ipmonitor.py

#!/usr/bin/env python3
"""
IP Monitor

Sends an e-mail if the network's external IP address has changed since last run.

SMTP credentials are read from the user's ~/.netrc file. It expects an entry
like this:

    machine smtp.example.com
    login johndoe@example.com
    password supersecret

where `machine` matches the `smtp_url` below and `login` is typically your
e-mail address with or without the domain depending on your SMTP server.

Typically, this program is run as a cron job by a crontab entry like this:

    0 */12 * * * ~/bin/ipmonitor.py

(Every day, at 0:00 and 12:00 in this example)

"""

# Configuration
smtp_url = 'mail.antoneliasson.se'
sender_address = 'relay@antoneliasson.se' # E-mail 'From' field
recipient_address = 'root@antoneliasson.se' # E-mail 'To' field

import urllib.request
import re
import os.path

from email.mime.text import MIMEText
from email.utils import formatdate

from netrc import netrc
from smtplib import SMTP

from socket import gethostname

def get_ip():
    with urllib.request.urlopen('http://dns.loopia.se/checkip/checkip.php') as f:
        html = f.read().decode('utf-8')
        ip = re.findall('[0-9]+.[0-9]+.[0-9]+.[0-9]+', html)[0]
    
    return ip

def compare(ip):
    home = os.path.expanduser("~")
    try:
        file = open(home + '/.ipmonitor', 'r')
        return file.read() == ip
    except IOError as e:
        return False

def update(ip):
    home = os.path.expanduser("~")
    try:
        file = open(home + '/.ipmonitor', 'w')
        file.write(ip)
        return True
    except IOError as e:
        return False

def compose(ip, wrote, sender, recipient):
    msg = MIMEText('')
    if not wrote:
        msg.set_payload('Unable to create config file!')
    msg['Subject'] = 'IP address change on {}: {}'.format(gethostname(), ip)
    msg['From'] = sender
    msg['To'] = recipient
    # RFC 2822, section 3.3: "The date and time-of-day SHOULD express local time."
    # Anton: "Use UTC and the world will be a better place."
    msg['Date'] = formatdate(localtime = False)
    
    return msg

def send(msg, server, port):
    username, _, password = netrc().authenticators(server)
    
    smtp = SMTP(server, port)
    smtp.starttls()
    smtp.login(username, password)
    smtp.send_message(msg) # Python >= 3.2
    # smtp.sendmail(msg['From'], msg['To'], str(msg)) # Python >= 3.1
    smtp.quit()

def main():
    ip = get_ip()
    if not compare(ip):
        print('New IP: {}'.format(ip))
        wrote = update(ip)
        if wrote:
            print('Wrote config')
        else:
            print('Failed to write config')
        msg = compose(ip, wrote, sender_address, recipient_address)
        send(msg, smtp_url, 587)
        print('Sent e-mail')

if __name__ == '__main__':
    main()