When binding to "0.0.0.0", how to find the actual interface IP address that my Java ServerSocket was bound to?

Question

So I'm bounding my Java ServerSocket to 0.0.0.0. Imagine my machine has 3 network interfaces, each one with its own IP address. Now I want to programmatically discover the IP address that my clients can use to connect to my recently created ServerSocket. Calling:

serverSocket.getLocalSocketAddress()

or

serverSocket.getInetAddress()

Returns "0.0.0.0", which is of course what I don't want.

Any ideas?

Answer 1

You can't get that information from the ServerSocket. You have to enumerate the actual interfaces separately, using NetworkInterface.getNetworkInterfaces() and NetworkInterface.getInetAddresses().

Per Listing Network Interface Addresses in the Java documentation:

One of the most useful pieces of information you can get from a network interface is the list of IP addresses that are assigned to it. You can obtain this information from a NetworkInterface instance by using one of two methods. The first method, getInetAddresses(), returns an Enumeration of InetAddress. The other method, getInterfaceAddresses(), returns a list of java.net.InterfaceAddress instances. This method is used when you need more information about an interface address beyond its IP address. For example, you might need additional information about the subnet mask and broadcast address when the address is an IPv4 address, and a network prefix length in the case of an IPv6 address.

The following example program lists all the network interfaces and their addresses on a machine:

import java.io.*;
import java.net.*;
import java.util.*;
import static java.lang.System.out;

public class ListNets {

    public static void main(String args[]) throws SocketException {
        Enumeration<NetworkInterface> nets = NetworkInterface.getNetworkInterfaces();
        for (NetworkInterface netint : Collections.list(nets))
            displayInterfaceInformation(netint);
    }

    static void displayInterfaceInformation(NetworkInterface netint) throws SocketException {
        out.printf("Display name: %s\n", netint.getDisplayName());
        out.printf("Name: %s\n", netint.getName());
        Enumeration<InetAddress> inetAddresses = netint.getInetAddresses();
        for (InetAddress inetAddress : Collections.list(inetAddresses)) {
            out.printf("InetAddress: %s\n", inetAddress);
        }
        out.printf("\n");
     }
}  

The following is sample output from the example program:

Display name: TCP Loopback interface
Name: lo
InetAddress: /127.0.0.1

Display name: Wireless Network Connection
Name: eth0
InetAddress: /192.0.2.0