draft-reddy-add-enterprise-split-dns-07.xml

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<rfc category="std" docName="draft-reddy-add-enterprise-split-dns-07"
     ipr="trust200902">
  <front>
    <title abbrev="Split-Horizon DNS Configuration">Split-Horizon DNS
    Configuration</title>

    <author fullname="Tirumaleswar Reddy" initials="T." surname="Reddy">
      <organization>Akamai</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 rather
      than a public 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, 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>The scope of the specification is split-horizon DNS names, which are
      not domains reserved for special use like ".local". Using these domains
      are difficult because bookmarks, advertising, and human behavior would
      need to change so that the special domains only work when connected to
      the correct network, which is difficult for users to discern on most
      endpoints. Furthermore, as special domains cannot obtain certificates
      from a public CA, authenticated TLS connections to those servers are
      fraught with (Trust on First Use) and certificate warnings, a
      longstanding problem in the industry.</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>
    </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 that requires 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>
    </section>

    <section anchor="dnsZones" title="Provisioning Domains dnsZones">
      <t>As discussed in <xref target="split-horizon"></xref>, internal
      resources in a network tend to have private DNS names. A 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>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. 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, and which are
      intended to be resolved using network-designated DNS servers. The
      private domains in dnsZones are only reachable by devices attached and
      authenticated 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>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>

      <t><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 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 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 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>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. A DNSSEC-validating
            client SHOULD apply the same validation policy to the NS query as
            it does to other queries. A client that does not validate DNSSEC
            SHOULD apply the same policy (if any) to the NS query as it does
            to other queries.</t>

            <t>The client checks that the NS RRset matches any one of the
            Authentication Domain Names (ADNs) of the discovered
            network-designated encrypted DNS resolvers. <list style="letters">
                <t>If the match fails and no ADN of the discovered
                network-designated encrypted DNS resolvers is a subdomain of
                NS RRset, 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 fails but any one of the ADNs of the
                discovered network-designated encrypted DNS resolvers is a
                subdomain of NS RRset, the client can then establish a TLS
                connection to that network-designated resolver. The client
                follows the mechanism discussed in Section 8 of <xref
                target="RFC8310"></xref> to authenticate the DNS server
                certificate using the ADN and checks if at least one of the
                values in subjectAltName matches NS RRset. If the server
                certificate validation and match succeeds, the client can
                subsequently resolve the domains in that subtree using the
                network-designated resolver. If the server certificate does
                not validate or match fails, the client can proceed as
                discussed in step 2 (A).</t>

                <t>If the match succeeds, the client can then establish a TLS
                connection to that network-designated resolver. The client
                follows the mechanism discussed in Section 8 of <xref
                target="RFC8310"></xref> to authenticate the DNS server
                certificate using the ADN.<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 proceed as discussed
                    in step 2 (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>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.</t>
          </list></t>

        <t>For example, if in a 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 of Split-Horizon DNS Configuration">
      <t>In a network the apex domain name is "example.com", which runs a
      different version of its global domain on its internal network. Today,
      on the Internet it publishes two NS records, "ns1.example.com" and
      "ns2.example.com".</t>

      <t>To add support for the mechanism described in this document, the
      network and endpoints first need to support <xref
      target="I-D.ietf-add-dnr"></xref> and <xref target="RFC8801"></xref>.
      Then, for each site the administrator would add DNS server names which
      end in "ns1.example.com" or "ns2.example.com" (the names published on
      the Internet). Also on its internal network, it uses a geographic naming
      scheme for its internal nameservers with a "service dot location" naming
      scheme. Assuming its two geographies are "us" and "uk". So the UK site
      would add "ns1.uk.ns1.example.com" and "ns2.uk.ns2.example.com". Note
      that these names can be added in addition to more traditional names that
      might already exist for those DNS servers (e.g., the common "service dot
      location" such as "ns1.uk.example.com"). All of those names would be
      advertised to the endpoints as described in <xref
      target="I-D.ietf-add-dnr"></xref>.</t>

      <t>The endpoints compliant with this specification can then determine
      the network's internal nameservers are owned and managed by the same
      entity that has published the NS records on the Internet, as described
      in the following paragraphs:</t>

      <t>Continuing with the UK site as the example, the endpoint will join
      the network, obtain an IPv6 address, and using <xref
      target="I-D.ietf-add-dnr"></xref> will discover the resolvers
      "ns1.uk.ns1.example.com" and "ns2.uk.ns2.example.com" and their IP
      addresses. Using <xref target="RFC8801"></xref> (which utilizes IPv6
      Router Advertisments), the endpoint will also discover the PvD FQDN is
      "pvd.uk.example.com".</t>

      <t>Continuing with the UK site as the example, the endpoint performs the
      following steps corresponding with the call flow diagram numbers:</t>

      <t><list style="hanging">
          <t hangText="Steps 1-2:">The client joins the network, obtain an IP
          address, and using <xref target="I-D.ietf-add-dnr"></xref> will
          discover the resolvers "ns1.uk.ns1.example.com" and
          "ns2.uk.ns1.example.com" and their IP addresses. Using <xref
          target="RFC8801"></xref>, the endpoint will also discover the PvD
          FQDN is "pvd.uk.example.com".</t>

          <t hangText="Steps 3-7:">The client establishes an encrypted DNS
          connection with "ns1.uk.ns1.example.com", validates its TLS
          certificate, and queries it for "pvd.uk.example.com" to retrieve the
          PvD JSON object. Note that <xref target="RFC8801"></xref> in Section
          4.1 mandates the PvD FQDN MUST be resolved using the DNS servers
          indicated by the associated PvD. The PvD contains:<figure>
              <artwork><![CDATA[ {
    "identifier": "pvd.uk.example.com",   
    "expires": "2020-05-23T06:00:00Z",
    "prefixes": ["2001:db8:1::/48", "2001:db8:4::/48"],
    "dnsZones:": ["example.com"]
  }]]></artwork>
            </figure>The JSON keys "identifier", "expires", and "prefixes" are
          defined in <xref target="RFC8801"></xref>.</t>

          <t hangText="Steps 8-9:">The client then uses an encrypted DNS
          connection to a public resolver (e.g., 1.1.1.1) to issue NS queries
          for the domains in dnsZones. The NS lookup for "example.com" will
          return "ns1.example.com" and "ns2.example.com".</t>

          <t hangText="Step 10:">As the network-provided nameservers are
          subdomains of those names retrieved from the public resolver and the
          network-designated resolver's certificate includes at least one of
          the names retrieved from the public resolver, the client has
          finished validation that the nameservers signaled in <xref
          target="I-D.ietf-add-dnr"></xref> and <xref target="RFC8801"></xref>
          are owned and managed by the same entity that published the NS
          records on the Internet. The endpoint will then use that information
          from <xref target="I-D.ietf-add-dnr"></xref> and <xref
          target="RFC8801"></xref> to resolve names within dnsZones.</t>
        </list></t>

      <t><xref target="poex"></xref> is a simple call flow diagram of the
      example discussed above.</t>

      <figure anchor="poex"
              title="An Example of Split-Horizon DNS Configuration">
        <artwork><![CDATA[+---------+                   +-------------------+  +------------+ +---------+ +---------+
| client  |                   | Network           |  | Network    | | Router  | | public  |
|         |                   | encrypted resolvr |  | PvD server | |         | | resolvr |
+---------+                   +-------------------+  +------------+ +---------+ +---------+
     |                                          |         |          |           |
     | Router Solicitation (1)                  |         |          |           |
     |-------------------------------------------------------------->|           |
     |                                          |         |          |           |
     |       Router Advertisement with DNR hostnames & PvD FQDN (2)  |           |
     |<--------------------------------------------------------------|           |
     | -------------------------------------\   |         |          |           |
     |-| now knows DNR hostnames & PvD FQDN |   |         |          |           |
     | |------------------------------------|   |         |          |           |
     |                                          |         |          |           |
     | TLS connection (3)                       |         |          |           |
     |----------------------------------------->|         |          |           |
     | ---------------------------\             |         |          |           |
     |-| validate TLS certificate |             |         |          |           |
     | |--------------------------|             |         |          |           |
     |                                          |         |          |           |
     | resolve pvd.uk.example.com  (4)          |         |          |           |
     |----------------------------------------->|         |          |           |
     |                                          |         |          |           |
     |                         AAAA records (5) |         |          |           |
     |<-----------------------------------------|         |          |           |
     |                                          |         |          |           |
     | https://pvd.uk.example.com/.well-known/pvd (6)     |          |           |
     |--------------------------------------------------->|          |           |
     |                                          |         |          |           |
     |       200 OK (JSON Additional Information) (7)     |          |           |
     |<---------------------------------------------------|          |           |
     | -----------------------\                 |         |          |           |
     |-| dnsZones=example.com |                 |         |          |           |
     | |----------------------|                 |         |          |           |
     |                                          |         |          |           |
     | TLS connection                           |         |          |           |
     |-------------------------------------------------------------------------->|
     | ---------------------------\             |         |          |           |
     |-| validate TLS certificate |             |         |          |           |
     | |--------------------------|             |         |          |           |
     |                                          |         |          |           |
     | NS? example.com  (8)                     |         |          |           |
     |-------------------------------------------------------------------------->|
     |                                          |         |          |           |
     |                               NS=ns1.example.com, ns2.example.com (9)     |
     |<--------------------------------------------------------------------------|
     | -------------------------------\         |         |          |           |
     |-| ns1.uk.ns1.example.com is    |         |         |          |           |
     | | subdomain of ns1.example.com |         |         |          |           |
     | ----------------------\--------|         |         |          |           |
     |-| finished validation |                  |         |          |           |
     | |---------------------|                  |         |          |           |
     |                                          |         |          |           |
     |  use network-designated resolver         |         |          |           |   
     |  for example.com (10)                    |         |          |           |
     |----------------------------------------->|         |          |           |
     |                                          |         |          |           |
]]></artwork>
      </figure>
    </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 <xref
      target="RFC8598"></xref> via the tunnel; other traffic not destined to
      the VPN does not traverse the tunnel. In contrast, a non-split- tunnel
      VPN configuration causes all traffic to traverse the tunnel into the
      VPN.</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 not, the client can take appropriate action like
      disconnecting from the network.</t>

      <t>In order to deploy DNSSEC operationally for split-horizon domains,
      the first three schemes described in <xref
      target="I-D.krishnaswamy-dnsop-dnssec-split-view"></xref> can be used
      but not the one discussed in Section 4.1.4. Alternatively, after
      validating the network authority over the dnsZones domains <xref
      target="Authority"></xref>, a DNSSEC-validating client can reconfigure
      its stub resolver to disable DNSSEC validation for the domains in
      dnsZones.</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, Paul Wouters 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'?>

      <?rfc include='reference.RFC.8310'?>
    </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.I-D.ietf-add-dnr'?>

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

      <?rfc include='reference.I-D.krishnaswamy-dnsop-dnssec-split-view'?>

      <?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>