Uses the Rackspace Apps control panel website to update the DNS A record for a subdomain.

update-dns.rb

#!/usr/bin/ruby

require 'rubygems'
require 'steam'
require 'net/http'
require 'resolv'
require 'mail'

class DnsUpdateBot
  def initialize
    # assumes that the HtmlUnit jar files are stored in an htmlunit directory
    Steam.config[:html_unit][:java_path] = File.dirname(__FILE__) + '/htmlunit/lib'

    @browser = Steam::Browser.create
  end
  
  def login(username, password)
    @browser.get('https://cp.rackspace.com')
    
    sanity_check

    @browser.fill_in('username', :with => username)
    @browser.fill_in('password', :with => password)
    @browser.click_button('login')
  
    sanity_check
  end

  # record_type is expected to be one of :a, :cname, :url, :frame or :txt
  def update_dns_entry(domain, hostname, record_type, address)
    @browser.get("https://cp.rackspace.com/no-borders/domains/domains/dns/dnsRecords.asp?domain=#{domain}")
    address_field = @browser.locate_in_browser(:field, :xpath => ".//*[@id='nonMailRecordTable']/descendant::input[@value='#{hostname}']/parent::td/parent::tr/descendant::select[contains(@name, 'Type')]/option[@selected and @value='#{record_type.to_s.upcase}']/parent::select/parent::td/parent::tr/descendant::input[contains(@name, 'Address')]")
    address_field.setText(address) rescue address_field.setValueAttribute(address)
    @browser.click_button('Save')
    
    sanity_check
    
    raise "Update failed" unless @browser.response.body.include?('Your records have been successfully updated!')
  end
  
  private
  
  def sanity_check
    raise "Something went wrong" unless @browser.response.status == 200
  end
end

class PublicIpBot
  def fetch
    try_three_times do
      Net::HTTP.get(URI.parse('http://whatismyip.org'))
    end
  end
  
  private 
  
  def try_three_times
    raise "A block is required" unless block_given?
    
    result = nil
    attempt_count = 1
    was_attempt_successful = false
    
    while (not was_attempt_successful) and (attempt_count < 3)
      begin
        result = yield
        was_attempt_successful = true
      rescue
        attempt_count += 1
      end
    end
    
    result
  end
end

begin
  public_ip_bot = PublicIpBot.new

  unless public_ip_bot.fetch == Resolv.getaddress('home.example.com')
    dns_bot = DnsUpdateBot.new
    dns_bot.login('user_name', 'sekret')
    dns_bot.update_dns_entry('example.com', 'home', :a, public_ip_bot.fetch)
  end
rescue Exception => error
  Mail.defaults do
  
    delivery_method :smtp, {
      :address => 'smtp.example.com',
      :domain => 'smtp.example.com',
      :user_name => '[email protected]',
      :password => 'sekret',  
      :authentication => nil,
      :enable_starttls_auto => true
    }
  end
  
  
  mail = Mail.new do
    from '[email protected]'
    to '[email protected]'
    subject '[alert] ip update failure'
    body error.to_s
  end
  
 
  mail.deliver
end