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).

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's 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 a subdomain cannot inherit the organizational domain policy (need to define a self policy)
  • "r" for relaxed: means you don't have to define a policy for each domain. Can inherit from the organizational domain.

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)
  • "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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