Network discovery in Java using multicasting

Question

I'm trying to make a client/server Java App. Both client and server will be running on the same wi-fi network. Server will be running on a specific port that client is aware of.

I am planning to send a multicast message from client through the network for that specific port to discover the server. However, I'm not too sure how I can find out which IP in my network received my message.

Do I need to create a socket on the client and listen to incoming packets once I send my multicast message in case server replies back?

Thanks in advance.

Answer 1

You could try using SSDP. It's what UPnP devices use to discover each other. It's multicast on port 1900 and just uses really simple packets to send around IPs and service information.

Cling is a UPnP lib you can pull from. Note I'm not recommending you move to UPnP - just the discovery protocol used.

Answer 2

You can try using java.net.MulticastSocket (available since Java 1.1). If you don't need the rich feature sets of libs like jgroups, hazelcast etc. that plain Java API might serve you well enough.

See also example pages here and here.

Answer 3

(1)server listens on a pre-arranged port

DatagramSocket s = new DatagramSocket(8888);
s.receive  //(1)
s.send     //(2)

(3)client sends a message to the port, on the broadcast IP, 255.255.255.255

DatagramSocket c = new DatagramSocket();
c.send(255.255.255.255:8888,msg)     //(3)
c.receive  //(4)

the client binds to a port too. we didn't specify it, so it's random chosen for us.

(3) will broadcast the message to all local machines, server at (1) receives message, with the client IP:port.

(2) server sends response message to client IP:port

(4) client gets the reponse message from server.

Answer 4

I would strongly recommend using JGroups. It has a lot of features and it will do all the UDP stuff. JBoss uses it for their clustering.