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Imported from mukul975/anthropic-cybersecurity-skills (skills/performing-dmarc-policy-enforcement-rollout/SKILL.md). Install upstream with npx skills add mukul975/anthropic-cybersecurity-skills --skill performing-dmarc-policy-enforcement-rollout. Copyright stays with the author (Apache-2.0).

Performing DMARC Policy Enforcement Rollout

Name: performing-dmarc-policy-enforcement-rollout
Author: mukul975

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

Use it

Copy one of these into your project. Installing also returns the manifest and these snippets.

yaml

targets:
  - https://api.opensmartroute.ai/api/v1/registry/mukul975-anthropic-cybersecurity-skills-performing-dmarc-e93ef8/manifest   # or paste the manifest below

Manifest

An Open Capability Manifest: the router reads it to know what this does, what it costs and when to pick it.

mukul975-anthropic-cybersecurity-skills-performing-dmarc-e93ef8.ocm.jsonjson

{
  "ocm": "1",
  "id": "mukul975-anthropic-cybersecurity-skills-performing-dmarc-e93ef8",
  "kind": "skill",
  "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.",
  "publisher": "mukul975",
  "version": "1.0.0",
  "capabilities": {
    "domains": [
      "general"
    ],
    "tags": [
      "skill-md",
      "dmarc",
      "spf",
      "dkim",
      "email-authentication",
      "anti-spoofing",
      "phishing",
      "dns",
      "email-security",
      "skills-sh"
    ],
    "languages": [
      "en"
    ]
  },
  "quality_prior": 0.6,
  "examples": [
    "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."
  ],
  "primary": false,
  "metadata": {
    "source": {
      "provider": "skills.sh",
      "repository": "https://github.com/mukul975/anthropic-cybersecurity-skills",
      "path": "skills/performing-dmarc-policy-enforcement-rollout/SKILL.md",
      "ref": "HEAD",
      "url": "https://github.com/mukul975/anthropic-cybersecurity-skills/blob/HEAD/skills/performing-dmarc-policy-enforcement-rollout/SKILL.md",
      "key": "mukul975/anthropic-cybersecurity-skills/skills/performing-dmarc-policy-enforcement-rollout/SKILL.md"
    },
    "license": "Apache-2.0"
  },
  "instructions": "# Performing DMARC Policy Enforcement Rollout\n\n## Overview\nDomain-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.\n\n\n## Wh",
  "cost": {
    "context_tokens": 1353
  }
}

Fetch it by URL: GET /api/v1/registry/mukul975-anthropic-cybersecurity-skills-performing-dmarc-e93ef8/manifest?version=1.0.0

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