idnits 2.17.00 (12 Aug 2021) /tmp/idnits36158/draft-schwartz-svcb-dns-03.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 1 instance of lines with non-RFC2606-compliant FQDNs in the document. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (19 April 2021) is 397 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-09) exists of draft-ietf-dnsop-svcb-https-02 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 add B. Schwartz 3 Internet-Draft Google LLC 4 Intended status: Standards Track 19 April 2021 5 Expires: 21 October 2021 7 Service Binding Mapping for DNS Servers 8 draft-schwartz-svcb-dns-03 10 Abstract 12 The SVCB DNS record type expresses a bound collection of endpoint 13 metadata, for use when establishing a connection to a named service. 14 DNS itself can be such a service, when the server is identified by a 15 domain name. This document provides the SVCB mapping for named DNS 16 servers, allowing them to indicate support for new transport 17 protocols. 19 Discussion Venues 21 This note is to be removed before publishing as an RFC. 23 Discussion of this document takes place on the ADD Working Group 24 mailing list (add@ietf.org), which is archived at 25 https://mailarchive.ietf.org/arch/browse/add/. 27 Source for this draft and an issue tracker can be found at 28 https://github.com/bemasc/svcb-dns. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at https://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on 21 October 2021. 47 Copyright Notice 49 Copyright (c) 2021 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 54 license-info) in effect on the date of publication of this document. 55 Please review these documents carefully, as they describe your rights 56 and restrictions with respect to this document. Code Components 57 extracted from this document must include Simplified BSD License text 58 as described in Section 4.e of the Trust Legal Provisions and are 59 provided without warranty as described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 64 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 65 3. Name form . . . . . . . . . . . . . . . . . . . . . . . . . . 3 66 4. Applicable existing SvcParamKeys . . . . . . . . . . . . . . 3 67 4.1. alpn . . . . . . . . . . . . . . . . . . . . . . . . . . 3 68 4.2. port . . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 4.3. Other applicable SvcParamKeys . . . . . . . . . . . . . . 4 70 5. New SvcParamKeys . . . . . . . . . . . . . . . . . . . . . . 4 71 5.1. dohpath . . . . . . . . . . . . . . . . . . . . . . . . . 4 72 6. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 4 73 7. Relationship to DNS URIs . . . . . . . . . . . . . . . . . . 5 74 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 5 75 9. Security Considerations . . . . . . . . . . . . . . . . . . . 5 76 9.1. Adversary on the query path . . . . . . . . . . . . . . . 5 77 9.2. Adversary on the transport path . . . . . . . . . . . . . 6 78 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 79 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 80 11.1. Normative References . . . . . . . . . . . . . . . . . . 7 81 11.2. Informative References . . . . . . . . . . . . . . . . . 8 82 Appendix A. Mapping Summary . . . . . . . . . . . . . . . . . . 8 83 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9 84 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 86 1. Introduction 88 The SVCB record type [SVCB] provides clients with information about 89 how to reach alternative endpoints for a service, which may have 90 improved performance or privacy properties. The service is 91 identified by a "scheme" indicating the service type, a hostname, and 92 optionally other information such as a port number. A DNS server is 93 often identified only by its IP address (e.g. in DHCP), but in some 94 contexts it can also be identified by a hostname (e.g. "NS" records, 95 manual resolver configuration) and sometimes also a non-default port 96 number. 98 Use of the SVCB record type requires a mapping document for each 99 service type, indicating how a client for that service can interpret 100 the contents of the SVCB SvcParams. This document provides the 101 mapping for the "dns" service type, allowing DNS servers to offer 102 alternative endpoints and transports, including encrypted transports 103 like DNS over TLS and DNS over HTTPS. 105 2. Conventions and Definitions 107 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 108 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 109 "OPTIONAL" in this document are to be interpreted as described in BCP 110 14 [RFC2119] [RFC8174] when, and only when, they appear in all 111 capitals, as shown here. 113 3. Name form 115 Names are formed using Port-Prefix Naming ([SVCB] Section 2.3). For 116 example, a DNS service identified by the name "dns1.example.com" and 117 (unusually) the non-default port number 5353 would be represented as 118 "_5353._dns.dns1.example.com.". A DNS service using the default port 119 number of 53 would be represented as "_dns.dns1.example.com.". 121 4. Applicable existing SvcParamKeys 123 4.1. alpn 125 This key indicates the set of supported protocols ([SVCB] 126 Section 6.1). There is no default protocol, so the "no-default-alpn" 127 key does not apply, and the "alpn" key MUST be present. 129 If the protocol set contains any HTTP versions (e.g. "h2", "h3"), 130 then the record indicates support for DNS over HTTPS [DOH], and the 131 "dohpath" key MUST be present (Section 5.1). All keys specified for 132 use with the HTTPS record are also permissible, and apply to the 133 resulting HTTP connection. 135 If the protocol set contains protocols with different default ports, 136 and no port key is specified, then protocols are contacted separately 137 on their default ports. Note that in this configuration, ALPN 138 negotiation does not defend against cross-protocol downgrade attacks. 140 4.2. port 142 This key is used to indicate the target port for connection (([SVCB] 143 Section 6.2)). If omitted, the client SHALL use the default port for 144 each transport protocol (853 for DNS over TLS [DOT], 443 for DNS over 145 HTTPS). 147 This key is automatically mandatory if present. (See Section 7 of 148 [SVCB] for the definition of "automatically mandatory".) 150 4.3. Other applicable SvcParamKeys 152 These SvcParamKeys apply to the "dns" scheme without modification: 154 * echconfig 156 * ipv4hint 158 * ipv6hint 160 5. New SvcParamKeys 162 5.1. dohpath 164 "dohpath" is a single-valued SvcParamKey whose value (both in 165 presentation and wire format) is a relative URI Template [RFC6570], 166 normally starting with "/". If the "alpn" SvcParamKey indicates 167 support for HTTP, clients MAY construct a DNS over HTTPS URI Template 168 by combining the prefix "https://", the service name, the port from 169 the "port" key if present, and the "dohpath" value. (The DNS 170 service's original port number MUST NOT be used.) 172 Clients SHOULD NOT query for any "HTTPS" RRs when using the 173 constructed URI Template. Instead, the SvcParams and address records 174 associated with this SVCB record SHOULD be used for the HTTPS 175 connection, with the same semantics as an HTTPS RR. However, for 176 consistency, service operators SHOULD publish an equivalent HTTPS RR, 177 especially if clients might learn this URI Template through a 178 different channel. 180 6. Limitations 182 This document is concerned exclusively with the DNS transport, and 183 does not affect or inform the construction or interpretation of DNS 184 messages. For example, nothing in this document indicates whether 185 the service is intended for use as a recursive or authoritative DNS 186 server. Clients must know the intended use in their context. 188 7. Relationship to DNS URIs 190 The "dns:" URI scheme [DNSURI] describes a way to represent DNS 191 queries as URIs. This scheme optionally includes an authority, 192 comprised of a host and port number (with a default of 53). DNS URIs 193 normally omit the authority, or specify an IP address, but a hostname 194 is allowed, in which case it is suitable for use with this mapping. 196 8. Examples 198 * A resolver at "simple.example" that supports DNS over TLS on port 199 853 (implicitly, as this is its default port): 201 _dns.simple.example. 7200 IN SVCB 1 alpn=dot simple.example. 203 * A resolver at "doh.example" that supports only DNS over HTTPS (DNS 204 over TLS is disabled): _dns.doh.example. 7200 IN SVCB 1 205 doh.example. ( alpn=h2 dohpath=/dns-query{?dns} ) 207 * A resolver at "resolver.example" that supports 209 - DNS over TLS on "resolver.example" ports 853 (implicit in 210 record 1) and 8530 (explicit in record 2), with 211 "resolver.example" as the Authentication Domain Name, 213 - DNS over HTTPS at "https://resolver.example/dns-query{?dns}" 214 (record 1), and 216 - an experimental protocol on "fooexp.resolver.example:5353" 217 (record 3): 219 $ORIGIN resolver.example. 220 _dns 7200 IN SVCB 1 @ alpn=dot,h2,h3 dohpath=/dns-query{?dns} 221 SVCB 2 @ alpn=dot port=8530 222 SVCB 3 fooexp port=5353 alpn=foo foo-info=... 224 * A nameserver at "ns.example" whose service configuration is 225 published on a different domain: 227 $ORIGIN example. 228 _dns.ns 7200 IN SVCB 0 _dns.ns.nic 230 9. Security Considerations 232 9.1. Adversary on the query path 234 This section considers an adversary who can add or remove responses 235 to the SVCB query. 237 Clients MUST authenticate the server to its name during secure 238 transport establishment. This name is the hostname used to construct 239 the original SVCB query, and cannot be influenced by the SVCB record 240 contents. Accordingly, this draft does not mandate the use of 241 DNSSEC. This draft also does not specify how clients authenticate 242 the name (e.g. selection of roots of trust), which might vary 243 according to the context. 245 Although this adversary cannot alter the authentication name of the 246 service, it does have control of the port number and "dohpath" value. 247 As a result, the adversary can direct DNS queries for $HOSTNAME to 248 any port on $HOSTNAME, and any path on "https://$HOSTNAME", even if 249 $HOSTNAME is not actually a DNS server. If the DNS client uses 250 shared TLS or HTTP state, the client could be correctly authenticated 251 (e.g. using a TLS client certificate or HTTP cookie). 253 This behavior creates a number of possible attacks for certain server 254 configurations. For example, if "https://$HOSTNAME/upload" accepts 255 any POST request as a public file upload, the adversary could forge a 256 SVCB record containing "dohpath=/upload". This would cause the 257 client to upload and publish every query, resulting in unexpected 258 storage costs for the server and privacy loss for the client. 260 To mitigate this attack, a client of this SVCB mapping MUST NOT 261 provide client authentication for DNS queries, except to servers that 262 it specifically knows are not vulnerable to such attacks, and a DoH 263 service operator MUST ensure that all unauthenticated DoH requests to 264 its origin maintain the DoH service's privacy guarantees, regardless 265 of the path. Also, if an alternative service endpoint sends an 266 invalid response to a DNS query, the client SHOULD NOT send more 267 queries to that endpoint. 269 9.2. Adversary on the transport path 271 This section considers an adversary who can modify network traffic 272 between the client and the SvcDomainName (i.e. the destination 273 server). 275 For a SVCB-reliant client ([SVCB] Section 3), this adversary can only 276 cause a denial of service. However, because DNS is unencrypted by 277 default, this adversary can execute a downgrade attack against SVCB- 278 optional clients. Accordingly, when use of this specification is 279 optional, clients SHOULD switch to SVCB-reliant behavior if SVCB 280 resolution succeeds. Specifications making using of this mapping MAY 281 adjust this fallback behavior to suit their requirements. 283 10. IANA Considerations 285 Per [SVCB] IANA would be directed to add the following entry to the 286 SVCB Service Parameters registry. 288 +========+=========+==============================+=================+ 289 | Number | Name | Meaning | Reference | 290 +========+=========+==============================+=================+ 291 | TBD | dohpath | DNS over HTTPS path template | (This | 292 | | | | document) | 293 +--------+---------+------------------------------+-----------------+ 295 Table 1 297 Per [Attrleaf], IANA would be directed to add the following entry to 298 the DNS Underscore Global Scoped Entry Registry: 300 +=========+============+===============+=================+ 301 | RR TYPE | _NODE NAME | Meaning | Reference | 302 +=========+============+===============+=================+ 303 | SVCB | _dns | DNS SVCB info | (This document) | 304 +---------+------------+---------------+-----------------+ 306 Table 2 308 11. References 310 11.1. Normative References 312 [DOH] Hoffman, P. and P. McManus, "DNS Queries over HTTPS 313 (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, 314 . 316 [DOT] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 317 and P. Hoffman, "Specification for DNS over Transport 318 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 319 2016, . 321 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 322 Requirement Levels", BCP 14, RFC 2119, 323 DOI 10.17487/RFC2119, March 1997, 324 . 326 [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., 327 and D. Orchard, "URI Template", RFC 6570, 328 DOI 10.17487/RFC6570, March 2012, 329 . 331 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 332 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 333 May 2017, . 335 [SVCB] Schwartz, B., Bishop, M., and E. Nygren, "Service binding 336 and parameter specification via the DNS (DNS SVCB and 337 HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf- 338 dnsop-svcb-https-02, 2 November 2020, 339 . 342 11.2. Informative References 344 [Attrleaf] Crocker, D., "Scoped Interpretation of DNS Resource 345 Records through "Underscored" Naming of Attribute Leaves", 346 BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019, 347 . 349 [DNSURI] Josefsson, S., "Domain Name System Uniform Resource 350 Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006, 351 . 353 Appendix A. Mapping Summary 355 This table serves as a non-normative summary of the DNS mapping for 356 SVCB. 358 +=================+========================================+ 359 +=================+========================================+ 360 | *Mapped scheme* | "dns" | 361 +-----------------+----------------------------------------+ 362 | *RR type* | SVCB (64) | 363 +-----------------+----------------------------------------+ 364 | *Name prefix* | "_dns" for port 53, else "_$PORT._dns" | 365 +-----------------+----------------------------------------+ 366 | *Required keys* | alpn | 367 +-----------------+----------------------------------------+ 368 | *Automatically | port | 369 | Mandatory Keys* | | 370 +-----------------+----------------------------------------+ 371 | *Special | Supports all HTTPS RR SvcParamKeys | 372 | behaviors* | | 373 +-----------------+----------------------------------------+ 374 | | Overrides the HTTPS RR for DoH | 375 +-----------------+----------------------------------------+ 376 | | Default port is per-transport | 377 +-----------------+----------------------------------------+ 378 | | No encrypted -> cleartext fallback | 379 +-----------------+----------------------------------------+ 381 Table 3 383 Acknowledgments 385 TODO acknowledge. 387 Author's Address 389 Benjamin Schwartz 390 Google LLC 392 Email: bemasc@google.com