aws_iam_policy.json

{
  "Statement": [
    {
      "Action": [
        "route53:ChangeResourceRecordSets",
        "route53:GetHostedZone",
        "route53:ListResourceRecordSets"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:route53:::hostedzone/<your hosted zone id>"
      ]
    }
  ]
}

aws_update_dns.rb

#!/usr/bin/env ruby
require 'aws-sdk'
require 'net/http'

AWS.config({
:access_key_id => '<iam user key>',
:secret_access_key => '<iam user secret>'
})

hostname = ARGV[0].to_s
domain = '<your domain name>'
zone = '<your hosted zone id>'
ttl = 60

metadata_endpoint = 'http://169.254.169.254/latest/meta-data/'
hostname_local = Net::HTTP.get( URI.parse( metadata_endpoint + 'local-hostname' ) )
hostname_public = Net::HTTP.get( URI.parse( metadata_endpoint + 'public-hostname' ) )

records = [{
  :alias => [ hostname, domain, '' ] * '.',
  :target => hostname_local
},{
  :alias => [ hostname + '-public', domain, '' ] * '.',
  :target => hostname_public
}]

#update DNS records
rrsets = AWS::Route53::HostedZone.new(zone).rrsets
records.each{ |record|
  rrset = rrsets[
    record[ :alias ],
    'CNAME'
  ]

  if rrset.exists?
    rrset.delete
  end

  rrset = rrsets.create(
    record[ :alias ],
    'CNAME',
    :ttl => ttl,
    :resource_records => [
      { :value => record[ :target ] }
    ]
  )
}