ads.txt (Authorized Digital Sellers) is an initiative from IAB Technology Laboratory. It specifies a text file that companies can host on their web servers (root directory), listing the other companies authorized to sell their products or services. This is designed to allow online buyers to check the validity of the sellers from whom they buy, for the purposes of internet fraud prevention.
Once google (adsense) adopted it, it got widespread usage by publishers.
The file ads.txt is a simple text-file placed in the root-directory of your website ( www.yoursite.com/ads.txt ).
With lines for each authorized publisher.
It look something like this.
google.com, pub-xxxxxxx078, DIRECT, f08xxxxx0
- 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
A complete guide can be found on: https://iabtechlab.com/ads-txt/.
You can also test the ads.txt-file on this site : https://www.adstxtvalidator.com