Navigating Active Directory Forest Recovery

Recovering an Active Directory (AD) forest is no small feat, especially given the numerous dependencies involved. If you find yourself facing a forest failure, it’s crucial to have prior experience with the recovery process; it shouldn’t be your first attempt. A 2022 poll during the “Hacked and Afraid – Dramatic Tales from AD Disaster Recovery Scenarios” session revealed that 45% of organizations had never tested their AD disaster recovery plans. We hope these organizations have since initiated annual tests. A thoroughly documented and regularly practiced recovery plan is vital for quickly restoring business operations.

Microsoft’s Active Directory Forest Recovery Guide serves as a solid foundation for the recovery process. It addresses key recovery steps but cannot cover every configuration scenario—every organization has unique requirements. Therefore, while it provides a starting point, your organization will need to develop a customized recovery plan.

Importance of AD-Aware Backups

Regular backups of AD domain controllers (DCs) using AD-aware methods are essential for recovery. These are the only recommended recovery methods. Alternatives, like restoring virtual hard disk images (VHDX), are unsupported and may introduce complications.

In this article, we’ll explore recommendations for AD forest recovery, proper backup techniques, protection for AD backups, and key operational dependencies to include in your recovery strategy.

Using Virtual Machines for DC Recovery

While there are methods for restoring DCs beyond AD-aware backups, Microsoft has enhanced virtualization capabilities starting with Windows Server 2012. Hosting DCs on virtual machines (VMs) allows for VM cloning and hypervisor snapshots. While recovering the first DC in a multi-domain forest using a hypervisor snapshot is supported, it’s not the recommended approach. The best practice remains to use AD-aware backup solutions.

Why avoid hypervisor snapshots for DC recovery? Reverting to a snapshot returns the VM to its state at the time the snapshot was taken. If malware was present then, it will be restored alongside the server.

With virtualization enhancements in Windows Server 2012, virtualized DCs can utilize VM-Generation ID (VMGenID). When a snapshot is applied, the DC checks the VMGenID from the hypervisor against the value stored in AD. If they don’t match, the DC activates protection mechanisms to prevent issues within AD, including resetting the invocation ID and invalidating the local RID pool.

If your environment meets VMGenID requirements, you’re less likely to face “USN (Update Sequence Number) rollback.” However, improper restore methods that don’t change the VMGenID can trigger this condition, leading to replication issues with changes made to objects on the affected DC.

Key Backup Considerations

While following the steps outlined in Microsoft’s Active Directory Forest Recovery Guide is essential, keep in mind that it may not account for every scenario unique to your environment. For example, if you rely on third-party DNS solutions, you might find gaps in the guidance provided.

The restoration process must utilize an AD-aware backup method. The recommended approach employs a backup utility that leverages Volume Shadow Copy Service (VSS), minimizing the risk of introducing new issues during the restoration.

Storing backups in a secure location is critical. They must be protected against ransomware, tampering, theft, and deletion. Compromised backups could hinder your ability to recover from a forest failure or allow attackers to gain sensitive information. If backups are encrypted by ransomware, recovery may be impossible without paying the ransom, and deleted backups eliminate any recovery options.

Ensure your restoration method can revert to a clean OS to avoid reintroducing malware. The backups should include only the files necessary for restoring AD, not the entire operating system or file system.

Your AD Forest recovery plan should also allow for restoring one DC in each domain to an isolated environment. This step is vital if an attacker has compromised your environment, providing an opportunity to clean up privileged group memberships and reset account passwords. In this environment, validate that the original threat has been mitigated and perform necessary metadata cleanup.

Addressing Operational Dependencies

Don’t overlook operational dependencies in your recovery plan. Have contact information for all key decision-makers and team members ready and ensure you can access this information even if AD is down.

Consider the deployment of your DCs—are they on-premises or in the cloud? Can you access hypervisors if AD is unavailable? If your DCs are on physical servers, can they be remotely reimaged, or will someone need to be onsite?

Also, verify that your recovery plan aligns with your organization’s Recovery Time Objective (RTO) and Recovery Point Objective (RPO):

• RTO: The time required to restore systems.
• RPO: The acceptable data loss threshold, indicating the maximum age of backups suitable for recovery.

Meeting both objectives is crucial to ensure AD can be restored swiftly and with minimal data loss.

Preparing for Success

AD forest recovery is complex, with numerous potential pitfalls that could arise during a real disaster. A well-prepared recovery plan that is regularly updated and tested at least annually is essential. Keeping this plan current will help mitigate surprises if a recovery becomes necessary.

Consider the cost of AD downtime. For example, one company has multiple “million-dollar-an-hour apps.” An AD outage would disrupt these applications, costing the company significantly. Additionally, many critical functions may rely on domain-joined servers.

Using improper restore methods can lead to USN rollback, reintroduced malware, or other issues that might not surface until weeks or months after recovery. It’s crucial to use AD-aware backup methods to prevent complications in the restored environment.

If a full forest recovery becomes necessary, ensure you have the appropriate approvals from decision-makers, as it can be disruptive.

While we hope you never face a full AD forest recovery in production, being well-prepared is vital. Continuous testing and updating of your recovery plan are essential to ensure a smooth recovery process when the need arises.