In this step, what has returned from the down host is returned, will be returned to the original client. For example if a 500 error occurs or if a bad request happens, all the HTTP status, body and header is exactly repeated to the original client.

But by your good hint, I am inspired to ban the down host temporairly or permanently. So do not send requests to hosts which are down. The definition of down can be configurable. For example a timeout error and all 500 family error can be considered down and all other error can be consider repeatable actions.