How to send multicast packets via a specfic interface in Linux

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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 packets to go on eth2. I tried all the following but when I do ping6 ff38:40:2001:dead:beef:cafe::1 the packets always goes on eth0. Things I have tried and have not worked (i.e., packet still goes out on eth0).

$> route add --inet6 ff38:40:2001:dead:beef:cafe::/96 gw 2003::100 dev eth2
$> route add --inet6 ff38:40:2001:dead:beef:cafe::/96 dev eth2
$> route add --inet6 ff38:40:2001:dead:beef:cafe::/96 metric 1 gw 2003::100 dev eth2

My routing table is

[root@dev ~]# route --inet6  |grep eth0
fe80::/64                                   *                                       U     256    0        0 eth0
ff00::/8                                    *                                       U     256    0        0 eth0

[root@dev ~]# route --inet6  |grep eth2
2003::/64                                   *                                       U     256    68       0 eth2
fe80::/64                                   *                                       U     256    0        0 eth2
ff38:40:2001:dead:beef:cafe::/96            2003::100                               UG    1      0        0 eth2
*/0                                         fe80::c671:feff:fe14:e482               UGDA  1024   0        0 eth2
ff00::/8                                    *                                       U     256    0        0 eth2

However, ping6 ff38:40:2001:dead:beef:cafe::1 -I eth2 work just fine. Moreover, I see this problem only on Linux machines (MAC is fine).

[root@dev ~]# ping6 ff38:40:2001:dead:beef:cafe::1 -I eth2
PING ff38:40:2001:dead:beef:cafe::1(ff38:40:2001:dead:beef:cafe:0:1) from cal eth2: 56 data bytes
64 bytes from 2012::1: icmp_seq=0 ttl=253 time=19.1 ms
64 bytes from 2012::1: icmp_seq=1 ttl=253 time=2.16 ms
64 bytes from 2012::1: icmp_seq=2 ttl=253 time=2.14 ms
64 bytes from 2012::1: icmp_seq=3 ttl=253 time=2.26 ms
64 bytes from 2012::1: icmp_seq=4 ttl=253 time=2.08 ms
64 bytes from 2012::1: icmp_seq=5 ttl=253 time=2.15 ms

root@dev ~]# uname -a
Linux 2.6.18-194.el5 #1 SMP Tue Mar 16 21:52:39 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux

Perhaps the problem has to do with the fact that there is a ff00::/8 for eth0. How do I overrule that route. I am not able to delete ff00::/8 route as well.

ipv6 multicast routing linux
2 Answers

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.

user_62089810
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