Discovery of Oblivious Services via Service Binding Records
RFC 9540



Internet Engineering Task Force (IETF)                          T. Pauly
Request for Comments: 9540                                    Apple Inc.
Category: Standards Track                                     T. Reddy.K
ISSN: 2070-1721                                                    Nokia
                                                           February 2024

      Discovery of Oblivious Services via Service Binding Records

Abstract

   This document defines a parameter that can be included in Service
   Binding (SVCB) and HTTPS DNS resource records to denote that a
   service is accessible using Oblivious HTTP, by offering an Oblivious
   Gateway Resource through which to access the target.  This document
   also defines a mechanism for learning the key configuration of the
   discovered Oblivious Gateway Resource.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9540.

Copyright Notice

   Copyright (c) 2024 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  Applicability
   4.  The "ohttp" SvcParamKey
     4.1.  Use in HTTPS Service RRs
     4.2.  Use in DNS Server SVCB RRs
       4.2.1.  Use with DDR
       4.2.2.  Use with DNR
   5.  Gateway Location
   6.  Key Configuration Fetching
   7.  Security and Privacy Considerations
     7.1.  Key Targeting Attacks
     7.2.  dohpath Targeting Attacks
   8.  IANA Considerations
     8.1.  SVCB Service Parameter
     8.2.  Well-Known URI
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Authors' Addresses

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 offers Oblivious HTTP 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 [DDR] and Discovery of Network-designated
   Resolvers [DNR].  Clients can access these gateways through trusted
   relays.

   This document defines a way to use DNS resource records (RRs) to
   advertise that an HTTP service supports Oblivious HTTP.  This
   advertisement is a parameter that can be included in Service Binding
   (SVCB) and HTTPS DNS RRs [SVCB] (Section 4).  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 suffix [WELLKNOWN], "ohttp-gateway", which is accessed on the
   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 a gateway's key
   configuration from the gateway (Section 6).

   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 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.  These include:

   *  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" its requests from non-proxied HTTP to 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
   needs to provide either (1) generic access to gateways or (2) a
   service to clients to allow them to check which gateways are
   accessible.

4.  The "ohttp" SvcParamKey

   The "ohttp" SvcParamKey is used to indicate that a service described
   in a SVCB RR 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 Section 6).

   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 RR.  Including the
   "ohttp" parameter without marking it mandatory advertises a service
   that is optionally available using Oblivious HTTP.  Note also that
   multiple SVCB RRs 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 RR.  This document defines
   the interpretation for the "https" scheme [SVCB] and the "dns" scheme
   [DNS-SVCB].  Other schemes that want to use this parameter MUST
   define the interpretation and meaning of the configuration.

4.1.  Use in HTTPS Service RRs

   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 [OHTTP] [BINARY-HTTP].

   For example, an HTTPS service RR for svc.example.com that supports
   Oblivious HTTP could look like this:

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

   A similar RR for a service that only supports Oblivious HTTP could
   look like this:

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

4.2.  Use in DNS Server SVCB RRs

   For the "dns" scheme, as defined in [DNS-SVCB], the presence of the
   "ohttp" parameter means that the DNS server being described has a
   DNS-over-HTTPS (DoH) service [DOH] 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
   [BINARY-HTTP], containing inner requests that use the "application/
   dns-message" media type [DOH].

   If the "ohttp" parameter is included in a DNS server SVCB RR, the
   "alpn" parameter 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 be either 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.

4.2.1.  Use with DDR

   Clients can discover that a DoH recursive resolver supports Oblivious
   HTTP using DDR, by either querying _dns.resolver.arpa to a locally
   configured resolver or querying using the name of a resolver [DDR].

   For example, a DoH service advertised over DDR can be annotated as
   supporting resolution via Oblivious HTTP using the following RR:

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

   Clients still need to perform verification of oblivious DoH servers
   -- specifically, the TLS certificate checks described in Section 4.2
   of [DDR].  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 Section 6
   and can be done either directly or via the relay or another proxy to
   avoid exposing client IP addresses.

   Opportunistic Discovery [DDR], 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 MUST 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 Section 7 for more discussion.

4.2.2.  Use with DNR

   The SvcParamKey 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 Section 7 for more discussion.

5.  Gateway Location

   Once a client has discovered that a service supports Oblivious HTTP
   via the "ohttp" parameter in a SVCB or HTTPS RR, it needs to be able
   to send requests via a relay to the correct gateway location.

   This document defines a well-known resource [WELLKNOWN], "/.well-
   known/ohttp-gateway", which is an Oblivious Gateway Resource
   available on the same host as the Target Resource.

   Some servers might not want to operate the gateway on a well-known
   URI.  In such cases, these servers can use 3xx (Redirection)
   responses (Section 15.4 of [HTTP]) to direct clients and relays to
   the correct location of the gateway.  Such redirects would apply to
   both (1) requests made to fetch key configurations (as defined in
   Section 6) and (2) 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.

6.  Key Configuration Fetching

   Clients also need to know the key configuration of a gateway before
   encapsulating and sending requests to the relay.

   If a client fetches the key configuration directly from the gateway,
   it will expose identifiers like a client IP address to the gateway.
   The privacy and security implications of fetching the key
   configuration are discussed more in Section 7.  Clients can use an
   HTTP proxy to hide their IP addresses when fetching key
   configurations.  Clients can also perform consistency checks to
   validate that they are not receiving unique key configurations, as
   discussed in Section 7.1.

   In order to fetch the key configuration of a gateway discovered in
   the manner described in Section 5, the client issues a GET request
   (either through a proxy or directly) to the URI of the gateway
   specifying the "application/ohttp-keys" media type [OHTTP] 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

   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".

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 HTTP support are just hints, a client can mitigate this by
   always checking for a gateway configuration (Section 6) on the well-
   known gateway location (Section 5).  Using encrypted DNS along with
   DNSSEC can also provide such a mitigation.

   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 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 Section 4.2.1 for more
   discussion, as well as Section 8 ("Security Considerations") of
   [DNS-SVCB].

7.1.  Key Targeting Attacks

   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 overview of the options
   for ensuring that 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 [CONSISTENCY].  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 so that other clients can avoid using this gateway
   in the future.

7.2.  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.

   A client can avoid these targeting attacks by only allowing a single
   "dohpath" value, such as the commonly used "/dns-query{?dns}" or
   another pre-known value.  If the client allows arbitrary "dohpath"
   values, it SHOULD mitigate targeting attacks with a consistency
   check, such as using one of the mechanisms described in [CONSISTENCY]
   to validate the "dohpath" value with another source.  Clients might
   choose to only employ a consistency check on a percentage of
   discovery events, depending on the capacity of consistency check
   options and their deployment threat model.

8.  IANA Considerations

8.1.  SVCB Service Parameter

   This document adds the following entry to the "Service Parameter Keys
   (SvcParamKeys)" registry [SVCB].  This parameter is defined in
   Section 4.

    +========+=======+=======================+============+===========+
    | Number | Name  | Meaning               | Change     | Reference |
    |        |       |                       | Controller |           |
    +========+=======+=======================+============+===========+
    | 8      | ohttp | Denotes that a        | IETF       | RFC 9540, |
    |        |       | service operates an   |            | Section 4 |
    |        |       | Oblivious HTTP target |            |           |
    +--------+-------+-----------------------+------------+-----------+

                                  Table 1

8.2.  Well-Known URI

   IANA has added one entry in the "Well-Known URIs" registry
   [WELLKNOWN].

   URI Suffix:  ohttp-gateway

   Change Controller:  IETF

   Reference:  RFC 9540

   Status:  permanent

   Related Information:  N/A

9.  References

9.1.  Normative References

   [BINARY-HTTP]
              Thomson, M. and C. A. Wood, "Binary Representation of HTTP
              Messages", RFC 9292, DOI 10.17487/RFC9292, August 2022,
              <https://www.rfc-editor.org/info/rfc9292>.

   [DDR]      Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", RFC 9462,
              DOI 10.17487/RFC9462, November 2023,
              <https://www.rfc-editor.org/info/rfc9462>.

   [DNR]      Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
              and T. Jensen, "DHCP and Router Advertisement Options for
              the Discovery of Network-designated Resolvers (DNR)",
              RFC 9463, DOI 10.17487/RFC9463, November 2023,
              <https://www.rfc-editor.org/info/rfc9463>.

   [DNS-SVCB] Schwartz, B., "Service Binding Mapping for DNS Servers",
              RFC 9461, DOI 10.17487/RFC9461, November 2023,
              <https://www.rfc-editor.org/info/rfc9461>.

   [DOH]      Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [OHTTP]    Thomson, M. and C. A. Wood, "Oblivious HTTP", RFC 9458,
              DOI 10.17487/RFC9458, January 2024,
              <https://www.rfc-editor.org/info/rfc9458>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [SVCB]     Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/info/rfc9460>.

   [WELLKNOWN]
              Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
              <https://www.rfc-editor.org/info/rfc8615>.

9.2.  Informative References

   [CONSISTENCY]
              Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
              "Key Consistency and Discovery", Work in Progress,
              Internet-Draft, draft-ietf-privacypass-key-consistency-01,
              10 July 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-privacypass-key-consistency-01>.

   [DNSSEC]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

Authors' Addresses

   Tommy Pauly
   Apple Inc.
   Email: tpauly@apple.com

   Tirumaleswar Reddy.K
   Nokia
   Email: kondtir@gmail.com