Call Home Script

A year or two ago when I first set up my old laptop as a linux server I created a script that logged into my router, parsed the external ip address, checked that against the ip address it got last time, and emailed me if they were different. That worked great for the intended purpose (telling me when the address of my server changed), but I was thinking recently that I could rewrite it to tell me whenever the external ip address of the computer changes, such as if someone stole one of my linux computers and booted it far enough for my cron job to be able to fire. That depends on the computer being connected to the internet when the script runs, but updating the script was interesting enough to be worth doing even if there’s very little chance of it being overly useful.

I’m building off of code I wrote a while ago, so the parts that work won’t have much commentary on what it took to write them.

Calling Home

The first (and most important part) is the script to notify you when it decides it needs to. I chose email because I expected it to be pretty easy (it was) and because always having an email in my inbox that contained the ip address of my server seemed useful.

A quick search on “python send email” turned up a docs.python.org page with the basics, so that’s might have been where I started. I wanted a general script that I could call from other places, but I didn’t want to include the login info for the server every time so I ended up with this script.

The username, password, and email address are obviously not actually all ‘*’s, but I didn’t particularly want to post my gmail info. The smtpuser that my version of the script uses is everything before the @ in my gmail address, and the smtppass has to be a valid password for your account, though interestingly it was willing to accept my last password (my password changed after I wrote the script and it still worked). Giving it an invalid password raised an exception at session.login(smtpuser, smtppass) with a link to a gmail support page.

The documentation example I linked above used another library for setting up the body of the message, but joining the header lines I have with line breaks works too. The "", # extra line break comment is a required blank line for it to parse correctly as an email.

Checking your IP

Once your script can notify you, it has to be able to tell when it needs to. My original script logged onto my router (a linksys), opened the status page, and parsed the external ip. That obviously depends on your computer being at home where it’s able to log into the router. A more general solution is to check a website for the ip address it sees you connecting from.

Since I have web hosting and wasn’t sure that sites like whatismyip would like being called regularly from a script I looked up the php to display the external ip of the requesting computer. I don’t really want my site to be hit be any random person looking for a machine-friendly ip address site, so it’s in a passworded section of my site, but here’s the html of my getip.php file:

<html><body>
<?php echo $_SERVER['REMOTE_ADDR']; ?>
</body></html>

Note: All block code formatting and syntax highlighting is done through code2html.

Since I’m fetching the ip address from an html page behind a password I had to find a way to have python log in. I ended up using urllib2:

uri = "http://192.168.1.1/"
username = "********"
password = "********"
page = "http://192.168.1.1/Status_Router.asp"

def get_ip() :
	pass_mgr = urllib2.HTTPPasswordMgrWithDefaultRealm()
	pass_mgr.add_password(None, uri, username, password)

	auth_handler = urllib2.HTTPBasicAuthHandler(pass_mgr)
	opener = urllib2.build_opener(auth_handler)
	urllib2.install_opener(opener)
	
	handle = urllib2.urlopen(page)
	site_text = handle.read()

	match = re.search("var wan_ip = \"([\d\.]+)\";", site_text)
	#print match.group(1)

	my_ip = match.group(1)

	return (time.asctime(), my_ip)

The uri and regex in that snippet are for my linksys router. For the php code earlier in this post I’m matching on "([\d\.]+)". The return statement gives the time the ip address was taken and the ip string. I keep a log file with that data pair so I can see when the ip address changed, but that’s just python file io, so I need the time at some point and making a tuple here was easiest.

The entire script (including the error handling I trimmed out of the snippet) is posted here.

Scheduling

No matter how fancy your script is, it’s useless if it doesn’t actually get run. Since I’m running it on linux cron seemed like the easiest way to go. Once the script was there to tell me if my ip address changed I didn’t want to run it too often, but I did want it to run regularly. I settled for running it once an hour, and I think I’ll stick with that:

# to deactivate (all cron jobs): crontab -r
# to activate : crontab ./cronip
MAILTO=*username*@localhost
0 * * * * /home/*username*/scripts/ipcheck.py

The *username* should be the login of the user who the script is to be run as. If I remember correctly any errors or other output from the script will be mailed to the specified local user. I think there is a way to mail to an external user, but it required more setup than just typing an email address.

For the path to the script I wrote out the full path, but a relative path from your home directory might also work. The script will be run from the home directory, so any file input and output will be done from there unless an absolute path is specified.

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