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.
Contents
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
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 website, you have to add a new TXT record to your websites DNS server. An example subdomain record like this:
_dmarc.example.com TXT v=DMARC1; p=quarantine; pct=20; adkim=s; aspf=r; fo=1; rua=mailto:postmaster@example.com; ruf=mailto:forensic@example.com;
DMARC options in detail
| Tag name | Purpose | Sample |
|---|---|---|
| v | Protocol version | v=DMARC1 |
| pct | Percentage of messages subjected to filtering | pct=20 |
| ruf | Reporting URI for forensic reports | ruf=mailto:forensic@example.com |
| rua | Reporting URI of aggregate reports | rua=mailto:postmaster@example.com |
| p | Policy for organizational domain | p=quarantine |
| sp | Policy for subdomains of the | sp=reject |
| adkim | Alignment mode for DKIM | adkim=s |
| aspf | Alignment mode for SPF | aspf=r |
| fo | Forensic report options | fo=1 |
| rf | Reporting format. either afrf or iodef | rf=afrf |
| ri | Reporting interval of aggregate reports. Often disregarded | ri=86400 |
The alignment modes for DKIM and SPF can be:
- "s" for strict
- "r" for relaxed
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 fail to produce a DMARC pass result
- "1" to generate reports if any mechanisms fail
- "d" to generate report if the DKIM signature failed to verify
- "s" if SPF failed.