Network T. Pauly
Internet-Draft Apple Inc.
Intended status: Standards Track P. Wouters
Expires: December 20, 2018 Red Hat
June 18, 2018
Split DNS Configuration for IKEv2
draft-ietf-ipsecme-split-dns-08
Abstract
This document defines two Configuration Payload Attribute Types for
the IKEv2 protocol that add support for private DNS domains. These
domains are intended to be resolved using DNS servers reachable
through an IPsec connection, while leaving all other DNS resolution
unchanged. This approach of resolving a subset of domains using non-
public DNS servers is referred to as "Split 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-
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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 20, 2018.
Copyright Notice
Copyright (c) 2018 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Exchange . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Configuration Request . . . . . . . . . . . . . . . . . . 4
3.2. Configuration Reply . . . . . . . . . . . . . . . . . . . 4
3.3. Mapping DNS Servers to Domains . . . . . . . . . . . . . 5
3.4. Example Exchanges . . . . . . . . . . . . . . . . . . . . 5
3.4.1. Simple Case . . . . . . . . . . . . . . . . . . . . . 5
3.4.2. Requesting Domains and DNSSEC trust anchors . . . . . 6
4. Payload Formats . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. INTERNAL_DNS_DOMAIN Configuration Attribute Type Request
and Reply . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. INTERNAL_DNSSEC_TA Configuration Attribute . . . . . . . 7
5. INTERNAL_DNS_DOMAIN Usage Guidelines . . . . . . . . . . . . 9
6. INTERNAL_DNSSEC_TA Usage Guidelines . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Split DNS is a common configuration for secure tunnels, such as
Virtual Private Networks in which host machines private to an
organization can only be resolved using internal DNS resolvers
[RFC2775]. In such configurations, it is often desirable to only
resolve hosts within a set of private domains using the tunnel, while
letting resolutions for public hosts be handled by a device's default
DNS configuration.
The Internet Key Exchange protocol version 2 [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 one or more
DNS domains that SHOULD be resolved only using the provided DNS
nameserver IP addresses, causing these requests to use the IPsec
connection.
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The INTERNAL_DNSSEC_TA attribute type is used to convey DNSSEC trust
anchors for those domains.
When only a subset of traffic is routed into a private network using
an IPsec SA, these Configuration Payload options can be used to
define which private domains are intended to be resolved through the
IPsec connection without affecting the client's global DNS
resolution.
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".
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. The client device
can 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 external DNS server.
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
captials, as shown here.
2. Background
Split DNS is a common configuration for enterprise VPN deployments,
in which only one or a few private DNS domains are accessible and
resolvable via an IPsec based VPN connection.
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.
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.
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
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has been negotiated, the existing 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 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 SHOULD also include one or more
INTERNAL_IP4_DNS and 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 Split DNS configuration.
To indicate support for DNSSEC, 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 SHOULD also include one or more
INTERNAL_DNS_DOMAIN attributes in the CFG_REQUEST. If the initiator
includes an INTERNAL_DNSSEC_TA attribute, but does not inclue 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. If it 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 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 SHOULD behave as if Split DNS configurations are not
supported by the server.
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Each INTERNAL_DNS_DOMAIN represents a domain that the DNS servers
address 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 in the DNS presentation format of a DS record as
specified in [RFC4034]. The INTERNAL_DNSSEC_TA attribute MUST
immediately follow the INTERNAL_DNS_DOMAIN attribute that it applies
to.
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 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.com".
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.
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CP(CFG_REQUEST) =
INTERNAL_IP4_ADDRESS()
INTERNAL_IP4_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_DNS_DOMAIN(example.com)
INTERNAL_DNS_DOMAIN(city.other.com)
3.4.2. Requesting Domains and DNSSEC trust anchors
In this example exchange, the initiator requests INTERNAL_IP4_DNS,
INTERNAL_DNS_DOMAIN and INTERNAL_DNSSEC_TA attributes in the
CFG_REQUEST.
Any subsequent DNS queries from the initiator for domains such as
"www.example.com" or "city.other.com" 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
would the requested domain. the "example.com" dommain has DNSSEC
trust anchors that are returned, while the "other.com" domain has no
DNSSEC trust anchors.
CP(CFG_REQUEST) =
INTERNAL_IP4_ADDRESS()
INTERNAL_IP4_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_DNS_DOMAIN(example.com)
INTERNAL_DNSSEC_TA(43547,8,1,B6225AB2CC613E0DCA7962BDC2342EA4...)
INTERNAL_DNSSEC_TA(31406,8,2,F78CF3344F72137235098ECBBD08947C...)
INTERNAL_DNS_DOMAIN(city.other.com)
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").
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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 optionally using IDNA [RFC5890] for
Internationalized Domain Names. 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) |
+-+-----------------------------+-------------------------------+
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 (0 or 2 octets) - Length of DNSSEC Trust Anchor data (4
octets plus the length of the Digest Data).
o DNSKEY Key Tag value (0 or 2 octets) - Delegation Signer (DS) Key
Tag as specified in [RFC4034] Section 5.1.
o DNSKEY Algorithm (0 or 1 octet) - DNSKEY algorithm value from the
IANA DNS Security Algorithm Numbers Registry.
o Digest Type (0 or 1 octet) - DS algorithm value from the IANA
Delegation Signer (DS) Resource Record (RR) Type Digest Algorithms
Registry.
o Digest Data (0 or more octets) - The DNSKEY digest as specified in
[RFC4034] Section 5.1 in presentation format.
INTERNAL_DNSSEC_TA payloads MUST immediately follow an
INTERNAL_DNS_DOMAIN payload. 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.
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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
number 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 sub-domains and it MUST NOT
attempt to resolve the provided DNS domains using its external DNS
servers.
If the initiator host is configured to block DNS answers containing
IP addresses from special IP address ranges such as those of
[RFC1918], the initiator SHOULD allow the DNS domains listed in the
INTERNAL_DNS_DOMAIN attributes to contain those Special IP addresses.
If a CFG_REPLY contains one or more INTERNAL_DNS_DOMAIN attributes
and its local policy does not forbid these values, the client MUST
configure its DNS resolver to resolve those domains and all their
subdomains using only the DNS resolver(s) listed in that CFG_REPLY
message. If those resolvers fail, those names MUST NOT be resolved
using any other DNS resolvers. Other domain names SHOULD be resolved
using some other external DNS resolver(s), 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 have
no guarantee of being answered. For example, if the
INTERNAL_DNS_DOMAIN attribute specifies "example.com", then
"example.com", "www.example.com" and "mail.eng.example.com" MUST be
resolved using the internal DNS resolver(s), but "anotherexample.com"
and "ample.com" SHOULD NOT be resolved using the internal resolver
and SHOULD use the system's external DNS resolver(s).
When an IKE 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.
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
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routed over the IPsec connection, the existing global
INTERNAL_IP4_DNS and INTERNAL_IP6_DNS can be used without creating
specific DNS exemptions.
6. INTERNAL_DNSSEC_TA Usage Guidelines
Installing an INTERNAL_DNSSEC_TA trust anchor can be seen as the
equivalent of installing an Enterprise Certificate Agency (CA)
certificate. It allows the remote IKE/IPsec server to modify DNS
answers including its DNSSEC cryptographic signatures by overriding
existing DNS information with trust anchor conveyed via IKE and
(temporarilly) installed on the IKE client. Of specific concern is
the overriding of [RFC6698] based TLSA records, which represent a
confirmation or override of an existing WebPKI TLS certificate.
Other DNS record types that convey cryptographic materials (public
keys or fingerprints) are OPENPGPKEY, SMIMEA, SSHP and IPSECKEY
records.
IKE clients MUST ignore any received INTERNAL_DNSSEC_TA requests for
a FDQN for which it did not receive and accept an INTERNAL_DNS_DOMAIN
Configuration Payload.
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
CA certificate or EE certificate to expect for a certain host name.
These records are protected by DNSSEC and thus can be trusted by the
application. Whether to trust TLSA records instead of the
traditional WebPKI 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. The same applies to
other public key or fingerprint based DNS records, such as
OPENPGPKEY, SMIMEA or IPSECKEY records.
In most deployment scenario's, the IKE client has an expectation that
it is connecting, using a split-network setup, to a specific
organisation or enterprise. 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. Security Considerations
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
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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.
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
public DNS view, for which it has not explicitely requested such
deletation by specifying the domain specifically using a
INTERNAL_DNS_DOMAIN(domain) request.
Deployments that configure INTERNAL_DNS_DOMAIN domains should pay
close attention to their use of indirect reference RRtypes such as
CNAME, DNAME, MX or SRV records so that resolving works as intended
when all, some, or none of the IPsec connections are established.
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.
8. 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 [this document]
26 INTERNAL_DNSSEC_TA YES 0 or more [this document]
Figure 1
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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>.
[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>.
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Authors' Addresses
Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014
US
Email: tpauly@apple.com
Paul Wouters
Red Hat
Email: pwouters@redhat.com
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