Distribution of Oblivious Configurations via Service Binding Records
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|Authors||Tommy Pauly , Tirumaleswar Reddy.K|
|Stream||Stream state||(No stream defined)|
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Oblivious HTTP Application Intermediation T. Pauly Internet-Draft Apple Inc. Intended status: Informational T. Reddy Expires: 6 September 2022 Akamai 5 March 2022 Distribution of Oblivious Configurations via Service Binding Records draft-pauly-ohai-svcb-config-00 Abstract This document defines a parameter that can be included in SVCB and HTTPS DNS resource records to denote that a service is accessible as an Oblivious HTTP target, along with one or more oblivious key configurations. 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-pauly-ohai-svcb-config/. Discussion of this document takes place on the Oblivious HTTP Application Intermediation Working Group mailing list (mailto:firstname.lastname@example.org), which is archived at https://mailarchive.ietf.org/arch/browse/ohai/. 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/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 6 September 2022. Pauly & Reddy Expires 6 September 2022 [Page 1] Internet-Draft Oblivious Configs in SVCB March 2022 Copyright Notice Copyright (c) 2022 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 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. The ohttp-configs and ohttp-path SvcParamKeys . . . . . . . . 3 3.1. Use in HTTPS service records . . . . . . . . . . . . . . 4 3.2. Use in DNS server SVCB records . . . . . . . . . . . . . 4 3.2.1. Use with DDR . . . . . . . . . . . . . . . . . . . . 4 3.2.2. Use with DNR . . . . . . . . . . . . . . . . . . . . 5 3.2.3. Handling Oblivious DoH Configurations . . . . . . . . 5 4. Deployment Considerations . . . . . . . . . . . . . . . . . . 6 5. Security and Privacy Considerations . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7.2. Informative References . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Oblivious HTTP [OHTTP] 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. Pauly & Reddy Expires 6 September 2022 [Page 2] Internet-Draft Oblivious Configs in SVCB March 2022 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 ([DDR]. Clients can work with trusted proxies to access these target servers. This document defines a mechanism to distribute Oblivious HTTP key configurations in DNS records, as a parameter that can be included in SVCB and HTTPS DNS resource records [SVCB]. The presence of this parameter indicates that a service is an oblivious target; see Section 3 of [OHTTP] for a description of oblivious targets. This mechanism does not aid in the discovery of proxies to use to access oblivious targets; the configurations of proxies is out of scope for this document. 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. The ohttp-configs and ohttp-path SvcParamKeys The "ohttp-configs" SvcParamKey Section 6 is used to convey one or more key configurations that can be used by clients to issue oblivious requests to a target server described by the SVCB record. In wire format, the value of the parameter is one or more KeyConfig structures [OHTTP] concatenated together. In presentation format, the value is the same concatenated KeyConfig structures encoded in Base64 [BASE64]. The meaning of the "ohttp-configs" parameter 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. The "ohttp-path" SvcParamKey Section 6 is used to convey the URI path of the oblivious target to which oblivious HTTP requests can sent. In both wire format and presentation format, this is a UTF-8 encoded Pauly & Reddy Expires 6 September 2022 [Page 3] Internet-Draft Oblivious Configs in SVCB March 2022 string that contains the path segment of a URI. If this path parameter is not present, oblivious requests can be made to the root "/" path. 3.1. Use in HTTPS service records For the "https" scheme, which uses the HTTPS RR type instead of SVCB, the presence of the "ohttp-configs" parameter means that the service being described is an Oblivious HTTP service that uses the default "message/bhttp" media type [OHTTP] [BINARY-HTTP]. When present in an HTTPS record, the "ohttp-configs" MUST be included in the mandatory parameter list, to ensure that implementations that do not understand the key do not interpret this service as a generic HTTP service. Clients MUST validate that they can parse the value of "ohttp- configs" as a valid key configuration before attempting to use the service. 3.2. Use in DNS server SVCB records For the "dns" scheme, as defined in [DNS-SVCB], the presence of the "ohttp-configs" 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" [DOH]. The "ohttp-configs" parameter is only defined for use with DoH, so the "alpn" SvcParamKey MUST indicate support for a version of HTTP and the "dohpath" SvcParamKey MUST be present. The "ohttp-configs" MUST also be included in the mandatory parameter list, to ensure that implementations that do not understand the key do not interpret this service as a generic DoH service. Clients MUST validate that they can parse the value of "ohttp- configs" as a valid key configuration before attempting to use the service. 3.2.1. 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 [DDR]. In the case of oblivious DNS servers, the client might not be able to directly use the verification mechanisms described in [DDR], which rely on checking for known resolver IP addresses or hostnames in TLS Pauly & Reddy Expires 6 September 2022 [Page 4] Internet-Draft Oblivious Configs in SVCB March 2022 certificates, since clients do not generally perform TLS with oblivious targets. A client MAY perform a direct connection to the oblivious target server to do this TLS check, however this may be impossible or undesirable if the client does not want to ever expose its IP address to the oblivious target. If the client does not use the standard DDR verification check, it MUST use some alternate mechanism to verify that it should use an oblivious target. For example, the client could have a local policy of known oblivious target names that it is allowed to use, or the client could coordinate with the oblivious proxy to either have the oblivious proxy check the properties of the target's TLS certificate or filter to only allow targets known and trusted by the proxy. Clients also need to ensure that they are not being targeted with unique key configurations that would reveal their identity. See Section 5 for more discussion. 3.2.2. 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 still need to ensure that they are not being targeted with unique key configurations that would reveal their identity. See Section 5 for more discussion. 3.2.3. Handling Oblivious DoH Configurations Oblivious DoH was originally defined in [ODOH]. This version of Oblivious DoH uses a different key configuration format than generic Oblivious HTTP. SVCB records using the "dns" scheme can include one or more ObliviousDoHConfig structures using the "odoh-configs" parameter. In wire format, the value of the "odoh-configs" parameter is one or more ObliviousDoHConfigs structures [ODOH] concatenated together. In presentation format, the value is the same structures encoded in Base64 [BASE64]. All other requirements for "ohttp-configs" in this document apply to "odoh-configs". Pauly & Reddy Expires 6 September 2022 [Page 5] Internet-Draft Oblivious Configs in SVCB March 2022 4. Deployment Considerations Deployments that add the "ohttp-configs" SvcParamKey need to be careful to add this only to services meant to be accessed using Oblivious HTTP. Information in a single SVCB record that contains "ohttp-configs" only applies to the oblivious service, not other HTTP services. If a service offers both traditional HTTP and oblivious HTTP, these can be represented by separate SVCB or HTTPS records, both with and without the "ohttp-configs" SvcParamKey. 5. Security and Privacy Considerations 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 Section 3.2.1 for more discussion. 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 targetting 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 targetting attack. One mitigation specific to this mechanism is validating that SVCB or HTTPS records including the "oblivious- configs" are protected by DNSSEC [DNSSEC]. This prevents attacks where a unique response is generated for each client of a resolver. 6. IANA Considerations IANA is requested to add the following entry to the SVCB Service Parameters registry ([SVCB]). Pauly & Reddy Expires 6 September 2022 [Page 6] Internet-Draft Oblivious Configs in SVCB March 2022 +========+===============+====================+===========+ | Number | Name | Meaning | Reference | +========+===============+====================+===========+ | TBD | ohttp-configs | Oblivious HTTP key | (This | | | | configurations | document) | +--------+---------------+--------------------+-----------+ | TBD | ohttp-path | Oblivious HTTP | (This | | | | request path | document) | +--------+---------------+--------------------+-----------+ | TBD | odoh-configs | Oblivious DoH key | (This | | | | configurations | document) | +--------+---------------+--------------------+-----------+ Table 1 7. References 7.1. Normative References [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <https://www.rfc-editor.org/rfc/rfc4648>. [BINARY-HTTP] Thomson, M. and C. A. Wood, "Binary Representation of HTTP Messages", Work in Progress, Internet-Draft, draft-ietf- httpbis-binary-message-01, 3 February 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis- binary-message-01>. [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-05, 31 January 2022, <https://datatracker.ietf.org/doc/html/ draft-ietf-add-ddr-05>. [DNR] Boucadair, M., Reddy, 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-05, 13 December 2021, <https://datatracker.ietf.org/doc/html/ draft-ietf-add-dnr-05>. [DNS-SVCB] Schwartz, B., "Service Binding Mapping for DNS Servers", Work in Progress, Internet-Draft, draft-ietf-add-svcb-dns- 02, 1 February 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-add- svcb-dns-02>. Pauly & Reddy Expires 6 September 2022 [Page 7] Internet-Draft Oblivious Configs in SVCB March 2022 [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>. [OHTTP] Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in Progress, Internet-Draft, draft-ietf-ohai-ohttp-01, 15 February 2022, <https://datatracker.ietf.org/doc/html/ draft-ietf-ohai-ohttp-01>. [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., 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-08, 12 October 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop- svcb-https-08>. 7.2. 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-02, 4 March 2022, <https://datatracker.ietf.org/doc/html/draft-wood- key-consistency-02>. [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>. [ODOH] Kinnear, E., McManus, P., Pauly, T., Verma, T., and C. A. Wood, "Oblivious DNS Over HTTPS", Work in Progress, Internet-Draft, draft-pauly-dprive-oblivious-doh-11, 17 February 2022, <https://datatracker.ietf.org/doc/html/ draft-pauly-dprive-oblivious-doh-11>. Authors' Addresses Pauly & Reddy Expires 6 September 2022 [Page 8] Internet-Draft Oblivious Configs in SVCB March 2022 Tommy Pauly Apple Inc. Email: email@example.com Tirumaleswar Reddy Akamai Email: firstname.lastname@example.org Pauly & Reddy Expires 6 September 2022 [Page 9]