[Nottingham] Easy linux network multicast?

[Charles Samuels]

Cam wrote, on Tuesday 2006 May 16 2:40 pm:
> Martin
>
> > So how can you easily multicast a few GBytes of binary data to multiple
> > machines on a LAN?
>
> I can't think of an easy way that's a true multicast. You could look at
> videolan for streaming but your few Gb might have to be an MPEG file.
>
> You could possibly multicast out a file using netcat -u, and receive it
> with netcat -l, but you'd have to
>
> 1. start and stop manually
> 2. verify integrity afterwards using rsync or similar

For true multicast, there's a few things you need to know about:
- you need to set up a specific route on the "multicast network"
- it needs to be UDP (duh!) 
- you need to account for dropped/out of order packets
- you send to the multicast address, not the recipients' address or a 
broadcast address
- it may be easier to broadcast and not multicast
- you need to "subscribe" as such:

	if (IN6_IS_ADDR_MULTICAST(addr6->sin6_addr.s6_addr))
	{ // ipv6
		struct ipv6_mreq mreq;
		mreq.ipv6mr_interface = 0;
		std::memcpy(&mreq.ipv6mr_multiaddr, &addr6->sin6_addr, sizeof(in6_addr));

		setsockopt(mFd, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
	}

	if(IN_MULTICAST(ntohl(addr4->sin_addr.s_addr)))
	{ // ipv4
		struct ip_mreq mreq;
		mreq.imr_multiaddr.s_addr = addr4->sin_addr.s_addr;
		mreq.imr_interface.s_addr = htonl(INADDR_ANY);

		setsockopt(mFd,IPPROTO_IP,IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
	}

(the client you are using needs to support multicast directly)

Unless you're serious about performance, go with TCP.
Unless you have a reason to use multicast, use broadcast
You'll probably need to write code to use multicast or broadcast unless you 
can find something on the 'net.

good luck!