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Chapter 3. Manually creating a custom schema file

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You can add custom attributes and object classes to Directory Server by extending the schema. You can extend the schema:

3.1. Workflow of a schema extension

Adding new schema elements requires:

  1. Planning and defining unique object identifiers (OID) for the new schema. Directory Server recognizes schema elements by their OID, but you must manage the OIDs manually.

    An OID is a dot-separated number that identifies the schema element to the server. OIDs can be hierarchical with a base OID that can be expanded to accommodate different branches. For example, the base OID could be 1, and there can be a branch for attributes at 1.1 and for object classes at 1.2.

    Important

    Even if not required, Red Hat recommends to use numeric OIDs for custom schemas for better forward compatibility and performance.

  2. Request OIDs from the Internet Assigned Numbers Authority (IANA). For details, see https://pen.iana.org/pen/PenApplication.page.
  3. Create a OID registry to track OID assignments and to ensure that no OID is used for more than one purpose. An OID registry is a list of all OIDs used in the directory schema including descriptions. Publish the OID registry with the custom schema.
  4. Define the new attributes.
  5. Define the object classes that contain the new attributes. However, never update the default schema. If you create new attributes, always add them to a custom object class.

Directory Server loads the schema when the instance starts. To load new schema files, restart the instance or initiate a reload task.

Keep the following rules in mind when customizing the Directory Server schema:

  • Keep the schema as simple as possible.
  • Reuse existing schema elements whenever possible.
  • Minimize the number of mandatory attributes defined for each object class.
  • Do not define more than one object class or attribute for the same purpose.
  • Do not modify any existing definitions of attributes or object classes.
Warning

Do not update or delete the standard schema to avoid compatibility problems with other directories or LDAP client applications.

3.2. Requirements for a schema file

Schema files use the LDIF format that define the cn=schema entry. Each attribute type and object class is added to this entry.

The following are the requirements for a schema file:

  • The file must start with the following entry: 

    dn: cn=schema
  • A schema file can include attribute types or object classes or both of them.
  • Object class definitions can use attributes defined in other schema files.

  • Depending on which instances should use a custom schema file, store it in one of the following locations:

    • /etc/dirsrv/slapd-instance_name/schema/ to make the schema file available to this specific instance
    • /usr/share/dirsrv/schema/ to make the schema file available to all instances running on this host

  • By default, Directory Server expects the custom schema in the 99user.ldif file. If you use a different file name:

    • The name must be alphabetically lower than 99user.ldif. For example, 99aaa.ldif is ok, but 99zzz.ldif is not.

    • The name must start with two digits and be higher than 01 because custom schema files must be loaded after the core schema files, which begin with 00 up to 98

      Directory Server reads schema files in alphabetical order. Therefore, for example, if you store a definition 99user.ldif, it will override definitions from standard files whose name begins with 00 and 01.

  • If you want to use a standard schema file from the /usr/share/dirsrv/data/ directory, copy the file to /etc/dirsrv/slapd-instance_name/schema/ or /usr/share/dirsrv/schema/ depending on which instances should use the file. However, use a different file name in the destination directory. Otherwise, Directory Server renames the file during an upgrade and appends the .bak suffix.

Example 3.1. Example of a custom schema file

dn: cn=schema
objectClasses: ( 2.16.840.1.1133730.2.1.123 NAME 'exampleperson' DESC 'An example
  person object class' SUP top STRUCTURAL MUST dateOfBirth X-ORIGIN 'user defined' )
attributeTypes: ( 2.16.840.1.1133730.2.1.99 NAME 'dateOfBirth' DESC 'For employee
  birthday' SYNTAX 1.3.6.1.4.1.1466.115.121.1.15 SINGLE-VALUE X-ORIGIN 'user defined' )

3.3. The definition of attributes in custom schema files

You define attributes in schema files as values of attributeTypes attributes.

Example 3.2. Definition of an attribute

attributeTypes: ( 2.16.840.1.1133730.2.1.123 NAME 'dateOfBirth' DESC 'For employee birthday' SYNTAX 1.3.6.1.4.1.1466.115.121.1.15 SINGLE-VALUE X-ORIGIN 'user defined' )

The attribute definition contains the following components:

  • A unique object identifier (OID) specified as a dot-separated number.
  • A unique name in the form of NAME attribute_name.
  • A description in the form of DESC description.
  • The OID for the syntax of the attribute values in the form SYNTAX OID. For details about the LDAP attribute syntaxes, see RFC 4517.
  • Optional: The source where the attribute is defined.

3.4. The definition of object classes in custom schema files

You define object classes in schema files as values of objectClasses attributes.

Example 3.3. Definition of an object class

objectClasses: ( 2.16.840.1.1133730.2.1.99 NAME 'exampleperson' DESC 'An example person object class' SUP top STRUCTURAL MUST dateOfBirth X-ORIGIN 'user defined' )

The object class definition contains the following components:

  • A unique object identifier (OID) specified as a dot-separated number.
  • A unique name in the form of NAME attribute_name.
  • A description in the form of DESC description.
  • The superior (parent) object class for this object class in the form SUP object_class. If there is no related parent, use SUP top.
  • The word STRUCTURAL defines the type of entry to which the object class applies. Any entry must belong to at least one STRUCTURAL object class. AUXILIARY means that it can apply to any entry.
  • A list of required attributes, preceded by the MUST keyword. To include multiple attributes, enclose the group in parentheses and separate the attributes with a [command]`$ ` (dollar sign and space).
  • A list of optional attributes, preceded by the MAY keyword. To include multiple attributes, enclose the group in parentheses and separate the attributes with a [command]`$ ` (dollar sign and space).

Only the name and OID is required, and other settings depend on the needs of the object class.

Additional resources

3.5. How Directory Server manages schema updates in a replication environment

When you update the directory schema in the cn=schema tree, Directory Server stores the changes in the /etc/dirsrv/slapd-instance_name/schema/99user.ldif file, including a change state number (CSN).

Directory Server does not directly replicate the schema changes to other replicas. Schema replication starts when directory content is updated in the replicated tree. For example, if you update a user after modifying the schema, the supplier compares the CSN stored in the nsSchemaCSN attribute with the one on the consumer. If the value of the nsSchemaCSN attribute on the consumer is lower than the one on the supplier, Directory Server replicates the schema to the consumer. For a successful replication, all object classes and attribute types on the supplier must be a superset of the consumer’s definition.

Example 3.4. Schema subsets and supersets

  • On server1, the example object class allows the a1, a2, and a3 attributes.
  • On server2, the example object class allows the a1 and a3 attributes.

In the previous example, the schema definition of the example object class on server1 is a superset of the object class on server2. During the validation phase, when Directory Server replicates or accepts the schema, the server retrieves the superset definitions. For example, if a consumer detects that an object class in the local schema allows less attributes than the object class in the supplier schema, Directory Server updates the local schema.

If the schema definitions are successfully replicated, the nsSchemaCSN attributes are identical on both servers and the schema definitions, such as object classes and attributes types, are no longer compared at the beginning of a replication session.

In the following scenarios, Directory Server does not replicate the schema:

  • The schema on one host is a subset of the schema of another host.

    For example, the schema definition of the example object class on server2 is a subset of the object class on server1. Subsets can also occur for attributes (a single-value attribute is a subset of a multi-value attribute) and attribute syntaxes.

  • When definitions in supplier schema and consumer schema need to be merged.
  • Directory Server does not support merging schemas. For example, if an object class on one server allows the a1, a2, and a3 attributes and a1, a3, and a4 on the other, the schemas are not subsets and cannot be merged.

  • You use schema files other than /etc/dirsrv/slapd-instance_name/schema/99user.ldif.

    Directory Server enables you to add additional schema files to the /etc/dirsrv/slapd-instance_name/schema/ directory. However, only the CSN in the /etc/dirsrv/slapd-instance_name/schema/99user.ldif file is updated. For this reasons, other schema file are used only locally and not automatically transferred to replication partners.

    Important

    To enable Directory Server to automatically replicate the schema and to avoid duplicate schema definitions, store the custom schema in the /etc/dirsrv/slapd-instance_name/schema/99user.ldif file.

3.6. Manually creating a custom schema file for an attribute and object class

If you want to manually create a custom schema, store it in the /etc/dirsrv/slapd-instance_name/schema/99user.ldif file. Using a different file name is possible, but causes drawbacks, such as schema definitions stored in other files are replicated, but then stored in /etc/dirsrv/slapd-instance_name/schema/99user.ldif on the replica. See How Directory Server manages schema updates in a replication environment.

This procedure adds:

  • A single-valued attribute named dateOfBirth with OID 2.16.840.1.1133730.2.1.123 and syntax Directory String (OID 1.3.6.1.4.1.1466.115.121.1.15)
  • An object class named exampleperson without parent object class (SUP top) that must contain the dateOfBirth attribute.

Procedure

  1. Add the following content below the dn: cn=schema entry in the /etc/dirsrv/slapd-instance_name/schema/99user.ldif file: 

    attributeTypes: ( 2.16.840.1.1133730.2.1.123 NAME 'dateOfBirth' DESC 'For employee
  birthday' SYNTAX 1.3.6.1.4.1.1466.115.121.1.15 SINGLE-VALUE X-ORIGIN 'user defined' )
objectClasses: ( 2.16.840.1.1133730.2.1.99 NAME 'exampleperson' DESC 'An example
  person object class' SUP top STRUCTURAL MUST dateOfBirth X-ORIGIN 'user defined' )

  2. Run a schema reload task: 

    # dsconf -D "cn=Directory Manager" ldap://server.example.com schema reload

Verification steps:

  • Monitor the /var/log/dirsrv/slapd-instance_name/errors file:

    • If the build succeeds, Directory Server logs: 

      [23/Sep/2021:13:47:33.334241406 +0200] - INFO - schemareload - schemareload_thread - Schema reload task starts (schema dir: default) ...
[23/Sep/2021:13:47:33.415692558 +0200] - INFO - schemareload - schemareload_thread - Schema validation passed.
[23/Sep/2021:13:47:33.454768148 +0200] - INFO - schemareload - schemareload_thread - Schema reload task finished.
    • If the build fails, Directory Server logs which step failed and why.
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