Selecting an Interface when Multicasting on Linux

Question

I'm working with a cluster of about 40 nodes running Debian 4. Each node runs a daemon which sits and listens on a multicast IP.

I wrote some client software to send out a multicast over the LAN with a client computer on the same switch as the cluster, so that each node in the cluster would receive the packet and respond.

It works great, except when I run the client software on a computer that has both LAN and WAN interfaces. If there is a WAN interface, the multicast doesn't work. So obviously, I figure the multicast is incorrectly going over the WAN interface (eth0), rather than the LAN (eth1.) So, I use the SO_BINDTODEVICE socket option to force the multicast socket to use eth1, and all is well.

But I thought that the kernel's routing table should determine that the LAN (eth1) is obviously a lower cost destination for the multicast. Is there some reason I have to explicitly force the socket to use eth1? And, is there some way (perhaps an ioctl call) that I can have the application automatically determine if a particular interface is a LAN or WAN?

Answer 1

If you don't explicitly bind to an interface, I believe Linux uses the interface for the default unicast route for multicast sending. So my guess is that your default route is via the WAN interface.

Richard Stevens' "Unix Network Programming, Vol. 1", chapter 17 (at least in the 3rd edition), has some good information and examples of how to enumerate the network interfaces.

Answer 2

If you don't explicitly bind to an interface, I believe Linux uses the interface for the default unicast route for multicast sending.

Linux needs a multicast route, if none exists you will get a EHOSTUNREACH or ENETUNREACH error. The LCM project documents this possible problem. The routing will be overridden if you use the socket option IP_MULTICAST_IF or IPV6_MULTICAST_IF. You are supposed be able to specify the interface via scope-id field in IPv6 addresses but not all platforms properly support it. As dameiss points out, Stevens' Unix Network Programming book covers these details, you can browse most of the chapter on multicast via Google Books for free.