Information about Active Directory

Aboobakar Sanjar
Microsoft Active Directory Related Information’s
Active Directory (AD) is a directory service created by Microsoft for Windows domain networks. It is included in most Windows Server operating systems. Server computers that run Active Directory are called domain controllers.Active Directory provides a central location for network administration and security. It authenticates and authorizes all users and computers in a Windows domain type network—assigning and enforcing security policies for all computers and installing or updating software. For example, when a user logs into a computer that is part of a Windows domain, Active Directory verifies the password and specifies whether the user is a system administrator or normal user.[1]

Active Directory uses Lightweight Directory Access Protocol (LDAP) versions 2 and 3, Kerberos and DNS.

History.
Active Directory was previewed in 1999, released first with Windows 2000 Server edition, and revised to extend functionality and improve administration in Windows Server 2003. Additional improvements were made in Windows Server 2003 R2, Windows Server 2008 and Windows Server 2008 R2, and with the release of the latter the domain controller role was renamed Active Directory Domain Services.
  1. Windows Server 2000
  2. Windows Server 2003
  3. Windows Server 2003 R2
  4. Windows Server 2008
  5. Windows Server 2008 R2
  6. Windows Server 2012
  7. Windows Server 2012 R2
  8. Windows Server 2016
 
Structure

Objects

An Active Directory structure is a hierarchical arrangement of information about objects. The objects fall into two broad categories: resources (e.g., printers) and security principals (user or computer accounts and groups). Security principals are assigned unique security identifiers (SIDs).

Each object represents a single entity—whether a user, a computer, a printer, or a group—and its attributes. Certain objects can contain other objects. An object is uniquely identified by its name and has a set of attributes—the characteristics and information that the object represents— defined by a schema, which also determines the kinds of objects that can be stored in Active Directory.

The schema object lets administrators extend or modify the schema when necessary. However, because each schema object is integral to the definition of Active Directory objects, deactivating or changing these objects can fundamentally change or disrupt a deployment. Schema changes automatically propagate throughout the system. Once created, an object can only be deactivated—not deleted. Changing the schema usually requires planning.

Sites

A Site object in Active Directory represents a geographic location that hosts networks.

Forests, trees, and domains

The Active Directory framework that holds the objects can be viewed at a number of levels. The forest, tree, and domain are the logical divisions in an Active Directory network.

Within a deployment, objects are grouped into domains. The objects for a single domain are stored in a single database (which can be replicated). Domains are identified by their DNS name structure, the namespace.

A tree is a collection of one or more domains and domain trees in a contiguous namespace, linked in a transitive trust hierarchy.

At the top of the structure is the forest. A forest is a collection of trees that share a common global catalog, directory schema, logical structure, and directory configuration. The forest represents the security boundary within which users, computers, groups, and other objects are accessible.

Organizational units

The objects held within a domain can be grouped into Organizational Units (OUs). OUs can provide hierarchy to a domain, ease its administration, and can resemble the organization’s structure in managerial or geographical terms. OUs can contain other OUs—domains are containers in this sense. Microsoft recommends using OUs rather than domains for structure and to simplify the implementation of policies and administration. The OU is the recommended level at which to apply group policies, which are Active Directory objects formally named Group Policy Objects (GPOs), although policies can also be applied to domains or sites (see below). The OU is the level at which administrative powers are commonly delegated, but delegation can be performed on individual objects or attributes as well.

Organizational Units are an abstraction for the administrator and do not function as containers; the underlying domain is the true container. It is not possible, for example, to create user accounts with an identical username (sAMAccountName) in separate OUs, such as “fred.staff-ou.domain” and “fred.student-ou.domain”, where “staff-ou” and “student-ou” are the OUs. This is so because sAMAccountName, a user object attribute, must be unique within the domain. However, two users in different OUs can have the same Common Name (CN), the first component of the Distinguished Name (DN) of the user. Thus from the point of view of the DN, OUs do function as containers.

As the number of users in a domain increases, conventions such as “first initial, middle initial, last name” (Western order) or the reverse (Eastern order) fail for common family names like Li, Smith or Garcia. Workarounds include adding a digit to the end of the username. Alternatives include creating a separate ID system of unique employee/student id numbers to use as account names in place of actual user’s names, and allowing users to nominate their preferred word sequence within an acceptable use policy.

Because duplicate usernames cannot exist within a domain, account name generation poses a significant challenge for large organizations that cannot be easily subdivided into separate domains, such as students in a public school system or university who must be able to use any computer across the network.

Single server operations

Flexible Single Master Operations Roles (FSMO, sometimes pronounced “fizz-mo”) operations are also known as operations master roles. Although domain controllers allow simultaneous updates in multiple places, certain operations are supported only on a single server. These operations are performed using the roles listed below:
Role Name Scope Description
Schema Master 1 per forest Schema modifications
Domain Naming Master 1 per forest Addition and removal of domains if present in root domain
PDC Emulator 1 per domain Provides backwards compatibility for NT4 clients for PDC operations (like password changes). The PDC runs domain specific processes such as the Security Descriptor Propagator (SDPROP), and is the master time server within the domain. It also handles external trusts, the DFS consistency check, holds current passwords and manages all GPOs as default server.
RID Master 1 per domain Allocates pools of unique identifiers to domain controllers for use when creating objects
Infrastructure Master 1 per domain/partition Synchronizes cross-domain group membership changes. The infrastructure master cannot run on a global catalog server (GCS) (unless all DCs are also GCs, or environment consists of a single domain).

Trusting

To allow users in one domain to access resources in another, Active Directory uses trusts.

Trusts inside a forest are automatically created when domains are created. The forest sets the default boundaries of trust, and implicit, transitive trust is automatic for all domains within a forest.

Terminology

One-way trust
One domain allows access to users on another domain, but the other domain does not allow access to users on the first domain.

Two-way trust
Two domains allow access to users on both domains.

Trusting domain
The domain that allows access to users from a trusted domain.

Trusted domain
The domain that is trusted; whose users have access to the trusting domain.

Transitive trust
A trust that can extend beyond two domains to other trusted domains in the forest.

Intransitive trust
A one way trust that does not extend beyond two domains.

Explicit trust
A trust that an admin creates. It is not transitive and is one way only.

Cross-link trust
An explicit trust between domains in different trees or in the same tree when a descendant/ancestor (child/parent) relationship does not exist between the two domains.

Shortcut
Joins two domains in different trees, transitive, one- or two-way.

Forest
Applies to the entire forest. Transitive, one- or two-way

Realm
Can be transitive or non-transitive, one- or two-way

External
Connect to other forests or non-AD domains. Non-transitive, one- or two-way.

Windows Server 2003 introduced the forest root trust. This trust can be used to connect Windows Server 2003 forests if they are operating at the 2003 forest functional level. Authentication across this type of trust is Kerberos based (as opposed to NTLM). Forest trusts are transitive for all the domains in the trusted forests. Forest trusts, however, are not transitive at a forest level. So where domains inside two trusting forests trust each other, forests A that trusts Forest B will not automatically trust Forest C because it is trusted by forest B. In that sense forest A wil not automatically (transatively) trust forest.

Lightweight Directory Service

Active Directory Lightweight Directory Service (AD LDS), formerly known as Active Directory Application Mode (ADAM), is a light-weight implementation of Active Directory. AD LDS is capable of running as a service on computers running Microsoft Windows Server. AD LDS shares the code base with Active Directory and provides the same functionality as Active Directory, including an identical API, but does not require the creation of domains or domain controllers.

Like Active Directory, AD LDS provides a Data Store for storage of directory data and a Directory Service with an LDAP Directory Service Interface. Unlike Active Directory, however, multiple AD LDS instances can be run on the same server.