A Service Principle Name (SPN) is the name by which a client uniquely identifies an instance of a service. By default your IIS machine has SPNs for its hostname, such as myServer, and its Fully Qualified Domain Name (FQDN), such as myServer.branch.example.com.
If the proxy machine is accessed by a name other than either of these, such as myServer.example.com, otherName.company.com, or its IP address, then Active Directory authentication will not work. Your choices are:
SPNs can be viewed and changed with the setspn.exe
tool, which
Microsoft provides as part of its Windows 2003 Support Tools
.
This package is available on the Windows 2003 CD, or, at the time of
writing, at http://go.microsoft.com/fwlink/?LinkId=100114
.