-domain options. Other "obvious" settings, such as managing bounces of sent-out reports, are not even mentioned. Finally, in order to complete DMARC installation, you need to parse aggregate reports received from your targets and possibly feed the database. You may want to consider transforming aggregate reports to more readable HTML using dmarc-xls.

• courier
• libgnutls or openssl*
• libopendbx,
• libnettle,
• libresolv,
• libidn2,
• libunistring,
• zlib,
• uuid,
• the pkg-config utility,
• libtool (a buggy requirement that will be removed from future versions; for v.1.5, libtool-bin is necessary to configure the package)
• In addition, the Public Suffix List is used for DMARC, if configured.

(*) Versions older than 3.0 require libopendkim

(Some old mumbling about where to get required software and how to set up Courier may still be valid.)

The complete installation automates sending DMARC aggregate reports.

As is well known, forwarding modified messages may require to rewrite From:. That is one of the means for mitigating DMARC damage to third party mail. This filter attempts to recognize transformations typical of mailing lists, and adds auxiliary header fields to ease such recognition. This feature seems to work if authors avoid signing list specific fields, such as MIME-Version:, Content-Type: and Content-Transfer-Encoding:.

For sending, there is a Python script to forward mail safely, shielding from strict DMARC policies. Designed for .courier files, the script does a Mailman style mitigation —possibly munge From: and save the original value in the Reply-To:. dmarc_shield3.py is published by Lindsay Haisley.

For receiving, zdkimfilter tries to revert that munging and verify the original author domain's signature. If it succeeds, it sets the header so that a Maildrop instruction can restore the original value of From: after any external forwarding (see zdkimfilter(8) man page).

Keep in mind that DKIM relies on patented technology (US6986049) that may require a separate licence.

As far as software copyright is concerned, zdkimfilter is free software: you can redistribute it and/or modify it under the terms of the GNU General Public Licence as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

zdkimfilter is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public Licence for more details.

As an additional permission under GNU GPLv3 section 7,

If you modify zdkimfilter, or any covered part of it, by linking or combining it with OpenSSL, OpenDKIM, Sendmail, or any software developed by The Trusted Domain Project or Sendmail Inc., containing parts covered by the applicable licence, the licensor of zdkimfilter grants you additional permission to convey the resulting work.