强制反向代理Nginx带来content-lenght标头响应

Question

我将nginx用作静态Web服务器和反向代理。

我想获取下一个响应头（连接：keepalive，Content-Leght，ETag）：

Server: nginx/1.10.3 (Ubuntu)
Date:   Sat, 07 Sep 2019 21:10:35 GMT-2s
Content-Type:   text/plain
Content-Length: 210 kilobytes
Last-Modified:  Sat, 07 Sep 2019 21:04:01 GMT-6m 36s
Connection: keep-alive
ETag:   "5d741b41-3480e"
Accept-Ranges:  bytes

我需要在实际配置中添加哪些指令？

它在nginx / 1.10.3 Ubuntu Server 16.04上运行。 通过我的实际配置，我获得了下一个响应头：

Server: nginx/1.10.3 (Ubuntu)
Date:   Sat, 07 Sep 2019 21:15:45 GMT-1s
Content-Type:   text/plain; charset=utf-8
Connection: close
Expires:    Sat, 07 Sep 2019 21:15:44 GMT-2s
Cache-Control:  no-cache

我的实际配置是：

upstream app_server {

server 127.0.0.1:5000 fail_timeout=0;
}

server {

listen 80;
server_name _;

# Path for static files WebUI
root /opt/static/www/current;

location = / {
    index index.html;
}

location ~* \.html$ {
    try_files $uri @proxy_to_app;
    expires -1;
}

# First files, then proxy to flask app
# If the original URI can't resolve into a existing file or directory,
# the request is redirected to the named location (proxy_to_app)

location / {
    try_files $uri @proxy_to_app;
    expires off;
}

# For SSE Server-Sent Events
location /status/events {

    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_set_header Connection '';
    proxy_http_version 1.1;
    proxy_redirect off;
    proxy_buffering off;
    chunked_transfer_encoding off;
    proxy_cache off;
    proxy_read_timeout 120s;

    expires -1;

    proxy_pass   http://app_server;
}

location /feeds/ {

    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_set_header Connection '';
    #proxy_http_version 1.1;
    proxy_redirect off;
    proxy_buffering off;
    chunked_transfer_encoding off;
    proxy_cache off;
    proxy_read_timeout 120s;

    proxy_set_header X-NginX-Proxy true;
    expires -1;

    gzip_static always;
    # Forward /feeds/ requests to the application server
    proxy_pass http://app_server;
}

# For content that should be handled by mw flask app
# The named location (proxy_to_app) passes the request to a proxied server 

（app_server）

location @proxy_to_app {

    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_redirect off;

    proxy_pass   http://app_server;
}

}

预先感谢