Read Only Domain Controllers

What Is an RODC? 

Read-only domain controllers (RODCs) are a new feature of Active Directory Domain Services (AD DS) in Windows Server 2008. RODCs are additional domain controllers for a domain that host complete, read-only copies of the partitions of the Active Directory database and a read-only copy of the SYSVOL folder contents. By selectively caching credentials, RODCs address some of the challenges that enterprises can encounter in branch offices and perimeter networks (also known as DMZs) that may lack the physical security that is commonly found in datacenters and hub sites. RODCs also offer a number of manageability improvements that are described in this guide. This section describes how RODCs work with the rest of the Active Directory environment, the main differences between RODCs and writable domain controllers, and the RODC features that can help resolve a number of security or manageability issues.

Prerequisites for Deploying an RODC 

 Applies To: Windows Server 2008, Windows Server 2008 R2, Windows Server 2012   Ensure that the forest functional level is Windows Server 2003 or higher, so that linked-value replication (LVR) is available. This provides a higher level of replication consistency. The domain functional level must be Windows Server 2003 or higher, so that Kerberos constrained delegation is available. If the forest functional level is Windows Server 2003, the domain functional level of all domains in the forest is Windows Server 2003 or higher.  Constrained delegation supports security calls that must be impersonated under the context of the caller. Delegation makes it possible for applications and services to authenticate to a remote resource on behalf of a user. Because it provides powerful capabilities, typically only domain controllers are enabled for delegation. For RODCs, applications and services must be able to delegate, but only constrained delegation is allowed because it prevents the target from impersonating again and making another hop. The user or computer must be cacheable at the RODC for constrained delegation to work. This restriction places limits on how a rogue RODC may be able to abuse cached credentials.  Run Adprep.exe commands to prepare your existing forest and domains for domain controllers that run Windows Server 2008 or Windows Server 2008 R2. The adprep commands extend the Active Directory schema and update security descriptors so that you can add the new domain controllers. There are different versions of Adprep.exe for Windows Server 2008 and Windows Server 2008 R2. For more information, see Running Adprep.exe (http://go.microsoft.com/fwlink/?LinkID=142597). 

   1.  Prepare the forest and domains. There are three adprep commands to complete and have the changes replicate throughout the forest. Run the three commands as follows:             >>>>>  Prepare the forest by running adprep /forestprep on the server that holds the schema master operations master (also known as flexible single master operations or FSMO) role to update the schema.         >>>>>>   Prepare the domain by running adprep /domainprep /gpprep on the server that holds the infrastructure operations master role.      >>>>>>>  If you are installing an RODC in an existing Windows Server 2003 domain, you must also run adprep /rodcprep. 

2. Install Active Directory Domain Services (AD DS). 

Deploy at least one writable domain controller running Windows Server 2008 or Windows Server 2008 R2 in the same domain as the RODC and ensure that the writable domain controller is also a DNS server that has registered a name server (NS) resource record for the relevant DNS zone. An RODC must replicate domain updates from a writable domain controller running Windows Server 2008 or Windows Server 2008 R2.   An RODC that runs Windows Server 2008 R2 can replicate the domain partition from a writable domain controller that runs Windows Server 2008 or Windows Server 2008 R2. But if an RODC that runs Windows Server 2008 R2 is added to a domain that has only a writable domain controller that runs Windows Server 2008, the RODC logs Event ID 2916 in the Directory Services log. This error can be disregarded, and it will not appear if there is a writable domain controller that runs Windows Server 2008 R2 in the domain.  For fault tolerance, deploy at least two writable domain controllers running Windows Server 2008 or Windows Server 2008 R2. An RODC can use the second domain controller for failover if the first domain controller is not available. The registration of the name server (NS) resource record is necessary to allow dynamic updates to replicate to the RODC by using a replicate-single-object (RSO) operation.