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### 3.8 Forbidden and redirected URLs

The web contains advertisements that a user-agent is supposed to download together with the requested pages. Not only do advertisements pollute the user’s brain, pushing them around takes time and uses up network bandwidth.

Many so-called content providers also track user activities by using web bugs, tiny embedded images that cause a server to log where they are requested from. Such images can be detected because they are usually uncacheable (see Cache transparency) and therefore logged by Polipo by default.

Polipo can be configured to prevent certain URLs from reaching the browser, either by returning a forbidden error message to the user, or by redirecting such URLs to some other URL.

Some content providers attempt to subvert content filtering as well as malware scans by tunnelling their questionable content as https or other encrypted protocols. Other content providers are so clueless as to inject content from external providers into supposedly safe webpages. Polipo has therefore the ability to selectively block tunneled connections based on hostname and port information.