Basic HelloWorld Nginx proxy_pass giving 502 bad gateway : WSL Ubuntu

I am simply trying to learn nginx load-balancing, and have a very basic nodejs hello world server, 4 instances of which are running in port 3001-3004.

I want to load-balance across them, but proxy_pass will not work. Can someone please help ?

Below is my nginx.conf file. Nginx is running in WSL ubuntu

events {

}

http {

  upstream allbackend {
    server 127.0.0.1:3001;
    server 127.0.0.1:3002;
    server 127.0.0.1:3003;
    server 127.0.0.1:3004;
  }

  access_log /path/to/log/nginx/access.log;
  error_log /path/to/log/nginx/error.log;

  server {

    listen 8888;
    location / {
      # return 200 "hello from nginx"; # only this works
      proxy_pass http://allbackend; # this fails
    }
  }
}

Fixed.

The issue was that I was running node servers in my local (non-wsl) windows, whereas nginx was running in WSL.

Once I started the node servers inside WSL, all things worked well.