NDP DoS attack (was Re: Anybody can participate in the IETF (Was: Why is IPv6 broken?))

Fri Jul 15 02:06:02 UTC 2011

On 07/14/2011 10:24 PM, Jimmy Hess wrote:
>> In this particular case, if the implementation enforces a limit on the
>> number of entries in the "INCOMPLETE" state, then only nodes that have
>> never communicated with the outside world could be affected by this
>> attack. And if those entries that are in the "INCOMPLETE" state are
>> pruned periodically (e.g. in a round-robin fashion), chances are that
> 
> Not only that but  it's possible to differentiate _how_ an entry is added to
> the table when the table reaches a "high water mark" it's possible
> to drop the packet that was attempting to cause a NDP discover, fail to add
> the INCOMPLETE entry to the table,  but _still_  send  the outgoing NDP
> neighbor solicitation, and complete the entry or "whitelist"  the destination
> if the neighbor advertises itself.

Agreed. I should double-check whether there's room in the current
specifications to do this -- however, whether the specs allow this or
not is irrelevant. At the point you're being hit with a DoS, you better
do something about it (particularly when it's possible, as in this case!)

> That is: if the destination is good, the neighbor will respond to the
> NDP solicit,
> even though the neighbor doesn't have an entry in the table.

Modulo that when the high water mark has not been hit, the router should
probably *not* create ND cache entries in response to this "gratuitous"
ND advertisements, since otherwise it would open the door to a DoS from
local attackers.

> It should be possible to mitigate this, so long as the attack does not actually
> originate from a neighbor on the same subnet as a router  IP interface on
> an IPv6 subnet with sufficient number of IPs.

Well, unless there's some layer-2 anti-spoofing mitigation in place,
with /64 subnets the "local attacker" typically *will* have enough
addresses.

Thanks,
-- 
Fernando Gont
e-mail: fernando at gont.com.ar || fgont at acm.org
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1