Introduction

This how-to is created by the Dutch Internet Standards Platform (the organization behind internet.nl) and is meant to provide practical information and guidance on implementing DMARC.

What is DMARC?

DMARC is short for Domain based Message Authentication, Reporting and Conformance and is described in RFC 7489. With DMARC the owner of a domain can, by means of a DNS record, publish a policy that states how to handle e-mail (deliver, quarantine, reject) which is not properly authenticated using SPF and/or DKIM.

At the same time DMARC also provides the means for receiving reports which allows a domain's administrator to detect whether their domainname is used for phishing or spam.

Why use DMARC?

Before DMARC, organizations already took several measures to determine the authenticity of an e-mail (like SPF and DKIM) to reduce the received amount of SPAM to a minimum. This is basically a good thing, but if these measures fail to choose whether or not an email is SPAM with a high level of certainty, the choice is redirected to the addressee (receiving party). This methodology is prone to abuse, since users are generally not equiped with the knowledge and/or means to classify incoming emails.

DMARC addresses this problem and enables the owner of a domain to take explicit responsiblity with regard to the actions taken by the sending party when the validity of an incoming email cannot be determined.

Tips, tricks and notices for implementation

Interoperabily issues: https://tools.ietf.org/html/rfc7960
DMARC does not require both DKIM or SPF.
Parked domain: “DMARC p=reject”. Make sure to include rua and ruf addresses, since this allows monitoring of possible abuse attempts.
RFC 7489 states that the tags dmarc-version ("v=") and dmarc-request ("p=") should be on the first and second position of the DMARC record. The order of the other tags does not matter: "components other than dmarc-version and dmarc-request may appear in any order".
Errata 5440 of RFC 7489 states that a semicolon should be included in the DMARC version tag. Correct: "v=DMARC1;". Incorrect: "v=DMARC1".

Creating a DMARC record

The DMARC policy is published by means of DNS TXT record. Overview

rua: aggregate reports ruf: forensic reports

DMARC configuration tag	Required?	Value(s)	Explanation
v	mandatory	DMARC1;
p	mandatory	none quaritine reject	None: don't do anything if DMARC verification fails (used for testing) quarantine: treat mail that fails DMARC check as suspicious reject: reject mail that fail DMARC check
rua	optional	rua@example.nl	This field contains the e-mail address used to send aggregate reports to
ruf	optional	ruf@example.nl	This field contains the e-mail address used to send forensic reports to
fo	mandatory	0 1 s d	Reporting options for failure reports. Generates a report if: - both SPF and DKIM tests fail (0) - either SPF or DKIM test fail (1) - SPF test fails (s) - DKIM test fails (d)
adkim	optional	s r	Controls how strict the result of DKIM verification should be intepreted. Strict or relaxed.
aspf	optional	s r	Controls how strict the result of SPF verification should be intepreted. Strict or relaxed.
pct	optional	0..100	Determine percentage of mail from your domain to have the DMARC verificaton done by other mail providers. Default is 100.
rf	optional
ri	optional
sp	optional

Reporting

to-do

Implementing DKIM with OpenDKIM for Postfix with SpamAssassin

Specifics for this setup

Linux Debian 9.8 (Stretch)
Postfix 3.4.5
BIND 9.10.3-P4-Debian
OpenDMARC v1.3.2

Assumptions

DNSSEC is used
Mail server is operational
Software packages are already installed

Outbound e-mail traffic

DMARC for outbound e-mail traffic can be accomplished by publishing a DMARC policy as a TXT record in a domain name's DNS zone.

Inbound e-mail traffic

DMARC for inbound e-mail traffic can be accomplished by setting up OpenDMARC and integrate it with Postfix.

Set up OpenDMARC

Make sure the file /etc/opendmarc.conf has a least the following configuration options.

AuthservID mail.example.nl
PidFile /var/run/opendmarc/opendmarc.pid
RejectFailures false
Syslog true
TrustedAuthservIDs mail.example.nl,mail2.example.nl,localhost,127.0.0.1
UMask 0002
UserID opendmarc:opendmarc
IgnoreAuthenticatedClients true
IgnoreHosts /etc/opendmarc/ignore.hosts
HistoryFile /var/run/opendmarc/opendmarc.dat
Socket inet:54321@localhost

Make sure the file /etc/opendmarc/ignore.hosts contains all hosts that you trust. The e-mail coming from these hosts will not be checked by OpenDMARC:

127.0.0.1
localhost

Make sure the default file /etc/default/opendmarc contains:

RUNDIR=/var/run/opendmarc
SOCKET=inet:54321@localhost
USER=opendmarc
GROUP=opendmarc
PIDFILE=$RUNDIR/opendmarc.pid

Integrate with Postfix

Now we need to tell Postfix to use OpenDMARC as a mail filter in order to use its functionality. This is done by making sure that /etc/postfix/main.cf contains the configuration values as listed below. Notice that the DKIM check (localhost:12301) is done before DMARC (localhost:54321) since DMARC relies on the DKIM results.

smtpd_milters = inet:localhost:12301,inet:localhost:54321
non_smtpd_milters = inet:localhost:12301,inet:localhost:54321

Set up reporting