Introduction

The Active Directory schema contains the blueprint for how objects are structured and secured, what data they can contain, and even how they can be viewed. Having a thorough understanding of the schema is paramount for any Active Directory administrator. Understanding key concepts, such as class inheritance, class types, attribute syntax, and attribute indexing options, is critical to being able to adequately design an Active Directory infrastructure and should be considered mandatory for any developer that is writing applications or automation scripts that utilize Active Directory.

If you are one of the lucky few who is designated as a schema administrator (i.e., member of the Schema Admins group), then the importance of the schema is already well known to you. This chapter serves a guide to accomplishing many of the day-to-day tasks you will need to do as a schema administrator. If you feel you need more nuts and bolts information on how the schema works, I suggest reading Chapter 4 of Active Directory, Second Edition (O'Reilly).

The Anatomy of Schema Objects

An interesting feature of Active Directory that is not common among other LDAP implementations is that the schema is stored within Active Directory as a set of objects. This means that you can use similar interfaces and programs to manage the schema as you would any other type of object.

All schema objects are stored in the Schema container (e.g., cn=schema,cn=configuration,<ForestRootDN>). The schema is comprised of two classes of objects, classSchema and attributeSchema. Unsurprisingly, the classSchema objects define classes and attributeSchema objects define attributes. The Schema container contains a third type of object called subSchema, also known as the abstract schema, which is defined in the LDAP v3 specification (RFC 2251). There is only a single subSchema object in the Schema container, named cn=Aggregate, and it contains a summary of the entire schema.

Table 10-1 and Table 10-2 contain useful attributes of classSchema objects and attributeSchema objects, respectively.

Table 10-1. Attributes of classSchema objects

Attribute	Description
adminDescription	Description of the class.
auxiliaryClass	Multivalued attribute containing any auxiliary classes defined for the class.
cn	Relative distinguished name of the class.
defaultHidingValue	Boolean that determines whether objects of this class are hidden by default in administrative GUIs.
defaultSecurityDescriptor	Default security descriptor applied to objects of this class.
governsID	Object identifier (OID) for the class.
isDefunct	Boolean that indicates whether the class is defunct (i.e., deactivated).
lDAPDisplayName	Name used when referencing the class in searches or when instantiating or modifying objects of this class.
mayContain	Multivalued attribute that contains a list of attributes that can be optionally set on the class.
mustContain	Multivalued attribute that contains a list of attributes that must be set on the class.
objectClassCategory	Integer representing the class's type. Can be one of 1 (structural), 2 (abstract), 3 (auxiliary), or 0 (88).
possibleInferiors	Multivalued list of other object classes this object can contain.
possSuperiors	Multivalued list of object classes this object can be subordinate to.
rDNAttID	Naming attribute (i.e., RDN) of instances of the class.
schemaIDGUID	GUID of the class.
showInAdvancedViewOnly	Boolean that indicates whether instances of this class should only be shown in Advanced mode in the administrative GUIs.
subClassOf	Parent class.
systemAuxiliaryClass	Multivalued attribute containing any auxiliary classes defined for the class. This can only be modified internally by Active Directory.
systemFlags	Integer representing additional properties of the class.
systemMayContain	Multivalued attribute that contains a list of attributes that can be optionally set on the class. This can only be modified internally by Active Directory.
systemMustContain	Multivalued attribute that contains a list of attributes that must be set on the class. This can only be modified internally by Active Directory.
systemPossSuperiors	Multivalued list of object classes this object can be subordinate to. This can only be modified internally by Active Directory.

Table 10-2. Attributes of attributeSchema objects

Attribute	Description
adminDescription	Description of the attribute.
attributeID	OID for the attribute.
attributeSecurityGUID	GUID to be used to apply security credentials to a set of objects.
attributeSyntax	OID representing the syntax of the attribute. This is used in conjunction with oMSyntax to define a unique syntax.
cn	Relative distinguished name of the attribute.
isDefunct	Boolean that indicates if the attribute is defunct (i.e., deactivated).
isMemberOfPartialAttributeSet	Boolean that indicates if the attribute is a member of the partial attribute set (i.e., the global catalog).
isSingleValued	Boolean that indicates whether the attribute is single valued or multivalued.
linkID	If this is populated, it will contain an integer that represents a link (either forward or backward) to another attribute.
lDAPDisplayName	Name used when referencing the attribute in searches or when populating it on objects. Note that this value may not be the same as cn.
oMSyntax	An integer representing the OM type of the attribute. This is used in conjunction with attributeSyntax to determine a unique syntax for the attribute.
schemaIDGUID	GUID of the attribute.
searchFlags	Integer representing special properties related to searching with the attribute. This includes how the attribute is indexed and if it is used in ANR searches.
systemFlags	Integer representing additional properties of the attribute.