ADD T. Reddy
Internet-Draft McAfee
Intended status: Standards Track D. Wing
Expires: September 29, 2021 Citrix
March 28, 2021
Split-Horizon DNS Configuration in Enterprise Networks
draft-reddy-add-enterprise-split-dns-02
Abstract
When split-horizon DNS is deployed by an enterprise, certain
enterprise domains are only resolvable by querying the network-
designated 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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 29, 2021.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scope of the Document . . . . . . . . . . . . . . . . . . . . 3
4. Split DNS . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. PvD dnsZones . . . . . . . . . . . . . . . . . . . . . . . . 6
6. PvD SplitDNSAllowed and ErrorSplitDNSBlocked Keys . . . . . . 6
7. An Example . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Roaming Enterprise Users . . . . . . . . . . . . . . . . . . 7
9. Upstream Encryption . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 10
13.2. Informative References . . . . . . . . . . . . . . . . . 10
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). 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 network-designated 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).
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.ietf-add-
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dnr] 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).
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]. 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).
The term "enterprise network" in this document extends to a wide
variety of deployment scenarios. For example, an "enterprise" can be
a Small Office, Home Office or Corporation. The clients that connect
to a enterprise network can securely authenticate that network and
the client is sure that it has connected to the network it was
expecting.
3. Scope of the Document
If a device is managed by an enterprise's IT department, the device
can be configured to use a specific encrypted DNS server. 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].
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If mobile device management (MDM) (e.g., [MDM-Apple]) secures a
device, MDM can configure OS/browser with a specific encrypted DNS
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 a specific
encrypted DNS server. In this case, MDM is not installed on the
device.
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
network-designated 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 network-designated 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 and split DNS configuration on
explicitly trusted networks. In this use case, the user has
authorized the client to override local DNS settings for a specific
network. It is similar to the way users explicitly disable VPN
connection in specific networks and VPN connection is enabled by
default in other networks for privacy. The unmanaged BYOD devices
typically 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.
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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-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.
An Enterprise can require 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 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.
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.
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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-designated 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. The private domains specified in the dnsZones key are
intended to be resolved using network-designated DNS servers. The
private domains in dnsZones are only reachable by devices
authenticated or attached to the Enterprise network. The global
domains specified in the dnsZones key have a different version in the
internal network. DNS resolution for other domains remains
unchanged.
The dnsZones PvD Key conveys the specified DNS domains that need to
be resolved using an network-designated 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.
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).
6. PvD SplitDNSAllowed and ErrorSplitDNSBlocked Keys
If an Enterprise network restricts all the DNS queries to be sent to
the network-designated DNS server, SplitDNSAllowed will be set to
false. If SplitDNSAllowed is set to false, it implies the network
will try to block DNS queries to DNS servers not provided by the
network and the ErrorSplitDNSBlocked key MUST contain a human-
friendly description for this block. This information is intended
for human consumption (not automated parsing).
Split DNS configurations may be preferable to sending all DNS queries
to an network-designated DNS server in some deployments. This allows
an endpoint to only send DNS queries for the enterprise to the
network-designated DNS servers. The Enterprise remains unaware of
all non-enterprise (DNS) activity of the user. It also allows the
network-designated DNS servers to only be configured for the
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enterprise DNS domains, which removes the legal and technical
responsibility of the enterprise to resolve every DNS domain
potentially asked for by the endpoints.
The SplitDNSAllowed key value set to false is a internal security
policy expression by the operator of the network but is not a policy
prescription to the endpoints to disable use of DNS servers not
provided by the network. If SplitDNSAllowed is set to false, the
client MUST NOT trust the SplitDNSAllowed key in case of connecting
to unknown or untrusted networks (e.g., coffee shops or hotel
networks). Most importantly, the endpoint 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"]
}
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 discovered encrypted DNS server to resolve
domain names for which the Enterprise network claims authority. For
non-split-tunnel VPN configurations, the endpoint uses the discovered
encrypted DNS server to resolve both global and private domain names.
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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-
known domains are not deploying DNSSEC, as the Enterprise network
could then even modify the DNS answers without detection. It is
similar to the mechanism discussed in Section 8 of [RFC8598].
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.
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 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 or to a network-
designated resolver whose response will be validated using DNSSEC
as described in [RFC6698].
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2. The client checks that the NS RRset matches, or is a subdomain
of, any one of the ADN of the discovered network-designated
encrypted DNS resolvers.
A. If the match fails, 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 succeeds, the client can then establish a secure
connection to that network-designated resolver and validate
its certificate.
+ If the server certificate does not validate and a secure
connection cannot be established to the network designated
resolver, the client can action as discussed in step 3
(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".
For example, if in an 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".
11. IANA Considerations
IANA is requested to add SplitDNSAllowed and ErrorSplitDNSBlocked PvD
Keys to the Additional Information PvD Keys registry
(https://www.iana.org/assignments/pvds/pvds.xhtml).
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12. Acknowledgements
Thanks to Mohamed Boucadair, Jim Reid, Ben Schwartz, Tommy Pauly,
Paul Vixie and Vinny Parla for the discussion and comments. The
authors would like to give special thanks to Ben Schwartz for his
help.
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>.
[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>.
[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>.
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[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.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>.
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[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>.
[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>.
[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: kondtir@gmail.com
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Dan Wing
Citrix Systems, Inc.
4988 Great America Pkwy
Santa Clara, CA 95054
USA
Email: danwing@gmail.com
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