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Networking - Proxy - PAC File

A **PAC** (Proxy Auto-Configuration) file is a JavaScript function that determines whether web browser requests (HTTP, HTTPS, and FTP) go directly to the destination or are forwarded to a web proxy server.

The JavaScript function contained in the PAC file defines the function:

function FindProxyForURL(url, host) {
  // ...
}

NOTE: Parameters:

url: The URL being accessed.
- The path and query components of https:// URLs are stripped.
  - In Chrome (versions 52 to 73), you can disable this by setting PacHttpsUrlStrippingEnabled to false in policy or by launching with the --unsafe-pac-url command-line flag.
  - In Chrome 74, only the flag works.
  - In Chrome, from version 75, onwards, there is no way to disable path-stripping.
  - In Chrome version 81, path-stripping does not apply to HTTP URLs, but there is interest in changing this behavior to match HTTPS;
  - In Firefox, the preference is network.proxy.autoconfig_url.include_path.

host: The hostname extracted from the URL.
- This is only for convenience; it is the same string as between :// and the first : or / after that.
- The port number is not included in this parameter; It can be extracted from the URL when necessary.