[syslog-ng] if no host; do use_dns

Hi list, With keep_hostname(yes), when the syslog-ng box receives a message which does not have a host field, you end up with the last hop IP as the hostname. Is there already a way to tell sysng to use_dns() when this situation is encountered? I'd like to continue using keep_hostname(yes) for all log messages which do have a hostname included, but in the instance that hostname is not included, have syslog-ng do a reverse dns lookup in order to resolve hostname. Thanks,