Active Directory
Active Directory is a directory service developed by Microsoft for Windows domain networks. Windows Server operating systems include it as a set of processes and services. Originally, only centralized domain management used Active Directory. However, it ultimately became an umbrella title for various directory-based identity-related services.
A domain controller is a server running the Active Directory Domain Services role. 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 which is part of a Windows domain, Active Directory checks the submitted username and password and determines whether the user is a system administrator or a non-admin user. Furthermore, it allows the management and storage of information, provides authentication and authorization mechanisms, and establishes a framework to deploy other related services: Certificate Services, Active Directory Federation Services, Lightweight Directory Services, and Rights Management Services.
Active Directory uses Lightweight Directory Access Protocol versions 2 and 3, Microsoft's version of Kerberos, and DNS.
Robert R. King defined it in the following way:
History
Like many information-technology efforts, Active Directory originated out of a democratization of design using Requests for Comments. The Internet Engineering Task Force oversees the RFC process and has accepted numerous RFCs initiated by widespread participants. For example, LDAP underpins Active Directory. Also, X.500 directories and the Organizational Unit preceded the Active Directory concept that uses those methods. The LDAP concept began to emerge even before the founding of Microsoft in April 1975, with RFCs as early as 1971. RFCs contributing to LDAP includeMicrosoft previewed Active Directory in 1999, released it first with Windows 2000 Server edition, and revised it to extend functionality and improve administration in Windows Server 2003. Active Directory support was also added to Windows 95, Windows 98, and Windows NT 4.0 via patch, with some unsupported features. Additional improvements came with subsequent versions of Windows Server. In Windows Server 2008, Microsoft added further services to Active Directory, such as Active Directory Federation Services. The part of the directory in charge of managing domains, which was a core part of the operating system, was renamed Active Directory Domain Services and became a server role like others. "Active Directory" became the umbrella title of a broader range of directory-based services. According to Byron Hynes, everything related to identity was brought under Active Directory's banner.
Active Directory Services
Active Directory Services consist of multiple directory services. The best known is Active Directory Domain Services, commonly abbreviated as AD DS or simply AD.Domain Services
Active Directory Domain Services is the foundation of every Windows domain network. It stores information about domain members, including devices and users, verifies their credentials, and defines their access rights. The server running this service is called a domain controller. A domain controller is contacted when a user logs into a device, accesses another device across the network, or runs a line-of-business Metro-style app sideloaded into a machine.Other Active Directory services and most Microsoft server technologies rely on or use Domain Services; examples include Group Policy, Encrypting File System, BitLocker, Domain Name Services, Remote Desktop Services, Exchange Server, and SharePoint Server.
The self-managed Active Directory DS must be distinct from managed Azure AD DS, a cloud product.
Lightweight Directory Services
Active Directory Lightweight Directory Services, previously called Active Directory Application Mode, implements the LDAP protocol for AD DS. It runs as a service on Windows Server and offers the same functionality as AD DS, including an equal API. However, AD [|LDS] does not require the creation of domains or domain controllers. It provides a Data Store for storing directory data and a Directory Service with an LDAP Directory Service Interface. Unlike AD DS, multiple AD LDS instances can operate on the same server.Certificate Services
Active Directory Certificate Services establishes an on-premises public key infrastructure. It can create, validate, revoke and perform other similar actions, public key certificates for internal uses of an organization. These certificates can be used to encrypt files, emails, and network traffic.AD CS predates Windows Server 2008, but its name was simply Certificate Services.
AD CS requires an AD DS infrastructure.
Federation Services
Active Directory Federation Services is a single sign-on service. With an AD FS infrastructure in place, users may use several web-based services or network resources using only one set of credentials stored at a central location, as opposed to having to be granted a dedicated set of credentials for each service. AD FS uses many popular open standards to pass token credentials such as SAML, OAuth or OpenID Connect. AD FS supports encryption and signing of SAML assertions. AD FS's purpose is an extension of that of AD DS: The latter enables users to authenticate with and use the devices that are part of the same network, using one set of credentials. The former enables them to use the same set of credentials in a different network.As the name suggests, AD FS works based on the concept of federated identity.
AD FS requires an AD DS infrastructure, although its federation partner may not.
Rights Management Services
Active Directory Rights Management Services, previously known as Rights Management Services or RMS before Windows Server 2008, is server software that allows for information rights management, included with Windows Server. It uses encryption and selective denial to restrict access to various documents, such as corporate e-mails, Microsoft Word documents, and web pages. It also limits the operations authorized users can perform on them, such as viewing, editing, copying, saving, or printing. IT administrators can create pre-set templates for end users for convenience, but end users can still define who can access the content and what actions they can take.Logical structure
Active Directory is a service comprising a database and executable code. It is responsible for managing requests and maintaining the database. The Directory System Agent is the executable part, a set of Windows services and processes that run on Windows 2000 and later. Accessing the objects in Active Directory databases is possible through various interfaces such as LDAP, ADSI, messaging API, and Security Accounts Manager services.Objects used
Active Directory structures consist of information about objects classified into two categories: resources and security principals. Each security principal is assigned a unique security identifier. An object represents a single entity, such as a user, computer, printer, or group, along with its attributes. Some objects may even contain other objects within them. Each object has a unique name, and its definition is a set of characteristics and information by a schema, which determines the storage in the Active Directory.Administrators can extend or modify the schema using the schema object when needed. However, because each schema object is integral to the definition of Active Directory objects, deactivating or changing them can fundamentally alter or disrupt a deployment. Modifying the schema affects the entire system automatically, and new objects cannot be deleted, only deactivated. Changing the schema usually requires planning.
Forests, trees, and domains
In an Active Directory network, the framework that holds objects has different levels: the forest, tree, and domain. Domains within a deployment contain objects stored in a single replicable database, and the DNS name structure identifies their domains, the namespace. A domain is a logical group of network objects such as computers, users, and devices that share the same Active Directory database.On the other hand, a tree is a collection of domains and domain trees in a contiguous namespace linked in a transitive trust hierarchy. The forest is at the top of the structure, a collection of trees with a standard global catalog, directory schema, logical structure, and directory configuration. The forest is a secure boundary that limits access to users, computers, groups, and other objects.
Organizational units
The objects held within a domain can be grouped into organizational units. 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 simplifying 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, although policies can also be applied to domains or sites. 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 do not each have a separate namespace. As a consequence, for compatibility with Legacy NetBios implementations, user accounts with an identical SamAccountName are not allowed within the same domain even if the accounts objects are in separate OUs. This is because SamAccountName, a user object attribute, must be unique within the domain. However, two users in different OUs can have the same common name, the name under which they are stored in the directory itself such as "fred.staff-ou.domain" and "fred.student-ou.domain", where "staff-ou" and "student-ou" are the OUs.
In general, the reason for this lack of allowance for duplicate names through hierarchical directory placement is that Microsoft primarily relies on the principles of NetBIOS, which is a flat-namespace method of network object management that, for Microsoft software, goes all the way back to Windows NT 3.1 and MS-DOS LAN Manager. Allowing for duplication of object names in the directory, or completely removing the use of NetBIOS names, would prevent backward compatibility with legacy software and equipment. However, disallowing duplicate object names in this way is a violation of the LDAP RFCs on which Active Directory is supposedly based.
As the number of users in a domain increases, conventions such as "first initial, middle initial, last name" or the reverse 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 users' 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.