ADD                                                             T. Reddy
Internet-Draft                                                    Akamai
Intended status: Standards Track                                 D. Wing
Expires: March 19, 2022                                           Citrix
                                                                K. Smith
                                                      September 15, 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 rather
   than a public 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

   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 March 19, 2022.

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
   ( in effect on the date of
   publication of this document.  Please review these documents

Reddy, et al.            Expires March 19, 2022                 [Page 1]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   carefully, as they describe your rights and restrictions with respect
   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.  An Example  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Split DNS Configuration for IKEv2 . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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].

   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.

Reddy, et al.            Expires March 19, 2022                 [Page 2]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   [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).  The network lists its claims of
   authority for DNS domains using the "dnsZones" PvD key (defined in

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.

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".

   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,

Reddy, et al.            Expires March 19, 2022                 [Page 3]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   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 that requires one or more
   DNS domains to be resolved via network-designated 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-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., "") 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, internal resources in a network tend to
   have private DNS names.  A network 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
   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 a 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

Reddy, et al.            Expires March 19, 2022                 [Page 4]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   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 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.

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

           +  If the server certificate does not validate and a secure
              connection cannot be established to the network designated
              resolver, the client can proceed 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 "" or ".local".

Reddy, et al.            Expires March 19, 2022                 [Page 5]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   4.  If the client uses a public resolver, authenticated denial of
       existence using NSEC3 or NSEC records can be used by a client to
       identify that the domain name does not exist in the global DNS.

   For example, if in a network the private domain names are defined
   under "".  The DnsZones PvD Key would
   indicate that "*" are private domain
   names.  The client can trigger a NS query of
   "" and the NS RRset returns that the
   nameserver is "".  The client would then connect
   to the network-designated encrypted resolver whose name is
   "", authenticate it using server certificate
   validation in TLS handshake, and use it for resolving the domains in
   the subtree of "*".

5.  An Example

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

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

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

6.  Split DNS Configuration for IKEv2

   The split-tunnel Virtual Private Network (VPN) configuration allows
   the endpoint to access resources that reside in the VPN [RFC8598] via
   the tunnel; other traffic not destined to the VPN does not traverse
   the tunnel.  In contrast, a non-split- tunnel VPN configuration
   causes all traffic to traverse the tunnel into the VPN.

   When the VPN tunnel is IPsec, the encrypted DNS resolver hosted by
   the VPN service provider 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 VPN provider claims authority.
   For non-split-tunnel VPN configurations, the endpoint uses the
   discovered encrypted DNS server to resolve both global and private
   domain names.  For split-tunnel VPN configurations, the IKE client
   can use the steps discussed in Section 4.1 to determine if the VPN

Reddy, et al.            Expires March 19, 2022                 [Page 6]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   service provider is authoritative over the INTERNAL_DNS_DOMAIN

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

7.  Security Considerations

   The content of dnsZones may be passed to another (DNS) program for
   processing.  As with any network input, the content SHOULD be
   considered untrusted and handled accordingly.  The client must
   perform the steps discussed in Section 4.1 to determine if the
   network-designated encrypted resolvers are authoritative over the
   domains in DnsZones.  If the network is lying, the client can take
   appropriate action like disconnecting from the network.

   As an additional precaution, 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

8.  IANA Considerations

   This document has no IANA actions..

9.  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

10.  References

10.1.  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,

Reddy, et al.            Expires March 19, 2022                 [Page 7]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   [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, <>.

   [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, <>.

   [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,

10.2.  Informative References

              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-03 (work in progress), May 2021.

              Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", draft-ietf-
              add-ddr-02 (work in progress), July 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-02 (work in progress), May 2021.

Reddy, et al.            Expires March 19, 2022                 [Page 8]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

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

   [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, <>.

   [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,

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

   [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,

Authors' Addresses

   Tirumaleswar Reddy
   Embassy Golf Link Business Park
   Bangalore, Karnataka  560071


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


Reddy, et al.            Expires March 19, 2022                 [Page 9]

Internet-Draft       Split-Horizon DNS Configuration      September 2021

   Kevin Smith
   Vodafone Group
   One Kingdom Street


Reddy, et al.            Expires March 19, 2022                [Page 10]