Ads.txt Implementation Guide


This document provides the details on Authorized Digital Sellers transparency to restrict fraud by IAB Tech Lab and guidelines for updating the ads.txt file.

Ads.txt is an open standard and technical specification defining how a publisher can publicly declare their list of “Authorized Digital Sellers,” in an “ads.txt” on their site. It is a simple, flexible, and secure way for content owners and distributors to declare who is authorized to sell their inventory, improving transparency for programmatic buyers. The data required to populate the file is readily available in the OpenRTB protocol, making it simple to populate and target. Because publishers sell their inventory through a variety of sales channels, ads.txt supports the following types of supplier relationships:

  • Domain owners who sell on exchanges through their own accounts.
  • Networks and sales houses who programmatically sell on behalf of domain owners.
  • Content syndication partnerships where multiple authorized sellers represent the same inventory.

How to Update Ads.txt

Ads.txt is a simple text file that publishers can host on the root domain of their website that includes information about approved seller relationships for their inventory. (It is similar in concept and hosting to a robots.txt file.)

The standard file format per IAB standard includes:

  • Field #1 = Domain name of the advertising system
  • Field #2 = Publisher’s Account ID
  • Field #3 = Type of Account Relationship
  • Field #4 = Certification Authority ID
Example: publishes ads.txt on their web server listing three exchanges as authorized to sell their inventory,’s seller account IDs within each of those exchanges.

#<SSP/Exchange Domain>, <SellerAccountID>, <PaymentsType>, <TAGID>, 12345, DIRECT, AEC242, 4536, DIRECT, 9675, RESELLER, 23456, RESELLER, AEC242

A buyer receiving a bid request claiming to be can verify if the exchange and SellerAccountID matches the authorized sellers listed in file.

Refer to IAB’s How to Ads.txt and IAB Ads.txt Specification documentation for more details.

How to Use Ads.txt

Buying platforms (DSPs, Networks, etc.) can crawl the ads.txt files to acquire the Authorized Digital Sellers list for every domain. This list informs the platform of the ‘tuple’ (a finite sequential list of elements) of information that provides an authorized way to buy a content owners true inventory. Additionally, ads.txt will tell the buyer if the PubID that they are buying the domain from is owned and operated by the publisher, or if it is an authorized seller.

Advertisers that work with buying platforms that support ads.txt and that allow them to target Authorized Digital Sellers can be secure in their digital media expenditure, and can eliminate the problems associated with buying inventory from unauthorized and unknown counter-parties.

While the IAB Tech Lab is actively thinking about solutions to the below, ads.txt does not currently solve for:

  • Mobile Apps
  • Video Syndication
  • Subdomains
  • Blind/anonymous Inventory

How to implement Ads.txt for PubMatic

Publishers can take the following steps in order to get assistance from PubMatic in implementing their ads.txt files.

  1. Contact your PubMatic account manager for a list of domains, Publisher Ids, and Types of Account relationship.
  2. PubMatic is a TAG member and requiring implementation of our TAG ID also, although it is an optional parameter. PubMatic TAG ID is 5d62403b186f2ace.
  3. Publishers who own their inventory should have their engineering teams update / install ads.txt file for all domains using the content provided.
  4. Authorized resellers will need to contact upstream publishers to have them update their ads.txt files.
  5. Ensure there are multiple lines in the ads.txt file in case of multiple types of relationship. Example below.
  6. Once updated publisher will send confirmation email to PubMatic account manager. 
    Example:, 13579, DIRECT, <5d62403b186f2ace>, 2468, RESELLER, <5d62403b186f2ace>

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