EC2 Instance Route53 Hostname registration init.d script. Instance needs to have the attached IAM instance role policy applied.

ec2hostname.rb

#!/usr/bin/ruby
# chkconfig: 35 99 01
# description: EC2 DNS registration
# processname: ec2hostname

require 'aws-sdk'
require 'net/http'

`touch /var/lock/subsys/ec2hostname`

hostname = '<hostname>'
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
}]

rrsets = AWS::Route53::HostedZone.new(zone).rrsets

case ARGV[0]
when 'start', 'restart'
records.each{ |record|
  rrset = rrsets[
    record[ :alias ],
    'CNAME'
  ]

  rrset.delete if rrset.exists?
 
  rrset = rrsets.create(
    record[ :alias ],
    'CNAME',
    :ttl => ttl,
    :resource_records => [
      { :value => record[ :target ] }
    ]
  )
}
when 'stop'
records.each{ |record|
  rrset = rrsets[
    record[ :alias ],
    'CNAME'
  ]

  rrset.delete if rrset.exists?
}
end

`rm -f /var/lock/subsys/ec2hostname` if ARGV[0] == 'stop'

iam-instance-role.json

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