Web Proxy Autodiscovery Protocol - Context

Context

In order for all browsers in an organization to be supplied the same proxy policy, without configuring each browser manually, both the below technologies are required:

  • Proxy auto-config (PAC) standard: create and publish one central proxy configuration file. Details are discussed in a separate article.
  • Web Proxy Autodiscovery Protocol (WPAD) standard: ensure that an organization's browsers will find this file without manual configuration. This is the topic of this article.

The WPAD standard defines two alternative methods the system administrator can use to publish the location of the proxy configuration file, using the Dynamic Host Configuration Protocol (DHCP) or the Domain Name System (DNS):

Before fetching its first page, a web browser implementing this method sends the local DHCP server a DHCPINFORM query, and uses the URL from the WPAD option in the server's reply. If the DHCP server does not provide the desired information, DNS is used. If, for example, the network name of the user's computer is pc.department.branch.example.com, the browser will try the following URLs in turn until it finds a proxy configuration file within the domain of the client:

  • http://wpad.department.branch.example.com/wpad.dat
  • http://wpad.branch.example.com/wpad.dat
  • http://wpad.example.com/wpad.dat
  • http://wpad.com/wpad.dat (in incorrect implementations, see note in Security below)

(Note: These are examples and may not be live URLs.)

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