Update a Route 53 record with your current external IP address

ddns.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from __future__ import print_function

import re
import sys
from argparse import ArgumentParser
from datetime import datetime
from subprocess import CalledProcessError
from subprocess import check_output

from botocore.session import get_session


IP_PATTERN = r'^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$'

RECORD_TYPE = 'A'


def log(msg):
    print('{}\t{}'.format(datetime.now().isoformat(), msg))


def get_external_ip():
    try:
        ip = check_output(['dig', '+short', 'myip.opendns.com', '@resolver1.opendns.com'])
        if re.match(IP_PATTERN, ip):
            return ip.strip()
        else:
            log('IP "{}" does not match pattern'.format(ip))
    except CalledProcessError as e:
        log(e.output)
    return None


def get_registered_ip(client, hosted_zone_id, record_name):
    res = client.list_resource_record_sets(
        HostedZoneId=hosted_zone_id,
        StartRecordName=record_name,
        StartRecordType=RECORD_TYPE,
        MaxItems='1',
    )
    resource_record_sets = res['ResourceRecordSets']
    if resource_record_sets:
        record_set = resource_record_sets[0]
        if record_set['Name'] == record_name and record_set['Type'] == RECORD_TYPE:
            resource_records = record_set['ResourceRecords']
            if resource_records:
                return resource_records[0]['Value']
    return None


def register_ip(client, hosted_zone_id, record_name, ip, ttl):
    res = client.change_resource_record_sets(
        HostedZoneId=hosted_zone_id,
        ChangeBatch={
            'Comment': datetime.now().isoformat(),
            'Changes': [
                {
                    'Action': 'UPSERT',
                    'ResourceRecordSet': {
                        'Name': record_name,
                        'ResourceRecords': [{'Value': ip}],
                        'TTL': ttl,
                        'Type': RECORD_TYPE,
                    },
                },
            ],
        },
    )
    return res['ResponseMetadata']['HTTPStatusCode'] == 200


if __name__ == '__main__':
    parser = ArgumentParser(description='Update Route 53 A record')
    parser.add_argument('hosted_zone_id', type=str)
    parser.add_argument('record_name', type=str)
    parser.add_argument('-t', '--ttl', default=3600, type=int)
    args = parser.parse_args()

    external_ip = get_external_ip()
    if external_ip is None:
        log('Unable to determine external IP address')
        sys.exit(1)

    route53 = get_session().create_client('route53')

    registered_ip = get_registered_ip(route53, args.hosted_zone_id, args.record_name)

    if registered_ip != external_ip:
        if register_ip(route53, args.hosted_zone_id, args.record_name, external_ip, args.ttl):
            log('Updated {} with address {}'.format(args.record_name, external_ip))
        else:
            log('Failed to update {}'.format(args.record_name))
            sys.exit(1)
    else:
        log('IP address unchanged for {}'.format(args.record_name))