DNS Resolver Information

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Table of Contents

1. Introduction

Historically, DNS stub resolvers communicated with recursive resolvers without needing to know anything about the features supported by these recursive resolvers. As more and more recursive resolvers expose different features that may impact the delivered DNS service, means to help stub resolvers to identify the capabilities of the resolver are valuable. Typically, stub resolvers can discover and authenticate encrypted DNS servers provided by a local network, for example, using the techniques specified in [I-D.ietf-add-dnr] and [I-D.ietf-add-ddr]. However, these stub resolvers need a means to retrieve information from the discovered recursive resolvers about their capabilities.¶

This document fills that void by specifying a method for stub resolvers to retrieve such information. To that aim, a new RRtype is defined for stub resolvers to query the recursive resolvers. The information that a resolver might want to give is defined in Section 5.¶

Retrieved information can be used to feed the server selection procedure.¶

2. Terminology

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.¶

This document makes use of the terms defined in [RFC8499].¶

'Encrypted DNS' refers to a DNS protocol that provides an encrypted channel between a DNS client and server (e.g., DoT, DoH, or DoQ).¶

3. Retrieving Resolver Information

A stub resolver that wants to retrieve the resolver information may use the RRtype "RESINFO" defined in this document (see Section 7.1).¶

The content of the RDATA in a response to RRtype query is defined in Section 5. If the resolver understands the RESINFO RRtype, the RRset in the Answer section MUST have exactly one record.¶

The client can retrieve the resolver information using the RESINFO RRtype and QNAME of the domain name that is used to authenticate the DNS server (referred to as ADN in [I-D.ietf-add-dnr]).¶

If the special use domain name "resolver.arpa" defined in [I-D.ietf-add-ddr] is used to discover the Encrypted DNS server, the client can retrieve the resolver information using the RESINFO RRtype and QNAME of the designated resolver.¶

4. Format of the Resolver Information

The resolver information is returned as a JSON object. Precisely, the JSON object MUST use the I-JSON message format [RFC7493].¶

• Note that [RFC7493] was based on [RFC7159], but [RFC7159] was replaced by [RFC8259]. Requiring the use of I-JSON instead of more general JSON format greatly increases the likelihood of interoperability.¶

The JSON object returned by a DNS query may contain any name/value pairs. All names MUST consist only of lower-case ASCII characters, digits, and hyphens (that is, Unicode characters U+0061 through 007A, U+0030 through U+0039, and U+002D). These names MUST be 63 characters or shorter.¶

All names in the returned object MUST either be defined in the IANA registry Section 7.2 or begin with the substring "temp-" for names defined for local use only.¶

5. Resolver Information

The resolver information includes the following attributes:¶

qnameminimization:

If the DNS server supports QNAME minimisation [RFC7816] to improve DNS privacy, the parameter value is set to true. This is a mandatory attribute.¶

extendeddnserror:

If the DNS server supports extended DNS error (EDE) [RFC8914] to return additional information about the cause of DNS errors, the parameter lists the possible extended DNS error codes that can be returned by the DNS server. This is an optional attribute.¶

resinfourl:

An URL that points to the generic unstructured resolver information (e.g., DoH APIs supported, possible HTTP status codes returned by the DoH server, how to report a problem) for troubleshooting purpose. The server MUST support the content-type 'text/html'. The DNS client MUST reject the URL if the scheme is not "https". The client MUST validate that both the encrypted DNS server and the resolver information server are owned and managed by the same entity by establishing a TLS connection to the domain name in the URL and checking if the subjectAltName entry in the server certificate includes the name of the encrypted DNS server. If this match fails, the client MUST ignore the resolver information. As such, the URL should be treated only as diagnostic information for IT staff. This is a mandatory attribute.¶

New attributes can be defined as per the procedure defined in Section 7.2.¶

As specified in [RFC7493], the I-JSON object is encoded as UTF8. [RFC7493] explicitly allows the returned objects to be in any order.¶

Figure 1 shows an example of resolver information.¶

{
  "qnameminimization": true,
  "extendeddnserror": [
    15,
    16,
    17
  ],
  "resinfourl": "https://resolver.example.com/guide",
}

6. Security Considerations

Unless a DNS request to retrieve the resolver information is encrypted (e.g., sent over DNS-over-TLS (DoT) [RFC7858] or DNS-over- HTTPS (DoH)) [RFC8484], the response is susceptible to forgery. The DNS resolver information can be retrieved after the encrypted connection is established to the DNS server or retrieved before the encrypted connection is established to the DNS server by using local DNSSEC validation.¶

7. IANA Considerations

• Note to the RFC Editor: Please update [RFCXXXX] with the RFC number to be assigned to this document.¶

Type: RESINFO
Value: TBD
Meaning: Resolver Information as an I-JSON
Reference: [RFCXXXX]

8. Acknowledgments

This specification leverages the work that has been documented in [I-D.pp-add-resinfo].¶

Thanks to Tommy Jensen, Vittorio Bertola, Vinny Parla, Chris Box, Ben Schwartz, Tony Finch, Daniel Kahn Gillmor, Eric Rescorla and Shashank Jain for the discussion and comments.¶

Authors' Addresses