Bridge not forwarding multicast traffic to the tap interface

Terry T terryhwt at gmail.com
Fri May 13 18:06:51 CEST 2016 

I have a Debian 8 64-bit machine set up as a server and apt-got the tinc
package. I configured tinc as a bridge and everything seems normal except
that the tunnel does not forward multicast traffic.

I used tcpdump to examine the br0, eth0 and tap interfaces. I could see
multicast packets on both br0 and eth0, but there is no such packet present
on the tap interface. I don't quite know why that is.

br0       Link encap:Ethernet  HWaddr 7a:22:d7:1b:04:2e
          inet addr:10.10.189.254  Bcast:10.10.189.255  Mask:255.255.255.0
          inet6 addr: fe80::d227:88ff:fee5:2078/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3569 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2773 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:1125028 (1.0 MiB)  TX bytes:429677 (419.6 KiB)

eth0      Link encap:Ethernet  HWaddr d0:27:88:e5:20:78
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:56890 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2869 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:26184692 (24.9 MiB)  TX bytes:444021 (433.6 KiB)

tap0      Link encap:Ethernet  HWaddr ae:a2:e1:cd:aa:68
          inet6 addr: fe80::aca2:e1ff:fecd:aa68/64 Scope:Link
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2336 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:500
          RX bytes:0 (0.0 B)  TX bytes:1015186 (991.3 KiB)

bridge name    bridge id        STP enabled    interfaces
br0        8000.7a22d71b042e    no        eth0
                                                               tap0

Samples of the tcpdump on the bridge.

root at lionrock:/home/terry# tcpdump -i br0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on br0, link-type EN10MB (Ethernet), capture size 262144 bytes
00:03:04.185117 IP 10.10.189.254.30022 > 116.19.81.143.53880: Flags [P.],
seq 3950669668:3950669856, ack 1682193163, win 317, options [nop,nop,TS val
425064 ecr 4674355], length 188
00:03:04.242176 IP 116.19.81.143.53880 > 10.10.189.254.30022: Flags [.],
ack 188, win 1066, options [nop,nop,TS val 4674385 ecr 425064], length 0
00:03:04.269393 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 211
00:03:04.292112 IP 10.93.112.115.47978 > 224.10.86.1.2013: UDP, length 11
00:03:04.385814 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011
00:03:04.406215 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011
00:03:04.420350 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011
00:03:04.422856 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011
00:03:04.424661 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011

root at lionrock:/home/terry# tcpdump -i eth0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
00:03:48.657143 IP 10.10.189.254.30022 > 116.19.81.143.53880: Flags [P.],
seq 3950676648:3950676836, ack 1682193487, win 317, options [nop,nop,TS val
436182 ecr 4685479], length 188
00:03:48.715103 IP 116.19.81.143.53880 > 10.10.189.254.30022: Flags [.],
ack 188, win 1313, options [nop,nop,TS val 4685503 ecr 436182], length 0
00:03:48.754522 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011
00:03:48.769103 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011
00:03:48.872717 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 361
00:03:48.941631 IP 10.68.150.37.54718 > 224.10.86.7.2014: UDP, length 11
00:03:48.993608 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 461
00:03:49.012100 IP 10.93.112.115.45907 > 224.10.86.3.2014: UDP, length 11
00:03:49.105880 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 161
00:03:49.115551 IP 10.68.150.38.59286 > 224.10.86.7.2003: UDP, length 11
00:03:49.155296 IP 10.93.112.51.57244 > 224.10.86.1.2001: UDP, length 1011

root at lionrock:/home/terry# tcpdump -i tap0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on tap0, link-type EN10MB (Ethernet), capture size 262144 bytes
00:04:41.417898 IP 10.10.189.1 > all-systems.mcast.net: igmp query v2
00:04:41.494373 IP 10.10.189.254.ntp > 10.10.189.255.ntp: NTPv4, Broadcast,
length 48
00:04:44.529568 IP 10.10.189.1 > pim-routers.mcast.net: PIMv2, Hello,
length 38
00:04:47.132901 IP 10.10.189.101 > 239.255.255.250: igmp v2 report
239.255.255.250
00:04:57.828645 IP 10.10.189.1 > pim-routers.mcast.net: PIMv2, DF Election,
length 20
00:04:57.828689 IP 10.10.189.1 > pim-routers.mcast.net: PIMv2, DF Election,
length 20
00:04:57.828847 IP 10.10.189.1 > pim-routers.mcast.net: PIMv2, DF Election,
length 20
00:04:57.828906 IP 10.10.189.1 > pim-routers.mcast.net: PIMv2, DF Election,
length 20
00:05:13.927163 IP 10.10.189.1 > pim-routers.mcast.net: PIMv2, Hello,
length 38
00:05:16.146586 IP 10.10.189.101.netbios-dgm > 10.10.189.255.netbios-dgm:
NBT UDP PACKET(138)

This really got me stumped. I've searched high and low for a clue but
haven't really come across with anything that helps.
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