| Internet-Draft | SVCB for DNS | October 2021 |
| Schwartz | Expires 4 April 2022 | [Page] |
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.¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the ADD Working Group mailing list (add@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/add/.¶
Source for this draft and an issue tracker can be found at https://github.com/bemasc/svcb-dns.¶
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 https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 4 April 2022.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
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.¶
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.¶
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 dns1.example.com would be located at _dns.dns1.example.com.¶
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 dns1.example.com:9953 would be located at _9953._dns.dns1.example.com. If two DNS services operating on different port numbers provide different behaviors, this arrangement allows them to preserve the distinction when specifying alternative endpoints.¶
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.¶
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".)¶
"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.¶
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.¶
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¶
resolver.example ports 853 (implicit in record 1) and 8530 (explicit in record 2), with resolver.example as the Authentication Domain Name,¶
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¶
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.¶
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.¶
Per [SVCB] IANA would be directed to add the following entry to the SVCB Service Parameters registry.¶
| Number | Name | Meaning | Reference |
|---|---|---|---|
| 7 | 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:¶
| RR TYPE | _NODE NAME | Meaning | Reference |
|---|---|---|---|
| SVCB | _dns | DNS SVCB info | (This document) |
This table serves as a non-normative summary of the DNS mapping for SVCB.¶
| 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.¶