draft-reddy-add-split-dns-00.xml

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<rfc category="std" docName="draft-reddy-add-split-dns-00" ipr="trust200902">
  <front>
    <title abbrev="Split-Horizon DNS Configuration">Split-Horizon DNS
    Configuration</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="Dan Wing" initials="D." surname="Wing">
      <organization abbrev="Citrix">Citrix Systems, Inc.</organization>

      <address>
        <postal>
          <street>4988 Great America Pkwy</street>

          <city>Santa Clara</city>

          <region>CA</region>

          <code>95054</code>

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

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

    <author fullname="Kevin Smith" initials="K." surname="Smith">
      <organization abbrev="Vodafone">Vodafone Group</organization>

      <address>
        <postal>
          <street>One Kingdom Street</street>

          <city>London</city>

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

        <email>kevin.smith@vodafone.com</email>
      </address>
    </author>

    <date />

    <workgroup>ADD</workgroup>

    <abstract>
      <t>When split-horizon DNS is deployed by a network, certain domains are
      only resolvable by querying the network-designated DNS server. DNS
      clients which use DNS servers not provided by the network need to route
      those DNS domain queries to the network-designated DNS server. This
      document informs DNS clients of split-horizon DNS, their DNS domains,
      and is compatible with encrypted DNS.</t>
    </abstract>
  </front>

  <middle>
    <section anchor="intro" title="Introduction">
      <t>Historically, an endpoint would utilize network-designated DNS
      servers upon joining a network (e.g., DHCP OFFER, IPv6 Router
      Advertisement). While it has long been possible to configure endpoints
      to ignore the network's suggestions and use a (public) DNS server on the
      Internet, this was seldom used because some networks block UDP/53 (in
      order to enforce their own DNS policies). Also, there has been an
      increase in the availability of "public resolvers" <xref
      target="RFC8499"></xref> which DNS clients may be pre-configured to use
      instead of the default network resolver for a variety of reasons (e.g.,
      offer a good reachability, support an encrypted transport, provide a
      claimed privacy policy, (lack of) filtering). With the advent of DoT and
      DoH, such network blocking is more difficult, but the endpoint is unable
      to (properly) resolve split-horizon DNS domains which must query the
      network-designated DNS server.</t>

      <t>This document specifies a mechanism to indicate which DNS zones are
      used for split-horizon DNS. DNS clients can discover and authenticate
      DNS servers provided by the network, for example using the techniques
      proposed in <xref target="I-D.ietf-add-dnr"></xref> and <xref
      target="I-D.ietf-add-ddr"></xref>.</t>

      <t>Provisioning Domains (PvDs) are defined in <xref
      target="RFC7556"></xref> as sets of network configuration information
      that clients can use to access networks, including rules for DNS
      resolution and proxy configuration. <xref target="RFC8801"></xref>
      defines a mechanism for discovering multiple Explicit PvDs on a single
      network and their Additional Information by means of an HTTP-over-TLS
      query using a URI derived from the PvD ID. This set of additional
      configuration information is referred to as a Web Provisioning Domain
      (Web PvD). The "dnsZones" PvD key defined in <xref
      target="RFC8801"></xref> is used to define the DNS domains for which the
      network claims authority.</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>. The terms "Private DNS", "Global DNS" and
      "Split DNS" are defined in <xref target="RFC8499"></xref>.</t>

      <t>'Encrypted DNS' refers to a DNS protocol that provides an encrypted
      channel between a DNS client and server (e.g., DoT, DoH, or DoQ).</t>

      <t>The term "enterprise network" in this document extends to a wide
      variety of deployment scenarios. For example, an "enterprise" can be a
      Small Office, Home Office or Corporation.</t>
    </section>

    <section anchor="split-horizon" title="Split DNS">
      <t><xref target="RFC2826"></xref> "does not preclude private networks
      from operating their own private name spaces" but notes that if private
      networks "wish to make use of names uniquely defined for the global
      Internet, they have to fetch that information from the global DNS naming
      hierarchy".</t>

      <t>There are various DNS deployments outside of the global DNS,
      including "split horizon" deployments and DNS usages on private (or
      virtual private) networks. In a split horizon, an authoritative server
      gives different responses to queries from the Internet than they do to
      network-designated DNS servers; while some deployments differentiate
      internal queries from public queries by the source IP address, the
      concerns in Section 3.1.1 of <xref target="RFC6950"></xref> relating to
      trusting source IP addresses apply to such deployments.</t>

      <t>When the internal address space range is private <xref
      target="RFC1918"></xref>, this makes it both easier for the server to
      discriminate public from private and harder for public entities to
      impersonate nodes in the private network. The use cases that motivate
      split-horizon DNS typically involve restricting access to some network
      services -- intranet resources such as internal web sites, development
      servers, or directories, for example -- while preserving the ease of use
      offered by domain names for internal users.</t>

      <t>A typical use case is an Enterprise network can require one or more
      DNS domains to be resolved via network-designated DNS servers. This can
      be a special domain, such as "corp.example.com" for an enterprise that
      is publicly known to use "example.com". In this case, the endpoint needs
      to be informed what the private domain names are and what the IP
      addresses of the network-designated DNS servers are. An Enterprise can
      also run a different version of its global domain on its internal
      network. In that case, the client is instructed to send DNS queries for
      the enterprise public domain (e.g., "example.com") to the
      network-designated DNS servers. A configuration for this deployment
      scenario is referred to as a Split DNS configuration. Another use case
      for split-horizon DNS is Cellular and Fixed-access networks (ISPs)
      typically offer private domains, including account status/controls, and
      free education initiatives <xref target="INS"></xref>.</t>

      <t>The PvD RA option defined in <xref target="RFC8801"></xref> SHOULD
      set the H-flag to indicate that Additional Information is available.
      This Additional Information JSON object SHOULD include the "dnsZones"
      key to define the DNS domains for which the network claims
      authority.</t>
    </section>

    <section anchor="dnsZones" title="PvD dnsZones">
      <t>As discussed in <xref target="split-horizon"></xref>, internal
      resources in an network tend to have private DNS names. An network can
      also run a different version of its global domain on its internal
      network, and require the use of network-designated DNS servers to get
      resolved.</t>

      <t>The PvD Key dnsZones is defined in <xref target="RFC8801"></xref>.
      The PvD Key dnsZones adds support for DNS domains for which the network
      claims authority. The private domains specified in the dnsZones key are
      intended to be resolved using network-designated DNS servers. The
      private domains in dnsZones are only reachable by devices authenticated
      or attached to the network. The global domains specified in the dnsZones
      key have a different version in the internal network. DNS resolution for
      other domains remains unchanged.</t>

      <t>The dnsZones PvD Key conveys the specified DNS domains that need to
      be resolved using an network-designated DNS server. The DNS root zone
      (".") MUST be ignored if it appears in dnsZones. Other generic or global
      domains, such as Top-Level Domains (TLDs), similarly MUST be ignored if
      they appear in dnsZones.</t>

      <t>For each dnsZones entry, the client can use the network-designated
      DNS servers to resolve the listed domains and its subdomains. Other
      domain names may be resolved using some other DNS servers that are
      configured independently. For example, if the dnsZones key specifies
      "example.test", then "example.test", "www.example.test", and
      "mail.eng.example.test" can be resolved using the network-designated DNS
      resolver(s), but "otherexample.test" and "ple.test" can be resolved
      using the system's public resolver(s).</t>

      <section anchor="Authority" title="Authority over the Domains">
        <t>To comply with <xref target="RFC2826"></xref> the split-horizon DNS
        zone must either not exist in the global DNS hierarchy or must be
        authoritatively delegated to the split-horizon DNS server to answer.
        The client can use the mechanism described in <xref
        target="I-D.ietf-add-dnr"></xref> to discover the network-designated
        resolvers. To determine if the network-designated encrypted resolvers
        are authoritative over the domains in DnsZones, the client performs
        the following steps for each domain in DnsZones:</t>

        <t><list style="numbers">
            <t>The client sends an NS query for the domain in DnsZones. This
            query MUST only be sent over encrypted DNS session to a public
            resolver that is configured independently or to a
            network-designated resolver whose response will be validated using
            DNSSEC as described in <xref target="RFC6698"></xref>.</t>

            <t>The client checks that the NS RRset matches, or is a subdomain
            of, any one of the ADN of the discovered network-designated
            encrypted DNS resolvers. <list style="letters">
                <t>If the match fails, the client determines the network is
                not authoritative for the indicated domain. It might log an
                error, reject the network entirely (because the network lied
                about its authority over a domain) or other action.</t>

                <t>If the match succeeds, the client can then establish a
                secure connection to that network-designated resolver and
                validate its certificate. <list style="symbols">
                    <t>If the server certificate does not validate and a
                    secure connection cannot be established to the network
                    designated resolver, the client can action as discussed in
                    step 3 (A).</t>

                    <t>If the server certificate validation is successful and
                    a secure connection is established, the client can
                    subsequently resolve the domains in that subtree using the
                    network-designated resolver.</t>
                  </list></t>
              </list></t>

            <t>As an exception to this rule, the client need not perform the
            above validation for domains reserved for special use <xref
            target="RFC6761"></xref> or <xref target="RFC6762"></xref> such as
            ".home.arpa" or ".local".</t>

            <t>If the client uses a public resolver, authenticated denial of
            existence using NSEC3 or NSEC records can be used by a client to
            identify that the domain name does not exist in the global DNS.
            <xref target="RFC7149"></xref> recommends validation of responses
            using NSEC3.</t>
          </list></t>

        <t>For example, if in an network the private domain names are defined
        under "internal.corp1.example.com". The DnsZones PvD Key would
        indicate that "*.internal.corp1.example.com" are private domain names.
        The client can trigger a NS query of "internal.corp1.example.com" and
        the NS RRset returns that the nameserver is "ns1.corp2.example.com".
        The client would then connect to the network-designated encrypted
        resolver whose name is "ns1.corp2.example.com", authenticate it using
        server certificate validation in TLS handshake, and use it for
        resolving the domains in the subtree of
        "*.internal.corp1.example.com".</t>
      </section>
    </section>

    <section title="An Example">
      <t>The following example shows how the JSON keys defined in this
      document can be used:</t>

      <t><figure>
          <artwork><![CDATA[   {
     "identifier": "cafe.example.com.",
     "expires": "2020-05-23T06:00:00Z",
     "prefixes": ["2001:db8:1::/48", "2001:db8:4::/48"],
     "dnsZones:": ["city.other.test", "example.com"]
   }]]></artwork>
        </figure></t>

      <t>The JSON keys "identifier", "expires", and "prefixes" are defined in
      <xref target="RFC8801"></xref>.</t>
    </section>

    <section anchor="VPN" title="Split DNS Configuration for IKEv2">
      <t>The split-tunnel Virtual Private Network (VPN) configuration allows
      the endpoint to access resources that reside in the VPN network <xref
      target="RFC8598"></xref> via the tunnel; other traffic not destined to
      the VPN network does not traverse the tunnel. In contrast, a non-split-
      tunnel VPN configuration causes all traffic to traverse the tunnel into
      the VPN network.</t>

      <t>When the VPN tunnel is IPsec, the encrypted DNS resolver hosted by
      the VPN service provider can be securely discovered by the endpoint
      using the ENCDNS_IP*_* IKEv2 Configuration Payload Attribute Types
      defined in <xref target="I-D.btw-add-ipsecme-ike"></xref>. For
      split-tunnel VPN configurations, the endpoint uses the discovered
      encrypted DNS server to resolve domain names for which the VPN provider
      claims authority. For non-split-tunnel VPN configurations, the endpoint
      uses the discovered encrypted DNS server to resolve both global and
      private domain names. For split-tunnel VPN configurations, the IKE
      client can use the steps discussed in <xref target="Authority"></xref>
      to determine if the VPN service provider is authoritative over the
      INTERNAL_DNS_DOMAIN domains.</t>

      <t>Other VPN tunnel types have similar configuration capabilities, not
      detailed here.</t>
    </section>

    <section anchor="Security" title="Security Considerations">
      <t>The content of dnsZones may be passed to another (DNS) program for
      processing. As with any network input, the content SHOULD be considered
      untrusted and handled accordingly. The client must perform the steps
      discussed in <xref target="Authority"></xref> to determine if the
      network-designated encrypted resolvers are authoritative over the
      domains in DnsZones. If the network is lying, the client can take
      appropriate action like disconnecting from the network.</t>

      <t>As an additional precaution, clients may want to preconfigure global
      domains for TLDs and Second-Level Domains (SLDs) to prevent malicious
      DNS redirections for well-known domains. This prevents users from
      unknowingly giving DNS queries to third parties. This is even more
      important if those well-known domains are not deploying DNSSEC, as the
      attached network could then even modify the DNS answers without
      detection. It is similar to the mechanism discussed in Section 8 of
      <xref target="RFC8598"></xref>.</t>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>This document has no IANA actions..</t>
    </section>

    <section title="Acknowledgements">
      <t>Thanks to Mohamed Boucadair, Jim Reid, Ben Schwartz, Tommy Pauly,
      Paul Vixie and Vinny Parla for the discussion and comments. The authors
      would like to give special thanks to Ben Schwartz for his help.</t>
    </section>
  </middle>

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

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

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

      <?rfc include='reference.RFC.8801'?>

      <?rfc include='reference.RFC.2826'?>

      <?rfc include='reference.RFC.1918'?>

      <?rfc include='reference.RFC.6761'?>

      <?rfc include='reference.RFC.6762'?>
    </references>

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

      <?rfc include='reference.RFC.8598'?>

      <?rfc include='reference.RFC.6950' ?>

      <?rfc include='reference.RFC.7556' ?>

      <?rfc include='reference.RFC.6698'?>

      <?rfc include='reference.RFC.7149'?>

      <?rfc include='reference.I-D.ietf-add-dnr'?>

      <?rfc include='reference.I-D.btw-add-ipsecme-ike'?>

      <?rfc include='reference.I-D.ietf-add-ddr'?>

      <!-- not yet in XML library           <?rfc include='reference.I-D.ietf-add-ddr'?> 
           <?rfc include='reference.I-D.ietf-add-dnr'?> -->

      <reference anchor="INS"
                 target="https://www.vodafone.com/about/vodafone-foundation/focus-areas/instant-schools">
        <front>
          <title>Vodafone Foundation Instant Schools for Sub-Saharan
          Africa</title>

          <author>
            <organization>The Unicode Consortium</organization>
          </author>

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

      <!---->
    </references>
  </back>
</rfc>