DNS lookup for dynamic domains in rack applications

custom_domain.rb

require File.dirname(__FILE__) + '/../../vendor/gems/net-dns-0.4/lib/net/dns/resolver'

# Custom Domain
#   A Rack middleware to to resolve the custom domain to original subdomain 
#   for your multi telent application.
#
#   It's all transperant to your application, it performs cname lookup and 
#   overwrite HTTP_HOST if needed
# 
#   www.example.org  =>  example.myapp.com
#
class CustomDomain
  @@cache = {}

  def initialize(app, default_domain)
    @app = app
    @default_domain = default_domain
  end

  def call(env)
    host = env['HTTP_HOST'].split(':').first
    env['HTTP_X_FORWARDED_HOST'] = [host, cname_lookup(host)].join(', ') if custom_domain?(host)
    @app.call(env)
  end

  def custom_domain?(host)
    domain = @default_domain.sub(/^\./, '')
    host !~ Regexp.new("#{domain}$", Regexp::IGNORECASE)
  end

  def cname_lookup(host)
    unless @@cache[host]
      Net::DNS::Resolver.new.query(host).each_cname do |cname| 
        @@cache[host] = cname if cname.include?(@default_domain)
      end
    end

    @@cache[host]
  end
end