This how-to is created by the Dutch Internet Standards Platform (the organization behind [internet.nl](https://internet.nl)) and is meant to provide practical information and guidance on implementing DMARC.
DMARC is short for **D**omain based **M**essage **A**uthentication, **R**eporting and **C**onformance and is described in [RFC 7489](https://tools.ietf.org/html/rfc7489). 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.
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.
* Parked domain: “DMARC p=reject”. Make sure to include rua and ruf addresses, since this allows monitoring of possible abuse attempts.
* RFC 7489 [states](https://tools.ietf.org/html/rfc7489#section-6.4) 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](https://www.rfc-editor.org/errata_search.php?rfc=7489) states that a semicolon should be included in the DMARC version tag. Correct: "v=DMARC1;". Incorrect: "v=DMARC1".
* When using office 365, the forwarding of calendar appointments from a DMARC projected domain fails. This is a known issue. Read more on the [Office 365 UserVoice forum](https://office365.uservoice.com/forums/264636-general/suggestions/34012756-forwarding-of-calendar-appointments-from-a-dmarc-p) and don't forget to submit your vote!
| p | mandatory | none<br>quaritine<br>reject | None: don't do anything if DMARC verification fails (used for testing)<br>quarantine: treat mail that fails DMARC check as suspicious<br>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 | <br>0<br>1<br>s<br>d | Reporting options for failure reports. Generates a report if:<br>- both SPF and DKIM tests fail (0)<br>- either SPF or DKIM test fail (1)<br>- SPF test fails (s)<br>- DKIM test fails (d) |
| adkim | optional | s<br>r | Controls how strict the result of DKIM verification should be intepreted. Strict or relaxed. |
| aspf | optional | s<br>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. |
For more information about these configuration parameters, take a look at [its man page](https://manpages.debian.org/unstable/opendmarc/opendmarc.conf.5.en.html).
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:
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 parameters as listed below. Notice that the DKIM check (localhost:12301) is done _before_ DMARC (localhost:54321) since DMARC relies on the DKIM results.
