[Bug 199078] IPv6 on a bridge only works if an attached interface has had a v6 address

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=199078

            Bug ID: 199078
           Summary: IPv6 on a bridge only works if an attached interface
                    has had a v6 address
           Product: Base System
           Version: 10.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: n+freebsd at nirf.de

Created attachment 155070
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=155070&action=edit
ifconfig and rc.conf

I've got a bridge (bridge0) with one interface (re0) attached. bridge0 has a v6
address.

Apparently (according to tcpdump) incoming IPv6 packets reach re0 but not
bridge0, except when they are coming from a link local address. Outgoing v6
packets successfully pass to their destination.

Example of ping6 google.com:
tcpdump -i bridge0 -n not tcp
01:28:02.114762 IP6 2a01:4f8:123:4567::2 > 2a00:1450:4001:803::100e: ICMP6,
echo request, seq 0, length 16
01:28:03.124809 IP6 2a01:4f8:123:4567::2 > 2a00:1450:4001:803::100e: ICMP6,
echo request, seq 1, length 16
01:28:04.144537 IP6 2a01:4f8:123:4567::2 > 2a00:1450:4001:803::100e: ICMP6,
echo request, seq 2, length 16

tcpdump -i re0 -n not tcp
01:29:02.200106 IP6 2a01:4f8:123:4567::2 > 2a00:1450:4001:803::100e: ICMP6,
echo request, seq 0, length 16
01:29:02.205795 IP6 2a00:1450:4001:803::100e > 2a01:4f8:123:4567::2: ICMP6,
echo reply, seq 0, length 16
01:29:03.224609 IP6 2a01:4f8:123:4567::2 > 2a00:1450:4001:803::100e: ICMP6,
echo request, seq 1, length 16
01:29:03.230286 IP6 2a00:1450:4001:803::100e > 2a01:4f8:123:4567::2: ICMP6,
echo reply, seq 1, length 16
01:29:04.222578 IP6 2a01:4f8:123:4567::2 > 2a00:1450:4001:803::100e: ICMP6,
echo request, seq 2, length 16
01:29:04.228249 IP6 2a00:1450:4001:803::100e > 2a01:4f8:123:4567::2: ICMP6,
echo reply, seq 2, length 16


Pinging to fe80::1%bridge0 works as intended.

However, the problem can be fixed by adding any v6 address out of the same
subnet to re0:
ifconfig re0 inet6 2a01:4f8:123:4567::42
# ipv6 now works
ifconfig re0 inet6 2a01:4f8:123:4567::42 delete
# ipv6 still works

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