Dophiのブログ

    Web Proxy Autodiscovery Protocol (WPAD) is developed by Microsoft. Clients use this protocol to download a proxy auto-config file and automatically configure proxy setting of web browsers. WPAD defines alternative ways for administrators to publish proxy auto-config files; They are Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS).

The procedures of WPAD in DHCP

1. A client with a web browser enabling proxy automatic configuration sends a DHCP Inform query to a local DHCP server.

2. The DHCP serve responses a DHCP ACK packet which includes DHCP option 252 (A URL containing a download path of auto-config file locates in option 252).

3. The client performs a HTTP GET to download the proxy auto-config file and configure the web browser.  

    Nortel ERS5500 (aka BayStack 5500) provides DHCP relay function which can forward both DHCP and BootP requests to a DHCP server. But, with software version 5.1.0, ERS5500 drops DHCP ACK corresponding to DHCP Inform queries of Windows Vista clients from a DHCP server.  To fix this issue, upgrade ERS5500 to version 5.1.3 above.

Note: Windows Vista supports DHCP option 121 and 249 (Classless Route Options), but Windows XP only supports option 249. This is the only one difference of DHCP Inform between Vista and XP.

Reference:  DHCP version 4 RFC 3442 ,  How to configure WPAD in DHCP