Which methods are supported by WPAD to automatically learn the URL of a PAC file?

The correct choice is DHCP, as it is one of the standard protocols used by the Web Proxy Auto-Discovery Protocol (WPAD) to automatically discover the URL of a Proxy Auto-Configuration (PAC) file. WPAD can leverage DHCP by using specific DHCP options to inform clients about the location of the PAC file, enabling them to configure their browsers to use an appropriate proxy without manual intervention.

In practice, devices on a network can send a DHCP request to the server, which can then respond with the necessary information, including the URL of the PAC file. This is particularly useful in environments where ensure proper proxy settings for web traffic can save considerable administrative effort.

While other methods like BOOTP and DNS are related to network configuration, they don't play a primary role in the context of WPAD for PAC file location discovery. BOOTP is generally used for network bootstrapping, and while DNS can be used in some contexts to locate a PAC file by resolving domain names, DHCP is explicitly designed to provide configuration information to hosts, which includes the PAC URL in a more straightforward manner compared to DNS. IPv6 autoconfiguration is more focused on the addressing (enabling devices to configure themselves with IP addresses) rather than retrieving PAC file information.