After Port Forwarding, how to get my external IP in Java ?

Question

I set up a static IP and did port forwarding on my notebook, and now I have a static IP address, but it's relatively static, every time I re-start the machine, I get another address, and since I have a "static" IP I can now do Paypal IPN messaging. But how can I get this static IP from my Java program ? One way I can think of is to visit : http://portforward.com/ and on that page it tells me what my external IP is, so I can extract it with Java code, is there any other way that I can do in my Java code to get this info ?

Answer 1

The best solution is probably dynamic DNS. Essentially, you run a program on your computer (or router) that notifies a DNS server when your IP changes. Then, you can just tell PayPal the domain name.

Answer 2

I see three ways of doing this:

1. As you discovered, querying an external server what IP you're apparently connecting from. This is simple but you require such a service to be available and, usually, that no transparent proxy messes with your results.
2. IGD, a sub-protocol of UPnP can give you the result very easily if your port forwarding devices supports it (many do). Google "IGD JAVA" for libraries and samples.
3. Register for a dynamic DNS service and then lookup your own DNS name.

Answer 3

You can use the NetworkInterface class:

Enumeration<NetworkInterface> ifcs = NetworkInterface.getNetworkInterfaces();
while(ifcs.hasMoreElements()){
  NetworkInterface ifc = ifcs.nextElement();
  System.out.println("Adresses of: "+ifc.getDisplayName());
  Enumeration<InetAddress> adresses = ifc.getInetAddresses();
  while(adresses.hasMoreElements()){
    System.out.println(adresses.nextElement().getHostAddress());
  }
}

This snippet will show you all of the interfaces and the IPs bound to them. You will need to look through the list to find the appropriate interface (see also NetworkInterface.getByName(String name)) And then look at the addresses for that interface. Once you have found the appropriate InetAdress you can use that to get the string or byte representation.