Really? I still have to do this in 2013?

rack-middleware-ip.rb

require 'ipaddr'

module Monitoring
  module Middleware
    class Ip
      def initialize(app, options = {})
        trusted = options[:trusted] || [ '127.0.0.1/32' ]
        @trusted = trusted.map { |network| IPAddr.new network }
        @app = app
      end

      def call(env)
        if env['HTTP_X_FORWARDED_FOR'] and trusted? env['REMOTE_ADDR']
          env['REMOTE_ADDR'] = env['HTTP_X_FORWARDED_FOR']
        end

        @app.call(env)
      end

      private
      def trusted?(ip)
        @trusted.each { |network| return true if network === ip }
        false
      end
    end
  end
end