Discovery of Oblivious Services via Service Binding Records

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Table of Contents

1. Introduction

Oblivious HTTP [OHTTP] 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 access 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 discover the client's identity.¶

Since Oblivious HTTP deployments will often involve very specific coordination between clients, relays, and gateways, the key configuration can often be shared in a bespoke fashion. However, some deployments involve clients discovering oblivious targets and their assoicated 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 ([DDR]). Clients can work with trusted relays to access these gateways.¶

This document defines a way to use DNS records to advertise that an HTTP service supports Oblivious HTTP. This indication is a parameter that can be included in SVCB and HTTPS DNS resource records [SVCB] (Section 4). The presence of this parameter indicates that a service can act as an oblivious target and has an oblivious gateway that can provide access to the target.¶

The client learns the URI to use for the oblivious gateway using a well-known URI [WELLKNOWN], "ohttp-gateway", which is accessed on the oblivious target (Section 5). 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 oblivious gateway's key configuration from the oblivious gateway (Section 6).¶

This mechanism does not aid in the discovery of oblivious relays; relay configuration is out of scope for this document. Models in which this discovery mechanism is applicable are described in Section 3.¶

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. Applicability

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

• Upgrading non-oblivious 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 an oblivious gateway that it offers to allow access using oblivious HTTP. Once the client learns about the oblivious 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 oblivious 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 oblivious target services if they are discovered, which may provide more optimized or more relevant responses.¶

In both of these deployment models, the client is assumed to already know of an oblivious relay that it trusts and works with. This oblivious relay either needs to provide generic access to oblivious gateways, or provide a service to clients to allow them to check which gateways are accessible.¶

4. The ohttp SvcParamKey

The "ohttp" SvcParamKey (Section 8) is used to indicate that a service described in an SVCB record can be accessed as an oblivious 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 obliviously, 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 Section 6).¶

Both the presentation and wire format values for the "ohttp" parameter MUST be empty.¶

The "ohttp" 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 mark the "ohttp" parameter 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 obliviously or directly SHOULD NOT mark the "ohttp" 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 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" [SVCB] and "dns" [DNS-SVCB] schemes. Other schemes that want to use this parameter MUST define the interpretation and meaning of the configuration.¶

svc.example.com. 7200  IN HTTPS 1 . ( alpn=h2 ohttp )

svc.example.com. 7200  IN HTTPS 1 . ( mandatory=ohttp ohttp )

_dns.resolver.arpa  7200  IN SVCB 1 doh.example.net (
     alpn=h2 dohpath=/dns-query{?dns} ohttp )

5. Gateway Location

Clients that know a service is available as an oblivious target via discovery through the "ohttp" parameter in a SVCB or HTTPS record need to know the location of the associated oblivious gateway before sending oblivious requests.¶

By default, the oblivious gateway for an oblivious target is defined as a well-known resource ([WELLKNOWN]) on the target, "/.well-known/ohttp-gateway".¶

Commonly, servers will not want to actually operate the oblivious gateway on a well-known URI. In such cases, servers can use 3xx redirection responses (Section 15.4 of [HTTP]) to direct clients and relays to the correct location of the oblivious gateway.¶

Generally, the first request a client will make will be a GET request to discover the key configuration, described in Section 6. This initial request also provides a convenient way for clients to learn about the redirect from the well-known resource, if there is a redirect. When clients work with their oblivious relays to send oblivious requests to the gateway, clients can communicate this redirected gateway URI.¶

6. Key Configuration Fetching

Clients also need to know the key configuration of an oblivious gateway before sending oblivious requests.¶

In order to fetch the key configuration of an oblivious gateway discovered in the manner described in Section 5, the client issues a GET request to the URI of the gateway specifying the "application/ohttp-keys" ([OHTTP]) 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:¶

GET /.well-known/ohttp-gateway HTTP/1.1
Host: svc.example.com
Accept: application/ohttp-keys

Oblivious gateways that coordinate with targets that advertise oblivious 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 Section 7 for more discussion of avoiding key targeting attacks.¶

7. Security and Privacy Considerations

Attackers on a network can remove SVCB information from cleartext DNS answers that are not protected by DNSSEC [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 gateway configuration Section 6 on the well-known gateway location Section 5. Use of encrypted DNS along with DNSSEC can also be used as a mitigation.¶

When discovering designated oblivious DoH 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 gateway server's TLS certificate. See Section 4.2.1 for more discussion, as well as the Security Considerations of [SVCBDNS].¶

For oblivious DoH servers, an attacker could use unique DoH path values to target or identify specific clients. Clients can mitigate such attacks in several ways. Some options include: only allow common DoH paths (such as the de-facto default "/dns-query{?dns}"); performing consistency checks by fetching the information about the resolver over multiple resolution paths; or coordinating with a trusted oblivious relay to validate that DoH paths are common across clients using the same gateway.¶

As discussed in [OHTTP], 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. [CONSISTENCY] 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 gateways that are detected to use targeted key configurations per-client MUST NOT be used.¶

When clients fetch a gateway's configuration (Section 6), 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 obfuscate its IP address or location (and not merely decouple its specific requests from its IP address), or revealing its IP address will increase the risk of a key targeting attack (if a gateway service is trying to differentiate traffic across client IP addresses), a proxy or similar mechanism can be used to fetch the gateway's configuration.¶

8. IANA Considerations

Authors' Addresses