Active Directory is a multi-master distributed database. This means that any DC can assume the role of a master for some task. These roles are called Flexible Single Master Operation roles or FSMO (“fizz-moh”) roles.
FSMO roles are required for certain critical operations such changing a domain name or modifying the AD design schema. Such changes must be carefully coordinated across all DCs. One DC is designated as the “master” for all such critical operations, and all the other DCs must defer to the DC that holds the master role.
If your AD contains only a single domain then the Primary Domain Controller (PDC) will typically hold all of the FSMO roles. This is the most common case.
The Seven FSMO Roles
There are seven FSMO roles defined in Active Directory:
- The Primary Domain Controller (PDC) emulator role, one per domain. The DC with this role coordinates changes to user passwords and secrets.
- The Relative Identifier (RID) Master role, one per domain. The DC with this role allocates RIDs for newly created users and groups.
- The Schema Master role, one per forest. The DC with this role coordinates adding new object classes to the AD design schema.
- The Domain Naming Master role, one per forest. The DC with this role coordinates adding or deleting domains and renaming domains.
- The Infrastructure Master role, one per domain. The DC with this role updates cross-domain references to renamed objects. (The Infrastructure Master role has special rules -- see below.)
- The Domain DNS Zone Master role, one per domain. The DC with this role coordinates adding or deleting any AD-integrated DNS zones on the DCs with DNS servers that host the domain.
- The Forest DNS Zone Master role, one per forest. The DC with this role coordinates adding or deleting the forest-wide records that list all DNS servers that host AD-integrated DNS zones.
To view which DCs own the FSMO roles, type the console command netdom query fsmo.
Verify that the DCs in your test network own the FSMO roles listed above and that at least one DC has the Global Catalog (GC).
Undocumented: The DNS Zone Master roles
Many AD books and websites describe five FSMO roles. There are actually seven. The two extra hidden roles are the Domain DNS Zone Master role and the Forest DNS Zone Master role. These two roles are not well documented and there is no way to display or transfer them without using advanced tools such as ADSIEdit.
U-Move will automatically display the ownership of these hidden roles, and it will offer to move them along with the other well-documented roles when you migrate AD to a new computer.
The Infrastructure Master role is special
The Infrastructure Master role has special rules that must be considered when moving the role to another DC. (Don't worry if you do not understand this section. U-Move will automatically check the rules for you during the migration and advise you on how to proceed.)
The Infrastructure role should be held by a DC that is not a GC in the same domain. This is because the GC holds a partial replica of every object in the forest. The Infrastructure Master role must be held by a DC that is not a GC in the same domain so that it can identify and fix discrepancies between the GC and its own domain objects.
You can safely ignore the Infrastructure Master role if either of the following are true.
- If all of the DCs in the domain are also GCs (which is a common configuration for the DCs in the forest root domain), or if none of the DCs in the domain are GCs (which is a common configuration for the DCs in other domains), then the Infrastructure Master role does not matter.
- You can disregard the Infrastructure Master role if you have enabled the Recycle Bin for the AD forest.
The all/none rule applies only to the DCs actually running. If, for example, you are testing AD changes in your lab then you will typically clone only a single DC to run your tests (the PDC) so the Infrastructure Master role does not matter.
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