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Flexible Single Master Operation

(also FSMO, pronounced “fiz-mo”)

Flexible Single Master Operation definition

Flexible Single Master Operation is a specialized set of roles in Microsoft Active Directory (AD), where a single domain controller hosts one or more services to prevent conflicts and ensure consistency across the AD environment.

See also: hybrid active directory, domain controller, global catalog, Lightweight Directory Access Protocol

FSMO roles

  • Forest-wide operations. These roles are unique in the entire AD forest.
    • Schema Master. Responsible updating the AD schema, which defines object classes and attributes within the AD forest.
    • Domain Naming Master. Responsible for controlling the addition and removal of domains in the AD forest.
  • Domain-wide operations. These roles are unique in each AD domain.
    • Relative ID (RID) Master. Allocates RID pools to domain controllers within a domain. RIDs are used to generate unique security identifiers (SIDs) for each object in the domain.
    • Primary Domain Controller (PDC) Emulator. Responsible for processing password changes and is the authoritative source for the time service.
    • Infrastructure Master. Responsible for maintaining references from objects in its domain to objects in other domains.

History of FSMO

Early 2000s:

  • Windows 2000 Server introduced FSMO roles as part of the new Active Directory feature.

Mid-2000s to Late 2010s:

  • With each release of Windows Server, the management and functionality of FSMO roles were enhanced to provide more features and to improve the stability and performance of Active Directory environments.
  • Microsoft developed various tools and utilities (like “ntdsutil”) for managing FSMO roles, allowing for transferring and seizing roles more efficiently.

Late 2010s to today:

  • Windows Server 2016 and 2019 continued to support FSMO roles, maintaining the same core roles established in Windows 2000 with improvements in management features and capabilities.
  • As Microsoft introduced Azure Active Directory and other cloud services, Active Directory and FSMO remained a fundamental aspect of hybrid cloud environments.