Avoid IP fragmentation in DNS
draft-fujiwara-dnsop-avoid-fragmentation-01

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Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Intended status: Best Current Practice                          P. Vixie
Expires: March 30, 2020                                         Farsight
                                                      September 27, 2019

                     Avoid IP fragmentation in DNS
              draft-fujiwara-dnsop-avoid-fragmentation-01

Abstract

   Path MTU discovery remains widely undeployed due to security issues,
   and IP fragmentation has exposed weaknesses in application protocols.
   Currently, DNS is known to be the largest user of IP fragmentation.
   It is possible to avoid IP fragmentation in DNS by limiting response
   size where possible, and signaling the need to upgrade from UDP to
   TCP transport where necessary.  This document proposes to avoid IP
   fragmentation in DNS.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 30, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Proposal to avoid IP fragmentation in DNS . . . . . . . . . .   3
   4.  Default maximum DNS/UDP payload size  . . . . . . . . . . . .   4
   5.  Incremental deployment  . . . . . . . . . . . . . . . . . . .   5
   6.  Request to zone operator  . . . . . . . . . . . . . . . . . .   5
   7.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   5
     10.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Appendix A.  How to retrieve path MTU value to a destination  . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   DNS has EDNS0 [RFC6891] mechanism.  It enables that DNS server can
   send large size response using UDP.  Now EDNS0 is widely deployed,
   and DNS (over UDP) is said to be the biggest user of IP
   fragmentation.

   However, "Fragmentation Considered Poisonous" [Herzberg2013] proposed
   effective off-path DNS cache poisoning attack vectors using IP
   fragmentation.  "IP fragmentation attack on DNS" [Hlavacek2013] and
   "Domain Validation++ For MitM-Resilient PKI" [Brandt2018] proposed
   that off-path attackers can intervene in path MTU discovery [RFC1191]
   to perform intentionally fragmented responses from authoritative
   servers.  [RFC7739] stated security implications of predictable
   fragment identification values.

   And more, Section 3.2 Message Side Guidelines of UDP Usage Guidelines
   [RFC8085] specifies that an application SHOULD NOT send UDP datagrams
   that result in IP packets that exceed the Maximum Transmission Unit
   (MTU) along the path to the destination.

   As a result, we cannot trust fragmented UDP packets, primarily due to
   the low level of entropy provided by UDP port numbers and DNS message
   identifiers, each of which being 16 bits in size.  By comparison, TCP
   is considered resistant against IP fragmentation attacks because TCP
   has a 32-bit sequence number and 32-bit acknowledgement number in
   each segment.

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