Domain-based Message Authentication, Reporting, and Conformance, or DMARC, is a technical standard that helps protect email senders and recipients from spam, spoofing, and phishing.
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Domain-based Message Authentication, Reporting, and Conformance, or DMARC, is a technical standard that helps protect email senders and recipients from spam, spoofing, and phishing. DMARC allows an organization to publish a policy that defines its authentification des e-mails practices and provides instructions to receiving mail servers for how to enforce them. In this edition of “DMARC Explained” you’ll learn what DMARC is and how it works.
Specifically, DMARC establishes a method for a domain owner to:
Publier ses pratiques d'authentification du courrier électronique
State what actions should be taken on mail that fails authentication checks
Permettre le signalement de ces actions prises sur le courrier prétendant provenir de son domaine
DMARC itself is not itself an email authentication protocol, but it builds on key authentication standards SPF and DKIM. With them, it supplements SMTP, the basic protocol used to send email, because SMTP does not itself include any mechanisms for implementing or defining policies for email authentication.
Comment fonctionne DMARC ?
DMARC relies on the established SPF and DKIM standards for email authentication. It also piggybacks on the well-established Domain Name System (DNS). In general terms, the process of DMARC validation works like this:
A domain administrator publishes the policy defining its email authentication practices and how receiving mail servers should handle mail that violates this policy. This DMARC policy is listed as part of the domain’s overall DNS records.
When an inbound mail server receives an incoming email, it uses DNS to look up the DMARC policy for the domain contained in the message’s “From” (RFC 5322) header. Le inbound server then checks evaluates the message for three key factors:
Does the message’s DKIM signature validate?
Le message provenait-il d'adresses IP autorisées par les enregistrements SPF du domaine d'envoi ?
Do the headers in the message show proper “domain alignment”?
With this information, the server is ready to apply the sending domain’s DMARC policy to decide whether to accept, reject, or otherwise flag the email message.
After using DMARC policy to determine the proper disposition for the message, the receiving mail server will report the outcome to the sending domain owner.
Qu'est-ce qu'un enregistrement DMARC ?
A DMARC record is included in an organization’s DNS database. An DMARC record is a specially-formatted version of a standard DNS TXT record with a particular name, namely “_dmarc.mydomain.com” (note the leading underscore). A DMARC record looks something like this: _dmarc.mydomain.com. IN TXT “v=DMARC1\; p=none\; rua=mailto:dmarc-aggregate@mydomain.com\; ruf=mailto:dmarc-afrf@mydomain.com\; pct=100”
En lisant de gauche à droite, cet enregistrement dit :
v=DMARC1 specifies the DMARC version
p=none specifies the preferred treatment, or DMARC policy
rua=mailto:dmarc-aggregate@mydomain.com is the mailbox to which aggregate reports should be sent
ruf=mailto:dmarc-afrf@mydomain.com is the mailbox to which forensic reports should be sent
pct=100 is the percentage of mail to which the domain owner would like to have its policy applied
Additional configuration options are available for a domain owner to use in its DMARC policy record as well, but these are the basics.
Que signifie l'alignement des domaines DMARC ?
“Domain alignment” is a concept in DMARC that expands the domain validation intrinsic to SPF and DKIM. DMARC domain alignment matches a message’s “from” domain with information relevant to these other standards:
Pour SPF, le domaine From du message et le domaine Return-Path doivent correspondre.
For DKIM, the message’s From domain and its DKIM d= domain must match
Le alignment can be relaxed (matching base domains, but allowing different subdomains) or strict (precisely matching the entire domain). This choice is specified in the published DMARC policy of the sending domain.
Que sont les politiques DMARC p= ?
The DMARC specification provides three choices for domain owners to use to specify their preferred treatment of mail that fails DMARC validation checks. These “p= policies” are:
none: treat the mail the same as it would be without any DMARC validation
la quarantaine : accept the mail but place it somewhere other than the recipient’s inbox (typically the spam folder)
rejeter : reject the message outright
Remember that the domain owner can only request, not force, enforcement of its DMARC record; it’s up à la inbound mail server to decide whether or not to honor the requested policy.
Qu'est-ce qu'un rapport DMARC ?
DMARC reports are generated by inbound mail servers as part of the DMARC validation process. There are two formats of DMARC reports:
Rapports agrégés, which are XML documents showing statistical data about the messages received that claimed to be from a particular domain. Date reported includes authentication results and message disposition. Aggregate reports are designed to be machine-readable.
Rapports médico-légaux, which are individual copies of messages which failed authentication, each enclosed in a full email message using a special format called AFRF. Forensic report can be useful both for troubleshooting a domain’s own authentication issues and for identifying malicious domains and web sites.
Quel est le lien entre DMARC et SPF, DKIM ou d'autres normes ?
DKIM, SPF et DMARC sont des normes qui permettent différents aspects de l'authentification du courrier électronique. Elles traitent de questions complémentaires.
SPF allows senders to define which IP addresses are allowed to send mail for a particular domain.
DKIM provides an encryption key and digital signature that verifies that an email message was not faked or altered.
DMARC unifies the SPF and DKIM authentication mechanisms into a common framework and allows domain owners to declare how they would like email from that domain to be handled if it fails an authorization test.
Ai-je besoin de DMARC ?
If you are a business sending commercial or transactional email, you definitely need to implement one or more forms of email authentication to verify that an email is actually from you or your business. Properly configuring DMARC helps receiving mail servers determine how to evaluate messages that claim to be from your domain, and it is one of the most important steps you can take to improve your deliverability.
However, standards like DMARC only go so far; MessageBird and other email experts recommend implementing a DMARC email authentication policy in context of a complete messaging strategy.