DMARC Definition

DMARC is an open email authentication protocol that provides domain-level protection of the email channel. DMARC authentication detects and prevents email spoofing techniques used in phishing, business email compromise (BEC) and other email-based attacks. Building on existing standards—SPF and DKIM—DMARC is the first and only widely deployed technology that can make the header “from” domain trustworthy. The domain owner can publish a DMARC record in the Domain Name System (DNS) and create a DMARC policy to tell receivers what to do with emails that fail authentication.

DMARC Examples

  • Domain spoofing—An attacker spoofs the domain of a company to make an email seem legitimate.
  • Email spoofing—A term for spoofing activities involving email.
  • Business email compromise (BEC)—An email that appears to come from a senior employee within an organisation requesting that money or sensitive information be sent.
  • Impostor email—A spoofed email sent by an impostor who claims to be someone they are not.
  • Email phishing—An email that tries to get victims to install malware or offer their credentials. A phishing email often looks like a familiar brand to appear legitimate.
  • Consumer phishing—Spoofed email sent to a consumer of a company claiming to be from that company with the intention of stealing credentials.
  • Partner spoofing—Business-based spoofed email between supply chain partners, which attempts to change payment details in order to siphon money.
  • Whaling email scam—Fraudulent email sent to a senior employee within an organisation aiming to get a large financial gain.

DMARC Standards

  • Domain-based Message Authentication Reporting and Conformance (DMARC)—An email validation system that detects and prevents email spoofing. It helps combat certain techniques often used in phishing and email spam, such as emails with forged sender addresses that appear to come from legitimate organisations.
  • Sender Policy Framework (SPF)—An email validation protocol designed to detect and block email. It allows receiving mail exchangers to verify that incoming mail from a domain comes from an IP address authorised by that domain's administrators.
  • DomainKeys Identified Mail (DKIM)—An email authentication method that detects email spoofing. It allows the receiver to check that an email that claims to come from a specific domain was authorised by the owner of that domain.
  • Binding Operational Directive 18-01—The Department of Homeland Security has issued Binding Operational Directive 18-01 for agencies to upgrade their email and web security. Agencies will need to implement SPF, DMARC, and STARTTLS efficiently.

SPF and DKIM Validation

Sender Policy Framework (SPF) is an email validation protocol that allows an organisation to specify who can send email from their domains. Organisations can authorise senders within an SPF record published in the Domain Name System (DNS). This record includes the approved IP addresses of email senders, including the IP addresses of service providers that are authorised to send email on the organisation's behalf. Publishing and checking SPF records is a reliable way to stop phishing and other email-based threats that forge “from” addresses and domains.

Domain Keys Identified Mail (DKIM) is an email authentication protocol that allows the receiver to check that an email from a specific domain was really authorised by the owner of that domain. It allows an organisation to take responsibility for transmitting a message by attaching a digital signature to it. Verification is done through cryptographic authentication using the signer’s public key published in the DNS. The signature ensures that parts of the email have not been modified since the time the digital signature was attached.

How to Verify & Test DMARC

For a message to pass DMARC authentication, it must pass SPF authentication and SPF alignment and/or pass DKIM authentication and DKIM alignment. If a message fails DMARC, senders can instruct receivers on what to do with that message via a DMARC policy. There are three policies the domain owner can enforce: none (the message is delivered to the recipient and the DMARC report is sent to the domain owner), quarantine (the message is moved to a quarantine folder) and reject (the message is not delivered at all).

The DMARC policy of “none” is a good first step. This way, the domain owner can ensure that all legitimate email is authenticating properly. The domain owner receives DMARC reports to help them make sure that all legitimate email is identified and passes authentication. Once the domain owner is confident they have identified all legitimate senders and have fixed authentication issues, they can move to a DMARC “reject” policy and block phishing, business email compromise, and other email fraud attacks. As an email receiver, an organisation can ensure that its secure email gateway enforces the DMARC policy implemented to the domain owner. This will protect employees against inbound email threats.

How DMARC Authentication Works

SPF authentication starts by identifying all legitimate IP addresses that should send email from a given domain and then publishes this list in the DNS. Before delivering a message, email providers will verify the SPF record by looking up the domain included in the “envelope from” address within the hidden technical header of the email. If the IP address sending an email on behalf of this domain is not listed in the domain’s SPF record, the message fails SPF authentication.

For DKIM authentication, the sender first identifies what fields they want to include in their DKIM signature. These fields can include the “from” address, the body of the email, the subject and more. These fields must remain unchanged in transit, or the message will fail DKIM authentication. Second, the sender’s email platform will create a hash of the text fields included in the DKIM signature. Once the hash string is generated, it is encrypted with a private key, which only the sender can access. After the email is sent, it’s up to the email gateway or consumer mailbox provider to validate the DKIM signature. This is done by locating a public key that is an exact match of the private key. Then the DKIM signature is decrypted back to its original hash string.

DMARC Tools and Best Practices

  • Due to the volume of DMARC reports that an email sender can receive and the lack of clarity provided within DMARC reports, fully implementing DMARC authentication can be difficult.
  • DMARC parsing tools can help organisations make sense of the information included within DMARC reports.
  • Additional data and insights beyond what’s included within DMARC reports help organisations to identify email senders faster and more accurately. This helps speed up the process of implementing DMARC authentication and reduces the risk of blocking legitimate email.
  • Professional services consultants with DMARC expertise can help organisations with DMARC implementation. Consultants can help identify all legitimate senders, fix authentication issues and can even work with email service providers to make sure they are authenticating properly.
  • Organisations can create a DMARC record in minutes and start gaining visibility through DMARC reports by enforcing a DMARC policy of “none.”
  • By properly identifying all legitimate email senders - including third-party email service providers—and fixing any authentication issues, organisations should reach a high confidence level before enforcing a DMARC policy of “reject.”