You can configure nginx to reverse proxy connections to port 81. If you’re going this route, it would make sense to allow access to your sinatra app through nginx only. Edit your app’s config and set it to listen on “127.0.0.1:8888” or any unused port above 1024 (any program that needs to listen on ports < 1024 must run as root which is not recommended for a website) and add the following nginx server block:

server {
    listen 80;
    server_name yourapp.com; # or server_name subdomain.yourapp.com;

    location / {
        proxy_pass http://localhost:8888;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_set_header X-NginX-Proxy true;

        # Enables WS support
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_redirect off;
    }
}

Hi Kamal Nasser,
actually this results in

 * Restarting NGINX nginx                                                                                                                       nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
nginx: [emerg] still could not bind()

as my apache is using 80 already.