What is ads.txt?

ads.txt is a plain-text file a publisher posts at the root of its domain, publicly listing the advertising systems and account IDs authorized to sell its ad inventory, so buyers can check that a seller is legitimate.

Also known as: Authorized Digital Sellers file

How it works

The idea is simple. A publisher writes down, in a public file on its own domain, which companies are allowed to sell its ad space. Buyers and their DSPs crawl that file, and when a bid request claims to come from that publisher, they check whether the seller is on the list. If it is not, they can decline to bid. The IAB Tech Lab specification says the aim is to help remove the profit from counterfeit inventory by giving content owners a way to declare who is authorized to sell it (ads.txt 1.1 specification).

The file lives at /ads.txt on the root domain, plus subdomains where needed, and is served as plain text. Each line is one record with comma-separated fields:

  1. Advertising system domain (required): the canonical domain of the SSP, exchange or similar system that buyers connect to.
  2. Publisher account ID (required): the seller account ID within that system, matching what it sends in bid requests.
  3. Relationship (required): DIRECT if the publisher controls the account, RESELLER if it has authorized another company to.
  4. Certification authority ID (optional): an ID from a certification body. The specification notes this field may be deprecated, because identifiers in sellers.json now supersede it.

Here is an illustrative file using placeholder domains:

# Illustrative only
exchange-one.example, 10001, DIRECT
exchange-two.example, pub-20002, RESELLER
CONTACT=adops@publisher.example
OWNERDOMAIN=publisher.example

The specification also defines variables, written as NAME=value lines. These include CONTACT, SUBDOMAIN, INVENTORYPARTNERDOMAIN, OWNERDOMAIN and MANAGERDOMAIN, which let a publisher point to subdomain files, name the business that owns the property and identify a primary monetization partner (ads.txt 1.1 specification).

Why it matters

For buyers, ads.txt is a first filter against spoofing, where a seller pretends to offer inventory from a site it has no right to sell. For publishers, it is a revenue question. If a legitimate partner's line is missing or has a typo, buyers who enforce ads.txt may skip that path entirely, and the publisher loses demand without an obvious error message. It also feeds supply path optimization, since buyers often favour DIRECT paths.

Example

Example (illustrative)

A small travel blog adds a new SSP but forgets to update ads.txt. Bids through that SSP stay low for weeks. When the blog's team checks, they find the missing line, add it with the correct account ID and relationship, and bidding through that partner returns to normal once buyers recrawl the file.

IncrementX perspective

ads.txt is where a publisher's sales relationships become visible to buyers. When a publisher works with a representation partner, every seller account used to sell its inventory needs to be authorized accurately, or buyers may never see that inventory. Keeping the file tidy is part of good web publisher representation. ads.txt belongs to the wider Authorized Digital Sellers family, and works hand in hand with sellers.json.