Q:

NGINX Reverse Proxy return 502 bad gateway when proxied server is down

I setup nginx as a reverse proxy for my apache tomcat. It works normally as I expected. However, I got confused when NGINX is always returning a 502 Bad Gateway when the Apache Tomcat server is down. Instead of returning a 504 Bad Gateway timeout? 

502 Bad Gateway:
      The server was acting as a gateway or proxy and received an invalid response from the upstream server.
504 Gateway Timeout
      The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.

user  root;
worker_processes  1;

events {
        worker_connections  1024;
}

http {
       include       mime.types;
       default_type  application/octet-stream;
       sendfile        on;

       ssl_session_cache   shared:SSL:20m;
       ssl_session_timeout 10m;
       keepalive_timeout  65;

       map $http_upgrade $connection_upgrade {
               default Upgrade;
               '' close;
       }

        server {
                listen          *:80;
                return 301      https://$host:443$request_uri;
        }

        server{
                listen       *:443; #Ip of client
                # Specifies the maximum accepted body size of a client request, as indicated by the request header Content-Length.
                client_max_body_size 1024M;
                # ssl config
                ssl                  on;
                ssl_certificate      server.crt;
                ssl_certificate_key  server.key;

                # for proxy timeout
                proxy_connect_timeout 75s;
                proxy_read_timeout 600s;
                proxy_send_timeout 600s;

                # not cache authorization
                proxy_no_cache $http_pragma $http_authorization;

                location /wss {
                        rewrite ^.*\/wss\/(?<api>.*) /$api break;
                        proxy_pass http://127.0.0.1:8071;

                        # for websocket
                       proxy_set_header Upgrade $http_upgrade;
                       proxy_set_header Connection $connection_upgrade;
                       proxy_http_version 1.1;
                       proxy_buffering off;
                       proxy_ignore_client_abort off;
                       proxy_read_timeout 1d;
                       proxy_send_timeout 1d;
                }

                location / {
                        proxy_buffering off;
                        proxy_pass http://127.0.0.1:8071;
                }
        }
}

Error log when accessing:

2015/10/19 10:10:03 [error] 29475#0: *44 connect() failed (111:
  Connection refused) while connecting to upstream, client:
  192.168.70.60, server: , request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:8071/", host: "192.168.70.161"
2015/10/19 10:10:03 [error] 29475#0: *44 connect() failed (111:
  Connection refused) while connecting to upstream, client:
  192.168.70.60, server: , request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:8071/", host: "192.168.70.161"

Can anyone explain why the NGINX returns a 502 HTTP error instead of a 504?
Or, are there problems with my configuration?
I think, I missed.
504 only happen when NGINX can't forward request to proxied server but the proxied server doesn't response in time as NGINX expected.
In my case:
proxy_connect_timeout 75s;
proxy_read_timeout 600s;
proxy_send_timeout 600s;

So in case of Proxied Server is down, NGINX will respond with the HTTP error code 502, 503?

A:

By default, the SELinux configuration does not allow NGINX to connect to a remote web, fastCGI, or other server. You can set permissive mode with setenforce 0 to check whether SELinux is to blame. If it is, All you have to do is use audit2allow to generate a set of policy rules that would allow the required actions：
grep nginx /var/log/audit/audit.log | audit2allow -M nginx
semodule -i nginx.pp
After that, remember to enable SELinux again with setenforce 1.

For more about that, you can see this acticle.

