I’ve just enabled DMARC support on my mail server (which already supported SPF and DKIM), and since it did not go as smoothly as I would have wanted, here are some notes for those interested.
The mail server is Postfix, running on a Debian Stable (“Jessie” at the time of this writing). The planned setup is as follows:
postfix-policyd-spf-pythonin Debian), called by Postfix when evaluating recipient restrictions;
In the above setup, OpenDMARC relies on pypolicyd-spf and OpenDKIM
adding both a
Authentication-Results header (one for SPF
validation and one for DKIM validation) to the processed message.
However, for some reasons,1 Postfix
never sends the first header of a message to the milters.
Authentication-Results header prepended
by policyd-spf cannot be seen by OpenDMARC.
There are several ways of fixing that problem:
I tested the first option and quickly discarded it, as SPF validation in OpenDMARC is rather buggy—and does not seem to be a priority for OpenDMARC’s developers, as this bug, which leads to false positive results, remains unfixed even though an obvious patch was provided. I also did a (admittedly quick) search for milter-based SPF implementations, but was not convinced by what I found (many of them seem abandoned).
I thus choose the third option, even though it’s kind of a (ugly) hack: I had Postfix prepend a dummy header even before attempting SPF validation, so that the header prepended by pypolicyd-spf could always be seen by the OpenDMARC milter.
I mainly use the default configuration of pypolicyd-spf as provided by the Debian package, save for the two following lines I added:
Header_Type = AR Authserv_Id = mail.incenp.org
The first line instructs pypolicyd-spf to generate a
Authentication-Results header instead of a
Received-SPF header. This is mainly cosmetic: both headers
are allowed by RFC
7208, and recent versions of OpenDMARC can extract SPF results from
both (older versions could not use the
The second line explicitly sets the Authentication Identifier field (RFC 7001, §2.4) that will also be used by OpenDKIM and OpenDMARC.
Integration with Postfix is done through the
smtpd_recipient_rstrictions directive in
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, check_recipient_access regexp:/etc/postfix/recipient_access, check_policy_service unix:private/policyd-spf
/etc/postfix/recipient_access contains the following
line, which prepends the aforementioned dummy header to all
/.\@./ PREPEND X-Incenp: Dummy header
OpenDMARC itself did not cause noticeable problems, save for one
little glitch: the
IgnoreAuthenticatedClients option is
ignored in OpenDMARC 1.3.0 (as provided in Debian Jessie). The bug has
been fixed in OpenDMARC 1.3.1, though, and I decided the bug was
annoying enough to justify pulling down OpenDMARC 1.3.1 from Debian