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Flexible Single Master Operations (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). As of 2005, the term FSMO has been deprecated in favour of operations masters.
FSMO is a specialized domain controller (DC) set of 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 synchronized 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.
The PDC Emulator (Primary Domain Controller) - This role is the most used of all FSMO roles and has the widest range of functions. The domain controller that holds the PDC Emulator role is crucial in a mixed environment where Windows NT 4.0 BDCs are still present. This is because the PDC Emulator role emulates the functions of a Windows NT 4.0 PDC. But even if you've migrated all your Windows NT 4.0 domain controllers to Windows 2000 or later, the domain controller that holds the PDC Emulator role still does a lot. The PDC Emulator is the domain source for time synchronization for all other domain controllers; in a multi-domain forest, the PDC Emulator in each domain synchronizes to the forest root PDC Emulator. All other domain member computers synchronize to their respective domain controllers. It's critically important that computer clocks are synchronized across your forest because if they're off by too much then Kerberos authentication will fail and users won't be able to log on to the network. In addition, all password changes occur on the PDC Emulator and receive priority replication.
The RID Master - This FSMO role owner is the single DC responsible for processing RID Pool requests from all DCs within a given domain. It is also responsible for moving an object from one domain to another during an interdomain object move. When a DC creates a security principal object such as a user or group, it attaches a unique SID to the object. This SID consists of a domain SID (the same for all SIDs created in a domain) and a relative ID (RID) that is unique for each security principal SID created in a domain. Each DC in a domain is allocated a pool of RIDs that it is allowed to assign to the security principals it creates. When a DC's allocated RID pool falls below a threshold, that DC issues a request for additional RIDs to the domain's RID Master FSMO role owner, the RID Master FSMO role owner responds to the request by retrieving RIDs from the domain's unallocated RID pool and assigns them to the pool of the requesting DC.
The Infrastructure Master - The purpose of this role is to ensure that cross-domain object references are correctly handled. For example, if you add a user from one domain to a security group from a different domain, the Infrastructure Master makes sure this is done properly. As you can guess however, if your Active Directory deployment has only a single domain, then the Infrastructure Master role does no work at all, and even in a multi-domain environment it is rarely used except when complex user administration tasks are performed, so the machine holding this role doesn't need to be powerful.
One per Microsoft Windows Forest of Domains
These roles are unique at enterprise level
The Schema Master - While the first three FSMO roles described above are domain-specific, the Schema Master role and the one following are forest-specific and are found only in the forest root domain (The first domain you create when you create a new forest). This means there is only one Schema Master in a forest, and the purpose of this role is to replicate schema changes to all other domain controllers in the forest. Since the schema of Active Directory is rarely changed however, the Schema Master role will rarely do any work. Typical scenarios where this role is used would be when you deploy Exchange Server onto your network, or when you upgrade domain controllers from Windows 2000 to Windows Server 2003, as these situations both involve making changes to the Active Directory schema.
The Domain Naming Master - The other forest-specific FSMO role is the Domain Naming Master, and this role also resides in the forest root domain. The Domain Naming Master role processes all changes to the namespace, for example adding the child domain vancouver.mycompany.com to the forest root domain mycompany.com requires that this role be available, so if you can't add a new child domain or new domain tree, check to make sure this role is running properly.
To summarize then, the Schema Master and Domain Naming Master roles are found only in the forest root domain, while the remaining roles are found in each domain of your 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, except for the PDC emulator Master operation and the Infrastructure Master Operation. 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 Naming 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 Naming 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 a 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.
^"Windows 2000 Active Directory FSMO roles". Microsoft Corporation. 2007-02-23. "To prevent conflicting updates in Windows 2000, the Active Directory performs updates to certain objects in a single-master fashion. [...] Because an Active Directory role is not bound to a single DC, it is referred to as a Flexible Single Master Operation (FSMO) role."