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Windows Server

Windows Server 2008: Understanding Read-Only Domain Controllers (part 1)

12/29/2010 9:50:18 AM
One of the new features that received close attention in Windows Server 2008 was a new breed of domain controllers referred to as Read-Only Domain Controllers, also known as RODCs. The RODC hosts a copy of the Active Directory (AD) database like any other writable domain controller, but as its name implies, the contents replica of the domain database residing on the domain controller is read-only and write operations are not supported. It is equally important to mention that the RODCs do not participate in Active Directory replication in the same fashion as writable domain controllers. The fundamental difference between RODC replication and the typical multimaster replication model between writable domain controllers is that RODC replication is unidirectional. This means all changes from a writable domain controller are propagated to the RODCs. As a result, the RODC receives changes, but does not partake in or perform outbound replication with other domain controllers. This characteristic of RODCs provides an extra layer of security as any unauthorized data changes, especially changes made with the intent to hurt the organization, will not replicate out to other domain controllers. Unidirectional replication also reduces the workload of bridgehead servers in the hub site and the effort required to monitor replication.

Another new RODC functionality that improves security is commonly witnessed when replication transpires between a writable domain controller and an RODC. Here, user account information is replicated, but account passwords are not replicated. This is a new phenomenon because of the existence of Windows domain controllers. Security is bolstered in this situation as the only password that resides on the RODC is the local administrator’s password and Krbtgt accounts (the account used for Kerberos authentication). In essence, the read-only philosophy of an RODC is similar to the NT 4.0 Backup Domain Controller (BDC); however, with the NT 4.0 BDC, all user information is replicated from the Primary Domain Controller (PDC), including passwords.

Note

If needed, it is also possible to configure credential caching of passwords for a specific user account to an RODC. Moreover, by default, security groups with high privileges such as Domain Administrators and Enterprise Administrators are configured to never allow their passwords to replicate to RODCs.


Although Microsoft fields numerous questions on this new Active Directory technology, the question that is asked the most is where does the RODC fit in? RODCs are most often used to provide Active Directory Domain Services (AD DS) to remote locations and branch offices where heightened security is essential, where Windows Active Directory administrators are lacking, and where the promise of physical security is practically nonexistent. In many cases, RODCs offer a practical headache-free solution for branch office environments that in the past had to endure solutions that always put them in compromising situations.

Organizations’ Branch Office Concerns and Dilemmas

The next section illustrates typical branch office concerns about having domain controllers onsite. This section makes it evident why the RODC is becoming popular if not extremely necessary for branch offices.

Lack of Physical Security at the Branch Office

Typically, branch office locations do not have the facilities to host a data center. For that reason, it is common to find domain controllers hiding in closets, tucked away in the kitchen next to the fridge, or even in a restroom. As such, branch offices lack physical security when it comes to storing domain controllers, which results in these servers being prime targets for thieves.

Domain Controllers Stolen from the Branch Office

With inadequate physical security in the branch offices, it was very common for domain controllers to be stolen. This posed a major security threat to organizations because domain controllers contain a copy of all the user accounts associated with the domain. Confidential items such as highly privileged administrator accounts, DNS records, and the Active Directory schema could fall into the hands of the wrong people in this situation.

Removing Domain Controllers from the Branch Office

Because of a lack of physical security and concerns over domain controller theft, branch offices often had their domain controllers removed from their site. After being removed, users were forced to authenticate over the WAN to a domain controller residing at their corporate headquarters or to the closest hub site. Although this action solved the security issue, it also cultivated a new problem. If the WAN link between the branch office and hub site was unreliable or unavailable, users could not log on to the workstations at the branch office or the amount of time required to log on was greatly increased. This resulted in a loss of productivity for users in the branch office or outages that resulted in downtime if the WAN link was severed. These types of outages commonly lasted for days.

Lack of Administration Role Separation at the Branch Office

In small branch offices, it is also very common for multiple server functions to be hosted on a single server to reduce costs. For example, a single server might provide domain controller, file, print, messaging, and other line-of-business (LOB) functionality. In such cases, it is necessary for the administrators of these applications to log on to the system to manage their applications. By granting administrators privileges to the domain controller, these individuals also received full access to the Active Directory domain, which is considered to be a major security risk.

Lack of IT Support Personnel at the Branch Office

It is very common for secretaries, receptionists, or even high-level personnel such as managers and directors without any prior knowledge of IT management or maintenance to manage servers in a branch office. Typically, these individuals get nominated or promoted to a branch office IT support role because a local IT administrator does not exist. Unfortunately, even when conducting basic administration tasks like restarting an unresponsive server, these individuals can inadvertently wreak havoc on the Active Directory domain when granted administrator privileges on a domain controller. In a Windows Server 2003 environment, there was little that could be done about this situation. You just had to be careful about who you promoted to the exclusive club of domain administrators.

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