How do I set-up reverse proxying with a Ruby Rack app?

Sure thing! Here is the simplified setup for nginx as a proxy to your ruby app.

Install nginx:

apt-get update;
apt-get install nginx

Then in your main virtualhost configuration: /etc/nginx/sites-enabled/default

Replace the existing Location / block with the following:

location / {
    proxy_pass       http://localhost:4567;
    proxy_set_header Host      $host;
    proxy_set_header X-Real-IP $remote_addr;
}

Making sure to change 4567 to the port your app is running on if it is different. Then restart nginx so your changes will take effect:

service nginx restart;