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Web Proxy Auto-Discovery Protocol
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Web Proxy Auto-Discovery Protocol
The Web Proxy Auto-Discovery (WPAD) Protocol is a method used by clients to locate the URL of a configuration file using DHCP and/or DNS discovery methods. Once detection and download of the configuration file is complete, it can be executed to determine the proxy for a specified URL.
The WPAD protocol only outlines the mechanism for discovering the location of this file, but the most commonly deployed configuration file format is the proxy auto-config format originally designed by Netscape in 1996 for Netscape Navigator 2.0. The WPAD protocol was drafted by a consortium of companies including Inktomi Corporation, Microsoft Corporation, RealNetworks, Inc., and Sun Microsystems, Inc. (now Oracle Corp.). WPAD is documented in an INTERNET-DRAFT which expired in December 1999. However, WPAD is still supported by all major browsers. WPAD was first included with Internet Explorer 5.0.
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:
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 a DHCPINFORM query to the local DHCP server, 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:
(Note: These are examples and are not "live" URLs due to them employing the reserved domain name of "example.com".)
Additionally on Windows if the DNS query is unsuccessful then Link-Local Multicast Name Resolution (LLMNR) and/or NetBIOS will be used.
DHCP has a higher priority than DNS: if DHCP provides the WPAD URL, no DNS lookup is performed. This only works with DHCPv4. In DHCPv6, there is no WPAD-Option defined.
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Web Proxy Auto-Discovery Protocol
The Web Proxy Auto-Discovery (WPAD) Protocol is a method used by clients to locate the URL of a configuration file using DHCP and/or DNS discovery methods. Once detection and download of the configuration file is complete, it can be executed to determine the proxy for a specified URL.
The WPAD protocol only outlines the mechanism for discovering the location of this file, but the most commonly deployed configuration file format is the proxy auto-config format originally designed by Netscape in 1996 for Netscape Navigator 2.0. The WPAD protocol was drafted by a consortium of companies including Inktomi Corporation, Microsoft Corporation, RealNetworks, Inc., and Sun Microsystems, Inc. (now Oracle Corp.). WPAD is documented in an INTERNET-DRAFT which expired in December 1999. However, WPAD is still supported by all major browsers. WPAD was first included with Internet Explorer 5.0.
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:
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 a DHCPINFORM query to the local DHCP server, 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:
(Note: These are examples and are not "live" URLs due to them employing the reserved domain name of "example.com".)
Additionally on Windows if the DNS query is unsuccessful then Link-Local Multicast Name Resolution (LLMNR) and/or NetBIOS will be used.
DHCP has a higher priority than DNS: if DHCP provides the WPAD URL, no DNS lookup is performed. This only works with DHCPv4. In DHCPv6, there is no WPAD-Option defined.