502：通过Nginx代理Ubuntu 18.04的Nodejs应用

Question

Setup

Running an Ubuntu 18.04 on a Virtual Private Server. Installed Nginx (to work as a proxy), serving Nodejs app using PM2.

Problem

Worked for a month now suddenly the server returns a 502:

2018/10/10 00:39:51 [error] 1009#1009: *19 connect() failed (111: Connection refused) while connecting to upstream, client: 176.104.---.---, server: foo.bar.com, request: "GET / HTTP/2.0", upstream: " http://127.0.0.1:4000/ ", host: "foo.bar.com"

2018/10/10 00:39:51 [error] 1009#1009: *19 no live upstreams while connecting to upstream, client: 176.104.---.---, server: foo.bar.com, request: "GET /favicon.ico HTTP/2.0", upstream: " http://localhost/favicon.ico ", host: "foo.bar.com", referrer: " https://foo.bar.com/ "

2018/10/10 01:10:15 [error] 1009#1009: *24 connect() failed (111: Connection refused) while connecting to upstream, client: 176.104.--.---, server: foo.bar.com, request: "GET / HTTP/2.0", upstream: " http://[::1]:4000/ ", host: "foo.bar.com"

2018/10/10 01:10:15 [error] 1009#1009: *24 connect() failed (111: Connection refused) while connecting to upstream, client: 176.104.---.---, server: foo.bar.com, request: "GET / HTTP/2.0", upstream: " http://127.0.0.1:4000/ ", host: "foo.bar.com"

2018/10/10 01:10:15 [error] 1009#1009: *24 no live upstreams while connecting to upstream, client: 176.104.---.---, server: foo.bar.com, request: "GET /favicon.ico HTTP/2.0", upstream: " http://localhost/favicon.ico ", host: "foo.bar.com", referrer: " https://foo.bar.com/ "

Configs:

• nginx.conf
• Per-site blocks

Important

The most important - this config worked perfectly for month. Then it stopped working. This happened when the new version of the app was uploaded. Have rolled back to the previous version (full copy of the whole system). But now even previous version (which worked) doesn't work with the same error. So scratching the head here.

Firewall

sudo ufw status
Status: active

To                         Action      From
--                         ------      ----
**/tcp                     ALLOW       ******
Nginx Full                 ALLOW       Anywhere
Nginx Full (v6)            ALLOW       Anywhere (v6)

Update

I have tried to serve just a static HTML file and it worked. I have tried to serve a dummy hello-world Expressjs app and it worked. So the problem seems to be with the Angular's generated server.js (Universal engine).

Answer 1

The 502 Bad gateway error means your backend is not reachable by NGINX.
Make sure pm2 runs the NodeJS app and listens on 4000/TCP.
You can simply check it by curl or netstat with these commands:

netstat -ntlp | grep :4000
curl http://127.0.0.1:4000