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© Rakhesh Sasidharan

[Aside] Multiple domain forests

Was reading about multiple domains/ child domains in a forest and came across these interesting posts. They talk pretty much the same stuff. 

Key points are:

  • In the past a domain was considered to be the security boundary. But since Windows 2000 a domain is no longer considered a security boundary, a forest is.
  • Domain Admins from child domain can gain access to control the forest. The posts don’t explain how but allude that it is possible and widely known.
  • Another reason for multiple domains was password policies. In Windows Server 2000 and 2003 password policies were per domain. But since Windows Server 2008 it is possible to define Fine-Grain Password Policies (FGPPs) that can override the default domain password policy. 
  • Multiple domains were also used when security was a concern. Maybe a remote location had poor security and IT Admins weren’t comfortable with having all the domain usernames and password replicating to DCs in such locations. Solution was the create a separate domain with just the users of that domain. But since Windows Server 2008 we have Read-Only Domain Controllers (RODCs) that do not store any password and can be set to cache passwords of only specified users.
  • Yet another reason for multiple domains was DNS replication. In Windows Server 2000 AD integrated DNS zones replicated to all DCs of the domain – that is, even DCs not holding the DNS role. To avoid such replication traffic multiple domains were created so the DNS replication was limited to only DCs of those domains. Again, starting Windows Server 2003 we have Application Partitions which can be set to replicate to specific DCs. In fact, Server 2003 introduced two Application Partitions specifically for DNS – a Forest DNS Zone partition, and a Domain DNS Zone partition (per domain). These replicate to all DCs that are also DNS servers in the forest and domain respectively, thus reducing DNS replication traffic. 
  • Something I wasn’t aware of until I read these articles was the Linked Value Replication (LVR). In Server 2000 whenever an attribute changed the entire attribute was replicated – for example, if a user is added to a group, the list of all group members is replicated – obviously too much traffic, and yet another reason for multiple domains (to contain the replication traffic). But since Server 2003 we have LVR which only replicates the change – thus, if a user is added to the group, only the addition is replicated. 

One recommendation (itself a matter of debate and recommended against in the above two posts) is to have two domains in the forest with one of them being a placeholder:

  1. A root domain, which will be the forest root domain and will contain the forest FSMO roles as well as Schema Admins and Enterprise Admins; and 
  2. A child domain, which will be the regular domain and will contain everything else (all the OUs, users, Domain Admins)

The root domain will not contain any objects except the Enterprise & Schema admins and the DCs. Check out this article for a nice picture and more details on this model. It’s worth pointing out that such a model is only recommended for medium to large domains, not small domains (because of overhead of maintaining two domains with the additional DCs).

Also check out this post on domain models in general. It is a great post and mentions the “placeholder forest root domain” model of above and how it is often used. From the comments I learnt why it’s better to create child domains rather than peer domains in case of the placeholder forest root domain model. If you create peers there’s no way to indicate a specific domain is the forest root – from the name they all appear the same – while if you create child domains you can easily identify who the forest root is. Also, with child domains you know that the parent forest root domain is important because you can’t remove that domain (without realizing its role) because the child domain namespace depends on it. Note that creating a domain as child to another does not give Domain Admins of the latter administrative rights to it (except of course if these Domain Admins are also Enterprise Admins). The domain is a child only in that its namespace is a child. The two domains have a two way trust relationship – be it peers or parent/ child – so users can logon to each domain using credentials from their domain, but they have no other rights unless explicitly granted. 

The authors of the “Active Directory (5th ed.)” book (a great book!) recommend keeping things simple and avoiding placeholder forest root domains.

[Aside] Multiple domain forests by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.