How does Secure Password Administrator (SPA) determine the closest domain controller (DC) to the client machine?
Secure Password Administrator makes a call to the system function DsGetDcName using the client's IP address. See the following link for more information on DsGetDcName:
Notice the comments about the SiteName parameter:
If this parameter is NULL, DsGetDcName attempts to return a domain controller in the site closest to the site of the computer specified by ComputerName. This parameter should be NULL, by default.