Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Intended status: Best Current Practice                          P. Vixie
Expires: October 15, 2020                                       Farsight
                                                          April 13, 2020

                     Fragmentation Avoidance 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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 15, 2020.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
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   include Simplified BSD License text as described in Section 4.e of
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   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 operator and DNS server operator  . . . . . .   5
   7.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Protocol compliance . . . . . . . . . . . . . . . . . . .   6
     7.2.  DNS packet size . . . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Appendix A.  How to retrieve path MTU value to a destination  . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 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.  By
   comparison, TCP is considered resistant against IP fragmentation

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   attacks because TCP has a 32-bit sequence number and 32-bit
   acknowledgement number in each segment.  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.  The takes advantage of the elasticity of TCP's
   packetizing process as to how much queued data will fit into the next
   segment.  In contrast, 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 space.

   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 and for an IPv6 interface is 1280 octets (see
   Section 5 of [RFC8200]).  These are theoretic limits and no modern

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   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 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 it been measurable.

   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 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, 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 estimated
      maximum DNS/UDP payload size, then the responder will 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].

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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
      answers received or transmitted by a full resolver, or else
      fallback to TCP will occur too often.  An operator of
      authoritative servers would be also 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 faraway 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.

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
   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 operator and DNS server operator

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

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   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.
      Notably, the signature size of ECDSA or EdDSA is smaller than RSA.

   o  Use 'minimal-responses' configuration: Some implementations have
      'minimal responses' configuration that enables that DNS servers
      make response packets smaller, mandatory and required data only.

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 noncompliant behaviour 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 noncompliant

7.2.  DNS packet size

   Many stub resolvers do not set the DNSSEC OK bit.  In this case,
   responses from full-service resolvers may be small.

   With 'minimal-response' configuration, DNS servers can be forced to
   emit small responses.

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  Server   |DNSSEC| Answer   | Response data         |response
   type    | OK   |   type   | Answer/Authority/Add. | size
  Resolver | No   | Exist    | RRSet//               |RRSet
  Resolver | No   | Not exist| /SOA/                 |SOA
  Resolver | Yes  | Exist    | RRSet+RRSIG//         |RRSet+RRSIG
  Resolver | Yes  | Not exist| /SOA+NSEC+RRSIG/      |SOA+NSEC*2+RRSIG*3
  Auth.    | No   | Referral | /NS/glue              |NS+glue
  Auth.    | No   | Exist    | RRSet//               |RRSet
  Auth.    | No   | Not exist| /SOA/                 |SOA
  Auth.    | Yes  | Referral | /DS+RRSIG+NS/glue     |NS+glue+DS+RRSIG
  Auth.    | Yes  | Referral | /NSEC+RRSIG+NS/glue   |NS+glue+NSEC+RRSIG
  Auth.    | Yes  | Exist    | RRSet+RRSIG//         |RRSet+RRSIG
  Auth.    | Yes  | Not exist| /SOA+NSEC*2+RRSIG/    |SOA+NSEC*2+RRSIG*3

   Non-existent answers with DNSSEC are largest.

   Without 'minimal responses' configuration, DNS servers may add
   unnecessary NS RRset in authority section and nameservers' A/AAAA
   RRSet in additional section.

   However, with 'minimal-responses' configuration, zone operators can
   control the authoritative server's response size (selection of DNSKEY
   algorithm and size, and number of resource records).

8.  IANA Considerations

   This document has no IANA actions.

9.  Security Considerations

10.  References

10.1.  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

   [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, <>.

   [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, <>.

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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.

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

              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, <

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: 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))

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

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

   Phone: +1 650 393 3994

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