This document discusses dynamically assigning hostnames to instances in an AWS auto-scaled environment. It outlines challenges in setting unique hostnames and tags for spawned instances. The author proposes an algorithm to sequentially number new instances based on the number of existing instances. A Python script would be called from CloudFormation templates to extract the next number in the sequence, set it as the hostname in a file and tag the instance. The script uses AWS APIs to get instances in the auto scaling group and set name tags. Questions from the audience are invited at the end.

1. Dynamically Assign Hostname in AWS Autoscaled Instance
By Kamal Maiti
Sr. Subject Matter Expert
Linux System Engineer
Amdocs Development Center, India
Dated : 22nd Feb, 2016
Agenda :
 Challenges to set hostname or tag of spawned instance
 How we can set hostname, any idea? What we need to consider?
 Set temporary hostname, Make permanent hostname
 Calling python script from CloudFormation
 Understanding python script
 Q/A

2. Dynamically Assign Hostname
 Challenges to set hostname or tag to spawned instance
 Hostname should be unique
 Required for puppet master, DNS and application mapping.
 AWS doesn’t have any ready made way to set dynamic
hostname
 New machine doesn’t understand which will be its name
 How will be sequence number of the new machine

3. Dynamically Assign Hostname
How we can set hostname, any idea? What we need to consider?
Algorithm :
1. If there is no instance in auto scaled group, pass a initial sequence number to
hostname.
2. If any instance is present, extract sequence number of it and add one to it.
3. Next set this new sequence to the spawned instance.
4. Pass the value to tag as well
5. To avoid race condition among instances, allow them to randomly sleep for few
seconds.

4. Dynamically Assign Hostname
1. Setup a temporary hostname for first instance/or instances which will be
initially built through CF.
2. Parse hostname and look for sequence number in temporary name. If doesn’t
exist then consider it’ll be first sequence for permanent hostname
Set Initial temporary hostname

5. Dynamically Assign Hostname
Calling python script from CF
Parameters
Script is called here
Saved hostname
Hostname setup

6. Dynamically Assign Hostname
less /root/scripts/aws-autoscale-ec2-instance-modify.py
Needed Modules
Function to extract sequence/server
number
Processing arguments

7. Dynamically Assign Hostname
Get all running Instances:
autoscaling connection object
Ec2 connection object
Extract the correct AS group
Retrieve all instance of AG

8. Dynamically Assign Hostname
Set Sequence Number
Take a list for saving all name tags
If list is blank, assign default value
Function is called
Next sequence number

9. Dynamically Assign Hostname
Set hostname in a file and TAG Instance Values those will be
used for hostname
Hostname is built
Setup Name tag to instance

10. Q/A?