ads.txt

What is ads.txt?

ads.txt is an IAB Tech Lab standard that gives website owners a public, crawlable way to declare which advertising systems may sell their inventory and under which account IDs. Each entry also states whether the relationship is direct, meaning the publisher controls the account, or reseller, meaning a third party is authorized to sell on its behalf. Buyers and verification vendors crawl these files and compare them with the seller information in each bid request. Together with sellers.json and the SupplyChain object, it forms the core of supply-chain transparency in programmatic advertising.

How ads.txt works

The publisher places the file at the root of its domain. Each line names a selling system and the publisher’s account in it. Buyers check the file before bidding.

1. The publisher collects the seller account IDs from each platform authorized to sell its inventory.
2. It writes one line per relationship: the ad system domain, the account ID, DIRECT or RESELLER, and an optional certification authority ID.
3. The file is hosted as plain text at the root of the domain so crawlers can fetch it.
4. Buyers crawl the file regularly and match each bid request’s seller ID against the authorized entries.
5. Requests from unlisted sellers are flagged or declined by buyers that enforce the standard.

Why it matters

It makes it harder for unauthorized parties to sell inventory that claims to come from a site. Buyers increasingly avoid supply that is not listed.

Practical example

Key considerations

• Outdated entries are a common cause of lost demand, so the file should be reviewed whenever a partner is added or removed.
• Listing too many resellers can dilute trust and invite unnecessary intermediaries.
• The file confirms authorization only; it does not prove traffic quality or viewability.
• Optional fields such as OWNERDOMAIN and MANAGERDOMAIN help clarify who owns and manages the inventory.