502 Bad Gateway when connecting to Nodejs app running express through Nginx - node.js

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