How can a cookie help to detect a proxy?
cannot detect IP with the help of cookies. However, when you first visit a web site, the IP (i.e. your proxy server s IP) is detected by the web server and then stored in the cookies. When you re-visit this site, the web server detects your IP again and checks it with the one stored in the cookies. If the IPs are not the same, the web server can make certain conclusions. And if you don’t disable cookies in your browser, no proxy will help you (anonymizers can disable cookies and stop relaying them to your machine).
