Where are SPNs stored in AD?

A. Each object has a servicePrincipalName attribute, which is a multivalue attribute in which all SPNs are stored. You can use ADSI Edit to view the attribute. If the SPN is for a machine's local System account, the SPN would be stored in the servicePrincipalName attribute of the Computers account in AD. You shouldn't write to this value directly. It should be updated only via the DsWriteAccountSpn call (but you can update it directly by using tools such as ADSI Edit).

When a client requests a connection to a service, the Key Distribution Center (KDC) searches the forest for a user or computer account for which the SPN is registered. If the KDC finds registration in more than one account, the request for authentication fails, indicating a rogue service registration