Discovery of Oblivious Services via Service Binding Records

Introduction Oblivious HTTP allows clients to encrypt messages exchanged with an Oblivious Target Resource (target). The messages are encapsulated in encrypted messages to an Oblivious Gateway Resource (gateway), which gates access to the target. The gateway is accessed via an Oblivious Relay Resource (relay), which proxies the encapsulated messages to hide the identity of the client. Overall, this architecture is designed in such a way that the relay cannot inspect the contents of messages, and the gateway and target cannot learn the client's identity from a single transaction. Since Oblivious HTTP deployments typically involve very specific coordination between clients, relays, and gateways, the key configuration is often shared in a bespoke fashion. However, some deployments involve clients discovering targets and their associated gateways more dynamically. For example, a network might operate a DNS resolver that provides more optimized or more relevant DNS answers and is accessible using Oblivious HTTP, and might want to advertise support for Oblivious HTTP via mechanisms like Discovery of Designated Resolvers (). Clients can access these gateways through trusted relays. This document defines a way to use DNS records to advertise that an HTTP service supports Oblivious HTTP. This advertisement is a parameter that can be included in SVCB and HTTPS DNS resource records (). The presence of this parameter indicates that a service can act as a target and has a gateway that can provide access to the target. The client learns the URI to use for the gateway using a well-known URI , "ohttp-gateway", which is accessed on the target (). This means that for deployments that support this kind of discovery, the gateway and target resources need to be located on the same host. This document also defines a way to fetch an gateway's key configuration from the gateway (). This mechanism does not aid in the discovery of relays; relay configuration is out of scope for this document. Models in which this discovery mechanism is applicable are described in .

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.

Applicability There are multiple models in which the discovery mechanism defined in this document can be used.

Upgrading regular (non-proxied) HTTP to Oblivious HTTP. In this model, the client intends to communicate with a specific target service, and prefers to use Oblivious HTTP if it is available. The target service has a gateway that it offers to allow access using Oblivious HTTP. Once the client learns about the gateway, it "upgrades" to using Oblivious HTTP to access the target service.
Discovering alternative Oblivious HTTP services. In this model, the client has a default target service that it uses. For example, this may be a public DNS resolver that is accessible over Oblivious HTTP. The client is willing to use alternative target services if they are discovered, which may provide more optimized or more relevant responses.

In both of these deployment models, the client is configured with a relay that it trusts for Oblivious HTTP transactions. This relay either needs to provide generic access to gateways, or provide a service to clients to allow them to check which gateways are accessible.

The ohttp SvcParamKey The "ohttp" SvcParamKey () is used to indicate that a service described in an SVCB record can be accessed as a target using an associated gateway. The service that is queried by the client hosts one or more target resources. In order to access the service's target resources using Oblivious HTTP, the client needs to send encapsulated messages to the gateway resource and the gateway's key configuration (both of which can be retrieved using the method described in ). Both the presentation and wire format values for the "ohttp" parameter MUST be empty. Services can include the "ohttp" parameter in the mandatory parameter list if the service is only accessible using Oblivious HTTP. Marking the "ohttp" parameter as mandatory will cause clients that do not understand the parameter to ignore that SVCB record. Including the "ohttp" parameter without marking it mandatory advertises a service that is optionally available using Oblivious HTTP. Note also that multiple SVCB records can be provided to indicate separate configurations. The media type to use for encapsulated requests made to a target 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 "ohttp" parameter means that the target being described is an Oblivious HTTP service that is accessible using the default "message/bhttp" media type . For example, an HTTPS service record for svc.example.com that supports a Oblivious HTTP could look like this: A similar record for a service that only supports Oblivious HTTP could look like this:

Use in DNS server SVCB records For the "dns" scheme, as defined in , the presence of the "ohttp" parameter means that the DNS server being described has a DNS over HTTP (DoH) service that can be accessed using Oblivious HTTP. Requests to the resolver are sent to the gateway using binary HTTP with the default "message/bhttp" media type , containing inner requests that use the "application/dns-message" media type . If the "ohttp" parameter is included in an DNS server SVCB record, the "alpn" MUST include at least one HTTP value (such as "h2" or "h3"). In order for DoH-capable recursive resolvers to function as Oblivious HTTP targets, their associated gateways need to be accessible via a client-trusted relay. DoH recursive resolvers used with the discovery mechanisms described in this section can either be publicly accessible, or specific to a network. In general, only publicly accessible DoH recursive resolvers will work as Oblivious HTTP targets, unless there is a coordinated deployment with a relay to access the network-specific DoH recursive resolvers.

Use with DDR Clients can discover that a DoH recursive resolvers support Oblivious HTTP using DDR, either by querying _dns.resolver.arpa to a locally configured resolver or by querying using the name of a resolver . For example, a DoH service advertised over DDR can be annotated as supporting resolution via Oblivious HTTP using the following record: Clients still need to perform verification of oblivious DoH servers, specifically the TLS certificate checks described in . Since the gateway and target resources for discovered oblivious services need to be on the same host, this means that the client needs to verify that the certificate presented by the gateway passes the required checks. These checks can be performed when looking up the configuration on the gateway as described in , which can either be done directly or via the relay or another proxy to avoid exposing client IP addresses. Opportunistic discovery , where only the IP address is validated, SHOULD NOT be used in general with Oblivious HTTP, since this mode primarily exists to support resolvers that use private or local IP addresses, which will usually not be accessible when using a relay. If a configuration occurs where the resolver is accessible, but cannot use certificate-based validation, the client needs to ensure that the relay only accesses the gateway and target using the unencrypted resolver's original IP address. For the case of DoH recursive resolvers, clients also need to ensure that they are not being targeted with unique DoH paths 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 that learn about DoH recursive resolvers still need to ensure that they are not being targeted with unique DoH paths that would reveal their identity. See for more discussion.

Gateway Location Once a client has discovered that a service supports Oblivious HTTP via the "ohttp" parameter in a SVCB or HTTPS record, it needs to be able to send requests via a relay to the correct gateway location. By default, the gateway for a target is defined as a well-known resource () on the target, "/.well-known/ohttp-gateway". Some servers might not want to operate the gateway on a well-known URI. In such cases, these servers can use 3xx redirection responses () to direct clients and relays to the correct location of the gateway. Such redirects would apply both to requests made to fetch key configurations (as defined in ) and to encapsulated requests made via a relay. If a client receives a redirect when fetching the key configuration from the well-known gateway resource, it MUST NOT communicate the redirected gateway URI to the relay as the location of the gateway to use. Doing so would allow the gateway to target clients by encoding unique or client-identifying values in the redirected URI. Instead, relays being used with dynamically discovered gateways MUST use the well-known gateway resource and follow any redirects independently of redirects that clients received. The relay can remember such redirects across oblivious requests for all clients in order to avoid added latency.

Key Configuration Fetching Clients also need to know the key configuration of a gateway before encapsulating and sending requests to the relay. In order to fetch the key configuration of a gateway discovered in the manner described in , the client issues a GET request to the URI of the gateway specifying the "application/ohttp-keys" () media type in the Accept header. For example, if the client knows an oblivious gateway URI, "https://svc.example.com/.well-known/ohttp-gateway", it could fetch the key configuration with the following request: Gateways that coordinate with targets that advertise Oblivious HTTP support SHOULD support GET requests for their key configuration in this manner, unless there is another out-of-band configuration model that is usable by clients. Gateways respond with their key configuration in the response body, with a content type of "application/ohttp-keys". Clients can either fetch this key configuration 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 HTTP support are just hints, a client can mitigate this by always checking for a gateway configuration on the well-known gateway location . Use of encrypted DNS along with DNSSEC can also be used as a mitigation. When clients fetch a gateway's configuration (), they can expose their identity in the form of an IP address if they do not connect via a proxy or some other IP-hiding mechanism. In some circumstances, this might not be a privacy concern, since revealing that a particular client IP address is preparing to use an Oblivious HTTP service can be expected. However, if a client is otherwise trying to hide its IP address or location (and not merely decouple its specific requests from its IP address), or if revealing its IP address facilitates key targeting attacks (if a gateway service uses IP addresses to associate specific configurations with specific clients), a proxy or similar mechanism can be used to fetch the gateway's configuration. When discovering designated oblivious DoH recursive resolvers using this mechanism, clients need to ensure that the designation is trusted in lieu of being able to directly check the contents of the gateway server's TLS certificate. See for more discussion, as well as the Security Considerations of .

Key Targeting Attacks 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 overview of the options for ensuring the key configurations are consistent between different clients. Clients SHOULD employ some technique to mitigate key targeting attacks, such as the option of confirming the key with a shared proxy as described in . If a client detects that a gateway is using per-client targeted key configuration, the client can stop using the gateway, and potentially report the targeting attack to let other clients avoid using this gateway in the future.

dohpath Targeting Attacks For oblivious DoH servers, an attacker could use unique dohpath values to target or identify specific clients. This attack is very similar to the generic OHTTP key targeting attack described above. Clients SHOULD mitigate such attacks. This can be done with a check for consistency, such as using a mechanism described in to validate the dohpath value with another source. It can also be done by limiting the the allowable values of dohpath to a single value, such as the commonly used "/dns-query{?dns}".

IANA Considerations

SVCB Service Parameter This document adds the following entry to the SVCB Service Parameters registry ().

Number	Name	Meaning	Reference
8 (Early Allocation)	ohttp	Denotes that a service operates an Oblivious HTTP target	(This document)

Well-Known URI IANA is requested to add one new entry in the "Well-Known URIs" registry . URI suffix: ohttp-gateway Change controller: IETF Specification document: This document Status: permanent Related information: N/A