CNAME Discovery of Local DoH Resolvers

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Table of Contents

1. Introduction

Some applications perform their own name resolution rather than using the system resolver, typically using an encrypted protocol such as DoH [RFC8484]. These applications have the choice of using either the same recursive resolver configured into the system or of using a resolver chosen out of a preconfigured list of trusted resolvers in an application, such as in [DOHTRR].¶

If all of the trusted resolvers are publicly available, then there are a number of mechanisms for choosing between them, for instance randomly or based on performance. [I-D.arkko-abcd-distributed-resolver-selection] describes a number of potential mechanisms. However, if the list of trusted resolvers includes Internet Service Providers (ISPs) and the client is on a network associated with such a provider, then it may be desirable to preferentially select the resolver associated with that provider. This provides the benefits both of using a DNS resolver with a known policy and using a resolver that has high quality local information about the local network topology.¶

This document describes a mechanism to address this situation. This mechanism is being tested in the Firefox browser with Comcast's resolvers.¶

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. DoH Resolver Discovery

The basic mechanism described in this document is straightforward and has been chosen for ease of implementation rather than architectural correctness.¶

                                +--------------+
Provision CNAME                 |              |
doh.test -> resolver.example -> | ISP Resolver |
                                |              |
                                +--------------+
                                    ^      |
                                    |      |
                          doh.test? |      | resolver.example
                                    |      |
                                    |      v
                                +--------------+
                                |              |
                                |    Client    |
                                |              |
                                +--------------+

A network provider can publish the fact that it has an associated DoH resolver on its network by configuring its own resolvers to serve a CNAME record at a well known domain name which cannot be otherwise registered. The current test deployment uses "doh.test" (see [RFC2606] for the definition of .test). This CNAME points to the domain name of the associated DoH resolver ("resolver.example" in the diagram above).¶

[[OPEN ISSUE: doh.test is probably the wrong domain. We may pick something else later.]]¶

A client which wishes to test for the presence of a DoH resolver on the network takes the following steps:¶

1. Do any testing for whether DoH should be disabled, such as looking for canary domains [I-D.grover-add-policy-detection] or checking for local enterprise configuration.¶
2. Do a CNAME query for "doh.test" using either the system resolver or by talking directly to the recursive resolver IP address configured into the system.¶
3. If the query succeeds, then look up the CNAME record value in the list of preconfigured resolvers. If an exact match is found, then use the resolver address for the matching preconfigured resolver. Otherwise fall back to the ordinary DoH resolver selection logic.¶
4. If the query fails, then no associated resolver is present; fall back to the ordinary DoH resolver selection logic.¶

As noted above, this mechanism was designed for ease of implementation.¶

Comcast's resolvers and authoritative servers have been configured with some additional records to support the Firefox applications and potential future applications. The DNS behavior is as follows, where example.com is the domain used for naming provider services:¶

1. doh.test IN CNAME doh-discovery.example.com¶
2. doh-discovery.example.com must have at least one A and/or AAAA RR (address does not matter - can be 127.0.0.1)¶
3. doh-discovery.example.com IN URI https://doh.example.com/dns-query (the ISP DoH URI - not currently used by Firefox as the URI is preconfigured in the application)¶

The next few sections describe the reasoning for some of the design choices.¶

Considering that many applications do not act as a DNS client and instead use platform functions such as getaddrinfo, the domain of the associated resolver SHOULD also have an A record, so the call to getaddrinfo does not fail.¶

4. Security Considerations

Because the initial request for discovery is done over insecure DNS (Do53), a local attacker or malicious local resolver can substitute their own response. However, because this mechanism only selects from a list of preconfigured trusted resolvers, an attacker can only redirect you to a different resolver out of that list, which by definition is also trusted. Note: the URI field potentially has different security properties depending on how it is used. As noted above; Firefox does not use it.¶

If the server which is redirected to is not publicly available, this mechanism can be used as a DoS attack. Application clients should test the selected server before committing to it and otherwise fall back to their ordinary DoH selection logic.¶

Any local discovery mechanism has potential privacy impacts: suppose that a user uses their mobile device on ISP A, which redirects it to their own resolver, and ISP B which does not. In that case, the user's DNS queries will be spread over both ISP A's resolver and one of the public trusted resolvers, which could have an impact on the user's privacy. This has to be balanced against the improvement obtained by using a local resolver and the level of metadata leakage that currently occurs to the ISP, but can be mitigated through trusted recursive resolver policies.¶

5. IANA Considerations

This document has no IANA actions.¶

Acknowledgments

TODO acknowledge.¶

Authors' Addresses