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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


   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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   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
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   to this document.  Code Components extracted from this document must

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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

   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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   This document proposes to avoid IP fragmentation in DNS/UDP.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   "Requestor" refers to the side that sends a request.  "Responder"
   refers to an authoritative, recursive resolver or other DNS component
   that responds to questions.  (Quoted from EDNS0 [RFC6891])

   "path MTU" is the minimum link MTU of all the links in a path between
   a source node and a destination node.  (Quoted from [RFC8201])

   Many of the specialized terms used in this document are defined in
   DNS Terminology [RFC8499].

3.  Proposal to avoid IP fragmentation in DNS

   TCP avoids fragmentation using its Maximum Segment Size (MSS)
   parameter, but each transmitted segment is header-size aware such
   that the size of the IP and TCP headers is known, as well as the far
   end's MSS parameter and the interface or path MTU, in order to send a
   smaller segment as necessary to keep the each IP datagram below a
   target size.  TCP's packetizing process is also elastic as to how
   much queued data will fit into the next segment.  DNS has no message
   size elasticity and lacks insight into IP header and option size, and
   so must make more conservative estimates about available UDP payload

   The minimum MTU for an IPv4 interface is 576 octets, and for an IPv6
   interface, 1280 octets.  These are theoretic limits and no modern
   networks implement them.  In practice, the smallest MTU witnessed in
   the operational DNS community is 1500 octets, the Ethernet maximum
   payload size.  While many networks such as Packet on SONET (PoS),
   Fiber Distributed Data Exchange (FDDI), and Ethernet Jumbo Frame,
   there is no reliable way of discovering such links in an IP
   transmission path.  Absent some kind of path MTU discovery result or
   a static configuration by the server or system operator, a
   conservative estimate must be chosen, even if less efficient.

   The methods to avoid IP fragmentation in DNS are described below:

   o  UDP requestors and responders SHOULD send DNS responses with
      IP_DONTFRAG / IPV6_DONTFRAG [RFC3542] options, which will yield

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      either a silent timeout, or a network (ICMP) error, if the path
      MTU is exceeded.  Upon a timeout, UDP requestors may retry using
      TCP or UDP, per local policy.

   o  The estimated maximum DNS/UDP payload size SHOULD be the actual or
      estimated path MTU minus the estimated header space.  When actual
      path MTU information is not available, use the default maximum
      DNS/UDP payload size described in following section.

   o  The maximum buffer size offered by an EDNS0 requestor SHOULD be no
      larger than the estimated maximum DNS/UDP payload size.  If the
      response cannot be reasonably expected fit into a buffer of that
      size, TCP should be used instead of UDP.

   o  Responders SHOULD compose UDP responses that result in IP packets
      that do not exceed the path MTU to the requestor.  Thus, if the
      requestor offers a buffer size larger than estimated maximum DNS/
      UDP payload size, then the responder will behave as though the
      requestor had specified a buffer size equal to the estimated
      maximum DNS/UDP payload size.

   o  Fragmented DNS/UDP messages may be dropped before IP assembly.  An
      ICMP error should should be sent in this case, with rate limiting
      to prevent this logic from becoming a DDoS amplification vector.
      If rate limiting is not possible, then no ICMP error should be
      sent.  (This is a countermeasure against DNS spoofing attacks
      using IP fragmentation.)

   The cause and effect of the TC bit is unchanged from EDNS0 [RFC6891].

4.  Default maximum DNS/UDP payload size

   o  [RFC4035] defines that "A security-aware name server MUST support
      the EDNS0 message size extension, MUST support a message size of
      at least 1220 octets".  Then, the smallest number of the maximum
      DNS/UDP payload size is 1220.

   o  However, in practice, the smallest MTU witnessed in the
      operational DNS community is 1500 octets.  The estimated size of a
      DNS message's UDP headers, IP headers, IP options, and one or more
      set of tunnel, IP-in-IP, VLAN, and virtual circuit headers, SHOULD
      be 100 octets.  Then, the maximum DNS/UDP payload size may be

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5.  Incremental deployment

   The proposed method supports incremental deployment.

   When a full-service resolver implements the proposed method, the
   full-service resolver becomes to avoid IP fragmentation in DNS.

   When an authoritative server implements the proposed method, the
   authoritative server becomes to avoid IP fragmentation in DNS.

6.  Request to zone operator

   Fat DNS responses come from fat configurations of zones.  Zone
   operator SHOULD consider small response size configurations.  For

   o  Use smaller number of name servers (13 may be too large)

   o  Use smaller number of A/AAAA RRs for a domain name

   o  Use smaller signature / public key size algorithm for DNSSEC.
      Signature size of ECDSA or EdDSA is smaller than RSA.

7.  Considerations

   In past researches ([Fujiwara2018] / dns-operations mailing list
   discussions), there are some authoritative servers that ignore EDNS0
   requestor's UDP payload size, and return large UDP responses.

   And it is known that there are some authoritative servers that do not
   support TCP transport.

8.  IANA Considerations

   This document has no IANA actions.

9.  Security Considerations

10.  References

10.1.  Normative References

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003,

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

10.2.  Informative References

              Brandt, M., Dai, T., Klein, A., Shulman, H., and M.
              Waidner, "Domain Validation++ For MitM-Resilient PKI",
              Proceedings of the 2018 ACM SIGSAC Conference on Computer
              and Communications Security , 2018.

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              Fujiwara, K., "Measures against cache poisoning attacks
              using IP fragmentation in DNS", OARC 30 Workshop , 2019.

              Herzberg, A. and H. Shulman, "Fragmentation Considered
              Poisonous", IEEE Conference on Communications and Network
              Security , 2013.

              Hlavacek, T., "IP fragmentation attack on DNS", RIPE 67
              Meeting , 2013, <https://ripe67.ripe.net/

Appendix A.  How to retrieve path MTU value to a destination

   Socket options: "IP_MTU (since Linux 2.2) Retrieve the current known
   path MTU of the current socket.  Valid only when the socket has been
   connected.  Returns an integer.  Only valid as a getsockopt(2)."
   (Quoted from Debian GNU Linux manual: man 7 ip)

   "IPV6_MTU getsockopt(): Retrieve the current known path MTU of the
   current socket.  Only valid when the socket has been connected.
   Returns an integer."  (Quoted from Debian GNU Linux manual: man 7

Authors' Addresses

   Kazunori Fujiwara
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065

   Phone: +81 3 5215 8451
   Email: fujiwara@jprs.co.jp

   Paul Vixie
   Farsight Security Inc
   177 Bovet Road, Suite 180
   San Mateo, CA  94402

   Phone: +1 650 393 3994
   Email: paul@redbarn.org

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