ADD                                                             T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 D. Wing
Expires: August 24, 2021                                          Citrix
                                                       February 20, 2021

         Split-Horizon DNS Configuration in Enterprise Networks


   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.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 24, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scope of the Document . . . . . . . . . . . . . . . . . . . .   4
   4.  Split DNS . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  PvD dnsZones  . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  PvD SplitDNSAllowed Key . . . . . . . . . . . . . . . . . . .   7
   7.  An Example  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Roaming Enterprise Users  . . . . . . . . . . . . . . . . . .   8
   9.  Upstream Encryption . . . . . . . . . . . . . . . . . . . . .   8
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     13.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     13.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   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.

   [RFC7626] discusses DNS privacy considerations in both "on the wire"
   (Section 2.4 of [RFC7626]) and "in the server" (Section 2.5 of
   [RFC7626]) contexts.  Also, there has been an increase in the
   availability of "public resolvers" [RFC8499] 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)

   If public encrypted DNS servers (e.g., DNS-over-TLS (DoT) [RFC7858]
   or DNS-over-HTTPS (DoH) [RFC8484]) 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,

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   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 achieve the claimed
   intent.  Nevertheless, network-provided 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.

   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.

   If a network security service blocks access to a public encrypted DNS
   server, there are incompatibilities with the privacy profiles
   discussed in [RFC8310]:

   o  If an endpoint has enabled strict privacy profile (Section 5 of
      [RFC8310]), the endpoint cannot resolve DNS names.

   o  If an endpoint has enabled opportunistic privacy profile
      (Section 5 of [RFC8310]), 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.

      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
      [I-D.arkko-farrell-arch-model-t] for a discussion on the need for
      more awareness about attacks from within closed domains.

   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.btw-add-home] and [I-D.ietf-add-ddr].  Discovery of encrypted
   DNS server for roaming enterprise endpoints is discussed in
   [I-D.btw-add-ipsecme-ike] (see Section 8).

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   Provisioning Domains (PvDs) are defined in [RFC7556] as sets of
   network configuration information that clients can use to access
   networks, including rules for DNS resolution and proxy configuration.
   [RFC8801] 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).

   This document defines one PvD Key:

   The SplitDNSAllowed PvD Key:  which determines if the Enterprise
      network allows split-horizon DNS.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document makes use of the terms defined in [RFC8499].  The terms
   "Private DNS", "Global DNS" and "Split DNS" are defined in [RFC8499].

   'Encrypted DNS' refers to a DNS protocol that provides an encrypted
   channel between a DNS client and server (e.g., DoT, DoH, or DoQ).

   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

3.  Scope of the Document

   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 [Firefox-Policy] for Windows and MacOS, and setting
   Chrome policies is discussed in [Chrome-Policy] and [Chrome-DoH].

   If mobile device management (MDM) (e.g., [MDM-Apple]) 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-

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   Air (OTA) enrollment [OTA] 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

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

   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.

   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 [PEAP], EAP-pwd [RFC8146], EAP-PSK [RFC4764]).

   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
      Windows 10 in S mode [win10s] or Chromebook [Chromebook], where
      applications are sandboxed (e.g., ransomware attack is not
      possible) and applications can only be installed via the OS store.

4.  Split DNS

   [RFC2826] "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".

   There are various DNS deployments outside of the global DNS,
   including "split horizon" deployments and DNS usages on private (or

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   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 [RFC6950] relating to
   trusting source IP addresses apply to such deployments.

   When the internal address space range is private [RFC1918], 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.

   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
   "" for an enterprise that is publicly known to use
   "".  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., "") to the network-
   provided DNS servers.  A configuration for this deployment scenario
   is referred to as a Split DNS configuration.

   The PvD RA option defined in [RFC8801] 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.

5.  PvD dnsZones

   As discussed in Section 4, 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.

   The PvD Key dnsZones is defined in [RFC8801].  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

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   The dnsZones PvD Key conveys the specified DNS domains must be
   resolved using an network-provided 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.

   For each dnsZones entry, the client MUST use the network-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 public resolvers.  Other domain names may be resolved
   using some other public resolvers that are configured independently.

6.  PvD SplitDNSAllowed Key

   If an Enterprise network restricts all the DNS queries to be sent to
   the network-provided DNS server, SplitDNSAllowed will be set to

   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.  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 network-provided
   DNS resolver(s), but "otherexample.test" and "ple.test" can be
   resolved using the system's public resolver(s).

   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.

7.  An Example

   The following example shows how the JSON keys defined in this
   document can be used:

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        "identifier": "",
        "expires": "2020-05-23T06:00:00Z",
        "prefixes": ["2001:db8:1::/48", "2001:db8:4::/48"],
        "SplitDNSAllowed": True,
        "dnsZones:": ["city.other.test", ""]

   The JSON keys "identifier", "expires", and "prefixes" are defined in

8.  Roaming Enterprise Users

   In this Enterprise scenario (Section 1.1.3 of [RFC7296]), 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 [RFC8598] 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.

   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 [I-D.btw-add-ipsecme-ike].  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.

   Other VPN tunnel types have similar configuration capabilities, not
   detailed here.

9.  Upstream Encryption

   If an Enterprise network is using a local encrypted DNS server
   configured as a Forwarding DNS server [RFC8499] 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.

   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

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10.  Security Considerations

   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.

   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.

11.  IANA Considerations

   IANA is requested to add SplitDNSAllowed PvD Key to the Additional
   Information PvD Keys registry (

12.  Acknowledgements

   Thanks to Mohamed Boucadair, Jim Reid and Vinny Parla for the
   discussion and comments.

13.  References

13.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2826]  Internet Architecture Board, "IAB Technical Comment on the
              Unique DNS Root", RFC 2826, DOI 10.17487/RFC2826, May
              2000, <>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8801]  Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W.
              Shao, "Discovering Provisioning Domain Names and Data",
              RFC 8801, DOI 10.17487/RFC8801, July 2020,

13.2.  Informative References

              The Unicode Consortium, "Chrome DNS over HTTPS (aka DoH)",

              The Unicode Consortium, "Chrome policies for users or
              browsers", <

              Microsoft, "Chromebook security",

              "Policy templates for Firefox",

              Arkko, J. and S. Farrell, "Challenges and Changes in the
              Internet Threat Model", draft-arkko-farrell-arch-model-
              t-04 (work in progress), July 2020.

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              Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for
              Encrypted DNS Discovery", draft-btw-add-home-12 (work in
              progress), January 2021.

              Boucadair, M., Reddy.K, T., Wing, D., and V. Smyslov,
              "Internet Key Exchange Protocol Version 2 (IKEv2)
              Configuration for Encrypted DNS", draft-btw-add-ipsecme-
              ike-01 (work in progress), September 2020.

              Livingood, J., Mayrhofer, A., and B. Overeinder, "DNS
              Privacy Requirements for Exchanges between Recursive
              Resolvers and Authoritative Servers", draft-ietf-dprive-
              phase2-requirements-02 (work in progress), November 2020.

              Apple, "Mobile Device Management",

   [OTA]      Apple, "Over-the-Air Profile Delivery Concepts", <https://

   [PEAP]     Microsoft, "[MS-PEAP]: Protected Extensible Authentication
              Protocol (PEAP)", <

   [RFC4764]  Bersani, F. and H. Tschofenig, "The EAP-PSK Protocol: A
              Pre-Shared Key Extensible Authentication Protocol (EAP)
              Method", RFC 4764, DOI 10.17487/RFC4764, January 2007,

   [RFC6950]  Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
              "Architectural Considerations on Application Features in
              the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013,

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <>.

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   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,

   [RFC8146]  Harkins, D., "Adding Support for Salted Password Databases
              to EAP-pwd", RFC 8146, DOI 10.17487/RFC8146, April 2017,

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

   [RFC8598]  Pauly, T. and P. Wouters, "Split DNS Configuration for the
              Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 8598, DOI 10.17487/RFC8598, May 2019,

   [win10s]   Microsoft, "Windows 10 in S mode",

Authors' Addresses

   Tirumaleswar Reddy
   McAfee, Inc.
   Embassy Golf Link Business Park
   Bangalore, Karnataka  560071


   Dan Wing
   Citrix Systems, Inc.
   4988 Great America Pkwy
   Santa Clara, CA  95054


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