How would I get only IPv4 addresses

Question

I have the following code which is supposed to get only the IPv4 addresses of all active interfaces, but it still returns an IPv6 address on some computers.

public static List<List> getIpAddress() {
    List<String> ip = new ArrayList<>();
    List<List> ipRefined = new ArrayList<>();
    try {
        Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
        while (interfaces.hasMoreElements()) {
            NetworkInterface iface = interfaces.nextElement();
            if (iface.isLoopback() || !iface.isUp())
                continue;
            Enumeration<InetAddress> addresses = iface.getInetAddresses();
            while(addresses.hasMoreElements()) {
                ip.add(addresses.nextElement().getHostAddress());
            }
        }
    } catch (SocketException e) {
        throw new RuntimeException(e);
    }
    for(int x = 0; x < ip.size(); x++){
        if(ip.get(x).contains("%")){
            try {
                if (ip.get(x + 1).contains(".")) {
                    List<String> tempList = new ArrayList<>();
                    tempList.add(ip.get(x).substring(ip.get(x).indexOf("%") + 1));
                    tempList.add(ip.get(x + 1));
                    ipRefined.add(tempList);
                }
            } catch (IndexOutOfBoundsException ae) {
            }
        }
    }
    return ipRefined;
}

I've tried to specify using only IPv4 by using System.setProperty("java.net.preferIPv4Stack" , "true");, but this only causes getIpAddress() to return an empty list. How should I be getting the IPv4 of active interfaces without the use of string manipulation?

EDIT:

Using System.setProperty("java.net.preferIPv4Stack" , "true"); always causes getIpAddress() to return an empty list.

Padnag

I'm not sure about this, but as you're reading through the addresses enumeration, what about checking if each address instanceof Inet4Address? – Albanese

I could try that, but I think the issue is that the interface is only returning its IPv6. Let me do some debugging real quick. – Padnag

It looks like I'm already filtering it so that only IPv4 can be returned when I do if (ip.get(x + 1).contains(".")). I rewrote some of my code to use instanceof instead of using contains(), but it won't change anything. I'm going to update the question with one more detail. – Padnag

Only IPv6 addresses contain '%' because IPv4 doesn't have scopeid. – Jefferson

Answer 1

From InterfaceAddress

This class represents a Network Interface address. In short it's an IP address, a subnet mask and a broadcast address when the address is an IPv4 one. An IP address and a network prefix length in the case of IPv6 address.

Here's my code:

Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
while (interfaces.hasMoreElements()) {
  NetworkInterface networkInterface = interfaces.nextElement();
  System.out.println(String.format("networkInterface: %s", networkInterface.toString()));

  if (!networkInterface.isUp()) {
    continue;
  }

  for (InterfaceAddress interfaceAddress : networkInterface.getInterfaceAddresses()) {
    int npf = interfaceAddress.getNetworkPrefixLength();
    InetAddress address = interfaceAddress.getAddress();
    InetAddress broadcast = interfaceAddress.getBroadcast();
    if (broadcast == null && npf != 8) {
      System.out.println(String.format("IPv6: %s; Network Prefix Length: %s", address, npf));
    } else {
      System.out.println(String.format("IPv4: %s; Subnet Mask: %s; Broadcast: %s", address, npf, broadcast));
    }
  }
}

Dunigan

A combination of this and substrings solved my question. – Padnag

Answer 2

My Kotlin solution:

  private suspend fun getAllActiveInterfaces(): Sequence<NetworkInterface> {
        return withContext(Dispatchers.IO) {
            NetworkInterface
                .getNetworkInterfaces()
                .asSequence()
                .filter { !it.isLoopback && it.isUp && !blackListedInterfaces.contains(it.name) }
        }
    }

    suspend fun getAllAddresses(): Result<Collection<InetAddress>> {
        return withContextCatching(Dispatchers.IO) {
            getAllActiveInterfaces()
                .map { networkInterface ->
                    networkInterface
                        .also { Timber.d("Interface: ${it.name}") }
                        .interfaceAddresses
                        .mapNotNull { it.address }
                        .filterIsInstance<Inet4Address>()
                }
                .flatten()
                .toSet()
        }
    }

private val blackListedInterfaces = setOf("dummy0")

Looney

Nice! The "timber" line threw me for a second. Some comments would be great. Much cleaner that what I had 5 years ago! :P – Padnag