What does Windows error code 8635 (ERROR_DS_FLAT_NAME_EXISTS_IN_FOREST) mean?

ERROR_DS_FLAT_NAME_EXISTS_IN_FOREST

The flat name of the trusted domain already exists in the forest.

ERROR_DS_FLAT_NAME_EXISTS_IN_FOREST is Win32 error 8635 (0x000021BB).

Likely impact: Legacy logon names and trust routing depend on this namespace being unique.

Typical causes

• another domain already uses the flat name.
• a stale trust object retains it.
• a migration preserved a conflicting NetBIOS name.
• the caller checks only DNS uniqueness.

Troubleshooting steps

1. Verify that the proposed flat name is unique across domains and trusted-domain metadata.
2. Collect proposed flat name and trustedDomain flatName attributes.

Useful evidence

• Collect stale object replication metadata.
• Collect trust wizard or API request.

Recovery and retry

Select a unique flat name or remove only verified stale trust metadata before recreating the trust.

Related errors

ERROR_DS_DOMAIN_NAME_EXISTS_IN_FOREST concerns the DNS name; this value concerns the short flat name

Example

Two companies merge with distinct DNS names but the same legacy NetBIOS name. Assigning a new flat name before trust creation resolves the collision.

References

• Microsoft: exact Win32 system error range
• Microsoft: AD DS troubleshooting
• Microsoft: dcdiag
• Microsoft: repadmin