feature spec toward subdomain ref: http://qiita.com/Oakbow/items/3da1b20ff17e5e192f47

feature_spec_toward_subdomain_rails_helper.rb

# You need to add your domain into Capybara's whitelist.
RSpec.configure do |config|
  Capybara::Webkit.configure do |c|
    c.block_unknown_urls
    c.allow_url("admin.lvh.me")
  end
  Capybara.default_selector  = :css
      :

users_spec.rb

require "rails_helper"

feature "Users", type: :feature, js: true do
  background(:all) do
    Capybara.default_host = 'http://subdomain.lvh.me'
    Capybara.app_host = 'http://subdomain.lvh.me'
    Capybara.always_include_port = true
  end

  after(:all) do
    Capybara.default_host = 'http://www.example.com'
    Capybara.app_host = nil
    Capybara.always_include_port = false
  end
   :

what_lvn.me_is.txt

$ nslookup lvh.me
Server:     192.168.100.254
Address:    192.168.100.254#53

Non-authoritative answer:
Name:   lvh.me
Address: 127.0.0.1