DNSOP WG                                                         D. Wing
Internet-Draft                                                    Citrix
Updates: 8914 (if approved)                                     T. Reddy
Intended status: Standards Track                                  Akamai
Expires: 29 October 2022                                         N. Cook
                                                            M. Boucadair
                                                           27 April 2022

                    Structured Data for Filtered DNS


   DNS filtering is widely deployed for network security, but filtered
   DNS responses lack information for the end user to understand the
   reason for the filtering.  Existing mechanisms to provide detail to
   end users cause harm especially if the blocked DNS response is to an
   HTTPS website.

   This document updates RFC8914's EXTRA-TEXT field to provide
   information on DNS filtering.  This information can be parsed by the
   client and displayed, logged, or used for other purposes.

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/.

   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 29 October 2022.

Copyright Notice

   Copyright (c) 2022 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 (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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  I-JSON in EXTRA-TEXT field  . . . . . . . . . . . . . . . . .   6
   4.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Client Generating Request . . . . . . . . . . . . . . . .   6
     4.2.  Server Generating Response  . . . . . . . . . . . . . . .   7
     4.3.  Client Processing Response  . . . . . . . . . . . . . . .   7
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   DNS filters are deployed for a variety of reasons including endpoint
   security, parental filtering, and filtering required by law
   enforcement.  Network-based security solutions such as firewalls and
   Intrusion Prevention Systems (IPS) rely upon network traffic
   inspection to implement perimeter-based security policies and operate
   by filtering DNS responses.  In a home, DNS filtering is used for the
   same reasons as above and additionally for parental control.
   Internet Service Providers typically block access to some DNS domains
   due to a requirement imposed by an external entity (e.g., law
   enforcement agency) also performed using DNS-based content filtering.

   Users of DNS services which perform filtering may wish to receive
   more information about such filtering to resolve problems with the
   filter -- for example to contact the administrator to allowlist a
   domain that was erroneously filtered or to understand the reason a
   particular domain was filtered.  With that information, the user can
   choose another network, open a trouble ticket with the DNS
   administrator to resolve erroneous filtering, log the information, or
   other uses.

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   DNS responses can be filtered by sending a bogus (also called,
   "forged") A or AAAA response, NXDOMAIN error or empty answer, or an
   extended DNS error (EDE) code defined in [RFC8914].  Each of these
   methods have advantages and disadvantages that are discussed below:

   1.  The DNS response is forged to provide a list of IP addresses that
       points to an HTTP(S) server alerting the end user about the
       reason for blocking access to the requested domain (e.g.,
       malware).  When an HTTP(S) enabled domain name is blocked, the
       network security device (e.g., CPE, firewall) presents a block
       page instead of the HTTP response from the content provider
       hosting that domain.  If an HTTP enabled domain name is blocked,
       the network security device intercepts the HTTP request and
       returns a block page over HTTP.  If an HTTPS enabled domain is
       blocked, the block page is also served over HTTPS.  In order to
       return a block page over HTTPS, man in the middle (MITM) is
       enabled on endpoints by generating a local root certificate and
       an accompanying (local) public/private key pair.  The local root
       certificate is installed on the endpoint while the network
       security device(s) stores a copy of the private key.  During the
       TLS handshake, the network security device modifies the
       certificate provided by the server and (re)signs it using the
       private key from the local root certificate.

       *  However, configuring the local root certificate on endpoints
          is not a viable option in several deployments like home
          networks, schools, Small Office/Home Office (SOHO), and Small/
          Medium Enterprise (SME).  In these cases, the typical behavior
          is that the filtered DNS response points to a server that will
          display the block page.  If the client is using HTTPS (via web
          browser or another application) this results in a certificate
          validation error which gives no information to the end-user
          about the reason for the DNS filtering.  Browsers will display
          errors such as "The security certificate presented by this
          website was not issued by a trusted certificate authority"
          (Internet Explorer/Edge"), "The site's security certificate is
          not trusted" (Chrome), "This Connection is Untrusted"
          (Firefox), "Safari can't verify the identity of the
          website..." (Safari on MacOS).  Applications might display
          even more cryptic error messages.

       *  Enterprise networks do not assume that all the connected
          devices are managed by the IT team or Mobile Device Management
          (MDM) devices, especially in the quite common Bring Your Own
          Device (BYOD) scenario.  In addition, the local root
          certificate cannot be installed on IoT devices without a
          device management tool.

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       *  An end user does not know why the connection was prevented
          and, consequently, may repeatedly try to reach the domain but
          with no success.  Frustrated, the end user may switch to an
          alternate network that offers no DNS filtering against malware
          and phishing, potentially compromising both security and
          privacy.  Furthermore, certificate errors train users to click
          through certificate errors, which is a bad security practice.
          To eliminate the need for an end user to click through
          certificate errors, an end user may manually install a local
          root certificate on a host device.  Doing so, however, is also
          a bad security practice as it creates a security vulnerability
          that may be exploited by a MITM attack.  When a manually
          installed local root certificate expires, the user has to
          (again) manually install the new local root certificate.

   2.  The DNS response is forged to provide a NXDOMAIN response to
       cause the DNS lookup to terminate in failure.  In this case, an
       end user does not know why the domain cannot be reached and may
       repeatedly try to reach the domain but with no success.
       Frustrated, the end user may use insecure connections to reach
       the domain, potentially compromising both security and privacy.

   3.  The extended error codes Blocked, Censored, and Filtered defined
       in Section 4 of [RFC8914] can be returned by a DNS server to
       provide additional information about the cause of an DNS error.
       If the extended error code "Forged Answer" defined in Section 4.5
       of [RFC8914] is returned by the DNS server, the client can
       identify the DNS response is forged together with the reason for
       HTTPS certificate error.

   4.  These extended error codes do not suffer from the limitations
       discussed in bullets (1) and (2), but the user still does not
       know the exact reason nor he/she is aware of the exact entity
       blocking the access to the domain.  For example, a DNS server may
       block access to a domain based on the content category such as
       "Adult Content" to enforce parental control, "Violence &
       Terrorism" due to an external requirement imposed by an external
       entity (e.g., Law Enforcement Agency), etc.  These content
       categories cannot be standardized because the classification of
       domains into content categories is vendor specific, typically
       ranges from 40 to 100 types of categories depending on the vendor
       and the categories keep evolving.  Furthermore, the threat data
       used to categorize domains may sometimes misclassify domains
       (e.g., domains wrongly classified as Domain Generation Algorithm
       (DGA) by deep learning techniques, domain wrongly classified as
       phishing due to crowd sourcing, new domains not categorized by
       the threat data).  A user needs to know the contact details of
       the IT/InfoSec team to raise a complaint.

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   5.  When a resolver or forwarder forwards the received EDE option,
       the EXTRA-TEXT field only conveys the source of the error
       (Section 3 of [RFC8914]) and does not provide additional textual
       information about the cause of the error.

   For both DNS filtering mechanisms described above, the DNS server can
   return extended error codes Blocked, Censored, Filtered, or Forged
   Answer defined in Section 4 of [RFC8914].  However, these codes only
   explain that filtering occurred but lack detail for the user to
   diagnose erroneous filtering.

   No matter which type of response is generated (forged IP address(es),
   NXDOMAIN or empty answer, even with an extended error code), the user
   who triggered the DNS query has little chance to understand which
   entity filtered the query, how to report a mistake in the filter, or
   why the entity filtered it at all.  This document describes a
   mechanism to provide such detail.

   One of the other benefits of this approach is to eliminate the need
   to "spoof" block pages for HTTPS resources.  This is achieved since
   clients implementing this approach would be able to display a
   meaningful error message, and would not need to connect to such a
   block page.  This approach thus avoids the need to install a local
   root certificate authority on those IT-managed devices.

   This document describes a format for computer-parsable data in the
   EXTRA-TEXT field of Extended DNS Errors [RFC8914].

   This document does not recommend DNS filtering but provides a
   mechanism for better transparency to explain to the users why some
   DNS queries are filtered.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   This document uses terms defined in DNS Terminology [RFC8499].

   "Requestor" refers to the side that sends a request.  "Responder"
   refers to an authoritative, recursive resolver or other DNS component
   that responds to questions.  Other terminology is used here as
   defined in the RFCs cited by this document.

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   "Encrypted DNS" refers to any encrypted scheme to convey DNS
   messages, for example, DNS-over-HTTPS [RFC8484], DNS-over-TLS
   [RFC7858], or DNS-over-QUIC [I-D.ietf-dprive-dnsoquic].

3.  I-JSON in EXTRA-TEXT field

   Servers compliant with this specification send I-JSON data in the
   EXTRA-TEXT field [RFC8914] using the Internet JSON (I-JSON) message
   format [RFC7493].

      |  Note that [RFC7493] was based on [RFC7159], but [RFC7159] was
      |  replaced by [RFC8259].

   This document defines the following JSON names:

   c: (contact)  The contact details of the IT/InfoSec team to report
      mis-classified DNS filtering.  This field is structured as an
      array of contact URIs (e.g., tel, sips, https).  At least one
      contact URI MUST be included.  This field is mandatory.

   j: (justification)  the textual justification for this particular DNS
      filtering.  This field is mandatory.

   o: (organization)  human-friendly name of the organization that
      filtered this particular DNS query.  This field is optional.

   New JSON names MUST be defined in the IANA registry (Section 7),
   consist only of lower-case ASCII characters, digits, and hyphens
   (that is, Unicode characters U+0061 through 007A, U+0030 through
   U+0039, and U+002D).  These names MUST be 63 characters or shorter
   and it is RECOMMENDED they be as short as possible.

   To reduce packet overhead the generated JSON SHOULD be as short as
   possible: short domain names, concise text in the values for the "j"
   and "o" names, and minified JSON (that is, without spaces or line
   breaks between JSON elements).

   The JSON data can be parsed to display to the user, logged, or
   otherwise used to assist the end-user or IT staff with
   troubleshooting and diagnosing the cause of the DNS filtering.

4.  Protocol Operation

4.1.  Client Generating Request

   When generating a DNS query, the client MUST include the OPT pseudo-
   RR [RFC6891] to elicit the Extended DNS Error option [RFC8914] in the
   DNS response.

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4.2.  Server Generating Response

   When the DNS server filters its DNS response to an A or AAAA record
   query, the DNS response MAY contain an empty answer, NXDOMAIN, or a
   forged A or AAAA response, as desired by the DNS server.  In
   addition, if the query contained the OPT pseudo-RR the DNS server MAY
   return more detail in the EXTRA-TEXT field as described in
   Section 4.3.

   Servers may decide to return small TTL values in filtered DNS
   responses (e.g., 2 seconds) to handle domain category and reputation

4.3.  Client Processing Response

   On receipt of a DNS response with an Extended DNS Error option, the
   following actions are performed if the EXTRA-TEXT field contains
   valid JSON:

   *  The response MUST be received over an encrypted DNS channel.  If
      not, the requestor MUST discard data in the EXTRA-TEXT field.

   *  The response MUST be received from a DNS server which advertised
      EDE support via RESINFO [I-D.reddy-add-resolver-info].

   *  Servers which don't support this specification might use plain
      text in the EXTRA-TEXT field so that requestors SHOULD properly
      handle both plaintext and JSON text in the EXTRA-TEXT field.

   *  The DNS response MUST also contain an extended error code of
      "Censored", "Blocked", "Filtered" or "Forged" [RFC8914], otherwise
      the EXTRA-TEXT field is discarded.

   *  If either of the mandatory JSON names "c" and "j" are missing or
      have empty values in the EXTRA-TEXT field, the entire JSON is

   *  If a DNS client has enabled opportunistic privacy profile
      (Section 5 of [RFC8310]) for DoT, the DNS client will either
      fallback to an encrypted connection without authenticating the DNS
      server provided by the local network or fallback to clear text
      DNS, and cannot exchange encrypted DNS messages.  Both of these
      fallback mechanisms adversely impacts security and privacy.  If
      the DNS client has enabled opportunistic privacy profile for DoT,
      the DNS client MUST ignore the EXTRA-TEXT field of the EDE
      responses, but SHOULD process other parts of the response.

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   *  If a DNS client has enabled strict privacy profile (Section 5 of
      [RFC8310]) for DoT, the DNS client requires an encrypted
      connection and successful authentication of the DNS server; this
      mitigates both passive eavesdropping and client redirection (at
      the expense of providing no DNS service if an encrypted,
      authenticated connection is not available).  If the DNS client has
      enabled strict privacy profile for DoT, the client MAY process the
      EXTRA-TEXT field of the DNS response.  Note that the strict and
      opportunistic privacy profiles as defined in [RFC8310] only apply
      to DoT; there has been no such distinction made for DoH.

   *  If the DNS client determines that the encrypted DNS server does
      not offer DNS filtering service, it MUST discard the EXTRA-TEXT
      field of the EDE response.  For example, the DNS client can learn
      whether the encrypted DNS resolver performs DNS-based content
      filtering or not by retrieving resolver information using the
      method defined in [I-D.reddy-add-resolver-info].

   *  When a forwarder receives an EDE option, whether or not (and how)
      to pass along JSON information in the EXTRA-TEXT on to their
      client is implementation dependent [RFC5625].  Implementations MAY
      choose to not forward the JSON information, or they MAY choose to
      create a new EDE option that conveys the information in the "c"
      and "j" fields encoded in the JSON object.

5.  Examples

   An example showing the nameserver at 'ns.example.net' that filtered a
   DNS "A" record query for 'example.org' is shown in Figure 1.

     "c": ["tel:+358-555-1234567", "sips:bob@bobphone.example.com",
       "j": "malware present for 23 days",
       "o": "example.net Filtering Service"

     Figure 1: JSON returned in EXTRA-TEXT field of Extended DNS Error

   In Figure 2 the same content is shown with minified JSON (no
   whitespace, no blank lines) with '\' line wrapping per [RFC8792].

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     ============== NOTE: '\' line wrapping per RFC 8792 ===============

     {"c":["tel:+358-555-1234567","sips:bob@bobphone.example.com", \
     "https://ticket.example.com?d=example.org&t=1650560748"], \
     "j":"malware present for 23 days","o":"example.net Filtering \

                        Figure 2: Minified response

6.  Security Considerations

   Security considerations in Section 6 of [RFC8914] apply to this

   To minimize impact of active on-path attacks on the DNS channel, the
   client validates the response as described in Section 4.3.

   A client might choose to display the information in the EXTRA-TEXT
   field if and only if the encrypted resolver has sufficient
   reputation, according to some local policy (e.g. user configuration,
   administrative configuration, or a built-in list of respectable
   resolvers).  This limits the ability of a malicious encrypted
   resolver to cause harm.  If the client decides not to display the all
   of the information in the EXTRA-TEXT field, it can be logged for
   diagnostics purpose and the client can only display the resolver
   hostname that blocked the domain and error description for the EDE
   code to the end-user.

   When displaying the free-form text of "c" and "j", the browser SHOULD
   NOT make any of those elements into actionable (clickable) links.

   An attacker might inject (or modify) the EDE EXTRA-TEXT field with an
   DNS proxy or DNS forwarder that is unaware of EDE.  Such a DNS proxy
   or DNS forwarder will forward that attacker-controlled EDE option.
   To prevent such an attack, clients supporting this document MUST
   discard the EDE option if their DNS server does not signal EDE
   support via RESINFO [I-D.reddy-add-resolver-info].  As recommended in
   [I-D.reddy-add-resolver-info], RESINFO should be retrieved over an
   encrypted DNS channel or integrity protected with DNSSEC.

7.  IANA Considerations

   This document requests IANA to register the "application/
   json+structured-dns-error" media type in the "Media Types" registry
   [IANA-MediaTypes].  This registration follows the procedures
   specified in [RFC6838]:

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      Type name: application

      Subtype name: json+structured-dns-error

      Required parameters: N/A

      Optional parameters: N/A

      Encoding considerations: as defined in Section NN of [RFCXXXX].

      Security considerations: See Section NNN of [RFCXXXX].

      Interoperability considerations: N/A

      Published specification: [RFCXXXX]

      Applications that use this media type: Section NNNN of [RFCXXXX].

      Fragment identifier considerations: N/A

      Additional information: N/A

      Person & email address to contact for further information: IETF,

      Intended usage: COMMON

      Restrictions on usage: none

      Author: See Authors' Addresses section.

      Change controller: IESG

      Provisional registration?  No

8.  Changes

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

8.1.  Changes from 02 to 03

   *  Require using RESINFO [I-D.reddy-add-resolver-info] in client
      processing and added discussion of attack mitigation of using

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   *  Removed validation of URI domain suffix, which we can't do for
      some URLs (e.g., tel:), is difficult/impossible for others when
      3rd party is handling level one support (e.g., sips:).  Instead
      rely on RESINFO telling us if EDE is supported by the DNS server
      and, if so, expect it to properly support EDE rather than blindly
      forward an unknown DNS option.

   *  Removed 'partial URI' text

8.2.  Changes from 01 to 02

   *  repurpose Extended DNS Error's EXTRA-TEXT field to carry JSON,
      which also means this document updates RFC8914

   *  clarified DNS forwarders might forward EXTRA-TEXT without change
      or might rewrite "j" and "d"

8.3.  Changes from 00 to 01

   *  removed support for multiple responsible parties

   *  one-character JSON names to minimize JSON length

   *  partial URI sent in "c" and "r" names, combined with "d" name sent
      in JSON to minimize attack surface and minimize JSON length

   *  moved EDNS(0) forgery-mitigation text, some Security
      Considerations text, and some other text from
      [I-D.reddy-dnsop-error-page] to this document

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

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

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   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,

   [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>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018,

9.2.  Informative References

              Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", Work in Progress, Internet-
              Draft, draft-ietf-dprive-dnsoquic-12, 20 April 2022,

              Reddy, T. and M. Boucadair, "DNS Resolver Information",
              Work in Progress, Internet-Draft, draft-reddy-add-
              resolver-info-05, 13 April 2022,

              Reddy, T., Cook, N., Wing, D., and M. Boucadair, "DNS
              Access Denied Error Page", Work in Progress, Internet-
              Draft, draft-reddy-dnsop-error-page-08, 4 June 2021,

              IANA, "Media Types",

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
              BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,

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   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [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>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,

Authors' Addresses

   Dan Wing
   Citrix Systems, Inc.
   United States of America
   Email: dwing-ietf@fuggles.com

   Tirumaleswar Reddy
   Email: kondtir@gmail.com

   Neil Cook
   United Kingdom
   Email: neil.cook@noware.co.uk

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   Mohamed Boucadair
   35000 Rennes
   Email: mohamed.boucadair@orange.com

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