Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

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Table of Contents

1. Introduction

The SVCB resource record (RR) type [SVCB] provides clients with information about how to reach alternative endpoints for a service. These endpoints may offer improved performance or privacy properties. The service is identified by a "scheme" indicating the service type, a hostname, and, optionally, other information such as a port number. A DNS server is often identified only by its IP address (e.g., in DHCP), but in some contexts it can also be identified by a hostname (e.g., "NS" records, manual resolver configuration) and sometimes also a non-default port number.¶

The use of the SVCB RR type requires a mapping document for each service type (Section 2.4.3 of [SVCB]), indicating how a client for that service can interpret the contents of the SVCB SvcParams. This document provides the mapping for the "dns" service type, allowing DNS servers to offer alternative endpoints and transports, including encrypted transports like DNS over TLS (DoT) [RFC7858], DNS over HTTPS (DoH) [RFC8484], and DNS over QUIC (DoQ) [RFC9250].¶

The SVCB mapping described in this document is intended as a general-purpose baseline. Subsequent specifications will adapt this mechanism as needed to support specific configurations (e.g., for communication between stub resolvers and recursive 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. Identities and Names

SVCB record names (i.e., QNAMEs) for DNS services are formed using Port Prefix Naming (Section 2.3 of [SVCB]), with a scheme of "dns". For example, SVCB records for a DNS service identified as dns1.example.com would be queried at _dns.dns1.example.com.¶

In some use cases, the name used for retrieving these DNS records is different from the server identity used to authenticate the secure transport. To distinguish between these, this document uses the following terms:¶

Binding authority:

The service name (Section 1.3 of [SVCB]) and optional port number used as input to Port Prefix Naming.¶

Authentication name:

The name used for secure transport authentication. This MUST be a DNS hostname or a literal IP address. Unless otherwise specified, this is the service name from the binding authority.¶

4. Applicable Existing SvcParamKeys

5. New SvcParamKey: "dohpath"

"dohpath" is a single-valued SvcParamKey whose value (in both presentation format and wire format) MUST be a URI Template in relative form ([RFC6570], Section 1.1) encoded in UTF-8 [RFC3629]. If the "alpn" SvcParam indicates support for HTTP, "dohpath" MUST be present. The URI Template MUST contain a "dns" variable, and MUST be chosen such that the result after DoH URI Template expansion (Section 6 of [RFC8484]) is always a valid and functional ":path" value ([RFC9113], Section 8.3.1).¶

When using this SVCB record, the client MUST send any DoH requests to the HTTP origin identified by the "https" scheme, the authentication name, and the port from the "port" SvcParam (if present). HTTP requests MUST be directed to the resource resulting from DoH URI Template expansion of the "dohpath" value.¶

Clients SHOULD NOT query for any HTTPS RRs when using "dohpath". Instead, the SvcParams and address records associated with this SVCB record SHOULD be used for the HTTPS connection, with the same semantics as an HTTPS RR. However, for consistency, service operators SHOULD publish an equivalent HTTPS RR, especially if clients might learn about this DoH service through a different channel.¶

6. Limitations

This document is concerned exclusively with the DNS transport and does not affect or inform the construction or interpretation of DNS messages. For example, nothing in this document indicates whether the service is intended for use as a recursive or authoritative DNS server. Clients need to know the intended use of services based on their context.¶

Not all features of this specification will be applicable or effective in all contexts:¶

• If the authentication name is received over an insecure channel (e.g., a glue NS record), this specification cannot prevent the client from connecting to an attacker.¶
• Different transports might prove to be popular for different purposes (e.g., querying a recursive resolver vs. an authoritative server). Implementors are not obligated to implement all the defined transports, although doing so is beneficial for compatibility.¶
• Where resolution speed is a high priority, the SVCB TargetName SHOULD follow the convention described in Section 10.2 of [SVCB], and the use of AliasMode records (Section 2.4.2 of [SVCB]) is NOT RECOMMENDED.¶

7. Examples

• A resolver known as simple.example that supports DNS over TLS on port 853 (implicitly, as this is its default port):¶

  _dns.simple.example. 7200 IN SVCB 1 simple.example. alpn=dot
  

  ¶

• A DoH-only resolver at https://doh.example/dns-query{?dns}. (DNS over TLS is not supported.):¶

  _dns.doh.example. 7200 IN SVCB 1 doh.example. (
        alpn=h2 dohpath=/dns-query{?dns} )
  

  ¶

• A resolver known as resolver.example that supports:¶

  • DoT on resolver.example ports 853 (implicit in record 1) and 8530 (explicit in record 2), with "resolver.example" as the Authentication Domain Name,¶
  • DoQ on resolver.example port 853 (record 1),¶
  • DoH at https://resolver.example/q{?dns} (record 1), and¶

  • an experimental protocol on fooexp.resolver.example:5353 (record 3):¶

    _dns.resolver.example.  7200 IN \
      SVCB 1 resolver.example. alpn=dot,doq,h2,h3 dohpath=/q{?dns}
      SVCB 2 resolver.example. alpn=dot port=8530
      SVCB 3 fooexp.resolver.example. port=5353 alpn=foo foo-info=...
    

    ¶

• A name server named ns.example. whose service configuration is published on a different domain:¶

  _dns.ns.example. 7200 IN SVCB 0 _dns.ns.nic.example.
  

  ¶

8. Security Considerations

9. IANA Considerations

Per [SVCB], IANA has added the following entry to the "Service Parameter Keys (SvcParamKeys)" registry.¶

Per [Attrleaf], IANA has added the following entry to the DNS "Underscored and Globally Scoped DNS Node Names" registry:¶

Appendix A. Mapping Summary

This table serves as a non-normative summary of the DNS mapping for SVCB.¶

Acknowledgments

Thanks to the many reviewers and contributors, including Andrew Campling, Peter van Dijk, Paul Hoffman, Daniel Migault, Matt Nordhoff, Eric Rescorla, Andreas Schulze, and Éric Vyncke.¶

Author's Address