Proxy auto-config

Modern web browsers implement several levels of automation; users can choose the level that is appropriate to their needs. The following methods are commonly implemented:

• Automatic proxy selection: Specify a host-name and a port number to be used for all URLs. Most browsers allow you to specify a list of domains (such as localhost) that will bypass this proxy.
• Proxy auto-configuration (PAC): Specify the URL for a PAC file with a JavaScript function that determines the appropriate proxy for each URL. This method is more suitable for laptop users who need several different proxy configurations, or complex corporate setups with many different proxies.
• Web Proxy Auto-Discovery Protocol (WPAD): Let the browser guess the location of the PAC file through DHCP and DNS lookups.

The Proxy auto-config file format was originally designed by Netscape in 1996 for the Netscape Navigator 2.0[1] and is a text file that defines at least one JavaScript function, FindProxyForURL(url, host), with two arguments:

• url is the URL of the object
• host is the host-name derived from that URL. Syntactically it is the same string as between :// and the first : or / after that.[2]

By convention, the PAC file is normally named proxy.pac. The WPAD standard uses wpad.dat.

To use it, a PAC file is published to a HTTP server, and client user agents are instructed to use it, either by entering the URL in the proxy connection settings of the browser or through the use of the WPAD protocol. The URL may also reference a local file as for example: file:///etc/proxy.pac.

Even though most clients will process the script regardless of the MIME type returned in the HTTP reply, for the sake of completeness and to maximize compatibility, the HTTP server should be configured to declare the MIME type of this file to be either application/x-ns-proxy-autoconfig or application/x-javascript-config.

There is little evidence to favor the use of one MIME type over the other. It would be, however, reasonable to assume that application/x-ns-proxy-autoconfig will be supported in more clients than application/x-javascript-config as it was defined in the original Netscape specification, the latter type coming into use more recently.

A very simple example of a PAC file is:

function FindProxyForURL(url, host)
{
	return "PROXY proxy.example.com:8080; DIRECT";
}

This function instructs the browser to retrieve all pages through the proxy on port 8080 of the server proxy.example.com. Should this proxy fail to respond, the browser contacts the Web-site directly, without using a proxy. The latter may fail if firewalls, or other intermediary network devices, reject requests from sources other than the proxy—a common configuration in corporate networks.

A more complicated example demonstrates some available JavaScript functions to be used in the FindProxyForURL function:

function FindProxyForURL(url, host) {
	// our local URLs from the domains below example.com don't need a proxy:
	if (shExpMatch(host, "*.example.com"))
	{
		return "DIRECT";
	}

	// URLs within this network are accessed through
	// port 8080 on fastproxy.example.com:
	if (isInNet(host, "10.0.0.0", "255.255.248.0"))
	{
		return "PROXY fastproxy.example.com:8080";
	}

	// All other requests go through port 8080 of proxy.example.com.
	// should that fail to respond, go directly to the WWW:
	return "PROXY proxy.example.com:8080; DIRECT";
}

By default, the PROXY keyword means that a proxy corresponding to the protocol of the original request, be it http, https, or ftp, is used. Other supported keyword and proxy types include:

SOCKS

Use a SOCKS proxy.

HTTP, HTTPS

Introduced in more recent versions of Firefox. Specifies an HTTP(S) proxy.

SOCKS4, SOCKS5

Introduced in more recent versions of Firefox. Specifies the SOCKS protocol version.

if (isInNet(host, dnsResolve(sampledomain), "255.255.248.0")) // .NET 2.0 will resolve proxy properly

if (isInNet(host, sampledomain, "255.255.248.0")) // .NET 2.0 will not resolve proxy properly

return "PROXY proxy1.example.com:80; PROXY proxy2.example.com:8080";

de Boyne Pollard, Jonathan (2004). "Automatic proxy HTTP server configuration in web browsers". Frequently Given Answers. Retrieved 2013-07-05.

• "Proxy Auto-Configuration (PAC) file". 2019-01-27.

• "Using the Client Autoconfiguration File". Netscape Proxy Server Administrator's Guide: Chapter 11. 1998-02-25. Archived from the original on 2004-08-10.

• "Chapter 26 - Using Automatic Configuration, Automatic Proxy, and Automatic Detection". Microsoft TechNet. Retrieved 2013-07-05.

• "Proxy Auto Config for Firefox (PAC). Fully working examples including anti-ad and anti-adult filter rules". 2012-05-12. Archived from the original on 2015-07-01. Retrieved 2009-05-24.