subnet prefix length > 64 breaks IPv6?

Jeff Wheeler jsw at
Wed Dec 28 21:08:27 UTC 2011

On Wed, Dec 28, 2011 at 10:19 AM, Ray Soucy <rps at> wrote:
> There are a few solutions that vendors will hopefully look into.  One
> being to implement neighbor discovery in hardware (at which point
> table exhaustion also becomes a legitimate concern, so the logic
> should be such that known associations are not discarded in favor of
> unknown associations).

Even if that is done you are still exposed to attacks -- imagine if a
downstream machine that is under customer control (not yours) has a
whole /64 nailed up on its Ethernet interface, and happily responds to
ND solicits for every address.  Your hardware table will fill up and
then your network has failed -- which way it fails depends on the
table eviction behavior.

Perhaps this is not covered very well in my slides.  There are design
limits here that cannot be overcome by any current or foreseen
technology.  This is not only about what is broken about current
routers but what will always be broken about them, in the absence of
clever work-arounds like limits on the number of ND entries allowed
per physical customer port, etc.

We really need DHCPv6 snooping and ND disabled for campus access
networks, for example.  Otherwise you could give out addresses from a
limited range in each subnet and use an ACL (like Owen DeLong suggests
for hosting environments -- effectively turning the /64 into a /120
anyway) but this is IMO much worse than just not configuring a /64.

On Wed, Dec 28, 2011 at 10:45 AM,  <sthaug at> wrote:
> I'm afraid I don't believe this is going to happen unless neighbor
> discovery based attacks become a serious problem. And even then it would
> take a long time.

The vendors seem to range from "huh?" to "what is everyone else
doing?" to Cisco (the only vendor to make any forward progress at all
on this issue.)  I think that will change as this topic is discussed
more and more on public mailing lists, and as things like DHCPv6
snooping, and good behavior when ND is disabled on a subnet/interface,
begin to make their way into RFPs.

As it stands right now, if you want to disable the IPv6 functionality
(and maybe IPv4 too if dual-stacked) of almost any datacenter /
hosting company offering v6, it is trivial to do that.  The same is
true of every IXP with a v6 subnet.  I think once some bad guys figure
this out, they will do us a favor and DoS some important things like
IXPs, or a highly-visible ISP, and give the vendors a kick in the
pants -- along with operators who still have the "/64 or bust"
mentality, since they will then see things busting due to trivial

Jeff S Wheeler <jsw at>
Sr Network Operator  /  Innovative Network Concepts

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