If your site shows ads, ads.txt is the public list of companies allowed to sell your ad space. Buyers check it before bidding, so it must be present, correct and updated whenever a partner changes.

One file at the root of the site

Every site that carries advertising should have a small plain text file at one fixed address: the domain name, followed by /ads.txt.

It lists the companies allowed to sell the site's ad space, each with the account number they sell it under. Nothing else.

Plenty of owners of small ad-supported sites have never opened theirs. Some do not have one, and some have a file that was correct two partners ago.

Try that address in a browser now. If what appears is a page from your website rather than a short list, that is the first finding.

Why buyers check it

Ad space on most sites is sold automatically, in auctions that run while the page loads. The buyer rarely deals with the site; it deals with exchanges and networks acting for it.

That chain made one kind of fraud easy: selling cheap or fake ad space while claiming it came from a well-known site. Buyers had no simple way to check the claim.

The IAB Tech Lab, an industry standards body, introduced ads.txt in 2017 to close that gap. The name is short for authorised digital sellers.

The site itself publishes who may sell its space, and anyone can read the list. Many of the platforms that buy ad space now check it before they bid.

What each line says

Each line is one authorisation, written in a fixed order with commas between the parts.

The third is the one that needs the most care. DIRECT means the business holds the account itself; RESELLER means a partner sells your space through an account of its own.

Some buyers treat the two differently, so the word has to match the actual arrangement, not the one that sounds better.

When the file is missing

With no file at all, a buyer has nothing to check your space against.

Some buyers carry on regardless. A growing number treat space they cannot verify as a risk, and bid less for it or leave it alone.

Nothing on your site shows this happening. The ads still appear, filled by whoever is willing, and the buyers who passed leave no trace in any report you see.

Which is why a missing file can go unnoticed for years. The cost is real and invisible.

When the file is wrong

A wrong file can do more harm than a missing one, because it tells buyers something definite.

Leave out a current partner, and buyers who check will treat that partner's offers of your space as unauthorised. The partner keeps trying to sell it; the careful buyers keep refusing.

Copy lines from another site's file and you have authorised strangers while leaving out your own accounts.

Small errors count too: a mistyped account number, a missing comma, the wrong relationship. A line a machine cannot read authorises nobody.

Where it has to live

The file belongs at the root of the domain, not inside a folder, and it has to come back as plain text.

Problems here are rarely visible to the owner. A security rule blocks the file, a redirect sends it to the home page, or the site serves a styled error page in its place.

On a WordPress site it is usually a separate file beside the site's own, or something a plugin produces. Either way it is not a page in the editor, so nobody thinks of it.

That is how it gets lost. A rebuild or a move to new hosting can leave the file behind, and removing a plugin during a clean-up can take it with it.

A worked example

A community news site covering a handful of small towns in the Interior carries ads through two partners, the second added last spring.

Over the summer the site was rebuilt on new hosting. The developer moved the articles, the images and the design. The ads.txt file had been uploaded by hand years earlier, and it was on nobody's list.

The new site went live with no file at all. The ads kept appearing, so nothing looked wrong.

It came to light in January, when the newer partner noticed its lines were missing. The editor typed the address into a browser and got the site's own error page.

The fix took ten minutes: current lines from each partner, a comment above each naming the partner and the date, and the file added to the maintenance checklist with the editor as its owner.

Keeping it current as partners change

The file is accurate only as of the day it was last edited, and ad partners change more often than most owners realise.

Adding a partner means adding its lines, exactly as the partner supplies them. Ending one means removing them, so an old account is no longer authorised to sell your space.

Partners also change their own arrangements, and may ask you to add lines for companies that resell through them. Those requests arrive by email and are easy to file and forget.

Treat each one like a change to your banking details: done promptly, by one named person, and checked afterwards in a browser.

Note which partner each line belongs to and when it was added. Comment lines in the file itself are the simplest place for that note.

The counter-case

Not every site with ads needs to think about this often.

A site that shows ads through a single partner, on an arrangement unchanged for years, has a short file and little to go wrong. A yearly check covers it.

Ads sold directly, such as a local business paying for a banner, never pass through automated auctions, so ads.txt plays no part in them.

And the file does not raise prices. It removes a reason for buyers to stay away, nothing more.

Have the file, keep it accurate, and check it whenever a partner changes.

This week

  1. Type your domain followed by /ads.txt.
  2. Confirm it shows a short plain text list.
  3. Ask each current partner for its current lines.
  4. Remove lines for partners you no longer use.
  5. Add a comment naming each partner and date.
  6. Name one person responsible for the file.
  7. Add it to the checklist for any rebuild or move.

Step seven is the one that stops the file vanishing again. Nobody deletes it on purpose; it is left behind by a move that nobody thought to check against it.

The ad accounts behind those lines belong on the same list as every login your website depends on.

Frequently asked questions

What is an ads.txt file?

A plain text file at the root of a domain listing every company allowed to sell the site's ad space, with the account each one sells it under.

Why do ad buyers check it?

Because automated ad sales pass through several companies, and fraudsters passed off fake space as well-known sites. The file lets buyers confirm that a seller is authorised.

What happens if my site has none?

Buyers have nothing to verify your space against. Some carry on bidding, while a growing number treat unverifiable space as a risk and bid less or not at all.

Can a wrong file be worse than none?

Yes. Leaving out a current partner tells checking buyers that partner is unauthorised, and copied or mistyped lines authorise nobody. A wrong file says something definite and false.

Why does the file go missing?

It sits outside the pages the site's editor shows, so rebuilds, hosting moves and plugin clean-ups can leave it behind without anyone noticing.

How often should it be updated?

Whenever a partner is added, removed or sends new lines, using the lines the partner supplies. Otherwise check it once a year and after any rebuild.

West Coast Media Solutions Inc. provides web design, web development, hosting, digital marketing, and business consulting to organisations across Canada, drawing on more than twenty-five years in the field.

Never opened your own ads.txt file?

Send us the address. We will check the file against what each of your current ad partners requires.

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