DMARC

Why DMARC

To prevent phishing emails and spam, we use SPF and DKIM. However, sometimes real messages may not authenticate properly, and other times fake messages may get accepted. Senders need some way to get feedback on how many emails are being sent and marked as fake. This helps with troubleshooting, improving delivery rates, and detecting fraud.

The Domain-based Message Authentication, Reporting and Conformance (DMARC) provides a way for mail senders and receivers to share this information.

DMARC helps:

reduce false positives
report on how much mail has authenticated
tell the receiver the sender's policy
reduce phishing

Inside a DMARC record, you tell the mail server:

if you are using DKIM, SPF, or both.
how to handle mail that doesn't validate.
if you want a feedback report, and how to report.

Note that DMARC uses DKIM and SPF; it does not replace either.

To use DMARC, you just add a TXT record in your DNS zone:

How it works

Tag	Indicates	Example	Meaning
v	DMARC version	v=DMARC1	First DMARC version; DMARC must be all uppercase; required
pct	Percent of mail to filter	pct=20	Filter 20% of mails; increase slowly over time to detect configurations mistakes gradually
ruf	Reporting URI for forensic reports	ruf=postmaster [snail] example [period] com	Report to postmaster@example.com Warning: make sure the address is inside the current zone or else you need an extra DMARC record
rua	Reporting URI of aggregate reports	rua=postmaster [snail] example [period] com	Report to postmaster@example.com Warning: make sure the address is inside the current zone or else you need an extra DMARC record
p	Policy for domain	p=<value>	Required; applies to domain (and subdomains if sp= not specified)
		p=none	No advice given
		p=quarantine	If checks fail, mail is suspicious
		p=reject	If checks fail, reject mail
sp	Policy for subdomains	sp=<value>	Same as above, but for subdomains only
adkim	Strictness of DKIM headers	adkim=<value>	(Optional; default adkim=r) Checks if d=name matches
		adkim=r	Relaxed; subdomains of d=name are accepted
		adkim=s	Strict; subdomains of d=name not accepted
aspf	Strictness of From headers	aspf=<value>	(Optional; default aspf=r) Checks MAIL FROM (SMTP) and From: header in message
		aspf=r	Relaxed; subdomains of d=name are accepted
		aspf=s	Strict; subdomains of d=name not accepted
fo	When to Report	fo=<value>	(Optional; default fo=0)
		fo=0	Send only if all requested checks fail
		fo=1	Send if any requested checks fail
		fo=d	Send if DKIM fails
		fo=s	Send if SPF fails

Example Records

TXT records are used to store DMARC information to avoid having to upgrade DNS software to support special resource record types.

Permit and Report Everything

_dmarc     IN    TXT "v=DMARC1;p=none;pct=0;fo=1;rua=mailto:postmaster@example.com;ruf=mailto:postmaster@example.com"

Between the two quotation marks "", we put in our DMARC information, which is made up of key=value pairs separated by semicolons ;.

Pair	Meaning
v=DMARC1	First DMARC version
p=none	No advice is given
pct=0	Filter 0% of mails
fo=1	Report all errors from DKIM and SPF
rua=postmaster [snail] example [period] com	Send user aggregate reports to postmaster@example.com
ruf=postmaster [snail] example [period] com	Send forensic reports to postmaster@example.com

This record will provide you with reports for both DKIM/SPF, but will not enforce any filtering whatsoever. This makes this entry very useful for testing out if a new mail server is configured properly. However, this loose configuration may allow more spammers to spoof your domain because bad email is not rejected.

Reject and Quarantine All Failed Mail

_dmarc     IN    TXT "v=DMARC1;p=reject;sp=quarantine;pct=100;fo=1;rua=mailto:postmaster@example.com;ruf=mailto:postmaster@example.com"

Pair	Meaning
v=DMARC1	First DMARC version
p=reject	Reject failed mail from example.com
sp=quarantine	Quarantine failed mail from <subdomain>.example.com
pct=100	Filter 100% of mails
fo=1	Report all errors from DKIM and SPF
rua=postmaster [snail] example [period] com	Send user aggregate reports to postmaster@example.com
ruf=postmaster [snail] example [period] com	Send forensic reports to postmaster@example.com

This rejects and quarantines all mail where DKIM and SPF are not perfectly configured. This is very good at stopping spam and phishing pretending to come from your domain.