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Active Directory

Updated: November 25, 2009

Applies To: Windows Server 2008 R2

Active Directory technologies define the data structure and services that provide organization, management, and security of accounts and resources in a Microsoft network.

Hierarchy of Managed Entities

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Managed Entities

Name Description

AD DS

Active Directory Domain Services (AD DS) defines the data structure and services that provide organization, management, and security of accounts and resources in a Microsoft network.

Forest

An Active Directory forest contains all the domains, sites, and trusts that are part of Active Directory Domain Services (AD DS). The forest acts as a security boundary for an organization, and it defines the scope of authority for administration. By default, a forest contains a single domain, which is called the forest root domain.

Domain

A domain is a distinct unit of administration and resource grouping in Active Directory Domain Services (AD DS).

Server

Domain controllers are servers that host Active Directory Domain Services (AD DS) resources. These servers host essential services in AD DS, including the following:

  • Kerberos Key Distribution Center (kdc)
  • NetLogon (Netlogon)
  • Windows Time (W32time)
  • Intersite Messaging (IsmServ)
  • File Replication (ntfrs): required if the forest functional level is lower than Windows Server 2008 or if an upgraded forest is at the Windows Server 2008 functional level and Distributed File System Replication (DFSR) is not yet configured
  • Distributed File System (Dfs): if the forest functional level is Windows Server 2008 and DFSR is in use

In addition, domain controllers host the SYSVOL share. Domain controllers must register Domain Controller Locator (DC Locator) records with Domain Name System (DNS) so that domain member computers can locate resources on the domain.

Directory Service

The directory service is a database with multiple data partitions, as well as the processes to maintain, manage, and secure the database. Domain controllers host and replicate the directory service database inside the forest. The directory service also provides services for managing and authenticating resources in the forest.

Interfaces

Lightweight Directory Access Protocol (LDAP) is the standard protocol that directory clients use to gain access to data that is held by directory servers. LDAP supports a relatively simple set of operations, such as bind, unbind, read, and modify. LDAP is the primary interface to Active Directory Domain Services (AD DS), and it is responsible for packaging and interpreting LDAP packets over the network.

Data (Database, Logs, SYSVOL, Partitions)

Active Directory data is replicated as a database that is separated into several partitions. These partitions represent the major object categories that organize, manage, and secure domain resources. Administrators and services can define custom data partitions. For example, Domain Name System (DNS) data partitions are created when DNS information is integrated with Active Directory Domain Services (AD DS). The partitions that are created by default include the following:

  • Domain naming context: Includes user, group, and computer accounts; network shares; and other resources for each domain in the forest.
  • Configuration container: Includes configuration information about the sites, domains, and services that are available across the forest.
  • Schema: Defines the type of information that can be stored.

The database itself consists of the Ntds.dit file and its related logs, which are stored in the NTDS folder on each domain controller by default. The folder location where the database is stored can be changed.

Another essential component of AD DS is the SYSVOL shared folder on each domain controller. The SYSVOL shared folder provides a location to which domain controllers replicate AD DS data to each other.

Partition Replication

Active Directory Domain Services (AD DS) data is logically partitioned so that all domain controllers in the forest do not store all objects in the directory. Active Directory objects are instances of schema-defined classes, which consist of named sets of attributes. When a change is made to an object in a directory partition, the value of the changed attribute or attributes must be updated on all domain controllers that store a replica of the same directory partition. Domain controllers communicate data updates automatically through Active Directory replication. Communication about updates is always specific to a single directory partition at a time.

Different categories of data are stored in replicas of different directory partitions, as follows:

  • Domain directory partition: Also known as the domain naming context (NC), contains domain-specific objects such as computer, user, and group accounts.
  • Configuration directory partition: Contains forest-wide data that controls site and replication operations.
  • Schema directory partition: Contains schema definitions for the forest.
  • Application directory partitions: Contain data that is particular to specific applications. Application directory partition replicas can be replicated to any set of domain controllers in a forest, irrespective of domain.

Schema

The Active Directory schema is the set of definitions that defines the kinds of objects, and the types of information about those objects, that can be stored in Active Directory Domain Services (AD DS). The definitions are themselves stored as objects so that AD DS can manage the schema objects with the same object management operations that are used for managing the rest of the objects in the directory.

There are two types of definitions in the schema: attributes and classes. Attributes and classes are also referred to as schema objects or metadata.

Attributes are defined separately from classes. Each attribute is defined only once and can be used in multiple classes. For example, the Description attribute is used in many classes, but it is defined once in the schema, which helps ensure consistency.

Classes, also referred to as object classes, describe the possible directory objects that can be created. Each class is a collection of attributes. When you create an object, the attributes store the information that describes the object. The User class, for example, is composed of many attributes, including Network Address, Home Directory, and so on. Every object in AD DS is an instance of an object class.

Schema Directory Partition

The schema is stored in its own partition (the schema directory partition). The schema directory partition is replicated among all the domain controllers in the forest, and any change that is made to the schema is replicated to every domain controller in the forest. Because the schema dictates how information is stored, and because any changes that are made to the schema affect every domain controller, changes to the schema should be made only when necessary — through a tightly controlled process — after testing has been performed to ensure that there will be no adverse effects on the rest of the forest.

Application Directory Partition

Active Directory Domain Services (AD DS) supports application directory partitions. An application directory partition can contain a hierarchy of any type of objects, except security principals. You can configure an application directory partition to replicate to any set of domain controllers in the forest. An application directory partition can replicate to domain controllers in different domains in the forest. Unlike a domain directory partition, an application directory partition is not required to replicate to all domain controllers in a domain.

Windows Time Service

The Windows Time service (W32time) automatically synchronizes the local computer's time with other computers on the network.

The Windows Time service architecture consists of the following components:

  • Service Control Manager
  • Windows Time Service Manager
  • Clock Discipline
  • Time providers

The Windows Time service internal time synchronization process involves the following steps:

  • Input providers request and receive time samples from configured NTP time sources.
  • These time samples are then passed to the Windows Time Service Manager, which collects all the samples and passes them to the clock discipline subcomponent.
  • The clock discipline subcomponent applies all NTP algorithms and selects the best time sample.
  • The clock discipline subcomponent adjusts the time of the system clock to the most accurate time by either adjusting the clock rate or directly changing the time.

If a computer has been designated as a time server, it can send the time on to any computer requesting time synchronization at any point in this process.

Time Source Peer

A time source peer is a server from which time samples are acquired. The time source for this varies, depending on whether the computer is joined to a domain in Active Directory Domain Services (AD DS) (domain heirarchy peers) or to a workgroup (manually configured peers).

Clock Manager

The Clock Manager receives updates from a peer and uses them to skew local system time within compliance.

Time Provider

A time source (also known as a time provider or an input provider) represents a source protocol for acquiring time samples. Windows includes a Network Time Protocol (NTP) time source with the Windows Time service.

SAM

The Security Accounts Manager (SAM) is a database that stores user accounts and security descriptors for users on the local computer.

Account Management

Account management includes all aspects of creating, modifying, and deleting user accounts. This includes detection of duplicate accounts and security identifiers (SIDs).

Database/Configuration

Database/configuration is a process that ensures that the security database is initialized, properly configured, and available for use by the system.

DB Upgrade/DC Promotion/DC Demotion

The Security Accounts Manager (SAM) database changes state (active or inactive):

  • During an operating system upgrade.
  • When a server becomes a domain controller.
  • When a server is no longer a domain controller.

The database upgrade, domain controller installation, and domain controller removal processes are designed to track events that are related to SAM state changes.

RID Manager

The relative ID (RID) manager is responsible for providing numbers that are used to create unique security identifiers (SIDs) for each account in a domain.

LSA Policy

You can use Local Security Authority (LSA) policy to manage trust relationships between domains. The LSA also provides a software interface for other software components when they query mappings of account names to security identifiers (SIDs) between the local domain and trusted domains.

Trusts

You can use Local Security Authority (LSA) policy to manage trust relationships between domains and forests.

NetLogon

The NetLogon service verifies NTLM logon requests, and it registers, authenticates, and locates domain controllers. Also, to maintain compatibility with older operating systems, NetLogon manages replication of the user account database to back up domain controllers running Windows NT 4.0 and earlier.

Global Catalog Server

The global catalog is a distributed data repository that facilitates searches and logons in an Active Directory forest. The Active Directory replication system builds global catalog data automatically.

One or more domain controllers in an Active Directory forest host the global catalog. The domain controllers that host the global catalog are called global catalog servers.

Users and applications can use the global catalog to locate objects in any domain in the forest by searching for an attribute of the object. For example, an administrator can use the global catalog to search for a user's last name to locate that user's account in the forest. A user can also use the global catalog to search the forest for a list of printers that are organized by location.

The global catalog facilitates logons by ensuring that membership in universal groups from all domains is represented in the user's access credentials (also known as the access token).

DFS Namespace

You can use Distributed File System (DFS) namespace to group shared folders that are located on different servers into one or more logically structured namespaces. Each namespace appears to users as a single shared folder with a series of subfolders. However, the underlying structure of the namespace can consist of numerous shared folders that are located on different servers and in multiple sites.

AD LDS

Organizations that have applications that require a directory for storing application data can use Active Directory Lightweight Directory Services (AD LDS) as the data store. AD LDS runs as a service that is independent of a domain's directory service. AD LDS does not require deployment on an Active Directory domain controller. In addition, you can install multiple instances of AD LDS and run them concurrently on a single computer.

Note: AD LDS was previously known as Active Directory Application Mode (ADAM).

Configuration

If there is a change to the configuration of an Active Directory Lightweight Directory Services (AD LDS) instance, the change must be registered with the instance's internal database as well as with the databases of any replication partners that are configured. Such updates are especially important when replication partners exist, because a local instance cannot receive any updates from its replication partners until the change is registered by the replication partners in their respective databases. The configuration changes that must be recorded and replicated include the following:

  • Host name modification of the server that hosts the AD LDS instance
  • Changes to the network communication port on which AD LDS services are offered
  • Changes to the service account that AD LDS is using

Note: AD LDS provides services by using the security credentials of a user account. A user account that a service uses is commonly known as a service account.

SCP

When Active Directory Lightweight Directory Services (AD LDS) is running on a computer that is joined to a domain, the AD LDS instance creates a serviceConnectionPoint (SCP) object in the domain so that other computers in the domain can locate the AD LDS instance.

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