DMARC (Domain-based Message Authentication, Reporting, and Conformance) is an email authentication protocol that builds on SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) to protect domains from unauthorized use, such as email spoofing, phishing, and other email-based attacks.
It helps organizations ensure that their email is trusted by recipients and provides mechanisms to monitor and control email delivery. Below is a concise explanation of how DMARC functions in a corporate network or email system:
How does DMARC functions ?
1. Email Authentication with SPF and DKIM:
• SPF: Checks if the sending email server’s IP address is authorized to send emails for the sender’s domain by referencing a DNS TXT record. It verifies the “envelope” sender (Return-Path or MAIL FROM).
• DKIM: Attaches a cryptographic signature to the email, linked to the sender’s domain. The receiving server verifies the signature using a public key published in the domain’s DNS records, ensuring the email’s integrity and authenticity.
• DMARC ensures both SPF and DKIM results align with the sender’s domain in the “From” header (visible to the recipient).
2. DMARC Policy Check:
• The sender’s domain publishes a DMARC policy in a DNS TXT record (e.g., _dmarc.example.com). The policy specifies how receiving mail servers should handle emails that fail authentication.
• Policy options include:
• None: Monitor only; no action taken (used for testing).
• Quarantine: Treat failed emails as suspicious (e.g., send to spam/junk folder).
• Reject: Block failed emails outright (highest protection).
• Example DMARC record: v=DMARC1; p=quarantine; rua=mailto:dmarc-reports@example.com;
3. Alignment Verification:
• DMARC checks identifier alignment to ensure the domain in the “From” header matches:
• The domain in the SPF record (SPF alignment).
• The domain in the DKIM signature (DKIM alignment).
• Alignment can be strict (exact domain match) or relaxed (subdomain match allowed), as specified in the DMARC policy.
4. Receiving Server Action:
• The receiving mail server performs SPF and DKIM checks, then applies the DMARC policy.
• If the email passes both SPF/DKIM checks and alignment, it’s delivered normally.
• If it fails (e.g., spoofed domain or misalignment), the server applies the DMARC policy (none, quarantine, or reject).
• For example, a phishing email claiming to be from example.com but sent from an unauthorized server would fail SPF/DKIM and be quarantined or rejected.
5. Reporting and Feedback:
• DMARC provides aggregate reports (sent to the rua address in the DMARC record) summarizing email authentication results, helping domain owners monitor legitimate and unauthorized email activity.
• Forensic reports (sent to the ruf address) provide detailed failure reports for individual emails, aiding in troubleshooting.
• Reports help organizations refine their email security policies and identify spoofing attempts.
Example Workflow in a Corporate Network:
1. A company (example.com) configures SPF, DKIM, and a DMARC policy in its DNS records.
• SPF: v=spf1 include:_spf.google.com ~all (authorizes Google’s servers).
• DKIM: A public key in DNS for signature verification.
• DMARC: v=DMARC1; p=reject; rua=mailto:dmarc-reports@example.com;
2. An email is sent claiming to be from user@example.com.
3. The receiving server (e.g., Gmail) checks:
• SPF: Is the sending server’s IP authorized by example.com’s SPF record?
• DKIM: Does the email’s signature match the public key in example.com’s DNS?
• DMARC: Do the SPF and DKIM domains align with example.com in the “From” header?
4. If the email passes all checks, it’s delivered. If it fails (e.g., sent from a hacker’s server), Gmail applies the DMARC policy (e.g., rejects the email).
5. The company receives daily DMARC reports showing which emails passed or failed, helping them detect spoofing attempts or misconfigurations.
Key Benefits in a Corporate Network:
• Prevents Spoofing: Stops attackers from impersonating the organization’s domain in phishing or spam emails.
• Improves Deliverability: Ensures legitimate emails pass authentication, reducing the chance of being marked as spam.
• Visibility: DMARC reports provide insights into email sources and potential abuse.
• Compliance: Helps meet regulatory requirements for data protection (e.g., GDPR, HIPAA).
• Brand Protection: Enhances trust by ensuring only authorized emails reach recipients.
Challenges:
• Complex Setup: Requires proper configuration of SPF, DKIM, and DMARC, which can be challenging for large organizations with multiple email sources.
• Third-Party Senders: Legitimate third-party services (e.g., marketing platforms) may fail DMARC if not properly configured.
• Monitoring Overhead: Analyzing DMARC reports requires expertise to identify and address issues.
• Initial Testing: Organizations often start with a “none” policy to avoid accidentally blocking legitimate emails.
Practical Considerations:
• Start with Monitoring: Set the DMARC policy to p=none initially to collect data without affecting email delivery.
• Third-Party Integration: Ensure third-party email services (e.g., SendGrid, Mailchimp) are included in SPF and DKIM configurations.
• Gradual Enforcement: Move to p=quarantine or p=reject after validating legitimate email flows.
• Use DMARC Tools: Platforms like Valimail, Dmarcian, or Proofpoint simplify DMARC setup and report analysis.