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Proxy auto-config

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Title: Proxy auto-config  
Author: World Heritage Encyclopedia
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Subject: Web browsers, Proxy servers, UltraBrowser, Netscape Navigator, MenuBox
Collection: Proxy Servers, Web Browsers
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Proxy auto-config

A proxy auto-config (PAC) file defines how web browsers and other user agents can automatically choose the appropriate proxy server (access method) for fetching a given URL.

A PAC file contains a JavaScript functionFindProxyForURL(url, host)”. This function returns a string with one or more access method specifications. These specifications cause the user agent to use a particular proxy server or to connect directly.

Multiple specifications provide a fall-back when a proxy fails to respond. The browser fetches this PAC file before requesting other URLs. The URL of the PAC file is either configured manually or determined automatically by the Web Proxy Autodiscovery Protocol.


  • Context 1
  • Proxy configuration 2
  • The PAC File 3
    • Limitations 3.1
      • PAC Character-Encoding 3.1.1
      • DnsResolve 3.1.2
      • myIpAddress 3.1.3
      • Security 3.1.4
      • Others 3.1.5
    • Advanced functionality 3.2
  • References 4
  • Further reading 5
  • External links 6


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 Autodiscovery Protocol (WPAD): Let the browser guess the location of the PAC file through DHCP and DNS lookups.

Proxy configuration

Computer operating systems (e.g., Microsoft Windows, Mac OS X, Linux) require a number of settings to communicate over the Internet. These settings are typically obtained from an Internet Service Provider (ISP). Either anonymous (proxy to use a proxy server) or real settings may be used to establish a network connection.

The PAC File

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 and host is the host-name derived from that URL. 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.

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; DIRECT";

This function instructs the browser to retrieve all pages through the proxy on port 8080 of the server 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 don't need a proxy:
        if (shExpMatch(host, "*"))
                return "DIRECT";

        // URLs within this network are accessed through
        // port 8080 on
        if (isInNet(host, "", ""))
                return "PROXY";

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


PAC Character-Encoding

Browsers, such as Mozilla Firefox and Internet Explorer, support only system default encoding PAC files, and cannot support Unicode encodings, such as UTF-8.


The function dnsResolve (and similar other functions) performs a DNS lookup that can block your browser for a long time if the DNS server does not respond.

Caching of proxy auto-configuration results by domain name in Microsoft's Internet Explorer 5.5 or newer limits the flexibility of the PAC standard. In effect, you can choose the proxy based on the domain name, but not on the path of the URL. Alternatively, you need to disable caching of proxy auto-configuration results by editing the registry, a process described by de Boyne Pollard (listed in further reading).

It is recommended to always use IP addresses instead of host domain names in the isInNet function for compatibility with other Windows components which make use of the Internet Explorer PAC configuration, such as .NET 2.0 Framework. For example,

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

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

The current convention is to fail over to direct connection when a PAC file is unavailable.

Shortly after switching between network configurations (e.g. when entering or leaving a VPN), dnsResolve may give outdated results due to DNS caching.

For instance, Firefox usually keeps 20 domain entries cached for 60 seconds. This may be configured via the network.dnsCacheEntries and network.dnsCacheExpiration configuration variables. Flushing the system's DNS cache may also help, which can be achieved e.g. in Linux with sudo service dns-clean start.


The myIpAddress function has often been reported to give incorrect or unusable results, e.g., the IP address of the localhost. It may help to remove on the system's host file (e.g. /etc/hosts on Linux) any lines referring to the machine host-name, while the line localhost can, and should, stay.

On Internet Explorer 9, isInNet("localHostName", "second.ip", "") returns true and can be used as a workaround.

The myIpAddress function assumes that the device has a single IPv4 address. The results are undefined if the device has more than one IPv4 address or has IPv6 addresses.


In 2013, researchers began warning about the security risks of proxy auto-config.[2] The threat involves using a PAC to redirect the victim's browser traffic to an attacker-controlled server instead.


Further limitations are related to the JavaScript engine on the local machine.

Advanced functionality

More advanced PAC files can reduce load on proxies, perform load balancing, fail over, or even black/white listing before the request is sent through the network. One can return multiple proxies:

return "PROXY; PROXY";


  1. ^ "Navigator Proxy Auto-Config File Format". Netscape Navigator Documentation. March 1996. Archived from the original on 2007-06-02. Retrieved 2013-07-05. 
  2. ^ Lemos, Robert (2013-03-06). "Cybercriminals Likely To Expand Use Of Browser Proxies". Retrieved 2013-07-05. 

Further reading

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

External links

  • "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".  
  • "Proxy Auto Config for Firefox (PAC). Fully working examples including anti-ad and anti-adult filter rules". 2012-05-12. 
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