Mostly complete schema specification chapter

[openldap] / doc / guide / admin / schema.sdf

# $OpenLDAP$
# Copyright 1999-2000, The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.

H1: Schema Specification

This chapter describes how to extend {{slapd}}(8) schema.  The
first section details how to extend schema using provided
schema files.  The second section details how to define
new schema items.

H2: 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 {{EX:include}} directive) in your {{slapd.conf}}(5)
file.  These schema files are normally installed in the
{{F:/usr/local/etc/openldap/schema}} directory.

!block table; colaligns="LR"; align=Center; \
        title="Table 6.1: Provided Schema Specifications"
File                            Description
{{F:core.schema}}               OpenLDAP {{core}} (required)
{{F:cosine.schema}}             Cosine and Internet X.500 (useful)
{{F:inetorgperson.schema}}      InetOrgPerson (useful)
{{F:misc.schema}}               Assorted (experimental)
{{F:nadf.schema}}               North American Directory Forum (FYI)
{{F:nis.schema}}                Network Information Services (FYI)
{{F:openldap.schema}}           OpenLDAP Project (experimental)
!endblock

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

>       # include schema
>       include /usr/local/etc/openldap/schema/core.schema
>       include /usr/local/etc/openldap/schema/cosine.schema
>       include /usr/local/etc/openldap/schema/inetorgperson.schema

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

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

H2: Extending Schema

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

There are five steps to defining new schema:
^       obtain Object Identifer
+       choose a naming prefix
+       create local schema file
+       define custom attribute types (if necessary)
+       define custom object classes

H3: Object Identifiers

Each schema element is identified by a globally unique
{{TERM[expand]OID}} ({{TERM:OID}}).  OIDs are also used to identify
other objects.
They are commonly found in protocols described by {{TERM:ASN.1}}.  In
particular, they are heavy used by {{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 {{EX:1.1}}, you could branch
the tree as follows:

!block table; colaligns="LR"; align=Center; \
        title="Table 6.2: Example OID hierarchy"
OID                     Assignment
{{EX:1.1}}              Organization's OID
{{EX:1.1.1}}            SNMP Elements
{{EX:1.1.2}}            LDAP Elements
{{EX:1.1.2.1}}          AttributeTypes
{{EX:1.1.2.1.1}}        myAttribute
{{EX:1.1.2.2}}          ObjectClasses
{{EX:1.1.2.2.1}}        myObjectClass
!endblock

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 a
something more sophisticated such as the {{OpenLDAP OID Registry}}
({{URL:http://www.openldap.org/faq/index.cgi?file=197}}).

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

.{{Under no circumstances should you use a fictious OID!}} 

To obtain a fully registered OID at {{no cost}}, apply for
a OID under {{ORG[expand]IANA}} maintained
{{Private Enterprise}} arch.  Any private enterprise (organization)
may request an OID to be assigned under this arch.  Just fill
out the form at {{URL: 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 {{EX:1.3.6.1.4.1.X}} were {{EX: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.


H3: Naming Prefix

In addition to assign 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 no 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 choosen a short prefix '{{EX:my}}'
(to save space).  Such a short would only be suitable for a
very large, global organization.  For a small, local
organization, we recommend something like '{{EX:deFirm}}'
(German company) or '{{EX:comExample}}' (elements associated
with organization associated with {{EX:example.com}}).


H3: Local schema file

The {{EX:objectclass}} and {{EX: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
{{F:local.schema}} in {{F:/usr/local/etc/openldap/schema/local.schema}}
and then include this file in your {{slapd.conf}}(5) file immediately
after other schema {{EX:include}} directives.

>       # include schema
>       include /usr/local/etc/openldap/schema/core.schema
>       include /usr/local/etc/openldap/schema/cosine.schema
>       include /usr/local/etc/openldap/schema/inetorgperson.schema
>       # include local schema
>       include /usr/local/etc/openldap/schema/local.schema


H3: 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 {{REF:RFC2252}}) used by the attributeTypes
attribute found in the subschema subentry, e.g.:

E:      attributetype <{{REF:RFC2252}} Attribute Type Description>

where Attribute Type Description is defined by the following
{{TERM: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 ] ; see section 4.3
>          [ "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 ('{{EX: }}'), numericoid is a globally unique
OID in numeric form (e.g. {{EX:1.2.3}}), qdescrs is one or more
names, woid is either the name or OID, and noidlen is a optional length
specifier (e.g {{EX:{10}}}).

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

>       attributeType: ( 2.5.4.41 NAME 'name'
>               EQUALITY caseIgnoreMatch
>               SUBSTR caseIgnoreSubstringsMatch
>               SYNTAX 1.3.6.1.4.1.1466.115.121.1.15{32768} )
>       attributeType: ( 2.5.4.3 NAME
>               ( 'cn' $ 'commonName' ) SUP name )

Notice that each defines the attribute's OID and descriptive
names.  Each name is an alias for the OID.  {{slapd}}(8) returns
the first listed name when returning results.

The first attribute, {{EX:name}}, has a syntax of directory string
(a UTF-8 encoded Unicode string) with a recommend maximun length.
Note that syntaxes is specified by OID.  In addition, the equality
and substring matching uses case ignore rules.  Below are tables of
{{slapd}}(8) supported syntax and matching rules.

The second attribute, {{EX:cn}}, is a subtype of {{EX:name}} hence
in inherits the syntax, matching rules, and usage of {{EX:name}}.
{{EX:commonName}} is an alternative name.

Neither attributes is restricted to a single value and both are
meant for usage by user applications.  You likely won't need
specify other parameters such as {{EX:OBSOLETE}}.

The following subsections provide a couple of examples.


H4: myUniqueName

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

>       attributetype ( 1.1.2.1.1 NAME 'myUniqueName'
>               DESC 'unique name with my organization' 
>               EQUALITY caseIgnoreMatch
>               SUBSTR caseIgnoreSubstringsMatch
>               SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
>               SINGLE-VALUE )

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

>       attributetype ( 1.1.2.1.1 NAME 'myUniqueName'
>               DESC 'unique name with my organization' 
>               SUP name )


H4: myPhoto

Many organizations maintain a photo of each each user.  A
{{EX:myPhoto}} attribute type could be defined to hold a photo.
Of course, one could use just use {{EX:jpegPhoto}} ({{REF: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 ( 1.1.2.1.2 NAME 'myPhoto'
>               DESC 'a photo (application defined format)' 
>               SYNTAX 1.3.6.1.4.1.1466.115.121.1.40
>               SINGLE-VALUE )

As noted in the description, LDAP has no knowledge of the
format of the photo.  It's assumed that all applications
accessing this attribute agree on the handling of values.

If you want to support multiple photo format, one could define
a separate attributes type for each format, prefix the photo
with some typing information, or describe the value using
{{TERM:ASN.1}} and use the {{EX:;binary}} transfer option.

Another alternative is for the attribute only to hold a URI
pointing to the photo.  You can model such an attribute after
{{EX:labeledURI}} ({{REF:RFC2079}}).


H3: 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 {{REF:RFC2252}}) used by the objectClasses
attribute found in the subschema subentry, e.g.:

E:      objectclass <{{REF:RFC2252}} Object Class Description>

where Object Class Description is defined by the following
{{TERM: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 ('{{EX: }}'), numericoid is a globally unique
OID in numeric form (e.g. {{EX:1.2.3}}), qdescrs is one or more
names, oids is one or more names and/or OIDs.


H4: myPhotoObject

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

>       objectclass ( 1.1.2.2.1 NAME 'myPhotoObject'
>               DESC 'mixin myPhoto'
>               AUXILIARY
>               MAY myPhoto )


H4: 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 {{EX:inetOrgPerson}}
{{REF:RFC2798}}, and add any additional attributes which
you desire.

>       objectclass ( 1.1.2.2.2 NAME 'myPerson'
>               DESC 'my person'
>               MUST ( 'myUniqueName' $ 'givenName' )
>               SUP inetOrgPerson
>               MAY 'myPhoto' )

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