    Internet Engineering Task Force (IETF)                          T. Pauly
    Request for Comments: 8598                                    Apple Inc.
    Category: Standards Track                                     P. Wouters
    ISSN: 2070-1721                                                  Red Hat
                                                                    May 2019

                            Split DNS Configuration
            for the Internet Key Exchange Protocol Version 2 (IKEv2)

    Abstract

       This document defines two Configuration Payload Attribute Types
       (INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA) for the Internet Key
       Exchange Protocol version 2 (IKEv2).  These payloads add support for
       private (internal-only) DNS domains.  These domains are intended to
       be resolved using non-public DNS servers that are only reachable
       through the IPsec connection.  DNS resolution for other domains
       remains unchanged.  These Configuration Payloads only apply to split-
       tunnel configurations.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8598.

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    Copyright Notice

       Copyright (c) 2019 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
       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  . . . . . . . . . . . . . . . . . . . . . . . .   3
         1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
       2.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.  Protocol Exchange . . . . . . . . . . . . . . . . . . . . . .   5
         3.1.  Configuration Request . . . . . . . . . . . . . . . . . .   5
         3.2.  Configuration Reply . . . . . . . . . . . . . . . . . . .   6
         3.3.  Mapping DNS Servers to Domains  . . . . . . . . . . . . .   7
         3.4.  Example Exchanges . . . . . . . . . . . . . . . . . . . .   7
           3.4.1.  Simple Case . . . . . . . . . . . . . . . . . . . . .   7
           3.4.2.  Requesting Domains and DNSSEC Trust Anchors . . . . .   7
       4.  Payload Formats . . . . . . . . . . . . . . . . . . . . . . .   9
         4.1.  INTERNAL_DNS_DOMAIN Configuration Attribute Type Request
               and Reply . . . . . . . . . . . . . . . . . . . . . . . .   9
         4.2.  INTERNAL_DNSSEC_TA Configuration Attribute  . . . . . . .   9
       5.  INTERNAL_DNS_DOMAIN Usage Guidelines  . . . . . . . . . . . .  11
       6.  INTERNAL_DNSSEC_TA Usage Guidelines . . . . . . . . . . . . .  12
       7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
       8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
       9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
         9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
         9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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    1.  Introduction

       Split-tunnel Virtual Private Network (VPN) configurations only send
       packets with a specific destination IP range, usually chosen from
       [RFC1918], via the VPN.  All other traffic is not sent via the VPN.
       This allows an enterprise deployment to offer remote access VPN
       services without needing to accept and forward all the non-
       enterprise-related network traffic generated by their remote users.
       Resources within the enterprise can be accessed by the user via the
       VPN, while all other traffic generated by the user is not sent over
       the VPN.

       These internal resources tend to only have internal-only DNS names
       and require the use of special internal-only DNS servers to get
       resolved.  Split DNS [RFC2775] is commonly configured as part of
       split-tunnel VPN configurations to allow remote access users to use
       special internal-only domain names.

       The IKEv2 protocol [RFC7296] negotiates configuration parameters
       using Configuration Payload Attribute Types.  This document defines
       two Configuration Payload Attribute Types that add support for
       trusted Split DNS domains.

       The INTERNAL_DNS_DOMAIN attribute type is used to convey that the
       specified DNS domain MUST be resolved using the provided DNS
       nameserver IP addresses as specified in the INTERNAL_IP4_DNS and
       INTERNAL_IP6_DNS Configuration Payloads, causing these requests to
       use the IPsec connection.

       The INTERNAL_DNSSEC_TA attribute type is used to convey a DNSSEC
       trust anchor for such a domain.  This is required if the external
       view uses DNSSEC, which would prove the internal view does not exist
       or would expect a different DNSSEC key on the different versions
       (internal and external) of the enterprise domain.

       If an INTERNAL_DNS_DOMAIN is sent by the responder, the responder
       MUST also include one or more INTERNAL_IP4_DNS or INTERNAL_IP6_DNS
       attributes that contain the IPv4 or IPv6 address of the internal DNS
       server.

       For the purposes of this document, DNS resolution servers accessible
       through an IPsec connection will be referred to as "internal DNS
       servers", and other DNS servers will be referred to as "external DNS
       servers".

       Other tunnel-establishment protocols already support the assignment
       of Split DNS domains.  For example, there are proprietary extensions
       to IKEv1 that allow a server to assign Split DNS domains to a client.

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       However, the IKEv2 standard does not include a method to configure
       this option.  This document defines a standard way to negotiate this
       option for IKEv2.

    1.1.  Requirements Language

       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 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

    2.  Applicability

       If the negotiated IPsec connection is not a split-tunnel
       configuration, the INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA
       Configuration Payloads MUST be ignored.  This prevents generic (non-
       enterprise) VPN services from overriding the public DNS hierarchy,
       which could lead to malicious overrides of DNS and DNSSEC.

       Such configurations SHOULD instead use only the INTERNAL_IP4_DNS and
       INTERNAL_IP6_DNS Configuration Payloads to ensure all of the user's
       DNS traffic is sent through the IPsec connection and does not leak
       unencrypted information onto the local network, as the local network
       is often explicitly exempted from IPsec encryption.

       For split-tunnel configurations, 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
       remote user 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 VPN 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.

       Split DNS configurations are often preferable to sending all DNS
       queries to the enterprise.  This allows the remote user 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.  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 remote user.

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       A client using these Configuration Payloads will be able to request
       and receive Split DNS configurations using the INTERNAL_DNS_DOMAIN
       and INTERNAL_DNSSEC_TA configuration attributes.  These attributes
       MUST be accompanied by one or more INTERNAL_IP4_DNS or
       INTERNAL_IP6_DNS configuration attributes.  The client device can
       then use the internal DNS server(s) for any DNS queries within the
       assigned domains.  DNS queries for other domains SHOULD be sent to
       the regular DNS service of the client unless it prefers to use the
       IPsec tunnel for all its DNS queries.  For example, the client could
       trust the IPsec-provided DNS servers more than the locally provided
       DNS servers, especially in the case of connecting to unknown or
       untrusted networks (e.g., coffee shops or hotel networks).  Or the
       client could prefer the IPsec-based DNS servers because they provide
       additional features compared to the local DNS servers.

    3.  Protocol Exchange

       In order to negotiate which domains are considered internal to an
       IKEv2 tunnel, initiators indicate support for Split DNS in their
       CFG_REQUEST payloads, and responders assign internal domains (and
       DNSSEC trust anchors) in their CFG_REPLY payloads.  When Split DNS
       has been negotiated, the INTERNAL_IP4_DNS and INTERNAL_IP6_DNS DNS
       server configuration attributes will be interpreted as internal DNS
       servers that can resolve hostnames within the internal domains.

    3.1.  Configuration Request

       To indicate support for Split DNS, an initiator includes one or more
       INTERNAL_DNS_DOMAIN attributes as defined in Section 4 as part of the
       CFG_REQUEST payload.  If an INTERNAL_DNS_DOMAIN attribute is included
       in the CFG_REQUEST, the initiator MUST also include one or more
       INTERNAL_IP4_DNS or INTERNAL_IP6_DNS attributes in the CFG_REQUEST.

       The INTERNAL_DNS_DOMAIN attribute sent by the initiator is usually
       empty but MAY contain a suggested domain name.

       The absence of INTERNAL_DNS_DOMAIN attributes in the CFG_REQUEST
       payload indicates that the initiator does not support or is unwilling
       to accept a Split DNS configuration.

       To indicate support for receiving DNSSEC trust anchors for Split DNS
       domains, an initiator includes one or more INTERNAL_DNSSEC_TA
       attributes as defined in Section 4 as part of the CFG_REQUEST
       payload.  If an INTERNAL_DNSSEC_TA attribute is included in the
       CFG_REQUEST, the initiator MUST also include one or more
       INTERNAL_DNS_DOMAIN attributes in the CFG_REQUEST.  If the initiator

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       includes an INTERNAL_DNSSEC_TA attribute but does not include an
       INTERNAL_DNS_DOMAIN attribute, the responder MAY still respond with
       both INTERNAL_DNSSEC_TA and INTERNAL_DNS_DOMAIN attributes.

       An initiator MAY convey its current DNSSEC trust anchors for the
       domain specified in the INTERNAL_DNS_DOMAIN attribute.  A responder
       can use this information to determine that it does not need to send a
       different trust anchor.  If the initiator does not wish to convey
       this information, it MUST use a length of 0.

       The absence of INTERNAL_DNSSEC_TA attributes in the CFG_REQUEST
       payload indicates that the initiator does not support or is unwilling
       to accept the DNSSEC trust anchor configuration.

    3.2.  Configuration Reply

       Responders MAY send one or more INTERNAL_DNS_DOMAIN attributes in
       their CFG_REPLY payload.  If an INTERNAL_DNS_DOMAIN attribute is
       included in the CFG_REPLY, the responder MUST also include one or
       both of the INTERNAL_IP4_DNS and INTERNAL_IP6_DNS attributes in the
       CFG_REPLY.  These DNS server configurations are necessary to define
       which servers can receive queries for hostnames in internal domains.
       If the CFG_REQUEST included an INTERNAL_DNS_DOMAIN attribute but the
       CFG_REPLY does not include an INTERNAL_DNS_DOMAIN attribute, the
       initiator MUST behave as if Split DNS configurations are not
       supported by the server, unless the initiator has been configured
       with local policy to define a set of Split DNS domains to use by
       default.

       Each INTERNAL_DNS_DOMAIN represents a domain that the DNS server
       addresses listed in INTERNAL_IP4_DNS and INTERNAL_IP6_DNS can
       resolve.

       If the CFG_REQUEST included INTERNAL_DNS_DOMAIN attributes with non-
       zero lengths, the content MAY be ignored or be interpreted as a
       suggestion by the responder.

       For each DNS domain specified in an INTERNAL_DNS_DOMAIN attribute,
       one or more INTERNAL_DNSSEC_TA attributes MAY be included by the
       responder.  This attribute lists the corresponding internal DNSSEC
       trust anchor information of a DS record (see [RFC4034]).  The
       INTERNAL_DNSSEC_TA attribute MUST immediately follow the
       INTERNAL_DNS_DOMAIN attribute that it applies to.

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    3.3.  Mapping DNS Servers to Domains

       All DNS servers provided in the CFG_REPLY MUST support resolving
       hostnames within all INTERNAL_DNS_DOMAIN domains.  In other words,
       the INTERNAL_DNS_DOMAIN attributes in a CFG_REPLY payload form a
       single list of Split DNS domains that applies to the entire list of
       INTERNAL_IP4_DNS and INTERNAL_IP6_DNS attributes.

    3.4.  Example Exchanges

    3.4.1.  Simple Case

       In this example exchange, the initiator requests INTERNAL_IP4_DNS,
       INTERNAL_IP6_DNS, and INTERNAL_DNS_DOMAIN attributes in the
       CFG_REQUEST but does not specify any value for either.  This
       indicates that it supports Split DNS but has no preference for which
       DNS requests will be routed through the tunnel.

       The responder replies with two DNS server addresses and two internal
       domains, "example.com" and "city.other.test".

       Any subsequent DNS queries from the initiator for domains such as
       "www.example.com" SHOULD use 198.51.100.2 or 198.51.100.4 to resolve.

       CP(CFG_REQUEST) =
         INTERNAL_IP4_ADDRESS()
         INTERNAL_IP4_DNS()
         INTERNAL_IP6_ADDRESS()
         INTERNAL_IP6_DNS()
         INTERNAL_DNS_DOMAIN()

       CP(CFG_REPLY) =
         INTERNAL_IP4_ADDRESS(198.51.100.234)
         INTERNAL_IP4_DNS(198.51.100.2)
         INTERNAL_IP4_DNS(198.51.100.4)
         INTERNAL_IP6_ADDRESS(2001:DB8:0:1:2:3:4:5/64)
         INTERNAL_IP6_DNS(2001:DB8:99:88:77:66:55:44)
         INTERNAL_DNS_DOMAIN(example.com)
         INTERNAL_DNS_DOMAIN(city.other.test)

    3.4.2.  Requesting Domains and DNSSEC Trust Anchors

       In this example exchange, the initiator requests INTERNAL_IP4_DNS,
       INTERNAL_IP6_DNS, INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA
       attributes in the CFG_REQUEST.

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       Any subsequent DNS queries from the initiator for domains such as
       "www.example.com" or "city.other.test" would be DNSSEC validated
       using the DNSSEC trust anchor received in the CFG_REPLY.

       In this example, the initiator has no existing DNSSEC trust anchors
       for the requested domain.  The "example.com" domain has DNSSEC trust
       anchors that are returned, while the "other.test" domain has no
       DNSSEC trust anchors.

       CP(CFG_REQUEST) =
         INTERNAL_IP4_ADDRESS()
         INTERNAL_IP4_DNS()
         INTERNAL_IP6_ADDRESS()
         INTERNAL_IP6_DNS()
         INTERNAL_DNS_DOMAIN()
         INTERNAL_DNSSEC_TA()

       CP(CFG_REPLY) =
         INTERNAL_IP4_ADDRESS(198.51.100.234)
         INTERNAL_IP4_DNS(198.51.100.2)
         INTERNAL_IP4_DNS(198.51.100.4)
         INTERNAL_IP6_ADDRESS(2001:DB8:0:1:2:3:4:5/64)
         INTERNAL_IP6_DNS(2001:DB8:99:88:77:66:55:44)
         INTERNAL_DNS_DOMAIN(example.com)
         INTERNAL_DNSSEC_TA(43547,8,1,B6225AB2CC613E0DCA7962BDC2342EA4...)
         INTERNAL_DNSSEC_TA(31406,8,2,F78CF3344F72137235098ECBBD08947C...)
         INTERNAL_DNS_DOMAIN(city.other.test)

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    4.  Payload Formats

       All multi-octet fields representing integers are laid out in big-
       endian order (also known as "most significant byte first" or "network
       byte order").

    4.1.  INTERNAL_DNS_DOMAIN Configuration Attribute Type Request and Reply

                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-----------------------------+-------------------------------+
       |R|         Attribute Type      |            Length             |
       +-+-----------------------------+-------------------------------+
       |                                                               |
       ~             Domain Name in DNS presentation format            ~
       |                                                               |
       +---------------------------------------------------------------+

       o  Reserved (1 bit) - Defined in IKEv2 RFC [RFC7296].

       o  Attribute Type (15 bits) - set to value 25 for
          INTERNAL_DNS_DOMAIN.

       o  Length (2 octets) - Length of domain name.

       o  Domain Name (0 or more octets) - A Fully Qualified Domain Name
          used for Split DNS rules, such as "example.com", in DNS
          presentation format and using an Internationalized Domain Names
          for Applications (IDNA) A-label [RFC5890].  Implementors need to
          be careful that this value is not null terminated.

    4.2.  INTERNAL_DNSSEC_TA Configuration Attribute

       An INTERNAL_DNSSEC_TA Configuration Attribute can either be empty, or
       it can contain one trust anchor by containing a non-zero Length with
       a DNSKEY Key Tag, DNSKEY Algorithm, Digest Type and Digest Data
       fields.

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       An empty INTERNAL_DNSSEC_TA CFG attribute:

                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-----------------------------+-------------------------------+
       |R|       Attribute Type        |       Length (set to 0)       |
       +-+-----------------------------+-------------------------------+

       o  Reserved (1 bit) - Defined in IKEv2 RFC [RFC7296].

       o  Attribute Type (15 bits) - set to value 26 for INTERNAL_DNSSEC_TA.

       o  Length (2 octets) - Set to 0 for an empty attribute.

       A non-empty INTERNAL_DNSSEC_TA CFG attribute:

                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-----------------------------+-------------------------------+
       |R|       Attribute Type        |            Length             |
       +-+-----------------------------+---------------+---------------+
       |        DNSKEY Key Tag         |  DNSKEY Alg   |  Digest Type  |
       +-------------------------------+---------------+---------------+
       |                                                               |
       ~                         Digest Data                           ~
       |                                                               |
       +---------------------------------------------------------------+

       o  Reserved (1 bit) - Defined in IKEv2 RFC [RFC7296].

       o  Attribute Type (15 bits) - set to value 26 for INTERNAL_DNSSEC_TA.

       o  Length (2 octets) - Length of DNSSEC trust anchor data (4 octets
          plus the length of the Digest Data).

       o  DNSKEY Key Tag (2 octets) - Delegation Signer (DS) Key Tag as
          specified in Section 5.1 of [RFC4034].

       o  DNSKEY Algorithm (1 octet) - DNSKEY algorithm value from the IANA
          DNS Security Algorithm Numbers Registry.

       o  Digest Type (1 octet) - DS algorithm value from the IANA
          Delegation Signer (DS) Resource Record (RR) Type Digest Algorithms
          Registry.

       o  Digest Data (1 or more octets) - The DNSKEY digest as specified in
          Section 5.1 of [RFC4034] in presentation format.

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       Each INTERNAL_DNSSEC_TA attribute in the CFG_REPLY payload MUST
       immediately follow a corresponding INTERNAL_DNS_DOMAIN attribute.  As
       the INTERNAL_DNSSEC_TA format itself does not contain the domain
       name, it relies on the preceding INTERNAL_DNS_DOMAIN to provide the
       domain for which it specifies the trust anchor.  Any
       INTERNAL_DNSSEC_TA attribute that is not immediately preceded by an
       INTERNAL_DNS_DOMAIN or another INTERNAL_DNSSEC_TA attribute applying
       to the same domain name MUST be ignored.

    5.  INTERNAL_DNS_DOMAIN Usage Guidelines

       If a CFG_REPLY payload contains no INTERNAL_DNS_DOMAIN attributes,
       the client MAY use the provided INTERNAL_IP4_DNS or INTERNAL_IP6_DNS
       servers as the default DNS server(s) for all queries.

       If a client is configured by local policy to only accept a limited
       set of INTERNAL_DNS_DOMAIN values, the client MUST ignore any other
       INTERNAL_DNS_DOMAIN values.

       For each INTERNAL_DNS_DOMAIN entry in a CFG_REPLY payload that is not
       prohibited by local policy, the client MUST use the provided
       INTERNAL_IP4_DNS or INTERNAL_IP6_DNS 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 its external DNS
       servers.  Other domain names SHOULD be resolved using some other
       external DNS resolver(s) that are configured independently from IKE.
       Queries for these other domains MAY be sent to the internal DNS
       resolver(s) listed in that CFG_REPLY message, but they have no
       guarantee of being answered.  For example, if the INTERNAL_DNS_DOMAIN
       attribute specifies "example.test", 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" MUST NOT be resolved using the internal resolver and MUST
       use the system's external DNS resolver(s).

       The initiator SHOULD allow the DNS domains listed in the
       INTERNAL_DNS_DOMAIN attributes to resolve to special IP address
       ranges, such as those of [RFC1918], even if the initiator host is
       otherwise configured to block a DNS answer containing these special
       IP address ranges.

       When an IKE Security Association (SA) is terminated, the DNS
       forwarding MUST be unconfigured.  This includes deleting the DNS
       forwarding rules; flushing all cached data for DNS domains provided
       by the INTERNAL_DNS_DOMAIN attribute, including negative cache
       entries; removing any obtained DNSSEC trust anchors from the list of
       trust anchors; and clearing the outstanding DNS request queue.

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       INTERNAL_DNS_DOMAIN attributes SHOULD only be used on split-tunnel
       configurations where only a subset of traffic is routed into a
       private remote network using the IPsec connection.  If all traffic is
       routed over the IPsec connection, the existing global
       INTERNAL_IP4_DNS and INTERNAL_IP6_DNS can be used without creating
       specific DNS or DNSSEC exemptions.

    6.  INTERNAL_DNSSEC_TA Usage Guidelines

       DNS records can be used to publish specific records containing trust
       anchors for applications.  The most common record type is the TLSA
       record specified in [RFC6698].  This DNS record type publishes which
       Certification Authority (CA) certificate or End Entity (EE)
       certificate to expect for a certain host name.  These records are
       protected by DNSSEC and thus are trustable by the application.
       Whether to trust TLSA records instead of the traditional Web PKI
       depends on the local policy of the client.  By accepting an
       INTERNAL_DNSSEC_TA trust anchor via IKE from the remote IKE server,
       the IPsec client might be allowing the remote IKE server to override
       the trusted certificates for TLS.  Similar override concerns apply to
       other public key or fingerprint-based DNS records, such as
       OPENPGPKEY, SMIMEA, or IPSECKEY records.

       Thus, installing an INTERNAL_DNSSEC_TA trust anchor can be seen as
       the equivalent of installing an Enterprise CA certificate.  It allows
       the remote IKE/IPsec server to modify DNS answers, including DNSSEC
       cryptographic signatures, by overriding existing DNS information with
       a trust anchor conveyed via IKE and (temporarily) installed on the
       IKE client.  Of specific concern is the overriding of TLSA records
       based on [RFC6698], which represents a confirmation or override of an
       existing Web PKI TLS certificate.  Other DNS record types that convey
       cryptographic materials (public keys or fingerprints) are OPENPGPKEY,
       SMIMEA, SSHP, and IPSECKEY records.

       IKE clients willing to accept INTERNAL_DNSSEC_TA attributes MUST use
       a whitelist of one or more domains that can be updated out of band.
       IKE clients with an empty whitelist MUST NOT use any
       INTERNAL_DNSSEC_TA attributes received over IKE.  Such clients MAY
       interpret receiving an INTERNAL_DNSSEC_TA attribute for a non-
       whitelisted domain as an indication that their local configuration
       may need to be updated out of band.

       IKE clients should take care to only whitelist domains that apply to
       internal or managed domains rather than to generic Internet traffic.
       The DNS root zone (".") MUST be ignored if it appears in a whitelist.
       Other generic or public domains, such as Top-Level Domains (TLDs),
       similarly MUST be ignored if they appear in a whitelist unless the
       entity actually is the operator of the TLD.  To determine this, an

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       implementation MAY interactively ask the user when a VPN profile is
       installed or activated to confirm this.  Alternatively, it MAY
       provide a special override keyword in its provisioning configuration
       to ensure non-interactive agreement can be achieved only by the party
       provisioning the VPN client, who presumably is a trusted entity by
       the end user.  Similarly, an entity might be using a special domain
       name, such as ".internal", for its internal-only view and might wish
       to force its provisioning system to accept such a domain in a Split
       DNS configuration.

       Any updates to this whitelist of domain names MUST happen via
       explicit human interaction or by a trusted automated provision system
       to prevent malicious invisible installation of trust anchors in case
       of an IKE server compromise.

       IKE clients SHOULD accept any INTERNAL_DNSSEC_TA updates for
       subdomain names of the whitelisted domain names.  For example, if
       "example.net" is whitelisted, then INTERNAL_DNSSEC_TA received for
       "antartica.example.net" SHOULD be accepted.

       IKE clients MUST ignore any received INTERNAL_DNSSEC_TA attributes
       for a Fully Qualified Domain Name (FQDN) for which it did not receive
       and accept an INTERNAL_DNS_DOMAIN Configuration Payload.

       In most deployment scenarios, the IKE client has an expectation that
       it is connecting to a specific organization or enterprise using a
       split-network setup.  A recommended policy would be to only accept
       INTERNAL_DNSSEC_TA directives from that organization's DNS names.
       However, this might not be possible in all deployment scenarios, such
       as one where the IKE server is handing out a number of domains that
       are not within one parent domain.

    7.  IANA Considerations

       This document defines two new IKEv2 Configuration Payload Attribute
       Types, which are allocated from the "IKEv2 Configuration Payload
       Attribute Types" namespace.

                                     Multi-
       Value    Attribute Type       Valued  Length      Reference
       ------   -------------------  ------  ----------  ---------------
       25       INTERNAL_DNS_DOMAIN   YES     0 or more  RFC 8598
       26       INTERNAL_DNSSEC_TA    YES     0 or more  RFC 8598

                                     Figure 1

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    8.  Security Considerations

       As stated in Section 2, if the negotiated IPsec connection is not a
       split-tunnel configuration, the INTERNAL_DNS_DOMAIN and
       INTERNAL_DNSSEC_TA Configuration Payloads MUST be ignored.
       Otherwise, generic VPN service providers could maliciously override
       DNSSEC-based trust anchors of public DNS domains.

       An initiator MUST only accept INTERNAL_DNSSEC_TAs for which it has a
       whitelist, since this mechanism allows the credential used to
       authenticate an IKEv2 association to be leveraged into authenticating
       credentials for other connections.  Initiators should ensure that
       they have sufficient trust in the responder when using this
       mechanism.  An initiator MAY treat a received INTERNAL_DNSSEC_TA for
       a non-whitelisted domain as a signal to update the whitelist via a
       non-IKE provisioning mechanism.  See Section 6 for additional
       security considerations for DNSSEC trust anchors.

       The use of Split DNS configurations assigned by an IKEv2 responder is
       predicated on the trust established during IKE SA authentication.
       However, if IKEv2 is being negotiated with an anonymous or unknown
       endpoint (such as for Opportunistic Security [RFC7435]), the
       initiator MUST ignore Split DNS configurations assigned by the
       responder.

       If a host connected to an authenticated IKE peer is connecting to
       another IKE peer that attempts to claim the same domain via the
       INTERNAL_DNS_DOMAIN attribute, the IKE connection SHOULD only process
       the DNS information if the two connections are part of the same
       logical entity.  Otherwise, the client SHOULD refuse the DNS
       information and potentially warn the end user.  For example, if a VPN
       profile for "Example Corporation" is installed that provides two
       IPsec connections, one covering 192.168.100.0/24 and one covering
       10.13.14.0/24, it could be that both connections negotiate the same
       INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA values.  Since these are
       part of the same remote organization (or provisioning profile), the
       Configuration Payloads can be used.  However, if a user installs two
       VPN profiles from two different unrelated independent entities, both
       could be configured to use the same domain -- for example,
       ".internal".  These two connections MUST NOT be allowed to be active
       at the same time.

       If the initiator is using DNSSEC validation for a domain in its
       public DNS view and it requests and receives an INTERNAL_DNS_DOMAIN
       attribute without an INTERNAL_DNSSEC_TA, it will need to reconfigure
       its DNS resolver to allow for an insecure delegation.  It SHOULD NOT
       accept insecure delegations for domains that are DNSSEC signed in the

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       public DNS view for which it has not explicitly requested such
       delegation, i.e., for which it has not used an INTERNAL_DNS_DOMAIN
       request to specify the domain.

       Deployments that configure INTERNAL_DNS_DOMAIN domains should pay
       close attention to their use of indirect reference RRtypes in their
       internal-only domain names.  Examples of such RRtypes are NS, CNAME,
       DNAME, MX, or SRV records.  For example, if the MX record for
       "internal.example.com" points to "mx.internal.example.net", then both
       "internal.example.com" and "internal.example.net" should be sent
       using an INTERNAL_DNS_DOMAIN Configuration Payload.

       IKE clients MAY want to require whitelisted domains for Top-Level
       Domains (TLDs) and Second-Level Domains (SLDs) to further 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 VPN service provider could then even modify the DNS
       answers without detection.

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

    9.  References

    9.1.  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,
                  <https://www.rfc-editor.org/info/rfc1918>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
                  Rose, "Resource Records for the DNS Security Extensions",
                  RFC 4034, DOI 10.17487/RFC4034, March 2005,
                  <https://www.rfc-editor.org/info/rfc4034>.

       [RFC5890]  Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document Framework",
                  RFC 5890, DOI 10.17487/RFC5890, August 2010,
                  <https://www.rfc-editor.org/info/rfc5890>.

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    RFC 8598            Split DNS Configuration for IKEv2           May 2019

       [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>.

       [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>.

       [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>.

    9.2.  Informative References

       [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
                  DOI 10.17487/RFC2775, February 2000,
                  <https://www.rfc-editor.org/info/rfc2775>.

       [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
                  Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
                  December 2014, <https://www.rfc-editor.org/info/rfc7435>.

    Authors' Addresses

       Tommy Pauly
       Apple Inc.
       One Apple Park Way
       Cupertino, California  95014
       United States of America

       Email: tpauly@apple.com

       Paul Wouters
       Red Hat

       Email: pwouters@redhat.com

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