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What Are Domain and Forest Trusts?

Updated: August 10, 2010

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2, Windows Server 2008, Windows Server 2008 R2

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Most organizations that have more than one domain have a legitimate need for users to access shared resources located in a different domain. Controlling this access requires that users in one domain can also be authenticated and authorized to use resources in another domain. To provide authentication and authorization capabilities between clients and servers in different domains, there must be a trust between the two domains. Trusts are the underlying technology by which secured Active Directory communications occur, and are an integral security component of the Windows Server 2003 network architecture.

When a trust exists between two domains, the authentication mechanisms for each domain trust the authentications coming from the other domain. Trusts help provide for controlled access to shared resources in a resource domain (the trusting domain) by verifying that incoming authentication requests come from a trusted authority (the trusted domain). In this way, trusts act as bridges that allow only validated authentication requests to travel between domains.

How a specific trust passes authentication requests depends on how it is configured; trust relationships can be one-way, providing access from the trusted domain to resources in the trusting domain, or two way, providing access from each domain to resources in the other domain. Trusts are also either nontransitive, in which case trust exists only between the two trust partner domains, or transitive, in which case trust automatically extends to any other domains that either of the partners trusts.

In some cases, trust relationships are automatically established when domains are created; in other cases, administrators must choose a type of trust and explicitly establish the appropriate relationships. The specific types of trusts used and the structure of the resulting trust relationships in a given trust implementation depend on such factors as how the Active Directory directory service is organized, and whether different versions of Windows coexist on the network.

Trust Scenarios

It is possible to create a number of different domain and forest trust configurations, depending on the Active Directory structure of the organization. Windows Server 2003 domains and forests can trust other Windows Server 2003 domains and forests, as well as Windows 2000 and Windows NT 4.0 domains. For example, trust configurations vary in nature and complexity in each of the following scenarios:

Trusts within a single Windows 2000 Server or Windows Server 2003 forest

By default, all domain trusts within a single Active Directory forest are two-way, transitive trusts. There are three types of transitive trusts that are used within a single Windows 2000 Server or Windows Server 2003 forest. The first is the tree-root trust, which is created by default when you create a new domain tree by using the Active Directory Installation Wizard. The two-way transitive nature of intra-forest trusts such as the tree-root trust allows all domains in one tree to trust all domains in any other tree within the same forest.

The second type of trust is a parent-child trust. It is created automatically when you create a new domain in an existing domain tree by using the Active Directory Installation Wizard. When a new child domain is created, a parent-child trust is established between the new domain and the domain that immediately precedes it in the namespace hierarchy.

The last type of trust that can be used between trees is a shortcut trust, and is used to speed up access times to resources in a domain that is deep within the tree hierarchy of another domain.

Trusts between two Windows Server 2003 forests

It is possible to extend the transitivity of domain trusts within a single Windows Server 2003 forest to another Windows Server 2003 forest by manually creating a one-way or two-way forest trust. A forest trust is a transitive trust between a forest root domain and a second forest root domain. A one-way forest trust allows all users in one forest to trust all domains in the other forest; a two-way forest trust forms a transitive trust relationship between every domain in both forests. The transitivity of forest trusts is limited to the two forest partners; the forest trust does not extend to additional forests trusted by either of the partners.

Trusts across Windows Server 2003 and Windows 2000 forests

Windows Server 2003 forest trusts cannot be created between a Windows Server 2003 forest and a Windows 2000 forest. You can, however, manually create a trust relationship between any domain in a Windows Server 2003 forest and any domain in a Windows 2000 forest by using one-way or two-way external trusts. External trusts are nontransitive and provide for access to resources in another domain outside the forest that is not already joined by a forest trust.

Trusts between Windows Server 2003 or Windows 2000 domains and Windows NT 4.0 domains

You can manually create a one-way or two-way external trust between Windows Server 2003 or Windows 2000 domains and Windows NT 4.0 domains so that users from either domain can be authenticated to access resources in the other domain.

Trusts between Windows 2000 or Windows Server 2003 domains and non-Windows Kerberos realms

Windows 2000 or Windows Server 2003 domains can be configured to trust non-Windows-brand operating system Kerberos realms, and non-Windows Kerberos realms can be configured to trust Windows Server 2003 domains by manually creating one-way or two-way realm trusts. Realm trusts can also be configured to be either nontransitive or transitive, depending on the level of interoperability you require with UNIX or Massachusetts Institute of Technology implementations of the Kerberos version 5 protocol.

When the direction of a one-way trust is from a non-Windows Kerberos realm to a Windows Server 2003 domain, the user in the Windows Server 2003 domain can access resources in the non-Windows Kerberos realm. When the direction of trust is from a Windows Server 2003 domain to a non-Windows Kerberos realm, users in the non-Windows Kerberos realm can access the resources in the Windows Server 2003 domain.

Technologies Related to Trusts

Trusts depend on the NTLM and Kerberos authentication protocols and on Windows-based authorization and access control mechanisms to help provide a secured communications infrastructure across Active Directory domains and forests. The following diagram illustrates how authentication and authorization technologies relate to trusts and other components of the Windows distributed security model.

Trusts and the Windows Distributed Security Model

Trusts & the Windows Distributed Security Mode

Applications and Net Logon

Both applications and the Net Logon service are components of the Windows distributed security channel model. Applications integrated with Windows Server 2003 and Active Directory use authentication protocols to communicate with the Net Logon service so that a secured path can be established over which authentication can occur.

Authentication Protocols

Active Directory domain controllers authenticate users and applications by using one of two protocols: either the Kerberos version 5 authentication protocol or the NTLM authentication protocol. When two Active Directory domains or forests are connected by a trust, authentication requests made using these protocols can be routed to provide access to resources in both forests.

NTLM

The NTLM protocol is the default protocol used for network authentication in the Windows NT 4.0 operating system. For compatibility reasons, it is used by Active Directory domains to process network authentication requests that come from earlier Windows-based clients and servers. Computers running Windows 2000, Windows XP or Windows Server 2003 use NTLM only when authenticating to servers running Windows NT 4.0 and when accessing resources in Windows NT 4.0 domains.

When the NTLM protocol is used between a client and a server, the server must contact a domain authentication service on a domain controller to verify the client credentials. The server authenticates the client by forwarding the client credentials to a domain controller in the client account domain. The authentication protocol of choice for Active Directory authentication requests, when there is a choice, is Kerberos version 5. When the Kerberos protocol is used, the server does not have to contact the domain controller. Instead, the client gets a ticket for a server by requesting one from a domain controller in the server account domain; the server validates the ticket without consulting any other authority.

Kerberos Version 5 Protocol

The Kerberos version 5 protocol is the default authentication protocol used by computers running Windows 2000, Windows XP Professional, or Windows Server 2003. This protocol is specified in RFC 1510 and is fully integrated with Active Directory, server message block (SMB), HTTP, and remote procedure call (RPC), as well as the client and server applications that use these protocols. In Active Directory domains, the Kerberos protocol is used to authenticate logons when any of the following conditions is true:

  • The user who is logging on uses a security account in an Active Directory domain.

  • The computer that is being logged on to is a Windows 2000, Windows XP or Windows Server 2003–based computer.

  • The computer that is being logged on to is joined to an Active Directory domain.

  • The computer account and the user account are in the same forest.

  • The computer from which the user is trying to access resources is located in a non-Windows Kerberos realm.

If any computer involved in a transaction does not support the Kerberos version 5 protocol, the NTLM protocol is used.

Authorization and Access Control

Authorization and trust technologies work together to help provide a secured communications infrastructure across Active Directory domains or forests. Authorization determines what level of access a user has to resources in a domain. Trusts facilitate cross-domain authorization of users by providing a path for authenticating users in other domains so their requests to shared resources in those domains can be authorized.

Once an authentication request made to a resource in a trusting domain is validated by the trusted domain, it is passed to the targeted resource computer, which determines, based on its access control configuration, whether to authorize the specific request made by the user, service, or computer in the trusted domain. In this way, trusts provide the mechanism by which validated authentication requests are passed to a trusting domain, while access control mechanisms on the resource computer determine the final level of access granted to the requestor in the trusted domain.

Note

  • “Access to resources” in any discussion of trust relationships always assumes the limitations of access control.

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