OpenDMARC

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OpenDMARC is an open source implementation of the Domain-based Message Authentication, Reporting and Conformance (DMARC) specification.

DMARC is a policy for mail transfer, which is already supported by some common mail providers. It depends on Sender Policy Framework and DKIM. DMARC provides a policy for outgoing mail and checks incoming mails for compliance with that policy. The policy is published via a DNS TXT record. It is explained in #DMARC Record. Validation is done in a daemon. For more info see RFC 7489.

Installation

Install the opendmarc package.

Configuration

Main configuration file is /etc/opendmarc/opendmarc.conf

Change the following options:

/etc/opendmarc/opendmarc.conf

Socket                  unix:/run/opendmarc/opendmarc.sock

Add the socket directory and set its credentials to be accessible to the STMP server user (likely postfix or mail:

# mkdir /run/opendmarc
# chown opendmarc:postfix /run/opendmarc

To have this socket directory created automatically, create the following file /etc/tmpfiles.d/opendmarc.conf:

 D /run/opendmarc 0750 opendmarc postfix

To run the opendmarc.service as the SMTP server user (default is opendmarc:mail) create /etc/systemd/system/opendmarc.service.d/override.conf:

[Service]
Group=
Group=postfix

Note: If you want to run your DMARC-Validator on a different machine, you should change the Socket field to inet:9999@10.0.0.4 with a sample host listening at port 9999 for an optional client 10.0.0.4 (can be omitted, listens on 0.0.0.0 then).

Enable and start the opendmarc.service. Read Daemons for more information.

Postfix integration

Add the following lines to main.cf:

/etc/postfix/main.cf

non_smtpd_milters   = unix:/run/opendkim/opendkim.sock, unix:/run/opendmarc/opendmarc.sock
smtpd_milters       = unix:/run/opendkim/opendkim.sock, unix:/run/opendmarc/opendmarc.sock

Make sure that the DMARC milter is declared after the DKIM milter.

DMARC Record

To enable DMARC for your domain, you have to add a new TXT record to your DNS zone.Here is an example of a DMARC policy, processed one step after another.

First testing, no harm as (sub)policy is "none", but start to receive aggregated reports and failing reports (SPF and DKIM):

_dmarc.example.com TXT v=DMARC1; rua=mailto:postmaster@example.com; ruf=mailto:forensic@example.com; adkim=s; fo=1

After a certain time, after analyzing these reports you can enable the policy for let us say 10% of e-mail traffic.

_dmarc.example.com TXT v=DMARC1; p=quarantine; rua=mailto:postmaster@example.com; ruf=mailto:forensic@example.com; adkim=s; fo=1; pct=10

Then slowly raise the percentage and finalize with policy 100% enabled and only failing reports:

_dmarc.example.com TXT v=DMARC1; p=quarantine; ruf=mailto:forensic@example.com; adkim=s; fo=1

DMARC options in detail

All fields are OPTIONAL unless "v" which is RECOMMENDED.

Tag name	Purpose	Sample
v	Protocol version	v=DMARC1
p	Policy for organizational domain (default "none")	p=quarantine
sp	Policy for subdomains (default value of "p" field)	sp=reject
rua	Reporting URI of aggregate reports	rua=mailto:postmaster@example.com
ruf	Reporting URI for forensic reports	ruf=mailto:forensic@example.com
adkim	Alignment mode for DKIM (default "r")	adkim=s
aspf	Alignment mode for SPF (default "r")	aspf=r
ri	Reporting interval of aggregate reports (default "86400" ; often disregarded to default value)	ri=86400
fo	Forensic report options (default "0")	fo=1
rf	Reporting format. (default "afrf")	rf=afrf
pct	Percentage of messages subjected to filtering (default 100)	pct=20

The alignment modes for DKIM and SPF can be:

"s" for strict: means "strict". Domains from From: shall match DKIM/SPF identifier.
"r" for relaxed: means "relaxed". Organizational domains from From: and DKIM/SPF shall match.

where the latter allows a subdomain in the "From" header while the former does not.

The domain policy (p) and subdomain policy (sp) might be one of:

"none" (for monitor mode)get at
"quarantine"
"reject"

The forensic report options are:

"0" to generate reports if all underlying authentication mechanisms (SPF and DKIM) fail to produce a DMARC pass result
"1" to generate reports if any mechanisms (SPF or DKIM) fail
"d" to generate report if the DKIM signature failed to verify
"s" if SPF failed.

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