How can I allow access to a single IP address via Nginx.conf?

modify your nginx.conf

  server {
    listen 80;
    server_name www.foo.bar;

    location / {
      root /path/to/rails/public/;
      passenger_enabled on;

      allow   my.public.ip.here;
      deny    all;
    }
  }

I know this is an old tread, but you an always use sudo nginx -t to get a more specific error message.