AwsDnsTool.rb

#!/usr/bin/env ruby
#
# This Ruby script will convert between Amazon EC2 public and private DNS names.
#
# The first command-line argument is used as a regular expression to perform pattern-matching
# against leading strings of all public and private DNS names returned from an EC2 'describe-instances'
# API call.  (ie, if the argument is 'domU', we will find all DNS names that match /domU.*/)
# Whenever a match is found, the "other" type of DNS name is printed.  (private <=> public)
#
# Any arguments after the first are ignored.
#
# Be sure to modify this script to add your AWS access and secret keys in the appropriate places.
#
# Requires the amazon-ec2 gem (gem install amazon-ec2)
#
# Author: Darren Lee (darren@bizo.com)

require 'rubygems'
require 'EC2'

ACCESS_KEY_ID = '(your AWS access key)'
SECRET_ACCESS_KEY = '(your AWS secret key)'

if ARGV.length < 1 then
  puts "Error: no leading string specified"
  exit
end

regex = /#{ARGV[0]}.*/

ec2 = EC2::Base.new(:access_key_id => ACCESS_KEY_ID, :secret_access_key => SECRET_ACCESS_KEY)

ec2.describe_instances.reservationSet.item.each do |instance|
  # Note that instance id is also available at instance.instancesSet.item[0]['instanceId']
  publicDns = instance.instancesSet.item[0]['dnsName']
  privateDns = instance.instancesSet.item[0]['privateDnsName']
  if !publicDns.nil? && !privateDns.nil? then
    if privateDns =~ regex then
      puts publicDns
    end
    if publicDns =~ regex then
      puts privateDns
    end
  end
end