ads.txt: A quick guide to ad inventory transparency

What is ads.txt?

ads.txt is a simple, text-based standard created by the IAB Tech Lab that publishers place at the root of their domain (for example: https://yourdomain.com/ads.txt). It lists which companies are authorized to sell that site's ad space.

Why it matters

This helps prevent unauthorized sales and ad fraud by making it easier for ad buyers to verify inventory.

How it works

• File format: a plain text file with one line per authorized seller.
• Each line typically includes: domain, selleraccountid, relationship, and optionally certificateauthorityid.
• The most common relationship values are DIRECT or RESELLER.
• The file is hosted at the root of the publisher's site (ads.txt) and can be indexed by crawlers.

How to implement

1. Create your ads.txt file with your authorized sellers.
2. Host it at the root of your domain (https://yourdomain.com/ads.txt).
3. Keep it up to date as you add or remove partners.
4. Validate using ad-tech platforms or crawlers.

Best practices

• Keep it accurate and up to date.
• Limit who can authorize sellers; regular audits.
• Use the certificate_id if your partners publish them.

Common questions

• Do I need ads.txt if I only sell directly? It's still recommended to maintain to support programmatic auctions.
• What happens if I don't have ads.txt? Some buyers may ignore your inventory or treat it as higher risk.