Fragmentation Avoidance in DNS
draft-ietf-dnsop-avoid-fragmentation-00

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Last updated 2020-06-30
Replaces draft-fujiwara-dnsop-avoid-fragmentation
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Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Intended status: Best Current Practice                          P. Vixie
Expires: January 1, 2021                                        Farsight
                                                           June 30, 2020

                     Fragmentation Avoidance in DNS
                draft-ietf-dnsop-avoid-fragmentation-00

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
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   This Internet-Draft will expire on January 1, 2021.

Copyright Notice

   Copyright (c) 2020 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
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   carefully, as they describe your rights and restrictions with respect
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Fujiwara & Vixie         Expires January 1, 2021                [Page 1]
Internet-Draft             avoid-fragmentation                 June 2020

   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.  Maximum DNS/UDP payload size  . . . . . . . . . . . . . . . .   5
   5.  Incremental deployment  . . . . . . . . . . . . . . . . . . .   5
   6.  Request to zone operators and DNS server operators  . . . . .   6
   7.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Protocol compliance . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  How to retrieve path MTU value to a destination from
                applications . . . . . . . . . . . . . . . . . . . .   9
   Appendix B.  Minimal-responses  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   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 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 the security implications of predictable
   fragment identification values.

   DNSSEC is a countermeasure against cache poisoning attacks that use
   IP fragmentation.  However, DNS delegation responses are not signed
   with DNSSEC, and DNSSEC does not have a mechanism to get the correct
   response if an incorrect delegation is injected.  This is a denial-
   of-service vulnerability that can yield failed name resolutions.  If
   cache poisoning attacks can be avoided, DNSSEC validation failures
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