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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5077 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 6347 (Obsoleted by RFC 9147) == Outdated reference: draft-ietf-dprive-dnsodtls has been published as RFC 8094 == Outdated reference: A later version (-07) exists of draft-ietf-tls-dnssec-chain-extension-01 -- Obsolete informational reference (is this intentional?): RFC 7626 (Obsoleted by RFC 9076) Summary: 3 errors (**), 0 flaws (~~), 3 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 dprive S. Dickinson 3 Internet-Draft Sinodun 4 Intended status: Standards Track D. Gillmor 5 Expires: April 10, 2017 ACLU 6 T. Reddy 7 Cisco 8 October 7, 2016 10 Authentication and (D)TLS Profile for DNS-over-(D)TLS 11 draft-ietf-dprive-dtls-and-tls-profiles-04 13 Abstract 15 This document describes how a DNS client can use a domain name to 16 authenticate a DNS server that uses Transport Layer Security (TLS) 17 and Datagram TLS (DTLS). Additionally, it defines (D)TLS profiles 18 for DNS clients and servers implementing DNS-over-TLS and DNS-over- 19 DTLS. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on April 10, 2017. 38 Copyright Notice 40 Copyright (c) 2016 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 57 3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5 59 4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 5 60 4.2. Usage Profiles . . . . . . . . . . . . . . . . . . . . . 6 61 4.2.1. DNS Resolution . . . . . . . . . . . . . . . . . . . 8 62 4.3. Authentication . . . . . . . . . . . . . . . . . . . . . 8 63 4.3.1. DNS-over-(D)TLS Bootstrapping Problems . . . . . . . 8 64 4.3.2. Credential Verification . . . . . . . . . . . . . . . 8 65 4.3.3. Implementation guidance . . . . . . . . . . . . . . . 9 66 5. Authentication in Opportunistic DNS-over(D)TLS Privacy . . . 9 67 6. Authentication in Strict DNS-over(D)TLS Privacy . . . . . . . 9 68 7. In Band Source of Domain Name: SRV Service Label . . . . . . 10 69 8. Out of Band Sources of Domain Name . . . . . . . . . . . . . 10 70 8.1. Full direct configuration . . . . . . . . . . . . . . . . 10 71 8.2. Direct configuration of name only . . . . . . . . . . . . 10 72 8.3. DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . 11 73 9. Credential Verification . . . . . . . . . . . . . . . . . . . 12 74 9.1. X.509 Certificate Based Authentication . . . . . . . . . 12 75 9.2. DANE . . . . . . . . . . . . . . . . . . . . . . . . . . 12 76 9.2.1. Direct DNS Lookup . . . . . . . . . . . . . . . . . . 13 77 9.2.2. TLS DNSSEC Chain extension . . . . . . . . . . . . . 13 78 10. Combined Credentials with SPKI Pinsets . . . . . . . . . . . 14 79 11. (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . . 14 80 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 81 13. Security Considerations . . . . . . . . . . . . . . . . . . . 15 82 13.1. Counter-measures to DNS Traffic Analysis . . . . . . . . 15 83 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 84 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 85 15.1. Normative References . . . . . . . . . . . . . . . . . . 16 86 15.2. Informative References . . . . . . . . . . . . . . . . . 17 87 Appendix A. Server capability probing and caching by DNS clients 19 88 Appendix B. Changes between revisions . . . . . . . . . . . . . 19 89 B.1. -04 version . . . . . . . . . . . . . . . . . . . . . . . 19 90 B.2. -03 version . . . . . . . . . . . . . . . . . . . . . . . 19 91 B.3. -02 version . . . . . . . . . . . . . . . . . . . . . . . 19 92 B.4. -01 version . . . . . . . . . . . . . . . . . . . . . . . 20 93 B.5. draft-ietf-dprive-dtls-and-tls-profiles-00 . . . . . . . 20 94 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 96 1. Introduction 98 DNS Privacy issues are discussed in [RFC7626]. Two documents that 99 provide DNS privacy between DNS clients and DNS servers are: 101 o Specification for DNS over Transport Layer Security (TLS) 102 [RFC7858], referred to here as simply 'DNS-over-TLS' 104 o DNS-over-DTLS (DNSoD) [I-D.ietf-dprive-dnsodtls], referred to here 105 simply as 'DNS-over-DTLS'. Note that this document has the 106 Intended status of Experimental. 108 Both documents are limited in scope to encrypting DNS messages 109 between stub clients and recursive resolvers and the same scope is 110 applied to this document (see Section 2 and Section 3). The 111 proposals here might be adapted or extended in future to be used for 112 recursive clients and authoritative servers, but this application is 113 out of scope for the DNS PRIVate Exchange (DPRIVE) Working Group per 114 its current charter. 116 This document defines two Usage Profiles (Strict and Opportunistic) 117 for DTLS [RFC6347] and TLS [RFC5246] which define the security 118 properties a user should expect when using that profile to connect to 119 the available DNS servers. In essence: 121 o the Strict Profile requires an encrypted connection and successful 122 authentication of the DNS server which provides strong privacy 123 guarantees (at the expense of providing no DNS service if this is 124 not available). 126 o the Opportunistic Profile will attempt, but does not require, 127 encryption and successful authentication; it therefore provides no 128 privacy guarantees but offers maximum chance of DNS service. 130 Additionally, a number of authentication mechanisms are defined that 131 specify how a DNS client should authenticate a DNS server based on a 132 domain name. In particular, the following is described: 134 o How a DNS client can obtain a domain name for a DNS server to use 135 for (D)TLS authentication. 137 o What are the acceptable credentials a DNS server can present to 138 prove its identity for (D)TLS authentication based on a given 139 domain name. 141 o How a DNS client can verify that any given credential matches the 142 domain name obtained for a DNS server. 144 It should be noted that [RFC7858] includes a description of a 145 specific case of a Strict Usage Profile using a single authentication 146 mechanism (SPKI pinning). This draft generalises the picture by 147 separating the Usage Profile, which is based purely on the security 148 properties it offers the user, from the specific mechanism that is 149 used for authentication. Therefore the "Out-of-band Key-pinned 150 Privacy Profile" described in the DNS-over-TLS draft would qualify as 151 a "Strict Usage Profile" that used SPKI pinning for authentication. 153 This document also defines a (D)TLS protocol profile for use with 154 DNS. This profile defines the configuration options and protocol 155 extensions required of both parties to optimize connection 156 establishment and session resumption for transporting DNS, and to 157 support the authentication mechanisms defined here. 159 2. Terminology 161 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 162 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 163 document are to be interpreted as described in [RFC2119]. 165 Several terms are used specifically in the context of this draft: 167 o DNS client: a DNS stub resolver or forwarder/proxy. In the case 168 of a forwarder, the term "DNS client" is used to discuss the side 169 that sends queries. 171 o DNS server: a DNS recursive resolver or forwarder/proxy. In the 172 case of a forwarder, the term "DNS server" is used to discuss the 173 side that responds to queries. 175 o Privacy-enabling DNS server: A DNS server that: 177 * MUST implement DNS-over-TLS [RFC7858] and MAY implement DNS- 178 over-DTLS [I-D.ietf-dprive-dnsodtls]. 180 * Can offer at least one of the credentials described in 181 Section 9. 183 * Implements the (D)TLS profile described in Section 11. 185 o (D)TLS: For brevity this term is used for statements that apply to 186 both Transport Layer Security [RFC5246] and Datagram Transport 187 Layer Security [RFC6347]. Specific terms will be used for any 188 statement that applies to either protocol alone. 190 o DNS-over-(D)TLS: For brevity this term is used for statements that 191 apply to both DNS-over-TLS [RFC7858] and DNS-over-DTLS 193 [I-D.ietf-dprive-dnsodtls]. Specific terms will be used for any 194 statement that applies to either protocol alone. 196 o Credential: Information available for a DNS server which proves 197 its identity for authentication purposes. Credentials discussed 198 here include: 200 * X.509 certificate 202 * DNSSEC validated chain to a TLSA record 204 but may also include SPKI pinsets. 206 o SPKI Pinsets: [RFC7858] describes the use of cryptographic digests 207 to "pin" public key information in a manner similar to HPKP 208 [RFC7469]. An SPKI pinset is a collection of these pins that 209 constrains a DNS server. 211 o Reference Identifier: a Reference Identifier as described in 212 [RFC6125], constructed by the DNS client when performing TLS 213 authentication of a DNS server. 215 3. Scope 217 This document is limited to domain-name-based authentication of DNS 218 servers by DNS clients (as defined in the terminology section), and 219 the (D)TLS profiles needed to support this. As such, the following 220 things are out of scope: 222 o Authentication of authoritative servers by recursive resolvers. 224 o Authentication of DNS clients by DNS servers. 226 o SPKI-pinset-based authentication. This is defined in [RFC7858]. 227 However, Section 10 does describe how to combine that approach 228 with the domain name based mechanism described here. 230 o Any server identifier other than domain names, including IP 231 address, organizational name, country of origin, etc. 233 4. Discussion 235 4.1. Background 237 To protect against passive attacks DNS privacy requires encrypting 238 the query (and response). Such encryption typically provides 239 integrity protection as a side-effect, which means on-path attackers 240 cannot simply inject bogus DNS responses. For DNS privacy to also 241 provide protection against active attackers pretending to be the 242 server, the client must authenticate the server. 244 This draft discusses Usage Profiles, which provide differing levels 245 of privacy guarantees to DNS clients, based on the requirements for 246 authentication and encryption, regardless of the context (for 247 example, which network the client is connected to). A Usage Profile 248 is a distinct concept to a usage policy or usage model, which might 249 dictate which Profile should be used in a particular context 250 (enterprise vs coffee shop), with a particular set of DNS Servers or 251 with reference to other external factors. A description of the 252 variety of usage policies is out of scope of this document, but may 253 be the subject of a future I-D. 255 4.2. Usage Profiles 257 A DNS client has a choice of privacy Usage Profiles available. This 258 choice is briefly discussed in both [RFC7858] and 259 [I-D.ietf-dprive-dnsodtls]. In summary, the usage profiles are: 261 o Strict Privacy: the DNS client requires both an encrypted and 262 authenticated connection to a privacy-enabling DNS Server. A hard 263 failure occurs if this is not available. This requires the client 264 to securely obtain information it can use to authenticate the 265 server. This profile can include some initial meta queries 266 (performed using Opportunistic Privacy) to securely obtain the IP 267 address and authentication information for the privacy-enabling 268 DNS server to which the DNS client will subsequently connect. The 269 rationale for this is that requiring Strict Privacy for such meta 270 queries would introduce significant deployment obstacles. This 271 profile provides strong privacy guarantees to the client. This is 272 Profile discussed in detail in Section 6. 274 o Opportunistic Privacy: the DNS client uses Opportunistic Security 275 as described in [RFC7435] 277 "... the use of cleartext as the baseline communication 278 security policy, with encryption and authentication negotiated 279 and applied to the communication when available." 281 The use of Opportunistic Privacy is intended to support 282 incremental deployment of security capabilities with a view to 283 widespread adoption of Strict Privacy. It should be employed when 284 the DNS client might otherwise settle for cleartext; it provides 285 the maximum protection available. As described in [RFC7435] it 286 might result in 288 * an encrypted and authenticated connection 289 * an encrypted connection 291 * a clear text connection 293 * hard failure 295 depending on the fallback logic of the client, the available 296 authentication information and the capabilities of the DNS Server. 297 In the first three cases the DNS client is willing to continue 298 with a connection to the DNS Server and perform resolution of 299 queries. 301 To compare the two Usage profiles the table below shows successful 302 Strict Privacy along side the 3 possible successful outcomes of 303 Opportunistic Privacy. In the best case scenario for Opportunistic 304 Privacy (an authenticated and encrypted connection) it is equivalent 305 to Strict Privacy. In the worst case scenario it is equivalent to 306 clear text. Clients using Opportunistic Privacy SHOULD try for the 307 best case but MAY fallback to intermediate cases and eventually the 308 worst case scenario in order to obtain a response. It therefore 309 provides no privacy guarantee to the user and varying protection 310 depending on what kind of connection is actually used. Note that 311 there is no requirement in Opportunistic to notify the user what type 312 of connection is actually used, the 'detection' described below is 313 only possible if such connection information is available. This is 314 discussed in Section 5. 316 +---------------+------------+------------------+-----------------+ 317 | Usage Profile | Connection | Passive Attacker | Active Attacker | 318 +---------------+------------+------------------+-----------------+ 319 | Strict | A, E | P | P | 320 | Opportunistic | A, E | P | P | 321 | Opportunistic | E | P | N (D) | 322 | Opportunistic | | N (D) | N (D) | 323 +---------------+------------+------------------+-----------------+ 325 P == protection; N == no protection; D == detection is possible; A == 326 Authenticated Connection; E == Encrypted Connection 328 Table 1: DNS Privacy Protection by Usage Profile and type of attacker 330 Since Strict Privacy provides the strongest privacy guarantees it is 331 preferable to Opportunistic Privacy. 333 However since the two profiles require varying levels of 334 configuration (or a trusted relationship with a provider) and DNS 335 server capabilities, DNS clients will need to carefully select which 336 profile to use based on their communication privacy needs. For the 337 case where a client has a trusted relationship with a provider it is 338 expected that the provider will provide either a domain name or SPKI 339 pinset via a secure out-of-band mechanism and therefore Strict 340 Privacy should be used. 342 4.2.1. DNS Resolution 344 A DNS client SHOULD select a particular usage profile when resolving 345 a query. A DNS client MUST NOT fallback from Strict Privacy to 346 Opportunistic Privacy during the resolution process as this could 347 invalidate the protection offered against active attackers. 349 4.3. Authentication 351 This document describes authentication mechanisms that can be used in 352 either Strict or Opportunistic Privacy for DNS-over-(D)TLS. 354 4.3.1. DNS-over-(D)TLS Bootstrapping Problems 356 Many (D)TLS clients use PKIX authentication [RFC6125] based on a 357 domain name for the server they are contacting. These clients 358 typically first look up the server's network address in the DNS 359 before making this connection. A DNS client therefore has a 360 bootstrap problem. DNS clients typically know only the IP address of 361 a DNS server. 363 As such, before connecting to a DNS server, a DNS client needs to 364 learn the domain name it should associate with the IP address of a 365 DNS server for authentication purposes. Sources of domains names are 366 discussed in Section 7 and Section 8. 368 One advantage of this domain name based approach is that it 369 encourages association of stable, human recognisable identifiers with 370 secure DNS service providers. 372 4.3.2. Credential Verification 374 The use of SPKI pinset verification is discussed in [RFC7858]. 376 In terms of domain name based verification, once a domain name is 377 known for a DNS server a choice of mechanisms can be used for 378 authentication. Section 9 discusses these mechanisms in detail, 379 namely X.509 certificate based authentication and DANE. 381 Note that the use of DANE adds requirements on the ability of the 382 client to get validated DNSSEC results. This is discussed in more 383 detail in Section 9.2. 385 4.3.3. Implementation guidance 387 Section 11 describes the (D)TLS profile for DNS-over(D)TLS. 388 Additional considerations relating to general implementation 389 guidelines are discussed in both Section 13 and in Appendix A. 391 5. Authentication in Opportunistic DNS-over(D)TLS Privacy 393 An Opportunistic Security [RFC7435] profile is described in [RFC7858] 394 which MAY be used for DNS-over-(D)TLS. 396 DNS clients issuing queries under an opportunistic profile which know 397 of a domain name or SPKI pinset for a given privacy-enabling DNS 398 server MAY choose to try to authenticate the server using the 399 mechanisms described here. This is useful for detecting (but not 400 preventing) active attack, since the fact that authentication 401 information is available indicates that the server in question is a 402 privacy-enabling DNS server to which it should be possible to 403 establish an authenticated, encrypted connection. In this case, 404 whilst a client cannot know the reason for an authentication failure, 405 from a privacy standpoint the client should consider an active attack 406 in progress and proceed under that assumption. Attempting 407 authentication is also useful for debugging or diagnostic purposes if 408 there are means to report the result. This information can provide a 409 basis for a DNS client to switch to (preferred) Strict Privacy where 410 it is viable. 412 6. Authentication in Strict DNS-over(D)TLS Privacy 414 To authenticate a privacy-enabling DNS server, a DNS client needs to 415 know the domain name for each server it is willing to contact. This 416 is necessary to protect against active attacks on DNS privacy. 418 A DNS client requiring Strict Privacy MUST either use one of the 419 sources listed in Section 8 to obtain a domain name for the server it 420 contacts, or use an SPKI pinset as described in [RFC7858]. 422 A DNS client requiring Strict Privacy MUST only attempt to connect to 423 DNS servers for which either a domain name or a SPKI pinset is known 424 (or both). The client MUST use the available verification mechanisms 425 described in Section 9 to authenticate the server, and MUST abort 426 connections to a server when no verification mechanism succeeds. 428 With Strict Privacy, the DNS client MUST NOT commence sending DNS 429 queries until at least one of the privacy-enabling DNS servers 430 becomes available. 432 A privacy-enabling DNS server may be temporarily unavailable when 433 configuring a network. For example, for clients on networks that 434 require registration through web-based login (a.k.a. "captive 435 portals"), such registration may rely on DNS interception and 436 spoofing. Techniques such as those used by DNSSEC-trigger 437 [dnssec-trigger] MAY be used during network configuration, with the 438 intent to transition to the designated privacy-enabling DNS servers 439 after captive portal registration. The system MUST alert by some 440 means that the DNS is not private during such bootstrap. 442 7. In Band Source of Domain Name: SRV Service Label 444 This specification adds a SRV service label "domain-s" for privacy- 445 enabling DNS servers. 447 Example service records (for TLS and DTLS respectively): 449 _domain-s._tcp.dns.example.com. SRV 0 1 853 dns1.example.com. 450 _domain-s._tcp.dns.example.com. SRV 0 1 853 dns2.example.com. 452 _domain-s._udp.dns.example.com. SRV 0 1 853 dns3.example.com. 454 8. Out of Band Sources of Domain Name 456 8.1. Full direct configuration 458 DNS clients may be directly and securely provisioned with the domain 459 name of each privacy-enabling DNS server. For example, using a 460 client specific configuration file or API. 462 In this case, direct configuration for a DNS client would consist of 463 both an IP address and a domain name for each DNS server. 465 8.2. Direct configuration of name only 467 A DNS client may be configured directly and securely with only the 468 domain name of its privacy-enabling DNS server. For example, using a 469 client specific configuration file or API. 471 A DNS client might learn of a default recursive DNS resolver from an 472 untrusted source (such as DHCP's DNS server option [RFC3646]). It 473 can then use opportunistic DNS connections to untrusted recursive DNS 474 resolver to establish the IP address of the intended privacy-enabling 475 DNS server by doing a lookup of SRV records. Such records MUST be 476 validated using DNSSEC. Private DNS resolution can now be done by 477 the DNS client against the configured privacy-enabling DNS server. 479 Example: 481 o A DNSSEC validating DNS client is configured with the domain name 482 dns.example.net for a privacy-enabling DNS server 484 o Using Opportunistic Privacy to a default DNS resolver (acquired, 485 for example, using DHCP) the client performs look ups for 487 * SRV record for _domain-s._tcp.dns.example.net to obtain the 488 server host name 490 * A and/or AAAA lookups to obtain IP address for the server host 491 name 493 o Client validates all the records obtained in the previous step 494 using DNSSEC. 496 o If the records successfully validate the client proceeds to 497 connect to the privacy-enabling DNS server using Strict Privacy. 499 A DNS client so configured that successfully connects to a privacy- 500 enabling DNS server MAY choose to locally cache the looked up 501 addresses in order to not have to repeat the opportunistic lookup. 503 8.3. DHCP 505 Some clients may have an established trust relationship with a known 506 DHCP [RFC2131] server for discovering their network configuration. 507 In the typical case, such a DHCP server provides a list of IP 508 addresses for DNS servers (see section 3.8 of [RFC2132]), but does 509 not provide a domain name for the DNS server itself. 511 In the future, a DHCP server might use a DHCP extension to provide a 512 list of domain names for the offered DNS servers, which correspond to 513 IP addresses listed. 515 Use of such a mechanism with any DHCP server when using an 516 Opportunistic profile is reasonable, given the security expectation 517 of that profile. However when using a Strict profile the DHCP 518 servers used as sources of domain names MUST be considered secure and 519 trustworthy. This document does not attempt to describe secured and 520 trusted relationships to DHCP servers. 522 [NOTE: It is noted (at the time of writing) that whilst some 523 implementation work is in progress to secure IPv6 connections for 524 DHCP, IPv4 connections have received little to no implementation 525 attention in this area.] 527 9. Credential Verification 529 9.1. X.509 Certificate Based Authentication 531 When a DNS client configured with a domain name connects to its 532 configured DNS server over (D)TLS, the server may present it with an 533 X.509 certificate. In order to ensure proper authentication, DNS 534 clients MUST verify the entire certification path per [RFC5280]. The 535 DNS client additionally uses [RFC6125] validation techniques to 536 compare the domain name to the certificate provided. 538 A DNS client constructs two Reference Identifiers for the server 539 based on the domain name: A DNS-ID and an SRV-ID [RFC4985]. The DNS- 540 ID is simply the domain name itself. The SRV-ID uses a "_domain-s." 541 prefix. So if the configured domain name is "dns.example.com", then 542 the two Reference Identifiers are: 544 DNS-ID: dns.example.com 546 SRV-ID: _domain-s.dns.example.com 548 If either of the Reference Identifiers are found in the X.509 549 certificate's subjectAltName extension as described in section 6 of 550 [RFC6125], the DNS client should accept the certificate for the 551 server. 553 A compliant DNS client MUST only inspect the certificate's 554 subjectAltName extension for these Reference Identifiers. In 555 particular, it MUST NOT inspect the Subject field itself. 557 9.2. DANE 559 DANE [RFC6698] provides mechanisms to root certificate and raw public 560 key trust with DNSSEC. However this requires the DNS client to have 561 a domain name for the DNS Privacy Server which must be obtained via a 562 trusted source. 564 This section assumes a solid understanding of both DANE [RFC6698] and 565 DANE Operations [RFC7671]. A few pertinent issues covered in these 566 documents are outlined here as useful pointers, but familiarity with 567 both these documents in their entirety is expected. 569 It is noted that [RFC6698] says 571 "Clients that validate the DNSSEC signatures themselves MUST use 572 standard DNSSEC validation procedures. Clients that rely on 573 another entity to perform the DNSSEC signature validation MUST use 574 a secure mechanism between themselves and the validator." 576 It is noted that [RFC7671] covers the following topics: 578 o Section 4.1: Opportunistic Security and PKIX Usages and 579 Section 14: Security Considerations, which both discuss the use of 580 PKIX-TA(0) and PKIX-EE(1) for OS. 582 o Section 5: Certificate-Usage-Specific DANE Updates and Guidelines. 583 Specifically Section 5.1 which outlines the combination of 584 Certificate Usage DANE-EE(3) and Selector Usage SPKI(1) with Raw 585 Public Keys [RFC7250]. Section 5.1 also discusses the security 586 implications of this mode, for example, it discusses key lifetimes 587 and specifies that validity period enforcement is based solely on 588 the TLSA RRset properties for this case. [QUESTION: Should an 589 appendix be added with an example of how to use DANE without X.509 590 certificates?] 592 o Section 13: Operational Considerations, which discusses TLSA TTLs 593 and signature validity periods. 595 The specific DANE record for a DNS Privacy Server would take the 596 form: 598 _853._tcp.[server-domain-name] for TLS 600 _853._udp.[server-domain-name] for DTLS 602 9.2.1. Direct DNS Lookup 604 The DNS client MAY choose to perform the DNS lookups to retrieve the 605 required DANE records itself. The DNS queries for such DANE records 606 MAY use opportunistic encryption or be in the clear to avoid trust 607 recursion. The records MUST be validated using DNSSEC as described 608 above in [RFC6698]. 610 9.2.2. TLS DNSSEC Chain extension 612 The DNS client MAY offer the TLS extension described in 613 [I-D.ietf-tls-dnssec-chain-extension]. If the DNS server supports 614 this extension, it can provide the full chain to the client in the 615 handshake. 617 If the DNS client offers the TLS DNSSEC Chain extension, it MUST be 618 capable of validating the full DNSSEC authentication chain down to 619 the leaf. If the supplied DNSSEC chain does not validate, the client 620 MUST ignore the DNSSEC chain and validate only via other supplied 621 credentials. 623 10. Combined Credentials with SPKI Pinsets 625 The SPKI pinset profile described in [RFC7858] MAY be used with DNS- 626 over-(D)TLS. 628 This draft does not make explicit recommendations about how a SPKI 629 pinset based authentication mechanism should be combined with a 630 domain based mechanism from an operator perspective. However it can 631 be envisaged that a DNS server operator may wish to make both an SPKI 632 pinset and a domain name available to allow clients to choose which 633 mechanism to use. Therefore, the following is guidance on how 634 clients ought to behave if they choose to configure both, as is 635 possible in HPKP [RFC7469]. 637 A DNS client that is configured with both a domain name and a SPKI 638 pinset for a DNS server SHOULD match on both a valid credential for 639 the domain name and a valid SPKI pinset if both are available when 640 connecting to that DNS server. 642 11. (D)TLS Protocol Profile 644 This section defines the (D)TLS protocol profile of DNS-over-(D)TLS. 646 There are known attacks on (D)TLS, such as machine-in-the-middle and 647 protocol downgrade. These are general attacks on (D)TLS and not 648 specific to DNS-over-TLS; please refer to the (D)TLS RFCs for 649 discussion of these security issues. Clients and servers MUST adhere 650 to the (D)TLS implementation recommendations and security 651 considerations of [RFC7525] except with respect to (D)TLS version. 652 Since encryption of DNS using (D)TLS is virtually a green-field 653 deployment DNS clients and server MUST implement only (D)TLS 1.2 or 654 later. 656 Implementations MUST NOT offer or provide TLS compression, since 657 compression can leak significant amounts of information, especially 658 to a network observer capable of forcing the user to do an arbitrary 659 DNS lookup in the style of the CRIME attacks [CRIME]. 661 Implementations compliant with this profile MUST implement all of the 662 following items: 664 o TLS session resumption without server-side state [RFC5077] which 665 eliminates the need for the server to retain cryptographic state 666 for longer than necessary. 668 o Raw public keys [RFC7250] which reduce the size of the 669 ServerHello, and can be used by servers that cannot obtain 670 certificates (e.g., DNS servers on private networks). 672 Implementations compliant with this profile SHOULD implement all of 673 the following items: 675 o TLS False Start [RFC7918] which reduces round-trips by allowing 676 the TLS second flight of messages (ChangeCipherSpec) to also 677 contain the (encrypted) DNS query 679 o Cached Information Extension [RFC7924] which avoids transmitting 680 the server's certificate and certificate chain if the client has 681 cached that information from a previous TLS handshake 683 Guidance specific to TLS is provided in [RFC7858] and that specific 684 to DTLS it is provided in[I-D.ietf-dprive-dnsodtls]. 686 12. IANA Considerations 688 This memo includes no request to IANA. 690 13. Security Considerations 692 Security considerations discussed in [RFC7525], 693 [I-D.ietf-dprive-dnsodtls] and [RFC7858] apply to this document. 695 13.1. Counter-measures to DNS Traffic Analysis 697 This section makes suggestions for measures that can reduce the 698 ability of attackers to infer information pertaining to encrypted 699 client queries by other means (e.g. via an analysis of encrypted 700 traffic size, or via monitoring of resolver to authoritative 701 traffic). 703 DNS-over-(D)TLS clients and servers SHOULD consider implementing the 704 following relevant DNS extensions 706 o EDNS(0) padding [RFC7830], which allows encrypted queries and 707 responses to hide their size. 709 DNS-over-(D)TLS clients SHOULD consider implementing the following 710 relevant DNS extensions 712 o Privacy Election using Client Subnet in DNS Queries [RFC7871]. If 713 a DNS client does not include an EDNS0 Client Subnet Option with a 714 SOURCE PREFIX-LENGTH set to 0 in a query, the DNS server may 715 potentially leak client address information to the upstream 716 authoritative DNS servers. A DNS client ought to be able to 717 inform the DNS Resolver that it does not want any address 718 information leaked, and the DNS Resolver should honor that 719 request. 721 14. Acknowledgements 723 Thanks to the authors of both [I-D.ietf-dprive-dnsodtls] and 724 [RFC7858] for laying the ground work that this draft builds on and 725 for reviewing the contents. The authors would also like to thank 726 John Dickinson, Shumon Huque, Melinda Shore, Gowri Visweswaran, Ray 727 Bellis, Stephane Bortzmeyer, Jinmei Tatuya, Paul Hoffman and 728 Christian Huitema for review and discussion of the ideas presented 729 here. 731 15. References 733 15.1. Normative References 735 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 736 Requirement Levels", BCP 14, RFC 2119, 737 DOI 10.17487/RFC2119, March 1997, 738 . 740 [RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure 741 Subject Alternative Name for Expression of Service Name", 742 RFC 4985, DOI 10.17487/RFC4985, August 2007, 743 . 745 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 746 "Transport Layer Security (TLS) Session Resumption without 747 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 748 January 2008, . 750 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 751 (TLS) Protocol Version 1.2", RFC 5246, 752 DOI 10.17487/RFC5246, August 2008, 753 . 755 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 756 Housley, R., and W. Polk, "Internet X.509 Public Key 757 Infrastructure Certificate and Certificate Revocation List 758 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 759 . 761 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 762 Verification of Domain-Based Application Service Identity 763 within Internet Public Key Infrastructure Using X.509 764 (PKIX) Certificates in the Context of Transport Layer 765 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 766 2011, . 768 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 769 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 770 January 2012, . 772 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 773 of Named Entities (DANE) Transport Layer Security (TLS) 774 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 775 2012, . 777 [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., 778 Weiler, S., and T. Kivinen, "Using Raw Public Keys in 779 Transport Layer Security (TLS) and Datagram Transport 780 Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, 781 June 2014, . 783 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 784 "Recommendations for Secure Use of Transport Layer 785 Security (TLS) and Datagram Transport Layer Security 786 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 787 2015, . 789 [RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based 790 Authentication of Named Entities (DANE) Protocol: Updates 791 and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671, 792 October 2015, . 794 [RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830, 795 DOI 10.17487/RFC7830, May 2016, 796 . 798 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 799 and P. Hoffman, "Specification for DNS over Transport 800 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 801 2016, . 803 15.2. Informative References 805 [CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", 2012. 807 [dnssec-trigger] 808 NLnetLabs, "Dnssec-Trigger", May 2014, 809 . 811 [I-D.ietf-dprive-dnsodtls] 812 Reddy, T., Wing, D., and P. Patil, "Specification for DNS 813 over Datagram Transport Layer Security (DTLS)", draft- 814 ietf-dprive-dnsodtls-12 (work in progress), September 815 2016. 817 [I-D.ietf-tls-dnssec-chain-extension] 818 Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE 819 Record and DNSSEC Authentication Chain Extension for TLS", 820 draft-ietf-tls-dnssec-chain-extension-01 (work in 821 progress), July 2016. 823 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 824 RFC 2131, DOI 10.17487/RFC2131, March 1997, 825 . 827 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 828 Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, 829 . 831 [RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic 832 Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, 833 DOI 10.17487/RFC3646, December 2003, 834 . 836 [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection 837 Most of the Time", RFC 7435, DOI 10.17487/RFC7435, 838 December 2014, . 840 [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning 841 Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 842 2015, . 844 [RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626, 845 DOI 10.17487/RFC7626, August 2015, 846 . 848 [RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W. 849 Kumari, "Client Subnet in DNS Queries", RFC 7871, 850 DOI 10.17487/RFC7871, May 2016, 851 . 853 [RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport 854 Layer Security (TLS) False Start", RFC 7918, 855 DOI 10.17487/RFC7918, August 2016, 856 . 858 [RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security 859 (TLS) Cached Information Extension", RFC 7924, 860 DOI 10.17487/RFC7924, July 2016, 861 . 863 Appendix A. Server capability probing and caching by DNS clients 865 This section presents a non-normative discussion of how DNS clients 866 might probe for and cache privacy capabilities of DNS servers. 868 Deployment of both DNS-over-TLS and DNS-over-DTLS will be gradual. 869 Not all servers will support one or both of these protocols and the 870 well-known port might be blocked by some middleboxes. Clients will 871 be expected to keep track of servers that support DNS-over-TLS and/or 872 DNS-over-DTLS, and those that have been previously authenticated. 874 If no server capability information is available then (unless 875 otherwise specified by the configuration of the DNS client) DNS 876 clients that implement both TLS and DTLS should try to authenticate 877 using both protocols before failing or falling back to a lower 878 security. DNS clients using opportunistic security should try all 879 available servers (possibly in parallel) in order to obtain an 880 authenticated encrypted connection before falling back to a lower 881 security. (RATIONALE: This approach can increase latency while 882 discovering server capabilities but maximizes the chance of sending 883 the query over an authenticated encrypted connection.) 885 Appendix B. Changes between revisions 887 [Note to RFC Editor: please remove this section prior to 888 publication.] 890 B.1. -04 version 892 Introduction: Add comment that DNS-over-DTLS draft is Experiments 894 Update 2 I-D references to RFCs. 896 B.2. -03 version 898 Section 9: Update DANE section with better references to RFC7671 and 899 RFC7250 901 B.3. -02 version 903 Introduction: Added paragraph on the background and scope of the 904 document. 906 Introduction and Discussion: Added more information on what a Usage 907 profiles is (and is not) the the two presented here. 909 Introduction: Added paragraph to make a comparison with the Strict 910 profile in RFC7858 clearer. 912 Section 4.2: Re-worked the description of Opportunistic and the 913 table. 915 Section 8.3: Clarified statement about use of DHCP in Opportunistic 916 profile 918 Title abbreviated. 920 B.4. -01 version 922 Section 4.2: Make clear that the Strict Privacy Profile can include 923 meta queries performed using Opportunistic Privacy. 925 Section 4.2, Table 1: Update to clarify that Opportunistic Privacy 926 does not guarantee protection against passive attack. 928 Section 4.2: Add sentence discussing client/provider trusted 929 relationships. 931 Section 5: Add more discussion of detection of active attacks when 932 using Opportunistic Privacy. 934 Section 8.2: Clarify description and example. 936 B.5. draft-ietf-dprive-dtls-and-tls-profiles-00 938 Re-submission of draft-dgr-dprive-dtls-and-tls-profiles with name 939 change to draft-ietf-dprive-dtls-and-tls-profiles. Also minor nits 940 fixed. 942 Authors' Addresses 944 Sara Dickinson 945 Sinodun Internet Technologies 946 Magdalen Centre 947 Oxford Science Park 948 Oxford OX4 4GA 949 UK 951 Email: sara@sinodun.com 952 URI: http://sinodun.com 953 Daniel Kahn Gillmor 954 ACLU 955 125 Broad Street, 18th Floor 956 New York NY 10004 957 USA 959 Email: dkg@fifthhorseman.net 961 Tirumaleswar Reddy 962 Cisco Systems, Inc. 963 Cessna Business Park, Varthur Hobli 964 Sarjapur Marathalli Outer Ring Road 965 Bangalore, Karnataka 560103 966 India 968 Email: tireddy@cisco.com