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Originally Posted by ULVideo
Perhaps my understanding of web bugs is lacking. I thought that they could provide:
The IP address of the computer that fetched the Web Bug
The URL of the page that the Web Bug is located on
The URL of the Web Bug image
The time the Web Bug was viewed
The type of browser that fetched the Web Bug image
A previously set cookie value
If that's the case, how does that track if someone has a dynamic IP and clears their cookies? (Isn't it really just cookie + IP?).
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Ok well, lets see
Let's say you accessed
http://www.someserver.com/someimage.gif , the server would send back a header that says the date it was last modified.
The web browser caches the image and remembers the date the image was last modified.
if you happened to load the pic again, the web browser would send a header that tells server to only send the image if the last modified date is later than the value that had been originally remembered by the web browser.
the way "web bugs" work is that they will send a fake unique last modified date, so when some surfer loads the image originally they will have a totally unique last modified date
if they go back to the page, the web browser will resend that fake/unique last modified date even if cookies are cleared and the ip is different