idnits 2.17.1 draft-ietf-dane-smtp-with-dane-12.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 : ---------------------------------------------------------------------------- -- The document has examples using IPv4 documentation addresses according to RFC6890, but does not use any IPv6 documentation addresses. Maybe there should be IPv6 examples, too? Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (August 17, 2014) is 3539 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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-16) exists of draft-ietf-dane-ops-06 ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) ** Obsolete normative reference: RFC 6125 (Obsoleted by RFC 9525) == Outdated reference: A later version (-06) exists of draft-dukhovni-opportunistic-security-03 == Outdated reference: A later version (-14) exists of draft-ietf-dane-srv-07 Summary: 2 errors (**), 0 flaws (~~), 4 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DANE V. Dukhovni 3 Internet-Draft Two Sigma 4 Intended status: Standards Track W. Hardaker 5 Expires: February 18, 2015 Parsons 6 August 17, 2014 8 SMTP security via opportunistic DANE TLS 9 draft-ietf-dane-smtp-with-dane-12 11 Abstract 13 This memo describes a downgrade-resistant protocol for SMTP transport 14 security between Mail Transfer Agents (MTAs) based on the DNS-Based 15 Authentication of Named Entities (DANE) TLSA DNS record. Adoption of 16 this protocol enables an incremental transition of the Internet email 17 backbone to one using encrypted and authenticated Transport Layer 18 Security (TLS). 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on February 18, 2015. 37 Copyright Notice 39 Copyright (c) 2014 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 55 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 56 1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 5 57 1.3. SMTP channel security . . . . . . . . . . . . . . . . . . 6 58 1.3.1. STARTTLS downgrade attack . . . . . . . . . . . . . . 6 59 1.3.2. Insecure server name without DNSSEC . . . . . . . . . 7 60 1.3.3. Sender policy does not scale . . . . . . . . . . . . 8 61 1.3.4. Too many certification authorities . . . . . . . . . 8 62 2. Identifying applicable TLSA records . . . . . . . . . . . . . 9 63 2.1. DNS considerations . . . . . . . . . . . . . . . . . . . 9 64 2.1.1. DNS errors, bogus and indeterminate responses . . . . 9 65 2.1.2. DNS error handling . . . . . . . . . . . . . . . . . 11 66 2.1.3. Stub resolver considerations . . . . . . . . . . . . 12 67 2.2. TLS discovery . . . . . . . . . . . . . . . . . . . . . . 13 68 2.2.1. MX resolution . . . . . . . . . . . . . . . . . . . . 14 69 2.2.2. Non-MX destinations . . . . . . . . . . . . . . . . . 15 70 2.2.3. TLSA record lookup . . . . . . . . . . . . . . . . . 17 71 3. DANE authentication . . . . . . . . . . . . . . . . . . . . . 19 72 3.1. TLSA certificate usages . . . . . . . . . . . . . . . . . 19 73 3.1.1. Certificate usage DANE-EE(3) . . . . . . . . . . . . 21 74 3.1.2. Certificate usage DANE-TA(2) . . . . . . . . . . . . 22 75 3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1) . . . . 23 76 3.2. Certificate matching . . . . . . . . . . . . . . . . . . 24 77 3.2.1. DANE-EE(3) name checks . . . . . . . . . . . . . . . 24 78 3.2.2. DANE-TA(2) name checks . . . . . . . . . . . . . . . 24 79 3.2.3. Reference identifier matching . . . . . . . . . . . . 25 80 4. Server key management . . . . . . . . . . . . . . . . . . . . 26 81 5. Digest algorithm agility . . . . . . . . . . . . . . . . . . 26 82 6. Mandatory TLS Security . . . . . . . . . . . . . . . . . . . 27 83 7. Note on DANE for Message User Agents . . . . . . . . . . . . 27 84 8. Interoperability considerations . . . . . . . . . . . . . . . 28 85 8.1. SNI support . . . . . . . . . . . . . . . . . . . . . . . 28 86 8.2. Anonymous TLS cipher suites . . . . . . . . . . . . . . . 28 87 9. Operational Considerations . . . . . . . . . . . . . . . . . 29 88 9.1. Client Operational Considerations . . . . . . . . . . . . 29 89 9.2. Publisher Operational Considerations . . . . . . . . . . 30 90 10. Security Considerations . . . . . . . . . . . . . . . . . . . 30 91 11. IANA considerations . . . . . . . . . . . . . . . . . . . . . 31 92 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31 93 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 94 13.1. Normative References . . . . . . . . . . . . . . . . . . 31 95 13.2. Informative References . . . . . . . . . . . . . . . . . 32 96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 98 1. Introduction 100 This memo specifies a new connection security model for Message 101 Transfer Agents (MTAs). This model is motivated by key features of 102 inter-domain SMTP delivery, in particular the fact that the 103 destination server is selected indirectly via DNS Mail Exchange (MX) 104 records and that neither email addresses nor MX hostnames signal a 105 requirement for either secure or cleartext transport. Therefore, 106 aside from a few manually configured exceptions, SMTP transport 107 security is of necessity opportunistic. 109 This specification uses the presence of DANE TLSA records to securely 110 signal TLS support and to publish the means by which SMTP clients can 111 successfully authenticate legitimate SMTP servers. This becomes 112 "opportunistic DANE TLS" and is resistant to downgrade and man-in- 113 the-middle (MITM) attacks. It enables an incremental transition of 114 the email backbone to authenticated TLS delivery, with increased 115 global protection as adoption increases. 117 With opportunistic DANE TLS, traffic from SMTP clients to domains 118 that publish "usable" DANE TLSA records in accordance with this memo 119 is authenticated and encrypted. Traffic from legacy clients or to 120 domains that do not publish TLSA records will continue to be sent in 121 the same manner as before, via manually configured security, (pre- 122 DANE) opportunistic TLS or just cleartext SMTP. 124 Problems with existing use of TLS in MTA to MTA SMTP that motivate 125 this specification are described in Section 1.3. The specification 126 itself follows in Section 2 and Section 3 which describe respectively 127 how to locate and use DANE TLSA records with SMTP. In Section 6, we 128 discuss application of DANE TLS to destinations for which channel 129 integrity and confidentiality are mandatory. In Section 7 we briefly 130 comment on potential applicability of this specification to Message 131 User Agents. 133 1.1. Terminology 135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 137 "OPTIONAL" in this document are to be interpreted as described in 138 [RFC2119]. 140 The following terms or concepts are used through the document: 142 Man-in-the-middle or MITM attack: Active modification of network 143 traffic by an adversary able to thereby compromise the 144 confidentiality or integrity of the data. 146 secure, bogus, insecure, indeterminate: DNSSEC validation results, 147 as defined in Section 4.3 of [RFC4035]. 149 Validating Security-Aware Stub Resolver and Non-Validating 150 Security-Aware Stub Resolver: 151 Capabilities of the stub resolver in use as defined in [RFC4033]; 152 note that this specification requires the use of a Security-Aware 153 Stub Resolver. 155 (pre-DANE) opportunistic TLS: Best-effort use of TLS that is 156 generally vulnerable to DNS forgery and STARTTLS downgrade 157 attacks. When a TLS-encrypted communication channel is not 158 available, message transmission takes place in the clear. MX 159 record indirection generally precludes authentication even when 160 TLS is available. 162 opportunistic DANE TLS: Best-effort use of TLS, resistant to 163 downgrade attacks for destinations with DNSSEC-validated TLSA 164 records. When opportunistic DANE TLS is determined to be 165 unavailable, clients should fall back to opportunistic TLS. 166 Opportunistic DANE TLS requires support for DNSSEC, DANE and 167 STARTTLS on the client side and STARTTLS plus a DNSSEC published 168 TLSA record on the server side. 170 reference identifier: (Special case of [RFC6125] definition). One 171 of the domain names associated by the SMTP client with the 172 destination SMTP server for performing name checks on the server 173 certificate. When name checks are applicable, at least one of the 174 reference identifiers MUST match an [RFC6125] DNS-ID (or if none 175 are present the [RFC6125] CN-ID) of the server certificate (see 176 Section 3.2.3). 178 MX hostname: The RRDATA of an MX record consists of a 16 bit 179 preference followed by a Mail Exchange domain name (see [RFC1035], 180 Section 3.3.9). We will use the term "MX hostname" to refer to 181 the latter, that is, the DNS domain name found after the 182 preference value in an MX record. Thus an "MX hostname" is 183 specifically a reference to a DNS domain name, rather than any 184 host that bears that name. 186 delayed delivery: Email delivery is a multi-hop store & forward 187 process. When an MTA is unable forward a message that may become 188 deliverable later the message is queued and delivery is retried 189 periodically. Some MTAs may be configured with a fallback next- 190 hop destination that handles messages that the MTA would otherwise 191 queue and retry. When a fallback next-hop is configured, messages 192 that would otherwise have to be delayed may be sent to the 193 fallback next-hop destination instead. The fallback destination 194 may itself be subject to opportunistic or mandatory DANE TLS as 195 though it were the original message destination. 197 original next hop destination: The logical destination for mail 198 delivery. By default this is the domain portion of the recipient 199 address, but MTAs may be configured to forward mail for some or 200 all recipients via designated relays. The original next hop 201 destination is, respectively, either the recipient domain or the 202 associated configured relay. 204 MTA: Message Transfer Agent ([RFC5598], Section 4.3.2). 206 MSA: Message Submission Agent ([RFC5598], Section 4.3.1). 208 MUA: Message User Agent ([RFC5598], Section 4.2.1). 210 RR: A DNS Resource Record 212 RRset: A set of DNS Resource Records for a particular class, domain 213 and record type. 215 1.2. Background 217 The Domain Name System Security Extensions (DNSSEC) add data origin 218 authentication, data integrity and data non-existence proofs to the 219 Domain Name System (DNS). DNSSEC is defined in [RFC4033], [RFC4034] 220 and [RFC4035]. 222 As described in the introduction of [RFC6698], TLS authentication via 223 the existing public Certification Authority (CA) PKI suffers from an 224 over-abundance of trusted parties capable of issuing certificates for 225 any domain of their choice. DANE leverages the DNSSEC infrastructure 226 to publish trusted public keys and certificates for use with the 227 Transport Layer Security (TLS) [RFC5246] protocol via a new "TLSA" 228 DNS record type. With DNSSEC each domain can only vouch for the keys 229 of its directly delegated sub-domains. 231 The TLS protocol enables secure TCP communication. In the context of 232 this memo, channel security is assumed to be provided by TLS. Used 233 without authentication, TLS provides only privacy protection against 234 eavesdropping attacks. With authentication, TLS also provides data 235 integrity protection to guard against MITM attacks. 237 1.3. SMTP channel security 239 With HTTPS, Transport Layer Security (TLS) employs X.509 certificates 240 [RFC5280] issued by one of the many Certificate Authorities (CAs) 241 bundled with popular web browsers to allow users to authenticate 242 their "secure" websites. Before we specify a new DANE TLS security 243 model for SMTP, we will explain why a new security model is needed. 244 In the process, we will explain why the familiar HTTPS security model 245 is inadequate to protect inter-domain SMTP traffic. 247 The subsections below outline four key problems with applying 248 traditional PKI to SMTP that are addressed by this specification. 249 Since SMTP channel security policy is not explicitly specified in 250 either the recipient address or the MX record, a new signaling 251 mechanism is required to indicate when channel security is possible 252 and should be used. The publication of TLSA records allows server 253 operators to securely signal to SMTP clients that TLS is available 254 and should be used. DANE TLSA makes it possible to simultaneously 255 discover which destination domains support secure delivery via TLS 256 and how to verify the authenticity of the associated SMTP services, 257 providing a path forward to ubiquitous SMTP channel security. 259 1.3.1. STARTTLS downgrade attack 261 The Simple Mail Transfer Protocol (SMTP) [RFC5321] is a single-hop 262 protocol in a multi-hop store & forward email delivery process. An 263 SMTP envelope recipient address does not correspond to a specific 264 transport-layer endpoint address, rather at each relay hop the 265 transport-layer endpoint is the next-hop relay, while the envelope 266 recipient address typically remains the same. Unlike the Hypertext 267 Transfer Protocol (HTTP) and its corresponding secured version, 268 HTTPS, where the use of TLS is signaled via the URI scheme, email 269 recipient addresses do not directly signal transport security policy. 270 Indeed, no such signaling could work well with SMTP since TLS 271 encryption of SMTP protects email traffic on a hop-by-hop basis while 272 email addresses could only express end-to-end policy. 274 With no mechanism available to signal transport security policy, SMTP 275 relays employ a best-effort "opportunistic" security model for TLS. 276 A single SMTP server TCP listening endpoint can serve both TLS and 277 non-TLS clients; the use of TLS is negotiated via the SMTP STARTTLS 278 command ([RFC3207]). The server signals TLS support to the client 279 over a cleartext SMTP connection, and, if the client also supports 280 TLS, it may negotiate a TLS encrypted channel to use for email 281 transmission. The server's indication of TLS support can be easily 282 suppressed by an MITM attacker. Thus pre-DANE SMTP TLS security can 283 be subverted by simply downgrading a connection to cleartext. No TLS 284 security feature, such as the use of PKIX, can prevent this. The 285 attacker can simply disable TLS. 287 1.3.2. Insecure server name without DNSSEC 289 With SMTP, DNS Mail Exchange (MX) records abstract the next-hop 290 transport endpoint and allow administrators to specify a set of 291 target servers to which SMTP traffic should be directed for a given 292 domain. 294 A PKIX TLS client is vulnerable to MITM attacks unless it verifies 295 that the server's certificate binds the public key to a name that 296 matches one of the client's reference identifiers. A natural choice 297 of reference identifier is the server's domain name. However, with 298 SMTP, server names are not directly encoded in the recipient address, 299 instead they are obtained indirectly via MX records. Without DNSSEC, 300 the MX lookup is vulnerable to MITM and DNS cache poisoning attacks. 301 Active attackers can forge DNS replies with fake MX records and can 302 redirect email to servers with names of their choice. Therefore, 303 secure verification of SMTP TLS certificates matching the server name 304 is not possible without DNSSEC. 306 One might try to harden TLS for SMTP against DNS attacks by using the 307 envelope recipient domain as a reference identifier and requiring 308 each SMTP server to possess a trusted certificate for the envelope 309 recipient domain rather than the MX hostname. Unfortunately, this is 310 impractical as email for many domains is handled by third parties 311 that are not in a position to obtain certificates for all the domains 312 they serve. Deployment of the Server Name Indication (SNI) extension 313 to TLS (see [RFC6066] Section 3) is no panacea, since SNI key 314 management is operationally challenging except when the email service 315 provider is also the domain's registrar and its certificate issuer; 316 this is rarely the case for email. 318 Since the recipient domain name cannot be used as the SMTP server 319 reference identifier, and neither can the MX hostname without DNSSEC, 320 large-scale deployment of authenticated TLS for SMTP requires that 321 the DNS be secure. 323 Since SMTP security depends critically on DNSSEC, it is important to 324 point out that consequently SMTP with DANE is the most conservative 325 possible trust model. It trusts only what must be trusted and no 326 more. Adding any other trusted actors to the mix can only reduce 327 SMTP security. A sender may choose to further harden DNSSEC for 328 selected high-value receiving domains by configuring explicit trust 329 anchors for those domains instead of relying on the chain of trust 330 from the root domain. However, detailed discussion of DNSSEC 331 security practices is out of scope for this document. 333 1.3.3. Sender policy does not scale 335 Sending systems are in some cases explicitly configured to use TLS 336 for mail sent to selected peer domains. This requires sending MTAs 337 to be configured with appropriate subject names or certificate 338 content digests to expect in the presented server certificates. 339 Because of the heavy administrative burden, such statically 340 configured SMTP secure channels are used rarely (generally only 341 between domains that make bilateral arrangements with their business 342 partners). Internet email, on the other hand, requires regularly 343 contacting new domains for which security configurations cannot be 344 established in advance. 346 The abstraction of the SMTP transport endpoint via DNS MX records, 347 often across organization boundaries, limits the use of public CA PKI 348 with SMTP to a small set of sender-configured peer domains. With 349 little opportunity to use TLS authentication, sending MTAs are rarely 350 configured with a comprehensive list of trusted CAs. SMTP services 351 that support STARTTLS often deploy X.509 certificates that are self- 352 signed or issued by a private CA. 354 1.3.4. Too many certification authorities 356 Even if it were generally possible to determine a secure server name, 357 the SMTP client would still need to verify that the server's 358 certificate chain is issued by a trusted Certification Authority (a 359 trust anchor). MTAs are not interactive applications where a human 360 operator can make a decision (wisely or otherwise) to selectively 361 disable TLS security policy when certificate chain verification 362 fails. With no user to "click OK", the MTA's list of public CA trust 363 anchors would need to be comprehensive in order to avoid bouncing 364 mail addressed to sites that employ unknown Certification 365 Authorities. 367 On the other hand, each trusted CA can issue certificates for any 368 domain. If even one of the configured CAs is compromised or operated 369 by an adversary, it can subvert TLS security for all destinations. 370 Any set of CAs is simultaneously both overly inclusive and not 371 inclusive enough. 373 2. Identifying applicable TLSA records 375 2.1. DNS considerations 377 2.1.1. DNS errors, bogus and indeterminate responses 379 An SMTP client that implements opportunistic DANE TLS per this 380 specification depends critically on the integrity of DNSSEC lookups, 381 as discussed in Section 1.3.2. This section lists the DNS resolver 382 requirements needed to avoid downgrade attacks when using 383 opportunistic DANE TLS. 385 A DNS lookup may signal an error or return a definitive answer. A 386 security-aware resolver must be used for this specification. 387 Security-aware resolvers will indicate the security status of a DNS 388 RRset with one of four possible values defined in Section 4.3 of 389 [RFC4035]: "secure", "insecure", "bogus" and "indeterminate". In 390 [RFC4035] the meaning of the "indeterminate" security status is: 392 An RRset for which the resolver is not able to determine whether 393 the RRset should be signed, as the resolver is not able to obtain 394 the necessary DNSSEC RRs. This can occur when the security-aware 395 resolver is not able to contact security-aware name servers for 396 the relevant zones. 398 Note, the "indeterminate" security status has a conflicting 399 definition in section 5 of [RFC4033]. 401 There is no trust anchor that would indicate that a specific 402 portion of the tree is secure. 404 To avoid further confusion, the adjective "anchorless" will be used 405 below to refer to domains or RRsets that are "indeterminate" in the 406 [RFC4033] sense, and the term "indeterminate" will be used 407 exclusively in the sense of [RFC4035]. 409 SMTP clients following this specification SHOULD NOT distinguish 410 between "insecure" and "anchorless" DNS responses. Both "insecure" 411 and "anchorless" RRsets MUST be handled identically: in either case 412 unvalidated data for the query domain is all that is and can be 413 available, and authentication using the data is impossible. In what 414 follows, the term "insecure" will also include the case of 415 "anchorless" domains that lie in a portion of the DNS tree for which 416 there is no applicable trust anchor. With the DNS root zone signed, 417 we expect that validating resolvers used by Internet-facing MTAs will 418 be configured with trust anchor data for the root zone, and that 419 therefore "anchorless" domains should be rare in practice. 421 As noted in section 4.3 of [RFC4035], a security-aware DNS resolver 422 MUST be able to determine whether a given non-error DNS response is 423 "secure", "insecure", "bogus" or "indeterminate". It is expected 424 that most security-aware stub resolvers will not signal an 425 "indeterminate" security status (in the sense of RFC4035) to the 426 application, and will signal a "bogus" or error result instead. If a 427 resolver does signal an RFC4035 "indeterminate" security status, this 428 MUST be treated by the SMTP client as though a "bogus" or error 429 result had been returned. 431 An MTA making use of a non-validating security-aware stub resolver 432 MAY use the stub resolver's ability, if available, to signal DNSSEC 433 validation status based on information the stub resolver has learned 434 from an upstream validating recursive resolver. Security-Oblivious 435 stub-resolvers MUST NOT be used. In accordance with section 4.9.3 of 436 [RFC4035]: 438 ... a security-aware stub resolver MUST NOT place any reliance on 439 signature validation allegedly performed on its behalf, except 440 when the security-aware stub resolver obtained the data in question 441 from a trusted security-aware recursive name server via a secure 442 channel. 444 To avoid much repetition in the text below, we will pause to explain 445 the handling of "bogus" or "indeterminate" DNSSEC query responses. 446 These are not necessarily the result of a malicious actor; they can, 447 for example, occur when network packets are corrupted or lost in 448 transit. Therefore, "bogus" or "indeterminate" replies are equated 449 in this memo with lookup failure. 451 There is an important non-failure condition we need to highlight in 452 addition to the obvious case of the DNS client obtaining a non-empty 453 "secure" or "insecure" RRset of the requested type. Namely, it is 454 not an error when either "secure" or "insecure" non-existence is 455 determined for the requested data. When a DNSSEC response with a 456 validation status that is either "secure" or "insecure" reports 457 either no records of the requested type or non-existence of the query 458 domain, the response is not a DNS error condition. The DNS client 459 has not been left without an answer; it has learned that records of 460 the requested type do not exist. 462 Security-aware stub resolvers will, of course, also signal DNS lookup 463 errors in other cases, for example when processing a "ServFail" 464 RCODE, which will not have an associated DNSSEC status. All lookup 465 errors are treated the same way by this specification, regardless of 466 whether they are from a "bogus" or "indeterminate" DNSSEC status or 467 from a more generic DNS error: the information that was requested 468 cannot be obtained by the security-aware resolver at this time. A 469 lookup error is thus a failure to obtain the relevant RRset if it 470 exists, or to determine that no such RRset exists when it does not. 472 In contrast to a "bogus" or an "indeterminate" response, an 473 "insecure" DNSSEC response is not an error, rather it indicates that 474 the target DNS zone is either securely opted out of DNSSEC validation 475 or is not connected with the DNSSEC trust anchors being used. 476 Insecure results will leave the SMTP client with degraded channel 477 security, but do not stand in the way of message delivery. See 478 section Section 2.2 for further details. 480 2.1.2. DNS error handling 482 When a DNS lookup failure (error or "bogus" or "indeterminate" as 483 defined above) prevents an SMTP client from determining which SMTP 484 server or servers it should connect to, message delivery MUST be 485 delayed. This naturally includes, for example, the case when a 486 "bogus" or "indeterminate" response is encountered during MX 487 resolution. When multiple MX hostnames are obtained from a 488 successful MX lookup, but a later DNS lookup failure prevents network 489 address resolution for a given MX hostname, delivery may proceed via 490 any remaining MX hosts. 492 When a particular SMTP server is securely identified as the delivery 493 destination, a set of DNS lookups (Section 2.2) MUST be performed to 494 locate any related TLSA records. If any DNS queries used to locate 495 TLSA records fail (be it due to "bogus" or "indeterminate" records, 496 timeouts, malformed replies, ServFails, etc.), then the SMTP client 497 MUST treat that server as unreachable and MUST NOT deliver the 498 message via that server. If no servers are reachable, delivery is 499 delayed. 501 In what follows, we will only describe what happens when all relevant 502 DNS queries succeed. If any DNS failure occurs, the SMTP client MUST 503 behave as described in this section, by skipping the problem SMTP 504 server, or the problem destination. Queries for candidate TLSA 505 records are explicitly part of "all relevant DNS queries" and SMTP 506 clients MUST NOT continue to connect to an SMTP server or destination 507 whose TLSA record lookup fails. 509 2.1.3. Stub resolver considerations 511 SMTP clients that employ opportunistic DANE TLS to secure connections 512 to SMTP servers MUST NOT use Security-Oblivious stub-resolvers. 514 A note about DNAME aliases: a query for a domain name whose ancestor 515 domain is a DNAME alias returns the DNAME RR for the ancestor domain 516 along with a CNAME that maps the query domain to the corresponding 517 sub-domain of the target domain of the DNAME alias [RFC6672]. 518 Therefore, whenever we speak of CNAME aliases, we implicitly allow 519 for the possibility that the alias in question is the result of an 520 ancestor domain DNAME record. Consequently, no explicit support for 521 DNAME records is needed in SMTP software; it is sufficient to process 522 the resulting CNAME aliases. DNAME records only require special 523 processing in the validating stub-resolver library that checks the 524 integrity of the combined DNAME + CNAME reply. When DNSSEC 525 validation is handled by a local caching resolver, rather than the 526 MTA itself, even that part of the DNAME support logic is outside the 527 MTA. 529 When a stub resolver returns a response containing a CNAME alias that 530 does not also contain the corresponding query results for the target 531 of the alias, the SMTP client will need to repeat the query at the 532 target of the alias, and should do so recursively up to some 533 configured or implementation-dependent recursion limit. If at any 534 stage of CNAME expansion an error is detected, the lookup of the 535 original requested records MUST be considered to have failed. 537 Whether a chain of CNAME records was returned in a single stub 538 resolver response or via explicit recursion by the SMTP client, if at 539 any stage of recursive expansion an "insecure" CNAME record is 540 encountered, then it and all subsequent results (in particular, the 541 final result) MUST be considered "insecure" regardless of whether any 542 earlier CNAME records leading to the "insecure" record were "secure". 544 Note that a security-aware non-validating stub resolver may return to 545 the SMTP client an "insecure" reply received from a validating 546 recursive resolver that contains a CNAME record along with additional 547 answers recursively obtained starting at the target of the CNAME. In 548 this case, the only possible conclusion is that some record in the 549 set of records returned is "insecure", and it is in fact possible 550 that the initial CNAME record and a subset of the subsequent records 551 are "secure". 553 If the SMTP client needs to determine the security status of the DNS 554 zone containing the initial CNAME record, it may need to issue a 555 separate query of type "CNAME" that returns only the initial CNAME 556 record. In particular in Section 2.2.2 when insecure A or AAAA 557 records are found for an SMTP server via a CNAME alias, it may be 558 necessary to perform an additional CNAME query to determine whether 559 the DNS zone in which the alias is published is signed. 561 2.2. TLS discovery 563 As noted previously (in Section 1.3.1), opportunistic TLS with SMTP 564 servers that advertise TLS support via STARTTLS is subject to an MITM 565 downgrade attack. Also some SMTP servers that are not, in fact, TLS 566 capable erroneously advertise STARTTLS by default and clients need to 567 be prepared to retry cleartext delivery after STARTTLS fails. In 568 contrast, DNSSEC validated TLSA records MUST NOT be published for 569 servers that do not support TLS. Clients can safely interpret their 570 presence as a commitment by the server operator to implement TLS and 571 STARTTLS. 573 This memo defines four actions to be taken after the search for a 574 TLSA record returns secure usable results, secure unusable results, 575 insecure or no results or an error signal. The term "usable" in this 576 context is in the sense of Section 4.1 of [RFC6698]. Specifically, 577 if the DNS lookup for a TLSA record returns: 579 A secure TLSA RRset with at least one usable record: A connection to 580 the MTA MUST be made using authenticated and encrypted TLS, using 581 the techniques discussed in the rest of this document. Failure to 582 establish an authenticated TLS connection MUST result in falling 583 back to the next SMTP server or delayed delivery. 585 A secure non-empty TLSA RRset where all the records are unusable: A 586 connection to the MTA MUST be made via TLS, but authentication is 587 not required. Failure to establish an encrypted TLS connection 588 MUST result in falling back to the next SMTP server or delayed 589 delivery. 591 An insecure TLSA RRset or DNSSEC validated proof-of-non-existent TLSA 592 records: 593 A connection to the MTA SHOULD be made using (pre-DANE) 594 opportunistic TLS, this includes using cleartext delivery when the 595 remote SMTP server does not appear to support TLS. The MTA MAY 596 retry in cleartext when delivery via TLS fails either during the 597 handshake or even during data transfer. 599 Any lookup error: Lookup errors, including "bogus" and 600 "indeterminate", as explained in Section 2.1.1 MUST result in 601 falling back to the next SMTP server or delayed delivery. 603 An SMTP client MAY be configured to require DANE verified delivery 604 for some destinations. We will call such a configuration "mandatory 605 DANE TLS". With mandatory DANE TLS, delivery proceeds only when 606 "secure" TLSA records are used to establish an encrypted and 607 authenticated TLS channel with the SMTP server. 609 When the original next-hop destination is an address literal, rather 610 than a DNS domain, DANE TLS does not apply. Delivery proceeds using 611 any relevant security policy configured by the MTA administrator. 612 Similarly, when an MX RRset incorrectly lists a network address in 613 lieu of an MX hostname, if an MTA chooses to connect to the network 614 address in the non-conformant MX record, DANE TLSA does not apply for 615 such a connection. 617 In the subsections that follow we explain how to locate the SMTP 618 servers and the associated TLSA records for a given next-hop 619 destination domain. We also explain which name or names are to be 620 used in identity checks of the SMTP server certificate. 622 2.2.1. MX resolution 624 In this section we consider next-hop domains that are subject to MX 625 resolution and have MX records. The TLSA records and the associated 626 base domain are derived separately for each MX hostname that is used 627 to attempt message delivery. DANE TLS can authenticate message 628 delivery to the intended next-hop domain only when the MX records are 629 obtained securely via a DNSSEC validated lookup. 631 MX records MUST be sorted by preference; an MX hostname with a worse 632 (numerically higher) MX preference that has TLSA records MUST NOT 633 preempt an MX hostname with a better (numerically lower) preference 634 that has no TLSA records. In other words, prevention of delivery 635 loops by obeying MX preferences MUST take precedence over channel 636 security considerations. Even with two equal-preference MX records, 637 an MTA is not obligated to choose the MX hostname that offers more 638 security. Domains that want secure inbound mail delivery need to 639 ensure that all their SMTP servers and MX records are configured 640 accordingly. 642 In the language of [RFC5321] Section 5.1, the original next-hop 643 domain is the "initial name". If the MX lookup of the initial name 644 results in a CNAME alias, the MTA replaces the initial name with the 645 resulting name and performs a new lookup with the new name. MTAs 646 typically support recursion in CNAME expansion, so this replacement 647 is performed repeatedly (up to the MTA's recursion limit) until the 648 ultimate non-CNAME domain is found. 650 If the MX RRset (or any CNAME leading to it) is "insecure" (see 651 Section 2.1.1), DANE TLS need not apply, and delivery MAY proceed via 652 pre-DANE opportunistic TLS. That said, the protocol in this memo is 653 an "opportunistic security" protocol, meaning that it strives to 654 communicate with each peer as securely as possible, while maintaining 655 broad interoperability. Therefore, the SMTP client MAY proceed to 656 use DANE TLS (as described in Section 2.2.2 below) even with MX hosts 657 obtained via an "insecure" MX RRset. For example, when a hosting 658 provider has a signed DNS zone and publishes TLSA records for its 659 SMTP servers, hosted domains that are not signed may still benefit 660 from the provider's TLSA records. Deliveries via the provider's SMTP 661 servers will not be subject to active attacks when sending SMTP 662 clients elect to make use of the provider's TLSA records. 664 When the MX records are not (DNSSEC) signed, an active attacker can 665 redirect SMTP clients to MX hosts of his choice. Such redirection is 666 tamper-evident when SMTP servers found via "insecure" MX records are 667 recorded as the next-hop relay in the MTA delivery logs in their 668 original (rather than CNAME expanded) form. Sending MTAs SHOULD log 669 unexpanded MX hostnames when these result from insecure MX lookups. 670 Any successful authentication via an insecurely determined MX host 671 MUST NOT be misrepresented in the mail logs as secure delivery to the 672 intended next-hop domain. When DANE TLS is mandatory (Section 6) for 673 a given destination, delivery MUST be delayed when the MX RRset is 674 not "secure". 676 Otherwise, assuming no DNS errors (Section 2.1.1), the MX RRset is 677 "secure", and the SMTP client MUST treat each MX hostname as a 678 separate non-MX destination for opportunistic DANE TLS as described 679 in Section 2.2.2. When, for a given MX hostname, no TLSA records are 680 found, or only "insecure" TLSA records are found, DANE TLSA is not 681 applicable with the SMTP server in question and delivery proceeds to 682 that host as with pre-DANE opportunistic TLS. To avoid downgrade 683 attacks, any errors during TLSA lookups MUST, as explained in 684 Section 2.1.1, cause the SMTP server in question to be treated as 685 unreachable. 687 2.2.2. Non-MX destinations 689 This section describes the algorithm used to locate the TLSA records 690 and associated TLSA base domain for an input domain not subject to MX 691 resolution. Such domains include: 693 o Each MX hostname used in a message delivery attempt for an 694 original next-hop destination domain subject to MX resolution. 695 Note, MTAs are not obligated to support CNAME expansion of MX 696 hostnames. 698 o Any administrator configured relay hostname, not subject to MX 699 resolution. This frequently involves configuration set by the MTA 700 administrator to handle some or all mail. 702 o A next-hop destination domain subject to MX resolution that has no 703 MX records. In this case the domain's name is implicitly also its 704 sole SMTP server name. 706 Note that DNS queries with type TLSA are mishandled by load balancing 707 nameservers that serve the MX hostnames of some large email 708 providers. The DNS zones served by these nameservers are not signed 709 and contain no TLSA records, but queries for TLSA records fail, 710 rather than returning the non-existence of the requested TLSA 711 records. 713 To avoid problems delivering mail to domains whose SMTP servers are 714 served by the problem nameservers the SMTP client MUST perform any A 715 and/or AAAA queries for the destination before attempting to locate 716 the associated TLSA records. This lookup is needed in any case to 717 determine whether the destination domain is reachable and the DNSSEC 718 validation status of the chain of CNAME queries required to reach the 719 ultimate address records. 721 If no address records are found, the destination is unreachable. If 722 address records are found, but the DNSSEC validation status of the 723 first query response is "insecure" (see Section 2.1.3), the SMTP 724 client SHOULD NOT proceed to search for any associated TLSA records. 725 With the problem domains, TLSA queries will lead to DNS lookup errors 726 and cause messages to be consistently delayed and ultimately returned 727 to the sender. We don't expect to find any "secure" TLSA records 728 associated with a TLSA base domain that lies in an unsigned DNS zone. 729 Therefore, skipping TLSA lookups in this case will also reduce 730 latency with no detrimental impact on security. 732 If the A and/or AAAA lookup of the "initial name" yields a CNAME, we 733 replace it with the resulting name as if it were the initial name and 734 perform a lookup again using the new name. This replacement is 735 performed recursively (up to the MTA's recursion limit). 737 We consider the following cases for handling a DNS response for an A 738 or AAAA DNS lookup: 740 Not found: When the DNS queries for A and/or AAAA records yield 741 neither a list of addresses nor a CNAME (or CNAME expansion is not 742 supported) the destination is unreachable. 744 Non-CNAME: The answer is not a CNAME alias. If the address RRset 745 is "secure", TLSA lookups are performed as described in 746 Section 2.2.3 with the initial name as the candidate TLSA base 747 domain. If no "secure" TLSA records are found, DANE TLS is not 748 applicable and mail delivery proceeds with pre-DANE opportunistic 749 TLS (which, being best-effort, degrades to cleartext delivery when 750 STARTTLS is not available or the TLS handshake fails). 752 Insecure CNAME: The input domain is a CNAME alias, but the ultimate 753 network address RRset is "insecure" (see Section 2.1.1). If the 754 initial CNAME response is also "insecure", DANE TLS does not 755 apply. Otherwise, this case is treated just like the non-CNAME 756 case above, where a search is performed for a TLSA record with the 757 original input domain as the candidate TLSA base domain. 759 Secure CNAME: The input domain is a CNAME alias, and the ultimate 760 network address RRset is "secure" (see Section 2.1.1). Two 761 candidate TLSA base domains are tried: the fully CNAME-expanded 762 initial name and, failing that, then the initial name itself. 764 In summary, if it is possible to securely obtain the full, CNAME- 765 expanded, DNSSEC-validated address records for the input domain, then 766 that name is the preferred TLSA base domain. Otherwise, the 767 unexpanded input-MX domain is the candidate TLSA base domain. When 768 no "secure" TLSA records are found at either the CNAME-expanded or 769 unexpanded domain, then DANE TLS does not apply for mail delivery via 770 the input domain in question. And, as always, errors, bogus or 771 indeterminate results for any query in the process MUST result in 772 delaying or abandoning delivery. 774 2.2.3. TLSA record lookup 776 Each candidate TLSA base domain (the original or fully CNAME-expanded 777 name of a non-MX destination or a particular MX hostname of an MX 778 destination) is in turn prefixed with service labels of the form 779 "_._tcp". The resulting domain name is used to issue a DNSSEC 780 query with the query type set to TLSA ([RFC6698] Section 7.1). 782 For SMTP, the destination TCP port is typically 25, but this may be 783 different with custom routes specified by the MTA administrator in 784 which case the SMTP client MUST use the appropriate number in the 785 "_" prefix in place of "_25". If, for example, the candidate 786 base domain is "mx.example.com", and the SMTP connection is to port 787 25, the TLSA RRset is obtained via a DNSSEC query of the form: 789 _25._tcp.mx.example.com. IN TLSA ? 790 The query response may be a CNAME, or the actual TLSA RRset. If the 791 response is a CNAME, the SMTP client (through the use of its 792 security-aware stub resolver) restarts the TLSA query at the target 793 domain, following CNAMEs as appropriate and keeping track of whether 794 the entire chain is "secure". If any "insecure" records are 795 encountered, or the TLSA records don't exist, the next candidate TLSA 796 base domain is tried instead. 798 If the ultimate response is a "secure" TLSA RRset, then the candidate 799 TLSA base domain will be the actual TLSA base domain and the TLSA 800 RRset will constitute the TLSA records for the destination. If none 801 of the candidate TLSA base domains yield "secure" TLSA records then 802 delivery MAY proceed via pre-DANE opportunistic TLS. SMTP clients 803 MAY elect to use "insecure" TLSA records to avoid STARTTLS downgrades 804 or even to skip SMTP servers that fail authentication, but MUST NOT 805 misrepresent authentication success as either a secure connection to 806 the SMTP server or as a secure delivery to the intended next-hop 807 domain. 809 TLSA record publishers may leverage CNAMEs to reference a single 810 authoritative TLSA RRset specifying a common Certification Authority 811 or a common end entity certificate to be used with multiple TLS 812 services. Such CNAME expansion does not change the SMTP client's 813 notion of the TLSA base domain; thus, when _25._tcp.mx.example.com is 814 a CNAME, the base domain remains mx.example.com and this is still the 815 reference identifier used together with the next-hop domain in peer 816 certificate name checks. 818 Note that shared end entity certificate associations expose the 819 publishing domain to substitution attacks, where an MITM attacker can 820 reroute traffic to a different server that shares the same end entity 821 certificate. Such shared end entity TLSA records SHOULD be avoided 822 unless the servers in question are functionally equivalent or employ 823 mutually incompatible protocols (an active attacker gains nothing by 824 diverting client traffic from one such server to another). 826 A better example, employing a shared trust anchor rather than shared 827 end-entity certificates, is illustrated by the DNSSEC validated 828 records below: 830 example.com. IN MX 0 mx1.example.com. 831 example.com. IN MX 0 mx2.example.com. 832 _25._tcp.mx1.example.com. IN CNAME tlsa201._dane.example.com. 833 _25._tcp.mx2.example.com. IN CNAME tlsa201._dane.example.com. 834 tlsa201._dane.example.com. IN TLSA 2 0 1 e3b0c44298fc1c149a... 836 The SMTP servers mx1.example.com and mx2.example.com will be expected 837 to have certificates issued under a common trust anchor, but each MX 838 hostname's TLSA base domain remains unchanged despite the above CNAME 839 records. Correspondingly, each SMTP server will be associated with a 840 pair of reference identifiers consisting of its hostname plus the 841 next-hop domain "example.com". 843 If, during TLSA resolution (including possible CNAME indirection), at 844 least one "secure" TLSA record is found (even if not usable because 845 it is unsupported by the implementation or support is 846 administratively disabled), then the corresponding host has signaled 847 its commitment to implement TLS. The SMTP client MUST NOT deliver 848 mail via the corresponding host unless a TLS session is negotiated 849 via STARTTLS. This is required to avoid MITM STARTTLS downgrade 850 attacks. 852 As noted previously (in Section Section 2.2.2), when no "secure" TLSA 853 records are found at the fully CNAME-expanded name, the original 854 unexpanded name MUST be tried instead. This supports customers of 855 hosting providers where the provider's zone cannot be validated with 856 DNSSEC, but the customer has shared appropriate key material with the 857 hosting provider to enable TLS via SNI. Intermediate names that 858 arise during CNAME expansion that are neither the original, nor the 859 final name, are never candidate TLSA base domains, even if "secure". 861 3. DANE authentication 863 This section describes which TLSA records are applicable to SMTP 864 opportunistic DANE TLS and how to apply such records to authenticate 865 the SMTP server. With opportunistic DANE TLS, both the TLS support 866 implied by the presence of DANE TLSA records and the verification 867 parameters necessary to authenticate the TLS peer are obtained 868 together. In contrast to protocols where channel security policy is 869 set exclusively by the client, authentication via this protocol is 870 expected to be less prone to connection failure caused by 871 incompatible configuration of the client and server. 873 3.1. TLSA certificate usages 875 The DANE TLSA specification [RFC6698] defines multiple TLSA RR types 876 via combinations of 3 numeric parameters. The numeric values of 877 these parameters were later given symbolic names in [RFC7218]. The 878 rest of the TLSA record is the "certificate association data field", 879 which specifies the full or digest value of a certificate or public 880 key. The parameters are: 882 The TLSA Certificate Usage field: Section 2.1.1 of [RFC6698] 883 specifies four values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and 884 DANE-EE(3). There is an additional private-use value: 885 PrivCert(255). All other values are reserved for use by future 886 specifications. 888 The selector field: Section 2.1.2 of [RFC6698] specifies two values: 889 Cert(0) and SPKI(1). There is an additional private-use value: 890 PrivSel(255). All other values are reserved for use by future 891 specifications. 893 The matching type field: Section 2.1.3 of [RFC6698] specifies three 894 values: Full(0), SHA2-256(1) and SHA2-512(2). There is an 895 additional private-use value: PrivMatch(255). All other values 896 are reserved for use by future specifications. 898 We may think of TLSA Certificate Usage values 0 through 3 as a 899 combination of two one-bit flags. The low bit chooses between trust 900 anchor (TA) and end entity (EE) certificates. The high bit chooses 901 between public PKI issued and domain-issued certificates. 903 The selector field specifies whether the TLSA RR matches the whole 904 certificate: Cert(0), or just its subjectPublicKeyInfo: SPKI(1). The 905 subjectPublicKeyInfo is an ASN.1 DER ([X.690]) encoding of the 906 certificate's algorithm id, any parameters and the public key data. 908 The matching type field specifies how the TLSA RR Certificate 909 Association Data field is to be compared with the certificate or 910 public key. A value of Full(0) means an exact match: the full DER 911 encoding of the certificate or public key is given in the TLSA RR. A 912 value of SHA2-256(1) means that the association data matches the 913 SHA2-256 digest of the certificate or public key, and likewise 914 SHA2-512(2) means a SHA2-512 digest is used. 916 Since opportunistic DANE TLS will be used by non-interactive MTAs, 917 with no user to "press OK" when authentication fails, reliability of 918 peer authentication is paramount. Server operators are advised to 919 publish TLSA records that are least likely to fail authentication due 920 to interoperability or operational problems. Because DANE TLS relies 921 on coordinated changes to DNS and SMTP server settings, the best 922 choice of records to publish will depend on site-specific practices. 924 The certificate usage element of a TLSA record plays a critical role 925 in determining how the corresponding certificate association data 926 field is used to authenticate server's certificate chain. The next 927 two subsections explain the process for certificate usages DANE-EE(3) 928 and DANE-TA(2). The third subsection briefly explains why 929 certificate usages PKIX-TA(0) and PKIX-EE(1) are not applicable with 930 opportunistic DANE TLS. 932 In summary, we recommend the use of either "DANE-EE(3) SPKI(1) 933 SHA2-256(1)" or "DANE-TA(2) Cert(0) SHA2-256(1)" TLSA records 934 depending on site needs. Other combinations of TLSA parameters are 935 either explicitly unsupported, or offer little to recommend them over 936 these two. 938 The mandatory to support digest algorithm in [RFC6698] is 939 SHA2-256(1). When the server's TLSA RRset includes records with a 940 matching type indicating a digest record (i.e., a value other than 941 Full(0)), a TLSA record with a SHA2-256(1) matching type SHOULD be 942 provided along with any other digest published, since some SMTP 943 clients may support only SHA2-256(1). If at some point the SHA2-256 944 digest algorithm is tarnished by new cryptanalytic attacks, 945 publishers will need to include an appropriate stronger digest in 946 their TLSA records, initially along with, and ultimately in place of, 947 SHA2-256. 949 3.1.1. Certificate usage DANE-EE(3) 951 Authentication via certificate usage DANE-EE(3) TLSA records involves 952 simply checking that the server's leaf certificate matches the TLSA 953 record. In particular the binding of the server public key to its 954 name is based entirely on the TLSA record association. The server 955 MUST be considered authenticated even if none of the names in the 956 certificate match the client's reference identity for the server. 958 Similarly, the expiration date of the server certificate MUST be 959 ignored, the validity period of the TLSA record key binding is 960 determined by the validity interval of the TLSA record DNSSEC 961 signature. 963 With DANE-EE(3) servers need not employ SNI (may ignore the client's 964 SNI message) even when the server is known under independent names 965 that would otherwise require separate certificates. It is instead 966 sufficient for the TLSA RRsets for all the domains in question to 967 match the server's default certificate. Of course with SMTP servers 968 it is simpler still to publish the same MX hostname for all the 969 hosted domains. 971 For domains where it is practical to make coordinated changes in DNS 972 TLSA records during SMTP server key rotation, it is often best to 973 publish end-entity DANE-EE(3) certificate associations. DANE-EE(3) 974 certificates don't suddenly stop working when leaf or intermediate 975 certificates expire, and don't fail when the server operator neglects 976 to configure all the required issuer certificates in the server 977 certificate chain. 979 TLSA records published for SMTP servers SHOULD, in most cases, be 980 "DANE-EE(3) SPKI(1) SHA2-256(1)" records. Since all DANE 981 implementations are required to support SHA2-256, this record type 982 works for all clients and need not change across certificate renewals 983 with the same key. 985 3.1.2. Certificate usage DANE-TA(2) 987 Some domains may prefer to avoid the operational complexity of 988 publishing unique TLSA RRs for each TLS service. If the domain 989 employs a common issuing Certification Authority to create 990 certificates for multiple TLS services, it may be simpler to publish 991 the issuing authority as a trust anchor (TA) for the certificate 992 chains of all relevant services. The TLSA query domain (TLSA base 993 domain with port and protocol prefix labels) for each service issued 994 by the same TA may then be set to a CNAME alias that points to a 995 common TLSA RRset that matches the TA. For example: 997 example.com. IN MX 0 mx1.example.com. 998 example.com. IN MX 0 mx2.example.com. 999 _25._tcp.mx1.example.com. IN CNAME tlsa201._dane.example.com. 1000 _25._tcp.mx2.example.com. IN CNAME tlsa201._dane.example.com. 1001 tlsa201._dane.example.com. IN TLSA 2 0 1 e3b0c44298fc1c14.... 1003 With usage DANE-TA(2) the server certificates will need to have names 1004 that match one of the client's reference identifiers (see [RFC6125]). 1005 The server MAY employ SNI to select the appropriate certificate to 1006 present to the client. 1008 SMTP servers that rely on certificate usage DANE-TA(2) TLSA records 1009 for TLS authentication MUST include the TA certificate as part of the 1010 certificate chain presented in the TLS handshake server certificate 1011 message even when it is a self-signed root certificate. At this 1012 time, many SMTP servers are not configured with a comprehensive list 1013 of trust anchors, nor are they expected to at any point in the 1014 future. Some MTAs will ignore all locally trusted certificates when 1015 processing usage DANE-TA(2) TLSA records. Thus even when the TA 1016 happens to be a public Certification Authority known to the SMTP 1017 client, authentication is likely to fail unless the TA certificate is 1018 included in the TLS server certificate message. 1020 TLSA records with selector Full(0) are discouraged. While these 1021 potentially obviate the need to transmit the TA certificate in the 1022 TLS server certificate message, client implementations may not be 1023 able to augment the server certificate chain with the data obtained 1024 from DNS, especially when the TLSA record supplies a bare key 1025 (selector SPKI(1)). Since the server will need to transmit the TA 1026 certificate in any case, server operators SHOULD publish TLSA records 1027 with a selector other than Full(0) and avoid potential 1028 interoperability issues with large TLSA records containing full 1029 certificates or keys. 1031 TLSA Publishers employing DANE-TA(2) records SHOULD publish records 1032 with a selector of Cert(0). Such TLSA records are associated with 1033 the whole trust anchor certificate, not just with the trust anchor 1034 public key. In particular, the SMTP client SHOULD then apply any 1035 relevant constraints from the trust anchor certificate, such as, for 1036 example, path length constraints. 1038 While a selector of SPKI(1) may also be employed, the resulting TLSA 1039 record will not specify the full trust anchor certificate content, 1040 and elements of the trust anchor certificate other than the public 1041 key become mutable. This may, for example, allow a subsidiary CA to 1042 issue a chain that violates the trust anchor's path length or name 1043 constraints. 1045 3.1.3. Certificate usages PKIX-TA(0) and PKIX-EE(1) 1047 As noted in the introduction, SMTP clients cannot, without relying on 1048 DNSSEC for secure MX records and DANE for STARTTLS support signaling, 1049 perform server identity verification or prevent STARTTLS downgrade 1050 attacks. The use of PKIX CAs offers no added security since an 1051 attacker capable of compromising DNSSEC is free to replace any PKIX- 1052 TA(0) or PKIX-EE(1) TLSA records with records bearing any convenient 1053 non-PKIX certificate usage. 1055 SMTP servers SHOULD NOT publish TLSA RRs with certificate usage PKIX- 1056 TA(0) or PKIX-EE(1). SMTP clients cannot be expected to be 1057 configured with a suitably complete set of trusted public CAs. 1058 Lacking a complete set of public CAs, clients would not be able to 1059 verify the certificates of SMTP servers whose issuing root CAs are 1060 not trusted by the client. 1062 Opportunistic DANE TLS needs to interoperate without bilateral 1063 coordination of security settings between client and server systems. 1064 Therefore, parameter choices that are fragile in the absence of 1065 bilateral coordination are unsupported. Nothing is lost since the 1066 PKIX certificate usages cannot aid SMTP TLS security, they can only 1067 impede SMTP TLS interoperability. 1069 SMTP client treatment of TLSA RRs with certificate usages PKIX-TA(0) 1070 or PKIX-EE(1) is undefined. SMTP clients should generally treat such 1071 TLSA records as unusable. 1073 3.2. Certificate matching 1075 When at least one usable "secure" TLSA record is found, the SMTP 1076 client MUST use TLSA records to authenticate the SMTP server. 1077 Messages MUST NOT be delivered via the SMTP server if authentication 1078 fails, otherwise the SMTP client is vulnerable to MITM attacks. 1080 3.2.1. DANE-EE(3) name checks 1082 The SMTP client MUST NOT perform certificate name checks with 1083 certificate usage DANE-EE(3); see Section 3.1.1 above. 1085 3.2.2. DANE-TA(2) name checks 1087 To match a server via a TLSA record with certificate usage DANE- 1088 TA(2), the client MUST perform name checks to ensure that it has 1089 reached the correct server. In all DANE-TA(2) cases the SMTP client 1090 MUST include the TLSA base domain as one of the valid reference 1091 identifiers for matching the server certificate. 1093 TLSA records for MX hostnames: If the TLSA base domain was obtained 1094 indirectly via a "secure" MX lookup (including any CNAME-expanded 1095 name of an MX hostname), then the original next-hop domain used in 1096 the MX lookup MUST be included as as a second reference 1097 identifier. The CNAME-expanded original next-hop domain MUST be 1098 included as a third reference identifier if different from the 1099 original next-hop domain. When the client MTA is employing DANE 1100 TLS security despite "insecure" MX redirection the MX hostname is 1101 the only reference identifier. 1103 TLSA records for Non-MX hostnames: If MX records were not used 1104 (e.g., if none exist) and the TLSA base domain is the CNAME- 1105 expanded original next-hop domain, then the original next-hop 1106 domain MUST be included as a second reference identifier. 1108 Accepting certificates with the original next-hop domain in addition 1109 to the MX hostname allows a domain with multiple MX hostnames to 1110 field a single certificate bearing a single domain name (i.e., the 1111 email domain) across all the SMTP servers. This also aids 1112 interoperability with pre-DANE SMTP clients that are configured to 1113 look for the email domain name in server certificates. For example, 1114 with "secure" DNS records as below: 1116 exchange.example.org. IN CNAME mail.example.org. 1117 mail.example.org. IN CNAME example.com. 1118 example.com. IN MX 10 mx10.example.com. 1119 example.com. IN MX 15 mx15.example.com. 1120 example.com. IN MX 20 mx20.example.com. 1121 ; 1122 mx10.example.com. IN A 192.0.2.10 1123 _25._tcp.mx10.example.com. IN TLSA 2 0 1 ... 1124 ; 1125 mx15.example.com. IN CNAME mxbackup.example.com. 1126 mxbackup.example.com. IN A 192.0.2.15 1127 ; _25._tcp.mxbackup.example.com. IN TLSA ? (NXDOMAIN) 1128 _25._tcp.mx15.example.com. IN TLSA 2 0 1 ... 1129 ; 1130 mx20.example.com. IN CNAME mxbackup.example.net. 1131 mxbackup.example.net. IN A 198.51.100.20 1132 _25._tcp.mxbackup.example.net. IN TLSA 2 0 1 ... 1134 Certificate name checks for delivery of mail to exchange.example.org 1135 via any of the associated SMTP servers MUST accept at least the names 1136 "exchange.example.org" and "example.com", which are respectively the 1137 original and fully expanded next-hop domain. When the SMTP server is 1138 mx10.example.com, name checks MUST accept the TLSA base domain 1139 "mx10.example.com". If, despite the fact that MX hostnames are 1140 required to not be aliases, the MTA supports delivery via 1141 "mx15.example.com" or "mx20.example.com" then name checks MUST accept 1142 the respective TLSA base domains "mx15.example.com" and 1143 "mxbackup.example.net". 1145 3.2.3. Reference identifier matching 1147 When name checks are applicable (certificate usage DANE-TA(2)), if 1148 the server certificate contains a Subject Alternative Name extension 1149 ([RFC5280]), with at least one DNS-ID ([RFC6125]) then only the DNS- 1150 IDs are matched against the client's reference identifiers. The CN- 1151 ID ([RFC6125]) is only considered when no DNS-IDs are present. The 1152 server certificate is considered matched when one of its presented 1153 identifiers ([RFC5280]) matches any of the client's reference 1154 identifiers. 1156 Wildcards are valid in either DNS-IDs or the CN-ID when applicable. 1157 The wildcard character must be entire first label of the DNS-ID or 1158 CN-ID. Thus, "*.example.com" is valid, while "smtp*.example.com" and 1159 "*smtp.example.com" are not. SMTP clients MUST support wildcards 1160 that match the first label of the reference identifier, with the 1161 remaining labels matching verbatim. For example, the DNS-ID 1162 "*.example.com" matches the reference identifier "mx1.example.com". 1163 SMTP clients MAY, subject to local policy allow wildcards to match 1164 multiple reference identifier labels, but servers cannot expect broad 1165 support for such a policy. Therefore any wildcards in server 1166 certificates SHOULD match exactly one label in either the TLSA base 1167 domain or the next-hop domain. 1169 4. Server key management 1171 Two TLSA records MUST be published before employing a new EE or TA 1172 public key or certificate, one matching the currently deployed key 1173 and the other matching the new key scheduled to replace it. Once 1174 sufficient time has elapsed for all DNS caches to expire the previous 1175 TLSA RRset and related signature RRsets, servers may be configured to 1176 use the new EE private key and associated public key certificate or 1177 may employ certificates signed by the new trust anchor. 1179 Once the new public key or certificate is in use, the TLSA RR that 1180 matches the retired key can be removed from DNS, leaving only RRs 1181 that match keys or certificates in active use. 1183 As described in Section 3.1.2, when server certificates are validated 1184 via a DANE-TA(2) trust anchor, and CNAME records are employed to 1185 store the TA association data at a single location, the 1186 responsibility of updating the TLSA RRset shifts to the operator of 1187 the trust anchor. Before a new trust anchor is used to sign any new 1188 server certificates, its certificate (digest) is added to the 1189 relevant TLSA RRset. After enough time elapses for the original TLSA 1190 RRset to age out of DNS caches, the new trust anchor can start 1191 issuing new server certificates. Once all certificates issued under 1192 the previous trust anchor have expired, its associated RRs can be 1193 removed from the TLSA RRset. 1195 In the DANE-TA(2) key management model server operators do not 1196 generally need to update DNS TLSA records after initially creating a 1197 CNAME record that references the centrally operated DANE-TA(2) RRset. 1198 If a particular server's key is compromised, its TLSA CNAME SHOULD be 1199 replaced with a DANE-EE(3) association until the certificate for the 1200 compromised key expires, at which point it can return to using a 1201 CNAME record. If the central trust anchor is compromised, all 1202 servers need to be issued new keys by a new TA, and an updated DANE- 1203 TA(2) TLSA RRset needs to be published containing just the new TA. 1205 SMTP servers cannot expect broad CRL or OCSP support from SMTP 1206 clients. As outlined above, with DANE, compromised server or trust 1207 anchor keys can be "revoked" by removing them from the DNS without 1208 the need for client-side support for OCSP or CRLs. 1210 5. Digest algorithm agility 1211 While [RFC6698] specifies multiple digest algorithms, it does not 1212 specify a protocol by which the SMTP client and TLSA record publisher 1213 can agree on the strongest shared algorithm. Such a protocol would 1214 allow the client and server to avoid exposure to any deprecated 1215 weaker algorithms that are published for compatibility with less 1216 capable clients, but should be ignored when possible. Such a 1217 protocol is specified in [I-D.ietf-dane-ops]. SMTP clients and 1218 servers that implement this specification MUST comply with the 1219 requirements outlined under "Digest Algorithm Agility" in 1220 [I-D.ietf-dane-ops]. 1222 6. Mandatory TLS Security 1224 An MTA implementing this protocol may require a stronger security 1225 assurance when sending email to selected destinations. The sending 1226 organization may need to send sensitive email and/or may have 1227 regulatory obligations to protect its content. This protocol is not 1228 in conflict with such a requirement, and in fact can often simplify 1229 authenticated delivery to such destinations. 1231 Specifically, with domains that publish DANE TLSA records for their 1232 MX hostnames, a sending MTA can be configured to use the receiving 1233 domains's DANE TLSA records to authenticate the corresponding SMTP 1234 server. Authentication via DANE TLSA records is easier to manage, as 1235 changes in the receiver's expected certificate properties are made on 1236 the receiver end and don't require manually communicated 1237 configuration changes. With mandatory DANE TLS, when no usable TLSA 1238 records are found, message delivery is delayed. Thus, mail is only 1239 sent when an authenticated TLS channel is established to the remote 1240 SMTP server. 1242 Administrators of mail servers that employ mandatory DANE TLS, need 1243 to carefully monitor their mail logs and queues. If a partner domain 1244 unwittingly misconfigures their TLSA records, disables DNSSEC, or 1245 misconfigures SMTP server certificate chains, mail will be delayed 1246 and may bounce if the issue is not resolved in a timely manner. 1248 7. Note on DANE for Message User Agents 1250 We note that the SMTP protocol is also used between Message User 1251 Agents (MUAs) and Message Submission Agents (MSAs) [RFC6409]. In 1252 [RFC6186] a protocol is specified that enables an MUA to dynamically 1253 locate the MSA based on the user's email address. SMTP connection 1254 security considerations for MUAs implementing [RFC6186] are largely 1255 analogous to connection security requirements for MTAs, and this 1256 specification could be applied largely verbatim with DNS MX records 1257 replaced by corresponding DNS Service (SRV) records 1258 [I-D.ietf-dane-srv]. 1260 However, until MUAs begin to adopt the dynamic configuration 1261 mechanisms of [RFC6186] they are adequately served by more 1262 traditional static TLS security policies. Specification of DANE TLS 1263 for Message User Agent (MUA) to Message Submission Agent (MSA) SMTP 1264 is left to future documents that focus specifically on SMTP security 1265 between MUAs and MSAs. 1267 8. Interoperability considerations 1269 8.1. SNI support 1271 To ensure that the server sends the right certificate chain, the SMTP 1272 client MUST send the TLS SNI extension containing the TLSA base 1273 domain. This precludes the use of the backward compatible SSL 2.0 1274 compatible SSL HELLO by the SMTP client. The minimum SSL/TLS client 1275 HELLO version for SMTP clients performing DANE authentication is SSL 1276 3.0, but a client that offers SSL 3.0 MUST also offer at least TLS 1277 1.0 and MUST include the SNI extension. Servers that don't make use 1278 of SNI MAY negotiate SSL 3.0 if offered by the client. 1280 Each SMTP server MUST present a certificate chain (see [RFC5246] 1281 Section 7.4.2) that matches at least one of the TLSA records. The 1282 server MAY rely on SNI to determine which certificate chain to 1283 present to the client. Clients that don't send SNI information may 1284 not see the expected certificate chain. 1286 If the server's TLSA records match the server's default certificate 1287 chain, the server need not support SNI. In either case, the server 1288 need not include the SNI extension in its TLS HELLO as simply 1289 returning a matching certificate chain is sufficient. Servers MUST 1290 NOT enforce the use of SNI by clients, as the client may be using 1291 unauthenticated opportunistic TLS and may not expect any particular 1292 certificate from the server. If the client sends no SNI extension, 1293 or sends an SNI extension for an unsupported domain, the server MUST 1294 simply send some fallback certificate chain of its choice. The 1295 reason for not enforcing strict matching of the requested SNI 1296 hostname is that DANE TLS clients are typically willing to accept 1297 multiple server names, but can only send one name in the SNI 1298 extension. The server's fallback certificate may match a different 1299 name acceptable to the client, e.g., the original next-hop domain. 1301 8.2. Anonymous TLS cipher suites 1303 Since many SMTP servers either do not support or do not enable any 1304 anonymous TLS cipher suites, SMTP client TLS HELLO messages SHOULD 1305 offer to negotiate a typical set of non-anonymous cipher suites 1306 required for interoperability with such servers. An SMTP client 1307 employing pre-DANE opportunistic TLS MAY in addition include one or 1308 more anonymous TLS cipher suites in its TLS HELLO. SMTP servers, 1309 that need to interoperate with opportunistic TLS clients SHOULD be 1310 prepared to interoperate with such clients by either always selecting 1311 a mutually supported non-anonymous cipher suite or by correctly 1312 handling client connections that negotiate anonymous cipher suites. 1314 Note that while SMTP server operators are under no obligation to 1315 enable anonymous cipher suites, no security is gained by sending 1316 certificates to clients that will ignore them. Indeed support for 1317 anonymous cipher suites in the server makes audit trails more 1318 informative. Log entries that record connections that employed an 1319 anonymous cipher suite record the fact that the clients did not care 1320 to authenticate the server. 1322 9. Operational Considerations 1324 9.1. Client Operational Considerations 1326 An operational error on the sending or receiving side that cannot be 1327 corrected in a timely manner may, at times, lead to consistent 1328 failure to deliver time-sensitive email. The sending MTA 1329 administrator may have to choose between letting email queue until 1330 the error is resolved and disabling opportunistic or mandatory DANE 1331 TLS for one or more destinations. The choice to disable DANE TLS 1332 security should not be made lightly. Every reasonable effort should 1333 be made to determine that problems with mail delivery are the result 1334 of an operational error, and not an attack. A fallback strategy may 1335 be to configure explicit out-of-band TLS security settings if 1336 supported by the sending MTA. 1338 SMTP clients may deploy opportunistic DANE TLS incrementally by 1339 enabling it only for selected sites, or may occasionally need to 1340 disable opportunistic DANE TLS for peers that fail to interoperate 1341 due to misconfiguration or software defects on either end. Some 1342 implementations MAY support DANE TLS in an "audit only" mode in which 1343 failure to achieve the requisite security level is logged as a 1344 warning and delivery proceeds at a reduced security level. Unless 1345 local policy specifies "audit only" or that opportunistic DANE TLS is 1346 not to be used for a particular destination, an SMTP client MUST NOT 1347 deliver mail via a server whose certificate chain fails to match at 1348 least one TLSA record when usable TLSA records are found for that 1349 server. 1351 9.2. Publisher Operational Considerations 1353 SMTP servers that publish certificate usage DANE-TA(2) associations 1354 MUST include the TA certificate in their TLS server certificate 1355 chain, even when that TA certificate is a self-signed root 1356 certificate. 1358 TLSA Publishers MUST follow the guidelines in the "TLSA Publisher 1359 Requirements" section of [I-D.ietf-dane-ops]. 1361 TLSA Publishers SHOULD follow the TLSA publication size guidance 1362 found in [I-D.ietf-dane-ops] under "DANE DNS Record Size Guidelines". 1364 10. Security Considerations 1366 This protocol leverages DANE TLSA records to implement MITM resistant 1367 opportunistic security ([I-D.dukhovni-opportunistic-security]) for 1368 SMTP. For destination domains that sign their MX records and publish 1369 signed TLSA records for their MX hostnames, this protocol allows 1370 sending MTAs to securely discover both the availability of TLS and 1371 how to authenticate the destination. 1373 This protocol does not aim to secure all SMTP traffic, as that is not 1374 practical until DNSSEC and DANE adoption are universal. The 1375 incremental deployment provided by following this specification is a 1376 best possible path for securing SMTP. This protocol coexists and 1377 interoperates with the existing insecure Internet email backbone. 1379 The protocol does not preclude existing non-opportunistic SMTP TLS 1380 security arrangements, which can continue to be used as before via 1381 manual configuration with negotiated out-of-band key and TLS 1382 configuration exchanges. 1384 Opportunistic SMTP TLS depends critically on DNSSEC for downgrade 1385 resistance and secure resolution of the destination name. If DNSSEC 1386 is compromised, it is not possible to fall back on the public CA PKI 1387 to prevent MITM attacks. A successful breach of DNSSEC enables the 1388 attacker to publish TLSA usage 3 certificate associations, and 1389 thereby bypass any security benefit the legitimate domain owner might 1390 hope to gain by publishing usage 0 or 1 TLSA RRs. Given the lack of 1391 public CA PKI support in existing MTA deployments, avoiding 1392 certificate usages 0 and 1 simplifies implementation and deployment 1393 with no adverse security consequences. 1395 Implementations must strictly follow the portions of this 1396 specification that indicate when it is appropriate to initiate a non- 1397 authenticated connection or cleartext connection to a SMTP server. 1398 Specifically, in order to prevent downgrade attacks on this protocol, 1399 implementation must not initiate a connection when this specification 1400 indicates a particular SMTP server must be considered unreachable. 1402 11. IANA considerations 1404 This specification requires no support from IANA. 1406 12. Acknowledgements 1408 The authors would like to extend great thanks to Tony Finch, who 1409 started the original version of a DANE SMTP document. His work is 1410 greatly appreciated and has been incorporated into this document. 1411 The authors would like to additionally thank Phil Pennock for his 1412 comments and advice on this document. 1414 Acknowledgments from Viktor: Thanks to Paul Hoffman who motivated me 1415 to begin work on this memo and provided feedback on early drafts. 1416 Thanks to Patrick Koetter, Perry Metzger and Nico Williams for 1417 valuable review comments. Thanks also to Wietse Venema who created 1418 Postfix, and whose advice and feedback were essential to the 1419 development of the Postfix DANE implementation. 1421 13. References 1423 13.1. Normative References 1425 [I-D.ietf-dane-ops] 1426 Dukhovni, V. and W. Hardaker, "Updates to and Operational 1427 Guidance for the DANE Protocol", draft-ietf-dane-ops-06 1428 (work in progress), August 2014. 1430 [RFC1035] Mockapetris, P., "Domain names - implementation and 1431 specification", STD 13, RFC 1035, November 1987. 1433 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1434 Requirement Levels", BCP 14, RFC 2119, March 1997. 1436 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 1437 Transport Layer Security", RFC 3207, February 2002. 1439 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1440 Rose, "DNS Security Introduction and Requirements", RFC 1441 4033, March 2005. 1443 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1444 Rose, "Resource Records for the DNS Security Extensions", 1445 RFC 4034, March 2005. 1447 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 1448 Rose, "Protocol Modifications for the DNS Security 1449 Extensions", RFC 4035, March 2005. 1451 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1452 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 1454 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 1455 Housley, R., and W. Polk, "Internet X.509 Public Key 1456 Infrastructure Certificate and Certificate Revocation List 1457 (CRL) Profile", RFC 5280, May 2008. 1459 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1460 October 2008. 1462 [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: 1463 Extension Definitions", RFC 6066, January 2011. 1465 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 1466 Verification of Domain-Based Application Service Identity 1467 within Internet Public Key Infrastructure Using X.509 1468 (PKIX) Certificates in the Context of Transport Layer 1469 Security (TLS)", RFC 6125, March 2011. 1471 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email 1472 Submission/Access Services", RFC 6186, March 2011. 1474 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the 1475 DNS", RFC 6672, June 2012. 1477 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 1478 of Named Entities (DANE) Transport Layer Security (TLS) 1479 Protocol: TLSA", RFC 6698, August 2012. 1481 [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify 1482 Conversations about DNS-Based Authentication of Named 1483 Entities (DANE)", RFC 7218, April 2014. 1485 [X.690] International Telecommunications Union, "Recommendation 1486 ITU-T X.690 (2002) | ISO/IEC 8825-1:2002, Information 1487 technology - ASN.1 encoding rules: Specification of Basic 1488 Encoding Rules (BER), Canonical Encoding Rules (CER) and 1489 Distinguished Encoding Rules (DER)", July 2002. 1491 13.2. Informative References 1493 [I-D.dukhovni-opportunistic-security] 1494 Dukhovni, V., "Opportunistic Security: Some Protection 1495 Most of the Time", draft-dukhovni-opportunistic- 1496 security-03 (work in progress), August 2014. 1498 [I-D.ietf-dane-srv] 1499 Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- 1500 Based Authentication of Named Entities (DANE) TLSA Records 1501 with SRV Records", draft-ietf-dane-srv-07 (work in 1502 progress), July 2014. 1504 [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July 1505 2009. 1507 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 1508 STD 72, RFC 6409, November 2011. 1510 Authors' Addresses 1512 Viktor Dukhovni 1513 Two Sigma 1515 Email: ietf-dane@dukhovni.org 1517 Wes Hardaker 1518 Parsons 1519 P.O. Box 382 1520 Davis, CA 95617 1521 US 1523 Email: ietf@hardakers.net