DMARC

Domain-based Message Authentication, Reporting and Conformance or DMARC is a method of email authentication, that is a way to mitigate email abuse. It expands on two existing mechanisms, the well-known Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM), coordinating their results on the alignment of the domain in the From: header field, which is often visible to end users. It allows specification of policies (the procedures for handling incoming mail based on the combined results) and provides for reporting of actions performed under those policies.

History

A group of leading organizations came together in the spring of 2011 to collaborate on a method for combating fraudulent email at Internet-scale. The mission was two-fold:

1. Enable senders to publish easily discoverable policies on unauthenticated email; and
2. Enable receivers to provide authentication reporting to senders so that they can improve and monitor their authentication infrastructure

Their common goal was to develop an operational specification, with the desire that it would be able to achieve formal standards status. The concept behind DMARC was based on practical experience during loose collaborations between some of the founding senders and receivers who agreed on how to interpret mail coming from domains supporting DKIM and SPF. The resulting DMARC specification was published on January 30, 2012 and subsequently publicly circulated as an Internet Draft on March 31, 2013. As of April 2, 2014 it was switched to the Independent Submissions track. (Please see Status of DMARC for more details.)^[1]

After one year, in 2013, DMARC was estimated to protect 60% of the world's mailboxes.^[2]

DMARC aims at standardizing how email receivers perform email authentication, so that senders will experience consistent authentication results for their messages at any email receiver implementing DMARC. In March 2013 the specification was moved to the IETF, intended for the independent stream. The specification creators hope this will encourage senders to more broadly authenticate their outbound email which can make email a more reliable way to communicate.

In April 2014, Yahoo changed its DMARC policy to p=reject, thereby causing misbehavior in several mailing lists.^[3]

Overview

A DMARC policy allows a sender to indicate that their emails are protected by SPF and/or DKIM, and tells a receiver what to do if neither of those authentication methods passes - such as junk or reject the message. DMARC removes guesswork from the receiver's handling of these failed messages, limiting or eliminating the user's exposure to potentially fraudulent & harmful messages. DMARC also provides a way for the email receiver to report back to the sender about messages that pass and/or fail DMARC evaluation.

DMARC is designed to fit into an organization's existing inbound email authentication process. The way it works is to help email receivers determine if the purported message aligns with what the receiver knows about the sender. If not, DMARC includes guidance on how to handle the "non-aligned" messages. DMARC doesn't directly address whether or not an email is spam or otherwise fraudulent. Instead, DMARC requires that a message not only pass DKIM or SPF validation, but that it also pass alignment. For SPF, the message must PASS the SPF check, and the domain in the From: header must match the domain used to validate SPF (must exactly match for strict alignment, or must be a sub-domain for relaxed alignment). For DKIM, the message must be validly signed and the d= domain of the valid signature must align with the domain in the From: header (must exactly match for strict alignment, or must be a sub-domain for relaxed alignment). Under DMARC a message can fail even if it passes SPF or DKIM, but fails alignment.^[4]

DMARC policies are published in the public Domain Name System (DNS) as text (TXT) resource records (RR) and announce what an email receiver should do with non-aligned mail it receives.

To ensure the sender trusts this process and knows the impact of publishing a policy different than p=none (monitor mode), the receiver sends daily aggregate reports indicating to the sender how many emails have been received and if these emails passed SPF and/or DKIM and were aligned.

DMARC may have a positive impact on deliverability for legitimate senders, at least Google recommends the use of DMARC for Bulk senders^[5]

Human policy

DMARC policies are published by Domain Owners and applied by Mail Receivers to the messages that don't pass the alignment test. The domain being queried is the author domain, that is the domain to the right of @ in the From: header field. The policy can be one of none the so-called monitor mode, quarantine to treat the message with suspicion according to the receiver capabilities, or reject to reject the message outright. Reject policy is fine for domains that don't have individual human users, or for companies with firm staff policies that all mail goes through the company mail server, and employees don't join mailing lists and the like using company addresses, or the company provides a separate less strictly managed domain for its staff mail. Strict policies will never be appropriate for public webmail systems where the users will use their mail addresses any way one can use a mail address.^[6]

In fact, human use of a mail address may involve email forwarding from a dismissed address, and mailing lists, which are frequent causes of legitimate breakage of DMARC alignment. Various workarounds have been proposed to cope with domains that publish strict policies unwittingly. For example, a mailing list manager should reject posts from authors who use problematic email domains. The latter behavior is the most respectful the communication protocols as well as the domain owner's will. However, it might cause inconveniences in the face of sudden policy changes. According to John Levine, a well known mail expert, the least intrusive way to temporarily mitigate the damage would be to rewrite the From: address in a predictable, comprehensible manner, such as the following:^[7]

 change
 From: John Doe <user@example.com>
  
 to
 From: John Doe <user@example.com.INVALID>

The .INVALID  top level domain is reserved by RFC 2606 for such kind of usage. In order to apply that change, before re-mailing a message, a mail agent must look up the TXT RR at _dmarc.example.com, if any, and check if it specifies a strict policy. If the change is applied, any recipient who wish to reply to the author can easily find out how to correct the address; in the same way, search engines that crawl mail archives can learn to discard the invalidating suffix.

For a more intrusive workaround, the From: field can be rewritten boldly, thereby taking ownership of the message. The original author's address can then be added to the Reply-To: field.^[8]

Making either change brings the message out of compliance with RFC 5322 section 3.6.2, "The "From:" field specifies the author(s) of the message, that is, the mailbox(es) of the person(s) or system(s) responsible for the writing of the message." Mailbox refers to the author's email address.

The Underscore Issue

Setting up DMARC on a domain requires to create sub-domains starting with an underscore. Unfortunately, some registrars such as 1&1 do not allow the creation of sub-domains starting with an underscore. Registrars allowing such sub-domains include GoDaddy, Network Solutions and OVH. Some registrars, such as Network Solutions, do not support underscores in cname records, which prevents the use of redirecting by cname.^[9][10]

Contributors

The contributors of the DMARC specification include:^[11][12]

• Receivers: AOL, Comcast, Gmail, Netease (163.com, 126.com, 188.com, yeah.net), Outlook.com and Hotmail,^[13] Yahoo! Mail, Mail.ru, XS4ALL, Yandex.ru
• Senders: American Greetings, Bank of America, Facebook, Fidelity Investments, LinkedIn,^[14] PayPal, JPMorganChase, Twitter^[15]
• Intermediaries & Vendors: Agari, Cloudmark, Dmarcian, Netcraft, Return Path, Trusted Domain Project, Symantec^[16]

See also

• Author Domain Signing Practices
• Demarcation point
• E-mail authentication
• Certified email
• Mail servers with DMARC

References

External links

• Official website
• DMARC Record Lookup
• Phishing Scorecard
• Google Apps DMARC documentation
• DMARC monitoring (free)
• DMARC monitoring & tools