[Bug 256813] no neighbor advertisement after repeatedly setting ipv6

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256813

            Bug ID: 256813
           Summary: no neighbor advertisement after repeatedly setting
                    ipv6
           Product: Base System
           Version: 13.0-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: jcaplan@blackberry.com

Overview
-------

Repeatedly setting the ipv6 address for an interface on two targets and then
trying to ping one from the other fails. The sender sends neighbor
solicitations but gets no reply. 


Steps to Reproduce
------------------
Two devices D1 and D2

D1> ifconfig vmx0 inet6 2001::2 up
D2> ifconfig vmx0 inet6 2001::3 up
D1> ping6   2001::3              
PING6(56=40+8+8 bytes) 2001::2 --> 2001::3

--- 2001::3 ping6 statistics ---
9 packets transmitted, 0 packets received, 100.0% packet loss

Check with tcpdump that D2 shows no received packets.
Actual Results
--------------

All packets lost


Expected Results
----------------
D2 replies with neighbor advertisement and ping works


Build Date & Hardware
---------------------
vmware

FreeBSD freebsd 13.0-RELEASE FreeBSD 13.0-RELEASE #6
releng/13.0-n244733-ea31abc261f: Sat Jun 19 06:23:53 UTC 2021



Additional Information
----------------------

- Produced same result with both em and vmx drivers.
- Flapping the interface does not fix the problem, only deleting the address
and then re-adding it.
- Sometimes it takes multiple address assignments for problem to manifest.
- DDB breakpoint for ip6_input is never hit when problem occurs
- Wireshark on linux host shows solicitation packets.

Issue is not reproduceable with Linux because they have:

sudo ifconfig vmnet21 inet6 add 2001::4/64
SIOCSIFADDR: File exists

while on freebsd it appears that ifconfig tries to delete and then re-add the
same interface

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