Split-Horizon DNS Configuration
draft-reddy-add-enterprise-split-dns-07
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (candidate for add WG) | |
|---|---|---|---|
| Authors | Tirumaleswar Reddy.K , Dan Wing , Kevin Smith | ||
| Last updated | 2021-11-06 (Latest revision 2021-09-15) | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text html xml htmlized pdfized bibtex | ||
| Stream | WG state | Call For Adoption By WG Issued | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-reddy-add-enterprise-split-dns-07
ADD T. Reddy
Internet-Draft Akamai
Intended status: Standards Track D. Wing
Expires: May 10, 2022 Citrix
K. Smith
Vodafone
November 6, 2021
Split-Horizon DNS Configuration
draft-reddy-add-enterprise-split-dns-07
Abstract
When split-horizon DNS is deployed by a network, certain domains are
only resolvable by querying the network-designated DNS server rather
than a public DNS server. DNS clients which use DNS servers not
provided by the network need to route those DNS domain queries to the
network-designated DNS server. This document informs DNS clients of
split-horizon DNS, their DNS domains, and is compatible with
encrypted DNS.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 10, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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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
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Split DNS . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Provisioning Domains dnsZones . . . . . . . . . . . . . . . . 4
4.1. Authority over the Domains . . . . . . . . . . . . . . . 5
5. An example of Split-Horizon DNS Configuration . . . . . . . . 6
6. Split DNS Configuration for IKEv2 . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Historically, an endpoint would utilize network-designated DNS
servers upon joining a network (e.g., DHCP OFFER, IPv6 Router
Advertisement). While it has long been possible to configure
endpoints to ignore the network's suggestions and use a public DNS
server on the Internet, this was seldom used because some networks
block UDP/53 in order to enforce their own DNS policies. Also, there
has been an increase in the availability of "public resolvers"
[RFC8499] which DNS clients may be pre-configured to use instead of
the default network resolver for a variety of reasons (e.g., offer a
good reachability, support an encrypted transport, provide a claimed
privacy policy, (lack of) filtering). With the advent of DoT and
DoH, the endpoint is unable to properly resolve split-horizon DNS
domains which must query the network-designated DNS server.
This document specifies a mechanism to indicate which DNS zones are
used for split-horizon DNS. DNS clients can discover and
authenticate DNS servers provided by the network, for example using
the techniques proposed in [I-D.ietf-add-dnr] and [I-D.ietf-add-ddr].
The scope of the specification is split-horizon DNS names, which are
not domains reserved for special use like ".local". Using these
domains are difficult because bookmarks, advertising, and human
behavior would need to change so that the special domains only work
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when connected to the correct network, which is difficult for users
to discern on most endpoints. Furthermore, as special domains cannot
obtain certificates from a public CA, authenticated TLS connections
to those servers are fraught with (Trust on First Use) and
certificate warnings, a longstanding problem in the industry.
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]. The terms
"Private DNS", "Global DNS" and "Split DNS" are defined in [RFC8499].
'Encrypted DNS' refers to a DNS protocol that provides an encrypted
channel between a DNS client and server (e.g., DoT, DoH, or DoQ).
3. Split DNS
[RFC2826] "does not preclude private networks from operating their
own private name spaces" but notes that if private networks "wish to
make use of names uniquely defined for the global Internet, they have
to fetch that information from the global DNS naming hierarchy".
There are various DNS deployments outside of the global DNS,
including "split horizon" deployments and DNS usages on private (or
virtual private) networks. In a split horizon, an authoritative
server gives different responses to queries from the Internet than
they do to network-designated DNS servers; while some deployments
differentiate internal queries from public queries by the source IP
address, the concerns in Section 3.1.1 of [RFC6950] relating to
trusting source IP addresses apply to such deployments.
When the internal address space range is private [RFC1918], this
makes it both easier for the server to discriminate public from
private and harder for public entities to impersonate nodes in the
private network. The use cases that motivate split-horizon DNS
typically involve restricting access to some network services --
intranet resources such as internal web sites, development servers,
or directories, for example -- while preserving the ease of use
offered by domain names for internal users.
A typical use case is an Enterprise network that requires one or more
DNS domains to be resolved via network-designated DNS servers. This
can be a special domain, such as "corp.example.com" for an enterprise
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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-designated DNS servers are. An
Enterprise can also run a different version of its global domain on
its internal network. In that case, the client is instructed to send
DNS queries for the enterprise public domain (e.g., "example.com") to
the network-designated DNS servers. A configuration for this
deployment scenario is referred to as a Split DNS configuration.
Another use case for split-horizon DNS is Cellular and Fixed-access
networks (ISPs) typically offer private domains, including account
status/controls, and free education initiatives [INS].
4. Provisioning Domains dnsZones
As discussed in Section 3, internal resources in a network tend to
have private DNS names. A network can also run a different version
of its global domain on its internal network, and require the use of
network-designated DNS servers to get resolved.
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.
The PvD Key dnsZones is defined in [RFC8801]. The PvD Key dnsZones
adds support for DNS domains for which the network claims authority,
and which are intended to be resolved using network-designated DNS
servers. The private domains in dnsZones are only reachable by
devices attached and authenticated to the network. The global
domains specified in the dnsZones key have a different version in the
internal network. DNS resolution for other domains remains
unchanged.
For each dnsZones entry, the client can use the network-designated
DNS servers to resolve the listed domains and its subdomains. Other
domain names may be resolved using some other DNS servers that are
configured independently. For example, if the dnsZones key specifies
"example.test", then "example.test", "www.example.test", and
"mail.eng.example.test" can be resolved using the network-designated
DNS resolver(s), but "otherexample.test" and "ple.test" can be
resolved using the system's public resolver(s).
[RFC8801] defines a mechanism for discovering multiple Explicit PvDs
on a single network and their Additional Information by means of an
HTTP-over-TLS query using a URI derived from the PvD ID. This set of
additional configuration information is referred to as a Web
Provisioning Domain (Web PvD). The PvD RA option defined in
[RFC8801] SHOULD set the H-flag to indicate that Additional
Information is available. This Additional Information JSON object
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SHOULD include the "dnsZones" key to define the DNS domains for which
the network claims authority.
4.1. Authority over the Domains
To comply with [RFC2826] the split-horizon DNS zone must either not
exist in the global DNS hierarchy or must be authoritatively
delegated to the split-horizon DNS server to answer. The 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.
The client can use the mechanism described in [I-D.ietf-add-dnr] to
discover the network-designated resolvers. To determine if the
network-designated encrypted resolvers are authoritative over the
domains in dnsZones, the client performs the following steps for each
domain in dnsZones:
1. The client sends an NS query for the domain in dnsZones. This
query MUST only be sent over encrypted DNS session to a public
resolver that is configured independently. A DNSSEC-validating
client SHOULD apply the same validation policy to the NS query as
it does to other queries. A client that does not validate DNSSEC
SHOULD apply the same policy (if any) to the NS query as it does
to other queries.
2. The client checks that the NS RRset matches any one of the
Authentication Domain Names (ADNs) of the discovered network-
designated encrypted DNS resolvers.
A. If the match fails and no ADN of the discovered network-
designated encrypted DNS resolvers is a subdomain of NS
RRset, the client determines the network is not authoritative
for the indicated domain. It might log an error, reject the
network entirely (because the network lied about its
authority over a domain) or other action.
B. If the match fails but any one of the ADNs of the discovered
network-designated encrypted DNS resolvers is a subdomain of
NS RRset, the client can then establish a TLS connection to
that network-designated resolver. The client follows the
mechanism discussed in Section 8 of [RFC8310] to authenticate
the DNS server certificate using the ADN and checks if at
least one of the values in subjectAltName matches NS RRset.
If the server certificate validation and match succeeds, the
client can subsequently resolve the domains in that subtree
using the network-designated resolver. If the server
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certificate does not validate or match fails, the client can
proceed as discussed in step 2 (A).
C. If the match succeeds, the client can then establish a TLS
connection to that network-designated resolver. The client
follows the mechanism discussed in Section 8 of [RFC8310] to
authenticate the DNS server certificate using the ADN.
+ If the server certificate does not validate and a secure
connection cannot be established to the network designated
resolver, the client can proceed as discussed in step 2
(A).
+ If the server certificate validation is successful and a
secure connection is established, the client can
subsequently resolve the domains in that subtree using the
network-designated resolver.
3. As an exception to this rule, the client need not perform the
above validation for domains reserved for special use [RFC6761]
or [RFC6762] such as ".home.arpa" or ".local".
4. Authenticated denial of existence using NSEC3 or NSEC records can
be used by a client to identify that the domain name does not
exist in the global DNS.
For example, if in a network the private domain names are defined
under "internal.corp1.example.com". The dnsZones PvD Key would
indicate that "*.internal.corp1.example.com" are private domain
names. The client can trigger a NS query of
"internal.corp1.example.com" and the NS RRset returns that the
nameserver is "ns1.corp2.example.com". The client would then connect
to the network-designated encrypted resolver whose name is
"ns1.corp2.example.com", authenticate it using server certificate
validation in TLS handshake, and use it for resolving the domains in
the subtree of "*.internal.corp1.example.com".
5. An example of Split-Horizon DNS Configuration
In a network the apex domain name is "example.com", which runs a
different version of its global domain on its internal network.
Today, on the Internet it publishes two NS records, "ns1.example.com"
and "ns2.example.com".
To add support for the mechanism described in this document, the
network and endpoints first need to support [I-D.ietf-add-dnr] and
[RFC8801]. Then, for each site the administrator would add DNS
server names which end in "ns1.example.com" or "ns2.example.com" (the
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names published on the Internet). Also on its internal network, it
uses a geographic naming scheme for its internal nameservers with a
"service dot location" naming scheme. Assuming its two geographies
are "us" and "uk". So the UK site would add "ns1.uk.ns1.example.com"
and "ns2.uk.ns2.example.com". Note that these names can be added in
addition to more traditional names that might already exist for those
DNS servers (e.g., the common "service dot location" such as
"ns1.uk.example.com"). All of those names would be advertised to the
endpoints as described in [I-D.ietf-add-dnr].
The endpoints compliant with this specification can then determine
the network's internal nameservers are owned and managed by the same
entity that has published the NS records on the Internet, as
described in the following paragraphs:
Continuing with the UK site as the example, the endpoint will join
the network, obtain an IPv6 address, and using [I-D.ietf-add-dnr]
will discover the resolvers "ns1.uk.ns1.example.com" and
"ns2.uk.ns2.example.com" and their IP addresses. Using [RFC8801]
(which utilizes IPv6 Router Advertisments), the endpoint will also
discover the PvD FQDN is "pvd.uk.example.com".
Continuing with the UK site as the example, the endpoint performs the
following steps corresponding with the call flow diagram numbers:
Steps 1-2: The client joins the network, obtain an IP address, and
using [I-D.ietf-add-dnr] will discover the resolvers
"ns1.uk.ns1.example.com" and "ns2.uk.ns1.example.com" and their IP
addresses. Using [RFC8801], the endpoint will also discover the
PvD FQDN is "pvd.uk.example.com".
Steps 3-7: The client establishes an encrypted DNS connection with
"ns1.uk.ns1.example.com", validates its TLS certificate, and
queries it for "pvd.uk.example.com" to retrieve the PvD JSON
object. Note that [RFC8801] in Section 4.1 mandates the PvD FQDN
MUST be resolved using the DNS servers indicated by the associated
PvD. The PvD contains:
{
"identifier": "pvd.uk.example.com",
"expires": "2020-05-23T06:00:00Z",
"prefixes": ["2001:db8:1::/48", "2001:db8:4::/48"],
"dnsZones:": ["example.com"]
}
The JSON keys "identifier", "expires", and "prefixes" are defined
in [RFC8801].
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Steps 8-9: The client then uses an encrypted DNS connection to a
public resolver (e.g., 1.1.1.1) to issue NS queries for the
domains in dnsZones. The NS lookup for "example.com" will return
"ns1.example.com" and "ns2.example.com".
Step 10: As the network-provided nameservers are subdomains of those
names retrieved from the public resolver and the network-
designated resolver's certificate includes at least one of the
names retrieved from the public resolver, the client has finished
validation that the nameservers signaled in [I-D.ietf-add-dnr] and
[RFC8801] are owned and managed by the same entity that published
the NS records on the Internet. The endpoint will then use that
information from [I-D.ietf-add-dnr] and [RFC8801] to resolve names
within dnsZones.
Figure 1 is a simple call flow diagram of the example discussed
above.
+---------+ +-------------------+ +------------+ +---------+ +---------+
| client | | Network | | Network | | Router | | public |
| | | encrypted resolvr | | PvD server | | | | resolvr |
+---------+ +-------------------+ +------------+ +---------+ +---------+
| | | | |
| Router Solicitation (1) | | | |
|-------------------------------------------------------------->| |
| | | | |
| Router Advertisement with DNR hostnames & PvD FQDN (2) | |
|<--------------------------------------------------------------| |
| -------------------------------------\ | | | |
|-| now knows DNR hostnames & PvD FQDN | | | | |
| |------------------------------------| | | | |
| | | | |
| TLS connection (3) | | | |
|----------------------------------------->| | | |
| ---------------------------\ | | | |
|-| validate TLS certificate | | | | |
| |--------------------------| | | | |
| | | | |
| resolve pvd.uk.example.com (4) | | | |
|----------------------------------------->| | | |
| | | | |
| AAAA records (5) | | | |
|<-----------------------------------------| | | |
| | | | |
| https://pvd.uk.example.com/.well-known/pvd (6) | | |
|--------------------------------------------------->| | |
| | | | |
| 200 OK (JSON Additional Information) (7) | | |
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|<---------------------------------------------------| | |
| -----------------------\ | | | |
|-| dnsZones=example.com | | | | |
| |----------------------| | | | |
| | | | |
| TLS connection | | | |
|-------------------------------------------------------------------------->|
| ---------------------------\ | | | |
|-| validate TLS certificate | | | | |
| |--------------------------| | | | |
| | | | |
| NS? example.com (8) | | | |
|-------------------------------------------------------------------------->|
| | | | |
| NS=ns1.example.com, ns2.example.com (9) |
|<--------------------------------------------------------------------------|
| -------------------------------\ | | | |
|-| ns1.uk.ns1.example.com is | | | | |
| | subdomain of ns1.example.com | | | | |
| ----------------------\--------| | | | |
|-| finished validation | | | | |
| |---------------------| | | | |
| | | | |
| use network-designated resolver | | | |
| for example.com (10) | | | |
|----------------------------------------->| | | |
| | | | |
Figure 1: An Example of Split-Horizon DNS Configuration
6. Split DNS Configuration for IKEv2
The split-tunnel Virtual Private Network (VPN) configuration allows
the endpoint to access resources that reside in the VPN [RFC8598] via
the tunnel; other traffic not destined to the VPN does not traverse
the tunnel. In contrast, a non-split- tunnel VPN configuration
causes all traffic to traverse the tunnel into the VPN.
When the VPN tunnel is IPsec, the encrypted DNS resolver hosted by
the VPN service provider can be securely discovered by the endpoint
using the ENCDNS_IP*_* IKEv2 Configuration Payload Attribute Types
defined in [I-D.btw-add-ipsecme-ike]. For split-tunnel VPN
configurations, the endpoint uses the discovered encrypted DNS server
to resolve domain names for which the VPN provider claims authority.
For non-split-tunnel VPN configurations, the endpoint uses the
discovered encrypted DNS server to resolve both global and private
domain names. For split-tunnel VPN configurations, the IKE client
can use the steps discussed in Section 4.1 to determine if the VPN
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service provider is authoritative over the INTERNAL_DNS_DOMAIN
domains.
Other VPN tunnel types have similar configuration capabilities, not
detailed here.
7. Security Considerations
The content of dnsZones may be passed to another (DNS) program for
processing. As with any network input, the content SHOULD be
considered untrusted and handled accordingly. The client must
perform the steps discussed in Section 4.1 to determine if the
network-designated encrypted resolvers are authoritative over the
domains in dnsZones. If not, the client can take appropriate action
like disconnecting from the network.
In order to deploy DNSSEC operationally for split-horizon domains,
the first three schemes described in
[I-D.krishnaswamy-dnsop-dnssec-split-view] can be used but not the
one discussed in Section 4.1.4. Alternatively, after validating the
network authority over the dnsZones domains Section 4.1, a DNSSEC-
validating client can reconfigure its stub resolver to disable DNSSEC
validation for the domains in dnsZones. The deployment model
discussed in this document assumes a hybrid DNS server that acts both
as a recursive server and is also authoritative for the domains in
dnsZones. It prevents DNS cache-poisoning attacks for the dnsZones
domains.
8. IANA Considerations
This document has no IANA actions..
9. Acknowledgements
Thanks to Mohamed Boucadair, Jim Reid, Ben Schwartz, Tommy Pauly,
Paul Vixie, Paul Wouters and Vinny Parla for the discussion and
comments. The authors would like to give special thanks to Ben
Schwartz for his help.
10. References
10.1. Normative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<https://www.rfc-editor.org/info/rfc1918>.
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[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>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[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>.
[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>.
[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>.
10.2. Informative References
[I-D.btw-add-ipsecme-ike]
Boucadair, M., Reddy, T., Wing, D., and V. Smyslov,
"Internet Key Exchange Protocol Version 2 (IKEv2)
Configuration for Encrypted DNS", draft-btw-add-ipsecme-
ike-03 (work in progress), May 2021.
[I-D.ietf-add-ddr]
Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", draft-ietf-
add-ddr-03 (work in progress), October 2021.
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[I-D.ietf-add-dnr]
Boucadair, M., Reddy, T., Wing, D., Cook, N., and T.
Jensen, "DHCP and Router Advertisement Options for the
Discovery of Network-designated Resolvers (DNR)", draft-
ietf-add-dnr-02 (work in progress), May 2021.
[I-D.krishnaswamy-dnsop-dnssec-split-view]
Krishnaswamy, S., "Split-View DNSSEC Operational
Practices", draft-krishnaswamy-dnsop-dnssec-split-view-04
(work in progress), March 2007.
[INS] The Unicode Consortium, "Vodafone Foundation Instant
Schools for Sub-Saharan Africa",
<https://www.vodafone.com/about/vodafone-foundation/focus-
areas/instant-schools>.
[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>.
[RFC7556] Anipko, D., Ed., "Multiple Provisioning Domain
Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
<https://www.rfc-editor.org/info/rfc7556>.
[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>.
Authors' Addresses
Tirumaleswar Reddy
Akamai
Embassy Golf Link Business Park
Bangalore, Karnataka 560071
India
Email: kondtir@gmail.com
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Internet-Draft Split-Horizon DNS Configuration November 2021
Dan Wing
Citrix Systems, Inc.
4988 Great America Pkwy
Santa Clara, CA 95054
USA
Email: danwing@gmail.com
Kevin Smith
Vodafone Group
One Kingdom Street
London
UK
Email: kevin.smith@vodafone.com
Reddy, et al. Expires May 10, 2022 [Page 13]