OpenDMARC
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.
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 make it accessible to the user the SMTP server run as (likely postfix or mail:
# mkdir /run/opendmarc # chown opendmarc:postfix /run/opendmarc
To have this socket directory created automatically, create the file /etc/tmpfiles.d/opendmarc.conf with the following contents:
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
Postfix integration
Add the following lines to :
Make sure that the DMARC milter is declared after the DKIM milter.
DMARC Record
To enable DMARC for a domain, add a new TXT record to its DNS zone. Below 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 enable the policy, for example, for 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.