One important use of proxy servers as I understand reading Wikipedia is to keep you anonymous from other servers by acting as a middle-man. If that is indeed the case, consider the following situation.

  1. You access a server without any proxy
  2. The server sends some cookies
  3. The cookies are saved on your computer
  4. You disconnects from the server and reconnects through the proxy

At this point wouldn't the server read the previous cookies and understand what your previous IP was? Now that the server has both IPs in its log, couldn't it tie this proxy and its real client to the same address in the future?

I must confess that my knowledge in the area is very poor. Except reading the online resources and general definitions, I have no way of knowing such specific details. It would be very helpful if you could provide an answer directly addressing the issue that I've mentioned (if there is one).

  • Cookies do not keep you anonymous. Quite the opposite, in fact. – Stephane Sep 19 '14 at 13:50
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    Most proxies do not exist to provide anonymity, instead they are used as security devices to filter traffic and protect the users behind them. Some are used to keep an eye on user traffic, so the opposite of anonymity. – GdD Sep 19 '14 at 13:51
  • @Stephane I didn't mention that in the question. – Renae Lider Sep 19 '14 at 13:52
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    @user3058846 Sorry. I failed at reading – Stephane Sep 19 '14 at 14:27

In the specific example you have outlined, yes, it would be possible to correlate this information to say that both IP addresses seem to be for the same user. Even without cookies, it may be possible to identify you across sessions, IP addresses, etc. For example, they could use browser fingerprinting or other techniques.

If you wanted to try to reduce this, you could clear your cookies between using proxies. You may also want to consider using two different browsers to reduce fingerprinting and use other techniques to make you less uniquely identifiable as a user.

When you hear about people using proxies, their main concern may not be anonymity. They may want to simply bypass the internal network filters or admin controlled proxies. They may want to go through a proxy in another country to get around country-based access restrictions.

If someone wants to be anonymous, they would likely use a lot more layers, including things like accessing from a changing public location, using TOR, using a locked down OS, etc. They may also utilize a number of proxies, or different proxies at different times. They may purposely try to make multiple unique fingerprints during this process to keep individual sessions isolated. An attacker may also take advantage of cross site attacks, OS level flaws/malware to tie together disparate sessions.

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You're not missing anything -- cookies absolutely can identify you when behind a proxy, and you absolutely can link proxy and non-proxy IPs. The catch is that proxies can have many users, so linking the IPs may not be correct -- not all connections from the proxy are from the person with that non-proxy IP address. But if you don't want a site to be able to identify you, merely using a proxy isn't enough (as they can also use cookies to trace you).

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