kern/60323: IPv6 NDP issue since upgrade to 11.0_RC4

["df4nx%gmx.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60323
>Category:       kern
>Synopsis:       Neighbor Discovery Protocol on 11.0_RC4
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Jun 12 07:40:00 +0000 2026
>Originator:     =09
>Release:        NetBSD 11.0_RC4
>Organization:
	home user
>Environment:
=09
=09
System: NetBSD at166.local 11.0_RC4 NetBSD 11.0_RC4 (GENERIC) #0: Tue May =
12 04:23:51 UTC 2026 mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/co=
mpile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:
	The host runs IPv6 with a static assigned IPv6 address. Trying
	to ping6 this address from a Linux host on the LAN failes.
	ping6 from a other NetBSD or OpenBSD host on the same LAN works.
	Ping6 and connect from the internet to the static IPv6 address
	works. `tcpdump icmp6` on the Linux host on the same LAN shows
	outgoing NDP Packets, but no incoming answer.
	The problem did not occur on NetBSD 10.1.
	For test purposes I have switched off NPF, but this did not fix
	the issue.
	It looks like a problem with the Neighbor Discovery Protocol on
	11.0_RC4.
	To double check I have two other hosts without static IPv6
	address. One 10.1, the other 11.0_RC4. The 10.1 host can be
	pinged, the 11.0_RC4 not. I used the dynnamic assigned IPv6
	address. None of these two systems run NPF.
	See also this blogpost:
	https://sirius-a.neocities.org/#v6issue
>How-To-Repeat:
	I used a installed Debian 13.5, a Debian live system (13.5 XFCE)
	and a Fedora Live 44-1.7 to test on the LAN.
	Ping6 and connect from the internet with:
	https://www.ipv6tech.ch/?pingv4v6
	https://www.ipv6tech.ch/?tcpportscan
	Tests with OpenBSD are done with the FuguIta live system:
	https://fuguita.org/
>Fix:
	Work around: Running the following command on the Linux host
	(Debian 13.5) works:
	ip -6 neigh replace XXXX:XXXX:XXXX:XXXX::1 lladdr yy:yy:yy:yy:yy:yy dev e=
no1
	with XXXX is my /64 prefix and yy is my MAC.

>Unformatted:
 =09
 =09