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Re: [DNSOP] Cache poisoning on DNSSEC

Mark Andrews <Mark_Andrews@isc.org> Wed, 27 August 2008 00:48 UTC

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To: Dean Anderson <dean@av8.com>
From: Mark Andrews <Mark_Andrews@isc.org>
In-reply-to: Your message of "Tue, 26 Aug 2008 15:02:07 -0400." <Pine.LNX.4.44.0808261445330.13198-100000@citation2.av8.net>
Date: Wed, 27 Aug 2008 10:48:15 +1000
Cc: "Blacka, David" <davidb@verisign.com>, Ralf Weber <denic@eng.colt.net>, "dnsop@ietf.org WG" <dnsop@ietf.org>
Subject: Re: [DNSOP] Cache poisoning on DNSSEC
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> Large UDP packets (think EDNSO DNSSEC as a good example of large UDP
> packets almost certain to be fragmented) suffer the same problem, as
> they can be fragmented by PMTU discovery. The server (operating system)
> has to maintain UDP state for PMTUD to work.  If the ICMP fragmentation
> needed is lost due to Anycast, PMTUD will fail. Lost UDP fragments are
> fatal to the UDP transaction.

	Actually you just turn off PMTUD on replies.  This is
	recommended for *all* nameservers.  It's pointless for
	authoritative nameservers to maintain PMTU state and may
	infact be a DoS vector if they do.

	IPv4 - Don't set FD.
	IPv6 - Fragment at the server at network MTU.

	The socket option IPV6_USE_MIN_MTU was a direct consequence
	of DNS operators looking at this issue over 10 years ago.

	http://www3.tools.ietf.org/html/draft-ietf-ipngwg-bsd-frag-01

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