Searches the directory for a user matching AUTH_LDAP_USER_SEARCH.
        Populates self._user_dn and self._user_attrs.

def _search_for_user_dn(self):
        """
        Searches the directory for a user matching AUTH_LDAP_USER_SEARCH.
        Populates self._user_dn and self._user_attrs.
        """
        search = self.settings.USER_SEARCH
        if search is None:
            raise ImproperlyConfigured(
                "AUTH_LDAP_USER_SEARCH must be an LDAPSearch instance."
            )

        results = search.execute(self.connection, {"user": self._username})
        if results is not None and len(results) == 1:
            (user_dn, self._user_attrs) = next(iter(results))
        else:
            user_dn = None

        return user_dn