Create CNAME records in Route 53 that refer to the running EC2 instance. Can be run at startup or on a cron job.

set_ec2_cnames.rb

#!/bin/env ruby

# Create CNAME records in Route 53 that refer to the running EC2 instance
# Unless specified by the first argument, a default TTL of 300 is assumed.
# Instance tags specify CNAMES to be set:
#  'set_cname:0' => 'name.example.com', 'set_cname:1' => 'name2.example.com', etc

# Uses IAM credentials found in the AWS CLI config file:
#  ~/.aws/config (use `aws configure` to create)
# Depends on ec2-metadata command to retrieve the current instance ID

# Invoking via rc.local or cron--You may need to set HOME and/or PATH:
# HOME=/root PATH=/opt/aws/bin:$PATH /full/path/to/set_ec2_cnames.rb

# Author:  Erik Elmore <erik@erikelmore.com>
# License:  Public Domain

require 'aws/ec2'
require 'aws/route_53'

# Fetch credentials from the AWS CLI config file "~/.aws/config"
# You can use the AWS CLI command `aws configure` to create this file.
# This requires ENV['HOME'] to be set (which might not be true during boot)
def aws_config( opts = {} )
  opts = {
    :profile => :default,
    :keys    => [:aws_access_key_id, :aws_secret_access_key, :region],
    :file    => File.expand_path('~/.aws/config')
  }.merge opts
  
  rx_for = ->(key){ /^\[#{opts[:profile]}\]\n[^\[]+\n#{key}\s*=\s*([^\s]+)$/m }
  find   = ->(key, body){ body.match(rx_for.(key)).captures[0] }
  body = opts[:body] || File.read(opts[:file])
  
  Hash[ opts[:keys].map{ |key|
    [key.to_s.gsub('aws_', '').to_sym, find.(key, body)]
  }]
end

def instance_id
  %x{ ec2-metadata -i |cut -f2 -d ' ' }.strip
end

ttl = (ARGV[0] || 300).to_i
AWS.config(aws_config)

box = AWS::EC2.new.instances[instance_id]
cnames = box.tags.select{ |tag, *|
  tag =~ /^set_cname\b/
}.map{|*, fqdn| fqdn.end_with?('.')? fqdn : fqdn + '.' }
Process.exit 0 if cnames.empty?

r53 = AWS::Route53.new
cnames.each do |cn|
  zone = r53.hosted_zones.detect{|z| cn.end_with? z.name }
  raise 'Cannot set CNAME %s because no matching Route 53 zone was found.' % cn.chop if zone.nil?
  rs = zone.resource_record_sets[cn, 'CNAME']
  if rs.exists? && rs.ttl == ttl && box.dns_name == rs.resource_records[0][:value]
    puts "No change required for CNAME #{cn}"
    next
  end
  rs.delete if rs.exists?
  zone.resource_record_sets.create(
    cn,
    'CNAME',
    :ttl => ttl, :resource_records => [{ :value => box.dns_name }]
  )
  puts 'Created CNAME %s -> %s with TTL %d' % [cn.chop, box.dns_name, ttl]
end