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

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 gateway's IP?

This is somewhat easy in .NET if you know your way around. But how do you do it in Java?

On Windows, OSX, Linux, etc then Chris Bunch's answer can be much improved by using

netstat -rn

in place of a traceroute command.

Your gateway's IP address will appear in the second field of the line that starts either default or 0.0.0.0.

This gets around a number of problems with trying to use traceroute:

1. on Windows traceroute is actually tracert.exe, so there's no need for O/S dependencies in the code
2. it's a quick command to run - it gets information from the O/S, not from the network
3. traceroute is sometimes blocked by the network

The only downside is that it will be necessary to keep reading lines from the netstat output until the right line is found, since there'll be more than one line of output.

EDIT: The Default Gateway's IP Address is in the second field of the line that starts with 'default' if you are on a MAC (tested on Lion), or in the third field of the line that starts with '0.0.0.0' (tested on Windows 7)

Windows:

Network Destination Netmask Gateway Interface Metric

0.0.0.0 0.0.0.0 192.168.2.254 192.168.2.46 10

Mac:

Destination Gateway Flags Refs Use Netif Expire

default 192.168.2.254 UGSc 104 4 en1

Java doesn't make this as pleasant as other languages, unfortunately. Here's what I did:

import java.io.*;
import java.util.*;

public class ExecTest {
    public static void main(String[] args) throws IOException {
        Process result = Runtime.getRuntime().exec("traceroute -m 1 www.amazon.com");

        BufferedReader output = new BufferedReader(new InputStreamReader(result.getInputStream()));
        String thisLine = output.readLine();
        StringTokenizer st = new StringTokenizer(thisLine);
        st.nextToken();
        String gateway = st.nextToken();
        System.out.printf("The gateway is %s\n", gateway);
    }
}

This presumes that the gateway is the second token and not the third. If it is, you need to add an extra st.nextToken(); to advance the tokenizer one more spot.

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

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

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

        String line = output.readLine();
        while(line != null){
            if ( line.trim().startsWith("default") == true || line.trim().startsWith("0.0.0.0") == true )
                break;      
            line = output.readLine();
        }
        if(line==null) //gateway not found;
            return;

        StringTokenizer st = new StringTokenizer( line );
        st.nextToken();
        st.nextToken();
        gateway = st.nextToken();
        System.out.println("gateway is: "+gateway);


    } catch( Exception e ) { 
        System.out.println( e.toString() );
        gateway = new String();
        adapter = new String();
    }

You may be better off using something like checkmyip.org, which will determine your public IP address - not necessarily your first hop router: at Uni I have a "real" IP address, whereas at home it is my local router's public IP address.

You can parse the page that returns, or find another site that allows you to just get the IP address back as the only string.

(I'm meaning load this URL in Java/whatever, and then get the info you need).

This should be totally platform independent.