How do parental-control app do web filtering and site blocking

I just wonder how does the parental-control sites do problematically the web filtering a开发者_运维问答nd the URL blocking in their applications?

I ask because I need to build something similar

Parent control applications are web proxies that look for particular strings in the web address. If the proxy encounters one of the strings on the "naughty" list, access to the web page is blocked.

Blocked means that the web proxy sends its own HTML page, rather than the "naughty" web page.

Some parent control applications have a white list of web pages, rather than trying to black list the "naughty" web pages. A white list means that only the web pages in the list will be shown, and all other web pages will be blocked.

The trick is making it easy for the parents to control the "naughty" list, without the kids being able to turn the proxy off.