How to send multicast packets via a specfic interface in Linux

Question

Having tried all possible ways couldn\'t find a work around for this problem. I have a machine with two interfaces eth0 and eth2. I want all ff38:40:2001:dead:beef:cafe::/96

Answer 1

Multicast is inherently a local link "broadcast". As such you must always indicate the zone or network interface it is sent to. There is no routing. If you have multiple interfaces, you should send it out to multiple interfaces. The way to do it is: ping(6) ip%zone . Now on that same network might be a router that receives the packets and might forward it to another zone, iff some node on the other zone has subscribed to that address, and the TTL of the packet is > 1. There are no routing tables involved in multicast routing, except on multicast routers.

Since this original question was from 2012, around that time user kernel space was fixed to make it illegal to send a multicast packet without a zone identifier. So the ping6 in the original question would not even work.

The situation for IPv4 is dire, since it is hard to bind to a specific interface for the outgoing multicast, unless the software used was correctly implemented.