What is ads.txt?
Ads.txt, short for Authorized Digital Sellers, is a plain text file that a publisher hosts at the root of its domain, for example example.com/ads.txt. It lists every ad system and account ID allowed to sell or resell that site's inventory. Buyers and DSPs check the file and can refuse to bid through sellers that are not listed.
The IAB Tech Lab introduced the standard in 2017 to fight domain spoofing, a type of ad fraud where low-quality inventory is passed off as a premium site. It is now a baseline expectation across programmatic buying.
How ads.txt works
Each line in the file describes one authorized seller with up to four comma-separated fields:
Domain of the ad system, such as the exchange or SSP domain.
Publisher account ID within that system.
Relationship type: DIRECT if the publisher controls the account, RESELLER if a third party sells on its behalf.
Certification authority ID (optional), usually the TAG ID of the ad system.
A typical line looks like this: adsystem.com, 12345, DIRECT, abc123. When a bid request arrives, the DSP compares the seller domain and account ID against the publisher's crawled ads.txt file. If there is no match, the impression is treated as unauthorized and most buyers will not bid on it.
The same idea was extended to apps with app-ads.txt. The developer hosts the file on the website listed in the app store, and it covers mobile and CTV apps.
Why ads.txt matters
Less spoofing: fraudsters cannot easily claim to sell a premium domain if they are not on its list.
Cleaner supply paths: buyers can see which sellers are direct and which are resellers, a starting point for supply path optimization.
Revenue protection for publishers: budget that would go to fake inventory has a better chance of reaching the real site.
Ads.txt is not a full fraud solution. It does not detect bots or confirm where an ad actually appeared, so it works best alongside verification and brand safety tools.
Ads.txt vs sellers.json
Ads.txt is the publisher side of the story: here are the sellers I authorize. Sellers.json is the ad system side: an SSP or exchange publishes a file listing the sellers it pays, with their seller IDs and whether each is a publisher or an intermediary. Together with the SupplyChain object, which records every hop in a bid request, they let buyers trace an impression from publisher to DSP and spot unnecessary resellers. All three are IAB Tech Lab standards.
Ads.txt with AdGeeks
DV360 lets buyers limit line items to authorized sellers, and supply quality checks apply across Amazon DSP and Adform as well. Our traders go further, favoring direct, authorized paths and reviewing placement reports so your budget lands on real publishers. If you want campaigns built on clean supply, request access.
