FSMO：Flexible single master operation

WiKipedia：Flexible single master operation

Flexible single master operation (FSMO, F is sometimes floating ; pronounced Fiz-mo), or just single master operation or operations master, is a feature of Microsoft's Active Directory (AD). Recently, as of 2005, the term FSMO has been deprecated in favour of operations masters.

FSMOs are specialised domain controller (DC) tasks, used where standard data transfer and update methods are inadequate. AD normally relies on multiple peer DCs, each with a copy of the AD database, being synchronised by multi-master replication. The tasks which are not suited to multi-master replication, and are viable only with a single-master database, are the FSMOs.

Domain-wide FSMO Roles:

Every domain in an Active Directory forest must contain one of each of the following FSMO roles:

• The Relative ID Master allocates security RIDs to DCs to assign to new AD security principals (users, groups or computer objects). It also manages objects moving between domains.
• The Infrastructure Master maintains security identifiers, GUIDs, and DNs for objects referenced across domains. Most commonly it updates user and group links.
• The PDC Emulator emulates a Windows NT Primary Domain Controller (PDC). It is also the favored DC for other DCs in replicating and confirming password information, and is the authoritative source of time in the domain.

Forest-wide FSMO Roles:

Regardless of the number of domains in an Active Directory forest, the following FSMO roles exist only once:

• The Schema Master maintains all modifications to the schema of the forest. The schema determines the types of objects permitted in the forest and the attributes of those objects.
• The Domain Name Master tracks the names of all domains in the forest and is required to add new domains to the forest or delete existing domains from the forest.

Moving FSMO Roles Between Domain Controllers

By default AD assigns all operations master roles to the first DC created in a forest. If new domains are created in the forest, the first DC in a new domain holds all of the domain-wide FSMO roles. This is not a satisfactory position. Microsoft recommends the careful division of FSMO roles, with standby DCs ready to take over each role. When a FSMO role is transferred to a different DC, the original FSMO holder and the new FSMO holder communicate to ensure no data is lost during the transfer. If the original FSMO holder experienced an unrecoverable failure, you can force another DC to seize the lost roles; however, there is a risk of data loss because of the lack of communications. If you seize a FSMO role instead of transferring the role, that domain controller can never be allowed to host that FSMO role again. Corruption can occur within Active Directory. FSMO roles can be easily moved between DCs using the AD snap-ins to the MMC or using ntdsutil which is a command line based tool.

Certain FSMO roles depend on the DC being a Global Catalog (GC) server as well. For example, the Infrastructure Master role must not be housed on a domain controller which also houses a copy of the global catalog in a multi-domain forest (unless all domain controllers in the domain are also global catalog servers), while the Domain Name Master role should be housed on a DC which is also a GC. When a Forest is initially created, the first Domain Controller is a Global Catalog server by default. The Global Catalog provides several functions. The GC stores object data information, manages queries of these data objects and their attributes as well as provides data to allow network logon.

The PDC emulator and the RID master should be on the same DC, if possible. The Schema Master and Domain Name Master should also be on the same DC. To provide fault tolerance, there should be at least 2 domain controllers available within each domain of the Forest. Furthermore, the Infrastructure Master role holder should not also be an Global Catalog Server, as the combination of these two roles on the same host will cause unexpected (and potentially damaging) behaviour in a multi-domain environment.(see "Phantoms, Tombstones and the Infrastructure Master", 248047)

Transferring or Seizing FSMO Roles

Here is the Knowledge Base article from Microsoft on transferring or seizing the FSMO Roles:

http://support.microsoft.com/kb/255504/en

Active Directory Support Tools

There are support tools that can test Active Directory to make sure the components are functioning correctly within the Forest. These tools can tell you the health of your Active Directory as they verify the various system components. The tools can be downloaded from the Microsoft web site or obtained from the Windows Server CD.

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