Internet-Draft SVCB for DNS July 2021
Schwartz Expires 27 January 2022 [Page]
Intended Status:
Standards Track
B. Schwartz
Google LLC

Service Binding Mapping for DNS Servers


The SVCB DNS record type expresses a bound collection of endpoint metadata, for use when establishing a connection to a named service. DNS itself can be such a service, when the server is identified by a domain name. This document provides the SVCB mapping for named DNS servers, allowing them to indicate support for new transport protocols.

Discussion Venues

This note is to be removed before publishing as an RFC.

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Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 27 January 2022.

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), with a scheme of "dns". For example, SVCB records for a DNS service identified as would be located at

3.1. Special case: non-default ports

Normally, a DNS service is identified by an IP address or a domain name. When connecting to the service using unencrypted DNS over UDP or TCP, clients use the default port number for DNS (53). However, in rare cases, a DNS service might be identified by both a name and a port number. For example, the dns: URI scheme [DNSURI] optionally includes an authority, comprised of a host and a port number (with a default of 53). DNS URIs normally omit the authority, or specify an IP address, but a hostname and non-default port number are allowed.

When a non-default port number is part of a service identifier, Port-Prefix Naming places the port number in an additional a prefix on the name. For example, SVCB records for a DNS service identified as would be located at If two DNS services operating on different port numbers provide different behaviors, this arrangement allows them to preserve the distinction when specifying alternative endpoints.

4. Applicable existing SvcParamKeys

4.1. alpn

This key indicates the set of supported protocols ([SVCB] Section 6.1). There is no default protocol, so the no-default-alpn key does not apply, and the alpn key MUST be present.

If the protocol set contains any HTTP versions (e.g. "h2", "h3"), then the record indicates support for DNS over HTTPS [DOH], and the "dohpath" key MUST be present (Section 5.1). All keys specified for use with the HTTPS record are also permissible, and apply to the resulting HTTP connection.

If the protocol set contains protocols with different default ports, and no port key is specified, then protocols are contacted separately on their default ports. Note that in this configuration, ALPN negotiation does not defend against cross-protocol downgrade attacks.

4.2. port

This key is used to indicate the target port for connection (([SVCB] Section 6.2)). If omitted, the client SHALL use the default port for each transport protocol (853 for DNS over TLS [DOT], 443 for DNS over HTTPS).

This key is automatically mandatory if present. (See Section 7 of [SVCB] for the definition of "automatically mandatory".)

4.3. Other applicable SvcParamKeys

These SvcParamKeys from [SVCB] apply to the "dns" scheme without modification:

Future SvcParamKeys may also be applicable.

5. New SvcParamKeys

5.1. dohpath

"dohpath" is a single-valued SvcParamKey whose value (both in presentation and wire format) is a relative URI Template [RFC6570], normally starting with "/". If the "alpn" SvcParamKey indicates support for HTTP, clients MAY construct a DNS over HTTPS URI Template by combining the prefix "https://", the service name, the port from the "port" key if present, and the "dohpath" value. (The DNS service's original port number MUST NOT be used.)

Clients SHOULD NOT query for any "HTTPS" RRs when using the constructed URI Template. 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 this URI Template 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 must know the intended use in their context.

7. Examples

8. Security Considerations

8.1. Adversary on the query path

This section considers an adversary who can add or remove responses to the SVCB query.

Clients MUST authenticate the server to its name during secure transport establishment. This name is the hostname used to construct the original SVCB query, and cannot be influenced by the SVCB record contents. Accordingly, this draft does not mandate the use of DNSSEC. This draft also does not specify how clients authenticate the name (e.g. selection of roots of trust), which might vary according to the context.

Although this adversary cannot alter the authentication name of the service, it does have control of the port number and "dohpath" value. As a result, the adversary can direct DNS queries for $HOSTNAME to any port on $HOSTNAME, and any path on "https://$HOSTNAME", even if $HOSTNAME is not actually a DNS server. If the DNS client uses shared TLS or HTTP state, the client could be correctly authenticated (e.g. using a TLS client certificate or HTTP cookie).

This behavior creates a number of possible attacks for certain server configurations. For example, if "https://$HOSTNAME/upload" accepts any POST request as a public file upload, the adversary could forge a SVCB record containing dohpath=/upload. This would cause the client to upload and publish every query, resulting in unexpected storage costs for the server and privacy loss for the client.

To mitigate this attack, a client of this SVCB mapping MUST NOT provide client authentication for DNS queries, except to servers that it specifically knows are not vulnerable to such attacks, and a DoH service operator MUST ensure that all unauthenticated DoH requests to its origin maintain the DoH service's privacy guarantees, regardless of the path. Also, if an alternative service endpoint sends an invalid response to a DNS query, the client SHOULD NOT send more queries to that endpoint.

8.2. Adversary on the transport path

This section considers an adversary who can modify network traffic between the client and the alternative service (identified by the TargetName).

For a SVCB-reliant client ([SVCB] Section 3), this adversary can only cause a denial of service. However, because DNS is unencrypted by default, this adversary can execute a downgrade attack against SVCB-optional clients. Accordingly, when use of this specification is optional, clients SHOULD switch to SVCB-reliant behavior if SVCB resolution succeeds. Specifications making using of this mapping MAY adjust this fallback behavior to suit their requirements.

9. IANA Considerations

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

Table 1
Number Name Meaning Reference
TBD dohpath DNS over HTTPS path template (This document)

Per [Attrleaf], IANA would be directed to add the following entry to the DNS Underscore Global Scoped Entry Registry:

Table 2
RR TYPE _NODE NAME Meaning Reference
SVCB _dns DNS SVCB info (This document)

10. References

10.1. Normative References

Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, , <>.
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, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Schwartz, B., Bishop, M., and E. Nygren, "Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-dnsop-svcb-https-06, , <>.

10.2. Informative References

Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, , <>.
Josefsson, S., "Domain Name System Uniform Resource Identifiers", RFC 4501, DOI 10.17487/RFC4501, , <>.

Appendix A. Mapping Summary

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

Table 3
Mapped scheme "dns"
RR type SVCB (64)
Name prefix _dns for port 53, else _$PORT._dns
Required keys alpn
Automatically Mandatory Keys port
Special behaviors Supports all HTTPS RR SvcParamKeys
  Overrides the HTTPS RR for DoH
  Default port is per-transport
  No encrypted -> cleartext fallback


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

Benjamin Schwartz
Google LLC