--- name: performing-dmarc-policy-enforcement-rollout description: Execute a phased DMARC rollout by inventorying sending sources, configuring SPF/DKIM alignment, and progressing DNS policy from p=none monitoring through p=quarantine to p=reject enforcement, ensuring all legitimate email sources authenticate before unauthorized senders are blocked. Use when deploying or advancing an organization's DMARC anti-spoofing posture, or when meeting bulk-sender authentication requirements from Google and Yahoo. domain: cybersecurity subdomain: phishing-defense tags: - dmarc - spf - dkim - email-authentication - anti-spoofing - phishing - dns - email-security version: '1.0' author: mahipal license: Apache-2.0 nist_csf: - PR.AT-01 - DE.CM-09 - RS.CO-02 - DE.AE-02 mitre_attack: - T1566 - T1598 - T1534 - T1036 --- # 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:dmarc-agg@company.com; ruf=mailto:dmarc-forensic@company.com; 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:dmarc@company.com; 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 --- Source attribution: mukul975 Original project: https://github.com/mukul975/Anthropic-Cybersecurity-Skills/blob/54a798831d2266a3ca61ce68a7acb80b81160d57/skills/performing-dmarc-policy-enforcement-rollout/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: mukul975 Original project: https://github.com/mukul975/Anthropic-Cybersecurity-Skills/blob/54a798831d2266a3ca61ce68a7acb80b81160d57/skills/performing-dmarc-policy-enforcement-rollout/SKILL.md Attribution basis: Source repository record Fields of work: Marketing, Sales Roles: Customer Success Manager, Email Marketer, Marketing Analyst, PR & Communications, SEO Specialist Specialties: Specialty not specified Tools: Dmarcly, Easydmarc, Valimail, Dmarcian Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 1e385c5e155b1148aa447d5d24376b896405e5680c9c95825a516c4bd3b78338