RFC 9606

DNS Resolver Information

📅 published June 2024, from ietf

Internet Engineering Task Force (IETF) 💖

Okay bestie, this RFC is literally about letting DNS resolvers say who they are 🎀✨. Here's the title block, so you know what you're looking at:

  • RFC number: 9606
  • Title: DNS Resolver Information 🦋
  • Authors: T. Reddy.K (Nokia) and M. Boucadair (Orange) 👑
  • Category: Standards Track 🔥
  • ISSN: 2070-1721
  • Date: June 2024 🌸
  • Obsoletes/Updates: none listed in the title block 💅

Abstract ✨

okay so this doc is about a method where DNS resolvers can publish info about themselves 💅. DNS clients can use that info to figure out what a resolver is capable of 🧚. How clients actually use the info? Not this doc's job, bestie 🙈.

Status of This Memo 🌈

This is an Internet Standards Track document 🔥.

It's a product of the Internet Engineering Task Force (IETF), so it represents the consensus of the IETF community 🫶. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG) 💜. If you want more on Internet Standards, check out Section 2 of RFC 7841 ✨.

For the current status, any errata, and how to give feedback, go to https://www.rfc-editor.org/info/rfc9606 🍓.

Copyright (c) 2024 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) as in effect on the date of publication 📜. Please review these carefully, because they describe your rights and restrictions. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions, and they're provided without warranty as described in the Revised BSD License 🙈.

Table of Contents 🦋

1.  Introduction
2.  Terminology
3.  Retrieving Resolver Information
4.  Format of the Resolver Information
5.  Resolver Information Keys/Values
6.  An Example
7.  Security Considerations
8.  IANA Considerations
  8.1.  RESINFO RR Type
  8.2.  DNS Resolver Information Keys Registration
  8.3.  Guidelines for the Designated Experts
9.  References
  9.1.  Normative References
  9.2.  Informative References
Acknowledgments
Authors' Addresses

1. Introduction 💖

Historically, DNS clients talked to recursive resolvers without needing to know anything about the features those resolvers supported 🌸. But now more and more recursive resolvers expose different features that can change the DNS services you get, like privacy preservation, filtering, transparent behavior, and so on 👀. DNS clients can discover and authenticate encrypted DNS resolvers that a local network provides, using the Discovery of Network-designated Resolvers (DNR) [RFC9463] and the Discovery of Designated Resolvers (DDR) [RFC9462] ✨. Still, those clients can't get info from the discovered resolvers about what they can do, which is a problem for the resolver selection process 😭. Instead of relying on opportunistic approaches, DNS clients need a more reliable way to discover the features configured on these resolvers 💅.

This document fills that gap 🧚. It specifies a mechanism for communicating DNS resolver information to DNS clients so they can make resolver selection decisions 🔥. For example, the selection procedure might use the retrieved info to prioritize privacy-preserving resolvers over ones that don't enable QNAME minimisation [RFC9156] 🦋. Another example: a DNS client can pick a resolver based on its filtering capability, like choosing one that filters domains according to a security policy using the Blocked (15) Extended DNS Error (EDE) [RFC8914] 🛡️. Or the client might have a policy of not selecting a resolver that forges responses using the Forged Answer (4) EDE 🙅‍♀️. Defining the selection procedure and policies is out of scope for this document, though 🙈. Once a client selects a resolver, and unless something is explicitly mentioned, this document doesn't interfere with that resolver's DNS operations 💜.

Specifically, this document defines a new resource record (RR) type that DNS clients use to query recursive resolvers 🎀. The initial information a resolver might want to expose is defined in Section 5 ✨. That info covers properties used to infer a resolver's privacy and transparency policies 👑. Other info can be registered in the future following the guidance in Section 8.2 🌈. This information is not meant for end-user consumption 💅.

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 uses the terms defined in [RFC9499]. These additional terms are also 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: Defined as "the estimation in which an identifiable actor is held, especially by the community or the Internet public generally" per Section 1 of [RFC7070] 🌸.

3. Retrieving Resolver Information 🦋

A DNS client that wants the resolver information can use the RR type "RESINFO" defined in this document 💖. The RDATA content 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 MUST have exactly one record 🎀. Invalid records MUST be silently ignored by DNS clients 🙈. 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 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 deal with the risk that a resolver doesn't support RESINFO 😬. The resolver might pass the query upstream, and then the client can receive a positive RESINFO response from either a legitimate DNS resolver or an attacker 💀.

⚠️ The DNS client MUST set the Recursion Desired (RD) bit of the query to 0. The DNS client MUST discard the response if the AA flag in the response is set to 0, indicating that the DNS resolver is not authoritative for the response 🚫.

If a group of resolvers shares the same ADN and/or anycast address, then these instances SHOULD expose a consistent RESINFO 👯‍♀️.

4. Format of the Resolver Information 💅

The resolver information record uses the same format as DNS TXT records 📝. The format rules for TXT records are defined in the base DNS specification (Section 3.3.14 of [RFC1035]) and are 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 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 to define future keys 🌸. If a DNS client sees unknown keys in a RESINFO RR type, it MUST silently ignore them 🙈. The same rules for the keys, as defined in Section 6.4 of [RFC6763], MUST be followed for RESINFO 💖.

Resolver information 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: The presence of this key indicates that the DNS resolver supports QNAME minimisation [RFC9156] to improve DNS privacy 🫶. Note that, per the rules for keys in Section 6.4 of [RFC6763], if there is no '=' in a key, then it's a boolean attribute, simply identified as being present, with no value 🧠.

The presence of this key indicates that the DNS resolver is configured to minimise the amount of privacy-sensitive data sent to an authoritative name server 🔒.

This is an optional attribute 🌈.

  • exterr: If the DNS resolver supports the EDE option defined in [RFC8914] to return additional information about the cause of DNS errors, the value of this key lists the possible EDE codes that this DNS resolver can return 📋. A value can be an individual EDE or a range of EDEs. Range values MUST be identified by "-". When multiple non-contiguous values are present, these values MUST be comma-separated 🎀.

Returned EDEs (e.g., Blocked (15), Censored (16), and Filtered (17)) indicate whether the DNS resolver is configured to reveal the reason a query was filtered or blocked when that happens 👀. If the resolver's capabilities are updated to include new similar error codes, the resolver can terminate the TLS session, prompting the client to start a new TLS connection and retrieve the resolver information again ✨. This lets the client become aware of the resolver's updated capabilities 💅. Alternatively, if the client gets an EDE for a DNS request but that EDE wasn't listed in "exterr", the client can query the resolver again to learn about the updated capabilities for returning new error codes 🔄. If a mismatch still exists, the client can identify that the resolver information is inaccurate and discard it 🗑️.

This is an optional attribute 🌸.

  • infourl: A URL that points to 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 the "resolver information server" 🖥️.

The resolver information server MUST support only the content-type "text/html" for the resolver information 📄. The DNS client MUST reject the URL as invalid if the scheme is not "https" 🔐. Invalid URLs MUST be ignored 🙅. The URL MUST be treated only as diagnostic information for IT staff 🧑‍💻. It's not intended for end-user consumption, since the URL can possibly provide misleading information ⚠️.

This key can be used by IT staff to retrieve other useful information about the resolver, and also the procedure to report problems (e.g., invalid filtering) 🧰.

This is an optional attribute 💖.

New keys can be defined following the procedure in Section 8.2 🌈.

Part 1 of 2. Part 2 picks up with Section 6 (An Example) and carries on through Security Considerations, IANA Considerations, the References, and the Authors' Addresses 💜.

  1. An Example 🎀

Okay so here's a Figure 1 example of a published resolver information record, and it's giving everything ✨

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

Figure 1: An Example of a Resolver Information Record

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 enables QNAME minimisation, 🧚
  • the resolver can return Blocked (15), Censored (16), and Filtered (17) EDEs, and 💅
  • more information can be retrieved from "https://resolver.example.com/guide". 🌸
  1. Security Considerations 🔥

Okay bestie, this section is SERIOUS 😭 DNS clients talking to discovered DNS resolvers MUST use one of the following measures to prevent DNS response forgery attacks, no exceptions 🙈

  1. Establish an authenticated secure connection to the DNS resolver. 🔐
  2. Implement local DNSSEC validation (Section 10 of [RFC9499]) 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". Literally the plot twist 💀

An encrypted resolver may return incorrect information in RESINFO. 😬 If the client cannot validate the attributes received from the resolver, which 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. It's giving protective, no cap 🛡️✨

  1. IANA Considerations 🌈

8.1. RESINFO RR Type 🦋

IANA has updated the "Resource Record (RR) TYPEs" registry under the "Domain Name System (DNS) Parameters" registry group [RRTYPE] as follows:

Type:  RESINFO
Value:  261
Meaning:  Resolver Information as Key/Value Pairs
Reference:  RFC 9606

8.2. DNS Resolver Information Keys Registration 🍓

IANA has created a new registry called "DNS Resolver Information Keys" under the "Domain Name System (DNS) Parameters" registry group [IANA-DNS]. This new registry contains definitions of the keys that can be used to provide the resolver information.

The registration procedure is Specification Required (Section 4.6 of [RFC8126]). Designated experts should carefully consider the security implications of allowing a resolver to include new keys in this registry. Additional considerations are provided in Section 8.3.

The structure of the registry is as follows:

Name:  The key name.  The name MUST conform to the definition in
   Section 4 of this document.  The IANA registry MUST NOT register
   names that begin with "temp-" so that these names can be used
   freely by any implementer.

Description:  A description of the registered key.

Reference:  The reference specification for the registered element.

The initial contents of this registry are provided in Table 1.

   +==========+=====================================+===========+
   | Name     | Description                         | Reference |
   +==========+=====================================+===========+
   | qnamemin | The presence of the key name        |  RFC 9606 |
   |          | indicates that QNAME minimisation   |           |
   |          | is enabled.                         |           |
   +----------+-------------------------------------+-----------+
   | exterr   | Lists the set of enabled extended   |  RFC 9606 |
   |          | DNS errors.  It must be an INFO-    |           |
   |          | CODE decimal value in the "Extended |           |
   |          | DNS Error Codes" registry           |           |
   |          | <https://www.iana.org/assignments/  |           |
   |          | dns-parameters/>.                   |           |
   +----------+-------------------------------------+-----------+
   | infourl  | Provides a URL that points to       |  RFC 9606 |
   |          | unstructured resolver information   |           |
   |          | that is used for troubleshooting.   |           |
   +----------+-------------------------------------+-----------+

     Table 1: Initial Contents of the DNS Resolver Information
                           Keys Registry

8.3. Guidelines for the Designated Experts 🧐

It is suggested that multiple designated experts be appointed for registry change requests.

Criteria that should be applied by the designated experts include determining whether the proposed registration duplicates existing entries and whether the registration description is clear and fits the purpose of this registry.

Registration requests are evaluated within a two-week review period on the advice of one or more designated experts. Within the review period, the designated experts will either approve or deny the registration request, communicating this decision to IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. ✨

  1. References 📚

9.1. Normative References 💜

[RFC1035]  Mockapetris, P., "Domain names - implementation and
           specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
           November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[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>.

[RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
           Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
           <https://www.rfc-editor.org/info/rfc6763>.

[RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
           Writing an IANA Considerations Section in RFCs", BCP 26,
           RFC 8126, DOI 10.17487/RFC8126, June 2017,
           <https://www.rfc-editor.org/info/rfc8126>.

[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>.

[RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
           Lawrence, "Extended DNS Errors", RFC 8914,
           DOI 10.17487/RFC8914, October 2020,
           <https://www.rfc-editor.org/info/rfc8914>.

[RFC9156]  Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
           Name Minimisation to Improve Privacy", RFC 9156,
           DOI 10.17487/RFC9156, November 2021,
           <https://www.rfc-editor.org/info/rfc9156>.

[RFC9462]  Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
           Jensen, "Discovery of Designated Resolvers", RFC 9462,
           DOI 10.17487/RFC9462, November 2023,
           <https://www.rfc-editor.org/info/rfc9462>.

[RFC9463]  Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
           and T. Jensen, "DHCP and Router Advertisement Options for
           the Discovery of Network-designated Resolvers (DNR)",
           RFC 9463, DOI 10.17487/RFC9463, November 2023,
           <https://www.rfc-editor.org/info/rfc9463>.

9.2. Informative References 🦋

[IANA-DNS] IANA, "Domain Name System (DNS) Parameters",
           <https://www.iana.org/assignments/dns-parameters/>.

[RESINFO]  Sood, P. and P. Hoffman, "DNS Resolver Information Self-
           publication", Work in Progress, Internet-Draft, draft-pp-
           add-resinfo-02, 27 June 2020,
           <https://datatracker.ietf.org/doc/html/draft-pp-add-
           resinfo-02>.

[RFC7070]  Borenstein, N. and M. Kucherawy, "An Architecture for
           Reputation Reporting", RFC 7070, DOI 10.17487/RFC7070,
           November 2013, <https://www.rfc-editor.org/info/rfc7070>.

[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>.

[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>.

[RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
           Dedicated QUIC Connections", RFC 9250,
           DOI 10.17487/RFC9250, May 2022,
           <https://www.rfc-editor.org/info/rfc9250>.

[RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
           RFC 9499, DOI 10.17487/RFC9499, March 2024,
           <https://www.rfc-editor.org/info/rfc9499>.

[RRTYPE]   IANA, "Resource Record (RR) TYPEs",
           <https://www.iana.org/assignments/dns-parameters/>.

Acknowledgments 🫶

This specification leverages the work that has been documented in [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 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, Arnt Gulbrandsen for the ART review, and Mallory Knodel for the gen-art review. 🧚

Thanks to Éric Vyncke for the AD review. 🌸

Thanks to Gunter Van de Velde, Erik Kline, Paul Wouters, Orie Steele, Warren Kumari, Roman Danyliw, and Murray Kucherawy for the IESG review. 💅

Authors' Addresses 🦋

Tirumaleswar Reddy.K
Nokia
India
Email: kondtir@gmail.com


Mohamed Boucadair
Orange
35000 Rennes
France
Email: mohamed.boucadair@orange.com
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