A content-filtering proxy is a proxy server, a server acting as a middleman between two computers, that filters the content it relays, limiting access to undesirable, forbidden, or dangerous content. Schools and workplaces may opt to use a content-filtering proxy to control what people view while in their facilities. In addition to filtering content, the proxy can also trap malware, viruses, and other unwanted software and eliminate them before they enter the network.
Proxy servers typically serve a network of computers, and act as a mediator for Internet access. When people on the Internet enter addresses, the request is relayed to the proxy so it can fetch the information and return it. One advantage to using a proxy server is the ability to cache frequently visited sites, thus speeding up network performance. The ability to filter content is another potential feature.
Content filtering is done for a number of different reasons. For productivity, workplaces may want to prevent employees from visiting sites not associated with work. Schools and libraries might want to filter content to prevent people from accessing porn and other potentially inappropriate content. If there is an acceptable use policy for Internet in the workplace or on campus, the content-filtering proxy can be used to enforce it, by banning access to specific sites or sites with particular strings of characters, such as keywords associated with pornography.
Typically, the content-filtering proxy maintains logs. These can allow people to see how the network is used, monitoring bandwidth usage and browsing activity. It can also be useful for when people complain about false positives, where they go to visit a legitimate website and find it blocked by the proxy because it contains content the proxy mistakenly flags. For example, someone in a medical office might have reason to be reading a website with keywords relating to the genitals, and the server might think the site was pornographic and block it.
The content-filtering proxy usually has whitelisting capabilities, allowing people to specify that some websites are always allowed, no matter what content appears to be present. This allows people to adjust the filters for false positives, to allow people to access sites they need for work, research, or school.
Critics of the use of content-filtering proxies focus on ethical concerns about restricting access to information and also point out that creative users can find ways to work around proxies. They may also point to the false positive rate as evidence that filtering is not as effective as might be desired.