About LDAP Syntaxes and backward compatibility…

In the LDAP information model, a syntax constrains the structure and format of attribute values. OpenDJ defines and implements a large number of syntaxes (you can discover them by reading the ldapSyntaxes attribute from the cn=Schema entry).

But infrequently, we receive enquiries on an obscure and non standard syntax, often in the form of “I’m having an error importing schema from this or that legacy directory server”, with an error message that ends with “No such syntax is configured for use in the Directory Server”.

As syntaxes are constraining the structure and format of attribute values, they are implemented as code, specifically Java code in OpenDJ. It’s possible to implement new syntaxes by implementing the org.opends.server.api.AttributeSyntax abstract class, and installing the classes or the JAR in OpenDJ classpath. But often, it’s easier and more convenient to define a syntax by configuration, and OpenDJ offers 3 possibilities to define new syntaxes. In term of backward compatibility, I will only focus on the 2 main ones, by substitution and by pattern (the 3rd one allows to define enumeration of values).

With OpenDJ, you can define a new syntax by configuration and delegating the contraints to an already implemented syntax. A simple example is the URI syntax (which was defined is some very old schema with the OID A URI is really an ASCII string, and it might be sufficient to accept attributes with URI syntax to verify that all characters are pure ASCII. The standard syntax for ASCII strings is IA5String aka

ldapSyntaxes: ( DESC ‘URI’ X-SUBST ‘’ )

Insert the above line in the schema LDIF file before the attributeTypes, and you’re done.

The other option is to define the syntax as a pattern, using regular expressions. This could be better when willing to enforce additional constraints on an URI, for example, verifying that the URI is an LDAP one.

ldapSyntaxes: ( 999.999.999.1 DESC 'LDAP URI Syntax' X-PATTERN '^ldap://[-a-zA-Z0-9+&@#/%?=~_|!:,.;]*[-a-zA-Z0-9+&@#/%=~_|]' )

So the next time you are trying to import some legacy schema to the OpenDJ directory server, and you have an error due to missing syntaxes, you know what to do to quickly resolve the problem.

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