Client Discovery of the KMS

By default, KMS clients query DNS for KMS information. The first time a KMS client queries DNS for KMS information, it randomly chooses a KMS host from the list of SRV RRs that DNS returns.

The address of a DNS server containing the SRV RRs can be listed as a suffixed entry on KMS clients, which allows advertisement of SRV RRs for KMS in one DNS server and allows KMS clients with other primary DNS servers to find KMS.

Also, priority and weight parameters can be added to the DnsDomainPublishList registry value for KMS. Doing so allows IT professionals to establish KMS host priority groupings and weighting within each group, which specify the KMS host to try first, to balance traffic among multiple KMS hosts. Only Windows 7 and Windows Server 2008 R2 use the priority and weight parameters.

If the KMS host that a client selects does not respond, the KMS client removes that KMS host from its list of SRV RRs and randomly selects another KMS host from the list. After a KMS host responds, the KMS client caches the name of the KMS host and uses it for subsequent activation and renewal attempts. If the cached KMS host does not respond on a subsequent renewal, the KMS client discovers a new KMS host by querying DNS for KMS SRV RRs.

By default, client computers connect to the KMS host for activation by using anonymous RPCs through TCP port 1688. (IT professionals can change the default port.) After establishing a TCP session with the KMS host, the client sends a single request packet. The KMS host responds with the activation count. If the count meets or exceeds the activation threshold for that operating system, the client is activated and the session is closed. The KMS client uses this same process for renewal requests. The communication each way is 250 bytes.