Detecting IP address changes

My brother recently asked me about writing a program that would detect when his IP address changes and email it to him. I thought "Oh, that should be easy to write".

I had it about half-way written out, when I realized a problem: it's easy to detect when the local computer's IP address changes, but with home routers these days, the IP address is going to be something like 192.168.0.2, and never change. What you really want is to detect when your router's IP address changes. That's harder than it sounds, though - the whole purpose of NAT is to effectively mask the fact that you aren't actually directly connected to the real internet.

I thought about it for a while but couldn't think of a solution that didn't involve contacting a remote server and having it tell you your IP address. It looks like dynamic DNS providers like DynDNS also have the same problem but no better solutions. It might be possible to connect to the router itself and get the IP from its status page, but that would be pretty router-specific.

In the end, this is the one-line C# program I came up with which will email you your internet-facing IP address:

new SmtpClient("smtp_server_name").Send(new MailMessage( "from@address.com", "to@address.com", "New IP (subject)", "Your new IP address is: " + new Regex( @"\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b" ).Match(new WebClient( ).DownloadString( "http://checkip.dyndns.org/")).Value));

Just compile that and set it as a scheduled task to run every 24 hours and you should be good to go . Of course, if you only want it to mail you when the IP address actually changes, or if your SMTP server requires authentication, then it might require a couple more lines of code...