Sample sites_available configuration for nginx where port 8123 redirects to another server (eg a NodeJS Express app) running on port 3218. This configuration considers WebSocket and CORs support.

another_available_site

server {
        listen 8123 default_server;
        listen [::]:8123 default_server;

        server_name your_server;
        add_header Strict-Transport-Security "max-age=31536000";
        access_log /var/log/nginx/some_app.access.log;
        error_log /var/log/nginx/some_app.error.log;

        location / {
                include /etc/nginx/mime.types;

                if ($request_method = OPTIONS) {
                   add_header Access-Control-Allow-Origin $http_origin;
                   add_header Access-Control-Allow-Methods "GET, POST, DELETE, PUT, OPTIONS";
                   add_header Access-Control-Allow-Headers "Authorization, Origin, X-Requested-With, Content-Type, Accept, Access-Control-Allow-Origin, x-uuid, X-JWT-Token, X-JWT-RefreshToken";
                   add_header Access-Control-Allow-Credentials "true";
                   add_header Content-Length 0;
                   add_header Content-Type text/plain;
                   return 200;
                }
                proxy_pass http://127.0.0.1:3218;
                proxy_buffering on;

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

}

Configuration included in article posted at https://medium.com/@JSantaCL/nginx-nodejs-and-spas-cc50c376b131