add B. Schwartz Internet-Draft Google LLC Intended status: Standards Track 19 April 2021 Expires: 21 October 2021 Service Binding Mapping for DNS Servers draft-schwartz-svcb-dns-03 Abstract The SVCB DNS record type expresses a bound collection of endpoint metadata, for use when establishing a connection to a named service. DNS itself can be such a service, when the server is identified by a domain name. This document provides the SVCB mapping for named DNS servers, allowing them to indicate support for new transport protocols. Discussion Venues This note is to be removed before publishing as an RFC. Discussion of this document takes place on the ADD Working Group mailing list (add@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/add/. Source for this draft and an issue tracker can be found at https://github.com/bemasc/svcb-dns. 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 21 October 2021. Schwartz Expires 21 October 2021 [Page 1] Internet-Draft SVCB for DNS April 2021 Copyright Notice Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Name form . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Applicable existing SvcParamKeys . . . . . . . . . . . . . . 3 4.1. alpn . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4.2. port . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.3. Other applicable SvcParamKeys . . . . . . . . . . . . . . 4 5. New SvcParamKeys . . . . . . . . . . . . . . . . . . . . . . 4 5.1. dohpath . . . . . . . . . . . . . . . . . . . . . . . . . 4 6. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 4 7. Relationship to DNS URIs . . . . . . . . . . . . . . . . . . 5 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5 9. Security Considerations . . . . . . . . . . . . . . . . . . . 5 9.1. Adversary on the query path . . . . . . . . . . . . . . . 5 9.2. Adversary on the transport path . . . . . . . . . . . . . 6 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 11.1. Normative References . . . . . . . . . . . . . . . . . . 7 11.2. Informative References . . . . . . . . . . . . . . . . . 8 Appendix A. Mapping Summary . . . . . . . . . . . . . . . . . . 8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 1. Introduction The SVCB record type [SVCB] provides clients with information about how to reach alternative endpoints for a service, which may have improved performance or privacy properties. The service is identified by a "scheme" indicating the service type, a hostname, and optionally other information such as a port number. A DNS server is often identified only by its IP address (e.g. in DHCP), but in some contexts it can also be identified by a hostname (e.g. "NS" records, Schwartz Expires 21 October 2021 [Page 2] Internet-Draft SVCB for DNS April 2021 manual resolver configuration) and sometimes also a non-default port number. Use of the SVCB record type requires a mapping document for each service type, indicating how a client for that service can interpret the contents of the SVCB SvcParams. This document provides the mapping for the "dns" service type, allowing DNS servers to offer alternative endpoints and transports, including encrypted transports like DNS over TLS and DNS over HTTPS. 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. Name form Names are formed using Port-Prefix Naming ([SVCB] Section 2.3). For example, a DNS service identified by the name "dns1.example.com" and (unusually) the non-default port number 5353 would be represented as "_5353._dns.dns1.example.com.". A DNS service using the default port number of 53 would be represented as "_dns.dns1.example.com.". 4. Applicable existing SvcParamKeys 4.1. alpn This key indicates the set of supported protocols ([SVCB] Section 6.1). There is no default protocol, so the "no-default-alpn" key does not apply, and the "alpn" key MUST be present. If the protocol set contains any HTTP versions (e.g. "h2", "h3"), then the record indicates support for DNS over HTTPS [DOH], and the "dohpath" key MUST be present (Section 5.1). All keys specified for use with the HTTPS record are also permissible, and apply to the resulting HTTP connection. If the protocol set contains protocols with different default ports, and no port key is specified, then protocols are contacted separately on their default ports. Note that in this configuration, ALPN negotiation does not defend against cross-protocol downgrade attacks. Schwartz Expires 21 October 2021 [Page 3] Internet-Draft SVCB for DNS April 2021 4.2. port This key is used to indicate the target port for connection (([SVCB] Section 6.2)). If omitted, the client SHALL use the default port for each transport protocol (853 for DNS over TLS [DOT], 443 for DNS over HTTPS). This key is automatically mandatory if present. (See Section 7 of [SVCB] for the definition of "automatically mandatory".) 4.3. Other applicable SvcParamKeys These SvcParamKeys apply to the "dns" scheme without modification: * echconfig * ipv4hint * ipv6hint 5. New SvcParamKeys 5.1. dohpath "dohpath" is a single-valued SvcParamKey whose value (both in presentation and wire format) is a relative URI Template [RFC6570], normally starting with "/". If the "alpn" SvcParamKey indicates support for HTTP, clients MAY construct a DNS over HTTPS URI Template by combining the prefix "https://", the service name, the port from the "port" key if present, and the "dohpath" value. (The DNS service's original port number MUST NOT be used.) Clients SHOULD NOT query for any "HTTPS" RRs when using the constructed URI Template. Instead, the SvcParams and address records associated with this SVCB record SHOULD be used for the HTTPS connection, with the same semantics as an HTTPS RR. However, for consistency, service operators SHOULD publish an equivalent HTTPS RR, especially if clients might learn this URI Template through a different channel. 6. Limitations This document is concerned exclusively with the DNS transport, and does not affect or inform the construction or interpretation of DNS messages. For example, nothing in this document indicates whether the service is intended for use as a recursive or authoritative DNS server. Clients must know the intended use in their context. Schwartz Expires 21 October 2021 [Page 4] Internet-Draft SVCB for DNS April 2021 7. Relationship to DNS URIs The "dns:" URI scheme [DNSURI] describes a way to represent DNS queries as URIs. This scheme optionally includes an authority, comprised of a host and port number (with a default of 53). DNS URIs normally omit the authority, or specify an IP address, but a hostname is allowed, in which case it is suitable for use with this mapping. 8. Examples * A resolver at "simple.example" that supports DNS over TLS on port 853 (implicitly, as this is its default port): _dns.simple.example. 7200 IN SVCB 1 alpn=dot simple.example. * A resolver at "doh.example" that supports only DNS over HTTPS (DNS over TLS is disabled): _dns.doh.example. 7200 IN SVCB 1 doh.example. ( alpn=h2 dohpath=/dns-query{?dns} ) * A resolver at "resolver.example" that supports - DNS over TLS on "resolver.example" ports 853 (implicit in record 1) and 8530 (explicit in record 2), with "resolver.example" as the Authentication Domain Name, - DNS over HTTPS at "https://resolver.example/dns-query{?dns}" (record 1), and - an experimental protocol on "fooexp.resolver.example:5353" (record 3): $ORIGIN resolver.example. _dns 7200 IN SVCB 1 @ alpn=dot,h2,h3 dohpath=/dns-query{?dns} SVCB 2 @ alpn=dot port=8530 SVCB 3 fooexp port=5353 alpn=foo foo-info=... * A nameserver at "ns.example" whose service configuration is published on a different domain: $ORIGIN example. _dns.ns 7200 IN SVCB 0 _dns.ns.nic 9. Security Considerations 9.1. Adversary on the query path This section considers an adversary who can add or remove responses to the SVCB query. Schwartz Expires 21 October 2021 [Page 5] Internet-Draft SVCB for DNS April 2021 Clients MUST authenticate the server to its name during secure transport establishment. This name is the hostname used to construct the original SVCB query, and cannot be influenced by the SVCB record contents. Accordingly, this draft does not mandate the use of DNSSEC. This draft also does not specify how clients authenticate the name (e.g. selection of roots of trust), which might vary according to the context. Although this adversary cannot alter the authentication name of the service, it does have control of the port number and "dohpath" value. As a result, the adversary can direct DNS queries for $HOSTNAME to any port on $HOSTNAME, and any path on "https://$HOSTNAME", even if $HOSTNAME is not actually a DNS server. If the DNS client uses shared TLS or HTTP state, the client could be correctly authenticated (e.g. using a TLS client certificate or HTTP cookie). This behavior creates a number of possible attacks for certain server configurations. For example, if "https://$HOSTNAME/upload" accepts any POST request as a public file upload, the adversary could forge a SVCB record containing "dohpath=/upload". This would cause the client to upload and publish every query, resulting in unexpected storage costs for the server and privacy loss for the client. To mitigate this attack, a client of this SVCB mapping MUST NOT provide client authentication for DNS queries, except to servers that it specifically knows are not vulnerable to such attacks, and a DoH service operator MUST ensure that all unauthenticated DoH requests to its origin maintain the DoH service's privacy guarantees, regardless of the path. Also, if an alternative service endpoint sends an invalid response to a DNS query, the client SHOULD NOT send more queries to that endpoint. 9.2. Adversary on the transport path This section considers an adversary who can modify network traffic between the client and the SvcDomainName (i.e. the destination server). For a SVCB-reliant client ([SVCB] Section 3), this adversary can only cause a denial of service. However, because DNS is unencrypted by default, this adversary can execute a downgrade attack against SVCB- optional clients. Accordingly, when use of this specification is optional, clients SHOULD switch to SVCB-reliant behavior if SVCB resolution succeeds. Specifications making using of this mapping MAY adjust this fallback behavior to suit their requirements. Schwartz Expires 21 October 2021 [Page 6] Internet-Draft SVCB for DNS April 2021 10. IANA Considerations Per [SVCB] IANA would be directed to add the following entry to the SVCB Service Parameters registry. +========+=========+==============================+=================+ | Number | Name | Meaning | Reference | +========+=========+==============================+=================+ | TBD | dohpath | DNS over HTTPS path template | (This | | | | | document) | +--------+---------+------------------------------+-----------------+ Table 1 Per [Attrleaf], IANA would be directed to add the following entry to the DNS Underscore Global Scoped Entry Registry: +=========+============+===============+=================+ | RR TYPE | _NODE NAME | Meaning | Reference | +=========+============+===============+=================+ | SVCB | _dns | DNS SVCB info | (This document) | +---------+------------+---------------+-----------------+ Table 2 11. References 11.1. Normative References [DOH] Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, . [DOT] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 2016, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., and D. Orchard, "URI Template", RFC 6570, DOI 10.17487/RFC6570, March 2012, . Schwartz Expires 21 October 2021 [Page 7] Internet-Draft SVCB for DNS April 2021 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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-02, 2 November 2020, . 11.2. Informative References [Attrleaf] Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019, . [DNSURI] Josefsson, S., "Domain Name System Uniform Resource Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006, . Appendix A. Mapping Summary This table serves as a non-normative summary of the DNS mapping for SVCB. Schwartz Expires 21 October 2021 [Page 8] Internet-Draft SVCB for DNS April 2021 +=================+========================================+ +=================+========================================+ | *Mapped scheme* | "dns" | +-----------------+----------------------------------------+ | *RR type* | SVCB (64) | +-----------------+----------------------------------------+ | *Name prefix* | "_dns" for port 53, else "_$PORT._dns" | +-----------------+----------------------------------------+ | *Required keys* | alpn | +-----------------+----------------------------------------+ | *Automatically | port | | Mandatory Keys* | | +-----------------+----------------------------------------+ | *Special | Supports all HTTPS RR SvcParamKeys | | behaviors* | | +-----------------+----------------------------------------+ | | Overrides the HTTPS RR for DoH | +-----------------+----------------------------------------+ | | Default port is per-transport | +-----------------+----------------------------------------+ | | No encrypted -> cleartext fallback | +-----------------+----------------------------------------+ Table 3 Acknowledgments TODO acknowledge. Author's Address Benjamin Schwartz Google LLC Email: bemasc@google.com Schwartz Expires 21 October 2021 [Page 9]