How Ads.txt Works and Best Practices

How Ads.txt Works and Best Practices
Since the ads.txt initiative was introduced in 2017, it has seen growing adoption rates throughout the digital publishing and advertising communities, and its impact is felt across the industry. In this post, we will go over what ads.txt is, its benefits, and the best implementation practices we’ve learned since its birth.

What is ads.txt

Ads.txt is an initiative from IAB Technology Laboratory aimed at improving the confidence of brands in buying authentic publisher inventory and preventing internet ad fraud. According to the IAB, its mission is to increase transparency in the programmatic advertising ecosystem.

Download the complete guide to Video Discovery

Ads.txt stands for Authorized Digital Sellers and is a simple, flexible, and secure method that publishers can use to publicly declare, which companies they authorize to sell their digital inventory. Its first version became available on June 27, 2017, and today over 1.2M domains are showing the file.

Who benefits from ads.txt

Well, the entire programmatic ecosystem. Ads.txt helps publishers, advertisers, and everyone in between. Publishers implementing ads.txt and advertisers requiring it, help the industry protect its ad supply chain from fraudsters trying to pose as the publisher's domain and commit what is known as domain spoofing.  

Domain spoofing is an act of misleading programmatic buyers into buying masked sites instead of the sites declared by the seller. It happens when an impostor site offers its inventory as if it was the actual site, leading programmatic buyers to purchase it instead of the legitimate inventory.

Ads.txt overcomes that by committing the site to authorized sellers only. Unfortunately, publishers who don't implement ads.txt are likely to be blocklisted by most buyers, who want to make sure they are buying only authorized traffic.

A publisher who implements ads.txt gives advertisers the assurance that they are buying authenticated inventory, allowing them to buy its inventory with confidence. It also gives the publisher control over its inventory in the market, ensuring the value of their impressions is not diluted by millions of spoofed impressions. A side effect is cleaning up the programmatic industry on the way. 


How ads.txt works

The ads.txt mechanism represents a text (.txt) file that companies host on their web servers, listing the companies authorized to sell their ad inventory. This text file is publicly accessible, allowing programmatic buyers to index and reference it by crawling the web for ads.txt files before purchasing inventory on the open markets. The buyers can match their retrieved data to the data provided in the bid request and make their purchasing decisions accordingly, filtering out unauthorized sellers.

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

ads.txt implementation

A publisher needs to create a text file, name it ads.txt and upload it to the root of its domain.

The file should contain one line for every authorized seller of the domain in the following format:

<Field #1>, <Field #2>, <Field #3>, <Field #4 – Optional >

The core syntax is a comma-separated format with three defined fields and one record per line, as the records are separated by line breaks.

The contents of each field should be filled according to this table (By IAB):

Ads.txt Table

*Comments are denoted by the character "#." Any line containing "#" should inform the data consumer to ignore the data after the "#" character to the end of the line.

You can visit the IAB for a more detailed reference.

Best Practices for ads.txt Implementation

Since ads.txt was adopted, we have learned a lot. Here are 3 of the most important tips we have to share:

  1. Stick to the exact IAB format. Make sure to use the precise number of spaces and commas. 

  2. Keep your ads.txt updated. The whole point of ads.txt is to make sure that only authorized parties can sell your inventory. If there are lines from old sellers you haven’t worked with for years, you leave an open edge. 

  3. Know who is selling your inventory by not adding too many lines. While there is no need to block partners from selling your inventory just to save line space, it is important to keep a handle on who is selling your traffic. If a potential partner gives you 50 lines to implement, make sure you understand why and what value it will give you. 

Introducing App-ads.txt

In order to improve app inventory transparency, the IAB Tech Lab released an extension to the ads.txt initiative customized for apps, the App-ads.txt.

The first beta was released on November 30, 2018, and like its predecessor, was enthusiastically adopted by the industry with a 5,550% growth in app-ads.txt adoption in 2019.

It's basically the app version of ads.txt, and it extends the original ads.txt standard to cover app inventory using a pretty similar method. Instead of posting the ads.txt file on each site's root domain, app publishers post the app-ads.txt file on their developer domain. Buyers can crawl developer websites and gather app-ads.txt data to match the data they receive with a bid request helping them validate the legitimacy of each seller.

Visit this page to learn more about it.

Bottom line(s)

Implementing ads.txt is almost mandatory for publishers wishing to sell their inventory programmatically. However, to get the most out of it, publishers need to keep it updated and make sure they know who the sellers they authorized to sell their inventory are. Ads.txt, together with sellers.json, has become a driving force in cleaning up the advertising landscape, and (finally) it’s becoming a more efficient and transparent ecosystem.