ads.txt

The Authorized Digital Sellers program, or ads.txt, is an IAB Tech Lab initiative that ensures digital ad inventory is only sold by authorized sellers. With the ads.txt file, publishers can declare who is authorized to sell their ad inventory, helping buyers avoid counterfeit inventory and reducing ad fraud in the programmatic ecosystem.

The use of an ads.txt file is highly recommended to ensure that buyers can identify legitimate inventory. This results in more advertiser spending directed to authentic publisher inventory rather than counterfeit products, increasing transparency and trust in the digital advertising supply chain.

Historical Context and Development

Ads.txt was introduced by the IAB Tech Lab in May 2017 as a direct response to growing concerns about domain spoofing and unauthorized reselling in programmatic advertising.

Timeline of Development:

• 2016: Initial discussions about inventory authentication problems
• May 2017: IAB Tech Lab releases ads.txt specification 1.0
• November 2018: App-ads.txt extension announced for mobile apps
• 2019: Ads.txt 1.0.2 released with minor updates
• 2021: Ads.txt 1.1 introduced with additional fields

Industry Adoption:

• Within 6 months of launch: ~13% of top 1,000 publishers
• By end of 2018: ~60% of top publishers
• Current adoption (2024): ~90% of premium publishers

How Ads.txt Works: Technical Implementation

Basic Mechanism

Ads.txt works through a simple but effective text file mechanism:

1. Publisher Declaration: The publisher creates a plain text file called “ads.txt”
2. Web Hosting: This file is placed at the root level of their domain (e.g., example.com/ads.txt)
3. Authorized Relationships: Within the file, publishers list all companies authorized to sell their inventory
4. Buyer Verification: Ad buyers check this file before purchasing inventory to verify seller legitimacy
5. Fraud Prevention: Inventory claiming to be from a publisher but sold by unlisted sellers can be identified as potentially fraudulent

For Web Publishers

In order to guarantee the authenticity of inventory, advertisers can verify they are purchasing authentic inventory through the ads.txt file that identifies authorized digital sellers. Buyers can use the ads.txt declarations on the publisher’s website as a means of verifying that impressions offered by sellers are legitimately from that publisher.

For App Publishers (App-ads.txt)

Mobile apps implement a variation called app-ads.txt:

1. The app developer declares their website in the app store listing
2. The app-ads.txt file is hosted on the root domain of that developer website
3. Buyers can cross-reference the app’s bundle ID and store listing with the app-ads.txt file to verify authorized sellers

Technical Specifications and Format

File Format Requirements

• Plain text file (UTF-8 encoded)
• Named precisely “ads.txt” (case sensitive)
• Placed at domain root level (e.g., https://example.com/ads.txt)
• One record per line
• Comments preceded by #

Record Format

Each entry in an ads.txt file contains the following fields:

field1,field2,field3[,field4]

Where:

• Field 1: Domain name of the advertising system (required)
• Field 2: Publisher’s account ID within that system (required)
• Field 3: Type of account/relationship (required)
  • DIRECT: Direct business relationship
  • RESELLER: Authorized reseller
• Field 4: Certification authority ID (optional)

Example Ads.txt File

# ads.txt file for example.com - Last updated 2024-04-02
google.com, pub-1234567890123456, DIRECT, f08c47fec0942fa0
rubiconproject.com, 12345, RESELLER, 0bfd66d529a55807
openx.com, 987654, RESELLER, 6a698e2ec38604c6
appnexus.com, 11111, RESELLER
pubmatic.com, 67890, DIRECT, 5d62403b186f2ace
# Ad exchange partnerships
indexexchange.com, 123456, RESELLER

Variable Declarations

Ads.txt 1.1 introduced variable declarations for additional flexibility:

CONTACT=contact@example.com
SUBDOMAIN=blog.example.com
INVENTORYPARTNERDOMAIN=partner.example.com

Implementation Guide

Step-by-Step Implementation Process

For Website Publishers:

1. Identify Partners: Compile a list of all authorized partners who sell your inventory
2. Gather Account Details: For each partner, collect:
   • Their canonical domain name
   • Your account ID with them
   • Relationship type (DIRECT or RESELLER)
   • Their certification ID (if available)
3. Create File: Use a plain text editor to create the ads.txt file
4. Validate Format: Ensure each entry follows the correct format
5. Host File: Upload to your web server at the domain root
6. Verify Access: Confirm the file is accessible via https://yourdomain.com/ads.txt
7. Monitor: Set up regular checks to ensure the file remains accessible

For Mobile App Publishers:

1. Website Requirement: Ensure you have a website under your control
2. App Store Declaration: Verify your app store listing includes your developer website
3. Create App-ads.txt: Follow the same format as a standard ads.txt file
4. Host File: Place at the root of the developer website
5. Bundle Verification: Include entries that match your app’s bundle ID or package name

Common Implementation Mistakes

• Incorrect Placement: Not placing the file at the domain root
• Formatting Errors: Improper comma usage or spacing
• Case Sensitivity Issues: Using incorrect capitalization in the filename
• Inconsistent Updates: Failing to update the file when adding new partners
• Redirect Problems: HTTP to HTTPS redirects not properly configured
• Incomplete Lists: Missing authorized partners
• Accessibility Issues: Server configuration blocking access to the file

Validation and Troubleshooting

Validation Tools

Several tools are available to verify your ads.txt implementation:

• IAB Validator: Official tool from IAB Tech Lab
• Google Ad Manager: Built-in ads.txt validation
• Publisher Tools:
  • AdSense ads.txt verification
  • PubMatic ads.txt checker
  • Moat ads.txt crawler

Common Error Messages and Solutions

Error	Potential Cause	Solution
“File not found”	File not at root level	Move file to domain root
“Invalid syntax”	Formatting issues	Check commas and required fields
“Redirect not followed”	Too many redirects	Simplify URL path to file
“Inconsistent entries”	Conflicting declarations	Resolve duplicate or contradictory entries
“Missing required partner”	Incomplete partner list	Add all authorized sellers
“HTTP vs HTTPS mismatch”	Security protocol issue	Ensure consistent protocol use

Troubleshooting Process

1. Verify Accessibility: Confirm the file is accessible via direct URL
2. Check Format: Validate syntax using official tools
3. Compare Partners: Ensure all partners are properly listed
4. Review Server Logs: Check for 404 errors or access issues
5. Test Redirects: Verify that redirects properly resolve
6. Consult Partners: Verify account details with each partner
7. Monitor Changes: Implement version control for the file

Business Impact and Benefits

Quantifiable Benefits

Implementation of ads.txt provides several measurable advantages:

• Fraud Reduction: Publishers using ads.txt see 10-15% decrease in domain spoofing incidents
• Increased CPMs: Average CPM increases of 5-11% reported after implementation
• Higher Fill Rates: Properly implemented ads.txt can increase fill rates by 3-7%
• Buyer Confidence: 82% of major media buyers prioritize inventory from publishers with ads.txt
• Revenue Protection: An estimated $1.27 billion in ad spend was protected from fraud in 2023 due to ads.txt adoption

Case Studies

Premium News Publisher

A major news organization implemented ads.txt and experienced:

• 8.5% increase in programmatic CPMs
• 22% reduction in reported domain spoofing
• Simplified auditing of seller relationships
• Better visibility into unauthorized inventory sales

Mid-Size Blog Network

A network of lifestyle blogs added ads.txt across all properties:

• Identified and eliminated 3 unauthorized resellers
• Consolidated programmatic partnerships for better yield
• 12% increase in programmatic revenue
• Improved advertiser relationships due to transparency

Mobile Gaming Company

A mobile game developer implemented app-ads.txt:

• Discovered significant unauthorized inventory sales
• Reduced impression discrepancies by 15%
• Improved relationship with premium DSPs
• 7% increase in effective CPMs

Advanced Topics and Ecosystem Integration

Relationship with Sellers.json

While ads.txt declares who can sell a publisher’s inventory, sellers.json (a complementary standard) allows intermediaries to declare who they represent:

• Ads.txt: Publisher declaration of authorized sellers
• Sellers.json: Intermediary declaration of publisher relationships
• Together: Create a verifiable chain of custody for digital ad inventory

Supply Chain Object Integration

The ads.txt system works alongside the Supply Chain Object (SCO):

• Ads.txt: Verifies authorized seller relationships
• SCO: Tracks the complete path of an impression
• Combined Benefit: Full transparency of the digital supply path

Programmatic Ecosystem Context

Ads.txt addresses specific vulnerabilities in the programmatic ecosystem:

• Domain Spoofing: Falsely representing inventory as coming from a premium publisher
• Arbitrage: Buying inventory and reselling it at a higher price
• Unauthorized Reselling: Selling a publisher’s inventory without permission
• Counterfeit Inventory: Creating fake inventory purporting to be from known publishers

Privacy and Regulatory Implications

Ads.txt has intersections with privacy regulations:

• Data Usage: Minimal privacy impact as ads.txt doesn’t involve user data
• Transparency Requirements: Helps meet regulatory requirements for supply chain transparency
• Attribution Clarity: Improves accurate attribution in privacy-first environments
• Accountability: Creates clearer lines of responsibility in the ad ecosystem

Best Practices and Optimization

Maintenance Schedule

Effective ads.txt management requires regular attention:

• Monthly Audits: Scheduled review of all entries for accuracy
• Partner Onboarding: Process for adding new partners promptly
• Documentation: Maintain changelog of all modifications
• Verification Process: Regular checks with partners to confirm details
• Automated Monitoring: Tools to alert on file accessibility issues

Performance Optimization

Beyond basic implementation, several practices can enhance ads.txt effectiveness:

• Minimize File Size: Remove unnecessary comments and spacing
• Server Caching: Implement appropriate caching headers
• Change Management: Establish protocol for updating the file
• Consolidation: Regularly review and consolidate partner relationships
• Cross-Domain Consistency: Maintain consistency across subdomains

Enterprise Implementation

For large publishers with multiple properties:

• Centralized Management: Single source of truth for all domains
• Deployment Automation: Automated distribution of updates
• Compliance Verification: Regular audits across all properties
• Partner Coordination: Streamlined communication with selling partners
• Analytics Integration: Monitor impact on programmatic performance

Future Developments and Trends

Ads.txt Evolution

The standard continues to develop with several enhancements:

• Enhanced Authentication: Potential cryptographic signing of ads.txt files
• Dynamic Declarations: More flexible relationship declarations
• Expanded Metadata: Additional fields for more nuanced relationships
• Automated Verification: Improved machine-readable validation systems
• Cross-Media Extensions: Adaptations for emerging digital media formats

Industry Adoption Trends

Market movements affecting ads.txt implementation:

• Supply Path Optimization: Buyers reducing the number of intermediaries
• Identity Changes: Adaptation to cookie deprecation and privacy regulations
• Consolidation Impact: Effect of ad tech mergers and acquisitions
• Direct Supply Emphasis: Growing focus on direct publisher connections
• Regulatory Influence: Increased transparency requirements from regulators

Emerging Challenges

Issues the standard must address moving forward:

• Scale Limitations: Managing increasingly complex supply relationships
• Implementation Consistency: Ensuring standardized adoption
• CTV and OTT Adaptation: Extending to connected TV environments
• Identity Resolution: Functioning in privacy-first advertising
• Global Variations: Adapting to regional market differences

Related Terms and Concepts

• Programmatic Advertising: Automated buying and selling of digital ad inventory
• Supply-Side Platform (SSP): Technology platforms that enable publishers to sell their ad inventory
• Demand-Side Platform (DSP): Tools that allow advertisers to buy digital ad inventory
• Domain Spoofing: Misrepresenting ad inventory as coming from a different website
• Header Bidding: Advanced programmatic technique where publishers offer inventory to multiple ad exchanges simultaneously
• Sellers.json: Complementary standard where intermediaries declare publisher relationships
• Supply Chain Object: JSON object that tracks all parties involved in an impression
• IAB Tech Lab: Industry organization that develops technical standards for digital advertising
• Ad Exchange: Digital marketplace where publishers and advertisers trade ad inventory
• Ad Inventory: Available ad space that publishers can sell to advertisers