protected function _vcl_director_backend($host, $port, $descriptor = '', $probeUrl = '', $options = array()) {
        $tpl = <<<EOS
    {
        .backend {$descriptor} = {
            .host = "{{host}}";
            .port = "{{port}}";
{{probe}}

EOS;
        $vars = array(
            'host'  => $host,
            'port'  => $port,
            'probe' => ''
        );
        if ( ! empty($probeUrl)) {
            $vars['probe'] = $this->_vcl_get_probe($probeUrl);
        }
        $str = $this->_formatTemplate($tpl, $vars);
        foreach ($options as $key => $value) {
            $str .= sprintf('            .%s = %s;', $key, $value).PHP_EOL;
        }
        $str .= <<<EOS
        }
    }
EOS;
        return $str;
    }