Web Caching

Proxy Auto-Configuration Script

The proxy auto-configuration (PAC) technique is designed to fix many of the manual configuration problems described previously. Instead of using static proxy addresses, the browser executes a function for every request. This function returns a list of proxy addresses that the browser tries until the request is successfully forwarded.

The PAC function is written in JavaScript. In theory, any browser that supports JavaScript can also support PAC. Netscape invented the PAC feature, and it was first available in Version 2 of their browser. Microsoft added PAC support to MSIE Version 3.

Both the Netscape and Microsoft browsers retrieve the PAC script as a URL. This is perhaps the biggest drawback to proxy auto-configuration. Setting the PAC URL requires someone to enter the URL in a pop-up window, or the browser must be preconfigured with the URL.

The best thing about proxy auto-configuration is that it allows administrators to reconfigure the browsers without further intervention from the users. If the proxy address changes, the administrator simply edits the PAC script to reflect the change. The browsers fetch the PAC URL every time they are started, but apparently not while the browser is running, unless the user forces a reload.

Another very nice feature is failure detection, coupled with the ability to specify multiple proxy addresses. If the first proxy in the list is not available, the browser tries the next entry, and so on until the end of the ...