ADD M. Boucadair, Ed.
Internet-Draft Orange
Intended status: Informational T. Reddy, Ed.
Expires: November 18, 2021 McAfee
D. Wing
Citrix
N. Cook
Open-Xchange
T. Jensen
Microsoft
May 17, 2021
Discovery of Encrypted DNS Resolvers: Deployment Considerations
draft-boucadair-add-deployment-considerations-00
Abstract
The document discusses some deployment considerations of the various
options to discover encrypted DNS servers (e.g., DNS-over-HTTPS, DNS-
over-TLS, DNS-over-QUIC).
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Sample Target Deployment Scenarios . . . . . . . . . . . . . 3
3.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Direct DNS . . . . . . . . . . . . . . . . . . . . . 5
3.1.2. Proxied DNS . . . . . . . . . . . . . . . . . . . . . 6
3.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. ISP-facing Unmanaged CPEs . . . . . . . . . . . . . . 7
3.2.2. Internal Unmanaged CPEs . . . . . . . . . . . . . . . 7
4. Hosting Encrypted DNS Forwarder in Local Networks . . . . . . 8
4.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. DNS Forwarders . . . . . . . . . . . . . . . . . . . 8
4.1.2. ACME . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 9
5. Legacy CPEs . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
[I-D.ietf-add-dnr] specifies how a local encrypted DNS server can be
discovered by connected hosts by means of DHCP [RFC2132], DHCPv6
[RFC8415], and IPv6 Router Advertisement (RA) [RFC4861] options.
These options are designed to convey the following information: the
DNS Authentication Domain Name (ADN), a list of IP addresses, and a
set of service parameters.
This document discusses deployment considerations for the discovery
of encrypted DNS servers such as DNS-over-HTTPS (DoH) [RFC8484], DNS-
over-TLS (DoT) [RFC7858], or DNS-over-QUIC (DoQ)
[I-D.ietf-dprive-dnsoquic] in local networks.
Sample target deployment scenarios are discussed in Section 3; both
managed and unmanaged Customer Premises Equipment (CPEs) are covered.
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It is out of the scope of this document to provide an exhaustive
inventory of deployments where Encrypted DNS options can be used.
Considerations related to hosting a DNS forwarder in a local network
are described in Section 4.
2. Terminology
This document makes use of the terms defined in [RFC8499]. The
following additional terms are used:
Do53: refers to unencrypted DNS.
Encrypted DNS: refers to a scheme where DNS exchanges are
transported over an encrypted channel. Examples of encrypted DNS
are DNS-over-TLS (DoT) [RFC7858], DNS-over-HTTPS (DoH) [RFC8484],
or DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic].
Encrypted DNS options: refers to the options defined in
[I-D.ietf-add-dnr].
Managed CPE: refers to a CPE that is managed by an Internet Service
Provider (ISP).
Unmanaged CPE: refers to a CPE that is not managed by an ISP.
DHCP: refers to both DHCPv4 and DHCPv6.
3. Sample Target Deployment Scenarios
ISPs traditionally provide DNS resolvers to their customers. To that
aim, ISPs deploy the following mechanisms to advertise a list of DNS
Recursive DNS server(s) to their customers:
o Protocol Configuration Options in cellular networks [TS.24008].
o DHCPv4 [RFC2132] (Domain Name Server Option) or DHCPv6
[RFC8415][RFC3646] (OPTION_DNS_SERVERS).
o IPv6 Router Advertisement [RFC4861][RFC8106] (Type 25 (Recursive
DNS Server Option)).
The communication between a customer's device (possibly via Customer
Premises Equipment (CPE)) and an ISP-supplied DNS resolver takes
place by using cleartext DNS messages (Do53). Some examples are
depicted in Figure 1. In the case of cellular networks, the cellular
network will provide connectivity directly to a host (e.g.,
smartphone, tablet) or via a CPE. Do53 mechanisms used within the
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Local Area Network (LAN) are similar in both fixed and cellular CPE-
based broadband service offerings.
Some ISPs rely upon external resolvers (e.g., outsourced service or
public resolvers); these ISPs provide their customers with the IP
addresses of these resolvers. These addresses are typically
configured on CPEs using dedicated management tools. Likewise, users
can modify the default DNS configuration of their CPEs (e.g.,
supplied by their ISP) to configure their favorite DNS servers. This
document permits such deployments.
(a) Fixed Networks
,--,--,--.
+-+ LAN +---+ ,-' `-.
|H+--------------+CPE+---+ ISP )
+-+ +---+ `-. ,-'
| `--'--'--'
| |
|<=============Do53============>|
| |
(b) Cellular Networks
| |
|<=============Do53============>|
| |
| ,--,--,-.
+-+ LAN +---+ ,-' .
|H+--------------+CPE+---+ \
+-+ +---+ ,' ISP `-.
( )
+-----+-. ,-'
+-+ | `--'--'--'
|H+----------------+ |
+-+ |
| |
|<=============Do53============>|
| |
Legend:
* H: refers to a host.
Figure 1: Sample Legacy Deployments
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3.1. Managed CPEs
This section focuses on CPEs that are managed by ISPs.
3.1.1. Direct DNS
ISPs have developed an expertise in managing service-specific
configuration information (e.g., CPE WAN Management Protocol
[TR-069]). For example, these tools may be used to provision the DNS
server's ADN to managed CPEs if an encrypted DNS is supported by a
local network similar to what is depicted in Figure 2.
For example, DoH-capable (or DoT) clients establish the DoH (or DoT)
session with the discovered DoH (or DoT) server.
The DNS client discovers whether the DNS server in the local network
supports DoH/DoT/DoQ by using the service paramters (ALPN).
(a) Fixed Networks
,--,--,--.
+-+ LAN +---+ ,-' `-.
|H+--------------+CPE+---+ ISP )
+-+ +---+ `-. ,-'
| `--'--'--'
| |
|<========Encrypted DNS========>|
| |
(b) Cellular Networks
| |
|<========Encrypted DNS========>|
| |
| ,--,--,-.
+-+ LAN +---+ ,-' .
|H+--------------+CPE+---+ \
+-+ +---+ ,' ISP `-.
( )
+-----+-. ,-'
+-+ | `--'--'--'
|H+----------------+ |
+-+ |
| |
|<========Encrypted DNS========>|
| |
Figure 2: Encrypted DNS in the WAN
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Figure 2 shows the scenario where the CPE relays the list of
encrypted DNS servers it learns for the network by using mechanisms
like DHCP or a specific Router Advertisement message. In such
context, direct encrypted DNS sessions will be established between a
host serviced by a CPE and an ISP-supplied encrypted DNS server (see
the example depicted in Figure 3 for a DoH/DoT-capable host).
,--,--,--. ,--,--,--.
,-' `-. ,-' ISP `-.
Host---( LAN CPE----( DNS Server )
| `-. ,-' `-. ,-'
| `--'--'--' `--'--'--'
| |
|<=========Encrypted DNS===========>|
Figure 3: Direct Encrypted DNS Sessions
3.1.2. Proxied DNS
Figure 4 shows a deployment where the CPE embeds a caching DNS
forwarder. The CPE advertises itself as the default DNS server to
the hosts it serves. The CPE relies upon DHCP or RA to advertise
itself to internal hosts as the default DoT/DoH/Do53 server. When
receiving a DNS request it cannot handle locally, the CPE forwards
the request to an upstream DoH/DoT/Do53 resolver. Such deployment is
required for IPv4 service continuity purposes (e.g., Section 5.4.1 of
[I-D.ietf-v6ops-rfc7084-bis]) or for supporting advanced services
within a local network (e.g., malware filtering, parental control,
Manufacturer Usage Description (MUD) [RFC8520] to only allow intended
communications to and from an IoT device). When the CPE behaves as a
DNS forwarder, DNS communications can be decomposed into two legs:
o The leg between an internal host and the CPE.
o The leg between the CPE and an upstream DNS resolver.
An ISP that offers encrypted DNS to its customers may enable
encrypted DNS in one or both legs as shown in Figure 4. Additional
considerations related to this deployment are discussed in Section 4.
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(a)
,--,--,--. ,--,--,--.
,-' `-. ,-' ISP `-.
Host---( LAN CPE----( DNS Server )
| `-. ,-'| `-. ,-'
| `--'--'--' | `--'--'--'
| | |
|<=====Encrypted=====>|<=Encrypted=>|
| DNS | DNS |
(b)
,--,--,--. ,--,--,--.
Legacy ,-' `-. ,-' ISP `-.
Host---( LAN CPE----( DNS Server )
| `-. ,-'| `-. ,-'
| `--'--'--' | `--'--'--'
| | |
|<=======Do53========>|<=Encrypted=>|
| | DNS |
Figure 4: Proxied Encrypted DNS Sessions
3.2. Unmanaged CPEs
3.2.1. ISP-facing Unmanaged CPEs
Customers may decide to deploy unmanaged CPEs (assuming the CPE is
compliant with the network access technical specification that is
usually published by ISPs). Upon attachment to the network, an
unmanaged CPE receives from the network its service configuration
(including the DNS information) by means of, e.g., DHCP. That DNS
information is shared within the LAN following the same mechanisms as
those discussed in Section 3.1. A host can thus establish DoH/DoT
session with a DoH/DoT server similar to what is depicted in Figure 3
or Figure 4.
3.2.2. Internal Unmanaged CPEs
Customers may also decide to deploy internal routers (called
hereafter, Internal CPEs) for a variety of reasons that are not
detailed here. Absent any explicit configuration on the internal CPE
to override the DNS configuration it receives from the ISP-supplied
CPE, an Internal CPE relays the DNS information it receives via DHCP/
RA from the ISP-supplied CPE to connected hosts. Encrypted DNS
sessions can be established by a host with the DNS servers of the ISP
(see Figure 5).
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,--,--,--. ,--,--,--.
,-' Internal ,-' ISP `-.
Host--( Network#A CPE----CPE---( DNS Server )
| `-. ,-' `-. ,-'
| `--'--'--' `--'--'--'
| |
|<==============Encrypted DNS=============>|
Figure 5: Direct Encrypted DNS Sessions with the ISP DNS Resolver
(Internal CPE)
Similar to managed CPEs, a user may modify the default DNS
configuration of an unmanaged CPE to use his/her favorite DNS servers
instead. Encrypted DNS sessions can be established directly between
a host and a 3rd Party DNS server (see Figure 6).
,--,--,--. ,--,
,' Internal ,-' '- 3rd Party
Host--( Network#A CPE----CPE---( ISP )--- DNS Server
| `. ,-' `-. -' |
| `-'--'--' `--' |
| |
|<=================Encrypted DNS==================>|
Figure 6: Direct Encrypted DNS Sessions with a Third Party DNS
Resolver
Section 4.2 discusses considerations related to hosting a forwarder
in the Internal CPE.
4. Hosting Encrypted DNS Forwarder in Local Networks
This section discusses some deployment considerations to host an
encrypted DNS forwarder within a local network.
4.1. Managed CPEs
The section discusses mechanisms that can be used to host an
encrypted DNS forwarder in a managed CPE (Section 3.1).
4.1.1. DNS Forwarders
The managed CPE should support a configuration parameter to instruct
the CPE whether it has to relay the encrypted DNS server received
from the ISP's network or has to announce itself as a forwarder
within the local network. The default behavior of the CPE is to
supply the encrypted DNS server received from the ISP's network.
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4.1.2. ACME
The ISP can assign a unique FQDN (e.g., "cpe1.example.com") and a
domain-validated public certificate to the encrypted DNS forwarder
hosted on the CPE. Automatic Certificate Management Environment
(ACME) [RFC8555] can be used by the ISP to automate certificate
management functions such as domain validation procedure, certificate
issuance and certificate revocation.
4.2. Unmanaged CPEs
The approach specified in Section 4.1 does not apply for hosting a
DNS forwarder in an unmanaged CPE.
The unmanaged CPE administrator can host an encrypted DNS forwarder
on the unmanaged CPE. This assumes the following:
o The encrypted DNS server certificate is managed by the entity in-
charge of hosting the encrypted DNS forwarder.
Alternatively, a security service provider can assign a unique
FQDN to the CPE. The encrypted DNS forwarder will act like a
private encrypted DNS server only be accessible from within the
local network.
o The encrypted DNS forwarder will either be configured to use the
ISP's or a 3rd party encrypted DNS server.
o The unmanaged CPE will advertise the encrypted DNS forwarder ADN
using DHCP/RA to internal hosts.
Figure 7 illustrates an example of an unmanaged CPE hosting a
forwarder which connects to a 3rd party encrypted DNS server. In
this example, the DNS information received from the managed CPE (and
therefore from the ISP) is ignored by the Internal CPE hosting the
forwarder.
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,--,--,--. ,--,
,' Internal Managed ,-' '- 3rd Party
Host--( Network#A CPE--------CPE------( ISP )--- DNS Server
| `. ,-'| | `-. -' |
| `-'--'--' | |<==DHCP==>|`--' |
| |<==DHCP==>| | |
|<======DHCP=======>| | |
| {RI, @i} | |
|<==Encrypted DNS==>|<==========Encrypted DNS==========>|
Legend:
* @i: IP address of the DNS forwarder hosted in the Internal
CPE.
Figure 7: Example of an Internal CPE Hosting a Forwarder
5. Legacy CPEs
Hosts serviced by legacy CPEs that can't be upgraded to support the
options defined in Sections 4, 5, and 6 of [I-D.ietf-add-dnr] won't
be able to learn the encrypted DNS server hosted by the ISP, in
particular. If the ADN is not discovered using DHCP/RA, such hosts
will have to fallback to use discovery using the resolver IP address
as defined in Section 4 of [I-D.ietf-add-ddr] to discover the
designated resolvers.
The guidance in Sections 4.1 and 4.2 of [I-D.ietf-add-ddr] related to
the designated resolver verification has to be followed in such case.
6. Security Considerations
DNR-related security considerations are discussed in Section 7 of
[I-D.ietf-add-dnr].
7. IANA Considerations
This document does not require any IANA action.
8. Acknowledgements
This text was initially part of [I-D.ietf-add-dnr].
9. References
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9.1. Normative References
[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-00 (work in progress), February 2021.
9.2. Informative References
[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-00 (work in progress), February 2021.
[I-D.ietf-dprive-dnsoquic]
Huitema, C., Mankin, A., and S. Dickinson, "Specification
of DNS over Dedicated QUIC Connections", draft-ietf-
dprive-dnsoquic-02 (work in progress), February 2021.
[I-D.ietf-v6ops-rfc7084-bis]
Martinez, J. P., "Basic Requirements for IPv6 Customer
Edge Routers", draft-ietf-v6ops-rfc7084-bis-04 (work in
progress), June 2017.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.
[RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
DOI 10.17487/RFC3646, December 2003,
<https://www.rfc-editor.org/info/rfc3646>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[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>.
[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 8106, DOI 10.17487/RFC8106, March 2017,
<https://www.rfc-editor.org/info/rfc8106>.
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[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[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>.
[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>.
[RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/info/rfc8520>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
[TR-069] The Broadband Forum, "CPE WAN Management Protocol",
December 2018, <https://www.broadband-
forum.org/technical/download/TR-069.pdf>.
[TS.24008]
3GPP, "Mobile radio interface Layer 3 specification; Core
network protocols; Stage 3 (Release 16)", December 2019,
<http://www.3gpp.org/DynaReport/24008.htm>.
Authors' Addresses
Mohamed Boucadair (editor)
Orange
Rennes 35000
France
Email: mohamed.boucadair@orange.com
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Tirumaleswar Reddy (editor)
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore, Karnataka 560071
India
Email: TirumaleswarReddy_Konda@McAfee.com
Dan Wing
Citrix Systems, Inc.
USA
Email: dwing-ietf@fuggles.com
Neil Cook
Open-Xchange
UK
Email: neil.cook@noware.co.uk
Tommy Jensen
Microsoft
USA
Email: tojens@microsoft.com
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