What is Proxy Auto-Config (PAC)?

A PAC (Proxy Auto-Config) file is a JavaScript file that tells web browsers how to route internet traffic, deciding whether requests for a specific URL go directly to the destination or through a proxy server, often used in corporate networks for efficient traffic management, including failover and load balancing. It contains the FindProxyForURL(url, host) function, which uses rules to determine the path, making proxy management dynamic and flexible.

How it works

JavaScript Logic: The PAC file contains JavaScript code that evaluates the requested URL and host.
Decision Making: Based on rules within the script (e.g., checking IP ranges, specific domains), it returns instructions like DIRECT (no proxy) or PROXY server_address:port (use a proxy).
Failover: Multiple proxy servers can be listed, so if the first one fails, the browser tries the next one, providing redundancy.
Automatic Configuration: Browsers fetch the PAC file (often via DHCP Option 252 or manual URL entry), and the script runs automatically for each connection.

Key benefits

Simplified Management: Centralizes proxy settings in one file, making updates easier for IT.
Granular Control: Allows complex routing rules, like bypassing proxies for internal sites or directing different users to different proxies.
Improved Performance: Directs traffic efficiently, reducing unnecessary proxy usage.