netdev - IPv6 neighbor discovery issues on 4.18

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Date:   Fri, 31 Aug 2018 10:49:18 -0400
From:   Brian Rak <brak@...eservers.com>
To:     netdev@...r.kernel.org
Subject: IPv6 neighbor discovery issues on 4.18

We've upgraded a few machines to a 4.18.3 kernel and we're running into 
weird IPv6 neighbor discovery issues.  Basically, the machines stop 
responding to inbound IPv6 neighbor solicitation requests, which very 
quickly breaks all IPv6 connectivity.

It seems like the routing table gets confused:

# ip -6 route get fe80::4e16:fc00:c7a0:7800 dev br0
RTNETLINK answers: Network is unreachable
# ping6 fe80::4e16:fc00:c7a0:7800 -I br0
connect: Network is unreachable
yet

# ip -6 route | grep fe80 | grep br0
fe80::/64 dev br0 proto kernel metric 256 pref medium

fe80::4e16:fc00:c7a0:7800 is the link-local IP of the server's default 
gateway.

In this case, br0 has a single adapter attached to it.

I haven't been able to come up with any sort of reproduction steps here, 
this seems to happen after a few days of uptime in our environment.  The 
last known good release we have here is 4.17.13.

Any suggestions for troubleshooting this?  Sometimes we see machines fix 
themselves, but we haven't been able to figure out what's happening that 
helps.