1 Introduction

When we automate an EC2 instance provision with CloudFormation, we can assign a name tag to the EC2 instance. Naturally, we would like to have the OS hostname match the name tag, but surprisingly, this is not the default AWS behavior and need some efforts to automate it.

2 The challenges

2.1 Hostname is derived from dynamic IP address

When we provision a new Amazon Linux EC2 instance on AWS, the default hostname is derived from the IP address that is dynamically assigned to the instance at startup.

For example, your hostname might look like this

ip-12-34-56-78

Very likely, this name is not desirable and need to be changed, but how?

Amazon has instruction on how to make the change manually, but it involve a reboot, something is not nice if we want to use script to automate this task.

2.2 Using on-premises AD service

On the other hand, changing the hostname is part of the story, we also need to update the DNS record.

If you are using Active Directory and extend the on-premises AD service to the AWS like the following picture, you would like to know how to automatic the DNS update too.

3 2 The solution

3.1 Enable DNS dynamic update

We need to enable dynamic update on the DNS service provided by the AD server.

The following enable dynamic updates using the Windows interface

Open DNS Manager.
In the console tree, right-click the applicable zone, and then click Properties.
On the General tab, verify that the zone type is either Primary or Active Directory-integrated.
In Dynamic Updates, click Nonsecure and secure.

In my example script, it only works with the “Nonsecure and secure” setting only. In case you need more security, you could set up Secure Dynamic Update, check out the details on this Microsoft document and modify the script accordingly.

3.2 Automate all the updates with a python script

I had written a script in Python to automate the task to update the hostname and DNS record based on the name tag assigned to the EC2 instance.

In details, this script perform the following:

Get the instance name from EC2 metadata, which is the EC2 name tag
Get the instance IP address from EC2 metadata
Update the /etc/sysconfig/network file with the name FQDN name
Update the /etc/hosts with the IP and FQDN
Set the hostname
Update the DNS record on AD

Following is the python script, you can include it in your bootstrap step script during the EC2 provision or in your Chef/Puppet/Ansible configuration steps.

#!/usr/bin/python

import boto3

import requests

import json

import re

import os

import subprocess

import StringIO

## Change the domain name to your need

DOMAIN='cloud.operative.com'

## replace content in a file

def replace(file, pattern, subst):

# Read contents from file as a single string

file_handle = open(file, 'r')

file_string = file_handle.read()

file_handle.close()

# Use RE package to allow for replacement (also allowing for (multiline) REGEX)

file_string = (re.sub(pattern, subst, file_string))

# Write contents to file.

# Using mode 'w' truncates the file.

file_handle = open(file, 'w')

file_handle.write(file_string)

file_handle.close()

## get the name tag of the ec2 instance

def get_instance_name():

## get the ec2 instance id and region

r = requests.get('http://169.254.169.254/latest/dynamic/instance-identity/document/')

r_arr=json.loads(r.text)

region = r_arr['region']

instance_id = r_arr['instanceId']

## get EC2 information

ec2 = boto3.resource('ec2', region_name=region)

ec2instance = ec2.Instance(instance_id)

instancename = ''

for tags in ec2instance.tags:

if tags["Key"] == 'Name':

instancename = tags["Value"]

return instancename

def get_ip_addr():

r = requests.get("http://169.254.169.254/latest/meta-data/local-ipv4")

return r.text

def grep(file,pattern):

for line in open(file).readlines():

if re.match(pattern, line):

return line

return ""

def append(file,data):

f = open(file, 'a')

f.write(data)

f.close()

def update_dns(hostname):

cmd='nslookup ' + hostname

args = cmd.split()

process = subprocess.Popen(args, shell=False, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

stdout, stderr = process.communicate()

exit_code = process.wait()

print "return code = " , exit_code

if exit_code == 0:

print "DNS record already existed"

cmd = """

else:

## update dns record

print "Update DNS record"

cmd = """

nsupdate << EOF

zone $ZONE

update delete $fqdn

update add $fqdn 86400 A $ip_addr

show

send

EOF

"""

os.system(cmd)

##### Main Program #####

instance_name = get_instance_name()

ip_addr = get_ip_addr()

fqdn = instance_name + '.' + DOMAIN

host_line = 'HOSTNAME=' + instance_name + '.' + DOMAIN

## Update /etc/sysconfig/network file

print "update /etc/sysconfig/network with " + host_line

replace("/etc/sysconfig/network", 'HOSTNAME=.*', host_line )

## Update /etc/hosts file

ret = grep("/etc/hosts", ip_addr)

ip_w_fqdn = ip_addr + " " + fqdn + " " + instance_name

if ret == "":

print "add fqdn info to /etc/hosts"

append("/etc/hosts", ip_w_fqdn + "\n")

else:

print "Replace line in /etc/hosts with '" + ip_w_fqdn + "'"

replace("/etc/hosts", ip_addr + '.*' , ip_w_fqdn)

## Run hostname command

cmd='hostname ' + instance_name

os.system(cmd)

## Update DNS record

update_dns(fqdn)

3.3 Additional tip

If your shop only allow one DNS server for dynamic update and it is not the one defined in /etc/resolv.conf, you can modify the script and add the server option for nsupdate like the following

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