Windows Server 2008: Defining AD Groups

1/29/2011 6:08:06 PM
The idea of groups has been around in the Microsoft world for much longer than OUs have been. As with the OU concept, groups serve to logically organize users into an easily identifiable structure. However, there are some major differences in the way that groups function as opposed to OUs. Among these differences are the following:
  • Group membership is viewable by users— Whereas OU visibility is restricted to administrators using special administrative tools, groups can be viewed by all users engaged in domain activities. For example, users who are setting security on a local share can apply permissions to security groups that have been set up on the domain level.

  • Membership in multiple groups— OUs are similar to a file system’s folder structure. In other words, a file can reside in only one folder or OU at a time. Group membership, however, is not exclusive. A user can become a member of any one of a number of groups, and her membership in that group can be changed at any time.

  • Groups as security principles— Each security group in AD DS has a unique security identifier (SID) associated with it upon creation. OUs do not have associated access control entries (ACEs) and consequently cannot be applied to object-level security. This is one of the most significant differences because security groups allow users to grant or deny security access to resources based on group membership. Note, however, that the exception to this is distribution groups, which are not used for security.

  • Mail-enabled group functionality— Through distribution groups and (with the latest version of Microsoft Exchange) mail-enabled security groups, users can send a single email to a group and have that email distributed to all the members of that group. The groups themselves become distribution lists, while at the same time being available for security-based applications.

Outlining Group Types: Security or Distribution

Groups in Windows Server 2008 R2 come in two flavors: security and distribution. In addition, groups can be organized into different scopes: machine local, domain local, global, and universal.

Security Groups

The type of group that administrators are most familiar with is the security group. This type of group is used to apply permissions to resources en masse so that large groups of users can be administered more easily. Security groups can be established for each department in an organization. For example, users in the Marketing department can be given membership in a Marketing security group, as shown in Figure 1. This group is then allowed to have permissions on specific directories in the environment.

Figure 1. Examining security group permission sharing.

As previously mentioned, security groups have a unique security identifier (SID) associated with them, much in the same way that individual users in AD DS have an SID. The uniqueness of the SID is utilized to apply security to objects and resources in the domain. This concept also explains why you cannot simply delete and rename a group to have the same permissions that the old group previously maintained.

Distribution Groups

The concept of distribution groups in Windows Server 2008 R2 was introduced in Windows 2000 Server along with its implementation of Active Directory. Essentially, a distribution group is a group whose members are able to receive Simple Mail Transfer Protocol (SMTP) mail messages that are sent to the group. Any application that can use AD DS for address book lookups (essentially LDAP lookups) can utilize this functionality in Windows Server 2008 R2.

Distribution groups are often confused with mail-enabled groups, a concept in environments with Exchange 2000/2003/2007/2010. In addition, in most cases distribution groups are not utilized in environments without Exchange Server because their functionality is limited to infrastructures that can support them.


In environments with Exchange Server, distribution groups can be used to create email distribution lists that cannot be used to apply security. However, if separation of security and email functionality is not required, you can make security groups mail-enabled.

Mail-Enabled Groups

AD DS includes a concept called mail-enabled groups. These groups are essentially security groups that are referenced by an email address, and can be used to send SMTP messages to the members of the group. This type of group is primarily used with Exchange Server, but can also be used with foreign mail systems integrated with AD DS.

Most organizations will find that mail-enabled security groups satisfy most of their needs, both security-wise and email-wise. For example, a single group called Marketing that contains all users in that department could also be mail-enabled to allow Exchange users to send emails to everyone in the department.

Understanding Group Scope

There are four primary scopes of groups in AD DS. Each scope is used for different purposes, but they simply serve to ease administration and provide a way to view or perform functions on large groups of users at a time. The group scopes are as follows:

Group scope can become one of the most confusing aspects of AD DS. However, if certain design criteria are applied to group membership and creation, the concept becomes more palatable.

Machine Local Groups

Machine local groups are essentially groups that are built in to the operating system and can be applied only to objects local to the machine in which they exist. In other words, they are the default local groups such as Power Users, Administrators, and the like created on a stand-alone system. Before networking simplified administration, local groups were used to control access to the resources on a server. The downside to this approach was that users needed to have a separate user account on each machine that they wanted to access. In a domain environment, using these groups for permissions is not recommended because the administrative overhead would be overwhelming.

Domain Local Groups

Domain local groups, a term that might seem contradictory at first, are domain-level groups that can be used to establish permissions on resources in the domain in which they reside. Essentially, domain local groups are the evolution of the old Windows NT local groups.

Domain local groups can contain members from anywhere in an AD DS forest or any trusted domain outside the forest. A domain local group can contain members from any of the following:

  • Global groups

  • User accounts

  • Universal groups

  • Other domain local groups

Domain local groups are primarily used for access to resources because different domain local groups are created for each resource and then other accounts and/or groups are added to them. This helps to readily determine which users and groups have access to a resource.

Global Groups

Global groups are the reincarnation of the legacy Windows NT global group, but with slightly different characteristics. These groups can contain the following types of objects:

  • User accounts

  • Global groups from their own domain

Global groups are primarily useful in sorting users into easily identifiable groupings and using them to apply permissions to resources. What separates global groups from universal groups, however, is that global groups stop their membership replication at the domain boundary, limiting replication outside the domain.

Universal Groups

The concept of universal groups was new with the release of Windows 2000 and is still useful in Windows Server 2008 R2. Universal groups are just that—universal. They can contain objects from any trusted domain and can be used to apply permissions to any resource in the domain.

Although simply making all groups within a domain into universal groups might seem practical, the limiting factor has always been that membership in universal groups is replicated across the entire forest. To make matters worse, Windows 2000 AD DS universal group objects contained a single multi-entry attribute that defined membership. This meant that any time membership was changed in a universal group, the entire group membership was re-replicated across the forest. Consequently, universal groups were limited in functionality.

Windows Server 2003 introduced the concept of incremental universal group membership replication, which accomplishes replication of membership in universal groups on a member-by-member basis. This drastically reduced the replication effects that universal groups had on an environment and made the concept of universal groups more feasible for distributed environments. This functionality is available in any domain functional level at or beyond Windows Server 2003 functional level.

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