From the IAB Tech site–here is all the info you need to learn how to post an ads.txt file.
Definition: Publisher IDs
What are publisher IDs? Sometimes called the seller network id or the account id,
these are the IDs associated with a Publisher’s account on an exchange or SSP platform. Content owners, publishers and their reseller networks and other businesses may all have accounts on these platforms. As a best practice this ID is transmitted as part of the OpenRTB protocol as the Publisher.ID along with the Publisher.Domain in the Publisher object. In other RTB protocols this may be called ‘seller_network_id’, member or seat ID.
Domain owners will need to post their Publisher ID for every exchange where they have an account. Exchanges already pass a Publisher ID in every call, and it represents the account owner that is getting paid for the media sold from it. Since this is already in every bid request it should be easy for exchanges to provide these IDs to domain owners and networks selling on their exchange.
As a best practice, exchanges should make it easy for domain owners and third party resellers to access their Publisher ID’s for sharing. If an exchange has multiple Publisher.ID’s for a single domain – for example if they have one for a header integration, and another for the tag in the domain owner’s ad server – they will need to provide the domain owner with both ID’s for publication.
Third party sales houses and aggregating networks
Third party sales houses and aggregating networks need to make sure that they provide the full list of exchange-based Publisher ID’s that they use to sell inventory, so that the domain owner can publish that information in their ads.txt file. If the partner doesn’t give the domain owner the correct information to post in the ads.txt file, buyers may not view the inventory as authorized and may value it lower than through a more transparent channel.
HOW TO USE ADS.TXT
Buying platforms (e.g. DSP’s and Networks) can crawl the web to acquire the Authorized Digital Sellers list for every domain. This list will identify the Exchange or SSP that the domain is authorized to be sold on, and the Publisher ID on each exchange that is explicitly authorized to sell the domain. Additionally, ads.txt enables the buyer to determine if the Publisher ID that they are buying the domain from is owned and operated by the publisher, or if it is an authorized seller.
ADS.TXT HELPS PUBLISHERS
Counterfeit inventory comes in many forms, but it typically results in real media spend not reaching legitimate and deserving publishers. Ads.txt helps publishers reclaim control of their media, brand, and rate card. This means more of an advertiser’s spend can get to the domain owner through their approved sales channels, and not be wasted on counterfeit inventory.
ADS.TXT HELPS SSPS/EXCHANGES
SSP’s and Exchanges with direct relationships with domain owners can be harmed by the amount of counterfeit and misrepresented inventory in the market today because it is usually cheaper than the real thing, or outperforms the real media due to less accountable KPI’s. Buyers who generally target domains across multiple exchanges are attracted to the low prices and seemingly “high performance” of the same domain on exchanges that don’t have direct relationships with the domain owners themselves. Sometimes, this cheap and high performing inventory is not actually inventory on the domain that the buyer wanted, but is misrepresenting the domain, format or serving environment to manufacture lower prices and higher performance. This means buyers may end up directing more of their spend to counterfeit inventory than to legitimate publishers on their preferred SSPs and Exchanges partners, harming honest SSPs, Exchanges and Publishers bottom line.
ADS.TXT HELPS ADVERTISERS
By using buying platforms that support ads.txt, Advertisers can be more confident that their working media budget is going to accountable media, and not counterfeit inventory.
Programmatic buying has historically focused on URLs and domains relying on the assumption that they represent authentic inventory. This has left the door open to various types of invalid and fraudulent activities such as creating and selling counterfeit inventory. Ads.txt helps address this problem by giving buyers the choice to only buy from authorized digital sellers of a participating domain.
ADS.TXT HELPS DSPS
DSPs that want to help their advertisers buy in a more secure fashion can now offer their Advertisers Authorized Digital Seller inventory. Ads.txt is not prescriptive about its use, so each DSP can form and create their own approach to advertisers to differentiate their offering from others.
ADS.TXT HELPS THIRD PARTY NETWORKS AND SALES HOUSES
Many domain owners turn to others for help monetizing their inventory. That’s why Ads.txt is built to provide networks and sales houses with a way to differentiate themselves in the market and become authorized sellers of specific domains. With ads.txt, they can make sure that their value proposition or relationship with a domain owner is uniquely valued, and cannot be undercut by counterfeit inventory or other fraudulent behaviors.