What is NTDS.DIT

Windows 2000 Active Directory data store, the actual database file, is %SystemRoot%\ntds\NTDS.DIT. The ntds.dit file is the heart of Active Directory including user accounts. Active Directory’s database engine is the Extensible Storage Engine (ESE ) which is based on the Jet database used by Exchange 5.5 and WINS.
The ESE has the capability to grow to 16 terabytes which would be large
enough for 10 million objects. Back to the real world. Only the Jet
database can maniuplate information within the AD data store.

The Active Directory ESE database, NTDS.DIT, consists of the following tables:

  • Schema table 
    the types of objects that can be created in the Active Directory, relationships between them, and the optional and mandatory attributes on each type of object. This table is fairly static and much smaller than the data table.
  • Link table 
    contains linked attributes, which contain values referring to other objects in the Active Directory. Take the MemberOf attribute on a user object. That attribute contains values that reference groups to which the user belongs. This is also far smaller than the data table.
  • Data table 
    users, groups,application-specific data, and any other data stored in the Active Directory. The data table can be thought of as having rows where each row represents an instance of an object such as a user, and columns where each column represents an attribute in the schema such as GivenName.

From a different perspective, Active Directory has three types of data

  • Schema information 
    definitional details about objects and attributes that one CAN store in the AD. Replicates to all domain controllers. Static in nature.
  • Configuration information 
    configuration data about forest and trees. Replicates to all domain controllers. Static as your forest is.
  • Domain information 
    object information for a domain. Replicates to all domain controllers within a domain. The object portion becomes part of Global Catalog. The attribute values (the actual bulk of data) only replicates within the

Although GUIDs are unique, they are large. AD uses distinguished
name tag ( DNT ). DNT is a 4-byte DWORD value which is incremented when
a new object is created in the store. The DNT represents the object’s
database row number. It is an example of a fixed column. Each object’s
parent relationship is stored as a parent distinguished name tag ( PDNT
). Resolution of parent-child relationships is optimized because the
DNT and PDNT are indexed fields in the database. 

The size of ntds.dit will often be different sizes across the domain controllers in a domain. Remember that Active Directory is a multi-master independent model where updates are occurring in each of the ADs with the changes being replicated over time to the other domain controllers. The changed data is replicated between domain controllers, not the database, so there is no guarantee that the files are going to be the same size across all domain controllers.

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