A redirector is an external process that rewrites URIs from client requests. For example, although a user requests the page http://www.example.com/page1.html, a redirector can change the request to something else, such as http://www.example.com/page2.html. Squid fetches the new URI automatically, as though the client originally requested it. If the response is cachable, Squid stores it under the new URI.
The redirector feature allows you to implement a number of interesting things with Squid. Many sites use them for access controls, removing advertisements, local mirrors, or even working around browser bugs.
One of the nice things about using a redirector for access control is that you can send the user to a page that explains exactly why her request is denied. You may also find that a redirector offers more flexibility than Squid's built-in access controls. As you'll see shortly, however, a redirector doesn't have access to the full spectrum of information contained in a client's request.
Many people use a redirector to filter out web page advertisements. In most cases, this involves changing a request for a GIF or JPEG advertisement image into a request for a small, blank image, located on a local server. Thus, the advertisement just "disappears" and doesn't interfere with the page layout.
So in essence, a redirector is really just a program that reads a URI and other information from its input and writes a new URI on its output. Perl and Python are popular languages for redirectors, although some authors use compiled languages such as C for better performance.
The Squid source code doesn't come with any redirector programs. As an administrator, you are responsible for writing your own or downloading one written by someone else. The first part of this chapter describes the interface between Squid and a redirector process. I also provide a couple of simple redirector examples in Perl. If you're interested in using someone else's redirector, rather than programming your own, skip ahead to Section 11.3.