Check anonymous bind
        :return: 'on', 'off', 'rootdse' or None

def _get_anon_bind(self):
        """Check anonymous bind
        :return: 'on', 'off', 'rootdse' or None
        """
        r = self._search(
            'cn=config',
            '(objectClass=*)',
            ['nsslapd-allow-anonymous-access'],
            scope=ldap.SCOPE_BASE
        )
        dn, attrs = r[0]
        state = attrs.get('nsslapd-allow-anonymous-access')[0].decode('utf-8', 'ignore')
        if state in ['on', 'off', 'rootdse']:
            r = state
        else:
            r = None
        self._anon_bind = r