Update AWS instance hosts file with all internal dns names for VPC members. Useful when dns discovery is not useable/available.

update_hosts_file.py

#!/usr/bin/env python

import boto3
import requests

ec2 = boto3.resource('ec2')

# fetch the instance id
response = requests.get('http://169.254.169.254/latest/meta-data/instance-id')
instance_id = response.text

# fetch the vpc_id
current_instance = ec2.Instance(instance_id)
vpc_id = current_instance.vpc_id

# loop through all instances, and get their details
hosts = []
for i in ec2.instances.all():
    if i.vpc_id != vpc_id:
        continue
    # append both the full hostname, as well as the short hostname
    hosts.append("%s %s\n" % (i.private_ip_address, i.private_dns_name))
    hosts.append("%s %s\n" % (i.private_ip_address, i.private_dns_name.split('.')[0]))

# open the current hosts file, read the first 3 lines, and append the new hosts
with open('/etc/hosts', 'r') as f:
    lines = f.readlines()
    start = lines[0:3]

    new_lines = start + hosts
    new_contents = "".join(new_lines)

    with open('/etc/hosts', 'w') as f:
        print new_contents
        f.write(new_contents)