Re: PR: MTU discovery fails with IPv6 sockets bound to IPv4 mapped address

[Frank Kardel <kardel%netbsd.org@localhost>]

On 08/08/21 06:55, Michael van Elst wrote:
> kardel%kardel.name@localhost (Frank Kardel) writes:
>
> > Is there anyone willing to look at the fix before I commit?
> > The MTU bug prohibits data transfer as the write(v) systemcall
> > fails with EMSGSIZE(40).
> > https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=56348
> Mapped IPv4 addresses are considered harmful on the wire, that's
> why /etc/rc.d/network sets a route that rejects such packets.
Ok that explains the idea of the entry. As far as I can see these 
packets would never make
it to the wire with IPv4 mapped IPv6 addresses IPv6 packets because they 
are sent via ip_output as IPv4
packets. Maybe that route helps in IPv6 routing restrictions. I haven't 
checked that.
> But this is just a (changeable) policy. How does the fix work on a
> machine without IPv4 where mapped addresses might be wanted (RFC1933) ?
I looked at RFC1933. RFC1933 talks about COMPATIBLE addresses These have 
the form of 96 zero bits and the last 32 bit
for the IPv4 address part. So RFC1933 does not apply here, thanks for 
the reference though.
IPv4 mapped IPv6 addresses have the form 80 zero bits, 16 one bits and 
then the 32 bits for the IPv4 address part.

The fix applies only to IPv4 mapped IPv6 addresses. So I expect no 
issues there.

Frank