Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Intended status: Best Current Practice                          P. Vixie
Expires: 25 December 2021                                       Farsight
                                                            23 June 2021


                     Fragmentation Avoidance in DNS
                draft-ietf-dnsop-avoid-fragmentation-05

Abstract

   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
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 25 December 2021.

Copyright Notice

   Copyright (c) 2021 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 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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   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
     3.1.  Recommendations for UDP responders  . . . . . . . . . . .   4
     3.2.  Recommendations for UDP requestors  . . . . . . . . . . .   4
     3.3.  Default Maximum DNS/UDP payload size  . . . . . . . . . .   4
   4.  Incremental deployment  . . . . . . . . . . . . . . . . . . .   6
   5.  Request to zone operators and DNS server operators  . . . . .   6
   6.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Protocol compliance . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Weaknesses of IP fragmentation . . . . . . . . . . .   9
   Appendix B.  Details of maximum DNS/UDP payload size
           discussions . . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix C.  How to retrieve path MTU value to a destination from
           applications  . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix D.  How to retrieve minimal MTU value to a
           destination . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix E.  Minimal-responses  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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.

   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.






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   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
   space.

   This document proposes to set IP_DONTFRAG / IPV6_DONTFRAG in DNS/UDP
   messages in order to avoid IP fragmentation, and describes how to
   avoid packet losses due to IP_DONTFRAG / IPV6_DONTFRAG.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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])

   "Path MTU discovery" is defined by [RFC1191], [RFC8201] and
   [RFC8899].

   IP_DONTFRAG option is not defined by any RFCs.  It is similar to
   IPV6_DONTFRAG option defined in [RFC3542].  IP_DONTFRAG option is
   used on BSD systems to set the Don't Fragment bit [RFC0791] when
   sending IPv4 packets.  On Linux systems this is done via
   IP_MTU_DISCOVER and IP_PMTUDISC_DO.

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

3.  Proposal to avoid IP fragmentation in DNS

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





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3.1.  Recommendations for UDP responders

   *  UDP responders SHOULD send DNS responses with IP_DONTFRAG /
      IPV6_DONTFRAG [RFC3542] options.

   *  If the UDP responder detects immediate error 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
      TC bit set.

   *  UDP responders MAY probe to discover the real MTU value per
      destination.

   *  UDP responders SHOULD compose UDP responses that result in IP
      packets that do not exceed the path MTU to the requestor.  If the
      path MTU discovery failed or is impossible, UDP responders SHOULD
      compose UDP responses that result in IP packets that do not exceed
      the default maximum DNS/UDP payload size described in Section 3.3.

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

3.2.  Recommendations for UDP requestors

   *  UDP requestors SHOULD send DNS requests with IP_DONTFRAG /
      IPV6_DONTFRAG [RFC3542] options.

   *  UDP requestors MAY probe to discover the real MTU value per
      destination.  Then, calculate their maximum DNS/UDP payload size
      as the reported path MTU minus IPv4/IPv6 header size (20 or 40)
      minus UDP header size (8).  If the path MTU discovery failed or is
      impossible, use the default maximum DNS/UDP payload size described
      in Section 3.3.

   *  UDP requestors SHOULD use the requestor's payload size as the
      calculated or the default maximum DNS/UDP payload size.

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

   *  DNS responses may be dropped by IP fragmentation.  Upon a timeout,
      UDP requestors may retry using TCP or UDP, per local policy.

3.3.  Default Maximum DNS/UDP payload size

   Fragmentation avoidance is achieved with the IP(V6)_DONTFRAG option.
   The purpose of packet size limitation is to decrease packet loss due
   to the effects of the IP(V6)_DONTFRAG option.



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   Default maximum DNS/UDP payload size depends on the connectivity of
   each node, it cannot be determined unconditionally.  However, there
   are good proposed values.

   Operators MAY select a good number from Table 1.  Details of proposed
   values are described in Appendix B.

       +========================+=============+===================+
       |                 Source |        IPv4 | IPv6              |
       +========================+=============+===================+
       | Minimal: RFC 4035 MUST |        1220 | 1220              |
       +------------------------+-------------+-------------------+
       |  Software developers / |        1232 | 1232 (1280-40-8)  |
       | DNSFlagDay2020 propose |             |                   |
       +------------------------+-------------+-------------------+
       |               Authors' |        1400 | 1400 (1500 -40 -8 |
       |         recommendation |             | - some headers)   |
       +------------------------+-------------+-------------------+
       |  Maximum: Ethernet MTU |        1472 | 1452 (1500-40-8)  |
       |      1500 [Huston2021] | (1500-20-8) |                   |
       +------------------------+-------------+-------------------+
       |               Measured |   MTU -20-8 | MTU -40-8         |
       +------------------------+-------------+-------------------+

              Table 1: Default maximum DNS/UDP payload size

   However, operators of DNS servers SHOULD measure their path MTU to
   the Internet at setting up DNS servers (and when network
   configuration changes).

   How to measure path MTU is described in Appendix D.

   Operators of authoritative servers (that offer global DNS zones) and
   full-service resolvers (that access authoritative servers of the
   global DNS) SHOULD measure their path MTU to well-known locations on
   the Internet, such as [a-m].root-servers.net or [a-m].gtld-
   servers.net.

   Operators of full-service resolvers 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.  Or,
   operators of ISPs know their customers' connectivity and customers'
   MTU to ISPs' servers.  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.





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   DNS clients (stub resolvers) need to specify an appropriate
   requestor's payload size when supporting EDNS0.  In case of CPEs,
   embedded devices, and user devices, network operators can not control
   them, developers may choose small values such as 1220 and 1232.

   Other DNS servers are out-of-scope of this document.  (For example,
   Forwarding only resolvers, or private DNS).

4.  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
   necessary.

5.  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,

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

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

   *  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 E).

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

6.  Considerations

6.1.  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.




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   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
   servers.

7.  IANA Considerations

   This document has no IANA actions.

8.  Security Considerations

9.  Acknowledgments

   The author would like to specifically thank Paul Wouters, Mukund
   Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson,
   Puneet Sood and Jim Reid for extensive review and comments.

10.  References

10.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc3542>.

   [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,
              <https://www.rfc-editor.org/info/rfc4035>.





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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,
              <https://www.rfc-editor.org/info/rfc6891>.

   [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,
              <https://www.rfc-editor.org/info/rfc8201>.

   [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>.

   [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, <https://www.rfc-editor.org/info/rfc8899>.

   [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,
              <https://www.rfc-editor.org/info/rfc8900>.

10.2.  Informative References

   [Brandt2018]
              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.

   [DNSFlagDay2020]
              "DNS flag day 2020", n.d., <https://dnsflagday.net/2020/>.

   [Fujiwara2018]
              Fujiwara, K., "Measures against cache poisoning attacks
              using IP fragmentation in DNS", OARC 30 Workshop , 2019.

   [Herzberg2013]
              Herzberg, A. and H. Shulman, "Fragmentation Considered
              Poisonous", IEEE Conference on Communications and Network
              Security , 2013.




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   [Hlavacek2013]
              Hlavacek, T., "IP fragmentation attack on DNS", RIPE 67
              Meeting , 2013, <https://ripe67.ripe.net/
              presentations/240-ipfragattack.pdf>.

   [Huston2021]
              Huston, G. and J. Damas, "Measuring DNS Flag Day 2020",
              OARC 34 Workshop , February 2021.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [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,
              <https://www.rfc-editor.org/info/rfc5155>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

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.









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   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 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.

Appendix B.  Details of maximum DNS/UDP payload size discussions

   There are many discussions for default path MTU size and maximum DNS/
   UDP payload size.

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

   *  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, 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 fit in 1232 octets.  The



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      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], 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 Buffer size to
      IPv4 1472 octets and IPv6 1452 octets.

Appendix C.  How to retrieve path MTU value to a destination from
             applications

   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))

   Section 3.4 of [RFC1122] specifies FIND_MAXSIZES() as one of
   "INTERNET/TRANSPORT LAYER INTERFACEs".

Appendix D.  How to retrieve minimal MTU value to a destination

   The Linux tool "tracepath" can be used to measure the path MTU to a
   destination.

   Or, "ping/ping6" command with "-D" Don't Fragment bit set / Disable
   IPv6 fragmentation options.

Appendix E.  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.



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   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

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

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


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

   Phone: +1 650 393 3994
   Email: vixie@fsi.io


























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