About automatic discovery
At first, it may seem like a simple task to configure the ISA Server computer to which Firewall clients and Web Proxy clients connect. However, subsequent modifications can be time consuming, particularly for roaming clients, who may require constant adjustments. The automatic discovery feature of ISA Server allows you to configure clients so that they automatically discover the appropriate ISA Server computer. In this way, roaming clients can connect to the ISA Server computer, as appropriate and when necessary. By enabling automatic discovery, Web Proxy and Firewall clients all automatically discover the appropriate ISA Server computer.
Configuring WPAD and WSPAD on the DNS or DHCP server
When automatic discovery is enabled, the Firewall client or the Web Proxy client requests an object from the ISA Server computer that is configured to fulfill requests. If the ISA Server does not respond and if automatic discovery is enabled for the client, it starts the automatic discovery process.
ISA Server uses the Web Proxy Autodiscovery Protocol (WPAD) entry to determine the appropriate Winsock Proxy Autodetect (WSPAD) entry. You do not have to configure the WSPAD entry explicitly on the DNS server.
For instructions, see Configure DNS for automatic discovery of ISA Server and Configure DHCP for automatic discovery of ISA Server.
How automatic discovery works
Automatic discovery is based on information that is stored in the DHCP server or DNS server. First, the client retrieves the name of the WPAD or WSPAD server from the DHCP server. Then, the client queries the server, to determine the actual ISA Server with which to work.
Modifying ISA Server for client using automatic discovery
In order to change the ISA Server used by a client, you change the entry in the DHCP or DNS server. However, because information is cached on the DNS or DHCP server, the client may actually receive the name of the previous ISA Server.
To ensure that the client receives the correct information when automatically detecting a server, perform the following steps:
- Set the auto-detection on the DHCP or DNS server to point to the IIS Server computer. Since the IIS computer contains wspad.dat and wpad.dat files, you will not have to update the DHCP or DNS automatic discovery information.
- Change the automatic discovery information in the wspad.dat file on the IIS computer, which acts as the dedicated redirector. The IIS machine is dedicated to act as the redirector.
Here is a sample of a wspad.dat file on an IIS computer:
[Common] Port=1745 [Servers Ip Addresses] Name=ISAServer.microsoft.com