Internet DRAFT - draft-stark-add-dns-forwarder-analysis
draft-stark-add-dns-forwarder-analysis
ADD Working Group B. Stark
Internet-Draft AT&T
Intended status: Informational C. Box
Expires: 17 December 2021 BT
15 June 2021
Analysis of DNS Forwarder Scenario Relative to DDR and DNR
draft-stark-add-dns-forwarder-analysis-00
Abstract
This draft analyzes the behaviors that residential end users and home
network owners (e.g., parents of young children) might experience
when operating systems and clients support [I-D.ietf-add-ddr] and/or
[I-D.ietf-add-dnr] for discovery of encrypted DNS services and the CE
router of the home network offers itself as the Do53 resolver. This
use case is explicitly mentioned in [I-D.ietf-add-requirements]
Section 3.2 and has several requirements related to it. This draft
has two goals: determine if the analysis it provides is accurate and,
if it is accurate, determine if the behavior is acceptable to the WG
or if there should be changes to either of the discovery mechanisms.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the mailing list
(add@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/add/.
Source for this draft and an issue tracker can be found at
https://github.com/bhstark2/dns-forwarder-analysis.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 17 December 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Scenario Analysis . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Scenario 1: No changes to CE router . . . . . . . . . . . 4
4.2. Scenario 2: CE router updated to provide DNR in DHCP/
RA . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Scenario 3: CE router updated to support opportunistic
encryption to its DNS forwarder . . . . . . . . . . . . . 6
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Questions for the WG . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This draft analyzes the behaviors that residential end users and home
network owners (e.g., parents of young children) might experience
when operating systems and clients support [I-D.ietf-add-ddr] and/or
[I-D.ietf-add-dnr] for discovery of encrypted DNS services and the CE
router of the home network offers itself as the Do53 resolver. This
use case is explicitly mentioned in [I-D.ietf-add-requirements]
Section 3.2 and has several requirements related to it.
This draft has two goals:
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* determine if the analysis it provides is accurate
* if it is accurate, determine if the behavior is acceptable to the
WG or if there should be changes to either of the discovery
mechanisms.
Becoming a WG draft is _not_ a goal of this draft. There is and will
be no request for adoption by any WG.
While DNS forwarders / proxies may exist in environments other than
home networks (e.g., hotspots, small businesses), this draft does not
attempt to examine those usages. This draft is focused on home
networks.
2. Conventions and Definitions
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.
3. Background
Having a DNS forwarder in the CE router that is advertised to the LAN
using DHCP and RDNSS options is a common deployment model for many
ISPs and is also the default in many retail consumer routers (e.g.,
Netgear).
[I-D.ietf-add-requirements] contains the following text related to
this use case:
"Many networks offer a Do53 resolver on an address that is not
globally meaningful, e.g. [RFC1918], link-local or unique local
addresses. To support the discovery of encrypted DNS in these
environments, a means is needed for the discovery process to work
from a locally-addressed Do53 resolver to an encrypted DNS resolver
that is accessible either at the same (local) address, or at a
different global address. Both options need to be supported."
"R4.1 If the local network resolver is a forwarder that does not
offer encrypted DNS service, an upstream encrypted resolver SHOULD be
retrievable via queries sent to that forwarder."
"R4.2 Achieving requirement 4.1 SHOULD NOT require any changes to DNS
forwarders hosted on non-upgradable legacy network devices."
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In the context of a home network, there are several reasons why this
deployment model is used. Some reasons are:
* Provide local name resolution
* Captive portal (Note that [RFC8952] defines an architecture that
does not rely on "breaking" DNS; however, there exist many legacy
devices with captive portals that do rely on "breaking" DNS.}
* Provide filtering (aka parental controls) and DNS-based
vulnerability assessment in the CE router. Note that
[I-D.ietf-add-requirements] describes this sort of filtering and
monitoring behavior as an attack; nonetheless, this functionality
is popular with many people -- especially parents.
* Caching responses to improve DNS performance
4. Scenario Analysis
The following sections will analyze what behavior a user is expected
to see when certain conditions exist. In all cases, it's assumed the
CE router is advertising itself as the Do53 server (using DHCP and/or
RA). The clients and OSs that are of interest in these scenarios are
using whatever Do53 server is advertised to them by DHCP/RA. There
may be clients and devices that use other Do53 servers; those are out
of scope of this analysis. Analyzing the behavior of clients that do
not support either DoH or DoT, or do not support any mechanism to
discover encrypted servers are also out of scope.
Assumptions common to all scenarios are:
* Common OSs support both DNR and DDR
* Some applications (e.g., browsers) support DDR
* No Certificate Authority will sign a certificate with a private IP
address in a SAN
4.1. Scenario 1: No changes to CE router
Assumptions:
* The CE router (including its DNS forwarder and DHCP/RA
capabilities) are not updated.
Expected behaviors:
* There will be no DHCP or RA advertisement of encrypted servers.
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* The DNS forwarder will forward DDR queries (dns://resolver.arpa)
to the DNS recursive resolver the CE router is configured to use.
* If that recursive resolver has the appropriate SVCB record, it
will provide that in the response that is returned.
* The querying OS/app will determine that the IP address of its
Unencrypted Resolver (the CE router) and the IP address of the
Unencrypted Resolver in the supplied certificate do not match and
will not do "authenticated discovery".
* The querying OS/app will determine that the IP address of its
Unencrypted Resolver (the CE router) does not match the IP address
of the Encrypted Resolver and will not do "opportunistic
discovery".
* The OS/app will not discover a local Encrypted Resolver.
The end result is that no Encrypted Resolver will be used by an OS or
app that uses DDR or DNR to discover an Encrypted Resolver, unless
the OS or app subsequently uses some non-standard mechanism to select
an Encrypted Resolver. Note that this suggests that the DDR and DNR
proposals in their current form do not satisfy the requirement "R4.2
Achieving requirement 4.1 SHOULD NOT require any changes to DNS
forwarders hosted on non-upgradable legacy network devices."
Also note that non-upgraded legacy routers will not satisfy the
[I-D.ietf-add-ddr] requirement that a "DNS forwarder SHOULD NOT
forward queries for "resolver.arpa" upstream." If the CE router were
updated to not forward queries for "resolver.arpa" upstream, the end
result would not change. Since this scenario provides the same end
result, it isn't broken out separately.
4.2. Scenario 2: CE router updated to provide DNR in DHCP/RA
Assumptions:
* The CE router is updated to provide Encrypted Resolver info in
DHCP/RA
* The CE router gets its Encrypted Resolver info from DHCP; getting
that was part of the update
* The upstream ISP has updated its core network resolver to support
encryption, and announces this resolver in DHCP
Expected behaviors:
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* OSs will use the Encrypted Resolver
* Applications that try "resolver.arpa" will not do their own
upgrade, because that will fail
Additional results:
* Local name resolution is broken?
* Legacy captive portal is now broken?
* Filtering in the CE router (parental controls and other security
mechanisms enabled by the home network owner) is now broken
* Any filtering deployed in the core network resolver continues to
operate
* No local caching
4.3. Scenario 3: CE router updated to support opportunistic encryption
to its DNS forwarder
Assumptions:
* The CE router supports encrypted connectivity to its DNS forwarder
* The CE router is updated to provide Encrypted Resolver info in
DHCP/RA
* The CE router is updated to reply to "dns://resolver.arpa"; SVCB
record points to the CE router with a self-signed certificate
Note that the effort do do these upgrades is considered to be rather
large.
Expected behaviors:
* Some OSs and applications accept DDR Opportunistic Discovery,
resulting in use of the CE router's Encrypted Resolver.
* Some OSs and applications do not.
* Across a range of households, and even within a single household,
there is inconsistent behavior.
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5. Conclusions
Since Scenario 3 is considered a large effort and the resulting
behavior is unpredictable, it is unlikely to be pursued.
Since Scenario 2 will break some of the functionality that a
significant number of home network owners have purposefully enabled
(e.g., router-based DNS-based parental controls), will break existing
captive portal implementations used to simplify setup of broadband
connections, and may break local name resolution (?) it is unlikely
to be pursued.
This leaves Scenario 1 (do nothing in routers that provide DNS
forwarder).
6. Questions for the WG
Are these the results we want to achieve with Encrypted Resolver
discovery mechanisms?
7. Security Considerations
Breaking the security mechanisms that many users currently have
enabled in their home network routers (e.g., DNS filtering) will
worsen the security of those users. While these mehanisms are not
perfect, and can easily be bypassed by client applications that run
DoH, this does not make them completely useless.
8. IANA Considerations
This document has no IANA actions.
9. References
9.1. Normative References
[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/rfc/rfc2119>.
[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/rfc/rfc8174>.
9.2. Informative References
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[I-D.ietf-add-ddr]
Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", Work in
Progress, Internet-Draft, draft-ietf-add-ddr-01, 14 June
2021, <https://tools.ietf.org/html/draft-ietf-add-ddr-01>.
[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)", Work in
Progress, Internet-Draft, draft-ietf-add-dnr-02, 17 May
2021, <https://tools.ietf.org/html/draft-ietf-add-dnr-02>.
[I-D.ietf-add-requirements]
Box, C., Pauly, T., Wood, C. A., Reddy, T., and D.
Migault, "Requirements for Discovering Designated
Resolvers", Work in Progress, Internet-Draft, draft-ietf-
add-requirements-00, 8 March 2021,
<https://tools.ietf.org/html/draft-ietf-add-requirements-
00>.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/rfc/rfc1918>.
[RFC8952] Larose, K., Dolson, D., and H. Liu, "Captive Portal
Architecture", RFC 8952, DOI 10.17487/RFC8952, November
2020, <https://www.rfc-editor.org/rfc/rfc8952>.
Acknowledgments
Thanks to ...
Authors' Addresses
Barbara Stark
AT&T
Austin, TX,
United States of America
Email: barbara.stark@att.com
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Chris Box
BT
Bristol
United Kingdom
Email: chris.box@bt.com
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