Performing DMARC Policy Enforcement Rollout
Overview
Domain-based Message Authentication, Reporting and Conformance (DMARC) is the cornerstone of email anti-spoofing protection. A DMARC rollout progresses through three phases: monitoring (p=none), quarantine (p=quarantine), and full enforcement (p=reject). When configured at p=reject, any email that fails both SPF and DKIM checks is outright rejected. Google and Yahoo now require DMARC for bulk senders (5,000+ emails), driving a 65% reduction in unauthenticated messages. The rollout typically takes 3-6 months for safe deployment.
When to Use
- When conducting security assessments that involve performing dmarc policy enforcement rollout
- When following incident response procedures for related security events
- When performing scheduled security testing or auditing activities
- When validating security controls through hands-on testing
Prerequisites
- Administrative access to DNS management for the domain
- Understanding of SPF, DKIM, and DMARC protocols (RFC 7208, 6376, 7489)
- Complete inventory of all legitimate email sending sources
- DMARC reporting analysis tool (EasyDMARC, DMARCLY, Valimail, or dmarcian)
- Email gateway with DMARC enforcement capability
Key Concepts
DMARC Policy Levels
| Policy | Behavior | Use Case |
|---|
| p=none | Monitor only, no action on failures | Discovery phase |
| p=quarantine | Send failing messages to spam/junk | Transition phase |
| p=reject | Block failing messages entirely | Full enforcement |
DMARC Record Anatomy
v=DMARC1; p=quarantine; pct=25; rua=mailto:[email protected]; ruf=mailto:[email protected]; adkim=r; aspf=r; fo=1
- p: Policy for organizational domain
- sp: Policy for subdomains
- pct: Percentage of messages subject to policy (for gradual rollout)
- rua: Aggregate report destination (daily XML reports)
- ruf: Forensic report destination (per-failure reports)
- adkim: DKIM alignment mode (r=relaxed, s=strict)
- aspf: SPF alignment mode (r=relaxed, s=strict)
- fo: Failure reporting options (0=both fail, 1=either fails)
SPF and DKIM Alignment
- SPF Alignment: The domain in the Return-Path (envelope sender) must match the From header domain
- DKIM Alignment: The d= domain in the DKIM signature must match the From header domain
- Relaxed: Organizational domain match (sub.example.com matches example.com)
- Strict: Exact domain match required
Workflow
Step 1: Inventory All Sending Sources (Week 1-2)
- Audit all systems sending email as your domain (marketing, CRM, ticketing, transactional)
- Document third-party services: Salesforce, Mailchimp, SendGrid, Zendesk, etc.
- Identify internal mail servers, applications, and relay hosts
- Check for shadow IT email sending (departments using unauthorized services)
Step 2: Configure SPF and DKIM (Week 2-4)
- Consolidate SPF record with all legitimate sending IPs and includes
- Ensure SPF record stays under 10 DNS lookup limit
- Generate and publish DKIM keys for each sending source
- Verify DKIM signing works for all outbound mail paths
- Test with MX Toolbox or dmarcian SPF/DKIM validators
Step 3: Deploy DMARC in Monitoring Mode (Week 4-6)
- Publish initial DMARC record:
v=DMARC1; p=none; rua=mailto:[email protected]; fo=1
- Wait 1-2 weeks to collect representative aggregate reports
- Analyze reports to identify unauthorized senders and alignment failures
- Fix SPF/DKIM for all legitimate sources showing failures
- Iterate until all legitimate mail passes DMARC
Step 4: Move to Quarantine with pct Tag (Week 6-12)
- Update to quarantine at 10%:
v=DMARC1; p=quarantine; pct=10; rua=...
- Monitor for false positives (legitimate mail being quarantined)
- Increase pct gradually: 10% -> 25% -> 50% -> 75% -> 100%
- Each increase: wait 1-2 weeks and review reports before advancing
- Fix any remaining alignment issues discovered at each stage
Step 5: Advance to Reject Policy (Week 12-20)
- After stable quarantine at 100%, move to reject at 10%:
v=DMARC1; p=reject; pct=10; rua=...
- Gradually increase pct: 10% -> 25% -> 50% -> 100%
- Monitor closely for legitimate mail being rejected
- Establish emergency rollback procedure (revert to quarantine)
- Apply subdomain policy:
sp=reject for subdomains
Step 6: Ongoing Monitoring and Maintenance
- Continuously monitor DMARC aggregate reports
- Add new sending sources before they start sending
- Review forensic reports for spoofing attempts
- Maintain SPF record as sending infrastructure changes
- Rotate DKIM keys annually
Tools & Resources
- EasyDMARC: DMARC monitoring dashboard with aggregate/forensic report analysis
- DMARCLY: SPF, DKIM, DMARC monitoring with auto-DNS updates
- dmarcian: DMARC deployment and management platform
- Valimail: Automated DMARC enforcement with hosted authentication
- MX Toolbox: DNS record lookup and DMARC validator
- Google Admin Toolbox: DMARC check and diagnostic tools
Validation
- DMARC record published and resolving correctly at _dmarc.domain.com
- All legitimate sending sources pass SPF and/or DKIM alignment
- Aggregate reports show >99% legitimate mail passing DMARC
- Spoofed messages from unauthorized senders are rejected
- No legitimate mail blocked after full p=reject enforcement
- Subdomain policy (sp=) also set to reject