Uniquely indentifying a user's machine

Question

My team and I have established a statistics service that allows our program to send data to a PHP script, which then stores the data in a MySQL table, but now we need a way to uniquely identify the user's computer so that duplicate occurrances don't appear and throw our statistics off.
We considered just building a huge string of all the System.getProperty() values, then calling hashCode() on it, but even that doesn't seem unique enough. We're completely out of ideas.
Can anyone think of a good way this could be accomplished?

Answer 1

Consider using the MAC address of the network interface. I use the following to get All the MAC addresses associated with a computer.import java.io.*;
import java.util.*;
import java.util.regex.*;
public final class MACAddress
    public final static List<String> getMacAddresses() throws IOException
        final ArrayList<String> macAddressList = new ArrayList<String>();
        final String os = System.getProperty("os.name");
        final String[] command;
        if(os.startsWith("Windows"))
            command = windowsCommand;
        else if(os.startsWith("Linux"))
            command = linuxCommand;
        else
            throw new IOException("Unknown operating system: " + os);
        final Process process = Runtime.getRuntime().exec(command);
        // Discard the stderr
        new Thread()
            public void run()
                try
                    InputStream errorStream = process.getErrorStream();
                    while (errorStream.read() != -1)
                    errorStream.close();
                catch (IOException e)
                    e.printStackTrace();
        }.start();
        // Extract the MAC addresses from stdout
        BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
        for (String line = null; (line = reader.readLine()) != null;)
            Matcher matcher = macPattern.matcher(line);
            if (matcher.matches())
                macAddressList.add(matcher.group(1).replaceAll("[-:]"," "));
        reader.close();
        return macAddressList;
    static private Pattern macPattern = Pattern.compile(".*((:?[0-9a-f]{2}[-:]){5}[0-9a-f]{2}).*", Pattern.CASE_INSENSITIVE);
    static final String[] windowsCommand = {"ipconfig","/all"};
    static final String[] linuxCommand = {"/sbin/ifconfig","-a"};
    public final static void main(String[] args)
        try
            for (String macAddress : getMacAddresses())
                System.out.println("  MAC Address: " + macAddress);
        catch(Throwable t)
            t.printStackTrace();
}