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

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<rfc category="std" docName="draft-reddy-add-intarea-enterprise-split-dns-00"
     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 enterprise's DNS
      server. DNS clients which use non-enterprise DNS servers need to route
      those DNS domain queries to the enterprise's 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
      enterprise's 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>If public encrypted DNS servers (e.g., DNS-over-TLS (DoT) <xref
      target="RFC7858"></xref> or DNS-over-HTTPS (DoH) <xref
      target="RFC8484"></xref>) are used instead of using local DNS servers,
      it can adversely impact Enterprise network-based security features.
      Indeed, various network security services are provided by Enterprise
      networks to protect endpoints (e.g., laptops, printers, IoT devices) and
      to enforce enterprise-specific policies. These policies may be necessary
      to protect employees, customers, or enterprise network. It is out of the
      scope of this memo to characterize such policies nor assess that they
      used to achieved the claimed intent. Nevertheless, Enterprise DNS
      servers in place for these purposes act on DNS messages involving
      endpoints connected to the Enterprise network to enforce these policies.
      Therefore, if an endpoint uses a public encrypted DNS server, the
      desired enterprise protection level and enforcement will be bypassed and
      thus nullified.</t>

      <t>In order to act on DNS messages involving endpoints connected to an
      Enterprise network, network security services can be configured to block
      DoT traffic by dropping outgoing packets to destination port number 853.
      Identifying DoH traffic is far more challenging than identifying DoT
      traffic. Network security services may try to identify the well-known
      DoH resolvers by their domain name and DoH traffic can be blocked by
      dropping outgoing packets to these domains. However, DoH traffic can not
      be fully identified without acting as a TLS proxy.</t>

      <t>If a network security service blocks access to a public encrypted DNS
      server, there are incompatibilities with the privacy profiles discussed
      in <xref target="RFC8310"></xref>: <list style="symbols">
          <t>If an endpoint has enabled strict privacy profile (Section 5 of
          <xref target="RFC8310"></xref>), the endpoint cannot resolve DNS
          names.</t>

          <t>If an endpoint has enabled opportunistic privacy profile (Section
          5 of <xref target="RFC8310"></xref>), the endpoint will either
          fallback to an encrypted connection without authenticating the DNS
          server provided by the local network or fallback to clear text DNS,
          and cannot exchange encrypted DNS messages. <vspace
          blankLines="1" />The fallback adversely impacts security and privacy
          as internal attacks are possible within Enterprise networks. For
          example, an internal attacker can modify the DNS responses to
          re-direct a client to malicious servers or pervasively monitor the
          DNS traffic. The reader may refer to Section 3.2.1 of <xref
          target="I-D.arkko-farrell-arch-model-t"></xref> for a discussion on
          the need for more awareness about attacks from within closed
          domains.</t>
        </list></t>

      <t>To overcome the above threats, this document specifies a mechanism to
      convey internal-only DNS names to endpoints connected to an Enterprise
      network and to determine if the Enterprise networks allows split-horizon
      DNS. DNS clients can discover and authenticate encrypted DNS servers
      provided by the Enterprise network, for example using the techniques
      proposed in <xref target="I-D.btw-add-home"></xref> and <xref
      target="I-D.pauly-add-deer"></xref>. 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.
      <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 an URI derived from
      the PvD ID. This document defines two PvD Keys:<list style="hanging">
          <t hangText="The InternalDNSDomains PvD Key:">which is used to
          convey that the specified DNS domains must be resolved using the
          Enterprise DNS server</t>

          <t hangText="The SplitDNSAllowed PvD Key:">which is used to
          determine 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>.</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>DNS resolution servers accessible only to devices attached to the
      Enterprise network will be referred to as "Internal DNS servers"; other
      DNS servers will be referred to as "external DNS servers".</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 Enterprise-provided encrypted DNS servers. 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>) is used to secure BYOD (Bring Your Own
      Device), MDM can be used to configure OS/browser with the Enterprise
      provided DoH/DoT 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 the Enterprise provided 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 out side 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 Enterprise
      Encrypted DNS server to only access internal-only DNS names and use
      another public DNS server for public domains or use the discovered
      Enterprise Encrypted DNS server to access both internal-only DNS names
      and public domains.</t>

      <t>The scope of this document is restricted to unmanaged BYOD devices
      without a configuration profile. The unmanaged BYOD devices 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 public 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
      internal 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
      internal 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
      internal-only domain names are and what the IP addresses of the internal
      DNS servers are. An enterprise can also run a different version of its
      public 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 internal DNS servers. A configuration for this
      deployment scenario is referred to as a Split DNS configuration.</t>
    </section>

    <section anchor="InternalDNSDomains" title="PvD InternalDNSDomains Key">
      <t>As discussed in <xref target="split-horizon"></xref>, the Enterprise
      internal resources tend to have internal-only DNS names and require the
      use of special internal-only DNS servers to get resolved.</t>

      <t>The PvD Key InternalDNSDomains adds support for private
      (internal-only) DNS domains. These domains are intended to be resolved
      using non-public DNS servers that are only reachable to the devices
      attached to the Enterprise network. DNS resolution for other domains
      remains unchanged.</t>

      <t>The InternalDnsDomains PvD Key is used to convey that the specified
      DNS domains must be resolved using an Enterprise DNS server. The DNS
      root zone (".") MUST be ignored if it appears in InternalDNSDomains.
      Other generic or public domains, such as Top-Level Domains (TLDs),
      similarly MUST be ignored if they appear in InternalDNSDomains.</t>

      <t>For each SplitDNSAllowed entry, the client MUST use the Enterprise
      provided DNS servers as the only resolvers for the listed domains and
      its subdomains and it MUST NOT attempt to resolve the provided DNS
      domains using external DNS servers. Other domain names may be resolved
      using some other external DNS resolver(s) that are configured
      independently.</t>
    </section>

    <section anchor="SplitDNSAllowed" title="PvD SplitDNSAllowed Key">
      <t>If an Enterprise network restricts all the DNS queries to be sent to
      the Internal DNS server, SplitDNSAllowed will be set to false.</t>

      <t>Split DNS configurations may be preferable to sending all DNS queries
      to an Enterprise DNS server in some deployments. This allows an endpoint
      to only send DNS queries for the enterprise to the internal DNS servers.
      The Enterprise remains unaware of all non-enterprise (DNS) activity of
      the user.</t>

      <t>It also allows the Enterprise 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. For example, if the SplitDNSAllowed key
      specifies "example.test" and SplitDNSAllowed is set to true, then
      "example.test", "www.example.test", and "mail.eng.example.test" must be
      resolved using the internal DNS resolver(s), but "otherexample.test" and
      "ple.test" can be resolved using the system's external DNS
      resolver(s).</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 external 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,
     "InternalDNSDomains:": ["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 Enterprise-provided encrypted DNS
      server to resolve internal-only domain names. For non-split-tunnel VPN
      configurations, the endpoint uses the Enterprise-provided encrypted DNS
      server to resolve both internal and external 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 external 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 InternalDnsDomains 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>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>IANA is requested to add InternalDNSDomains and SplitDNSAllowed 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 for the discussion and comments.</t>
    </section>
  </middle>

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

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

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

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

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

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

      <?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.I-D.ietf-dprive-phase2-requirements'?>

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

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

      <!--      <?rfc include='reference.I-D.ietf-dnsop-terminology-ter'?> -->

      <?rfc include='reference.I-D.arkko-farrell-arch-model-t'?>

      <?rfc include='reference.I-D.pauly-add-deer'?>

      <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>