Analysis of DNS Forwarder Scenario Relative to DDR and DNR

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Table of Contents

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:¶

• 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."¶

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¶

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.¶

Acknowledgments

Thanks to ...¶

Authors' Addresses