In reverse proxy, when an upstream causes an error (e.g. closes the connection abruptly) the next upstream is contacted. That is not logged but since it is an error it should be logged with ERROR status as it is crucial to help debug our upstreams. The only relevant log I've found is: ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,"http next upstream, %xi", ft_type);by sivann - Other discussion