Nginx WebSocket proxying?

Question

Nginx

Where did I screw up?
nginx config

server {
    listen 80;

    server_name subdomain.domain.com;

    location /{
        access_log off;

        proxy_pass http://localhost:3030;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
    }
}

knocking ws://subdomain.domain.com - not working
knocking ws://xxx.xxx.xxx.xxx:3030 - all ok
knocking ws://localhost:3030 - not working

Answer 1

Most likely IPv6 is enabled and the connection goes not to 127.0.0.1 but to ::1 which resolves to localhost.
or vice versa the server listens on IPv6 only
see netstat -pln