Service Binding Mapping for DNS Servers

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Table of Contents

1. Introduction

The SVCB resource record type [SVCB] provides clients with information about how to reach alternative endpoints for a service, which may have 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.¶

Use of the SVCB resource record 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 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 1.4 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 SvcParamKeys

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., stub resolution vs. iterative resolution). 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/dns-query{?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=/dns-query{?dns} )
    _dns.resolver.example.  7200 IN SVCB 2 resolver.example. (
        alpn=dot port=8530 )
    _dns.resolver.example.  7200 IN SVCB 3 fooexp (
          port=5353 alpn=foo foo-info=... )
    

    ¶

• A nameserver 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 is directed to add the following entry to the SVCB Service Parameters registry.¶

Per [Attrleaf], IANA is directed to add the following entry to the DNS Underscore Global Scoped Entry 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 Norhoff, Eric Rescorla, Andreas Schulze, and Éric Vyncke.¶

Author's Address