Discovery of Oblivious Services via Service Binding Records

Introduction Oblivious HTTP allows clients to encrypt messages exchanged with an HTTP server accessed via a proxy, in such a way that the proxy cannot inspect the contents of the message and the target HTTP server does not discover the client's identity. In order to use Oblivious HTTP, clients need to possess a key configuration to use to encrypt messages to the oblivious target. Since Oblivious HTTP deployments will often involve very specific coordination between clients, proxies, and targets, the key configuration can often be shared in a bespoke fashion. However, some deployments involve clients discovering oblivious targets more dynamically. For example, a network may want to advertise a DNS resolver that is accessible over Oblivious HTTP and applies local network resolution policies via mechanisms like Discovery of Designated Resolvers (. Clients can work with trusted proxies to access these target servers. This document defines a mechanism to advertise that an HTTP service supports Oblivious HTTP using DNS records, as a parameter that can be included in SVCB and HTTPS DNS resource records . The presence of this parameter indicates that a service has an oblivious target; see for a description of oblivious targets. This document also defines a well-known URI , "oblivious-configs", that can be used to look up key configurations on a service that is known to have an oblivious target. This mechanism does not aid in the discovery of proxies to use to access oblivious targets; the configuration of proxies is out of scope for this document.

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 when, and only when, they appear in all capitals, as shown here.

The oblivious SvcParamKey The "oblivious" SvcParamKey is used to indicate that a service described in an SVCB record can act as an oblivious target. Clients can issue requests to this service through an oblivious proxy once they learn the key configuration to use to encrypt messages to the oblivious target. Both the presentation and wire format values for the "oblivious" parameter MUST be empty. The "oblivious" parameter can be included in the mandatory parameter list to ensure that clients that do not support oblivious access do not try to use the service. Services that include mark oblivious support as mandatory can, therefore, indicate that the service might not be accessible in a non-oblivious fashion. Services that are intended to be accessed either as an oblivious target or directly SHOULD NOT mark the "oblivious" parameter as mandatory. Note that since multiple SVCB responses can be provided for a single query, the oblivious and non-oblivious versions of a single service can have different SVCB records to support different names or properties. The scheme to use for oblivious requests made to a service depends on the scheme of the SVCB record. This document defines the interpretation for the "https" and "dns" schemes. Other schemes that want to use this parameter MUST define the interpretation and meaning of the configuration.

Use in HTTPS service records For the "https" scheme, which uses the HTTPS RR type instead of SVCB, the presence of the "oblivious" parameter means that the service being described is an Oblivious HTTP service that uses the default "message/bhttp" media type . For example, an HTTPS service record for svc.example.com that supports an oblivious target could look like this: A similar record for a service that only support oblivious connectivity could look like this:

Use in DNS server SVCB records For the "dns" scheme, as defined in , the presence of the "oblivious" parameter means that the DNS server being described is an Oblivious DNS over HTTP (DoH) service. The default media type expected for use in Oblivious HTTP to DNS resolvers is "application/dns-message" . In order for DNS servers to function as oblivious targets, they need to be accessible via an oblivious proxy. Encrypted DNS servers used with the discovery mechanisms described in this section can either be publicly accessible, or specific to a network. In general, only publicly accessible DNS servers will work as Oblivious DNS servers, unless there is a coordinated deployment with an oblivious proxy that is also hosted within a network.

Use with DDR Clients can discover an oblivious DNS server configuration using DDR, by either querying _dns.resolver.arpa to a locally configured resolver or querying using the name of a resolver . For example, a DoH service advertised over DDR can be annotated as supporting oblivious resolution using the following record: Clients still need to perform some verification of oblivious DNS servers, such as the TLS certificate check described in . This certificate check can be done when looking up the configuration on the resolver using the well-known URI (), which can either be done directly, or via a proxy to avoid exposing client IP addresses. Clients also need to ensure that they are not being targeted with unique key configurations that would reveal their identity. See for more discussion.

Use with DNR The SvcParamKeys defined in this document also can be used with Discovery of Network-designated Resolvers (DNR) . In this case, the oblivious configuration and path parameters can be included in DHCP and Router Advertisement messages. While DNR does not require the same kind of verification as DDR, clients still need to ensure that they are not being targeted with unique key configurations that would reveal their identity. See for more discussion.

Configuration Well-Known URI Clients that know a service is available as an oblivious target, e.g., either via discovery through the "oblivious" parameter in a SVCB or HTTPS record, or by configuration, need to know the key configuration before sending oblivious requests. This document defines a well-known URI , "oblivious-configs", that allows a target to host its configurations. The URI is constructed using the TargetName in the associated ServiceMode SVCB record. For example, the URI for the following record: would be "https://svc.example.com/.well-known/oblivious-configs". As another example, the URI for the following record: would be "https://doh.example.net/.well-known/oblivious-configs". The content of this resource is expected to be "application/ohttp-keys", as defined in . Before being able to use a server as an oblivious target, clients need to use this URI to fetch the configuration. They can either fetch it directly, or do so via a proxy in order to avoid the server discovering information about the client's identity. See for more discussion of avoiding key targeting attacks.

Security and Privacy Considerations Attackers on a network can remove SVCB information from cleartext DNS answers that are not protected by DNSSEC . This can effectively downgrade clients. However, since SVCB indications for oblivious support are just hints, a client can mitigate this by always checking for oblivious target information. Use of encrypted DNS or DNSSEC also can be used as mitigations. When discovering designated oblivious DNS servers using this mechanism, clients need to ensure that the designation is trusted in lieu of being able to directly check the contents of the target server's TLS certificate. See for more discussion, as well as the Security Considerations of . As discussed in , client requests using Oblivious HTTP can only be linked by recognizing the key configuration. In order to prevent unwanted linkability and tracking, clients using any key configuration discovery mechanism need to be concerned with attacks that target a specific user or population with a unique key configuration. There are several approaches clients can use to mitigate key targeting attacks. provides an analysis of the options for ensuring the key configurations are consistent between different clients. Clients SHOULD employ some technique to mitigate key targeting attack. Oblivious targets that are detected to use targeted key configurations per-client MUST NOT be used. When clients fetch a target's configuration using the well-known URI, they can expose their identity in the form of an IP addres if they do not connect via a proxy or some other IP-hiding mechanism. Clients SHOULD use a proxy or similar mechanism to avoid exposing client IPs to a target.

IANA Considerations

SVCB Service Parameter IANA is requested to add the following entry to the SVCB Service Parameters registry ().

Number	Name	Meaning	Reference
TBD	oblivious	Describes if a service has an oblivious target	(This document)

Well-Known URI IANA is requested to add a new entry in the "Well-Known URIs" registry with the following information: URI suffix: oblivious-configs Change controller: IETF Specification document: This document Status: permanent Related information: N/A