Ignoring your own UDP broadcasts in Java

Question

In my program I'm sending out UDP broadcasts and reacting to them. I need a way to ignore the UDP broadcasts that I send out, but react to ones that aren't from my machine.

I did try using: if (NetworkInterface.getByInetAddress(packet.getAddress()) != null) but this generated IOExceptions in some cases (java.net.SocketException: no network interface is bound to such an IP address)

Anyone any ideas?

Also: getInetAddress() on my socket throws a NullPointerException

Answer 1

Can't you just do an equals() between packet.getAddress() an an Address object for your own host?

Answer 2

This is not exactly an answer to your question. But for handling UDP broadcasts you should have a look at JGroups:

JGroups is a toolkit for reliable multicast communication. (Note that this doesn't necessarily mean IP Multicast, JGroups can also use transports such as TCP). It can be used to create groups of processes whose members can send messages to each other. The main features include

• Group creation and deletion. Group members can be spread across LANs or WANs
• Joining and leaving of groups
• Membership detection and notification about joined/left/crashed members
• Detection and removal of crashed members
• Sending and receiving of member-to-group messages (point-to-multipoint)
• Sending and receiving of member-to-member messages (point-to-point)

Answer 3

I think there's a little discrepancy between the javadoc and the actual implementation of NetworkInterface getByInetAddress(). The javadoc seems to suggest that getByInetAddress would return null if no match was found, yet the implementation either returns a match, either throws a SocketException.

JavaDoc

public static NetworkInterface getByInetAddress(InetAddress addr)
                                               throws SocketException

Returns: A NetworkInterface or null if there is no network interface with the specified IP address.

Implementation

  public static NetworkInterface getByInetAddress (InetAddress addr)
    throws SocketException
  {
    if (networkInterfaces == null)
      networkInterfaces = getRealNetworkInterfaces ();

    for (Enumeration interfaces = networkInterfaces.elements ();
         interfaces.hasMoreElements (); )
      {
        NetworkInterface tmp = (NetworkInterface) interfaces.nextElement ();

        for (Enumeration addresses = tmp.inetAddresses.elements ();
             addresses.hasMoreElements (); )
          {
            if (addr.equals ((InetAddress) addresses.nextElement ()))
              return tmp;
          }
      }

    throw new SocketException (
      "no network interface is bound to such an IP address");
  }

I suggest to either catch the exception and treat it as an answer from a 3rd party, either re-implement it using the getNetworkInterfaces() method.

Answer 4

The "industrial strength" solution to this, of course, would be to generate yourself a random UUID for each server, and to include this ID (it could be an int or maybe a long) in each packet. If that ID matches your own, you can drop it.

I'm not enamored of this solution because it wastes a number of bytes in each datagram packet. But it's simple and effective.