Dropping IPv6 Fragments

Mark Andrews marka at isc.org
Thu Oct 4 21:14:07 UTC 2012


In message <C7E7DE67-F668-45B4-9D64-2058400DC161 at doubleshotsecurity.com>, Merik
e Kaeo writes:
> 
> On Oct 4, 2012, at 7:36 AM, Dobbins, Roland wrote:
> 
> >=20
> > On Oct 4, 2012, at 9:26 PM, Sander Steffann wrote:
> >=20
> >> The closer you get to the edge the more common it might become...
> >=20
> > iACLs should be implemented at the network edge to drop all IPv4 and =
> IPv6 traffic - including non-initial fragments - directed towards =
> point-to-point links, loopbacks, and other internal infrastructure with =
> exceptions made for cases where there's a legitimate need for sources =
> outside your network to be able to communicate with your infrastructure.
> >=20
> > As mentioned previously on the thread, this has nothing to do with =
> transit data-plane traffic, which should be left untouched unless it's =
> specifically classified as attack traffic or other undesirable traffic.
> 
> +1
> 
> > There's an apparently common misperception that fragmented traffic is =
> somehow bad.  It isn't.  It's normal, under most circumstances.  Protect =
> your infrastructure proactively, deal with anything else on a =
> case-by-case basis.
> 
> Same misconception as ICMP is bad....historical artifact from attacks in =
> early 90's that just perpetuate in mythical best practice.   =20

And it really hurts modern DNS where UDP responses often exceed
Ethernet MTU.  For IPv6 UDP DNS responses are often fragmented at
1280 to prevent PMTUD being needed.  For IPv4 PMTUD should be off
if your vendor followed the RFC's (know exception are Linux and
Solaris boxes).

Mark
-- 
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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