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If the destination server is not using SSL/TLS, then the entire content of traffic is vulnerable as well. Therefore, the internet service provider (ISP) or any man-in-the-middle can see connection details, including the destination IP address.
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By merging the data, this means users are distinguishable and easily identifiable. Even if the user's IP address was replaced by a proxy without any leaks, proxies do not ensure that users have an uniform appearance on the Internet see Data Collection Techniques.
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Whonix ™ is a leak shield to force all connections to use the Tor anonymity network. There is no project that is actively working on implementing a leak shield for proxies. In case of Tor, projects such as Tor Browser and Whonix ™ are entirely focused to provide a leak shield. A leak shield is required to make sure all applications are used by a tunnel such as a proxy, VPN or Tor. Proxies are usually used without a leak shield. Web browser are notoriously known to leak the user's real IP address through WebRTC, browser plugins and a number of other techniques. Configuration an application such as a web browser to reliably use a proxy for all connections is very difficult.
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