连接到通过 Nginx 运行 express 的 Nodejs 应用程序时出现 502 Bad Gateway

Question

I'm having issues connecting to my node app that is running on port 8081.
My setup is as follows (everything runs on a Raspberry Pi):

NGINX

events {
  worker_connections  1024;
}

http {
server {
  root /data/web;

  location / {
  }

  location /pub {
    proxy_pass http://localhost:8081;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
  }
}
}

I'm serving static files with the first location (which seems to be working fine), and I would like the second location to reroute to my node app. which is running on port 8081.

My node app looks like this:

app.get('/', function(req, res){
  res.send("Hello World!");
});
var server = app.listen(8081, '192.168.0.178');

And I'm testing my connection using a simple wget from another pc in the LAN:

wget http://192.168.0.178/pub

The full error I get is this:

http://192.168.0.178/pub
Connecting to 192.168.0.178:80... connected.
HTTP request sent, awaiting response... 502 Bad Gateway
2018-01-14 15:42:27 ERROR 502: Bad Gateway.

SOLUTION
The accepted answer was indeed the problem I was having.
Another thing I added was a rewrite in my /pub location because '/pub' needs to be cut off from the url going to the Node app. So the final nginx conf looks like this:

http {
access_log /data/access_log.log;
error_log /data/error_log.log debug;

upstream backend {
  server localhost:8081;
}

server {
  root /data/web;

  location / {
  }

  location /pub {
    proxy_pass http://localhost:8081;
    rewrite /pub(.*) /$1; break;
  }
}
}

Answer 1

The problem seems related to the network interface you are exposing the nodejs app. You have setup the app to listen to port 8081 on the interface with ip 192.168.0.178 , but the nginx is proxying trough the loopback interface, given the instruction

proxy_pass http://localhost:8081;

You can solve this issue exposing the nodejs app on the loopback interface:

var server = app.listen(8081, 'localhost');

The node app should be no more reachable directly on port 8081 from any other machine except the one the app is running