| Internet-Draft | SVCB for DNS | February 2022 |
| Schwartz | Expires 5 August 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.¶
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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.¶
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 "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 them, we use the following terms:¶
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 "dns1.example.com:9953", the client would query for SVCB records 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 (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.¶
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".)¶
"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:¶
$HOST be the authentication name encoded as a "host" value (Section 3.2.2 of [RFC3986]).¶
$PORT be the port from the "port" key if present, otherwise 443. (The binding authority's port number MUST NOT be used.)¶
$DOHPATH be the "dohpath" value, decoded from UTF-8.¶
"https://$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.¶
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):¶
_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.¶
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.¶
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.¶
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.¶
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.¶
Per [SVCB] IANA is 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 is 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.¶