This problem is solved. Except reset from backend server, if we dispatch request from host of http header，and find no frontend, 503 with NOSRV and SC can also occur.
Thank for you help, @lukastribus.
This problem is solved. Except reset from backend server, if we dispatch request from host of http header，and find no frontend, 503 with NOSRV and SC can also occur.
Thank for you help, @lukastribus.