draft-reddy-add-enterprise-split-dns-02.xml

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<rfc category="std" docName="draft-reddy-add-enterprise-split-dns-02"
     ipr="trust200902">
  <front>
    <title abbrev="Split-Horizon DNS Configuration">Split-Horizon DNS
    Configuration in Enterprise Networks</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>

    <date />

    <workgroup>ADD</workgroup>

    <abstract>
      <t>When split-horizon DNS is deployed by an enterprise, certain
      enterprise 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). 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><xref target="RFC7626"></xref> discusses DNS privacy considerations
      in both "on the wire" (Section 2.4 of <xref target="RFC7626"></xref>)
      and "in the server" (Section 2.5 of <xref target="RFC7626"></xref>)
      contexts. 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).</t>

      <t>This document specifies a mechanism to indicate which DNS zones are
      used for split-horizon DNS. DNS clients can discover and authenticate
      encrypted DNS servers provided by the Enterprise network, for example
      using the techniques proposed in [I-D.ietf-add-dnr] and
      [I-D.ietf-add-ddr]. Discovery of encrypted DNS server for roaming
      enterprise endpoints is discussed in <xref
      target="I-D.btw-add-ipsecme-ike"></xref> (see <xref
      target="VPN"></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).</t>

      <t>This document defines one PvD Key:<list style="hanging">
          <t hangText="The SplitDNSAllowed PvD Key:">which determines if the
          Enterprise network allows split-horizon DNS.</t>
        </list></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. The clients that connect to a
      enterprise network can securely authenticate that network and the client
      is sure that it has connected to the network it was expecting.</t>
    </section>

    <section anchor="Scope" title="Scope of the Document">
      <t>If a device is managed by an enterprise's IT department, the device
      can be configured to use a specific encrypted DNS server. This
      configuration may be manual or rely upon whatever deployed device
      management tool in an Enterprise network. For example, customizing
      Firefox using Group Policy to use the Enterprise DoH server is discussed
      in <xref target="Firefox-Policy"></xref> for Windows and MacOS, and
      setting Chrome policies is discussed in <xref
      target="Chrome-Policy"></xref> and <xref
      target="Chrome-DoH"></xref>.</t>

      <t>If mobile device management (MDM) (e.g., <xref
      target="MDM-Apple"></xref>) secures a device, MDM can configure
      OS/browser with a specific encrypted DNS server. If an endpoint is
      on-boarded, for example, using Over-The-Air (OTA) enrollment <xref
      target="OTA"></xref> to provision the device with a certificate and
      configuration profile, the configuration profile can include the
      authentication domain name (ADN) of the encrypted DNS server. The
      OS/Browser can use the configuration profile to use a specific encrypted
      DNS server. In this case, MDM is not installed on the device.</t>

      <t>Provisioning IT-managed devices, BYOD devices with MDM or
      configuration profile with the Split DNS configuration is outside the
      scope of this document.</t>

      <t>Typically, Enterprise networks do not assume that all devices in
      their network are managed by the IT team or MDM, especially in the quite
      common BYOD scenario. The endpoint can use the discovered
      network-designated DNS server to only access DNS names for which the
      Enterprise network claims authority and use another public DNS server
      for global domains or use the discovered network-designated DNS server
      to access both private domains and global domains.</t>

      <t>The scope of this document is restricted to unmanaged BYOD devices
      without a configuration profile and split DNS configuration on
      explicitly trusted networks. In this use case, the user has authorized
      the client to override local DNS settings for a specific network. It is
      similar to the way users explicitly disable VPN connection in specific
      networks and VPN connection is enabled by default in other networks for
      privacy. The unmanaged BYOD devices typically use the credentials (user
      name and password) provided by the IT admin to mutually authenticate to
      the Enterprise WLAN Access Point (e.g., PEAP-MSCHAPv2 <xref
      target="PEAP"></xref>, EAP-pwd <xref target="RFC8146"></xref>, EAP-PSK
      <xref target="RFC4764"></xref>).</t>

      <t><list style="hanging">
          <t hangText="Note: ">Many users have privacy and personal data
          sovereignty concerns with employers installing MDM on their personal
          devices; they are concerned that admin can glean personal
          information and could control how they use their devices. When users
          do not install MDM on their devices, IT admins do not get visibility
          into the security posture of those devices. To overcome this
          problem, a host agent can cryptographically attest the security
          status associated with device, such as minimum pass code length,
          biometric login enabled, OS version etc. This approach is fast
          gaining traction especially with the advent of closed OS like <xref
          target="win10s">Windows 10 in S mode</xref> or <xref
          target="Chromebook">Chromebook</xref>, where applications are
          sandboxed (e.g., ransomware attack is not possible) and applications
          can only be installed via the OS store.</t>
        </list></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>An Enterprise 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.</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 both the
      "dnsZones" and "SplitDNSAllowed" keys to define the DNS domains for
      which the Enterprise network claims authority and to indicate if the
      Enterprise network allows split-horizon DNS.</t>
    </section>

    <section anchor="dnsZones" title="PvD dnsZones">
      <t>As discussed in <xref target="split-horizon"></xref>, the Enterprise
      internal resources tend to have private DNS names. An enterprise 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
      Enterprise 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 Enterprise 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>

    <section anchor="SplitDNSAllowed"
             title="PvD SplitDNSAllowed and ErrorSplitDNSBlocked Keys">
      <t>If an Enterprise network restricts all the DNS queries to be sent to
      the network-designated DNS server, SplitDNSAllowed will be set to false.
      If SplitDNSAllowed is set to false, it implies the network will try to
      block DNS queries to DNS servers not provided by the network and the
      ErrorSplitDNSBlocked key MUST contain a human-friendly description for
      this block. This information is intended for human consumption (not
      automated parsing).</t>

      <t>Split DNS configurations may be preferable to sending all DNS queries
      to an network-designated DNS server in some deployments. This allows an
      endpoint to only send DNS queries for the enterprise to the
      network-designated DNS servers. The Enterprise remains unaware of all
      non-enterprise (DNS) activity of the user. It also allows the
      network-designated DNS servers to only be configured for the enterprise
      DNS domains, which removes the legal and technical responsibility of the
      enterprise to resolve every DNS domain potentially asked for by the
      endpoints.</t>

      <t>The SplitDNSAllowed key value set to false is a internal security
      policy expression by the operator of the network but is not a policy
      prescription to the endpoints to disable use of DNS servers not provided
      by the network. If SplitDNSAllowed is set to false, the client MUST NOT
      trust the SplitDNSAllowed key in case of connecting to unknown or
      untrusted networks (e.g., coffee shops or hotel networks). Most
      importantly, the endpoint can choose to use a alternate network to
      resolve the global domain names.</t>
    </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"],
     "SplitDNSAllowed": True, 
      "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="Roaming Enterprise Users">
      <t>In this Enterprise scenario (Section 1.1.3 of <xref
      target="RFC7296"></xref>), a roaming user connects to the Enterprise
      network through an VPN tunnel (e.g., IPsec, SSL, Wireguard). The
      split-tunnel Virtual Private Network (VPN) configuration allows the
      endpoint to access hosts that reside in the Enterprise network <xref
      target="RFC8598"></xref> using that tunnel; other traffic not destined
      to the Enterprise does not traverse the tunnel. In contrast, a
      non-split- tunnel VPN configuration causes all traffic to traverse the
      tunnel into the Enterprise.</t>

      <t>When the VPN tunnel is IPsec, the encrypted server hosted by the
      Enterprise network 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 Enterprise network claims authority.
      For non-split-tunnel VPN configurations, the endpoint uses the
      discovered encrypted DNS server to resolve both global and private
      domain names.</t>

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

    <section title="Upstream Encryption">
      <t>If an Enterprise network is using a local encrypted DNS server
      configured as a Forwarding DNS server <xref target="RFC8499"></xref>
      relying upon the upstream resolver (e.g., at an ISP) to perform
      recursive DNS lookups, DNS messages exchanged between the local
      encrypted DNS server and the recursive resolver MUST be encrypted.</t>

      <t>If the Enterprise network is using the local encrypted DNS server
      configured as a recursive DNS server, DNS messages exchanges between the
      recursive resolver and authoritative servers SHOULD be encrypted to
      conform to the requirements discussed in <xref
      target="I-D.ietf-dprive-phase2-requirements"></xref>.</t>
    </section>

    <section anchor="Security" title="Security Considerations">
      <t>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 Enterprise 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>

      <t>The content of dnsZones and SplitDNSAllowed may be passed to another
      (DNS) program for processing. As with any network input, the content
      SHOULD be considered untrusted and handled accordingly. The split DNS
      configuration assigned by an anonymous or unknown network (e.g., coffee
      shops) MUST be ignored by the client.</t>

      <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 [I-D.ietf-add-dnr] 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>
        </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 anchor="IANA" title="IANA Considerations">
      <t>IANA is requested to add SplitDNSAllowed and ErrorSplitDNSBlocked PvD
      Keys to the Additional Information PvD Keys registry
      (https://www.iana.org/assignments/pvds/pvds.xhtml).</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.7626' ?>

      <?rfc include='reference.RFC.7296' ?>

      <?rfc include='reference.RFC.8146' ?>

      <?rfc include='reference.RFC.4764' ?>

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

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

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

      <?rfc include='reference.I-D.ietf-dprive-phase2-requirements'?>

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

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

      <reference anchor="Firefox-Policy"
                 target="https://github.com/mozilla/policy-templates/blob/master/README.md#dnsoverhttps">
        <front>
          <title>Policy templates for Firefox</title>

          <author></author>

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

      <reference anchor="Chrome-Policy"
                 target="https://support.google.com/chrome/a/answer/2657289?hl=en">
        <front>
          <title>Chrome policies for users or browsers</title>

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

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

      <reference anchor="Chrome-DoH"
                 target="https://www.chromium.org/developers/dns-over-https">
        <front>
          <title>Chrome DNS over HTTPS (aka DoH)</title>

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

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

      <reference anchor="win10s"
                 target="https://www.microsoft.com/en-us/windows/s-mode">
        <front>
          <title>Windows 10 in S mode</title>

          <author>
            <organization>Microsoft</organization>
          </author>

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

      <reference anchor="Chromebook"
                 target="https://support.google.com/chromebook/answer/3438631?hl=en">
        <front>
          <title>Chromebook security</title>

          <author>
            <organization>Microsoft</organization>
          </author>

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

      <reference anchor="MDM-Apple"
                 target="https://developer.apple.com/documentation/devicemanagement">
        <front>
          <title>Mobile Device Management</title>

          <author>
            <organization>Apple</organization>
          </author>

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

      <reference anchor="OTA"
                 target="https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/iPhoneOTAConfiguration/OTASecurity/OTASecurity.html">
        <front>
          <title>Over-the-Air Profile Delivery Concepts</title>

          <author>
            <organization>Apple</organization>
          </author>

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

      <reference anchor="PEAP"
                 target="https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-peap/5308642b-90c9-4cc4-beec-fb367325c0f9">
        <front>
          <title>[MS-PEAP]: Protected Extensible Authentication Protocol
          (PEAP)</title>

          <author>
            <organization>Microsoft</organization>
          </author>

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

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