rbl.rb

require 'rubygems'
require 'net/dns/resolver'

DEFAULT_SERVERS = [ 'dnsbl.ahbl.org' ]

module Rack
  class RBL
    # List of RBL servers to check against
    attr_accessor :servers
    
    def initialize(app, options = {})
      @app = app
      self.servers = options[:servers] || DEFAULT_SERVERS
    end

    def call(env)
      if blacklisted? env['REMOTE_ADDR']
        [403, {"Content-Type" => "text/html"}, "Sorry, you are on a blacklist."]
      else
        @app.call(env)
      end
    end

    def blacklisted?(addr)
      # reverse IP address octets
      raddr = addr.split('.').reverse.join('.')
      servers.each do |s|
        return true unless Resolver("#{raddr}.#{s}").answer.empty?
      end
      false
    end
  end
end