A Proxy Auto-Config (PAC) file is a text file containing JavaScript code that tells web browsers and other software applications which proxy server to use for fetching a specific URL. Instead of routing all your internet traffic through a single, rigid proxy server, a PAC file evaluates each request in real time. It determines if the request should go through a specific corporate proxy, a failover backup server, or connect directly to the public internet. How a PAC File Works
Every time you type a web address or click a link, your browser executes a specific JavaScript function inside the PAC file called FindProxyForURL(url, host). javascript
function FindProxyForURL(url, host) { // JavaScript logic goes here } Use code with caution.
The browser feeds two pieces of data into this function: the full website URL (e.g., https://example.com) and the host name (e.g., example.com). The script instantly evaluates these parameters against a set of predefined conditions and returns a string instruction back to the browser. The response string typically takes one of three forms:
DIRECT: The browser bypasses all proxies and connects straight to the target server.
PROXY ://example.com: The browser forwards the web traffic through the designated proxy address and port.
SOCKS ://example.com: The browser routes the request using a SOCKS proxy server.
A PAC file can also chain multiple options together for redundancy (e.g., RETURN “PROXY p1.com:8080; PROXY p2.com:8080; DIRECT”). If the first proxy fails to respond, the browser seamlessly falls back to the second one, and finally resorts to a direct connection if all proxies fail. Core Syntax and Conditional Examples
Administrators use built-in functions designed specifically for the PAC environment to route traffic efficiently. Some of the most common conditions include:
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