Service Binding Mapping for DNS Servers

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Table of Contents

1. Introduction

The SVCB 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 record type requires a mapping document for each service type, 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) and DNS over HTTPS (DoH).¶

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.¶

7. Examples

• A resolver at "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 at "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,¶
  • 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,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 at "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 Daniel Migault, Paul Hoffman, Matt Norhoff, Peter van Dijk, Eric Rescorla, and Andreas Schulze.¶

Author's Address