8. Schema Specification

This chapter describes how to extend the user schema used by slapd(8). The chapter assumes the reader is familar with the LDAP/X.500 information model.

The first section, Distributed Schema Files details optional schema definitions provided in the distribution and where to obtain other definitions. The second section, Extending Schema, details how to define new schema items.

This chapter does not discuss how to extend system schema used by slapd(8) as this requires source code modification. System schema includes all operational attribute types or any object class which allows or requires an operational attribute (directly or indirectly).

8.1. Distributed Schema Files

OpenLDAP is distributed with a set of schema specifications for your use. Each set is defined in a file suitable for inclusion (using the include directive) in your slapd.conf(5) file. These schema files are normally installed in the /usr/local/etc/openldap/schema directory.

Table 8.1: Provided Schema Specifications
File Description
core.schema OpenLDAP core (required)
cosine.schema Cosine and Internet X.500 (useful)
inetorgperson.schema InetOrgPerson (useful)
misc.schema Assorted (experimental)
nis.schema Network Information Services (FYI)
openldap.schema OpenLDAP Project (experimental)

To use any of these schema files, you only need to include the desired file in the global definitions portion of your slapd.conf(5) file. For example:

        # include schema

Additional files may be available. Please consult the OpenLDAP FAQ (http://www.openldap.org/faq/).

Note: You should not modify any of the schema items defined in provided files.

8.2. Extending Schema

Schema used by slapd(8) may be extended to support additional syntaxes, matching rules, attribute types, and object classes. This chapter details how to add user application attribute types and object classes using the syntaxes and matching rules already supported by slapd. slapd can also be extended to support additional syntaxes, matching rules and system schema, but this requires some programming and hence is not discussed here.

There are five steps to defining new schema:

  1. obtain Object Identifer
  2. choose a name prefix
  3. create local schema file
  4. define custom attribute types (if necessary)
  5. define custom object classes

8.2.1. Object Identifiers

Each schema element is identified by a globally unique Object Identifier (OID). OIDs are also used to identify other objects. They are commonly found in protocols described by ASN.1. In particular, they are heavily used by the Simple Network Management Protocol (SNMP). As OIDs are hierarchical, your organization can obtain one OID and branch it as needed. For example, if your organization were assigned OID 1.1, you could branch the tree as follows:

Table 8.2: Example OID hierarchy
OID Assignment
1.1 Organization's OID
1.1.1 SNMP Elements
1.1.2 LDAP Elements AttributeTypes myAttribute ObjectClasses myObjectClass

You are, of course, free to design a hierarchy suitable to your organizational needs under your organization's OID. No matter what hierarchy you choose, you should maintain a registry of assignments you make. This can be a simple flat file or something more sophisticated such as the OpenLDAP OID Registry (http://www.openldap.org/faq/index.cgi?file=197).

For more information about Object Identifers (and a listing service) see http://www.alvestrand.no/harald/objectid/.

To obtain a registered OID at no cost, apply for an OID under the Internet Assigned Numbers Authority (IANA) maintained Private Enterprise arc. Any private enterprise (organization) may request an OID to be assigned under this arc. Just fill out the IANA form at http://www.iana.org/cgi-bin/enterprise.pl and your official OID will be sent to you usually within a few days. Your base OID will be something like where X is an integer.

Note: Don't let the "MIB/SNMP" statement on the IANA page confuse you. OIDs obtained using this form may be used for any purpose including identifying LDAP schema elements.

Alternatively, OID name space may be available from a national authority (e.g., ANSI, BSI).

For private experiments, OIDs under 1.1 may be used. The OID 1.1 arc is regarded as dead name space.

8.2.2. Name Prefix

In addition to assigning a unique object identifier to each schema element, you should provide a least one textual name for each element. The name should be both descriptive and not likely to clash with names of other schema elements. In particular, any name you choose should not clash with present or future Standard Track names.

To reduce (but not eliminate) the potential for name clashes, the convention is to prefix names of non-Standard Track with a few letters to localize the changes to your organization. The smaller the organization, the longer your prefix should be.

In the examples below, we have chosen a short prefix 'my' (to save space). Such a short prefix would only be suitable for a very large, global organization. In general, we recommend something like 'deFirm' (German company) or 'comExample' (elements associated with organization associated with example.com).

8.2.3. Local schema file

The objectclass and attributeTypes configuration file directives can be used to define schema rules on entries in the directory. It is customary to create a file to contain definitions of your custom schema items. We recommend you create a file local.schema in /usr/local/etc/openldap/schema/local.schema and then include this file in your slapd.conf(5) file immediately after other schema include directives.

        # include schema
        # include local schema
        include /usr/local/etc/openldap/schema/local.schema

8.2.4. Attribute Type Specification

The attributetype directive is used to define a new attribute type. The directive uses the same Attribute Type Description (as defined in RFC2252) used by the attributeTypes attribute found in the subschema subentry, e.g.:

        attributetype <RFC2252 Attribute Type Description>

where Attribute Type Description is defined by the following BNF:

      AttributeTypeDescription = "(" whsp
            numericoid whsp              ; AttributeType identifier
          [ "NAME" qdescrs ]             ; name used in AttributeType
          [ "DESC" qdstring ]            ; description
          [ "OBSOLETE" whsp ]
          [ "SUP" woid ]                 ; derived from this other
                                         ; AttributeType
          [ "EQUALITY" woid              ; Matching Rule name
          [ "ORDERING" woid              ; Matching Rule name
          [ "SUBSTR" woid ]              ; Matching Rule name
          [ "SYNTAX" whsp noidlen whsp ] ; Syntax OID
          [ "SINGLE-VALUE" whsp ]        ; default multi-valued
          [ "COLLECTIVE" whsp ]          ; default not collective
          [ "NO-USER-MODIFICATION" whsp ]; default user modifiable
          [ "USAGE" whsp AttributeUsage ]; default userApplications
          whsp ")"

      AttributeUsage =
          "userApplications"     /
          "directoryOperation"   /
          "distributedOperation" / ; DSA-shared
          "dSAOperation"          ; DSA-specific, value depends on server

where whsp is a space (' '), numericoid is a globally unique OID in dotted-decimal form (e.g. 1.1.0), qdescrs is one or more names, woid is either the name or OID optionally followed by a length specifier (e.g {10}).

For example, the attribute types name and cn are defined in core.schema as:

        attributeType ( NAME 'name'
                DESC 'name(s) associated with the object'
                EQUALITY caseIgnoreMatch
                SUBSTR caseIgnoreSubstringsMatch
                SYNTAX{32768} )
        attributeType ( NAME ( 'cn' 'commonName' )
                DESC 'common name(s) assciated with the object'
                SUP name )

Notice that each defines the attribute's OID, provides a short name, and a brief description. Each name is an alias for the OID. slapd(8) returns the first listed name when returning results.

The first attribute, name, holds values of directoryString (UTF-8 encoded Unicode) syntax. The syntax is specified by OID ( identifies the directoryString syntax). A length recommendation of 32768 is specified. Servers should support values of this length, but may support longer values The field does NOT specify a size constraint, so is ignored on servers (such as slapd) which don't impose such size limits. In addition, the equality and substring matching uses case ignore rules. Below are tables listing commonly used syntax and matching rules (OpenLDAP supports these and many more).

Table 8.3: Commonly Used Syntaxes
Name OID Description
boolean boolean value
directoryString Unicode (UTF-8) string
distinguishedName LDAP DN
integer integer
numericString numeric string
OID object identifier
octetString arbitary octets

Table 8.4: Commonly Used Matching Rules
Name Type Description
booleanMatch equality boolean
caseIgnoreMatch equality case insensitive, space insensitive
caseIgnoreOrderingMatch ordering case insensitive, space insensitive
caseIgnoreSubstringsMatch substrings case insensitive, space insensitive
caseExactMatch equality case sensitive, space insensitive
caseExactOrderingMatch ordering case sensitive, space insensitive
caseExactSubstringsMatch substrings case sensitive, space insensitive
distinguishedNameMatch equality distinguished name
integerMatch equality integer
integerOrderingMatch ordering integer
numericStringMatch equality numerical
numericStringOrderingMatch ordering numerical
numericStringSubstringsMatch substrings numerical
octetStringMatch equality octet string
octetStringOrderingStringMatch ordering octet string
octetStringSubstringsStringMatch ordering octet string
objectIdentiferMatch equality object identifier

The second attribute, cn, is a subtype of name hence it inherits the syntax, matching rules, and usage of name. commonName is an alternative name.

Neither attribute is restricted to a single value. Both are meant for usage by user applications. Neither is obsolete nor collective.

The following subsections provide a couple of examples. myUniqueName

Many organizations maintain a single unique name for each user. Though one could use displayName (RFC2798), this attribute is really meant to be controlled by the user, not the organization. We could just copy the definition of displayName from inetorgperson.schema and replace the OID, name, and description, e.g:

        attributetype ( NAME 'myUniqueName'
                DESC 'unique name with my organization'
                EQUALITY caseIgnoreMatch
                SUBSTR caseIgnoreSubstringsMatch
                SINGLE-VALUE )

However, if we want this name to be included in name assertions [e.g. (name=*Jane*)], the attribute could alternatively be defined as a subtype of name, e.g.:

        attributetype ( NAME 'myUniqueName'
                DESC 'unique name with my organization'
                SUP name ) myPhoto

Many organizations maintain a photo of each each user. A myPhoto attribute type could be defined to hold a photo. Of course, one could use just use jpegPhoto (RFC2798) (or a subtype) to hold the photo. However, you can only do this if the photo is in JPEG File Interchange Format. Alternatively, an attribute type which uses the Octet String syntax can be defined, e.g.:

        attributetype ( NAME 'myPhoto'
                DESC 'a photo (application defined format)'
                SINGLE-VALUE )

In this case, the syntax doesn't specify the format of the photo. It's assumed (maybe incorrectly) that all applications accessing this attribute agree on the handling of values.

If you wanted to support multiple photo formats, you could define a separate attribute type for each format, prefix the photo with some typing information, or describe the value using ASN.1 and use the ;binary transfer option.

Another alternative is for the attribute to hold a URI pointing to the photo. You can model such an attribute after labeledURI (RFC2079) or simply create a subtype, e.g.:

        attributetype ( NAME 'myPhotoURI'
                DESC 'URI and optional label referring to a photo'
                SUP labeledURI )

8.2.5. Object Class Specification

The objectclasses directive is used to define a new object class. The directive uses the same Object Class Description (as defined in RFC2252) used by the objectClasses attribute found in the subschema subentry, e.g.:

        objectclass <RFC2252 Object Class Description>

where Object Class Description is defined by the following BNF:

        ObjectClassDescription = "(" whsp
                numericoid whsp      ; ObjectClass identifier
                [ "NAME" qdescrs ]
                [ "DESC" qdstring ]
                [ "OBSOLETE" whsp ]
                [ "SUP" oids ]       ; Superior ObjectClasses
                [ ( "ABSTRACT" / "STRUCTURAL" / "AUXILIARY" ) whsp ]
                        ; default structural
                [ "MUST" oids ]      ; AttributeTypes
                [ "MAY" oids ]       ; AttributeTypes
                whsp ")"

where whsp is a space (' '), numericoid is a globally unique OID in numeric form (e.g. 1.1.0), qdescrs is one or more names, and oids is one or more names and/or OIDs. myPhotoObject

To define an auxiliary object class which allows myPhoto to be added to any existing entry.

        objectclass ( NAME 'myPhotoObject'
                DESC 'mixin myPhoto'
                MAY myPhoto ) myPerson

If your organization would like have a private structural object class to instantiate users, you can subclass one of the existing person classes, such as inetOrgPerson (RFC2798), and add any additional attributes which you desire.

        objectclass ( NAME 'myPerson'
                DESC 'my person'
                SUP inetOrgPerson
                MUST ( myUniqueName $ givenName )
                MAY myPhoto )

The object class inherits the required/allowed attribute types of inetOrgPerson but requires myUniqueName and givenName and allows myPhoto.

8.2.6. OID Macros

To ease the management and use of OIDs, slapd(8) supports Object Identifier macros. The objectIdentifier directive is used to equate a macro (name) with a OID. The OID may possibly be derived from a previously defined OID macro. The slapd.conf(5) syntax is:

        objectIdentifier <name> { <oid> | <name>[:<suffix>] }

The following demonstrates definition of a set of OID macros and their use in defining schema elements:

        objectIdentifier myOID  1.1
        objectIdentifier mySNMP myOID:1
        objectIdentifier myLDAP myOID:2
        objectIdentifier myAttributeType        myLDAP:1
        objectIdentifier myObjectClass  myLDAP:2
        attributetype ( myAttributeType:3 NAME 'myPhotoURI'
                DESC 'URI and optional label referring to a photo'
                SUP labeledURI )
        objectclass ( myObjectClass:1 NAME 'myPhotoObject'
                DESC 'mixin myPhoto'
                MAY myPhoto )