The Definitive Guide to ads.txt

What is ads.txt?

Ads.txt stands for Authorized Digital Sellers. It is a simple text file that web publishers host on their root domain to publicly declare which companies are authorized to sell their digital advertising inventory. Introduced by the IAB Tech Lab in May 2017, ads.txt was created as a direct countermeasure to widespread domain spoofing and unauthorized inventory reselling in the programmatic advertising ecosystem.

Before ads.txt, bad actors could easily misrepresent inventory in programmatic auctions. A fraudster could claim to be selling ad impressions on a premium publisher's website when, in reality, the impressions were on a low-quality or fabricated site. This cost advertisers an estimated $8+ billion annually and eroded trust in the entire programmatic supply chain. Ads.txt solved this by giving publishers a simple, open mechanism to authorize legitimate sellers.

How ads.txt Works: A Technical Deep Dive

The mechanics of ads.txt are deliberately simple. A publisher creates a plain text file and places it at the root of their domain, accessible at https://example.com/ads.txt. Each line in the file represents one authorized seller and follows a strict format:

<advertising_system_domain>, <publisher_account_id>, <relationship_type>, <certification_authority_id>

For example:

google.com, pub-1234567890, DIRECT, f08c47fec0942fa0
openx.com, 123456789, RESELLER, 6a698e2ec38604c6
pubmatic.com, 987654, DIRECT, 5d62403b186f2ace

The Four Fields Explained

• Advertising System Domain: The canonical domain of the SSP, exchange, or ad network. This must match the domain used in bid requests. Examples include google.com, openx.com, pubmatic.com, and rubiconproject.com.
• Publisher Account ID: The unique identifier assigned to the publisher within that advertising system. This is the ID the SSP uses in bid requests to identify the inventory owner.
• Relationship Type: Either DIRECT or RESELLER. DIRECT means the publisher holds the account themselves. RESELLER means an intermediary is authorized to sell on the publisher's behalf.
• Certification Authority ID: An optional but strongly recommended field. Typically, this is the seller's TAG (Trustworthy Accountability Group) ID, which provides an additional layer of verification.

How Buyers Use ads.txt

Demand-side platforms (DSPs) and programmatic buyers crawl ads.txt files across the web. When they receive a bid request, they cross-reference the selling system and account ID against the publisher's ads.txt file. If the seller is not listed, the buyer can choose to reject the bid, protecting their advertiser's budget from unauthorized sellers.

This simple cross-referencing mechanism has dramatically reduced domain spoofing. Major DSPs including Google's DV360, The Trade Desk, and Amazon DSP now enforce ads.txt validation, meaning unauthorized sellers are effectively locked out of quality demand.

The ads.txt Adoption Landscape in 2026

Since its introduction in 2017, ads.txt adoption has grown enormously. According to Red Volcano's proprietary crawl data across 32M+ publisher domains:

• Widespread among top publishers: The vast majority of top-traffic publisher websites now maintain an ads.txt file. Among the top 10,000 sites by traffic, adoption exceeds 90%.
• Growing in emerging markets: While adoption was initially concentrated in the US, UK, and Western Europe, markets like India, Brazil, and Southeast Asia have seen rapid growth as DSPs began enforcing ads.txt globally.
• Long-tail gaps remain: Among the broader long tail of publishers, many still lack an ads.txt file or maintain outdated ones, creating opportunities for unauthorized reselling.
• Average file size is growing: The typical ads.txt file has grown from 20-30 entries in 2018 to 100+ entries today, reflecting the complex, multi-SSP monetization strategies publishers use.

Implementing ads.txt: Step-by-Step

Whether you are a publisher setting up ads.txt for the first time or an ad tech professional helping clients, the implementation process follows these steps:

Step 1: Audit Your Advertising Relationships

Start by documenting every SSP, ad network, exchange, and intermediary that monetizes your inventory. For each partner, you will need their canonical domain name and your publisher account ID within their system.

Step 2: Determine Relationship Types

For each entry, determine whether the relationship is DIRECT (you hold the account) or RESELLER (an intermediary holds the account and sells on your behalf). Getting this distinction right matters for supply path optimization.

Step 3: Collect Certification Authority IDs

Ask each partner for their TAG ID or other certification authority identifier. While this field is optional, including it significantly strengthens the verification chain.

Step 4: Create and Upload the File

Create a plain UTF-8 text file named ads.txt and place it at your root domain. Key requirements:

• The file must be at the root: example.com/ads.txt, not www.example.com/ads.txt (though both should work via redirects)
• Use one entry per line
• Lines beginning with # are treated as comments
• The server must return the file with a 200 HTTP status code and appropriate content type

Step 5: Validate and Monitor

After uploading, validate the file using the IAB's official ads.txt validator or a platform like Red Volcano. Monitor your ads.txt regularly to ensure it stays current as you add or remove partners.

Common ads.txt Mistakes and How to Avoid Them

Despite its simplicity, ads.txt files frequently contain errors that can cost publishers revenue or leave gaps in fraud protection:

• Incorrect account IDs: Typos in account IDs are the most common error. A single wrong digit means buyers cannot verify the relationship, potentially blocking legitimate demand.
• Stale entries: Publishers often forget to remove entries for partners they no longer work with. While this does not directly harm revenue, it bloats the file and can mislead buyers about active relationships.
• Missing entries: Conversely, failing to add a new partner to ads.txt means their bid requests will be rejected by DSPs that enforce ads.txt, directly costing the publisher revenue.
• Wrong relationship type: Marking a RESELLER as DIRECT (or vice versa) creates inconsistencies that sophisticated buyers flag. Accuracy here supports supply path optimization efforts.
• Subdomain issues: The ads.txt file must be on the root domain. If your site runs on www.example.com, you still need the file at example.com/ads.txt (or a proper redirect from root to www).
• Encoding problems: The file should be plain UTF-8 text. Uploading it as HTML or with a byte order mark (BOM) can cause parsing failures.

ads.txt and sellers.json: The Complete Picture

Ads.txt works hand-in-hand with sellers.json. While ads.txt is publisher-side (declaring which SSPs can sell the publisher's inventory), sellers.json is SSP-side (declaring which publishers and resellers the SSP works with). Together, they create a bidirectional verification system:

• ads.txt = publisher says "SSP X is authorized to sell my inventory"
• sellers.json = SSP X says "publisher Y is in my network with account ID Z"

When both match, buyers have strong assurance that the supply path is legitimate. The combination of ads.txt and sellers.json, along with the SupplyChain Object (schain) in OpenRTB, creates what the industry calls full supply chain transparency.

How Red Volcano Helps with ads.txt

Red Volcano crawls and parses ads.txt files across 32M+ publisher domains every week, building the industry's most comprehensive ads.txt dataset. Here is how the platform helps different stakeholders:

For SSPs and Ad Networks

• Verify publisher authorization: Instantly check whether any publisher lists you in their ads.txt, with historical change tracking.
• Monitor your footprint: See exactly how many publishers authorize your domain, track growth over time, and compare against competitors.
• Identify unauthorized resellers: Detect unauthorized entities listing your domain in their ads.txt files.

For Agencies and Brands

• Supply path validation: Verify that the supply paths your DSP uses are fully ads.txt-authorized.
• Publisher due diligence: Before placing spend with a publisher, check their ads.txt to understand their monetization stack.

For Publishers

• Competitive intelligence: See which SSPs your competitors authorize and identify potential new monetization partners.
• Error detection: Red Volcano flags common ads.txt errors including mismatched account IDs and stale entries.