DNS Resolver Information

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Table of Contents

1. Introduction

Historically, DNS clients communicated with recursive resolvers without needing to know anything about the features supported by these resolvers. However, recent developments (e.g., Extended Error Reporting [RFC8914] or encrypted DNS) imply that earlier assumption no longer generally applies. Typically, DNS clients can discover and authenticate encrypted DNS resolvers provided by a local network (e.g., using the Discovery of Network-designated Resolvers (DNR) [RFC9463] and the Discovery of Designated Resolvers (DDR) [RFC9462]), however, these DNS clients can't retrieve information from the discovered recursive resolvers about their capabilities. Instead of depending on opportunistic approaches, DNS clients need a more reliable mechanism to discover the features that are supported by resolvers.¶

This document fills that void by specifying a method for stub resolvers to retrieve such information. To that aim, a new resource record (RR) type is defined for DNS clients to query the recursive resolvers. The information that a resolver might want to expose is defined in Section 5.¶

Retrieved information can be used to feed the server selection procedure. However, that selection procedure is out of the scope of this document.¶

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]. The following additional terms are used:¶

Encrypted DNS:

Refers to a DNS scheme where DNS exchanges are transported over an encrypted channel between a DNS client and server (e.g., DNS over HTTPS (DoH) [RFC8484], DNS over TLS (DoT) [RFC7858], or DNS over QUIC (DoQ) [RFC9250]).¶

Encrypted DNS resolver:

Refers to a DNS resolver that supports any encrypted DNS scheme.¶

Reputation:

"The estimation in which an identifiable actor is held, especially by the community or the Internet public generally" (Section 1 of [RFC7070].¶

3. Retrieving Resolver Information

A DNS client that wants to retrieve the resolver information may use the RR type "RESINFO" defined in this document.¶

The content of the RDATA in a response to a query for RESINFO RR QTYPE is defined in Section 5. If the resolver understands the RESINFO RR type, the RRSet in the Authority section MUST have exactly one record. RESINFO is a property of the resolver and is not subject to recursive resolution.¶

A DNS client can retrieve the resolver information using the RESINFO RR type and the QNAME of the domain name that is used to authenticate the DNS resolver (referred to as the Authentication Domain Name (ADN) in DNR [RFC9463]).¶

If the Special-Use Domain Name "resolver.arpa", defined in [RFC9462], is used to discover an encrypted DNS resolver, the client can retrieve the resolver information using the RESINFO RR type and QNAME of "resolver.arpa". In this case, a client has to contend with the risk that a resolver does not support RESINFO. The resolver might pass the query upstream, and then the client can receive a positive RESINFO response either from a legitimate DNS resolver or an attacker. The DNS client MUST set the Recursion Desired (RD) bit of the query to 0 to ensure that the response is provided by the resolver. If the resolver does not support RESINFO, it will return an authoritative name error.¶

4. Format of the Resolver Information

The resolver information record uses the same format as DNS TXT records. As a reminder, the format rules for TXT records are defined in the base DNS specification (Section 3.3.14 of [RFC1035]) and further elaborated in the DNS-based Service Discovery (DNS-SD) specification (Section 6.1 of [RFC6763]). The recommendations to limit the TXT record size are discussed in Section 6.1 of [RFC6763].¶

Similar to DNS-SD, the RESINFO RR type uses "key/value" pairs to convey the resolver information. Each "key/value" pair is encoded using the format rules defined in Section 6.3 of [RFC6763]. Using standardized "key/value" syntax within the RESINFO RR type makes it easier for future keys to be defined. If a DNS client sees unknown keys in a RESINFO RR type, it MUST silently ignore them. The same rules for the keys as those defined in Section 6.4 of [RFC6763] MUST be followed for RESINFO.¶

Keys MUST either be defined in the IANA registry (Section 8.2) or begin with the substring "temp-" for names defined for local use only.¶

5. Resolver Information Keys/Values

The following resolver information keys are defined:¶

qnamemin:

If the DNS resolver supports QNAME minimisation [RFC9156] to improve DNS privacy, the key is present. Note that, as per the rules for the keys defined in Section 6.4 of [RFC6763], if there is no '=' in a key, then it is a boolean attribute, simply identified as being present, with no value.¶

This is an optional attribute.¶

exterr:

If the DNS resolver supports extended DNS errors (EDE) option [RFC8914] to return additional information about the cause of DNS errors, the value of this key lists the possible extended DNS error codes that can be returned by this DNS resolver. When multiple values are present, these values MUST be comma-separated.¶

This is an optional attribute.¶

infourl:

An URL that points to the generic unstructured resolver information (e.g., DoH APIs supported, possible HTTP status codes returned by the DoH server, or how to report a problem) for troubleshooting purposes. The server that exposes such information is called "resolver information server".¶

The resolver information server MUST support the content-type 'text/html'. The DNS client MUST reject the URL if the scheme is not "https". The URL SHOULD be treated only as diagnostic information for IT staff. It is not intended for end user consumption as the URL can possibly provide misleading information. A DNS client MAY choose to display the URL to the end user, if and only if the encrypted resolver has sufficient reputation, according to some local policy (e.g., user configuration, administrative configuration, or a built-in list of respectable resolvers).¶

This is an optional attribute.¶

New keys can be defined as per the procedure defined in Section 8.2.¶

6. An Example

Figure 1 shows an example of a published resolver information record.¶

resolver.example.net. 7200 IN RESINFO qnamemin exterr=15,16,17
                      infourl=https://resolver.example.com/guide

As mentioned in Section 3, a DNS client that discovers the ADN "resolver.example.net" of its resolver using DNR will issue a query for RESINFO RR QTYPE for that ADN and will learn that the resolver supports:¶

• QNAME minimisation,¶

• Blocked (15), Censored (16), and Filtered (17) EDEs, and¶

• that more information can be retrieved from https://resolver.example.com/guide.¶

7. Security Considerations

DNS clients communicating with discovered DNS resolvers MUST use one of the following measures to prevent DNS response forgery attacks:¶

1. Establish an authenticated secure connection to the DNS resolver.¶

2. Implement local DNSSEC validation (Section 10 of [RFC8499]) to verify the authenticity of the resolver information.¶

It is important to note that, of these two measures, only the first one can apply to queries for 'resolver.arpa'.¶

An encrypted resolver may return incorrect information in RESINFO. If the client cannot validate the attributes received from the resolver, that will be used for resolver selection or displayed to the end-user, the client should process those attributes only if the encrypted resolver has sufficient reputation according to local policy (e.g., user configuration, administrative configuration, or a built-in list of reputable resolvers). This approach limits the ability of a malicious encrypted resolver to cause harm with false claims.¶

8. IANA Considerations

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

Type: RESINFO
Value: 261
Meaning: Resolver Information as Key/Value Pairs
Reference: RFCXXXX

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, Shashank Jain, Florian Obser, Richard Baldry, and Martin Thomson for the discussion and comments.¶

Thanks to Mark Andrews, Joe Abley, Paul Wouters, and Tim Wicinski for the discussion on the RR formatting rules.¶

Special thanks to Tommy Jensen for the careful and thoughtful Shepherd review.¶

Thanks to Johan Stenstam and Jim Reid for the dns-dir reviews, Ray Bellis for the RRTYPE allocation review, and Arnt Gulbrandsen for the ART review.¶

Thanks to Eric Vyncke for the AD review.¶

Authors' Addresses