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Fragmentation Avoidance in DNS

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Kazunori Fujiwara , Paul A. Vixie
Last updated 2020-07-27 (Latest revision 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 29, 2021                                       Farsight
                                                           July 28, 2020

                     Fragmentation Avoidance in DNS


   EDNS0 enables a DNS server to send large responses using UDP and is
   widely deployed.  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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   This Internet-Draft will expire on January 29, 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
   ( 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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   to this document.  Code Components extracted from this document must
   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 . . . . . . . . . . . . . . . . . . . . .   6
   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  . . . . . . . . . . . . . . . . . . . . . . .   9

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
   will be avoided.

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   In Section 3.2 (Message Side Guidelines) of UDP Usage Guidelines
   [RFC8085] we are told 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.

   A DNS message receiver cannot trust fragmented UDP datagrams
   primarily due to the small amount of entropy provided by UDP port
   numbers and DNS message identifiers, each of which being only 16 bits
   in size, and both likely being in the first fragment of a packet, if
   fragmentation occurs.  By comparison, TCP protocol stack controls
   packet size and avoid IP fragmentation under ICMP NEEDFRAG attacks.
   In TCP, fragmentation should be avoided for performance reasons,
   whereas for UDP, fragmentation should be avoided for resiliency and
   authenticity reasons.

   [I-D.ietf-intarea-frag-fragile] summarized that IP fragmentation
   introduces fragility to Internet communication.  The transport of DNS
   messages over UDP should take account of the observations stated in
   that document.

   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, so that the
   segment size can be chosen so as to keep the each IP datagram below a
   target size.  This takes advantage of the elasticity of TCP's

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   packetizing process as to how much queued data will fit into the next
   segment.  In contrast, DNS over UDP has little datagram 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 68 octets, and all receivers
   must be able to receive and reassemble datagrams at least 576 octets
   in size (see Section 2.1, NOTE 1 of [I-D.ietf-intarea-frag-fragile]).
   The minimum MTU for an IPv6 interface is 1280 octets (see Section 5
   of [RFC8200]).  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 non-Ethernet networks exist such as Packet
   on SONET (PoS), Fiber Distributed Data Exchange (FDDI), and Ethernet
   Jumbo Frame, there is currently 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 it is less
   efficient than the path MTU would have been had that been

   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
      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
      discovered or estimated path MTU minus the estimated header space.
      Path MTU discovery [RFC1191], [RFC8201] and
      [I-D.ietf-tsvwg-datagram-plpmtud] may discover real path MTU value
      to destinations.  One method to retrieve path MTU value is
      described in Appendix A.  When discovered path MTU information is
      not available, a message sender SHOULD use the default maximum
      DNS/UDP payload size described in following section.

   o  The maximum buffer size offered by an EDNS0 initiator SHOULD be no
      larger than the estimated maximum DNS/UDP payload size.  If the
      desired response cannot be reasonably expected to fit into a
      buffer of that size, the initiator should use TCP 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 responder's discovered
      or estimated maximum DNS/UDP payload size, then the responder will

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      behave as though the requestor had specified a buffer size equal
      to the responder's estimated maximum DNS/UDP payload size.

   o  Fragmented DNS/UDP messages may be dropped without IP reassembly.
      An ICMP error 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

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

4.  Maximum DNS/UDP payload size

   o  Most of the Internet and especially the inner core has an MTU of
      at least 1500 octets.  An operator of a full resolver would be
      well advised to measure their path MTU to several authority name
      servers and to a random sample of their expected stub resolver
      client networks, to find the upper boundary on IP/UDP packet size
      in the average case.  This limit should not be exceeded by most
      messages received or transmitted by a full resolver, or else
      fallback to TCP will occur too often.  An operator of
      authoritative servers would also be well advised to measure their
      path MTU to several full-service resolvers.  The Linux tool
      "tracepath" can be used to measure the path MTU to well known
      authority name servers such as [a-m] or [a-
      m]  If the reported path MTU is for example no
      smaller than 1460, then the maximum DNS/UDP payload would be 1432
      for IP4 (which is 1460 - IP4 header(20) - UDP header(8)) and 1412
      for IP6 (which is 1460 - IP6 header(40) - UDP header(8)).  To
      allow for possible IP options and distant tunnel overhead, a
      useful default for maximum DNS/UDP payload size would be 1400.

   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  DNS flag day 2020 proposed 1232 as an EDNS buffer size.
      [DNSFlagDay2020] By the above reasoning, this proposal is either
      too small or too large.

5.  Incremental deployment

   The proposed method supports incremental deployment.

   When a full-service resolver implements the proposed method, its stub
   resolvers (clients) and the authority server network will no longer

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   observe IP fragmentation or reassembly from that server, and will
   fall back to TCP when necessary.

   When an authoritative server implements the proposed method, its full
   service resolvers (clients) will no longer observe IP fragmentation
   or reassembly from that server, and will fall back to TCP when

6.  Request to zone operators and DNS server operators

   Large DNS responses are the result of zone configuration.  Zone
   operators SHOULD seek configurations resulting in small responses.
   For example,

   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 'minimal-responses' configuration: Some implementations have
      'minimal responses' configuration that causes DNS servers to make
      response packets smaller, containing only mandatory and required
      data (Appendix B).

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

7.  Considerations

7.1.  Protocol compliance

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

   It is also well known that there are some authoritative servers that
   do not support TCP transport.

   Such non-compliant behavior cannot become implementation or
   configuration constraints for the rest of the DNS.  If failure is the
   result, then that failure must be localized to the non-compliant

8.  IANA Considerations

   This document has no IANA actions.

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9.  Security Considerations

10.  Acknowledgments

   The author would like to specifically thank Paul Wouters, Mukund
   Sivaraman for extensive review and comments.

11.  References

11.1.  Normative References

              Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile", draft-
              ietf-intarea-frag-fragile-17 (work in progress), September

              Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
              T. Voelker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", draft-ietf-tsvwg-datagram-plpmtud-22
              (work in progress), June 2020.

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

   [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,

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,

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   [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, <>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [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, <>.

11.2.  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.

              "DNS flag day 2020", n.d., <>.

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

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              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, <

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

   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: ip(7))

   "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: ipv6(7))

Appendix B.  Minimal-responses

   Some implementations have 'minimal responses' configuration that
   causes a DNS server to make response packets smaller, containing only
   mandatory and required data.

   Under the minimal-responses configuration, DNS servers compose
   response messages using only RRSets corresponding to queries.  In
   case of delegation, DNS servers compose response packets with
   delegation NS RRSet in authority section and in-domain (in-zone and
   below-zone) glue in the additional data section.  In case of non-
   existent domain name or non-existent type, the start of authority
   (SOA RR) will be placed in the Authority Section.

   In addition, if the zone is DNSSEC signed and a query has the DNSSEC
   OK bit, signatures are added in answer section, or the corresponding
   DS RRSet and signatures are added in authority section.  Details are
   defined in [RFC4035] and [RFC5155].

Authors' Addresses

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   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

   Paul Vixie
   Farsight Security Inc
   177 Bovet Road, Suite 180
   San Mateo, CA  94402
   United States of America

   Phone: +1 650 393 3994

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