gistfile1.rb

module Rack
  class ServerNameHeader
    attr_reader :app, :hostname
    def initialize(app)
      @app = app
      @hostname = `hostname`.strip
    end
    def call(env)
      status, headers, body = *app.call(env)
      headers['X-Server-Name'] = hostname
      [status, headers, body]
    end
  end
end

gistfile2.rb

# config/environments/production.rb
require File.expand_path('../../../lib/rack/server_name_header', __FILE__)
My::Application.configure do
  config.middleware.use Rack::ServerNameHeader
  # all the other stuff
end