Internet DRAFT - draft-pauly-add-requirements
draft-pauly-add-requirements
Network Working Group T. Pauly
Internet-Draft E. Kinnear
Intended status: Informational Apple Inc.
Expires: 22 February 2021 C.A. Wood
Cloudflare
P. McManus
Fastly
T. Jensen
Microsoft
21 August 2020
Adaptive DNS Discovery Requirements
draft-pauly-add-requirements-00
Abstract
This document describes several use cases for discovering DNS
resolvers that support encrypted transports, and discusses how
solutions for these use cases can be designed to use common
mechanisms. It also considers the requirements for privacy and
security when designing resolver discovery mechanisms.
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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This Internet-Draft will expire on 22 February 2021.
Copyright Notice
Copyright (c) 2020 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.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements . . . . . . . . . . . . . . 3
2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Network-provisioned resolvers . . . . . . . . . . . . . . 3
2.2. Client-selected resolvers . . . . . . . . . . . . . . . . 3
2.3. VPN resolvers . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Encrypted resolvers for private names . . . . . . . . . . 4
2.5. Encrypted resolvers for local or home content . . . . . . 5
2.6. Encrypted resolvers for content providers . . . . . . . . 5
3. Discovery mechanisms . . . . . . . . . . . . . . . . . . . . 6
4. Privacy and security requirements . . . . . . . . . . . . . . 6
4.1. On opportunistic encryption . . . . . . . . . . . . . . . 7
4.2. Handling exceptions and failures . . . . . . . . . . . . 8
5. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Several protocols for protecting DNS traffic with encrypted
transports have been defined, such as DNS-over-TLS (DoT) [RFC7858]
and DNS-over-HTTPS (DoH) [RFC8484]. Encrypted DNS can provide many
security and privacy benefits for network clients.
While it is possible for clients to hard-code encrypted DNS resolvers
to use, dynamic discovery and provisioning of encrypted resolvers can
expand the usefulness and applicability of encrypted DNS to many more
use cases.
This document first describes several use cases for discovering DNS
resolvers that support encrypted transports (Section 2).
Next, it discusses how solutions for these use cases can be grouped
and categorized to point to the usefulness of common mechanisms
(Section 3).
Last, it considers the requirements for privacy and security when
designing resolver discovery mechanisms (Section 4).
This document is designed to aid in discussion of the Adaptive DNS
Discovery (ADD) working group as defines mechanism requirements.
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1.1. Specification of Requirements
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.
2. Use Cases
This section describes various use cases for which it is possible to
discover an encrypted resolver. For each use case, the privacy and
security benefits of adding encrypted resolution are briefly
described.
2.1. Network-provisioned resolvers
DNS servers are often provisioned by a network as part of DHCP
options [RFC2132] or IPv6 Router Advertisement (RA) options
[RFC8106]. These options describe one or more DNS resolver IP
addresses, to be used for traditional unencrypted DNS.
Using an encrypted resolver that is provisioned by the network can
provide several benefits that are not possible if only unencrypted
DNS is used:
* Prevent other devices on the network from observing client DNS
messages
* Verify that answers come from the selected DNS resolver
* Authenticate that the DNS resolver is the one provisioned by the
network
Often, network-provisioned resolvers are forwarders running on a
local router. The discovered encrypted resolvers in these cases may
either be local fowarders themselves, or an associated resolver that
is in the network (thus bypassing the router's DNS forwarder).
2.2. Client-selected resolvers
Client devices often allow a user or administrator to select a
specific DNS resolver to use on certain networks, or on all networks.
Historically, this selection was specified only with an IP address.
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Discovering if the selected resolver supports encryption, along with
the configuration for the encrypted resolver, allows the client to
"upgrade" connections to use encrypted DNS. This can provide several
benefits:
* Prevent devices along the network path to the selected resolver
from observing client DNS messages
* Verify that answers come from the selected DNS resolver
* Authenticate that the DNS resolver is the one selected by the
client
2.3. VPN resolvers
Virtual Private Networks (VPNs) also can provision DNS resolvers. In
addition to being able to use DHCP or RAs, VPNs can provision DNS
information in an explicit configuration message. For example, IKEv2
can provision DNS servers using Configuration Attributes [RFC7296].
VPNs can also configure Split DNS rules to limit the use of the
configured resolvers to specific domain names [RFC8598].
Discovering an encrypted resolver that is provisioned by a VPN can
provide the same benefits as doing so for a local network, but
applied to the private network. When using Split DNS, it becomes
possible to use a one encrypted resolver for private domains, and
another for other domains.
2.4. Encrypted resolvers for private names
Similar to how VPN DNS configurations can use Split DNS for private
names, other network environments can support resolution of private
names. For example, an enterprise-managed Wi-Fi network might be
able to access both the Internet an a private intranet. In such a
scenario, the private domains managed by the enterprise might only be
resolvable using a specific DNS resolver.
Discovering an encrypted resolver for private domains allows a client
to perform Split DNS while maintaining the benefits of encrypted DNS.
For example, a client could use a client-selected encrypted resolver
for most domains, but use a different encrypted resolver for
enterprise-private domains.
This has the privacy benefit of only exposing DNS queries to the
enterprise that fall within a limited set of domains, if there is a
more preferred option for generic Internet traffic.
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Using encrypted DNS for private names also opens up the possibility
of doing private name resolution outside of the content of a VPN or
managed network. If the DNS resolver authenticates clients, it can
offer its resolver for private names on a publicly accessible server,
while still limiting the visibility of the DNS traffic.
2.5. Encrypted resolvers for local or home content
Accessing locally-hosted content can require the use of a specific
resolver. For example, captive networks or networks with walled-
garden content like media on airplane Wi-Fi networks can rely on
using a resolver hosted on the local network.
In cases where a client is using an encrypted resolver provisioned by
a network, and that encrypted resolver is able to resolve names local
content, this can fall into the use case described in Section 2.1.
However, it might be necessary to discover a local encrypted resolver
along with specific domains if:
* the network-provisioned encrypted resolver is not able to resolve
local-only names, or
* the client has a more-preferred encrypted resolver for generic
traffic, and would otherwise not be able to access local content
This case also include accessing content specific to a home network.
2.6. Encrypted resolvers for content providers
Content Delivery Networks (CDNs), and content-providers more broadly,
can also provide encrypted DNS resolvers that can be used by clients
over the public Internet. These resolvers can either allow
resolution of all public names (like normal recursive resolvers), or
be designed to serve a subset of names managed by the content
provider (like an authoritative resolver). Using these resolvers can
allow the content provider to directly control how DNS answers are
used for load balancing and address selection, which could improve
performance of connections to the content provider.
Using a content-provider's encrypted resolver can also provide
several privacy and security benefits:
* Prevent devices along the network path to the content-provider's
resolver from observing client DNS messages
* Verify that answers come from the entity that manages the domains
being resolved
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* Reduce the number of entities able to monitor the specific names
accessed by a client to only the client and the content provider,
assuming that the content provider would already see the names
upon a secure connection later being made based on the DNS answers
(e.g., in the TLS SNI extension)
3. Discovery mechanisms
The use cases described in Section 2 do not all necessarily require
separate mechanisms.
Generally, the use cases can be summarized in two categories:
1. Resolver upgrade: Discover encrypted resolvers equivalent to (or
associated with) unencrypted resolvers. Examples include
network-provisioned, client-selected, and VPN-configured
resolvers.
2. Domain-specific resolvers: Discover encrypted resolvers
applicable to a limited set of domains. Examples include
resolvers for enterprise or private names, local content, and CDN
content.
Resolver upgrade mechanisms can either add new parameters to existing
provisioning mechanisms (adding necessary information to use DoT or
DoH to options in DHCP, RAs, or IKEv2) or else provide a way to
communicate with a provisioned unencrypted DNS resolver and discover
the equivalent or associated encrypted DNS resolver.
Domain-specific resolver discovery mechanisms additionally need to
provide some information about the applicability and capabilities of
encrypted resolvers. This information can either be provisioned or
can be discovered based on clients actively trying to access content.
4. Privacy and security requirements
Encrypted DNS improves the privacy and security of DNS queries and
answers in the presence of malicious attackers. Such attackers are
assumed to interfere with or otherwise impede DNS traffic and
corresponding discovery mechanisms. They may be on-path or off-path
between the client and entities with which the client communicates
[RFC3552]. These attackers can inject, tamper, or otherwise
interfere with traffic as needed. Given these capabilities, an
attacker may have a variety of goals, including, though not limited
to:
* Monitor and profile clients by observing unencrypted DNS traffic
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* Modify unencrypted DNS traffic to filter or augment the user
experience
* Block encrypted DNS
Clients cannot assume that their network does not have such an
attacker unless given some means of authenticating or otherwise
trusting the communication with their DNS resolver.
Given this type of attacker, resolver discovery mechanisms must be
designed carefully to not worsen a client's security or privacy
posture. In particular, attackers must not be able to:
* Redirect DNS traffic to themselves.
* Override or interfere with the resolver preferences of a user or
administrator.
* Cause clients to use a discovered resolver which has no
authenticated delegation from a client-known entity.
* Influence automatic discovery mechanisms such that a client uses
one or more resolvers that are not otherwise involved with
providing service to the client, such as: a network provider, a
VPN server, a content provider being accessed, or a server that
the client has manually configured.
Beyond these requirements, standards describing resolver discovery
mechanisms must not place any requirements on clients to select
particular resolvers over others.
4.1. On opportunistic encryption
Opportunistic encrypted DNS, when the client cannot authenticate the
entity that provides encrypted DNS, does not meet the requirements
laid out here for resolver discovery. While opportunistic encryption
can provide some benefits, specifically in reducing the ability for
other entities to observe traffic, it is not a viable solution
against an on-path attacker.
Performing opportunistic encrypted DNS does not require specific
discovery mechanisms. Section 4.1 of [RFC7858] already describes how
to use DNS-over-TLS opportunistically.
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4.2. Handling exceptions and failures
Even with encrypted DNS resolver discovery in place, clients must be
prepared to handle certain scenarios where encrypted DNS cannot be
used. In these scenarios, clients must consider if it is appropriate
to fail open by sending the DNS queries without encryption, fail
closed by not doing so, or presenting a choice to a user or
administrator. The exact behavior is a local client policy decision.
Some networks that use Captive Portals will not allow any Internet
connectivity until a client has interacted with the portal
[I-D.ietf-capport-architecture]. If these networks do not use
encrypted DNS for their own resolution, a client will need to perform
unencrypted DNS queries in order to get out of captivity. Many
operating systems have specific client code responsible for detecting
and interacting with Captive Portals; these system components may be
good candidates for failing open, since they do not generally
represent user traffic.
Other networks may not allow any use of encrypted DNS, or any use of
encrypted DNS to resolvers other than a network-provisioned resolver.
Clients should not silently fail open in these cases, but if these
networks are trusted by or administered by the user, the user may
want to specifically follow the network's DNS policy instead of what
the client would do on an unknown or untrusted network.
5. Informative References
[I-D.ietf-capport-architecture]
Larose, K., Dolson, D., and H. Liu, "Captive Portal
Architecture", Work in Progress, Internet-Draft, draft-
ietf-capport-architecture-09, 8 August 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-capport-
architecture-09.txt>.
[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>.
[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>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
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[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>.
[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>.
[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>.
[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>.
[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
Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Email: tpauly@apple.com
Eric Kinnear
Apple Inc.
One Apple Park Way
Cupertino, California 95014,
United States of America
Email: ekinnear@apple.com
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Christopher A. Wood
Cloudflare
101 Townsend St
San Francisco,
United States of America
Email: caw@heapingbits.net
Patrick McManus
Fastly
Email: mcmanus@ducksong.com
Tommy Jensen
Microsoft
Email: tojens@microsoft.com
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