Not forwarding multicast (Linux kernel (config) issue?)

[T-X]

Setup:

Same as in #13.

Issue:

router2 receives PIM Join/Prune from router1 and receives ICMPv6 multicast messages from client2 but does not forward these to router1.

Details:

Ignoring the issues discovered for router0 in #13 for now, router2 should still forward to router1 as at least router1 sends correct pim-join-prune messages to router2 due to the listener on client2. However:

Packets are received on lan0 of router2 from client2 fine:

router2$ timeout 15 tcpdump -c3 -i lan0 -n -v -l icmp6
tcpdump: listening on lan0, link-type EN10MB (Ethernet), capture size 262144 bytes
18:44:28.336339 IP6 (flowlabel 0x65e89, hlim 1, next-header ICMPv6 (58) payload length: 64) fd5c:725:2841:103::2 > ff13:23:42:ffff::123: [icmp6 sum ok] ICMP6, echo request, seq 245
18:44:29.360333 IP6 (flowlabel 0x65e89, hlim 1, next-header ICMPv6 (58) payload length: 64) fd5c:725:2841:103::2 > ff13:23:42:ffff::123: [icmp6 sum ok] ICMP6, echo request, seq 246
18:44:30.388044 IP6 (flowlabel 0x65e89, hlim 1, next-header ICMPv6 (58) payload length: 64) fd5c:725:2841:103::2 > ff13:23:42:ffff::123: [icmp6 sum ok] ICMP6, echo request, seq 247
3 packets captured
3 packets received by filter
0 packets dropped by kernel

However they are not forwarded to router1 on wan0:

router2$ timeout 15 tcpdump -c3 -i wan0 -n -v -l icmp6
tcpdump: listening on wan0, link-type EN10MB (Ethernet), capture size 262144 bytes

0 packets captured
0 packets received by filter
0 packets dropped by kernel

The pim6sd internal routing table on router2 looks like this:

Multicast Routing Table
 Source          Group           RP-addr         Flags
---------------------------(*,G)----------------------------
 IN6ADDR_ANY     ff13:23:42:ffff::123 fd5c:725:2841:1::3 WC RP
Joined   oifs: j..                 
Pruned   oifs: ...                 
Leaves   oifs: ...                 
Asserted oifs: ...                 
Outgoing oifs: o..                 
Incoming     : ..I                 
Upstream nbr: NONE

TIMERS: Entry=185 JP=40 RS=0 Assert=0
  MIF   0   1   2   3   4   5   6   7   8   9
    0 185   0   0
---------------------------(S,G)----------------------------
 fd5c:725:2841:103::2 ff13:23:42:ffff::123 fd5c:725:2841:1::3 SPT CACHE SG
Joined   oifs: ...                 
Pruned   oifs: ...                 
Leaves   oifs: ...                 
Asserted oifs: ...                 
Outgoing oifs: o..                 
Incoming     : .I.                 
Upstream nbr: NONE

TIMERS: Entry=195 JP=40 RS=0 Assert=0
  MIF   0   1   2   3   4   5   6   7   8   9
    0   0   0   0

And there is an entry in "ip -6 mroute show":

router2$ ip -6 mroute show
(fd5c:725:2841:103::2,ff13:23:42:ffff::123) Iif: lan0       Oifs: wan0  State: resolved

Also, multicast forwarding seems to be correctly enabled in the kernel:

router2$ cat /proc/sys/net/ipv6/conf/wan0/mc_forwarding 
1
router2$ cat /proc/sys/net/ipv6/conf/lan0/mc_forwarding 
1
router2$ cat /proc/sys/net/ipv6/conf/all/mc_forwarding 
1

So I'm a bit confused why the ICMPv6 multicast packets are not forwarded from router2 to router1. The Linux kernel in use is a 4.19.28 kernel as provided by Debian Sid. (I'm probably missing something obvious?)

[troglobit]

Weird, everything seems to look OK, from what my tired eyes can see atm.

You've checked the TTL?

[T-X]

Urgh! That was it :D.

Sorry, had read your excellent blog posts on multicast several times and you did mention to watch the TTL... but I forgot :-).

Now the ping6 from client2 arrives on client1 via router2->router1 fine and this echo request is then replied to via unicast and arrives back on client2 again.

[troglobit]

Ha ha ha, awesome! ;-)