So it is nice to be able to remotely connect to your home computer. Many (included my) ISPs, give dynamic IP addresses, and although these don’t change often, they do change and are a pain to remember anyway. My solution is to have my computer e-mail me whenever it discovers it has a new IP address! You can easily configure exim4 to send emails from a linux machine through gmail, a great guide is here.

Once you have that you just need a little script, mine does this with python by asking http://icanhazip.com. and writing a temporary file with the old ip in /var/tmp. It only sends you a message when the IP changes, no need to spam your email box!

#!/usr/bin/python

''' A super simple script to email changes in a dynamic IP to
an email address. If you drop this in /etc/cron.* , it will periodically
poll a website to check if your iP has changed, and if it has it well
send you an e-mail with the new one!

Alex Knaust - 2/28/2013
'''

import os, smtplib, urllib2, re
from email.mime.text import MIMEText

# file to save the last IP address in
IPLOC = '/var/tmp/ip'

# site from which to get IP, if you change this you might need to alter get_live_ip
IPSITE = 'http://icanhazip.com'

# Message to send yourself with the IP
MAILSTR = '''Hi, My new IP address is {0}\n'''

# Address from which the email is being sent (this should probably match your SMTP setup
FROMADDR = 'you@host.com'

# Address to which to send the message
TOADDR = 'you@host.com'


#####################################################
############### CODE ################################
#####################################################
def save_ip(ipstr):
    '''Save the Ip to IPLOC, return true if successful, false otherwise'''
    try:
        with open(IPLOC, 'w') as f:
            f.write(ipstr)
    except:
        return False
    return True
    
def get_old_ip():
    '''Gets the old IP by reading IPLOC, returns None if it could not be read'''
    try:
        with open(IPLOC, 'r') as f:
            ip = f.read()
        return ip.strip()
    except IOError:
        return None
    
def get_live_ip():
    '''Return the IP address of this computer by polling IPSITE
    
    returns None if not successful
    '''
    try:
        f = urllib2.urlopen(IPSITE)
        return f.read().strip()
    except urllib2.URLError:
        return None

def send_mail(ipstr):
    '''Tries to send an email about the change of IP to the TOADDR
    
    returns true if successful, false otherwise
    '''
    msg = MIMEText(MAILSTR.format(ipstr))
    msg['Subject'] = 'IP Address change'
    msg['From'] = FROMADDR
    msg['To'] = TOADDR
    
    try:
        s = smtplib.SMTP('localhost')
        s.sendmail(FROMADDR, [TOADDR, ], msg.as_string())
        s.quit()
    except:
        return False
    return True

def main():
    oldip = get_old_ip()
    newip = get_live_ip()
    
    if newip is not None:
        if oldip is not None:
            if newip != oldip:
                send_mail(newip)
                save_ip(newip)
        else:
            send_mail(newip)
            save_ip(newip)

if __name__ == '__main__':
    main()