USN RollBack in Active Directory
This blog will help describe a condition that occurs when a domain controller that is running Windows 2000, Windows Server 2003, Windows Server 2008, or Windows Server 2008 R2 starts from an Active Directory database that has been incorrectly restored or copied into place. This condition is known as an update sequence number rollback or USN rollback.
When a USN rollback occurs, modifications to objects and attributes that occur on one domain controller do not replicate to other domain controllers in the forest. Because replication partners believe that they have an up-to-date copy of the Active Directory database, monitoring and troubleshooting tools such as Repadmin.exe do not report any replication errors.
After hotfix 875495 or Windows Server 2003 Service Pack 1 is installed, a Microsoft Windows Server 2003 domain controller logs Directory Services event 2095 when it encounters a USN rollback. The text of the event message directs administrators to this article to learn about recovery options.
Because it is difficult to detect and recover from a USN rollback, we recommend that administrators install hotfix 875495 or the latest service pack that is available) on Windows Server 2003 RTM. The hotfix is included in Windows Server 2003 SP1 as well as in Windows Server 2008 and Windows Server 2008 R2.
This blog will in detail help you to understand the below points:
- Supported methods to back up Active Directory on domain controllers that are running Windows Server 2003, Windows Server 2008, or Windows Server 2008 R2
- Typical behavior that occurs when you restore an Active Directory-aware system state backup
- How copying a previous Active Directory database into the folder that contains the current Active Directory database without restoring the system state can lead to a USN rollback
- How Active Directory replication is affected when a Microsoft Windows Server 2003-based domain controller experiences a USN rollback
- Ways to recover an Active Directory domain controller after it experiences a USN rollback
- Enhancements in hotfix 875495 (and in Windows Server 2003 Service Pack 1, Windows Server 2008, and Windows Server 2008 R2) to detect USN rollbacks and to quarantine affected domain controllers
Over a domain controller’s lifecycle, you may have to restore, or “roll back,” the contents of the Active Directory database to a known good point in time. Or, you may have to roll back elements of a domain controller’s host operating system, including Active Directory, to a known good point.
The following are advised methods that you can use to roll back the contents of Active Directory:
- Use an Active Directory-aware backup and restoration utility which non-authoritatively or authoritatively restore a system state backup. The backup that is restored should originate from the same operating system installation and from the same physical or virtual computer that is being restored.
- Use an Active Directory-aware backup and restoration utility that uses Microsoft Volume Shadow Copy Service which backs up and restores the domain controller system state. The Volume Shadow Copy Service supports creating single point-in-time shadow copies of single or multiple volumes on computers that are running Windows Server 2003, Windows Server 2008, or Windows Server 2008 R2. Single point-in-time shadow copies are also known as snapshots. For more information, visit the following Microsoft Web site and search for “Volume Shadow Copy Service”:
- Restore the system state. Evaluate whether valid system state backups exist for this domain controller. If a valid system state backup was made before the rolled-back domain controller was incorrectly restored and if the backup contains recent changes that were made on the domain controller, restore the system state from the most recent backup.
Typical behavior that occurs when you restore an Active Directory-aware system state backup:
- Windows Server 2003 domain controllers use USNs together with the invocation IDs to track updates that must be replicated between replication partners in an Active Directory forest.
- Source domain controllers use USNs to determine what changes have already been received by the destination domain controller that is requesting changes. Destination domain controllers use USNs to determine what changes should be requested from source domain controllers.
- The invocation ID identifies the version or the instantiation of the Active Directory database that is running on a given domain controller.
- When Active Directory is restored on a domain controller by using the APIs and methods that Microsoft has designed and tested, the invocation ID is correctly reset on the restored domain controller. Domain controllers in the forest receive notification of the invocation reset. Therefore, they adjust their high watermark values accordingly.
Software and methodologies that cause USN rollbacks
When the following environments, programs, or subsystems are used, administrators can bypass the checks and validations that Microsoft has designed to occur when the domain controller system state is restored:
Starting an Active Directory domain controller whose Active Directory database file was restored (copied) into place by using an imaging program such as Norton Ghost.
Starting a previously saved virtual hard disk image of a domain controller. The following scenario can cause a USN rollback:
- Promote a domain controller in a virtual hosting environment.
- Create a snapshot or alternative version of the virtual hosting environment.
- Let the domain controller continue to inbound replicate and to outbound replicate.
- Start the domain controller image file that you created earlier.
Starting an Active Directory domain controller that is located on a volume where the disk subsystem loads by using previously saved images of the operating system without requiring a system state restoration of Active Directory. Scenario A: Starting multiple copies of Active Directory that are located on a disk subsystem that stores multiple versions of a volume
- Promote a domain controller. Locate the Ntds.dit file on a disk subsystem that can store multiple versions of the volume that hosts the Ntds.dit file.
- Use the disk subsystem to create a snapshot of the volume that hosts the Ntds.dit file for the domain controller.
- Continue to let the domain controller load Active Directory from the volume that you created in step 1.
- Start the domain controller that the Active Directory database saved earlier.
Scenario B: Starting Active Directory from other drives in a broken mirror
- Promote a domain controller. Locate the Ntds.dit file on a mirrored drive.
- Break the mirror.
- Continue to inbound replicate and outbound replicate by using the Ntds.dit file on the first drive in the mirror.
Start the domain controller by using the Ntds.dit file on the second drive in the mirror
Even if not intended, each of these scenarios can cause domain controllers to roll back to an older version of the Active Directory database by unsupported methods. The only supported way to roll back the contents of Active Directory or the local state of an Active Directory domain controller is to use an Active Directory-aware backup and restoration utility to restore a system state backup that originated from the same operating system installation and the same physical or virtual computer that is being restored.
Microsoft does not support any other process that takes a snapshot of the elements of an Active Directory domain controller’s system state and copies elements of that system state to an operating system image. Unless an administrator intervenes, such processes cause a USN rollback. This USN rollback causes the direct and transitive replication partners of an incorrectly restored domain controller to have inconsistent objects in their Active Directory databases.
The effects of a USN rollback
When USN rollbacks occur, modifications to objects and attributes are not inbound replicated by destination domain controllers that have previously seen the USN.
Because these destination domain controllers believe they are up to date, no replication errors are reported in Directory Service event logs or by monitoring and diagnostic tools.
USN rollback may affect the replication of any object or attribute in any partition. The most frequently observed side effect is that user accounts and computer accounts that are created on the rollback domain controller do not exist on one or more replication partners. Or, the password updates that originated on the rollback domain controller do not exist on replication partners.
The following steps show the sequence of events that may cause a USN rollback. A USN rollback occurs when the domain controller system state is rolled back in time using an unsupported system state restoration.
- An administrator promotes three domain controllers in a domain. (In this example, the domain controllers are RXDC1, RXDC2, and RXDC3, and the domain is Remotex.com.) RXDC1 and RXDC2 are direct replication partners. RXDC2 and RXDC3 are also direct replication partners. RXDC1 and RXDC3 are not direct replication partners but receive originating updates transitively through RXDC2.
- An administrator creates 10 user accounts that correspond to USNs 1 through 10 on RXDC1. All these accounts replicate to RXDC2 and RXDC3.
- A disk image of an operating system is captured on RXDC1. This image has a record of objects that correspond to local USNs 1 through 10 on RXDC1.
The following changes are made in Active Directory
- The passwords for all 10 user accounts that were created in step 2 are reset on RXDC1. These passwords correspond to USNs 11 through 20. All 10 updated passwords replicate to RXDC2 and RXDC3.
- 10 new user accounts that correspond to USNs 21 through 30 are created on RXDC1. These 10 user accounts replicate to RXDC2 and RXDC3.
- 10 new computer accounts that correspond to USNs 31 through 40 are created on RXDC1. These 10 computer accounts replicate to RXDC2 and RXDC3.
- 10 new security groups that correspond to USNs 41 through 50 are created on RXDC1. These 10 security groups replicate to RXDC2 and RXDC3.
- RXDC1 experiences a hardware failure or a software failure. The administrator uses a disk imaging utility to copy the operating system image that was created in step 3 into place. RXDC1 now starts with an Active Directory database that has knowledge of USNs 1 through 10. Because the operating system image was copied into place, and a supported method of restoring the system state was not used, DC1 continues to use the same invocation ID that created the initial copy of the database and all changes up to USN 50. DC2 and DC3 also maintain the same invocation ID for RXDC1 well as an up-to-date vector of USN 50 for RXDC1. (An up-to-date vector is the current status of the latest originating updates to occur on all domain controllers for a given directory partition.)
- Unless an administrator intervenes, RXDC2 and RXDC3 do not inbound-replicate the changes that correspond to local USN 11 through 50 that originate from DC1. Also, according to the invocation ID that RXDC2 uses, DC1 already has knowledge of the changes that correspond to USN 11 to 50. Therefore, RXDC2 does not send those changes. Because the changes in step 4 do not exist on RXDC1, logon requests fail with an “access denied” error. This error occurs either because passwords do not match or because the account does not exist when the newer accounts randomly authenticate with RXDC1.
- Administrators who monitor replication health in the forest note the following situations:
- The Repadmin /showreps command-line tool reports that two-way Active Directory replication between RXDC1 and RXDC2 and between RXDC2 and RXDC3 is occurring without error. This situation makes any replication inconsistency difficult to detect.
- Replication events in the directory service event logs of domain controllers that are running Windows Server do not indicate any replication failures in the directory service event logs. This situation makes any replication inconsistency difficult to detect.
- Active Directory Users and Computers or the Active Directory Administration Tool (Ldp.exe) show a different count of objects and different object metadata when the domain directory partitions on RXDC2 and RXDC3 are compared to the partition on RXDC1. The difference is the set of changes that map to USN changes 11 through 50 in step 4.Note In this example, the different object count applies to user accounts, computer accounts, and security groups. The different object metadata represents the different user account passwords.
- User authentication requests for the 10 user accounts that were created in step 2 occasionally generate an “access denied” or “incorrect password” error. This error may occur because a password mismatch exists between these user accounts on RXDC1 and the accounts on RXDC2 and RXDC3. The user accounts that experience this problem corresponds to the user accounts that were created in step 4. The user accounts and password resets in step 4 did not replicate to other domain controllers in the domain.
- RXDC2 and RXDC3 start to inbound-replicate originating updates that correspond to USN numbers that are greater than 50 from RXDC1. This replication proceeds normally without administrative intervention because the previously recorded up-to-dateness vector threshold, USN 50, has been exceeded. (USN 50 was the up-to-dateness vector USN recorded for RXDC1 on RXDC2 and RXDC3 before RXDC1 was taken offline and restored.) However, the new changes that corresponded to USNs 11 through 50 on the originating RXDC1 after the unsupported restore will never replicate to RXDC2, RXDC3, or their transitive replication partners.
Although the symptoms that are mentioned in step 6 represent some of the effects that a USN rollback can have on user and computer accounts, a USN rollback can prevent any object type in any Active Directory partition from replicating. These object types include the following:
- The Active Directory replication topology and schedule
- The existence of domain controllers in the forest and the roles that these domain controllers hold. Note These roles include the global catalog, relative identifier (RID) allocations, and operations master roles. (Operations master roles are also known as flexible single master operations or FSMO.)
- The existence of domain and application partitions in the forest
- The existence of security groups and their current group memberships
- DNS record registration in Active Directory-integrated DNS zones
The size of the USN hole may represent hundreds, thousands, or even tens of thousands of changes to users, computers, trusts, passwords, and security groups. (The USN hole is defined by the difference between the highest USN number that existed when the restored system state backup was made and the number of originating changes that were created on the rolled-back domain controller before it was taken offline.)