(Strange?) behavior of -current kernel on "ICMP6, packet too big, mtu 1450"

[Markus Kilbinger <mk%kilbi.de@localhost>]

Hi!

Recently I tried to connect (ssh) my -current aarch64 machine over
ipv6 and a vxlan tunnel what didn't work as expected (connection
timeout).

I digged further:

On my target -current aarch64 machine I've found (tcpdump) a corresponding:

  08:35:29.735126 IP6 fd01::1 > fd01::2: ICMP6, packet too big, mtu
1450, length 1240

which produced a routing table entry on the netbsd target machine:

# netstat  -f inet6 -rna | grep fd01
  ...
  fd01:01                link#1                         UGHD        -
      -   1450  awge0

resulting in a connection block for my source machine:

# ping6 fd01:1
  PING6(56=40+8+8 bytes) fd01::2 --> fd01:1
  ping6: sendmsg: Address family not supported by protocol family
  ping6: wrote fd01:1 16 chars, ret=-1

Deleting the 'false' routing table entry manually on the netbsd target
machine made the connection work again.

So, is this the intended reaction of a netbsd kernel on such ICMP6 packets?
My misunderstanding?

Initiating such ssh connections from an available Linux machine does
not show this problem / behavior (== works).

Any help / comment appreciated.
send-pr?

Regards, Markus