4.2 Attributes (attributeSchema Objects)

Just as class information is stored in Active Directory as instances of the class called classSchema, attributes are represented by instances of the class called attributeSchema. As with all objects, the attributeSchema class has a number of attributes that can be set when specifying a new instance. The attributeSchema class inherits attributes from the class called Top. However, most of the Top attributes are not really relevant here. Table 4-1 shows the defining attributes of an instance of the attributeSchema class (i.e., an attribute) that can be set.

Table 4-1. The defining attributes of an attributeSchema object instance

Attribute	Syntax	Mandatory	Multivalued	Description
attributeId	OID	Yes	No	The OID that uniquely identifies this attribute.
cn	Unicode string	Yes	No	The Relative Distinguished Name (RDN).
isSingleValued	Boolean	Yes	No	Whether this attribute is multivalued.
lDAPDisplayName	Unicode string	Yes	No	The name by which LDAP clients identify this attribute.
attributeSyntax	OID	Yes	No	Half of a pair of properties that define the syntax of an attribute. This one is an OID.
oMSyntax	Integer	Yes	No	Half of a pair of properties that define the syntax of an attribute. This one is an integer.
schemaIDGUID	Octet string	Yes	No	Globally Unique Identifier (GUID) to uniquely identify this attribute.
objectClass	OID	Yes	Yes	This will hold the values "attributeSchema" and "Top" to indicate that the value is an instance of those classes.
accessCategory	Integer	No	No	Used by the system.
attributeSecurityGUID	GUID	No	No	GUID used by Active Directory to identify the security of this attribute.
classDisplayName	Unicode string	No	No	The name displayed when viewing instances of the attribute.
defaultHidingValue	Boolean	No	No	Whether the object is to be hidden or displayed within tools by default.
description	Unicode string	No	No	A description of the attribute.
extendedCharsAllowed	Boolean	No	No	Whether extended characters are allowed in the value of this attribute.
isDefunct	Boolean	No	No	Whether the attribute is marked as disabled (i.e., unusable) in Active Directory.
isEphemeral	Boolean	No	No	Used by the system.
isMemberOfPartialAttributeSet	Boolean	No	No	Whether the attribute is held in the GC.
linkID	Integer	No	No	Whether the attribute is linked with another attribute (e.g., memberOf and members).
mAPIDisplayType	Integer	No	No	The integer by which MAPI clients identify this attribute.
oIDType	Integer	No	No	Used by the system.
oMObjectClass	Octet string	No	No	Used by the system.
rangeLower	Integer	No	No	For strings, this is the minimum character length; for integers, it is the minimum value; otherwise, it is unused. It must be less than range- Upper.
rangeUpper	Integer	No	No	For strings, this is the maximum character length; for integers, it is the maximum value; otherwise, it is unused.
schemaFlags	Integer	No	No	Used by the system.
schemaFlagsEx	Integer	No	No	Used by the system.
searchFlags	Integer	No	No	Whether this attribute is indexed: 0=Not Indexed, 1=Indexed.[2]
systemOnly	Boolean	No	No	If true, once the initial value has been set, only the system can create instances of this attribute. Administrators cannot create instances of the attribute if this is set, but they can add this attribute to new or existing classes as required. The default is false.

^[2] Indexing an object allows you to locate instances of the object or objects containing a particular value of an attribute by consulting the index rather than looking at each object. As with databases, index-aided searches run much faster than other searches.

The syntax of an attribute indicates the type of data that it holds, which we'll cover in a moment. The "Mandatory" column indicates whether the attribute must be set when initially creating an attributeSchema object. Attributes that are not mandatory do not have to be set when creating the object and can be defined later, if they are needed at all. The "Multi-valued" column indicates whether the particular attribute can accept an array of values or whether it accepts only a single value; there are no multivalued attributes here other than objectClass.

4.2.1 Dissecting an Example Attribute

The userPrincipalName (UPN) attribute is used on user objects to provide a unique method of identifying each user across a forest. Users can log on to a workstation in any domain in the forest using the UPN if they so desire. The UPN attribute, in fact, accepts valid RFC 822 (email) addresses, so the UPN for user tpood in the emea.mycorp.com domain could be either tpood@mycorp.com or tpood@emea.mycorp.com. In fact, any UPN suffix, such as @mycorp.com, can be used in a forest. The only requirement is that the UPN value for a user is unique across all users in a forest.

Active Directory does not enforce uniqueness of a UPN when it is set. If two different users in the same forest are assigned the same UPN, neither will be able to log on.

To dissect the attribute, we need to find out what values had been set for it. Perhaps the easiest way to do this is to use ADSI Edit from the Windows Support Tools, which can be installed from a Windows Server CD by running \Support\Tools\setup.exe. Table 4-2 shows the values of attributes that have been set for the userPrincipalName attribute.

Table 4-2. userPrincipalName's attributes

Attribute lDAPDisplayName	Attribute syntax	Attribute value
adminDescription	CASE_IGNORE_ STRING	User-Principal-Name
adminDisplayName	CASE_IGNORE_ STRING	User-Principal-Name
cn	CASE_IGNORE_ STRING	User-Principal-Name
distinguishedName	DN_STRING	cn=User-Principal-Name, cn=Schema, cn=Configuration,dc=mycorp,dc=com
instanceType	INTEGER	4
name	CASE_IGNORE_ STRING	User-Principal-Name
nTSecurityDescriptor	SECURITY_ DESCRIPTOR	<SID>
objectCategory	DN_STRING	cn=Attribute-Schema, cn=Schema, cn=Configuration, dc=mycorp,dc=com
objectClass	CASE_IGNORE_ STRING	top; attributeSchema (two values of a multi-valued attribute)
objectGUID	OCTET_STRING	<GUID>
showInAdvancedViewOnly	BOOLEAN	True
systemFlags	INTEGER	18
uSNChanged	LARGE_INTEGER	USN when last changed
uSNCreated	LARGE_INTEGER	USN when created
whenChanged	UTC_TIME	Time when last changed
whenCreated	UTC_TIME	Time when created
attributeID	CASE_IGNORE_ STRING	1.2.840.113556.1.4.656
attributeSecurityGUID	OCTET_STRING	<GUID>
attributeSyntax	CASE_IGNORE_ STRING	2.5.5.12
isMemberOfPartialAttributeSet	BOOLEAN	True
isSingleValued	BOOLEAN	True
lDAPDisplayName	CASE_IGNORE_ STRING	userPrincipalName
oMSyntax	INTEGER	64
schemaIDGUID	OCTET_STRING	<GUID>
searchFlags	INTEGER	1
systemOnly	BOOLEAN	False

We can see that the name of the attribute is User-Principal-Name (adminDescription, adminDisplayName, cn, name), that it is an instance of the attributeSchema class (objectCategory and objectClass), that it inherits attributes from both top and attributeSchema (objectClass), and that the UPN attribute is not visible to casual browsing (showInAdvancedViewOnly).

The userPrincipalName attributes show the following:

• It is to be stored in the GC (isMemberOfPartialAttributeSet).

• It is to be indexed (searchFlags).

• It has an OID of 1.2.840.113556.1.4.656 (attributeID).

• When binding to it with ADSI, we should use userPrincipalName (lDAPDisplayName).

• Instances can be created by anyone (systemOnly).

• It stores single (isSingleValued) Unicode strings (attributeSyntax and oMSyntax).

In Figure 4-3, you can see many of the values for the UPN attribute. We have indicated which attributes are changed by checking or unchecking each checkbox.

Figure 4-3. The UPN attribute as viewed by the Active Directory Schema snap-in