Sep 17, 2018

Proxy Auto-Configuration (PAC) files

Proxy Auto-Configuration (PAC) files are files that contain the text of a single JavaScript function, FindProxyForURL(). In theory, every time that a web object is about to be fetched, the JavaScript function is invoked (by the web browser) with two arguments: the URL of the object and the hostname derived from that URL. The result of the function is a string comprising a semi-colon-separated sequence of one or more instructions that determine whence the web browser is to fetch the object from:
Instruction Meaning
DIRECT Fetch the object directly from the content HTTP server denoted by its URL
PROXY name:port Fetch the object via the proxy HTTP server at the given location (name and port)
SOCKS name:port Fetch the object via the SOCKS server at the given location (name and port)