How to track users across domains?

Question

We got pitched this idea yesterday. A user visits our site and are marked. Then when they visit other sites like CNN they are targeted with adds for our site. So once they are exposed to us, they start to see us everywhere, creating the illusion we are bigger than we are.

The person pitching it said it was done by cookies. I was very skeptical since I don't believe there to be anyway to see what cookies a different domain has set. So I wanted to try an figure out how it was accomplished. The salesman called this technology pixel tracking.

I have never heard of pixel tracking but from my research I have found that it is placing a 1 pixel image that references a script on another domain with parameters to be executed. My first thought was, OK maybe its possible this way.. But I still don't know how?

Can anyone explain how they are able to mark you as visited our site, and then see this mark on another site? Is it from your IP?

Answer 1

Included at the bottom of the (CNN) website in this case is an img tag like:

<img src="http://www.webmarketingCompany.com/pixel.php?ID=623489593479"&gt;

When a user visits the (CNN) website, and the browser renders the page, it sends http requests for all the images as well, including a request to http://www.webmarketingCompany.com for the image pixel.php which includes the ID as a get parameter. pixel.php not only returns an image, typically a 1x1 transparent gif (so it isn't visible in the rendered page), but can do a whole host of additional processing using the ID value; and it also has access to any webmarketingCompany.com cookies, which are also sent with the http request.

Of course, CNN have to agree to include the img tag in their html. Typically it's used as a tracker by third party marketing companies working on behalf of CNN to identify who is visiting their site, what pages they're viewing, etc.

But because it's a PHP script, it can do a whole host of extras, such as setting further cookies. If webmarketingCompany.com also handle ad-serving on behalf of CNN, they can do some creative selection of the ads that they choose to serve.

Such cross-client "pollination" is frowned upon, certainly here in the UK.

Answer 2

What you are describing is pretty standard for all advertisement networks. The only difference here is that they will place that cookie on your site as well.

As long as the browser has "accept third party cookies" set to true, this will work as the salesman said. Most browsers has the setting set to true by default, the only exception I can think of is Safari.