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In hybrid environments, separate Active Directory forests are often used to isolate resources or administrative control. A cross-forest trust is a relationship that allows users in one forest to securely access resources in another forest. These trusts can be one-way (where only one forest trusts the other) or two-way (where both forests trust each other).
You can create these trust relationships using either the graphical Active Directory Domains and Trusts tool or PowerShell cmdlets like New-ADTrust. Before creating the trust, you must ensure that DNS resolution works between the forests, typically by setting up conditional forwarders. After creation, you must verify the secure channel—the encrypted connection used for authentication—is functioning correctly, which can be tested with commands like Test-ComputerSecureChannel.
Two critical settings govern how a trust operates. SID filtering is a security feature that prevents a user from bringing potentially malicious security identifiers (SIDs) from one forest into another; it is usually enabled by default. Trust transitivity determines whether a trust can be extended through a third forest. For example, if Forest A trusts Forest B, and Forest B trusts Forest C, a transitive trust would allow Forest A to trust Forest C automatically. This is a key design decision when planning your forest relationships.
Common problems include authentication failures and issues related to SID filtering. To troubleshoot, start by checking the Windows Security event logs on domain controllers for specific error codes. Next, verify the trust path and test the secure channel again. If SID filtering is blocking necessary access (such as during a user migration where SID history is required), you may need to adjust the SIDFilterQuarantine attribute, though this should be done cautiously due to security implications.
Once configured, validate that the trust works as intended by having a test user from one forest attempt to access a shared resource in the other. Regular monitoring is important, as network changes or expired trust passwords can break the secure channel. Using tools like Test-Availability can help proactively check the health of the trust relationship.
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