Privoxy

Privoxy is a web proxy with advanced filtering capabilities for enhancing privacy. Privoxy can be paired with Squid to provide filtering to it.

Emerge
To install Privoxy:

OpenRC
To start privoxy:

To make privoxy start at boot:

Configuration
Browsers need to set the proxy configuration to address 127.0.0.1 and port 8118 in the browser proxy configuration or in a proxy plugin (like foxyproxy). Privoxy default configuration block ads and further configuration should edit files in.

Test filters
There is a way to test the filters that are applied to a specific site.

If privoxy is working correctly the next URL should be intercepted by privoxy

http://config.privoxy.org/show-url-info

In this page it's possible to check why a page is filtered or what filters are applied to that site.