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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 2023-08-25 (Latest revision 2023-07-10)
Replaces draft-fujiwara-dnsop-avoid-fragmentation
RFC stream Internet Engineering Task Force (IETF)
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Document shepherd Tim Wicinski
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Responsible AD Warren "Ace" Kumari
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Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Intended status: Best Current Practice                          P. Vixie
Expires: 11 January 2024                                    AWS Security
                                                            10 July 2023

                     Fragmentation Avoidance in DNS


   EDNS0 enables a DNS server to send large responses using UDP and is
   widely deployed.  Large DNS/UDP responses are fragmented, and IP
   fragmentation has exposed weaknesses in application protocols.  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 techniques 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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 January 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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 to this document.  Code Components
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   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Proposal to avoid IP fragmentation in DNS . . . . . . . . . .   4
     3.1.  Recommendations for UDP responders  . . . . . . . . . . .   4
     3.2.  Recommendations for UDP requestors  . . . . . . . . . . .   4
   4.  Recommendations for zone operators and DNS server
           operators . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Protocol compliance . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Weaknesses of IP fragmentation . . . . . . . . . . .   9
   Appendix B.  Details of requestor's maximum UDP payload size
           discussions . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix C.  Minimal-responses  . . . . . . . . . . . . . . . . .  10
   Appendix D.  Known Implementations  . . . . . . . . . . . . . . .  11
     D.1.  BIND 9  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     D.2.  Knot DNS and Knot Resolver  . . . . . . . . . . . . . . .  11
     D.3.  PowerDNS Authoritative Server, PowerDNS Recursor, PowerDNS
           dnsdist . . . . . . . . . . . . . . . . . . . . . . . . .  12
     D.4.  PowerDNS Authoritative Server . . . . . . . . . . . . . .  12
     D.5.  Unbound . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   DNS has an EDNS0 [RFC6891] mechanism.  It enables a DNS server to
   send large responses using UDP.  EDNS0 is now widely deployed, and
   DNS over UDP relies on IP fragmentation when the EDNS buffer size is
   set to a value larger than the path MTU.

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   Fragmented DNS UDP responses have systemic weaknesses, which expose
   the requestor to DNS cache poisoning from off-path attackers.  (See
   Appendix A for references and details.)

   [RFC8900] 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.

   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
   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

   This document proposes that implementations set the "Don't Fragment
   (DF) bit" [RFC0791] on IPv4 and not use the "Fragment header"
   [RFC8200] on IPv6 in DNS/UDP messages in order to avoid IP
   fragmentation.  It also describes how to avoid packet losses due to
   DF bit and small MTU links.

   A path MTU different from the recommended value could be obtained
   from static configuration, or server routing hints, or some future
   discovery protocol; that would be the subject of a future
   specification and is beyond our scope here.

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 server, 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])

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   In this document, the term "Path MTU discovery" includes both
   Classical Path MTU discovery [RFC1191], [RFC8201], and Packetization
   Layer Path MTU discovery [RFC8899].

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

3.  Proposal to avoid IP fragmentation in DNS

   These recommendations are intended for nodes with global IP addresses
   on the Internet.  Private networks or local networks are out of the
   scope of this document.

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

3.1.  Recommendations for UDP responders

   1.  UDP responders SHOULD send DNS responses without "Fragment
       header" [RFC8200] on IPv6.

   2.  UDP responders are RECOMMENDED to set IP "Don't Fragment flag
       (DF) bit" [RFC0791] on IPv4.

   3.  UDP responders SHOULD compose response packets that fit in the
       offered requestor's maximum UDP payload size [RFC6891], the
       interface MTU, and the RECOMMENDED maximum DNS/UDP payload size

   4.  If the UDP responder detects an immediate error indicating that
       the UDP packet cannot be sent beyond the path MTU size
       (EMSGSIZE), the UDP responder MAY recreate response packets fit
       in path MTU size, or with the TC bit set.

   5.  UDP responders SHOULD limit the response size when UDP responders
       are located on small MTU (<1500) networks.

   The cause and effect of the TC bit are unchanged from EDNS0

3.2.  Recommendations for UDP requestors

   1.  UDP requestors SHOULD limit the requestor's maximum UDP payload
       size to the RECOMMENDED size of 1400 or a smaller size.

   2.  UDP requestors MAY drop fragmented DNS/UDP responses without IP
       reassembly to avoid cache poisoning attacks.

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   3.  DNS responses may be dropped by IP fragmentation.  Upon a
       timeout, to avoid resolution failures, UDP requestors MAY retry
       using TCP or UDP with a smaller EDNS requestor's maximum UDP
       payload size per local policy.

4.  Recommendations for 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,

   1.  Use a smaller number of name servers (13 may be too large)

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

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

   4.  Use a smaller signature / public key size algorithm for DNSSEC.
       Notably, the signature sizes of ECDSA and EdDSA are smaller than
       those usually used for RSA.

5.  Considerations

5.1.  Protocol compliance

   Prior research [Fujiwara2018] has shown that some authoritative
   servers ignore the EDNS0 requestor's maximum UDP payload size, and
   return large UDP responses.

   It is also well known that some authoritative servers 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

6.  IANA Considerations

   This document has no IANA actions.

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7.  Security Considerations

   When avoiding fragmentation, a DNS/UDP requestor behind a small-MTU
   network may experience UDP timeouts which would reduce performance
   and which may lead to TCP fallback.  This would indicate prior
   reliance upon IP fragmentation, which is universally considered to be
   harmful to both the performance and stability of applications,
   endpoints, and gateways.  Avoiding IP fragmentation will improve
   operating conditions overall, and the performance of DNS/TCP has
   increased and will continue to increase.

   If a UDP response packet is dropped (for any reason), it increases
   the attack window for poisoning the requestor's cache.

8.  Acknowledgments

   The author would like to specifically thank Paul Wouters, Mukund
   Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson,
   Puneet Sood, Jim Reid, Petr Spacek, Andrew McConachie, Joe Abley,
   Daisuke Higashi and Joe Touch for extensive review and comments.

9.  References

9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [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,

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

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

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   [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, <>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <>.

9.2.  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, <

              Huston, G. and J. Damas, "Measuring DNS Flag Day 2020",
              OARC 34 Workshop , February 2021.

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              Andrews, M. P., Huque, S., Wouters, P., and D. Wessels,
              "DNS Glue Requirements in Referral Responses", Work in
              Progress, Internet-Draft, draft-ietf-dnsop-glue-is-not-
              optional-09, 14 June 2023,

              Schwartz, B. M., Bishop, M., and E. Nygren, "Service
              binding and parameter specification via the DNS (DNS SVCB
              and HTTPS RRs)", Work in Progress, Internet-Draft, draft-
              ietf-dnsop-svcb-https-12, 11 March 2023,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,

   [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,

   [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, <>.

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   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,

Appendix A.  Weaknesses of IP fragmentation

   "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.

   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 avoids 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.

Appendix B.  Details of requestor's maximum UDP payload size discussions

   There are many discussions for default path MTU size and requestor's
   maximum UDP payload size.

   *  The minimum MTU for an IPv6 interface is 1280 octets (see
      Section 5 of [RFC8200]).  So, we can use it as the default path
      MTU value for IPv6.  The corresponding minimum MTU for an IPv4
      interface is 68 (60 + 8) [RFC0791].

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   *  Most of the Internet and especially the inner core has an MTU of
      at least 1500 octets.  Maximum DNS/UDP payload size for IPv6 on
      MTU 1500 ethernet is 1452 (1500 minus 40 (IPv6 header size) minus
      8 (UDP header size)).  To allow for possible IP options and
      distant tunnel overhead, the authors' recommendation of default
      maximum DNS/UDP payload size is 1400.

   *  [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.

   *  In order to avoid IP fragmentation, [DNSFlagDay2020] proposed that
      the UDP requestors set the requestor's payload size to 1232, and
      the UDP responders compose UDP responses so they fit in 1232
      octets.  The size 1232 is based on an MTU of 1280, which is
      required by the IPv6 specification [RFC8200], minus 48 octets for
      the IPv6 and UDP headers.

   *  [Huston2021] analyzed the result of [DNSFlagDay2020] and reported
      that their measurements suggest that in the interior of the
      Internet between recursive resolvers and authoritative servers the
      prevailing MTU is at 1,500 and there is no measurable signal of
      use of smaller MTUs in this part of the Internet, and proposed
      that their measurements suggest setting the EDNS0 requestor's UDP
      payload size to 1472 octets for IPv4, and 1452 octets for IPv6.

Appendix C.  Minimal-responses

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

   Under the minimal-responses configuration, a DNS server composes
   responses containing only nessesary RRs.  For delegations, see
   [I-D.ietf-dnsop-glue-is-not-optional].  In case of a non-existent
   domain name or non-existent type, the authority section will contain
   an SOA record and the answer section is empty. (defined in Section 2
   of [RFC2308]).

   Some resource records (MX, SRV, SVCB, HTTTPS) require additional A,
   AAAA, and SVCB records in the Additional Section defined in
   [RFC1035], [RFC2782] and [I-D.ietf-dnsop-svcb-https].

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

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Appendix D.  Known Implementations

   This section records the status of known implementations of these
   best practices defined by this specification at the time of
   publication, and any deviation from the specification.

   Please note that the listing of any individual implementation here
   does not imply endorsement by the IETF.  Furthermore, no effort has
   been spent to verify the information presented here that was supplied
   by IETF contributors.

D.1.  BIND 9

   BIND 9 does not implement the recommendations 1 and 2 in Section 3.1.

   BIND 9 on Linux sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with a
   fallback to IP_PMTUDISC_DONT.

   BIND 9 on systems with IP_DONTFRAG (such as FreeBSD), IP_DONTFRAG is

   Accepting PATH MTU Discovery for UDP is considered harmful and
   dangerous.  BIND 9's settings avoid attacks to path MTU discovery.

   For recommendation 3, BIND 9 will honor the requestor's size up to
   the configured limit (max-udp-size).  The UDP response packet is
   bound to be between 512 and 4096 bytes, with the default set to 1232.
   BIND 9 supports the requestor's size up to the configured limit (max-

   In the case of recommendation 4, and the send fails with EMSGSIZE,
   BIND 9 set the TC bit and try to send a minimal answer again.

   In the first recommendation of Section 3.2, BIND 9 uses the edns-buf-
   size option, with the default of 1232.

   BIND 9 does implement recommendation 2 of Section 3.2.

   For recommendation 3, after two UDP timeouts, BIND 9 will fallback to

D.2.  Knot DNS and Knot Resolver

   Both Knot servers set IP_PMTUDISC_OMIT to avoid path MTU spoofing.
   UDP size limit is 1232 by default.

   Fragments are ignored if they arrive over an XDP interface.

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   TCP is attempted after repeated UDP timeouts.

   Minimal responses are returned and are currently not configurable.

   Smaller signatures are used, with ecdsap256sha256 as the default.

D.3.  PowerDNS Authoritative Server, PowerDNS Recursor, PowerDNS dnsdist

   *  IP_PMTUDISC_OMIT with fallback to IP_PMTUDISC_DONT

   *  default EDNS buffer size of 1232, no probing for smaller sizes

   *  no handling of EMSGSIZE

   *  Recursor: UDP timeouts do not cause a switch to TCP.  "Spoofing
      nearmisses" do.

D.4.  PowerDNS Authoritative Server

   *  the default DNSSEC algorithm is 13

   *  responses are minimal, this is not configurable

D.5.  Unbound

   Unbound sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with fallback to
   IP_PMTUDISC_DONT.  It also disables IP_DONTFRAG on systems that have
   it, but not on Apple systems.  On systems that support it Unbound
   sets IPV6_USE_MIN_MTU, with a fallback to IPV6_MTU at 1280, with a
   fallback to IPV6_USER_MTU.  It also sets IPV6_MTU_DISCOVER to
   IPV6_PMTUDISC_OMIT with a fallback to IPV6_PMTUDISC_DONT.

   Unbound requests UDP size 1232 from peers, by default.  The
   requestors size is limited to a max of 1232.

   After some timeouts, Unbound retries with a smaller size, if that is
   smaller, at size 1232 for IPv6 and 1472 for IPv4.  This does not do
   anything since the flag day change to 1232.

   Unbound has minimal responses as an option, default on.

Authors' Addresses

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

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   Phone: +81 3 5215 8451

   Paul Vixie
   AWS Security
   11400 La Honda Road
   Woodside, CA,  94062
   United States of America
   Phone: +1 650 393 3994

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