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Versions: 00 01 02 03 04 05 06                                          
ADD                                                             T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 D. Wing
Expires: December 19, 2021                                        Citrix
                                                                K. Smith
                                                           June 17, 2021

                    Split-Horizon DNS Configuration


   When split-horizon DNS is deployed by a network, certain 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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 19, 2021.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Split DNS . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  PvD dnsZones  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Authority over the Domains  . . . . . . . . . . . . . . .   5
   5.  PvD NetworkDNSOnly and ErrorNetworkDNSOnly Keys . . . . . . .   6
     5.1.  Scope of NetworkDNSOnly Key . . . . . . . . . . . . . . .   7
   6.  An Example  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Roaming Enterprise Users  . . . . . . . . . . . . . . . . . .   9
   8.  Upstream Encryption . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Historically, an endpoint would utilize network-designated DNS
   servers upon joining a network (e.g., DHCP OFFER, IPv6 Router
   Advertisement).  While it has long been possible to configure
   endpoints to ignore the network's suggestions and use a (public) DNS
   server on the Internet, this was seldom used because some networks
   block UDP/53 (in order to enforce their own DNS policies).  Also,
   there has been an increase in the availability of "public resolvers"
   [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) filtering).  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.

   This document specifies a mechanism to indicate which DNS zones are
   used for split-horizon DNS.  DNS clients can discover and
   authenticate DNS servers provided by the network, for example using
   the techniques proposed in [I-D.ietf-add-dnr] 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 7).

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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 two PvD Key:

   The NetworkDNSOnly PvD Key:  which determines if network will block,
      or attempt to block, DNS queries sent to DNS servers that were not
      signaled by the network.

   The ErrorNetworkDNSOnly PvD Key:  which contains a human-friendly
      description of the reason to block access to DNS servers that were
      not signaled by the network.

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.  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".

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   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 [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.

   A typical use case is an Enterprise network 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.
   Another use case for split-horizon DNS is Cellular and Fixed-access
   networks (ISPs) typically offer private domains, including account
   status/controls, and free education initiatives [INS].

   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 the "dnsZones" key to define
   the DNS domains for which the network claims authority.

4.  PvD dnsZones

   As discussed in Section 3, 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.

   The PvD Key dnsZones is defined in [RFC8801].  The PvD Key dnsZones
   adds support for DNS domains for which the network claims authority.
   The private domains specified in the dnsZones key are intended to be

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   resolved using network-designated DNS servers.  The private domains
   in dnsZones are only reachable by devices authenticated or attached
   to the network.  The global domains specified in the dnsZones key
   have a different version in the internal network.  DNS resolution for
   other domains remains unchanged.

   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.

   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).

4.1.  Authority over the Domains

   To comply with [RFC2826] 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:

   1.  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 [RFC6698].

   2.  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.

       A.  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.

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       B.  If the match succeeds, the client can then establish a secure
           connection to that network-designated resolver and validate
           its certificate.

           +  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

           +  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.

   3.  As an exception to this rule, the client need not perform the
       above validation for domains reserved for special use [RFC6761]
       or [RFC6762] such as ".home.arpa" or ".local".

   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".

5.  PvD NetworkDNSOnly and ErrorNetworkDNSOnly Keys

   Some enterprise networks require clients to query the network-
   designated DNS servers, it sets the PvD NetworkDNSOnly key to True,
   otherwise sets NetworkDNSOnly to False.  If NetworkDNSOnly is set to
   True, it implies the network will block, or attempt to block, DNS
   queries sent to DNS servers that were not signaled by the network.
   If NetworkDNSOnly is True, the ErrorNetworkDNSOnly key MUST contain a
   human-friendly description for this block.  This information is
   intended for human consumption (not automated parsing).  The
   ErrorNetworkDNSOnly key is useful when the client does not use DNS
   lookup to reach the DNS servers not provided by the network.  For
   example, the client can be pre-configured with both the domain name
   and IP addresses of the DNS server not provided by the network
   (Section 7.1 in [RFC8310]) or the client can be pre-configured with
   the IP address of the resolver, and it uses IP address in the
   certificate as identifier (see [RFC8738]).  In this case, the
   extended error code "Blocked" defined in [RFC8914] cannot be returned

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   to the client to provide additional information about the cause for
   the block.

   The NetworkDNSOnly set to True is an internal security policy
   expression by the operator of the network but is not a policy
   prescription to the endpoints to disable its use of its other
   configured DNS servers; that is, the endpoint can ignore
   NetworkDNSOnly set to True.  If joining an un-trusted network (e.g.,
   coffeeshop, hotel, airport network), a True value of NetworkDNSOnly
   MUST be ignored.  The mechanism the client uses to determine 'trusted
   network' to assist the user MUST involve authenticated identity of
   the network (not merely matching SSID in the case of WiFi), such as
   802.1X or confirming the network-designated encrypted resolver name
   is pre-configured in the Operating System and TLS handshake with it
   succeeds.  For example, the client can determine "Open" (unencrypted)
   wireless networks are untrusted networks, notify the user that using
   a shared and public Pre-Shared Key (PSK) for wireless authentication
   is a untrusted network.  If the pre-shared-key is the same for all
   clients that connect to the same WLAN, the shared key will be
   available to all nodes, including attackers, so it is possible to
   mount an active on-path attack (e.g., [Evil-Twin], [Krack],
   [Dragonblood]).  For example, coffee shops and air ports use PSK and
   are unwilling to perform complex configuration on their networks.  In
   addition, customers are generally unwilling to do complicated
   provisioning on their devices just to obtain free Wi-Fi.  This type
   of networks can be tagged as "untrusted networks" with minimal human
   intervention.  In such cases the endpoint MAY choose to use an
   alternate network (e.g., cellular) to resolve the global domain

5.1.  Scope of NetworkDNSOnly Key

   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-
   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

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   encrypted DNS server.  In this case, MDM is not installed on the

   Provisioning IT-managed devices, BYOD devices with MDM or
   configuration profile with network-designated DNS server 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-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.

   The scope of NetworkDNSOnly key is restricted to unmanaged BYOD
   devices without a configuration profile 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 use mutual authentication of the
   client and the enterprise network.  The client is typically
   authenticated with their user credentials (e.g., username and
   password).  The network is typically authenticated with a certificate
   (e.g., PEAP-MSCHAPv2 [PEAP]) or a mutually-authenticated key exchange
   which is well-defended from offline attacks (e.g., EAP-pwd [RFC8146],
   EAP-PSK [RFC4764]).  Importantly, WPA-PSK and WPA2-PSK are not well-
   defended from offline attacks and MUST NOT be used in conjunction
   with NetworkDNSOnly set to True.

   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.

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6.  An Example

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

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

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

7.  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.

8.  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.

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   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

9.  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 attached network could
   then even modify the DNS answers without detection.  It is similar to
   the mechanism discussed in Section 8 of [RFC8598].

   The content of dnsZones and NetworkDNSOnly may be passed to another
   (DNS) program for processing.  As with any network input, the content
   SHOULD be considered untrusted and handled accordingly.

10.  IANA Considerations

   IANA is requested to add NetworkDNSOnly and ErrorSplitDNSBlocked PvD
   Keys to the Additional Information PvD Keys registry

11.  Acknowledgements

   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

12.  References

12.1.  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,

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   [RFC2826]  Internet Architecture Board, "IAB Technical Comment on the
              Unique DNS Root", RFC 2826, DOI 10.17487/RFC2826, May
              2000, <https://www.rfc-editor.org/info/rfc2826>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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,

12.2.  Informative References

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

              The Unicode Consortium, "Chrome policies for users or
              browsers", <https://support.google.com/chrome/a/

              Microsoft, "Chromebook security",

              The Unicode Consortium, "Dragonblood: Analyzing the
              Dragonfly Handshake of WPA3 and EAP-pwd",

              The Unicode Consortium, "Evil twin (wireless networks)",

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              "Policy templates for Firefox",

              Boucadair, M., Reddy, T., Wing, D., and V. Smyslov,
              "Internet Key Exchange Protocol Version 2 (IKEv2)
              Configuration for Encrypted DNS", draft-btw-add-ipsecme-
              ike-02 (work in progress), February 2021.

              Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", draft-ietf-
              add-ddr-00 (work in progress), February 2021.

              Boucadair, M., Reddy, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for the
              Discovery of Network-designated Resolvers (DNR)", draft-
              ietf-add-dnr-00 (work in progress), February 2021.

              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.

   [INS]      The Unicode Consortium, "Vodafone Foundation Instant
              Schools for Sub-Saharan Africa",

   [Krack]    The Unicode Consortium, "Key Reinstallation Attacks",
              2017, <https://www.krackattacks.com/>.

              Apple, "Mobile Device Management",

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

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   [PEAP]     Microsoft, "[MS-PEAP]: Protected Extensible Authentication
              Protocol (PEAP)", <https://docs.microsoft.com/en-

   [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,

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [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, <https://www.rfc-editor.org/info/rfc7296>.

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

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

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [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,

Reddy, et al.           Expires December 19, 2021              [Page 13]

Internet-Draft       Split-Horizon DNS Configuration           June 2021

   [RFC8738]  Shoemaker, R., "Automated Certificate Management
              Environment (ACME) IP Identifier Validation Extension",
              RFC 8738, DOI 10.17487/RFC8738, February 2020,

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,

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

Authors' Addresses

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

   Email: kondtir@gmail.com

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

   Email: danwing@gmail.com

   Kevin Smith
   Vodafone Group
   One Kingdom Street

   Email: kevin.smith@vodafone.com

Reddy, et al.           Expires December 19, 2021              [Page 14]