Set hostname from EC2 Tags (Name, Domain)

ec2_hostname.rb

#!/usr/bin/env ruby

require 'net/http'
require 'open-uri'
require 'uri'
require 'base64'
require 'openssl'
require 'cgi'

def query_aws(host, path, action, params = {}, method = "GET")
	params["SignatureVersion"] = 2
	params["SignatureMethod"] = "HmacSHA1"
	params["Action"] = action
	params["Version"] = "2012-06-15"
	params["Expires"] = (Time.now + 60).iso8601

	sec_token = params.delete "SecurityToken"

	sign_string = "#{method}\n"
	sign_string << "#{host}\n"
	sign_string << "#{path}\n"
	sign_string << params.sort.collect { |key, value| [CGI.escape(key.to_s), CGI.escape(value.to_s)].join('=') }.join('&')

	key = sec_token
	hmac = Base64.encode64(OpenSSL::HMAC.digest(OpenSSL::Digest::Digest.new('sha1'), sec_token, sign_string)).strip

	params['Signature'] = hmac

	base_url = "http://#{host}#{path}?"
	querystring = params.collect { |key, value| [CGI.escape(key.to_s), CGI.escape(value.to_s)].join('=') }.join('&')

	url = "#{base_url}#{querystring}"
	resp = open(url).read
	
	resp 
end

# IAM identity for tag-query user
AWS_ACCESS_KEY = ""
AWS_SECRET = ""

# fetch the system's current instance_id
instance_id = open("http://169.254.169.254/latest/meta-data/instance-id").read

# fetch the system's region
az = open("http://169.254.169.254/latest/meta-data/placement/availability-zone").read

ipv4 = open("http://169.254.169.254/latest/meta-data/local-ipv4").read

region = az.gsub(/[A-Za-z]+$/,"")

# query the AWS API tags
resp = query_aws "ec2.#{region}.amazonaws.com", "/", "DescribeTags", {"AWSAccessKeyId" => AWS_ACCESS_KEY, "SecurityToken" => AWS_SECRET, 
	"Filter.1.Name" => "resource-type", "Filter.1.Value.1" => "instance", "Filter.2.Name" => "resource-id", "Filter.2.Value.1" => instance_id}

hostname = resp.match(/<key>Name<\/key>\s+<value>(.*)<\/value>/)[1] rescue nil
domain = resp.match(/<key>Domain<\/key>\s+<value>(.*)<\/value>/)[1] rescue nil

# run system hostname config
File.open("/etc/hostname", "w") {|file| file.write(hostname)}
`hostname -F /etc/hostname`

# set FQDN for further queries

host_file = "127.0.0.1 localhost

# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

# Added by EC2 hostname resolver
#{ipv4}	#{hostname}.#{domain} #{hostname}"

File.open("/etc/hosts", "w") { |file| file.write(host_file)}