How can I determine the IP of my router/gateway in Java?

Question

How can I determine the IP of my router/gateway in Java? I can get my IP easily enough. I can get my internet IP using a service on a website. But how can I determine my gat

Answer 1

You can use netstat -rn command which is available on Windows, OSX, Linux, etc platform. Here is my code:

private String getDefaultAddress() {
        String defaultAddress = "";
        try {
            Process result = Runtime.getRuntime().exec("netstat -rn");

            BufferedReader output = new BufferedReader(new InputStreamReader(
                    result.getInputStream()));

            String line = output.readLine();
            while (line != null) {
                if (line.contains("0.0.0.0")) {

                    StringTokenizer stringTokenizer = new StringTokenizer(line);
                    stringTokenizer.nextElement(); // first element is 0.0.0.0
                    stringTokenizer.nextElement(); // second element is 0.0.0.0
                    defaultAddress = (String) stringTokenizer.nextElement();
                    break;
                }

                line = output.readLine();

            } // while
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return defaultAddress;

} // getDefaultAddress

Answer 2

On windows parsing the output of IPConfig will get you the default gateway, without waiting for a trace.

Answer 3

Try shelling out to traceroute if you have it.

'traceroute -m 1 www.amazon.com' will emit something like this:

traceroute to www.amazon.com (72.21.203.1), 1 hops max, 40 byte packets
 1  10.0.1.1 (10.0.1.1)  0.694 ms  0.445 ms  0.398 ms

Parse the second line. Yes, it's ugly, but it'll get you going until someone posts something nicer.

Answer 4

I'm not sure if it works on every system but at least here I found this:

import java.net.InetAddress;
import java.net.UnknownHostException;
public class Main
{
    public static void main(String[] args)
    {
        try
        {
            //Variables to find out the Default Gateway IP(s)
            String canonicalHostName = InetAddress.getLocalHost().getCanonicalHostName();
            String hostName = InetAddress.getLocalHost().getHostName();

            //"subtract" the hostName from the canonicalHostName, +1 due to the "." in there
            String defaultGatewayLeftover = canonicalHostName.substring(hostName.length() + 1);

            //Info printouts
            System.out.println("Info:\nCanonical Host Name: " + canonicalHostName + "\nHost Name: " + hostName + "\nDefault Gateway Leftover: " + defaultGatewayLeftover + "\n");
            System.out.println("Default Gateway Addresses:\n" + printAddresses(InetAddress.getAllByName(defaultGatewayLeftover)));
        } catch (UnknownHostException e)
        {
            e.printStackTrace();
        }
    }
    //simple combined string out of the address array
    private static String printAddresses(InetAddress[] allByName)
    {
        if (allByName.length == 0)
        {
            return "";
        } else
        {
            String str = "";
            int i = 0;
            while (i < allByName.length - 1)
            {
                str += allByName[i] + "\n";
                i++;
            }
            return str + allByName[i];
        }
    }
}

For me this produces:

Info:
Canonical Host Name: PCK4D-PC.speedport.ip
Host Name: PCK4D-PC
Default Gateway Leftover: speedport.ip

Default Gateway Addresses:
speedport.ip/192.168.2.1
speedport.ip/fe80:0:0:0:0:0:0:1%12

I'd require more tests on other Systems/Configurations/PC-Gateway-Setups to confirm if it works everywhere. Kind of doubt it but this was the first I found.

Answer 5

Matthew: Yes, that is what I meant by "I can get my internet IP using a service on a website." Sorry about being glib.

Brian/Nick: Traceroute would be fine except for the fact that lots of these routers have ICMP disabled and thus it always stalls.

I think a combination of traceroute and uPnP will work out. That is what I was planning on doing, I as just hoping I was missing something obvious.

Thank you everyone for your comments, so it sounds like I'm not missing anything obvious. I have begun implementing some bits of uPnP in order to discover the gateway.

Answer 6

In windows you can just use the following command:

ipconfig | findstr /i "Gateway"

Which will give you output like:

Default Gateway . . . . . . . . . : 192.168.2.1
Default Gateway . . . . . . . . . : ::

However I can't run this command with Java, gonna post when I figure this out.