I'm at my wits' end, so any help is appreciated.

I have an IPv6 host (Linux 4.15.1-gentoo SMP x86_64) that randomly stops sending neighbour advertisements. Running tcpdump shows a lot of neighbour solicitation requests and almost zero reaction to those requests. Occasionally, the host will still send NA, but only after a couple dozen ignored NS requests. Obviously, this completely breaks IPv6 connectivity.

I don't know if it's relevant, but IPv6 is configured on a bridge interface (a couple lxc containers are running on that bridge as well). The bridge is a typical brctl bridge with STP off.

IPv6 is configured statically (both host and gateway).

Manually flooding the network with unsolicited neighbour advertisements (using ndsend from vzctl for example) can mitigate the problem a little, but it's obviously not a solution.

What's even weirder, disabling and re-enabling ipv6 on the interface via procfs (/proc/sys/net/ipv6/conf/br0/disable_ipv6) and reconfiguring it (ip -6 addr add, etc) temporarily "fixes" the problem. It happens again in a day or two though.

For the sake of completeness, there's an nftables firewall running on the host, but it explicitly allows all icmpv6 traffic (via ip6 nexthdr ipv6-icmp accept everywhere). Disabling the firewall when the problem manifests doesn't change anything.

So, here's the question: what can I do to pinpoint the underlying issue?

UPDATE: For me, the problem disappeared after a few kernel updates, but there are reports of similar problems on later kernel versions, particularly with large routing tables and/or a large number of neighbours. Reportedly, one possible culprit here is the small limit on ipv6 route/neighbour cache size in kernel. If you're having similar issues, try raising net.ipv6.route.max_size sysctl parameter to a relatively large value (e.g. 1048576), for instance by running sysctl -w net.ipv6.route.max_size=1048576 and/or by editing /etc/sysctl.conf. You also will likely want to raise net.ipv6.route.gc_thresh to avoid running the garbage collector too often. Also, check net.ipv6.neigh.default.gc_thresh1, net.ipv6.neigh.default.gc_thresh2 and net.ipv6.neigh.default.gc_thresh3 if you have particularly many records in the neighbour cache. See https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt for what all those options do.

  • You did not say which kernel version and did you try other ones? Mar 21, 2018 at 2:31
  • I've seen this manifest at least in 4.12.0, 4.12.5, 4.12.10, 4.14.7 and 4.15.1. Did not as of yet observe it on 4.15.12 after updating this weekend, so maybe you're onto something.
    – lierdakil
    Mar 27, 2018 at 23:16
  • How many IP addresses does the host have? I have seen similar symptoms when there were too many IP addresses assigned (around 4000 as far as I recall).
    – kasperd
    Apr 16, 2018 at 21:17
  • @kasperd, not that many. Anyway, it seems like this was a kernel problem for me at least.
    – lierdakil
    Apr 17, 2018 at 8:40

2 Answers 2


I just discovered there is a bug in multicast_snooping in linux vlan aware bridges. It will not touch router advertisements, but it will block neighbor discovery even if multicast_flooding is on. What happens is that on boot of a system it will do dad, and that dad will stay in the multicast forwarding database. But that expires after 200 or 300s. After that any neighbor discovery multicast packet will be dropped to that port. This only happens with neighbour discovery, not with router adverteisment. You can witness it by doing:

bridge mdb show

If it shows you entries, it will have multicast_snooping turned on. And you might/will experience the bug. In my case it is about 80% of the systems that I set up started blocking neighbor discovery multicast only. Any other multicast is flooded, or correctly snooped.

The solution for now is to turn off multicast_snooping:

echo 0 > /sys/net/devicename/bridge/multicast_snooping

When I have time I will make a test set up. This bug has been biting in my behind for 2 years now, and I finally had the time during emergency maintenance to fully grasp the problem.


I was having the same issue with a 4.16.2-gentoo kernel. But in my case it turned out to be completely unrelated to the kernel.

The box in question, served as a ipv6 VPN-Gateway and was having a stable connection. Even the subnet-router behind it was perfectly fine, just the routed subnet itself was constantly losing connection.

firewalld was the culprit in my case. The ipv6 rpfilter setting filtered the neighor-solicitations of my subnet router.

Found out about it by enabling the logging in /etc/firewalld/firewalld.conf


which resulted in loglines like (MAC and SRC shortened and obfuscated):

kernel: rpfilter_DROP: IN=enp6s0.100 OUT= MAC=XX:…:XX SRC=fe80:…:beaf DST=ff02:0000:0000:0000:0000:0001:ff00:0001 LEN=72 TC=0 HOPLIMIT=255 FLOWLBL=0 PROTO=ICMPv6 TYPE=135 CODE=0

I've just disabled the ipv6 rpfilter until i'm able to find out why this is happening. The setup is quite simple and everything looks fine to me, but maybe it's an issue with the interface beeing a vlan...

  • Linux has reverse path filtering for IPv4 built into the kernel and it works properly. But it doesn't have IPv6 reverse path filtering, so firewalld fakes it with some firewall rules which don't quite work right. It is indeed best to turn this off in firewalld until they come up with better rules, or the reverse path filtering for IPv6 is built into the kernel. Aug 15, 2019 at 18:44

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