Internet-Draft SVCB for DNS February 2022
Schwartz Expires 5 August 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 5 August 2022.

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. Identities and Names

SVCB record names (i.e. QNAMEs) 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 "" would be queried at "".

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 them, we use 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. It must be a DNS hostname or a literal IP address. Unless otherwise specified, it is the service name from the binding authority.

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 the binding authority specifies a non-default port number, Port-Prefix Naming places the port number in an additional a prefix on the name. For example, if the binding authority is "", the client would query for SVCB records 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 (Section 6.1 of [SVCB]). 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 (Section 6.2 of [SVCB]). 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) MUST be a URI Template [RFC6570] encoded in UTF-8 [RFC3629]. If the "alpn" SvcParamKey indicates support for HTTP, "dohpath" MUST be present, and clients MAY construct a DNS over HTTPS URI Template as follows:

  1. Let $HOST be the authentication name encoded as a "host" value (Section 3.2.2 of [RFC3986]).
  2. Let $PORT be the port from the "port" key if present, otherwise 443. (The binding authority's port number MUST NOT be used.)
  3. Let $DOHPATH be the "dohpath" value, decoded from UTF-8.
  4. The DNS over HTTPS URI Template is "https://$HOST:$PORT$DOHPATH".

The "dohpath" value MUST be chosen such that the resulting URI Template is valid for use with DNS over HTTPS. For example, DNS over HTTPS servers are required to support requests using GET and POST methods. The GET method relies on the "dns" URI Template parameter, and the POST method does not use it. Therefore, the URI Template is required to make use of a "dns" variable, and result in a valid URI whether or not "dns" is defined.

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

  • 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 resolver at "doh.example" that supports only DNS over HTTPS (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:

    • 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):

      _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

8.1. Adversary on the query path

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

During secure transport establishment, clients MUST authenticate the server to its authentication name, which is not 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.

8.1.1. Downgrade attacks

This attacker cannot impersonate the secure endpoint, but it can forge a response indicating that the requested SVCB records do not exist. For a SVCB-reliant client ([SVCB], Section 3) this only results in a denial of service. However, SVCB-optional clients will generally fall back to insecure DNS in this case, exposing all DNS traffic to attacks.

8.1.2. Redirection attacks

SVCB-reliant clients always enforce the authentication domain name, but they are still subject to attacks using the transport, port number, and "dohpath" value, which are controlled by this adversary. By changing these values in the SVCB answers, the adversary can direct DNS queries for $HOSTNAME to any port on $HOSTNAME, and any path on "https://$HOSTNAME". 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. Similarly, if two DoH endpoints are available on the same origin, and the service has designated one of them for use with this specification, this adversary can cause clients to use the other endpoint instead.

To mitigate redirection attacks, 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. If an endpoint sends an invalid response to a DNS query, the client SHOULD NOT send more queries to that endpoint. DNS services that are identified by a hostname (Section 3) MUST ensure that all unauthenticated DNS requests to that name receive any promised privacy and security guarantees, regardless of transport, port number, or HTTP path.

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, 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 is directed to add the following entry to the SVCB Service Parameters registry.

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

Per [Attrleaf], IANA is 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, , <>.
Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, , <>.
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <>.
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-08, , <>.

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