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 and DNS over HTTPS.¶

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. Name form

Names are formed using Port-Prefix Naming ([SVCB] Section 2.3). For example, a DNS service identified by the name "dns1.example.com" and (unusually) the non-default port number 5353 would be represented as _5353._dns.dns1.example.com.. A DNS service using the default port number of 53 would be represented as _dns.dns1.example.com..¶

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 must know the intended use in their context.¶

7. Relationship to DNS URIs

The dns: URI scheme [DNSURI] describes a way to represent DNS queries as URIs. This scheme optionally includes an authority, comprised of a host and port number (with a default of 53). DNS URIs normally omit the authority, or specify an IP address, but a hostname is allowed, in which case it is suitable for use with this mapping.¶

8. 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 alpn=dot simple.example.
  

  ¶
• A resolver at doh.example that supports only DNS over HTTPS (DNS over TLS is disabled): _dns.doh.example. 7200 IN SVCB 1 doh.example. ( alpn=h2 dohpath=/dns-query{?dns} )¶

• A resolver at resolver.example that supports¶

  • DNS over TLS on resolver.example ports 853 (implicit in record 1) and 8530 (explicit in record 2), with resolver.example as the Authentication Domain Name,¶
  • DNS over HTTPS at https://resolver.example/dns-query{?dns} (record 1), and¶

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

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

    ¶

• A nameserver at ns.example whose service configuration is published on a different domain:¶

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

  ¶

9. Security Considerations

10. IANA Considerations

Per [SVCB] IANA would be directed to add the following entry to the SVCB Service Parameters registry.¶

Per [Attrleaf], IANA would be 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

TODO acknowledge.¶

Author's Address