draft-reddy-add-enterprise-split-dns-01.xml

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<rfc category="std" docName="draft-reddy-add-enterprise-split-dns-01"
     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-provided
      DNS server. DNS clients which use DNS servers not provided by the
      network need to route those DNS domain queries to the network-provided
      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-provided 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-provided 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-provided 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-provided 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-provided 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-provided 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-provided 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-provided 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-provided 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. These domains are intended to be
      resolved using network-provided DNS servers that are only reachable to
      the devices attached to the Enterprise 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-provided DNS server. The private-only PvD
      Key conveys whether the domain is private or global but with a different
      version to the devices attached to the network. 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-provided DNS
      servers to resolve the listed domains and its subdomains. Other domain
      names may be resolved using some other public resolvers 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-provided 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 Key">
      <t>If an Enterprise network restricts all the DNS queries to be sent to
      the network-provided DNS server, SplitDNSAllowed will be set to
      false.</t>

      <t>Split DNS configurations may be preferable to sending all DNS queries
      to an network-provided DNS server in some deployments. This allows an
      endpoint to only send DNS queries for the enterprise to the
      network-provided DNS servers. The Enterprise remains unaware of all
      non-enterprise (DNS) activity of the user. It also allows the
      network-provided 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>If SplitDNSAllowed is set to false, the client should not trust the
      SplitDNSAllowed key in case of connecting to unknown or untrusted
      networks (e.g., coffee shops or hotel networks). For example, if
      SplitDNSAllowed is set to false, client 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:": [{
          "name": "city.other.test",
          "private-only": true
      },{
          "name": "example.com",
          "private-only": false
      }]
   }]]></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.</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. To
      determine if the zone does not exist, the client can query a public
      resolver to identify if the domains specified in the dnsZones Key do not
      exist in the global DNS. Authenticated denial of existence using NSEC3
      or NSEC records can be used by a client to validate that a certain
      domain name does not exist in the global DNS. <xref
      target="RFC7149"></xref> recommends validation of responses using NSEC3.
      To handle scenario where the public resolver is not reachable (e.g.,
      resolver is down or blocked by the network), the client can store the
      top well-known domain names and TLDs (e.g., <xref target="TOP"></xref>)
      locally (e.g., using Bloom filters) to determine if the entries in the
      dnsZones Key do not exist in the global DNS. The local database of
      well-known domain names and TLDs needs to be updated periodically.</t>

      <t>An enterprise can run a different version of its public domain on its
      internal network. If the domain specified in the dnsZones Key exists in
      the global DNS, the client needs to identify if any one of the
      network-provided encrypted resolvers [I-D.ietf-add-dnr] is authoritative
      of the domain. The client can trigger a NS query of the domain and if
      the NS RRset matches the ADN of any one of the discovered
      network-provided encrypted DNS resolvers, the client can identify that
      the network-provided resolver whose ADN matches the NS response is
      authoritative for that domain. The client can then establish a secure
      connection to authenticate the network-provided resolver and
      subsequently resolve the domains in the subtree using the
      network-provided resolver.</t>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>IANA is requested to add SplitDNSAllowed PvD Key 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 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'?>
    </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.7149'?>

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

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

      <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>

      <reference anchor="TOP"
                 target="http://s3-us-west-1.amazonaws.com/umbrella-static/index.html">
        <front>
          <title>Top 1 million domains and TLDs</title>

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

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

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