ruby-on-rails - Rack-Attack: Array of IP addresses

Question

I'm trying to create an array of IP addresses so that when the application is ran Rack-Attack can identify from the set of IP addresses that are allowed to access the application. So what I have done is as followed:

  a = "127.0.0.1"
  Rack::Attack.blacklist('allow from localhost') do |req|
    p "#{'127.0.0.1' == req.ip} "
   a != req.ip 
  end

The above works, so localhost can access the application but I have tried the following below which seems to not work what so ever:

a = "127.0.0.1", "1.2.3.4"
  Rack::Attack.blacklist('allow from localhost') do |req|
    a.select{|x| x != req.ip}.join("")
  end

Can someone explain what the correct way would be to do this. You can see that I create an array. I want Rack::Attack to detect whether the IP address in the array has access or not.