Discovery of Oblivious Services via Service Binding Records
draft-ietf-ohai-svcb-config-01

Oblivious HTTP Application Intermediation                       T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                                T. Reddy
Expires: 6 September 2023                                          Nokia
                                                            5 March 2023

      Discovery of Oblivious Services via Service Binding Records
                     draft-ietf-ohai-svcb-config-01

Abstract

   This document defines a parameter that can be included in 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 to learn the key configuration of the discovered Oblivious
   Gateway Resource.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ohai-svcb-config/.

   Discussion of this document takes place on the Oblivious HTTP
   Application Intermediation Working Group mailing list
   (mailto:ohai@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/ohai/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/ohai/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-ohai/draft-ohai-svcb-config.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  The ohttp SvcParamKey . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Use in HTTPS service records  . . . . . . . . . . . . . .   5
     4.2.  Use in DNS server SVCB records  . . . . . . . . . . . . .   5
       4.2.1.  Use with DDR  . . . . . . . . . . . . . . . . . . . .   6
       4.2.2.  Use with DNR  . . . . . . . . . . . . . . . . . . . .   7
   5.  Gateway Location  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Key Configuration Fetching  . . . . . . . . . . . . . . . . .   7
   7.  Security and Privacy Considerations . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  SVCB Service Parameter  . . . . . . . . . . . . . . . . .   9
     8.2.  Well-Known URI  . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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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.

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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.

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   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.

4.1.  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 [OHTTP] [BINARY-HTTP].

   For example, an HTTPS service record for svc.example.com that
   supports an oblivious gateway could look like this:

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

   A similar record for a service that only supports oblivious
   connectivity could look like this:

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

4.2.  Use in DNS server SVCB records

   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 HTTP (DoH) [DOH] service that can be accessed using Oblivious
   HTTP.  Requests to the resolver are sent to the oblivious 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 an DNS server SVCB record,
   the "alpn" MUST include at least one HTTP value (such as "h2" or
   "h3").

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   In order for DoH servers to function as oblivious targets, their
   associated gateways need to be accessible via an oblivious relay.
   DoH 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 DoH servers will work as
   oblivious targets, unless there is a coordinated deployment with an
   oblivious relay that is also hosted within a network.

4.2.1.  Use with DDR

   Clients can discover that a DoH server 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 [DDR].

   For example, a DoH service advertised over DDR can be annotated as
   supporting oblivious resolution using the following record:

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

   Clients still need to perform some verification of oblivious DoH
   servers, such as the TLS certificate check described in [DDR].  This
   certificate check can be done when looking up the configuration on
   the gateway as described in Section 6, which can either be done
   directly, or via the relay or another proxy to avoid exposing client
   IP addresses.  Since the oblivious gateway that is discovered
   dynamically uses a well-known URI on the same host as the target, as
   described in Section 6, the certificate evaluation for the connection
   to well-known gateway URI also covers the name of the target DoH
   server.

   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
   an oblivious relay.  If a configuration occurs where the resolver is
   accessible, but cannot use certificate-based validation, the client
   needs to ensure that the oblivious relay only accesses the gateway
   and target using the unencrypted resolver's original IP address.

   For the case of DoH servers, 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.

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4.2.2.  Use with DNR

   The SvcParamKeys defined in this document also can be used with
   Discovery of Network-designated Resolvers (DNR) [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 servers 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

   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.

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   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

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   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

8.1.  SVCB Service Parameter

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

     +========+=======+=================================+===========+
     | Number | Name  | Meaning                         | Reference |
     +========+=======+=================================+===========+
     | TBD    | ohttp | Denotes that a service operates | (This     |
     |        |       | an Oblivious HTTP target        | document) |
     +--------+-------+---------------------------------+-----------+

                                 Table 1

8.2.  Well-Known URI

   IANA is requested to add one new entry in the "Well-Known URIs"
   registry [WELLKNOWN].

   URI suffix: ohttp-gateway

   Change controller: IETF

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   Specification document: This document

   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", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-binary-message-06, 6 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              binary-message-06>.

   [DDR]      Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", Work in
              Progress, Internet-Draft, draft-ietf-add-ddr-10, 5 August
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              add-ddr-10>.

   [DNR]      Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for the
              Discovery of Network-designated Resolvers (DNR)", Work in
              Progress, Internet-Draft, draft-ietf-add-dnr-13, 13 August
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              add-dnr-13>.

   [DNS-SVCB] Schwartz, B. M., "Service Binding Mapping for DNS
              Servers", Work in Progress, Internet-Draft, draft-ietf-
              add-svcb-dns-07, 11 August 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-add-
              svcb-dns-07>.

   [DOH]      Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc9110>.

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   [OHTTP]    Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
              Progress, Internet-Draft, draft-ietf-ohai-ohttp-06, 15
              December 2022, <https://datatracker.ietf.org/doc/html/
              draft-ietf-ohai-ohttp-06>.

   [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/rfc/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/rfc/rfc8174>.

   [SVCB]     Schwartz, B. M., Bishop, M., and E. Nygren, "Service
              binding and parameter specification via the DNS (DNS SVCB
              and HTTPS RRs)", Work in Progress, Internet-Draft, draft-
              ietf-dnsop-svcb-https-11, 11 October 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
              svcb-https-11>.

   [WELLKNOWN]
              Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
              <https://www.rfc-editor.org/rfc/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-wood-key-consistency-03, 17 August
              2022, <https://datatracker.ietf.org/doc/html/draft-wood-
              key-consistency-03>.

   [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/rfc/rfc4033>.

   [SVCBDNS]  Schwartz, B. M., "Service Binding Mapping for DNS
              Servers", Work in Progress, Internet-Draft, draft-ietf-
              add-svcb-dns-07, 11 August 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-add-
              svcb-dns-07>.

Authors' Addresses

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

   Tirumaleswar Reddy
   Nokia
   Email: kondtir@gmail.com

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