Active Directory Schema

This chapter is from the book

Inside Active Directory: A System Administrator's Guide

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This chapter is from the book

This chapter is from the book 

Inside Active Directory: A System Administrator's Guide

Learn More Buy

Attributes and Syntaxes

In this section we discuss the attributes that define the attributes in the schema.

As with classes, there is a schema object for each attribute, which means a total of 863 attributeSchema objects in the base schema. Each of these 863 attributes is characterized by the attributes of its schema object and those attributeSchema objects themselves use 94 attributes out of the 863. Figure 8.15 shows the relationship between the attribute schema objects and their attributes.

Figure 8.15 The attributeSchema class has 863 instances (i.e., attributeSchema objects) in the base schema. Each of these 863 objects defines the syntax and other aspects of one attribute. The definitions reside in the attributes of the objects, which means that the attributeSchema objects define attributes also for themselves. This figure shows three attributeSchema objects, which result in 3 3 arrows on the right side.

Of the 94 attributes that the attributeSchema class uses, it inherits 73 attributes from top and defines 21 on its own level. As with classes, most of the inherited attributes are quite general (after all, they apply to any Active Directory object). We cover only 13 of the inherited attributes and all 21 "own" attributes for a total of 34 attributes.

We divide the 34 attributes into four categories.

• Names and identifiers. You probably remember that classSchema objects have 14 various names, descriptions, and identifiers. attributeSchema objects have even more: 17 names, descriptions, and identifiers.

• Syntax and content rules. These 7 attributes define the kind of data the attribute accepts and the values that are possible for it.

• Searches. The 2 attributes in this category control things such as indexing of the attribute and whether it is part of the global catalog.

• Miscellaneous. There are 8 miscellaneous attributes.

Table 8.14 shows all 34 attributes listed by their LDAP names. If the common name is quite different from the LDAP name, it appears in parentheses. The four sections following Table 8.14 describe the attributes in more detail. The Type column indicates whether the attribute is mandatory or optional. The Source Class column contains either attributeSchema or top—the latter meaning the attribute was inherited from the top class. The Base Schema Attributes column indicates the kinds of values the 863 attribute objects have for the attributes in the table. Finally, the LDAP Display Name Attribute Example column lists the values for the lDAPDisplayName attribute.

TABLE 8.14 Attributes of an attributeSchema Object

Names and Identifiers

Most of the names, descriptions, and identifiers for attribute schema objects are exactly the same as names, descriptions, and identifiers for class schema objects. Consequently, there is no need to repeat the descriptions of the following nine attributes:

• lDAPDisplayName
• cn (Common-Name)
• adminDisplayName
• name (RDN)
• distinguishedName (Obj-Dist-Name)
• canonicalName
• adminDescription
• schemaIDGUID
• objectGUID

Also, the following four attributes are not used, as they were not used with class schema objects:

• displayName
• displayNamePrintable
• classDisplayName
• description

This leaves us with four not-yet-familiar attributes. They are described in Table 8.15.

TABLE 8.15 Some Name and Identifier Attributes of an attributeSchema Object

Linked Attributes

When an attribute refers to another object in the directory, it is often beneficial that the target object has a reference to the first object. An example is a user's membership in a group. The group has a member attribute, which includes the user (a forward link) and the user has a memberOf attribute, which includes the group (a back link). Figure 8.16 shows an example.

Figure 8.16 The member attribute is a forward link from a group object to a user object. The memberOf attribute is a back link from the user to the group. The relationship is defined in the corresponding attributeSchema objects.

An even value in the linkID attribute denotes a forward link and a larger-by-1 odd value denotes a back link.

The base schema contains the following 13 linked attribute pairs:

• member—memberOf (Is-Member-Of-DL)
• manager—directReports
• siteObject—siteObjectBL
• nonSecurityMember—nonSecurityMemberBL
• queryPolicyObject—queryPolicyBL
• privilegeHolder—isPrivilegeHolder
• managedBy—managedObjects
• hasMasterNCs—masteredBy
• serverReference—serverReferenceBL
• bridgeheadTransportList—bridgeheadServerListBL
• netbootServer—netbootSCPBL
• frsComputerReference—frsComputerReferenceBL
• fRSMemberReference—fRSMemberReferenceBL

All back-link attributes, such as memberOf, are "system only"—that is, users or administrators cannot modify them. Active Directory is responsible for updating these attributes, maintaining referential integrity in the process. If either of the referenced objects is moved, Active Directory modifies the reference accordingly.

A forward-link attribute must use one of the following syntaxes: DN, DN with Unicode string, DN with binary, access point DN, and OR name. A back-link attribute must be of syntax DN.

Syntax and Content Rules

Seven attributeSchema attributes control the type of data and the values each attribute accepts. Table 8.16 describes these syntax and content attributes.

TABLE 8.16 Syntax and Content Attributes of an attributeSchema Object

The data type is called syntax in this context and it is defined by three attributes: attributeSyntax, oMSyntax, and oMObjectClass. Naturally, the three attributes must be consistent, as you see in Tables 8.17 through 8.20 where we describe all the syntax choices.

Several attributes are necessary to express the syntax because more than one standard is involved. The first attribute (attributeSyntax) defines an X.500 syntax, and the two others a XOM syntax. "XOM" stands for "XAPIA X/Open Object Management," an interface for ASN.1 messaging defined by X.400 API Association (XAPIA) and X/Open (a former vendor standards organization). Currently, "X/Open" is a trademark that belongs to Open Group (http://www.opengroup.org/).

Syntax Choices

Active Directory supports 23 syntaxes, but it uses only 19 of them in the base schema. Because the syntax choices are hard-coded and don't appear as objects in Active Directory, you cannot add syntaxes.

We can divide the syntaxes into the following categories:

• Simple data types (4 syntaxes)

• String data types (10 syntaxes)

• Time data types (2 syntaxes), which are actually also strings

• Reference data types (7 syntaxes), which are various object references

Tables 8.17 through 8.20 describe the syntaxes of each category. Some syntaxes have two names, in which case we mention the second name in parentheses.

TABLE 8.17 Syntaxes for Simple Data Types

TABLE 8.18 Syntaxes for String Data Types

TABLE 8.19 Syntaxes for Time Data Types

TABLE 8.20 Syntaxes for Reference Data Types

All seven syntaxes in Table 8.20 have oMSyntax = 127, which means that they must have oMObjectClass also defined. The latter attribute distinguishes them, because some of them have identical attributeSyntax and oMSyntax. We list the oMObjectClass values in Table 8.21.

TABLE 8.21 oMObjectClass Values for "127" Syntaxes

Multivalued Attributes

Each multivalued attribute can have up to 850 values, except for linked multivalued attributes, such as group members, which can have 5,000 values (i.e., group members).

When one value is added, deleted, or modified, the whole attribute is replicated to other domain controllers. Therefore, it is not a good idea to make multivalued attributes too large. Another reason to favor relatively small multivalued attributes is that when you read the attribute, all values are normally returned together.

The values are returned in random order; if you write a program or script to read the values, you cannot depend on the order.

Even though returning all values of a multivalued attribute is the normal behavior, LDAP allows specifying the range of values. This is explained in Chapter 6.

Searches

Some attributes are indexed, which allows fast searches based on those attributes, and some attributes are part of the global catalog. The indexed and global catalog attributes are defined with two attributeSchema attributes, which are described in Table 8.22.

TABLE 8.22 Search Attributes of an attributeSchema Object

SearchFlags is a bit-field attribute. It contains 32 bits, 5 of which have a meaning. Those 5 bits are the least significant ones, which means that they are the rightmost bits if you use Windows Calculator to convert a decimal number to binary. The bits are described in Table 8.23. The bit value "1" means True—that is, the setting is on.

TABLE 8.23 SearchFlags Bits

Ambiguous Name Resolution

Ambiguous Name Resolution (ANR), which is mentioned in Table 8.23, needs a few words of explanation. ANR is an LDAP feature that allows using a simple LDAP filter instead of a complex filter in certain LDAP searches. You can use ANR manually with LDAP filters (discussed in Chapter 6). Also, when you perform a search with Windows 2000 Address Book, it will use ANR for you.

The following eight attributes are part of the ANR set in the base schema:

• displayName

• givenName

• legacyExchangeDN

• name (RDN)

• physicalDeliveryOfficeName (user properties, General tab, Office)

• proxyAddresses

• sAMAccountName (user properties, Account tab, Pre-Windows 2000 name)

• sn (Surname)

ANR gives flexibility in two ways:

• Only a partial match is required from the beginning of the text. If you search for "Brown," you will get both "Brown" and "Brownfield."

• The search is performed on several attributes (those eight just listed). If you search for "Brown," it can be the starting part of not only the surname (last name), but also displayName, sAMAccountName, or any of the other five attributes.

If you search for two words, ANR works as just described, with one addition. It will also try if the words are "first name—last name" or "last name—first name." If you want to find Jack Brown, you can type either "jac bro" or "brow jack." The corresponding LDAP filters would be

(anr=jac bro)
(anr=brow jack)

It is possible to suppress the first name/last name functionality and/or the last name/first name functionality. You do this by modifying the same dSHeuristics attribute that is used to enable the List Object permission. We explained the procedure in Chapter 4 in the "Enabling and Using the List Object Permission" section.

Miscellaneous Characteristics for Attributes

Most of the miscellaneous attributeSchema attributes are also classSchema attributes. Because we described the classSchema attributes when discussing classes, there is no need to repeat the discussion of the following attributes:

• nTSecurityDescriptor
• isDefunct
• objectClass
• objectCategory
• schemaFlagsEx

One new attribute, isEphemeral, is not used, so we are left with only two attributes to address here: systemOnly and systemFlags. Although those two attributes also appeared in the discussion of classes, they have some new aspects. Table 8.24 describes the attributes systemOnly and systemFlags, and Table 8.25 continues the description of the systemFlags.

TABLE 8.24 Miscellaneous Attributes of an attributeSchema Object

TABLE 8.25 SystemFlags Bits

AttributeSchema Object Property Pages

There is only one property page for attributeSchema objects in the Schema Manager snap-in, and there is no Security tab. Figure 8.17 shows the property page for the object lDAPDisplayName.

Of the 34 attributeSchema attributes that we have discussed in this section, Figure 8.17 shows the following 11, with the values in parentheses (we didn't count showInAdvancedViewOnly). To see all of the attributes, you need ADSI Edit.

Figure 8.17 With the Schema Manager snap-in you can see the attributes of various attributes, such as lDAPDisplayName.

• lDAPDisplayName ("lDAPDisplayName")

• adminDescription ("LDAP-Display-Name")

• cn ("LDAP-Display-Name")

• attributeID ("1.2.840.113556.1.2.460")

• attributeSyntax, oMSyntax, oMObjectClass ("Unicode String")

• rangeLower ("1")

• rangeUpper ("256")

• isSingleValued ("This attribute is single-valued")

• showInAdvancedViewOnly (Note that even though checking the "Show objects of this class while browsing" box sets this attribute to False, it doesn't have any practical meaning.)

• isDefunct ("Deactivate this attribute" not checked)

• searchFlags ("Index this attribute in the Active Directory" checked, "ANR" not checked, "Attribute is copied when duplicating a user" not checked)

• isMemberOfPartialAttributeSet ("Replicate this attribute to the Global Catalog" not checked)