[DNSOP] Introduction section of draft-ietf-dnsop-avoid-fragmentation

[Brian Dickson <[e-mail]>]

Fujiwara-san,

I have a suggestion on tweaking the wording of Section 1, Introduction.
The intent is to simplify it a bit. Feel free to ignore this if it doesn't
work,
or use whatever parts of it you feel are useful.

DNS has EDNS0 [RFC6891] mechanism.  It enables a DNS server to send
   large responses using UDP.  EDNS0 is now widely deployed, and DNS
   (over UDP) is said to be the biggest source of IP fragments.

Fragmented DNS UDP responses have systemic weaknesses, which expose
the requestor to DNS cache poisoning from off-path attackers. (See appendix
for references and details.) The primary weakness is the UDP checksum
itself,
and the fact that the UDP header and DNS header will be in the first
fragment only.
In addition, the IPv4 fragmentation and reassembly relies solely on the
IPv4 ID,
a 16-bit value which is weak enough to permit feasible brute force attacks.

UDP has no inherent ability to react to ICMP messages that would otherwise
alert
the sender to dropped packets with DF=1, due to MTU being exceeded.
Additionally, fragmentation can adversely affect TCP performance, thus the
same logic
is applicable to both UDP and TCP. Fragmentation SHOULD be avoided.

This document proposes to set IP_DONTFRAG / IPV6_DONTFRAG in DNS/UDP
   messages in order to avoid IP fragmentation, and describes how to
   avoid packet losses due to IP_DONTFRAG / IPV6_DONTFRAG.

(Place the rest of the original introduction text and references into an
Appendix.)

Feedback concerning this suggestion is welcome.

Sincerely,
Brian Dickson