upstream prematurely closed connection while reading response header from upstre

请求对方用nginx做了代理：但是error.log报
upstream prematurely closed connection while reading response header from upstream, client
找了半天，host没set进去

原来配置

location / {
            proxy_pass     http://pool_xxx;
            proxy_redirect     off;
            proxy_set_header   Host             $http_host;
            #proxy_set_header   X-Real-IP        $remote_addr;
            #proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
        }

修改后

location / {
            proxy_pass     http://pool_xxx;
            proxy_redirect     off;
            proxy_set_header   Host             "xxx.xxxxx.com";
            #proxy_set_header   X-Real-IP        $remote_addr;
            #proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
        }