sync_session.py

"""
Python 3 classes for synchronising the actual IP address of the current host
with its domain name A record.

Idea of using resolver1.opendns.com and myip.opendns.com taken from Krinkle:
https://unix.stackexchange.com/questions/22615
Code for using dnspython adapted from koblas:
https://stackoverflow.com/questions/5235569

License Notice
--------------
Copyright 2018 Amanda Wee

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at:
http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
"""

import socket

import dns.resolver


class SyncError(Exception):
    """
    Exception to be raised on failure to synchronise the actual IP address of
    the current host with its domain name A record.
    """
    def __init__(self, message):
        self.message = message

    def __str__(self):
        return 'Error: ' + self.message


class SyncSession:
    """
    Models a synchronisation session to synchronise the actual IP address of
    the current host ("my IP" address) with its domain name A record.
    """
    DNS_RESOLVER_SERVER = 'resolver1.opendns.com'
    SELF_IP_HOST_NAME = 'myip.opendns.com'

    def __init__(self, domain_record_api):
        if not domain_record_api:
            raise SyncError('domain record API not specified')
        self._domain_record_api = domain_record_api

    def find_my_ip(self):
        """
        Returns the actual IP address of the current host, or None if the
        resolver does not provide such an IP address.
        """
        resolver = dns.resolver.Resolver()
        resolver.nameservers = [socket.gethostbyname(self.DNS_RESOLVER_SERVER)]
        results = resolver.query(self.SELF_IP_HOST_NAME, 'A')
        return results[0].address if results else None

    def sync_my_ip_with_dns(self, host_name):
        """
        Synchronises the actual IP address of the current host with its domain
        name A record. Returns None.
        """
        actual_ip = self.find_my_ip()
        if actual_ip is None:
            raise SyncError('could not determine actual IP address')

        domain_record = self._domain_record_api.find(host_name)
        if domain_record is None:
            self._domain_record_api.create(host_name, actual_ip)
        elif domain_record['ip'] != actual_ip:
            self._domain_record_api.update(domain_record, actual_ip)