CoRE                                                       M. S. Lenders
Internet-Draft                                                 FU Berlin
Intended status: Standards Track                               C. Amsüss
Expires: 27 April 2023
                                                             C. Gündoğan
                                                     Huawei Technologies
                                                           T. C. Schmidt
                                                             HAW Hamburg
                                                             M. Wählisch
                                                               FU Berlin
                                                         24 October 2022


                          DNS over CoAP (DoC)
                    draft-ietf-core-dns-over-coap-01

Abstract

   This document defines a protocol for sending DNS messages over the
   Constrained Application Protocol (CoAP).  These CoAP messages are
   protected by DTLS-Secured CoAP (CoAPS) or Object Security for
   Constrained RESTful Environments (OSCORE) to provide encrypted DNS
   message exchange for constrained devices in the Internet of Things
   (IoT).

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (core@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/draft-dns-over-coap.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.






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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 27 April 2023.

Copyright Notice

   Copyright (c) 2022 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
   extracted from this document must include Revised BSD License text as
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Selection of a DoC Server . . . . . . . . . . . . . . . . . .   4
   4.  Basic Message Exchange  . . . . . . . . . . . . . . . . . . .   4
     4.1.  The "application/dns-message" Content-Format  . . . . . .   4
     4.2.  DNS Queries in CoAP Requests  . . . . . . . . . . . . . .   5
       4.2.1.  Request Format  . . . . . . . . . . . . . . . . . . .   5
       4.2.2.  Support of CoAP Caching . . . . . . . . . . . . . . .   5
       4.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  DNS Responses in CoAP Responses . . . . . . . . . . . . .   6
       4.3.1.  Response Codes and Handling DNS and CoAP errors . . .   6
       4.3.2.  Support of CoAP Caching . . . . . . . . . . . . . . .   6
       4.3.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   7
   5.  CoAP/CoRE Integration . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Observing the DNS Resource  . . . . . . . . . . . . . . .   7
     5.2.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Considerations for Unencrypted Use  . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  New "application/dns-message" Content-Format  . . . . . .   8
     8.2.  New "core.dns" Resource Type  . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  11



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     A.1.  Since draft-ietf-core-dns-over-coap-00  . . . . . . . . .  11
     A.2.  Since draft-lenders-dns-over-coap-04  . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   This document defines DNS over CoAP (DoC), a protocol to send DNS
   [RFC1035] queries and get DNS responses over the Constrained
   Application Protocol (CoAP) [RFC7252].  Each DNS query-response pair
   is mapped into a CoAP message exchange.  Each CoAP message is secured
   by DTLS [RFC9147] or Object Security for Constrained RESTful
   Environments (OSCORE) [RFC8613] to ensure message integrity and
   confidentiality.

   The application use case of DoC is inspired by DNS over HTTPS
   [RFC8484] (DoH).  DoC, however, aims for the deployment in the
   constrained Internet of Things (IoT), which usually conflicts with
   the requirements introduced by HTTPS.

   To prevent TCP and HTTPS resource requirements, constrained IoT
   devices could use DNS over DTLS [RFC8094].  In contrast to DNS over
   DTLS, DoC utilizes CoAP features to mitigate drawbacks of datagram-
   based communication.  These features include: block-wise transfer,
   which solves the Path MTU problem of DNS over DTLS (see [RFC8094],
   section 5); CoAP proxies, which provide an additional level of
   caching; re-use of data structures for application traffic and DNS
   information, which saves memory on constrained devices.

   To prevent resource requirements of DTLS or TLS on top of UDP (e.g.,
   introduced by DNS over QUIC [RFC9250]), DoC allows for lightweight
   end-to-end payload encryption based on OSCORE.

                   . FETCH coaps://[2001:db8::1]/
                  /
                 /
                CoAP request
   +--------+   [DNS query]   +--------+   DNS query    +--------+
   |  DoC   |---------------->|  DoC   |--- --- --- --->|  DNS   |
   | Client |<----------------| Server |<--- --- --- ---| Server |
   +--------+  CoAP response  +--------+  DNS response  +--------+
               [DNS response]

                      Figure 1: Basic DoC architecture

   The most important components of DoC can be seen in Figure 1: A DoC
   client tries to resolve DNS information by sending DNS queries
   carried within CoAP requests to a DoC server.  That DoC server may or



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   may not resolve that DNS information itself by using other DNS
   transports with an upstream DNS server.  The DoC server then replies
   to the DNS queries with DNS responses carried within CoAP responses.

2.  Terminology

   A server that provides the service specified in this document is
   called a "DoC server" to differentiate it from a classic "DNS
   server".  Correspondingly, a client using this protocol to retrieve
   the DNS information is called a "DoC client".

   The term "constrained nodes" is used as defined in [RFC7228].

   The terms "CoAP payload" and "CoAP body" are used as defined in
   [RFC7959], Section 2.

   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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Selection of a DoC Server

   In this document, it is assumed that the DoC client knows the DoC
   server and the DNS resource at the DoC server.  Possible options
   could be manual configuration of a URI [RFC3986] or CRI
   [I-D.ietf-core-href], or automatic configuration, e.g., using a CoRE
   resource directory [RFC9176], DHCP or Router Advertisement options
   [I-D.ietf-add-dnr].  Automatic configuration SHOULD only be done from
   a trusted source.

   When discovering the DNS resource through a link mechanism that
   allows describing a resource type (e.g., the Resource Type Attribute
   in [RFC6690]), the resource type "core.dns" can be used to identify a
   generic DNS resolver that is available to the client.

4.  Basic Message Exchange

4.1.  The "application/dns-message" Content-Format

   This document defines a CoAP Content-Format number for the Internet
   media type "application/dns-message".  This media type is defined as
   in [RFC8484] Section 6, i.e., a single DNS message encoded in the DNS
   on-the-wire format [RFC1035].  Both DoC client and DoC server MUST be
   able to parse contents in the "application/dns-message" format.





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4.2.  DNS Queries in CoAP Requests

   A DoC client encodes a single DNS query in one or more CoAP request
   messages that use the CoAP FETCH [RFC8132] method.  Requests SHOULD
   include an Accept option to indicate the type of content that can be
   parsed in the response.

   Since CoAP provides reliability of the message layer (e.g.  CON) the
   retransmission mechanism of the DNS protocol as defined in [RFC1035]
   is not needed.

4.2.1.  Request Format

   When sending a CoAP request, a DoC client MUST include the DNS query
   in the body of the CoAP request.  As specified in [RFC8132]
   Section 2.3.1, the type of content of the body MUST be indicated
   using the Content-Format option.  This document specifies the usage
   of Content-Format "application/dns-message" (details see
   Section 4.1).  A DoC server MUST be able to parse requests of
   Content-Format "application/dns-message".

4.2.2.  Support of CoAP Caching

   The DoC client SHOULD set the ID field of the DNS header always to 0
   to enable a CoAP cache (e.g., a CoAP proxy en-route) to respond to
   the same DNS queries with a cache entry.  This ensures that the CoAP
   Cache-Key (see [RFC8132] Section 2) does not change when multiple DNS
   queries for the same DNS data, carried in CoAP requests, are issued.

4.2.3.  Examples

   The following example illustrates the usage of a CoAP message to
   resolve "example.org.  IN AAAA" based on the URI
   "coaps://[2001:db8::1]/".  The CoAP body is encoded in "application/
   dns-message" Content Format.

   FETCH coaps://[2001:db8::1]/
   Content-Format: application/dns-message
   Accept: application/dns-message
   Payload: 00 00 01 20 00 02 00 00 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            01 00 01                                        [binary]









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4.3.  DNS Responses in CoAP Responses

   Each DNS query-response pair is mapped to a CoAP REST request-
   response operation.  DNS responses are provided in the body of the
   CoAP response.  A DoC server MUST be able to produce responses in the
   "application/dns-message" Content-Format (details see Section 4.1)
   when requested.  A DoC client MUST understand responses in
   "application/dns-message" format when it does not send an Accept
   option.  Any other response format than "application/dns-message"
   MUST be indicated with the Content-Format option by the DoC server.

4.3.1.  Response Codes and Handling DNS and CoAP errors

   A DNS response indicates either success or failure in the Response
   code of the DNS header (see [RFC1035] Section 4.1.1).  It is
   RECOMMENDED that CoAP responses that carry any valid DNS response use
   a "2.05 Content" response code.

   CoAP responses use non-successful response codes MUST NOT contain a
   DNS response and MUST only be used on errors in the CoAP layer or
   when a request does not fulfill the requirements of the DoC protocol.

   Communication errors with a DNS server (e.g., timeouts) SHOULD be
   indicated by including a SERVFAIL DNS response in a successful CoAP
   response.

   A DoC client might try to repeat a non-successful exchange unless
   otherwise prohibited.  The DoC client might also decide to repeat a
   non-successful exchange with a different URI, for instance, when the
   response indicates an unsupported Content-Format.

4.3.2.  Support of CoAP Caching

   The DoC server MUST ensure that any sum of the Max-Age value of a
   CoAP response and any TTL in the DNS response is less or equal to the
   corresponding TTL received from an upstream DNS server.  This also
   includes the default Max-Age value of 60 seconds (see [RFC7252],
   section 5.10.5) when no Max-Age option is provided.  The DoC client
   MUST then add the Max-Age value of the carrying CoAP response to all
   TTLs in a DNS response on reception and use these calculated TTLs for
   the associated records.

   The RECOMMENDED algorithm to assure the requirement for the DoC is to
   set the Max-Age option of a response to the minimum TTL of a DNS
   response and to subtract this value from all TTLs of that DNS
   response.  This prevents expired records unintentionally being served
   from an intermediate CoAP cache.  Additionally, it allows for the
   ETag value for cache validation, if it is based on the content of the



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   response, not to change even if the TTL values are updated by an
   upstream DNS cache.  If only one record set per DNS response is
   assumed, a simplification of this algorithm is to just set all TTLs
   in the response to 0 and set the TTLs at the DoC client to the value
   of the Max-Age option.

4.3.3.  Examples

   The following examples illustrate the replies to the query
   "example.org.  IN AAAA record", recursion turned on.  Successful
   responses carry one answer record including address
   2001:db8:1::1:2:3:4 and TTL 58719.

   A successful response:

   2.05 Content
   Content-Format: application/dns-message
   Max-Age: 58719
   Payload: 00 00 81 a0 00 01 00 01 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01 c0 0c 00 [binary]
            1c 00 01 00 01 37 49 00 10 20 01 0d b8 00 01 00 [binary]
            00 00 01 00 02 00 03 00 04                      [binary]

   When a DNS error (SERVFAIL in this case) is noted in the DNS
   response, the CoAP response still indicates success:

   2.05 Content
   Content-Format: application/dns-message
   Payload: 00 00 81 a2 00 01 00 00 00 00 00 00 07 65 78 61 [binary]
            6d 70 6c 65 03 6f 72 67 00 00 1c 00 01          [binary]

   When an error occurs on the CoAP layer, the DoC server SHOULD respond
   with an appropriate CoAP error, for instance "4.15 Unsupported
   Content-Format" if the Content-Format option in the request was not
   set to "application/dns-message" and the Content-Format is not
   otherwise supported by the server.

5.  CoAP/CoRE Integration

5.1.  Observing the DNS Resource

   There are use cases where updating a DNS record might be necessary on
   the fly.  Examples of this include e.g.  [RFC8490], Section 4.1.2,
   but just saving messages by omitting the query for a subscribed name
   might also be valid.  As such, the DNS resource MAY be observable as
   specified in [RFC7641].





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5.2.  OSCORE

   It is RECOMMENDED to carry DNS messages end-to-end encrypted using
   OSCORE [RFC8613].  The exchange of the security context is out of
   scope of this document.

6.  Considerations for Unencrypted Use

   While not recommended, DoC can be used without any encryption (e.g.,
   in very constrained environments where encryption is not possible or
   necessary).  It can also be used when lower layers provide secure
   communication between client and server.  In both cases, potential
   benefits of unencrypted DoC usage over classic DNS are e.g. block-
   wise transfer or alternative CoAP Content-Formats to overcome link-
   layer constraints.  For unencrypted DoC usage the ID field MUST not
   be set to a fixed value as suggested in Section 4.2.2, but changed
   with every query.

7.  Security Considerations

   When using unencrypted CoAP (see Section 6), setting the ID of a DNS
   message to 0 as specified in Section 4.2.2 opens the DNS cache of a
   DoC client to cache poisoning attacks via response spoofing.  Because
   of that, this documents requires the ID to be changed with every
   query when CoAP is not secured (see Section 6).

   For encrypted usage with DTLS or OSCORE the impact of a fixed ID on
   security is limited, as both harden against injecting spoofed
   responses.  Consequently, it is of little concern to leverage the
   benefits of CoAP caching by setting the ID to 0.

   TODO more security

8.  IANA Considerations

8.1.  New "application/dns-message" Content-Format

   IANA is requested to assign CoAP Content-Format ID for the DNS
   message media type in the "CoAP Content-Formats" sub-registry, within
   the "CoRE Parameters" registry [RFC7252], corresponding to the
   "application/dns-message" media type from the "Media Types" registry:

   Media-Type: application/dns-message

   Encoding: -

   Id: TBD




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   Reference: [TBD-this-spec]

8.2.  New "core.dns" Resource Type

   IANA is requested to assign a new Resource Type (rt=) Link Target
   Attribute, "core.dns" in the "Resource Type (rt=) Link Target
   Attribute Values" sub-registry, within the "CoRE Parameters" register
   [RFC6690].

   Attribute Value: core.dns

   Description: DNS over CoAP resource.

   Reference: [TBD-this-spec] Section 3

9.  References

9.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/rfc/rfc1035>.

   [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/rfc/rfc2119>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/rfc/rfc7228>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7252>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7641>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/rfc/rfc7959>.




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   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/rfc/rfc8132>.

   [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/rfc/rfc8174>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/rfc/rfc8613>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/rfc/rfc9147>.

9.2.  Informative References

   [I-D.ietf-add-dnr]
              Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for the
              Discovery of Network-designated Resolvers (DNR)", Work in
              Progress, Internet-Draft, draft-ietf-add-dnr-13, 13 August
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              add-dnr-13>.

   [I-D.ietf-core-href]
              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-11, 7 September 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              href-11>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/rfc/rfc6690>.







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   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/rfc/rfc8094>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/rfc8484>.

   [RFC8490]  Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              RFC 8490, DOI 10.17487/RFC8490, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8490>.

   [RFC9176]  Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
              P. van der Stok, "Constrained RESTful Environments (CoRE)
              Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
              2022, <https://www.rfc-editor.org/rfc/rfc9176>.

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,
              <https://www.rfc-editor.org/rfc/rfc9250>.

Appendix A.  Change Log

A.1.  Since draft-ietf-core-dns-over-coap-00
      (https://datatracker.ietf.org/doc/html/draft-ietf-core-dns-over-
      coap-00)

   *  SVGify ASCII art

   *  Move section on "DoC Server Considerations" (was Section 5.1) to
      its own draft (draft-lenders-dns-cns
      (https://datatracker.ietf.org/doc/draft-lenders-dns-cns/))

   *  Replace layer violating statement for CON with statement of fact

   *  Add security considerations on ID=0

A.2.  Since draft-lenders-dns-over-coap-04
      (https://datatracker.ietf.org/doc/html/draft-lenders-dns-over-
      coap-04)

   *  Removed change log of draft-lenders-dns-over-coap






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Acknowledgments

   TODO acknowledge.

Authors' Addresses

   Martine Sophie Lenders
   Freie Universität Berlin
   Takustrasse 9
   D-14195 Berlin
   Germany
   Email: m.lenders@fu-berlin.de


   Christian Amsüss
   Email: christian@amsuess.com


   Cenk Gündoğan
   Huawei Technologies
   Riesstrasse 25
   D-80992 Munich
   Germany
   Email: cenk.gundogan@huawei.com


   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   D-20099 Hamburg
   Germany
   Email: t.schmidt@haw-hamburg.de


   Matthias Wählisch
   Freie Universität Berlin
   Takustrasse 9
   D-14195 Berlin
   Germany
   Email: m.waehlisch@fu-berlin.de











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