EC2 DNS load-balancing init.d script. Instances automatically register themselves in Route53 RecordSets and properly update their records when starting/shutting down. Instances need to use attached IAM role allowing them to modify the Route53 zone.

ec2hostname.rb

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

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

`touch /var/lock/subsys/ec2hostname`

HOSTNAME = '<hostname>'
DOMAIN = '<your domain name>'
ZONE = '<your hosted zone id>'
TYPE = 'A'
TTL = 60

metadata_endpoint = 'http://169.254.169.254/latest/meta-data/'
ip_local = Net::HTTP.get( URI.parse( metadata_endpoint + 'local-ipv4' ) )
ip_public = Net::HTTP.get( URI.parse( metadata_endpoint + 'public-ipv4' ) )

records = []
records << { :target => ip_local, :alias => [ HOSTNAME, DOMAIN, '' ] * '.' }
records << { :target => ip_public, :alias => [ HOSTNAME + '-public', DOMAIN, '' ] * '.' }

changes = []

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

records.each{ |record|
  rrset = rrsets[
    record[:alias],
    TYPE
  ]

  existing_records = []

  if rrset.exists?
    existing_records = rrset.resource_records.select{ |r| r[:value] != record[:target] }
    changes << AWS::Route53::DeleteRequest.new(record[:alias], TYPE, :ttl => TTL, :resource_records => rrset.resource_records)
  end

  existing_records << { :value => record[:target] } if ARGV[0] != 'stop'
  changes << AWS::Route53::CreateRequest.new(record[:alias], TYPE, :ttl => TTL, :resource_records => existing_records) unless existing_records.empty?
}

r53 = AWS::Route53.new
response = r53.client.change_resource_record_sets({
  hosted_zone_id: ZONE,
  change_batch: { changes: changes }
})

`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>" ]
	]
}

Can I define the below 3 in Ec2 Tags and fetch it in program?

HOSTNAME = '<hostname>'
DOMAIN = '<your domain name>'
ZONE = '<your hosted zone id>'