What is ads.txt? Everything You Need to Know

What is ads.txt?

Ads.txt (Authorized Digital Sellers) is a simple text file that website publishers place on their domain to publicly declare which advertising companies are authorized to sell ads on their site. If you have ever navigated to a URL like https://cnn.com/ads.txt and seen a list of cryptic entries, you have seen ads.txt in action.

Created by the IAB Tech Lab in 2017, ads.txt was designed to solve one of digital advertising's biggest problems: domain spoofing. Before ads.txt, there was no standardized way for a buyer to verify that the company selling them ad space on a premium website was actually authorized by that website to do so.

The Problem ads.txt Solves

Imagine you are an advertiser who wants to buy ads on a popular news website. In the programmatic advertising world, your ad buying software (a DSP) receives millions of bid requests per second, each claiming to be from different websites. But how do you know the bid request claiming to be from nytimes.com is actually from the New York Times?

Before ads.txt, you could not know for certain. Fraudsters exploited this gap by creating fake bid requests that claimed to represent premium publishers. They would sell this "inventory" to advertisers, pocket the money, and the ads would run on low-quality or non-existent sites. This practice, known as domain spoofing, cost the industry billions of dollars annually.

Ads.txt provides a simple, elegant solution: the publisher themselves declares who is allowed to sell their inventory, in a publicly accessible file that anyone can check.

How ads.txt Works (Simply Explained)

The concept behind ads.txt is straightforward:

1. Publisher creates a file: The website owner creates a plain text file called ads.txt and lists every advertising company authorized to sell their inventory.
2. File goes on the website: The file is uploaded to the root of the website's domain, making it accessible at example.com/ads.txt.
3. Buyers check the file: When a buyer's software receives a bid request claiming to be from example.com, it checks example.com/ads.txt to verify the seller is listed.
4. Unauthorized sellers blocked: If the seller is not in the ads.txt file, the buyer can reject the bid, protecting the advertiser's budget.

What an ads.txt Entry Looks Like

Each line in an ads.txt file represents one authorized seller. Here is an example:

google.com, pub-1234567890123456, DIRECT, f08c47fec0942fa0

This breaks down as:

• google.com — the advertising platform
• pub-1234567890123456 — the publisher's account ID on that platform
• DIRECT — the publisher has a direct relationship with Google (as opposed to going through a reseller)
• f08c47fec0942fa0 — Google's TAG certification ID (an industry trust verification)

DIRECT vs. RESELLER: What is the Difference?

Every ads.txt entry includes a relationship type — either DIRECT or RESELLER:

• DIRECT means the publisher has a direct business account with that advertising platform. They signed up, they manage the account, and the revenue flows directly to them.
• RESELLER means the publisher has authorized a third party (a reseller, ad network, or intermediary) to sell their inventory through that platform. The reseller manages the relationship with the advertising platform on the publisher's behalf.

Both are legitimate, but the distinction matters for supply path optimization. Buyers generally prefer DIRECT entries because they indicate a shorter, more efficient supply path with fewer intermediary fees.

ads.txt Adoption: Where It Stands in 2026

Since its introduction in 2017, ads.txt adoption has grown dramatically:

• Premium publishers: Virtually all major publishers maintain ads.txt files. Among the top 10,000 websites globally, adoption exceeds 90%.
• Mid-tier and long-tail: Adoption decreases among smaller publishers, but the trend is upward as more DSPs enforce ads.txt in their buying logic.
• Global reach: While adoption started in the US and Europe, it has spread globally. Red Volcano tracks ads.txt adoption across 190+ countries.
• File complexity growing: The average ads.txt file has grown from dozens to over 100 entries as publishers work with more monetization partners.

Common ads.txt Mistakes

Despite its simplicity, ads.txt files often contain errors:

• Typos in account IDs: A single wrong character means buyers cannot verify the relationship.
• Outdated entries: Partners that have been dropped but not removed from the file.
• Missing new partners: Forgetting to add a new SSP to ads.txt means their bids get rejected, costing the publisher revenue.
• Wrong domain: The file must be on the root domain (example.com/ads.txt), not a subdomain.
• Formatting errors: Extra spaces, wrong delimiters, or encoding issues that cause parsing failures.

Regular auditing is essential. Tools like Red Volcano can automate this process across millions of domains.

ads.txt in the Broader Ecosystem

Ads.txt is part of a family of transparency standards:

• sellers.json: The SSP-side counterpart to ads.txt. While ads.txt says "these SSPs can sell my inventory," sellers.json says "these publishers are in my network."
• app-ads.txt: The equivalent of ads.txt for mobile apps.
• SupplyChain Object: A bid-request-level signal that traces the full path of each impression through intermediaries.

Together, these standards create what the industry calls supply chain transparency — the ability for any buyer to verify every link between their ad budget and the publisher's content.

How Red Volcano Helps with ads.txt

Red Volcano crawls ads.txt files across 32M+ publisher domains every week, making it the most comprehensive ads.txt dataset available. The platform helps:

• SSPs and ad networks: See which publishers authorize your domain, track your footprint growth, and identify onboarding opportunities.
• Agencies and buyers: Verify supply paths and ensure your programmatic spend flows through authorized sellers.
• Publishers: Benchmark your ads.txt against competitors and identify potential new monetization partners.
• Compliance teams: Monitor ads.txt changes at scale, flag errors, and detect unauthorized sellers.