draft-reddy-dnsop-dns-error-page-07.xml

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<rfc category="std" docName="draft-reddy-dnsop-error-page-07"
     ipr="trust200902">
  <front>
    <title abbrev="DNS Access Denied Error Page">DNS Access Denied Error
    Page</title>

    <author fullname="Tirumaleswar Reddy" initials="T." surname="Reddy">
      <organization abbrev="McAfee">McAfee, Inc.</organization>

      <address>
        <postal>
          <street>Embassy Golf Link Business Park</street>

          <city>Bangalore</city>

          <region>Karnataka</region>

          <code>560071</code>

          <country>India</country>
        </postal>

        <email>kondtir@gmail.com</email>
      </address>
    </author>

    <author fullname="Neil Cook" initials="N." surname="Cook">
      <organization>Open-Xchange</organization>

      <address>
        <postal>
          <street></street>

          <country>UK</country>
        </postal>

        <email>neil.cook@noware.co.uk</email>
      </address>
    </author>

    <author fullname="Dan Wing" initials="D." surname="Wing">
      <organization abbrev="Citrix">Citrix Systems, Inc.</organization>

      <address>
        <postal>
          <street></street>

          <country>USA</country>
        </postal>

        <email>dwing-ietf@fuggles.com</email>
      </address>
    </author>

    <author fullname="Mohamed Boucadair" initials="M." surname="Boucadair">
      <organization>Orange</organization>

      <address>
        <postal>
          <street></street>

          <city>Rennes</city>

          <code>35000</code>

          <country>France</country>
        </postal>

        <email>mohamed.boucadair@orange.com</email>
      </address>
    </author>

    <date />

    <workgroup>DNSOP WG</workgroup>

    <abstract>
      <t>When a DNS server filters a query, the response to such query conveys
      no detailed explanation that elaborates why that query was blocked,
      leading thus to end-user confusion. A solution to this problem is needed
      in order to enhance the user experience.</t>

      <t>This document defines a method to return an URI that explains the
      reason why a DNS query was filtered by a DNS server.</t>
    </abstract>
  </front>

  <middle>
    <section anchor="intro" title="Introduction">
      <t>DNS filters are deployed for a variety of reasons, including endpoint
      security, parental filtering, and filtering required by law enforcement.
      Some of these reasons are discussed in more detail below:</t>

      <t><list style="symbols">
          <t>Various network security services are provided by Enterprise
          networks to protect endpoints (e.g., Hosts including IoT devices).
          Network-based security solutions such as firewalls and Intrusion
          Prevention Systems (IPS) rely upon network traffic inspection to
          implement perimeter-based security policies. The network security
          services may, for example, prevent malware download, block known
          malicious domains, block phishing sites, etc. <vspace
          blankLines="1" />These network security services act on DNS queries
          originating from endpoints. For example, DNS firewalls, a method of
          expressing DNS response policy information inside specially
          constructed DNS zones, known as Response Policy Zones (RPZs) allows
          DNS servers to modify their DNS responses in real time in order to
          stop access to malware and phishing domains. Note that some of the
          commonly known types of malware are viruses, worms, trojans, bots,
          ransomware, backdoors, spyware, and adware.</t>

          <t>Network devices in a home network offer network security to
          protect the devices within the home network by performing DNS-based
          content filtering. The network security service may, for example,
          block access to specific domains to enforce parental control, block
          access to malware sites, etc.</t>

          <t>Internet Service Providers (ISPs) typically block access to some
          DNS domains due to a requirement imposed by an external entity
          (e.g., Law Enforcement Agency). Such blocking is performed using
          DNS-based content filtering.</t>
        </list></t>

      <t>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 <xref target="RFC8914"></xref>.
      Each of these methods have advantages and disadvantages that are
      discussed below:</t>

      <t><list style="numbers">
          <t>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. <list style="symbols">
              <t>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 forged DNS response directs the user towards a server
              hosted to display the block page which breaks the TLS
              connection. For web-browsing this then results in an HTTPS
              certificate error message indicating that a secure connection
              could not be established, which gives no information to the
              end-user about the reason for the error.<vspace
              blankLines="1" />The typical errors are "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), &ldquo;Safari can't verify
              the identity of the website&hellip;&rdquo; (Safari on
              MacOS)".</t>

              <t>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.</t>

              <t>An end user does not know why the connection was reset 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-level protection 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 (e.g. <xref
              target="Chrome-Install-Cert"></xref>). 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.</t>
            </list></t>

          <t>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.</t>

          <t>The extended error codes Blocked, Censored, and Filtered defined
          in Section 4 of <xref target="RFC8914"></xref> 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 <xref target="RFC8914"></xref> is returned by the DNS
          server, the client can identify the DNS response is forged together
          with the reason for HTTPS certificate error. <vspace
          blankLines="1" />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 &amp;
          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.</t>

          <t>The EXTRA-TEXT field of the EDE option defined in Section 2 of
          <xref target="RFC8914"></xref> can include additional textual
          information about the cause of the error, but the information could
          be provided in a language that is not understood by the user. When a
          resolver or forwarder forwards the received EDE option, the
          EXTRA-TEXT field only conveys the source of the error (Section 3 of
          <xref target="RFC8914"></xref>) and does not provide additional
          textual information about the cause of the error. Most importantly,
          EDE option does not offer authenticated information; it can thus be
          spoofed by an attacker. In addition, the additional textual
          information may not be able to convey all of the required
          information about the cause of the DNS error because lengthy
          EXTRA-TEXT content would be truncated to prevent fragmentation
          (Section 3 of <xref target="RFC8914"></xref>).</t>
        </list></t>

      <t>No matter which type of response is generated (forged IP address(es),
      NXDOMAIN or empty answer, or 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 an URI which, when accessed, provides such information to the
      user.</t>

      <t>One of the other benefits of this approach is to eliminate the need
      to "spoof" block pages for HTTPS resources. This is achieved as the
      block page no longer needs to create a signed certificate when blocking
      a destination. This approach avoids the need to install a local root
      certificate authority on those IT-managed devices.</t>
    </section>

    <section anchor="notation" title="Terminology">
      <t>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
      <xref target="RFC2119"></xref><xref target="RFC8174"></xref> when, and
      only when, they appear in all capitals, as shown here.</t>

      <t>This document makes use of the terms defined in <xref
      target="RFC8499"></xref>.</t>

      <t>'Encrypted DNS' refers to any encrypted scheme to convey DNS
      messages, for example, DNS-over-HTTPS <xref target="RFC8484"></xref>,
      DNS-over-TLS <xref target="RFC7858"></xref>, or DNS-over-QUIC <xref
      target="I-D.ietf-dprive-dnsoquic"></xref>.</t>
    </section>

    <section anchor="format-sec" title="Error Page URI EDNS0 Option Format">
      <t>This document uses an EDNS0 <xref target="RFC6891"></xref> option to
      include the URI that provides additional information in a DNS response
      about the cause of blocking access to a requested domain. This option is
      structured as depicted in <xref target="format"></xref>.</t>

      <figure align="center" anchor="format"
              title="Error Page URI EDNS0 Option Format">
        <artwork align="center"><![CDATA[                                             1   1   1   1   1   1
     0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                         OPTION-CODE                           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                        OPTION-LENGTH                          |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |                ERROR-PAGE-URI-LENGTH  (fixed size)            |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   /                 ERROR-PAGE-URI (variable size)                /
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
]]></artwork>
      </figure>

      <t></t>

      <t>The description of the fields is as follows:</t>

      <t><list style="symbols">
          <t>OPTION-CODE: TBD, indicates the code assigned for Error Page URI
          (Section 6.1.2 of <xref target="RFC6891"></xref>). [RFC Editor:
          change TBD to the proper code once assigned by IANA.]</t>

          <t>OPTION-LENGTH: See Section 6.1.2 of <xref
          target="RFC6891"></xref>. This field contains the length of the
          payload (everything after OPTION-LENGTH) in octets. The variability
          of the option length stems from the variable-length ERROR-PAGE-URI
          field.</t>

          <t>ERROR-PAGE-URI-LENGTH: This 16-bit field indicates the length of
          ERROR-PAGE-URI. It MUST NOT be set to 0.</t>

          <t>ERROR-PAGE-URI: A variable length UTF-8 encoded <xref
          target="RFC5198"></xref> text field containing the URI Template
          <xref target="RFC6570"></xref> that gives additional information
          about the cause of blocking access to a domain. The ERROR-PAGE-URI
          field MUST NOT be zero octets in length.</t>
        </list></t>

      <t>The Error Page URI option can be included in any response (SERVFAIL,
      NXDOMAIN, REFUSED, and even NOERROR, etc.) to a query that includes OPT
      Pseudo-RR <xref target="RFC6891"></xref>.</t>

      <t>The URI Template defined in ERROR-PAGE-URI describes how to construct
      the URL to fetch the error page. The agent acting as the HTTPS client on
      the endpoint encodes an FQDN to which access is denied into an HTTP GET
      request to retrieve the error page. The HTTPS server returning the error
      page defines the URI used by the HTTP GET request through the use of a
      URI Template. The URI Template is processed with a defined variable
      "target-domain" whose value is set to the FQDN to which access is
      denied.</t>

      <t>The FQDN is encoded using base64url <xref target="RFC4648"></xref>
      and then provided as the variable value for "target-domain" to expand
      the URI Template into an URI reference in the HTTP GET request. Padding
      characters for base64url MUST NOT be included.</t>

      <t>An example is illustrated below:<list style="empty">
          <t>If the URI Template is
          "https://resolver.example.net/block-page{?target-domain}" for the
          HTTPS server returning the error page and access to the target
          domain "example.com" is blocked by the encrypted DNS server, the
          variable "target-domain" has the value "example.com" base64url
          encoded into an HTTP GET request. In the above example, the
          expansion of the above URI Template is
          "https://resolver.example.net/block-page?target-domain=ZXhhbXBsZS5jb20".</t>
        </list></t>

      <t>HTTP/2 <xref target="RFC7540"></xref> is the minimum RECOMMENDED HTTP
      version to use to retrieve the error page. The HTTPS client retrieving
      the error page MUST verify the entire certification path as per <xref
      target="RFC5280"></xref>. The HTTPS client additionally uses validation
      techniques described in <xref target="RFC6125"></xref> to compare the
      domain name in the error page URI to the server certificate provided in
      TLS handshake. See <xref target="RFC7525"></xref> for additional TLS
      recommendations.</t>
    </section>

    <section title="Error Page URI Processing">
      <t>The DNS client MUST follow the rules below to process the Error Page
      URI EDNS0 option:<list style="symbols">
          <t>If the DNS response contains more than one Error Page URI EDNS0
          option, the DNS client MUST discard all Error Page URI EDNS0 options
          in the DNS response.</t>

          <t>The Error Page URI EDNS0 option MUST be processed by the DNS
          client for a "Censored", &ldquo;Blocked", &ldquo;Filtered&rdquo; or
          &ldquo;Forged" extended error codes and MUST be ignored for any
          other type of extended DNS error code. When "Censored",
          &ldquo;Blocked&rdquo;, "Filtered&rdquo; or &ldquo;Forged&rdquo;
          extended error code is returned in conjunction with an Error Page
          URI EDNS0 option, any other resource records in the answer MUST be
          ignored by clients supporting this specification.</t>

          <t>The DNS client MUST reject the error page URI if the scheme is
          not "https".</t>
        </list></t>

      <section title="Mitigating EDNS0 Forgery">
        <t>The Error Page URI EDNS0 option is susceptible to forgery. An
        attacker (e.g., a man in the middle (MITM)) could insert an extended
        Error Page URI EDNS0 option into the DNS response causing a client to
        attempt to visit that URI. For instance, the attacker can be located
        between the stub resolver and DNS recursive server or between the DNS
        proxy and the upstream resolver. To mitigate that attack, the
        following measures are enforced:</t>

        <t><list style="symbols">
            <t>The DNS client MUST NOT process the DNS response with Error
            Page URI EDNS0 option unless DNS messages exchanged are
            cryptographically protected using encrypted DNS.</t>

            <t>If a DNS client has enabled opportunistic privacy profile
            (Section 5 of <xref target="RFC8310"></xref>) 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 Error Page URI
            EDNS0 option in responses, but SHOULD process other parts of the
            response.</t>

            <t>If a DNS client has enabled strict privacy profile (Section 5
            of <xref target="RFC8310"></xref>) 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
            can process the DNS response with Error Page URI EDNS0 option.
            Note that the strict and opportunistic privacy profiles as defined
            in <xref target="RFC8310"></xref> only applies to DoT protocol,
            there has been no such distinction made for DoH protocol.</t>

            <t>If the DNS client determines that the encrypted DNS server does
            not offer DNS filtering service, it MUST reject the Error Page URI
            EDNS0 option. 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
            <xref target="I-D.reddy-add-resolver-info"></xref>.</t>

            <t>DNS forwarders (or DNS proxies) are supposed to propagate
            unknown EDNS0 options (Sections 4.1 and 4.4.1 of <xref
            target="RFC5625"></xref>), the Error Page URI EDNS0 option may get
            propagated by a DNS server that has not implemented this
            specification. To detect this scenario, the DNS client MUST verify
            the domain name in the Error Page URI matches the domain name of
            the encrypted DNS resolver. If this match fails, the DNS client
	    MUST ignore Error Page URI EDNS0 option in the response, but
	    SHOULD process other parts of the response.</t>
	</list></t>
      </section>
    </section>

    <section title="Error Page">
      <t>The following outlines the RECOMMENDED contents of an error page to
      assist the operator developing the error page:</t>

      <t><list style="symbols">
          <t>The exact reason for blocking access to the domain. If the domain
          is blocked based on some threat data, the threat type associated
          with the blocked domain can be provided/displayed to the end user.
          For example, the reason can indicate the type of malware blocked
          like spyware and the damage it can do the security and privacy of
          the user.</t>

          <t>The domain name blocked.</t>

          <t>If query was blocked by regulation, a pointer to a regulatory
          text that mandates this query block.</t>

          <t>The entity (or organization) blocking the access to the domain
          and contact details of the IT/InfoSec team to raise a complaint.</t>

          <t>The blocked error page to not include Ads and dynamic
          content.</t>
        </list>The content of the error page discussed above is non-normative,
      the above text only provides the guidelines and template for the error
      page and:<list style="symbols">
          <t>does not attempt to offer an exhaustive list for the contents of
          an error page.</t>

          <t>it is not intended to form the basis of any legal/compliance for
          developing the error page.</t>
        </list></t>
    </section>

    <section title="Usability Considerations">
      <t>The error page SHOULD be returned in the user's preferred language as
      expressed by the Accept-Language HTTP header.</t>
    </section>

    <section anchor="Security" title="Security Considerations">
      <t>Security considerations in Section 6 of <xref
      target="RFC8914"></xref> and <xref target="RFC8624"></xref> need to be
      taken into consideration.</t>

      <t>The Error Page URI EDNS0 option causes an HTTPS retrieval by the
      client. To prevent forgery of the Error Page URI EDNS0 option, this
      specification requires it only be sent only over an encrypted DNS
      channel with an authorized DNS server.</t>

      <t>The client knows it is connecting to a HTTPS server returning the
      error page. To reduce threat surface the client can retrieve the Error
      Page URL using, for example, an isolated environment and take other
      precautions such as clearly labeling the page as untrusted or prevent
      user interaction with the page. Such isolation should prevent
      transmitting cookies, block JavaScript, block auto-fill of credentials
      or personal information, and be isolated from the user's normal
      environment.</t>

      <t>Browsers perform some of the above restrictions when accessing
      captive portals (Section 5 of <xref target="RFC8910"></xref> or <xref
      target="Safari-Cookie"></xref>), during private browsing, or using
      containerization <xref target="Facebook-Container"></xref>.</t>

      <t>Note that the means to use a sandbox environment and a user interface
      presenting the error page are not covered in this document. By its
      nature, these aspects are implementation specific and best left to the
      application and user interface designers.</t>

      <t>The encrypted DNS session provides transport security for the
      interaction between the DNS client and server, but DNSSEC signing and
      validation is not possible for the Error Page URI EDNS0 option returning
      the Error Page URI Template. However, this specification mandates the
      DNS client to not process DNS response with Error Page URI EDNS0 option
      if domain name in the Error Page URI does not match the domain name of
      the encrypted DNS server. The validation ensures both the servers are
      operated by the same entity and have the same origin (similar to the
      Same Origin Policy (SOP)).</t>

      <t>By design, the object referenced by the error page URL potentially
      exposes additional information about the DNS resolution process that may
      leak information. An example of this is the reason for blocking the
      access to the domain name and the entity blocking access to the
      domain.</t>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <section title="A New Error Page URI EDNS Option">
        <t>This document defines a new EDNS(0) option, entitled "Error Page
        URI", assigned a value of TBD from the "DNS EDNS0 Option Codes (OPT)"
        registry [to be removed upon publication:
        [http://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-11]</t>

        <t><figure>
            <artwork><![CDATA[Value  Name                 Status    Reference
-----  ----------------     ------    ------------------
 TBD   Error Page URI      Standard       [ This document ]]]></artwork>
          </figure></t>
      </section>
    </section>

    <section title="Acknowledgements">
      <t>Thanks to Vittorio Bertola, Wes Hardaker, Ben Schwartz, Erid Orth,
      Viktor Dukhovni, Warren Kumari and Bob Harold for the comments.</t>
    </section>
  </middle>

  <!--  *****BACK MATTER ***** -->

  <back>
    <references title="Normative References">
      <?rfc include='reference.RFC.2119'?>

      <?rfc include='reference.RFC.8174'?>

      <?rfc include='reference.RFC.4648'?>

      <?rfc include='reference.RFC.7540'?>

      <?rfc include='reference.RFC.6891'?>

      <?rfc include='reference.RFC.8310'?>

      <?rfc include='reference.RFC.8624'?>

      <?rfc include='reference.RFC.5198'?>

      <?rfc include='reference.RFC.5280'?>

      <?rfc include='reference.RFC.6125'?>

      <?rfc include='reference.RFC.7525'?>

      <?rfc include='reference.RFC.5625'?>

      <?rfc include='reference.RFC.6570'?>

      <?rfc include='reference.RFC.8914' ?>
    </references>

    <references title="Informative References">
      <?rfc include='reference.RFC.8499'?>

      <?rfc include='reference.RFC.8484'?>

      <?rfc include='reference.RFC.7858'?>

      <?rfc include='reference.RFC.8910'?>

      <?rfc include='reference.I-D.ietf-dprive-dnsoquic'?>

      <?rfc include='reference.I-D.reddy-add-resolver-info'?>

      <reference anchor="Chrome-Install-Cert"
                 target="support.securly.com/hc/en-us/articles/206081828-How-to-manually-install-the-Securly-SSL-certificate-in-Chrome">
        <front>
          <title>How to manually install the Securly SSL certificate in
          Chrome</title>

          <author></author>

          <date day="" month="" year="" />
        </front>
      </reference>

      <reference anchor="Facebook-Container"
                 target="https://www.mozilla.org/en-US/firefox/facebookcontainer/">
        <front>
          <title>Facebook container for Firefox</title>

          <author></author>

          <date day="" month="" year="" />
        </front>
      </reference>

      <reference anchor="Safari-Cookie"
                 target="https://support.apple.com/en-us/HT205732">
        <front>
          <title>Isolated cookie store (CVE-2016-1730)</title>

          <author></author>

          <date day="" month="" year="" />
        </front>
      </reference>
    </references>
  </back>
</rfc>