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draft-reddy-add-intarea-enterprise-split-dns-00.txt
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ADD T. Reddy
Internet-Draft McAfee
Intended status: Standards Track D. Wing
Expires: August 22, 2021 Citrix
February 18, 2021
Split-Horizon DNS Configuration in Enterprise Networks
draft-reddy-add-enterprise-split-dns-00
Abstract
When split-horizon DNS is deployed by an enterprise, certain
enterprise domains are only resolvable by querying the enterprise's
DNS server. DNS clients which use non-enterprise DNS servers need to
route those DNS domain queries to the enterprise's DNS server. This
document informs DNS clients of split-horizon DNS, their DNS domains,
and is compatible with encrypted DNS.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 22, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Reddy & Wing Expires August 22, 2021 [Page 1]
Internet-Draft Split-Horizon DNS Configuration February 2021
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Scope of the Document . . . . . . . . . . . . . . . . . . . . 4
4. Split DNS . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. PvD dnsZones . . . . . . . . . . . . . . . . . . . . . . . . 6
6. PvD SplitDNSAllowed Key . . . . . . . . . . . . . . . . . . . 7
7. An Example . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Roaming Enterprise Users . . . . . . . . . . . . . . . . . . 8
9. Upstream Encryption . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
13.1. Normative References . . . . . . . . . . . . . . . . . . 9
13.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Historically, an endpoint would utilize network-provided DNS servers
upon joining a network (e.g., DHCP OFFER, IPv6 Router Advertisement).
While it has long been possible to configure endpoints to ignore the
network's suggestions and use a (public) DNS server on the Internet,
this was seldom used because some networks block UDP/53 (in order to
enforce their own DNS policies). With the advent of DoT and DoH,
such network blocking is more difficult, but the endpoint is unable
to (properly) resolve split-horizon DNS domains which must query the
enterprise's DNS server.
[RFC7626] discusses DNS privacy considerations in both "on the wire"
(Section 2.4 of [RFC7626]) and "in the server" (Section 2.5 of
[RFC7626]) contexts. Also, there has been an increase in the
availability of "public resolvers" [RFC8499] which DNS clients may be
pre-configured to use instead of the default network resolver for a
variety of reasons (e.g., offer a good reachability, support an
encrypted transport, provide a claimed privacy policy, (lack of)
filtering).
If public encrypted DNS servers (e.g., DNS-over-TLS (DoT) [RFC7858]
or DNS-over-HTTPS (DoH) [RFC8484]) are used instead of using local
DNS servers, it can adversely impact Enterprise network-based
security features. Indeed, various network security services are
provided by Enterprise networks to protect endpoints (e.g., laptops,
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Internet-Draft Split-Horizon DNS Configuration February 2021
printers, IoT devices) and to enforce enterprise-specific policies.
These policies may be necessary to protect employees, customers, or
enterprise network. It is out of the scope of this memo to
characterize such policies nor assess that they achieve the claimed
intent. Nevertheless, Enterprise DNS servers in place for these
purposes act on DNS messages involving endpoints connected to the
Enterprise network to enforce these policies. Therefore, if an
endpoint uses a public encrypted DNS server, the desired enterprise
protection level and enforcement will be bypassed and thus nullified.
In order to act on DNS messages involving endpoints connected to an
Enterprise network, network security services can be configured to
block DoT traffic by dropping outgoing packets to destination port
number 853. Identifying DoH traffic is far more challenging than
identifying DoT traffic. Network security services may try to
identify the well-known DoH resolvers by their domain name and DoH
traffic can be blocked by dropping outgoing packets to these domains.
However, DoH traffic can not be fully identified without acting as a
TLS proxy.
If a network security service blocks access to a public encrypted DNS
server, there are incompatibilities with the privacy profiles
discussed in [RFC8310]:
o If an endpoint has enabled strict privacy profile (Section 5 of
[RFC8310]), the endpoint cannot resolve DNS names.
o If an endpoint has enabled opportunistic privacy profile
(Section 5 of [RFC8310]), the endpoint will either fallback to an
encrypted connection without authenticating the DNS server
provided by the local network or fallback to clear text DNS, and
cannot exchange encrypted DNS messages.
The fallback adversely impacts security and privacy as internal
attacks are possible within Enterprise networks. For example, an
internal attacker can modify the DNS responses to re-direct a
client to malicious servers or pervasively monitor the DNS
traffic. The reader may refer to Section 3.2.1 of
[I-D.arkko-farrell-arch-model-t] for a discussion on the need for
more awareness about attacks from within closed domains.
To overcome the above threats, this document specifies a mechanism to
indicate which DNS zones are used for split-horizon DNS. DNS clients
can discover and authenticate encrypted DNS servers provided by the
Enterprise network, for example using the techniques proposed in
[I-D.btw-add-home] and [I-D.ietf-add-ddr]. Discovery of encrypted
DNS server for roaming enterprise endpoints is discussed in
[I-D.btw-add-ipsecme-ike] (see Section 8).
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Provisioning Domains (PvDs) are defined in [RFC7556] as sets of
network configuration information that clients can use to access
networks, including rules for DNS resolution and proxy configuration.
[RFC8801] defines a mechanism for discovering multiple Explicit PvDs
on a single network and their Additional Information by means of an
HTTP-over-TLS query using a URI derived from the PvD ID. This set of
additional configuration information is referred to as a Web
Provisioning Domain (Web PvD).
This document defines one PvD Key:
The SplitDNSAllowed PvD Key: which determines if the Enterprise
network allows split-horizon DNS.
2. Terminology
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.
This document makes use of the terms defined in [RFC8499].
'Encrypted DNS' refers to a DNS protocol that provides an encrypted
channel between a DNS client and server (e.g., DoT, DoH, or DoQ).
3. Scope of the Document
If a device is managed by an enterprise's IT department, the device
can be configured to use Enterprise-provided encrypted DNS servers.
This configuration may be manual or rely upon whatever deployed
device management tool in an Enterprise network. For example,
customizing Firefox using Group Policy to use the Enterprise DoH
server is discussed in [Firefox-Policy] for Windows and MacOS, and
setting Chrome policies is discussed in [Chrome-Policy] and
[Chrome-DoH].
If mobile device management (MDM) (e.g., [MDM-Apple]) secures a
device, MDM can configure OS/browser with the Enterprise provided
DoH/DoT server. If an endpoint is on-boarded, for example, using
Over-The-Air (OTA) enrollment [OTA] to provision the device with a
certificate and configuration profile, the configuration profile can
include the authentication domain name (ADN) of the encrypted DNS
server. The OS/Browser can use the configuration profile to use the
Enterprise provided encrypted DNS server. In this case, MDM is not
installed on the device.
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Provisioning IT-managed devices, BYOD devices with MDM or
configuration profile with the Split DNS configuration is outside the
scope of this document.
Typically, Enterprise networks do not assume that all devices in
their network are managed by the IT team or MDM, especially in the
quite common BYOD scenario. The endpoint can use the discovered
Enterprise Encrypted DNS server to only access DNS names for which
the Enterprise network claims authority and use another public DNS
server for global domains or use the discovered Enterprise Encrypted
DNS server to access both private domains and global domains.
The scope of this document is restricted to unmanaged BYOD devices
without a configuration profile. The unmanaged BYOD devices use the
credentials (user name and password) provided by the IT admin to
mutually authenticate to the Enterprise WLAN Access Point (e.g.,
PEAP-MSCHAPv2 [PEAP], EAP-pwd [RFC8146], EAP-PSK [RFC4764]).
Note: Many users have privacy and personal data sovereignty
concerns with employers installing MDM on their personal devices;
they are concerned that admin can glean personal information and
could control how they use their devices. When users do not
install MDM on their devices, IT admins do not get visibility into
the security posture of those devices. To overcome this problem,
a host agent can cryptographically attest the security status
associated with device, such as minimum pass code length,
biometric login enabled, OS version etc. This approach is fast
gaining traction especially with the advent of closed OS like
Windows 10 in S mode [win10s] or Chromebook [Chromebook], where
applications are sandboxed (e.g., ransomware attack is not
possible) and applications can only be installed via the OS store.
4. Split DNS
[RFC2826] "does not preclude private networks from operating their
own private name spaces" but notes that if private networks "wish to
make use of names uniquely defined for the global Internet, they have
to fetch that information from the global DNS naming hierarchy".
There are various DNS deployments outside of the global DNS,
including "split horizon" deployments and DNS usages on private (or
virtual private) networks. In a split horizon, an authoritative
server gives different responses to queries from the Internet than
they do to network-provided DNS servers; while some deployments
differentiate internal queries from public queries by the source IP
address, the concerns in Section 3.1.1 of [RFC6950] relating to
trusting source IP addresses apply to such deployments.
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When the internal address space range is private [RFC1918], this
makes it both easier for the server to discriminate public from
private and harder for public entities to impersonate nodes in the
private network. The use cases that motivate split-horizon DNS
typically involve restricting access to some network services --
intranet resources such as internal web sites, development servers,
or directories, for example -- while preserving the ease of use
offered by domain names for internal users.
An Enterprise can require one or more DNS domains to be resolved via
network-provided DNS servers. This can be a special domain, such as
"corp.example.com" for an enterprise that is publicly known to use
"example.com". In this case, the endpoint needs to be informed what
the private domain names are and what the IP addresses of the
network-provided DNS servers are. An Enterprise can also run a
different version of its global domain on its internal network. In
that case, the client is instructed to send DNS queries for the
enterprise public domain (e.g., "example.com") to the network-
provided DNS servers. A configuration for this deployment scenario
is referred to as a Split DNS configuration.
The PvD RA option defined in [RFC8801] SHOULD set the H-flag to
indicate that Additional Information is available. This Additional
Information JSON object SHOULD include both the "dnsZones" and
"SplitDNSAllowed" keys to define the DNS domains for which the
Enterprise network claims authority and to indicate if the Enterprise
network allows split-horizon DNS.
5. PvD dnsZones
As discussed in Section 4, the Enterprise internal resources tend to
have private DNS names, an enterprise can also run a different
version of its global domain on its internal network, and require the
use of network-provided DNS servers to get resolved.
The PvD Key dnsZones is defined in [RFC8801]. The PvD Key dnsZones
adds support for DNS domains for which the Enterprise network claims
authority. These domains are intended to be resolved using network-
provided DNS servers that are only reachable to the devices attached
to the Enterprise network. DNS resolution for other domains remains
unchanged.
The dnsZones PvD Key conveys the specified DNS domains must be
resolved using an Enterprise DNS server. The DNS root zone (".")
MUST be ignored if it appears in dnsZones. Other generic or global
domains, such as Top-Level Domains (TLDs), similarly MUST be ignored
if they appear in dnsZones.
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For each dnsZones entry, the client MUST use the network-provided 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 public resolvers. Other domain names may be resolved
using some other public resolvers that are configured independently.
6. PvD SplitDNSAllowed Key
If an Enterprise network restricts all the DNS queries to be sent to
the network-provided DNS server, SplitDNSAllowed will be set to
false.
Split DNS configurations may be preferable to sending all DNS queries
to an Enterprise DNS server in some deployments. This allows an
endpoint to only send DNS queries for the enterprise to the network-
provided 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 endpoints. For example, if the
SplitDNSAllowed key specifies "example.test" and SplitDNSAllowed is
set to true, then "example.test", "www.example.test", and
"mail.eng.example.test" must be resolved using the network-provided
DNS resolver(s), but "otherexample.test" and "ple.test" can be
resolved using the system's public resolver(s).
If SplitDNSAllowed is set to false, the client should not trust the
SplitDNSAllowed key in case of connecting to unknown or untrusted
networks (e.g., coffee shops or hotel networks). For example, if
SplitDNSAllowed is set to false, client can choose to use a alternate
network to resolve the global domain names.
7. An Example
The following example shows how the JSON keys defined in this
document can be used:
{
"identifier": "cafe.example.com.",
"expires": "2020-05-23T06:00:00Z",
"prefixes": ["2001:db8:1::/48", "2001:db8:4::/48"],
"SplitDNSAllowed": True,
"dnsZones:": ["city.other.test", "example.com"]
}
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The JSON keys "identifier", "expires", and "prefixes" are defined in
[RFC8801].
8. Roaming Enterprise Users
In this Enterprise scenario (Section 1.1.3 of [RFC7296]), a roaming
user connects to the Enterprise network through an VPN tunnel (e.g.,
IPsec, SSL, Wireguard). The split-tunnel Virtual Private Network
(VPN) configuration allows the endpoint to access hosts that reside
in the Enterprise network [RFC8598] using that tunnel; other traffic
not destined to the Enterprise does not traverse the tunnel. In
contrast, a non-split- tunnel VPN configuration causes all traffic to
traverse the tunnel into the Enterprise.
When the VPN tunnel is IPsec, the encrypted server hosted by the
Enterprise network can be securely discovered by the endpoint using
the ENCDNS_IP*_* IKEv2 Configuration Payload Attribute Types defined
in [I-D.btw-add-ipsecme-ike]. For split-tunnel VPN configurations,
the endpoint uses the Enterprise-provided encrypted DNS server to
resolve domain names for which the Enterprise network claims
authority. For non-split-tunnel VPN configurations, the endpoint
uses the Enterprise-provided encrypted DNS server to resolve both
global and private domain names.
Other VPN tunnel types have similar configuration capabilities, not
detailed here.
9. Upstream Encryption
If an Enterprise network is using a local encrypted DNS server
configured as a Forwarding DNS server [RFC8499] relying upon the
upstream resolver (e.g., at an ISP) to perform recursive DNS lookups,
DNS messages exchanged between the local encrypted DNS server and the
recursive resolver MUST be encrypted.
If the Enterprise network is using the local encrypted DNS server
configured as a recursive DNS server, DNS messages exchanges between
the recursive resolver and authoritative servers SHOULD be encrypted
to conform to the requirements discussed in
[I-D.ietf-dprive-phase2-requirements].
10. Security Considerations
Clients may want to preconfigure global domains for TLDs and Second-
Level Domains (SLDs) to prevent malicious DNS redirections for well-
known domains. This prevents users from unknowingly giving DNS
queries to third parties. This is even more important if those well-
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known domains are not deploying DNSSEC, as the Enterprise network
could then even modify the DNS answers without detection.
The content of dnsZones and SplitDNSAllowed may be passed to another
(DNS) program for processing. As with any network input, the content
SHOULD be considered untrusted and handled accordingly. The split
DNS configuration assigned by an anonymous or unknown network (e.g.,
coffee shops) MUST be ignored by the client.
11. IANA Considerations
IANA is requested to add SplitDNSAllowed PvD Key to the Additional
Information PvD Keys registry (https://www.iana.org/assignments/pvds/
pvds.xhtml).
12. Acknowledgements
Thanks to Mohamed Boucadair and Vinny Parla for the discussion and
comments.
13. References
13.1. 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>.
[RFC2826] Internet Architecture Board, "IAB Technical Comment on the
Unique DNS Root", RFC 2826, DOI 10.17487/RFC2826, May
2000, <https://www.rfc-editor.org/info/rfc2826>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[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>.
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[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC8801] Pfister, P., Vyncke, E., Pauly, T., Schinazi, D., and W.
Shao, "Discovering Provisioning Domain Names and Data",
RFC 8801, DOI 10.17487/RFC8801, July 2020,
<https://www.rfc-editor.org/info/rfc8801>.
13.2. Informative References
[Chrome-DoH]
The Unicode Consortium, "Chrome DNS over HTTPS (aka DoH)",
<https://www.chromium.org/developers/dns-over-https>.
[Chrome-Policy]
The Unicode Consortium, "Chrome policies for users or
browsers", <https://support.google.com/chrome/a/
answer/2657289?hl=en>.
[Chromebook]
Microsoft, "Chromebook security",
<https://support.google.com/chromebook/
answer/3438631?hl=en>.
[Firefox-Policy]
"Policy templates for Firefox",
<https://github.com/mozilla/policy-templates/blob/master/
README.md#dnsoverhttps>.
[I-D.arkko-farrell-arch-model-t]
Arkko, J. and S. Farrell, "Challenges and Changes in the
Internet Threat Model", draft-arkko-farrell-arch-model-
t-04 (work in progress), July 2020.
[I-D.btw-add-home]
Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
Jensen, "DHCP and Router Advertisement Options for
Encrypted DNS Discovery", draft-btw-add-home-12 (work in
progress), January 2021.
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Internet-Draft Split-Horizon DNS Configuration February 2021
[I-D.btw-add-ipsecme-ike]
Boucadair, M., Reddy.K, T., Wing, D., and V. Smyslov,
"Internet Key Exchange Protocol Version 2 (IKEv2)
Configuration for Encrypted DNS", draft-btw-add-ipsecme-
ike-01 (work in progress), September 2020.
[I-D.ietf-dprive-phase2-requirements]
Livingood, J., Mayrhofer, A., and B. Overeinder, "DNS
Privacy Requirements for Exchanges between Recursive
Resolvers and Authoritative Servers", draft-ietf-dprive-
phase2-requirements-02 (work in progress), November 2020.
[MDM-Apple]
Apple, "Mobile Device Management",
<https://developer.apple.com/documentation/
devicemanagement>.
[OTA] Apple, "Over-the-Air Profile Delivery Concepts", <https://
developer.apple.com/library/archive/documentation/Networki
ngInternet/Conceptual/iPhoneOTAConfiguration/OTASecurity/
OTASecurity.html>.
[PEAP] Microsoft, "[MS-PEAP]: Protected Extensible Authentication
Protocol (PEAP)", <https://docs.microsoft.com/en-
us/openspecs/windows_protocols/ms-peap/5308642b-90c9-4cc4-
beec-fb367325c0f9>.
[RFC4764] Bersani, F. and H. Tschofenig, "The EAP-PSK Protocol: A
Pre-Shared Key Extensible Authentication Protocol (EAP)
Method", RFC 4764, DOI 10.17487/RFC4764, January 2007,
<https://www.rfc-editor.org/info/rfc4764>.
[RFC6950] Peterson, J., Kolkman, O., Tschofenig, H., and B. Aboba,
"Architectural Considerations on Application Features in
the DNS", RFC 6950, DOI 10.17487/RFC6950, October 2013,
<https://www.rfc-editor.org/info/rfc6950>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/info/rfc7556>.
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Internet-Draft Split-Horizon DNS Configuration February 2021
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<https://www.rfc-editor.org/info/rfc7626>.
[RFC8146] Harkins, D., "Adding Support for Salted Password Databases
to EAP-pwd", RFC 8146, DOI 10.17487/RFC8146, April 2017,
<https://www.rfc-editor.org/info/rfc8146>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[RFC8598] Pauly, T. and P. Wouters, "Split DNS Configuration for the
Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 8598, DOI 10.17487/RFC8598, May 2019,
<https://www.rfc-editor.org/info/rfc8598>.
[win10s] Microsoft, "Windows 10 in S mode",
<https://www.microsoft.com/en-us/windows/s-mode>.
Authors' Addresses
Tirumaleswar Reddy
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore, Karnataka 560071
India
Email: [email protected]
Dan Wing
Citrix Systems, Inc.
4988 Great America Pkwy
Santa Clara, CA 95054
USA
Email: [email protected]
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