Route Table Assigner for AWS Nat instances

route_table_modifier.py

#!/bin/env python

# This script should run at boot (set as user data) on NAT instances
# - Disables the source/dest check for the current instance
# - Modifies default route for the specified route table to the instance running this 


import argparse
import boto
import boto.utils
import os
import sys

dry_run = False

def set_up():
    parser = argparse.ArgumentParser()
    parser.add_argument("route_table_id", help="VPC route table id you want to modify")
    parser.parse_args()

    # Authentication via iam role
    vpc_conn = boto.connect_vpc()
    ec2_conn = boto.connect_ec2()

    try:
        instance_id = boto.utils.get_instance_metadata()['instance-id']
        instance_region = boto.utils.get_instance_metadata()['placement']['availability-zone'][:-1]
    except:
        fail("Could not get EC2 instance metadata!")

def disable_src_dest_check():
    source_dest_check = ec2_conn.get_instance_attribute(instance_id, 'sourceDestCheck')['sourceDestCheck']

    print "Source/Dest check: %s" % source_dest_check

    if source_dest_check:
        print "Instance must have source/dest checking disabled to NAT properly!"
        try:
            ec2_conn.modify_instance_attribute(instance_id, 'sourceDestCheck', False, dry_run=dry_run)
        except Exception, e:
            print "Could not modify source/dest check: %s" % e
            sys.exit(1)

def config_route_table():
    print "Modifying route table {0}".format(route_table_id)

    try:
        vpc_conn.replace_route(route_table_id, '0.0.0.0/0', instance_id=instance_id, dry_run=dry_run)
    except Exception, e:
        fail("Issue setting route table [{0}]: {1}".format(route_table_id, e))

    print "Gateway set!"

def fail(error_msg="An unexpected error occurred"):
    print error_msg
    sys.exit(1)


set_up()
disable_src_dest_check()
config_route_table()