gistfile1.rb

class CanonicalHost
  def initialize(app, host=nil, &block)
    @app = app
    @host = (block_given? && block.call) || host
  end

  def call(env)
    if url = url(env)
      [301, { 'Location' => url }, ['Redirecting...']]
    else
      @app.call(env)
    end
  end

  def url(env)
    if @host && env['SERVER_NAME'] != @host
      url = Rack::Request.new(env).url
      url.sub(%r{\A(https?://)(.*?)(:\d+)?(/|$)}, "\\1#{@host}\\3/")
    end
  end
  private :url
end