The builtin class MulticastSocket [1] should satisfy your needs. It's
an extension of DatagramSocket [2] that adds several features, two of
which you'll need to accomplish your goal.
Your question states that you're trying to use the broadcast address
"255.255.255.255", so we'll create a socket that's ready to receive
and send packets on the "multicast group" 255.255.255.255.
// basic multicast datagram socket.
MulticastSocket socket = new MulticastSocket();
Next, we use joinGroup [3] to join the multicast group on a given
interface; this limits the broadcasts received to a specific
interface, as desired.
// DatagramPacket.send() also takes this network address.
InetAddress broadcast = InetAddress.getByName("255.255.255.255");
// get the interface object for interface "1.2.3.4".
NetworkInterfaceAddr ifaddr = InetAddress.getByName("1.2.3.4");
NetworkInterface if = NetworkInterface.getByInetAddress(ifaddr);
// bind the socket's receiving side to "255.255.255.255", on
interface "1.2.3.4".
socket.joinGroup(broadcast, if);
If you like, you can choose the outgoing interface for given broadcast
packets as well.
// bind the socket's sending side to interface "1.2.3.4".
socket.setNetworkInterface(if);
Further information on the classes used above can be found at Sun's
J2SE 1.4 reference [4]; I suggest browsing the methods provided in
classes NetworkInterface [5] and MulticastSocket [1].
[1] http://java.sun.com/j2se/1.4/docs/api/java/net/MulticastSocket.html
[2] http://java.sun.com/j2se/1.4/docs/api/java/net/DatagramInterface.html
[3] http://java.sun.com/j2se/1.4/docs/api/java/net/MulticastSocket.html#joinGroup(java.net.SocketAddress,%20java.net.NetworkInterface)
[4] http://java.sun.com/j2se/1.4/docs/api/overview-summary.html
[5] http://java.sun.com/j2se/1.4/docs/api/java/net/NetworkInterface.html
Much of the information provided in this answer was found through the
following Google search [6] and result URL [7], once I realized that
MulticastSocket could be used to send and receive on the broadcast
address.
[6] ://www.google.com/search?q=multicastsocket+%22255.255.255.255%22
[7] http://www.cee.hw.ac.uk/~mjc/teaching/3ne3/5/13+14.htm |
Request for Answer Clarification by
bigjosh-ga
on
26 Dec 2002 19:43 PST
Unfortunately that does not work. The MulticastSocket stuff only works
with multicast addresses, not broadcast ones, or the problem would be
easy.
If you try the following program...
NetworkInterface netif = NetworkInterface.getByName( "eth0" );
InetAddress INADDR_ANY = InetAddress.getByName("255.255.255.255");
MulticastSocket sock = new MulticastSocket();
sock.joinGroup( new InetSocketAddress(INADDR_ANY, 20560), netif );
byte[] data = new byte[ 1500 ];
DatagramPacket pack = new DatagramPacket( data, data.length );
sock.receive( pack );
You'll get...
Exception in thread "main" java.net.SocketException: Not a multicast
address
at java.net.MulticastSocket.joinGroup(MulticastSocket.java:350)
at NetIf.main(NetIf.java:35)
Becuase in the java.net.MulticastSocket.java source it says...
if (!((InetSocketAddress)mcastaddr).getAddress().isMulticastAddress())
{
throw new SocketException("Not a multicast address");
}
Oh well...
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Request for Answer Clarification by
bigjosh-ga
on
26 Dec 2002 19:46 PST
The SO_BINDTODEVICE stuff if interesting and we've been trying to put
together a kernal that will support it. I guess if no Java-possible
solution surfaces, I'll have no choice but to use this stuff with a
magic kernal and JNI. Thanks for the info.
|
Clarification of Answer by
coral-ga
on
26 Dec 2002 21:03 PST
Hm, I looked in the source code and found that [1]; thanks for the
error message. If you overload isMulticastAddress to accept
"255.255.255.255", does it work? Some example code here, in case it
helps.
class allowMulticastBroadcasts extends InetAddress {
public boolean isMulticastAddress () { return (address ==
0xffffffff || ((address & 0xf0000000) == 0xe0000000)); }
}
[1] http://216.239.39.100/search?q=cache:V9PBMhQTx2cC:guma.ii.fmph.uniba.sk/jdk/java/net/InetAddress.java.html+%22ismulticastaddress%22+%22255.255.255.255%22&hl=en&ie=UTF-8#:107
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Request for Answer Clarification by
bigjosh-ga
on
27 Dec 2002 10:31 PST
That just makes things worse. In the end, multicast and broadcast
listeners are completely different and there is no way to recieve a
broadcast UDP packet on a multicast socket.
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Clarification of Answer by
coral-ga
on
27 Dec 2002 12:54 PST
Alright, time for a different tack; my apologies for the diversion
down the multicast route.
The constructor for DatagramSocket notes that you have the option to
bind to both a port and a local address on a given machine. If you
bind your DatagramSocket to the IP of a local interface, and then
.setBroadcast(true), does the socket properly receive and send
broadcast packets only on the given local address?
[1] http://java.sun.com/j2se/1.4/docs/api/java/net/DatagramSocket.html#constructor_summary
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