Windows Server 2012 : Active Directory Domain Services Primer - Understanding Domain Trusts

9/28/2013 7:29:44 PM

Conceptualizing Transitive Trusts

Two-way transitive trusts are automatically established upon the creation of a subdomain or with the addition of a domain tree into an AD DS forest. Transitive trusts are normally two-way, with each domain trusting the other domain. In other words, users in each domain can access resources such as printers or servers in the other domain if they are explicitly given rights in those domains. Bear in mind that just because two domains have a trust relationship does not mean that users from one domain can automatically access all the resources in the other domain; it is simply the first step in accessing those resources. The proper permissions still need to be applied.

Understanding Explicit Trusts

Explicit trusts are those that are set up manually, similar to the way that Windows NT trusts were constructed. A trust can be set up to join two unrelated domain trees into the same forest, for example. Explicit trusts are one-way, but two explicit trusts can be established to create a two-way trust. In Figure 1, an explicit trust has been established between the companyabc domain and the companyxyz domain to join them into the same forest structure.


Figure 1. Explicit trust between two domain trees.

When an explicit trust is set up to expedite the flow of trusts from one subdomain to another, it is known as a shortcut trust. Shortcut trusts simply allow authentication verifications to be processed faster, as opposed to having to move up and down a domain tree. In Figure 2, even though a transitive trust exists between the and the domains, a shortcut trust has been created to minimize authentication time for access between the two subdomains of this organization.


Figure 2. Shortcut trust between two subdomains in a forest.

Another possible use for explicit trusts is to allow connectivity between an AD DS forest and an external domain. These types of explicitly defined trusts are known as external trusts, and they allow different forests to share information without actually merging schema information or global catalogs.

Determining Domain Usage Versus OU Usage

As previously mentioned, some administrators try to apply the AD DS domain structure to political boundaries within the organization. The dry-erase markers come out and, very soon, well-meaning managers get involved, organizing the AD DS structure based on political boundaries. Subdomains start to become multiple layers deep, with each department taking its own subdomain. The AD DS structure allows for this type of administrative granularity without division into multiple domains. In fact, the rule of thumb when designing domains is to start with a single domain and add additional domains only when necessary. In a nutshell, the type of administrative control required by many organizations can be realized by division of groups into separate OUs rather than into separate domains.

OUs can, therefore, be structured to allow for separate departments to have various levels of administrative control over their own users. For example, a secretary in the Engineering department can be delegated control of resetting passwords for users within his own OU. Another advantage of OU use in these situations is that users can be easily dragged and dropped from one OU to another. For example, if users are moved from one department to another, moving them into their new department’s OU is extremely simple.

It is important to keep in mind that OU structure can be modified when an administrator feels fit to make structural changes, within certain constraints (namely after mapping out any group policies and administrative permissions that have been applied to the OU structure). This gives AD DS the added advantage of being forgiving for OU design flaws because changes can be made at any time.

Choosing Between OUs and Groups

Whereas OUs are primarily used to segregate administrative function, groups are useful for logical organization of security functions. Translated, OUs are created if there is a need for a department or physical location to have some certain type of administrative control over its own environment. For example, an organization with offices in Japan could organize its Japanese users into a separate OU and give a local administrator password-change and account-creation privileges for that OU. Groups, however, can be used to organize users to more easily apply security permissions. For example, you can create a group named Japanese Office Users that contains all the users from the office in Japan. Security permissions can then be established on objects in AD DS using that group. They could, for example, be given privileges to folders in the main corporate location, something that could not be done at the OU level.

To summarize, the basic differences between OUs and groups is that groups can be used when applying security to objects, whereas OUs exist when certain administrative functionality needs to be delegated.

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