Check your ads.txt before it quietly costs you revenue.

An ads.txt file is a plain text list, published at the root of your site, of the companies allowed to sell your ad space. Buyers check it to avoid paying a fraudster who pretends to sell your pages. If it is missing or malformed, legitimate demand can skip you, and nobody tells you.

What the file looks like

Each line has up to four fields separated by commas, plus an optional comment after a #:

google.com, pub-0000000000000000, DIRECT, f08c47fec0942fa0

1. The advertising system’s domain, here Google.
2. Your account ID on that system. For Google AdSense it is pub- followed by 16 digits.
3. The relationship, DIRECT when you hold the account yourself or RESELLER when someone else sells on your behalf.
4. The certification authority ID, optional in the standard but used by Google. Google’s own is f08c47fec0942fa0.

The standard also defines variables such as CONTACT=, SUBDOMAIN=, OWNERDOMAIN= and MANAGERDOMAIN=. The checker recognizes them and does not count them as records.

What this checker does

It requests https://your-domain/ads.txt from our server, follows up to six redirects and reports the chain. It then checks:

• the HTTP status and the content type (it should be text/plain);
• whether the file begins with a byte order mark, a common cause of a broken first line;
• whether the server returned an HTML page, which means a soft 404 or a catch-all route;
• every line: field count, domain syntax, relationship value, duplicates, unknown variables;
• every Google line: the publisher ID format and the certification authority ID.

It keeps nothing. The result lives in your browser and the request in a server log that rotates away after about 14 days.

The errors we see most

Where to put the file

On a static host, drop ads.txt into the folder that is served at the root (public/ in most frameworks). On WordPress, upload it through your host’s file manager or use the plugin your ad network provides. On nginx, the file simply lives in the web root; if a catch-all rule such as try_files $uri /index.html serves your single-page app, make sure it does not swallow /ads.txt.

The other tab: write the Google line

If you only need the line, switch to the second tab, paste your publisher ID and copy the result. It checks the format as you type, so a pasted ca-pub- ID is caught immediately.

For the guide that explains how this file fits into an AdSense setup, read ads.txt explained, and if you are still preparing your first application, start with the important pages for AdSense approval.