ec2masq.py

#!/usr/bin/python

import boto3
import socket

# You have to supply this value or come up with a clever way to figure it out at runtime.
# It's the IP of your local VPC's DNS server - usually your VPC network address "+2".
LOCAL_DNS="10.0.0.2"

ec2 = boto3.resource('ec2')
config_template = '''# Auto generated by ec2masq.py
except-interface=lo
listen-address=%(localip)s
bind-interfaces
no-resolv
log-queries

# White-listed domains we have to allow to make certain things like EMR work
# We'll just forward these queries to the locally-enabled VPC DNS server
# You can add more stuff here if you have another DNS server in your infrastructure you trust
server=/s3.amazonaws.com/%(localdns)s
server=/kms.amazonaws.com/%(localdns)s
server=/kms.us-east-1.amazonaws.com/%(localdns)s
server=/repo.us-east-1.amazonaws.com/%(localdns)s

# And here are all the programatically added entries
'''

local_ip = socket.gethostbyname(socket.gethostname())
config = config_template % {"localip": local_ip, "localdns": LOCAL_DNS})

# Get all the running instances in your account
instances = ec2.instances.filter(Filters=[{'Name': 'instance-state-name', 'Values': ['running']}])

# Make some records and add to the config
for instance in instances:
    reverse_ip = '.'.join(instance.private_ip_address.split('.')[::-1])
    record = 'address=/%s/%s\n' % (instance.private_dns_name, instance.private_ip_address)
    record += 'ptr-record=%s.in-addr.arpa,%s' % (reverse_ip, instance.private_dns_name)
    config += '%s\n' % record

# Printing is still cool, right?
print config