CODEWITHSUNDEEP
javanetwork-programmingmulticastdiscovery
Virkom
Virkom

Reputation: 403

How to discover multicast addresses range in Java?

Maybe this question is strange. I don't know. For example, getting multicast packets in Java:

byte[] buf = new byte[1500];
MulticastSocket socket = new MulticastSocket(1234);
InetAddress group = InetAddress.getByName("233.0.0.1");
socket.joinGroup(group);

while (true) {
   DatagramPacket packet = new DatagramPacket(buf, buf.length);
   socket.receive(packet);
   // doing something with packet
}
socket.leaveGroup(group);
socket.close();

But it works well if I know multicast group address. And what if I don't know? I need to scan the network and find all available multicast streams by port. I don't know address but I know port. Is it possible?

Suppose I have 3 streams with addresses 233.0.0.1:1234, 233.0.0.2:1234, 233.0.0.3:1234. I need to discover network by port 1234 and find this addresses. How can I do it without going through all the multicast addresses in the loop and joining every group?

Upvotes: 1

Views: 1285

Answers (1)

Stephen C
Stephen C

Reputation: 718788

How to discover multicast addresses in Java?

AFAIK, there is no way to do this from Java (currently).

The problem is that multicast routing is essentially receiver driven. It goes something like this:

  • When an application initiates a multicast group, it uses one of the PIM protocols to figure out how route its outbound packets. This causes a distribution tree to be created.
  • When an application joins a multicast group, the local host sends an IGMP request onto the network. This flows through the switches in the local routing domain.
  • If one of the switches recognizes the group, the local host is added to the distribution tree for the group.
  • If the IGMP requests the gateway to the internet without being recognized (and the multicast IP isn't private), the gateway will then use MBGP to try and find the group.
  • If the BGMP request finds the group, the gateway joins the distribution tree.

As you can see, this works without any centralized authority. Indeed, if nothing in your local routing domain is currently using a given multicast group, there may be no trace of it on the network. It is possible to snoop the IGMP traffic ... but that only tells you about groups that are in use locally, not all available groups.

For more information, the Wikipedia article on IP Multicast is a good place to start.

Normally, most of the multicast routing and associate protocols are implemented in your network switches. However, there is linux-based software that supports multicasts routing, etcetera; see http://troglobit.com/howto/multicast/. You may be able to use this to do your on IGMP snooping ... though this has limitations; see above.

Your code doesn't appear to be doing multicast address discovery at all. Rather, it is joining a specific group, and receiving multicast traffic on a the IP address.

AFAIK, you cannot monitor to "all multicast traffic" from standard Java. Java doesn't support promiscuous mode. However, it may be possible to do it using the jpcap library ... with the caveat that you will only see traffic that lands on your local network segment.

Upvotes: 2

Related Questions