Deploying OpenDMARC on your Postfix mailserver

As a MTA lover, I always try to encourage people (especially “IT-people”) to host their own mailserver. Mostly so they actually can learn something and also that I do not like how the big providers like Google, Microsoft, Amazon etc keep eating up the market. Diversity is a key to a healthy market – but that is another topic.

This guide will mostly apply to Debian-based distros like Debian (9 or newer), Ubuntu (16.04 or newer) or any other “serverdistro”. I do assume that you already have a working mailserver that do both deliver and receive emails that are DKIM signed (or atleast perform validation with OpenDKIM), otherwhise you can read my short guide here (coming soon).

First, install OpenDMARC from the repository.

apt update
apt install opendmarc -y

Verify that the user and group opendmarc has been created by checking /etc/passwd and /etc/group. Otherwhise, create them. When you have installed it, verify the installation by running this:

opendmarc -V

You will get something like this (the version number is not that important yet):

opendmarc: OpenDMARC Filter v1.3.2       
    SMFI_VERSION 0x1000001       
    libmilter version 1.0.1       
    Active code options:               
    WITH_SPF               
    WITH_SPF2

Great! Let's proceed to configuring opendmarc First, take a backup of the current opendmarc.conf, it will save some headache in the future if you want to redo it:

cp /etc/opendmarc.conf /etc/opendmarc.conf.BAK

Edit /etc/opendmarc.conf with the following:

AuthservID [SERVERHOSTNAME]
FailureReports true
PidFile /var/run/opendmarc.pid
RejectFailures false
SPFSelfValidate yes
Socket inet:8893@localhost
SoftwareHeader true
Syslog true
SyslogFacility mail
TrustedAuthservIDs [SERVERHOSTNAME]
HistoryFile /var/run/opendmarc/opendmarc.dat
UMask 0002
UserID opendmarc

Dont forget to restart opendmarc

service opendmarc restart 

Proceed with adding opendmarc as a milter in postfix. I am assuming that you already have opendkim enabled as a milter like this:

smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

We now need to add the opendmarc milter into the postfix configuration, it is important that you add it AFTER the opendkim milter, otherwhise opendmarc will not be able to check if the DKIM key is valid.

smtpd_milters = inet:localhost:8891,inet:localhost:8893
non_smtpd_milters = inet:localhost:8891,inet:localhost:8893
milter_default_action = accept

The last one is pretty important, so if one of your milters does not work for some reason – Postfix will still let it throu. Restart postfix

service postfix restart

We should now be able to test the configuration by sending an email from example a gmail.com account to an email address on your email-server and check your logs if opendmarc actually works.

tail -f /var/log/mail.log | grep "opendmarc"

You should be able to see this:

Apr 26 12:16:38 mx opendmarc[31490]: 5155751C32: SPF(mailfrom): dmarctest@linux.pizza pass
Apr 26 12:16:39 mx opendmarc[31490]: 5155751C32: linux.pizza pass

Great! Your server does now validate DMARC policies! If you just wanted this basic functionality, you are done now. But there is always room for improvement!

Adding a Public-suffix list

This can be achieved in the following simple steps: Create a catalogue (and change ownership) for the list to be downloaded to:

mkdir -p /etc/opendmarc/
chown opendmarc: /etc/opendmarc

Set up a cronjob to download the suffix list once a week

crontab -u opendmarc -e

And this line:

@weekly/usr/bin/wget -k -q -N -P /etc/opendmarc https://publicsuffix.org/list/effective_tld_names.dat

Also, just download the list so you have it before you configure opendmarc to use it:

wget -k -q -N -P /etc/opendmarc https://publicsuffix.org/list/effective_tld_names.dat 

Finally, configure opendmarc to actually use that list, put this on the bottom in /etc/opendmarc.conf and restart opendmarc

PublicSuffixList /etc/opendmarc/effective_tld_names.dat
service opendmarc restart 

Awesome! You are now done with the OpenDMARC. Next up – adding DMARC reporting, this will be in an upcoming post.