dynamic ip addresses in ec2

http://ec2dream.blogspot.com/search/label/Networking%20Multi-Tier%20Applications

#!/usr/bin/ruby

require 'rubygems'
require 'right_aws'

AMAZON_PUBLIC_KEY=<public key>
AMAZON_PRIVATE_KEY=<private key>
#
# update hosts file from S3 copy
#
print "Getting /etc/hosts file from S3\n"
s3= RightAws::S3Interface.new(AMAZON_PUBLIC_KEY, AMAZON_PRIVATE_KEY)
print "Getting /etc/hosts file from S3"
hosts = File.new('/etc/hosts', 'wb')
rhdr = s3.get('', 'hosts') do |chunk|
  hosts.write(chunk)
end
hosts.close
#
# match ip address with hosts file and write a local_hostname file to the
# /root folder
#
print "Setting /root/local_hostname file\n"
begin
  File.delete('/root/local_hostname')
  rescue Errno::ENOENT
    print ""
end
ip_address = %x(hostname -i)
print "ip address " + ip_address
ip_address[ip_address.length-1] = ""
hostFile = "/etc/hosts"
File.open(hostFile).each_line{ |s|
  a = s.split(' ')
  if a[0] == ip_address
    print "hostname is " + a[1] + "\n"
    File.open('/root/local_hostname.sh', 'w') {|f| f.write("export LOCAL_HOSTNAME="+a[1]) }
    system('chmod 700 /root/local_hostname.sh')
  end
}