Para o wiki do Samba4 :
This document attempts to describe some of requirements that a 'general purpose' LDAP directory server must meet to have Samba4 successfully use it as a 'directory backend'.
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This is no longer an area of active development in Samba4, and is not supported, or expected to be supported
Eu sugeriria mudar qualquer referência ao seu LDAP externo para, em vez disso, apontar para o Samba4 (que implementa o AD) e não o contrário.