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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.
Pauly & Wouters Standards Track [Page 1]
RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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RFC 8598 Split DNS Configuration for IKEv2 May 2019
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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