Want to see who's serving and tracking an ad? Enter Live HTTP Headers
The Firefox plugin, Live HTTP Headers is a great tool if you want to see if certain links are getting redirected the way you want them to be, but you might also be able to see which ad network is serving a banner or text ad and also what ad server the advertiser is using.
Online ads generally redirect to another URL upon click. What users don't see is that often when they click on an ad, they are successively getting redirected to multiple URLs - that is, one URL redirects to another, which redirects to another with the host server of each URL tracking the click. What gets sent to the browser when a URL is requested is called a "header," which sometimes is followed by html code.
The header contains things like the type of server the URL is hosted on, a cookie perhaps as well as something called a status code. When a web page gets loaded, that is a '200 OK' code. Everyone has the experience of trying to load a URL but the server is unable to find that page - that is a '404 Not Found' code. When a URL redirects, it uses either a '301 Moved permanently' or a '302 Found' code. These basically tell the browser to go to another, specified URL. The server will log the request in the process. So I could create a redirecting URL on my server, perhaps agencyvendors.com followed by /redirect/123 and then point that at another URL. Then, I'd be able to see when someone visited my redirecting URL, which I could then use to monitor clicks.
Live HTTP Headers displays the requests that your browser sends to a server and the headers that get sent back. If a particular banner ad or link redirects 5 times before resolving to the landing page, you will see all those redirects. I just visited msnbc.com and found an ad for Infinity, so I clicked on it with Live HTTP Headers turned on. The first redirect is a path on cdn1.eyewonder.com, which redirects to an eyewonderlabs.com URL, which then redirects to an ad.doubleclick.net URL before resolving to infinity.com. I know that eyewonder.com and doubleclick.net both belong to ad serving companies, so I know that the advertiser is using these ad servers to serve and track the ads. Further, I know that if two ad servers are used, they serve different purposes. I know that eyewonder is known for rich media ad serving, so I can tell that it is likely that the advertiser is using doubleclick for their general ad serving needs and eyewonder for rich media. I can then use this same technique to see if an ad network (if any) is serving the ad. It is also useful to see what cookies (if any) are getting served with each requested URL.