Glue to keep specific entries up to date in /etc/hosts

hosts_updater.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

'''
Superglue to keep /etc/hosts up-to-date of given entries.
If the entry doesn't exists, we won't add it. We only keep
the EXISTING entries updated.

 * depends on python-dnspython

'''

import dns.resolver
import sys

# set the hosts you want to keep updated in the hosts file
hosts = [ 'www.google.com', 'www.gmail.com' ]
# set the dns resolver(s) you want to use
set_dns = [ '8.8.8.8' ]

##########################################################

resolver = dns.resolver.Resolver()
resolver.nameservers = set_dns
resolver.timeout = 5
local_hosts = '/etc/hosts'
results = {}

for host in hosts:
    try:
        answers = resolver.query(host, 'A')
        ips = []
        for data in answers:
            ips.append(data.address)
        # Sort the addresses as we only take the first ONE, even if there's
        # multiple given by the resolver. Sorting's there to recude the amount
        # of unnecessary editing due RR.
        results[host] = sorted(ips)

    except dns.resolver.NXDOMAIN:
        sys.stderr.write('Domain not found: %s\n' % host)
        sys.exit(1)
    except dns.resolver.Timeout:
        sys.stderr.write('Timeout while trying to resolv: %s\n' % host)
        sys.exit(1)
    except dns.exception.DNSException:
        sys.stderr.write('Unhandled exception\n')
        sys.exit(1)

# read in the hosts file to hosts_file list, strip \n and split on whitespace
hosts_file = [line.strip().split() for line in open(local_hosts, 'r')]
change_detected = None

for entry in hosts_file:
    try:
        for host in results:
            if entry[1] == host and entry[0] != results[host][0]:
                print '%s address changed from %s to %s' % (host, entry[0],
                        results[host][0] )
                entry[0] = results[host][0]
                change_detected = True
    except IndexError:
        pass

if change_detected:
    # Write the hosts file back if we've spotted a change
    with open(local_hosts,'w') as filu:
        filu.write('\n'.join(' '.join(row) for row in hosts_file))