Turn off IPV6_USE_MIN_MTU?

Mark Andrews marka at isc.org
Tue May 31 14:31:06 UTC 2016

In message <92E4FD9B-12A4-4773-B785-B3AA40F3E6B5 at bondis.org>, =?utf-8?Q?Jo=C3=A3o_Damas?= writes:
> Mark,
> > On 31 May 2016, at 13:32, Mark Andrews <marka at isc.org> wrote:
> >
> >
> > In message <20160531161631.7ca7f650 at pallas.home.time-travellers.org
> <mailto:20160531161631.7ca7f650 at pallas.home.time-travellers.org>>, Shane
> Kerr writes:
> >>
> >> Hello,
> >>
> >> Geoff Huston has done some work recently looking at IPv6 fragmentation,
> >> and there seem to be a number of cases where IPv6 fragments do not
> >> work:
> >>
> >> https://ripe72.ripe.net/archives/video/164/
> >>
> >> (The link says "video" but there are also slides there.)
> >>
> >> This is likely because of middleboxes; if you want to do deep-packet
> >> inspection, you need to look at the reassembled packets, which mean
> >> additional state and processing for the boxes. A simpler solution is
> >> just to disable IPv6 fragments, since TCP doesn't need it and UDP is
> >> unreliable anyway. :-P
> >
> > UDP is reliable if you don't have idiots breaking the protocol by
> > dropping fragments.
> Agreed
> >  Additionally DPI is going away as more and
> > more traffic is encrypted.
> Perhaps
> > Just because you can DPI something
> > doesn't mean that you should.
> Agreed, no only for this case.
> Now… with that baseline established, there is a fact that Shane pointed
> out that I think should be the overriding argument:
> measurements of reality indicate that using larger packets, stuffing as
> much data as possible into the UDP packet, in a world that overwhelmingly
> uses 1500(-tunnel) as its MTU, increases the chances of success of the
> communication.
> if you remember, the 1280 magic number comes from pure gut-feeling on the
> side of the RFC author/editor, not hard evidence.
> Please consider adoption of the change that Shane suggests as it will
> increase the chances of DNS lookups succeeding and we can continue to
> educate people about DPI, broken firewall rules, etc while getting the
> DNS to work better over UDP/IPv6
> Joao

And if you live behind a tunnel you want the fragmentation @1280.

Too many idiots with authoritative servers block PTB messages and
DNS/UDP doesn't resend automatically on receipt of PTB like TCP
does.  Even if the site doesn't deliberately block PTB, broken load
balancer that don't forward PTB corretly, etc. means that you don't
want to rely on PTB working.  This even impacts DNS/TCP.

You don't need to do reassembly and DPI to let fragmentmented
responses through.  Just open a slit in the firewall for non initial
fragment based on <src-addr,dst-addr,proto> along with the full
tuple for the initial fragement <src-addr,src-port,dst-addr,dst-port,proto>
and let the host reassemble the packet.  It doesn't have to be all

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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