Forest trusts

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

Forest trusts

In a Windows Server 2003 forest, you can link two disjoined Windows Server 2003 forests together to form a one-way or two-way, transitive trust relationships. A two-way, forest trust is used to form a transitive trust relationship between every domain in both forests.

Forest trusts can provide the following benefits:

  • Simplified management of resources across two Windows Server 2003 forests by reducing the number of external trusts necessary to share resources.

  • Complete two-way trust relationships with every domain in each forest.

  • Use of user principal name (UPN) authentication across two forests.

  • Use of both the Kerberos V5 and NTLM authentication protocols to improve the trustworthiness of authorization data transferred between forests.

  • Flexibility of administration. Administrative tasks can be unique to each forest.

Forest trusts can only be created between two forests and cannot be implicitly extended to a third forest. This means that if a forest trust is created between forest 1 and forest 2, and a forest trust is also created between forest 2 and forest 3, forest 1 will not have an implicit trust with forest 3. For more information about the requirements needed for a forest trust, see When to create a forest trust.


  • In a Windows 2000 forest, if users in one forest need to access resources in another forest, an administrator can create an external trust relationship between the two domains. External trusts can be one-way or two-way and are nontransitive, and therefore, limit the ability for trust paths to extend to other domains. For more information about external trusts, see Trust types.

  • All trusts in Windows Server 2003 Active Directory use security identifier (SID) filtering to some degree. External trusts are quarantined by default, which prevents any domain SIDs other than those of the quarantined trusted domain from traversing the trust relationship. SID filtering is used to prevent attacks from malicious users who might try to grant elevated user rights to another user account. SID filtering on forest trusts does not prevent migrations to domains within the same forest from using SID history and will not affect your universal group access control strategy. For more information about SID filtering, see When to create an external trust.

Managing a multiple forest environment

Forest trusts help you to manage a segmented Active Directory infrastructure within your organization by providing support for accessing resources and other objects across multiple forests. For more information about accessing resources across multiple forests, see Accessing resources across forests.

Because each forest is administered separately, adding additional forests to your organization increases your organization's management needs. For more information, see Creating a new forest.

Reasons to create multiple forests in your organization include:

  • To secure data within each forest. Sensitive data can be protected so that only users within that forest can access it.

  • To isolate directory replication within each forest. Schema changes, configuration changes, and the addition of new domains to a forest only have forest-wide impact within that forest, not on a trusting forest.

Delegating forest-wide administrative control

Active Directory data that is stored in the schema and configuration containers is replicated to every domain controller in the forest. Since changes to the schema and configuration containers will affect all domains in the forest, administrative control for forest-wide changes should be entrusted to highly trained or experienced administrators. All domain data contained in the forest root domain should also be regarded as highly sensitive data.

The following groups provide forest-wide administrative control in each forest:

  • Enterprise Admins

  • Domain Admins (in the forest root domain)

  • Schema Admins

Since membership in any of these groups can affect forest-wide behavior, add users with caution. As a security best practice, avoid adding users from another forest to any of these forest-wide administrative groups. For more information about these groups, see Default groups.

Synchronizing data across forests

You can synchronize global address lists (GALs) and objects across forests using Microsoft Metadirectory Services (MMS) or another supported synchronization tool. Common data types that need synchronization across forests include:

  • GALs (Exchange)

  • Public folders

  • Directory objects

Synchronizing this data across forests will help end users view address lists and other data the same way as they do when viewing this information within their own forest.

For more information about MMS, see Microsoft Metadirectory Services.