当服务器或客户端绑定到本地 IPV4 地址时，DatagramChannel 不接收多播数据包

Question

This is the server

 public static void main(String[] args)throws Exception
 {
  try(DatagramChannel server=DatagramChannel.open())
  {
   server.bind(new InetSocketAddress("192.168.1.5",5000));
   
   //Receive from any address
   ByteBuffer buffer=ByteBuffer.allocate(10);  
   InetSocketAddress addr=(InetSocketAddress)server.receive(buffer); 
   buffer.flip();
   
   //print data
   byte[] data=new byte[buffer.remaining()];
   buffer.get(data);
   System.out.println(addr+"/"+new String(data));
   
   //Send back to client address 
   server.send(ByteBuffer.wrap("World".getBytes()),addr);  
  } 
 }

And this is the client

public static void main(String[] args)throws Exception
 {
  //open with IPV4 Protocol
  try(DatagramChannel client=DatagramChannel.open(StandardProtocolFamily.INET))
  {
   //Send to any address
   client.send(ByteBuffer.wrap("Hello".getBytes()),new InetSocketAddress("192.168.1.5",5000));
   
   //Receive from any address
   ByteBuffer buffer=ByteBuffer.allocate(10);  
   InetSocketAddress addr=(InetSocketAddress)client.receive(buffer); 
   buffer.flip();
   
   //print data
   byte[] data=new byte[buffer.remaining()];
   buffer.get(data);
   System.out.println(addr+"/"+new String(data));
  } 
 }

I run the server first then the client as 2 seperate processes in 2 diffrent JVM's

The Address "192.168.1.5" is my PC IPV4 address which i obtained on windows 10 by

1. opening cmd as administrator

2. typing ipconfig

3. copy the IPV4 address of my adapter

Everything works fine

Server Output : /192.168.1.5:64640/Hello
Client Output : /192.168.1.5:5000/World

So i assume the address 192.168.1.5 is working

Now i want to perform the same test for multicast sockets

Here is the server

public static void main(String[] args)throws Exception
 {
  try(DatagramChannel server=DatagramChannel.open(StandardProtocolFamily.INET))
  {
   //Not mandatory using only for debugging
   server.bind(new InetSocketAddress(5000));
   
   //set it to broadcast on specific adapter
   server.setOption(StandardSocketOptions.IP_MULTICAST_IF,supportMulticast());
   
   //send to an specific multi cast address
   server.send(ByteBuffer.wrap("OK".getBytes()),new InetSocketAddress("224.0.0.1",7000));
  }
 }

Here is the code to get the 1st supported multicast interface

private static NetworkInterface supportMulticast()throws Exception
 {
  Enumeration<NetworkInterface> netInterfaces=NetworkInterface.getNetworkInterfaces();
  while(netInterfaces.hasMoreElements())
  {
   NetworkInterface netInterface=netInterfaces.nextElement();

   if(!netInterface.isLoopback() || !netInterface.isUp()){continue;}
  
   if(netInterface.supportsMulticast()){return netInterface;}   
  }
  return null;
 }

And here is the client

public static void main(String[] args)throws Exception
 {
  try(DatagramChannel client=DatagramChannel.open(StandardProtocolFamily.INET))
  {
    //allow multiple clients to reuse the below bounded address to allow multiple members on that same address
   client.setOption(StandardSocketOptions.SO_REUSEADDR,true);
    
   //must be bound to an local address and match the port of the sender send() address
   client.bind(new InetSocketAddress(7000));
   
   //not source specific i.e does not block any specific addresses
   MembershipKey key=client.join(InetAddress.getByName("224.0.0.1"),supportMulticast());
  
   //read the data
   ByteBuffer buffer=ByteBuffer.allocate(10);
   System.out.println("RECEIVED FROM="+client.receive(buffer));
   buffer.flip();
   System.out.println("MSG="+new String(buffer.array(),0,buffer.limit()));
   
   //leave the group
   key.drop();
  }
 } 

I Run the client first then the server as 2 separate processes[separate JVM's] and i get the expected output on the client side

RECEIVED FROM=/127.0.0.1:5000
MSG=OK

So far so good until i change the bind method on my server side from this

server.bind(new InetSocketAddress(5000));

To this

server.bind(new InetSocketAddress("192.168.1.5",5000));

Then the client does not receive any packets and hangs forever

If i don't alter the server code but instead make the client bind to the address 192.168.1.5 then again the client gets no packets and hangs forever.

Wire shark confirms that the packets get sent

The local IPV4 address 192.168.1.5 works for regular DatagramChannels but does not work Multicast groups. Any idea whats happening?

Answer 1

So the answer was in the adapter my server sends packets in and the adapter which the client uses to join the group

In this method

 private static NetworkInterface supportMulticast()throws Exception
 {
  Enumeration<NetworkInterface> netInterfaces=NetworkInterface.getNetworkInterfaces();
  while(netInterfaces.hasMoreElements())
  {
   NetworkInterface netInterface=netInterfaces.nextElement();

   if(!netInterface.isLoopback() || !netInterface.isUp()){continue;}
  
   if(netInterface.supportsMulticast()){return netInterface;}   
  }
  return null;
 }

This line

if(!netInterface.isLoopback() || !netInterface.isUp()){continue;}

will only allow an Loopback[ie an localhost] adapter which supports multicast to be returned. The Localhost adapter can only send/receive packets within the localhost and not outside it. What i need is the adapter which does the exact opposite so i changed it to this

//skip loop back adapter & adapters which are not working
if(netInterface.isLoopback() || !netInterface.isUp()){continue;}

This returns me the adapter with the interface address 192.168.1.5 and everything works perfectly now

Also after a bit more research to allow an packet to reach outside an certain scope and further distances we need to set the TTL for the multicast packets sent

/*
    0->Node Local
    1->Link Local
    1->32=Site Local
    32->64=Region Local
    64->255=Global
    For sending packets outside localhost
 */
 server.setOption(StandardSocketOptions.IP_MULTICAST_TTL,0); 

0 was enough for my case