7Internet Engineering Task Force (IETF) T. Finch
8Request for Comments: 7673 University of Cambridge
9Category: Standards Track M. Miller
10ISSN: 2070-1721 Cisco Systems, Inc.
16 Using DNS-Based Authentication of Named Entities (DANE)
17 TLSA Records with SRV Records
21 The DNS-Based Authentication of Named Entities (DANE) specification
22 (RFC 6698) describes how to use TLSA resource records secured by
23 DNSSEC (RFC 4033) to associate a server's connection endpoint with
24 its Transport Layer Security (TLS) certificate (thus enabling
25 administrators of domain names to specify the keys used in that
26 domain's TLS servers). However, application protocols that use SRV
27 records (RFC 2782) to indirectly name the target server connection
28 endpoints for a service domain name cannot apply the rules from RFC
29 6698. Therefore, this document provides guidelines that enable such
30 protocols to locate and use TLSA records.
34 This is an Internet Standards Track document.
36 This document is a product of the Internet Engineering Task Force
37 (IETF). It represents the consensus of the IETF community. It has
38 received public review and has been approved for publication by the
39 Internet Engineering Steering Group (IESG). Further information on
40 Internet Standards is available in Section 2 of RFC 5741.
42 Information about the current status of this document, any errata,
43 and how to provide feedback on it may be obtained at
44 http://www.rfc-editor.org/info/rfc7673.
58Finch, et al. Standards Track [Page 1]
60RFC 7673 TLSA and SRV October 2015
65 Copyright (c) 2015 IETF Trust and the persons identified as the
66 document authors. All rights reserved.
68 This document is subject to BCP 78 and the IETF Trust's Legal
69 Provisions Relating to IETF Documents
70 (http://trustee.ietf.org/license-info) in effect on the date of
71 publication of this document. Please review these documents
72 carefully, as they describe your rights and restrictions with respect
73 to this document. Code Components extracted from this document must
74 include Simplified BSD License text as described in Section 4.e of
75 the Trust Legal Provisions and are provided without warranty as
76 described in the Simplified BSD License.
80 1. Introduction ....................................................3
81 2. Terminology .....................................................4
82 3. DNS Checks ......................................................4
83 3.1. SRV Query ..................................................4
84 3.2. Address Queries ............................................5
85 3.3. TLSA Queries ...............................................6
86 3.4. Impact on TLS Usage ........................................6
87 4. TLS Checks ......................................................7
88 4.1. SRV Records Only ...........................................7
89 4.2. TLSA Records ...............................................8
90 5. Guidance for Protocol Authors ...................................8
91 6. Guidance for Server Operators ...................................8
92 7. Guidance for Application Developers .............................9
93 8. Internationalization Considerations .............................9
94 9. Security Considerations ........................................10
95 9.1. Mixed Security Status .....................................10
96 9.2. Certificate Subject Name Matching .........................10
97 10. References ....................................................11
98 10.1. Normative References .....................................11
99 10.2. Informative References ...................................12
100 Appendix A. Examples ..............................................13
101 A.1. IMAP .......................................................13
102 A.2. XMPP .......................................................13
103 Appendix B. Rationale .............................................14
104 Acknowledgements ..................................................15
105 Authors' Addresses ................................................16
114Finch, et al. Standards Track [Page 2]
116RFC 7673 TLSA and SRV October 2015
121 The base DNS-Based Authentication of Named Entities (DANE)
122 specification [RFC6698] describes how to use TLSA resource records
123 secured by DNSSEC [RFC4033] to associate a target server's connection
124 endpoint with its Transport Layer Security (TLS) certificate (thus
125 enabling administrators of domain names to specify the keys used in
126 that domain's TLS servers). Some application protocols locate
127 connection endpoints indirectly via SRV records [RFC2782]. As a
128 result of this indirection, the rules specified in [RFC6698] cannot
129 be directly applied to such application protocols. (Rules for SMTP
130 [RFC5321], which uses MX resource records instead of SRV records, are
131 described in [RFC7672].)
133 This document describes how to use DANE TLSA records with SRV
134 records. To summarize:
136 o We rely on DNSSEC to secure SRV records that map the desired
137 service, transport protocol, and service domain name to the
138 corresponding target server connection endpoints (i.e., the target
139 server hostnames and port numbers returned in the SRV records for
142 o Although in accordance with [RFC2782] a service domain name can
143 advertise a number of SRV records (some of which might map to
144 connection endpoints that do not support TLS), the intent of this
145 specification is for a client to securely discover connection
146 endpoints that support TLS.
148 o The TLSA records for each connection endpoint are located using
149 the transport protocol, port number, and hostname for the target
150 server (not the service domain name).
152 o When DNSSEC-validated TLSA records are published for a given
153 connection endpoint, clients always use TLS when connecting (even
154 if the connection endpoint supports cleartext communication).
156 o If there is at least one usable TLSA record for a given connection
157 endpoint, the connection endpoint's TLS certificate or public key
158 needs to match at least one of those usable TLSA records.
160 o If there are no usable TLSA records for a given connection
161 endpoint, the target server hostname is used as one of the
162 acceptable reference identifiers, as described in [RFC6125].
163 Other reference identifiers might arise through CNAME expansion of
164 either the service domain name or target server hostname, as
165 detailed in [RFC7671].
170Finch, et al. Standards Track [Page 3]
172RFC 7673 TLSA and SRV October 2015
175 o If there are no usable TLSA records for any connection endpoint
176 (and thus the client cannot securely discover a connection
177 endpoint that supports TLS), the client's behavior is a matter for
178 the application protocol or client implementation; this might
179 involve a fallback to non-DANE behavior using the public key
180 infrastructure [RFC5280].
184 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
185 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
186 "OPTIONAL" in this memo are to be interpreted as described in
189 This document uses the definitions for "secure", "insecure", "bogus",
190 and "indeterminate" from Section 4.3 of [RFC4035]. This document
191 uses the acronyms from [RFC7218] for the values of TLSA fields where
194 Additionally, this document uses the following terms:
196 connection endpoint: A tuple of a fully qualified DNS hostname,
197 transport protocol, and port number that a client uses to
198 establish a connection to the target server.
200 service domain name: The fully qualified DNS domain name that
201 identifies an application service; corresponds to the term "source
202 domain" from [RFC6125].
204 This document uses the term "target server hostname" in place of the
205 term "derived domain" from the so-called CertID specification
212 When the client makes an SRV query, a successful result will
213 typically be a list of one or more SRV records (or possibly a chain
214 of CNAME/DNAME aliases leading to such a list).
216 NOTE: Implementers need to be aware that unsuccessful results can
217 occur because of various DNS-related errors; guidance on avoiding
218 downgrade attacks can be found in Section 2.1 of [RFC7672].
226Finch, et al. Standards Track [Page 4]
228RFC 7673 TLSA and SRV October 2015
231 For this specification to apply, the entire chain of DNS RRset(s)
232 returned MUST be "secure" according to DNSSEC validation (Section 5
233 of [RFC4035]). In the case where the answer is obtained via a chain
234 of CNAME and/or DNAME aliases, the whole chain of CNAME and DNAME
235 RRsets MUST also be secure.
237 If the SRV lookup fails because the RRset is "bogus" (or the lookup
238 fails for reasons other than no records), the client MUST abort its
239 attempt to connect to the desired service. If the lookup result is
240 "insecure" (or no SRV records exist), this protocol does not apply
241 and the client SHOULD fall back to its non-DNSSEC, non-DANE (and
242 possibly non-SRV) behavior.
244 When the lookup returns a "secure" RRset (possibly via a chain of
245 "secure" CNAME/DNAME records), the client now has an authentic list
246 of target server connection endpoints with weight and priority
247 values. It performs server ordering and selection using the weight
248 and priority values without regard to the presence or absence of
249 DNSSEC or TLSA records. It also takes note of the DNSSEC validation
250 status of the SRV response for use when checking certificate names
251 (see Section 4). The client can then proceed to making address
252 queries on the target server hostnames as described in the following
257 For each SRV target server connection endpoint, the client makes
258 A and/or AAAA queries, performs DNSSEC validation on the address
259 (A or AAAA) response, and continues as follows, based on the results:
261 o If a returned RRSet is "secure", the client MUST perform a TLSA
262 query for that target server connection endpoint, as described in
265 o If no returned RRsets are "secure", the client MUST NOT perform a
266 TLSA query for that target server connection endpoint; the TLSA
267 query will most likely fail or produce spurious results.
269 o If the address record lookup fails (a validation status of either
270 "bogus" or "indeterminate"), the client MUST NOT connect to this
271 connection endpoint; instead, it uses the next most appropriate
272 SRV target. This helps prevent downgrade attacks.
282Finch, et al. Standards Track [Page 5]
284RFC 7673 TLSA and SRV October 2015
289 The client SHALL construct the TLSA query name as described in
290 Section 3 of [RFC6698], based on the fields from the SRV record: the
291 port number from the SRV RDATA, the transport protocol from the SRV
292 query name, and the TLSA base domain from the SRV target server
295 For example, the following SRV record for IMAP (see [RFC6186])
297 _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
299 leads to the TLSA query shown below:
301 _9143._tcp.imap.example.net. IN TLSA ?
3033.4. Impact on TLS Usage
305 The client SHALL determine if the TLSA records returned in the
306 previous step are usable according to Section 4.1 of [RFC6698]. This
307 affects the use of TLS as follows:
309 o If the TLSA response is "secure" and usable, then the client MUST
310 use TLS when connecting to the target server. The TLSA records
311 are used when validating the server's certificate as described in
314 o If the TLSA response is "bogus" or "indeterminate" (or the lookup
315 fails for reasons other than no records), then the client MUST NOT
316 connect to the target server (the client can still use other SRV
319 o If the TLSA response is "insecure" (or no TLSA records exist),
320 then the client SHALL proceed as if the target server had no TLSA
321 records. It MAY connect to the target server with or without TLS,
322 subject to the policies of the application protocol or client
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340RFC 7673 TLSA and SRV October 2015
345 When connecting to a server, the client MUST use TLS if the responses
346 to the SRV and TLSA queries were "secure" as described above. The
347 rules described in the next two sections -- Section 4.2 for cases
348 where there is at least one usable TLSA record, and Section 4.1
349 otherwise -- apply to such secure responses.
353 If the client received zero usable TLSA certificate associations, it
354 SHALL validate the server's TLS certificate using the normal PKIX
355 rules [RFC5280] or protocol-specific rules (e.g., following
356 [RFC6125]) without further input from the TLSA records. In this
357 case, the client uses the information in the server certificate and
358 the DNSSEC validation status of the SRV query in its authentication
359 checks. It SHOULD use the Server Name Indication extension (TLS SNI)
360 [RFC6066] or its functional equivalent in the relevant application
361 protocol (e.g., in the Extensible Messaging and Presence Protocol
362 (XMPP) [RFC6120], this is the 'to' address of the initial stream
363 header). The preferred name SHALL be chosen as follows, and the
364 client SHALL verify the identity asserted by the server's certificate
365 according to Section 6 of [RFC6125], using a list of reference
366 identifiers constructed as follows (note again that in RFC 6125 the
367 terms "source domain" and "derived domain" refer to the same things
368 as "service domain name" and "target server hostname" in this
369 document). The examples below assume a service domain name of
370 "im.example.com" and a target server hostname of
371 "xmpp23.hosting.example.net".
373 SRV is insecure: The reference identifiers SHALL include the service
374 domain name and MUST NOT include the SRV target server hostname
375 (e.g., include "im.example.com" but not
376 "xmpp23.hosting.example.net"). The service domain name is the
377 preferred name for TLS SNI or its equivalent.
379 SRV is secure: The reference identifiers SHALL include both the
380 service domain name and the SRV target server hostname (e.g.,
381 include both "im.example.com" and "xmpp23.hosting.example.net").
382 The service domain name is still the preferred name for TLS SNI or
383 its equivalent (this reduces code complexity and the possibility
384 of interoperability problems).
386 In the latter case, the client will accept either identity to ensure
387 compatibility with servers that support this specification as well as
388 servers that do not support this specification.
394Finch, et al. Standards Track [Page 7]
396RFC 7673 TLSA and SRV October 2015
401 If the client received one or more usable TLSA certificate
402 associations, it SHALL process them as described in Section 2.1 of
405 If the TLS server's certificate -- or the public key of the server's
406 certificate -- matches a usable TLSA record with certificate usage
407 DANE-EE, the client MUST ignore validation checks from [RFC5280] and
408 reference identifier checks from [RFC6125]. The information in such
409 a TLSA record supersedes the non-key information in the certificate.
4115. Guidance for Protocol Authors
413 This document describes how to use DANE with application protocols in
414 which target servers are discovered via SRV records. Although this
415 document attempts to provide generic guidance applying to all such
416 protocols, additional documents for particular application protocols
417 could cover related topics, such as:
419 o Fallback logic in the event that a client is unable to connect
420 securely to a target server by following the procedures defined in
423 o How clients ought to behave if (1) they do not support SRV lookups
424 or (2) they do support SRV lookups and encounter service domain
425 names that do not offer SRV records.
427 o Whether or not the application protocol has a functional
428 equivalent for TLS SNI that is preferred within that protocol.
430 o The use of SRV records with additional discovery technologies,
431 such as the use of both SRV records and NAPTR records [RFC3403]
432 for transport selection in the Session Initiation Protocol (SIP).
434 For example, [XMPP-DNA] covers such topics for XMPP.
4366. Guidance for Server Operators
438 To conform to this specification, the published SRV records and
439 subsequent address (A and AAAA) records MUST be secured with DNSSEC.
440 There SHOULD also be at least one TLSA record published that
441 authenticates the server's certificate.
443 When using TLSA records with certificate usage DANE-EE, it is not
444 necessary for the deployed certificate to contain an identifier for
445 either the source domain or target server hostname. However,
446 operators need to be aware that servers relying solely on validation
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452RFC 7673 TLSA and SRV October 2015
455 using certificate usage DANE-EE TLSA records might prevent clients
456 that do not support this specification from successfully connecting
459 For TLSA records with certificate usage types other than DANE-EE, the
460 certificate(s) MUST contain an identifier that matches:
462 o the service domain name (the "source domain" in [RFC6125] terms,
463 which is the SRV query domain), and/or
465 o the target server hostname (the "derived domain" in [RFC6125]
466 terms, which is the SRV target hostname).
468 Servers that support multiple service domain names (i.e., so-called
469 "multi-tenanted environments") can implement TLS SNI [RFC6066] or its
470 functional equivalent to determine which certificate to offer.
471 Clients that do not support this specification will indicate a
472 preference for the service domain name, while clients that support
473 this specification will indicate the target server hostname.
474 However, the server determines what certificate to present in the TLS
475 handshake; e.g., the presented certificate might only authenticate
476 the target server hostname.
4787. Guidance for Application Developers
480 Developers of application clients that depend on DANE-SRV often would
481 like to prepare as quickly as possible for making a connection to the
482 intended service, thus reducing the wait time for end users. To make
483 this optimization possible, a DNS library might perform the address
484 queries and TLSA queries in parallel. (Because a TLSA record can be
485 ignored if it turns out that the address record on which it depends
486 is not secure, performing the TLSA queries in parallel with the
487 address queries is not harmful from a security perspective and can
488 yield some operational benefits.)
4908. Internationalization Considerations
492 If any of the DNS queries are for an internationalized domain name,
493 then they need to use the A-label form [RFC5890].
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508RFC 7673 TLSA and SRV October 2015
5119. Security Considerations
5139.1. Mixed Security Status
515 We do not specify that all of the target server connection endpoints
516 for a service domain name need to be consistent in whether they have
517 or do not have TLSA records. This is so that partial or incremental
518 deployment does not break the service. Different levels of
519 deployment are likely if a service domain name has a third-party
520 fallback server, for example.
522 The SRV sorting rules are unchanged; in particular, they have not
523 been altered in order to prioritize secure connection endpoints over
524 insecure connection endpoints. If a site wants to be secure, it
525 needs to deploy this protocol completely; a partial deployment is not
526 secure, and we make no special effort to support it.
5289.2. Certificate Subject Name Matching
530 Section 4 of the TLSA specification [RFC6698] leaves the details of
531 checking names in certificates to higher-level application protocols,
532 though it suggests the use of [RFC6125].
534 Name checks are not necessary if the matching TLSA record is of
535 certificate usage DANE-EE. Because such a record identifies the
536 specific certificate (or public key of the certificate), additional
537 checks are superfluous and potentially conflicting.
539 Otherwise, while DNSSEC provides a secure binding between the server
540 name and the TLSA record, and the TLSA record provides a binding to a
541 certificate, this latter step can be indirect via a chain of
542 certificates. For example, a certificate usage PKIX-TA TLSA record
543 only authenticates the Certification Authority (CA) that issued the
544 certificate, and third parties can obtain certificates from the same
545 CA. Therefore, clients need to check to see whether or not the
546 server's certificate matches one of the expected reference
547 identifiers to ensure that the certificate was issued by the CA to
548 the server the client expects (naturally, this is in addition to
549 standard certificate-related checks as specified in [RFC5280],
550 including but not limited to certificate syntax, certificate
551 extensions such as name constraints and extended key usage, and
552 handling of certification paths).
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564RFC 7673 TLSA and SRV October 2015
56910.1. Normative References
571 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
572 Requirement Levels", BCP 14, RFC 2119,
573 DOI 10.17487/RFC2119, March 1997,
574 <http://www.rfc-editor.org/info/rfc2119>.
576 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
577 specifying the location of services (DNS SRV)", RFC 2782,
578 DOI 10.17487/RFC2782, February 2000,
579 <http://www.rfc-editor.org/info/rfc2782>.
581 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
582 Rose, "DNS Security Introduction and Requirements",
583 RFC 4033, DOI 10.17487/RFC4033, March 2005,
584 <http://www.rfc-editor.org/info/rfc4033>.
586 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
587 Rose, "Protocol Modifications for the DNS Security
588 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
589 <http://www.rfc-editor.org/info/rfc4035>.
591 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
592 Housley, R., and W. Polk, "Internet X.509 Public Key
593 Infrastructure Certificate and Certificate Revocation List
594 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
595 <http://www.rfc-editor.org/info/rfc5280>.
597 [RFC5890] Klensin, J., "Internationalized Domain Names for
598 Applications (IDNA): Definitions and Document Framework",
599 RFC 5890, DOI 10.17487/RFC5890, August 2010,
600 <http://www.rfc-editor.org/info/rfc5890>.
602 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
603 Extensions: Extension Definitions", RFC 6066,
604 DOI 10.17487/RFC6066, January 2011,
605 <http://www.rfc-editor.org/info/rfc6066>.
607 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
608 Verification of Domain-Based Application Service Identity
609 within Internet Public Key Infrastructure Using X.509
610 (PKIX) Certificates in the Context of Transport Layer
611 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
612 March 2011, <http://www.rfc-editor.org/info/rfc6125>.
618Finch, et al. Standards Track [Page 11]
620RFC 7673 TLSA and SRV October 2015
623 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
624 of Named Entities (DANE) Transport Layer Security (TLS)
625 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
626 August 2012, <http://www.rfc-editor.org/info/rfc6698>.
628 [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
629 Conversations about DNS-Based Authentication of Named
630 Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218,
631 April 2014, <http://www.rfc-editor.org/info/rfc7218>.
633 [RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
634 Authentication of Named Entities (DANE) Protocol: Updates
635 and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
636 October 2015, <http://www.rfc-editor.org/info/rfc7671>.
638 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
639 Opportunistic DNS-Based Authentication of Named Entities
640 (DANE) Transport Layer Security (TLS)", RFC 7672,
641 DOI 10.17487/RFC7672, October 2015,
642 <http://www.rfc-editor.org/info/rfc7672>.
64410.2. Informative References
646 [RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
647 Part Three: The Domain Name System (DNS) Database",
648 RFC 3403, DOI 10.17487/RFC3403, October 2002,
649 <http://www.rfc-editor.org/info/rfc3403>.
651 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
652 DOI 10.17487/RFC5321, October 2008,
653 <http://www.rfc-editor.org/info/rfc5321>.
655 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
656 Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
657 March 2011, <http://www.rfc-editor.org/info/rfc6120>.
659 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
660 Submission/Access Services", RFC 6186,
661 DOI 10.17487/RFC6186, March 2011,
662 <http://www.rfc-editor.org/info/rfc6186>.
664 [XMPP-DNA] Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name
665 Associations (DNA) in the Extensible Messaging and
666 Presence Protocol (XMPP)", Work in Progress,
667 draft-ietf-xmpp-dna-11, September 2015.
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676RFC 7673 TLSA and SRV October 2015
681 In the following, most of the DNS resource data is elided for
687 _imap._tcp.example.com. SRV 10 0 9143 imap.example.net.
688 example.com. RRSIG SRV ...
690 ; target server hostname
691 imap.example.net. A 192.0.2.1
692 imap.example.net. RRSIG A ...
694 imap.example.net. AAAA 2001:db8:212:8::e:1
695 imap.example.net. RRSIG ...
697 ; TLSA resource record
698 _9143._tcp.imap.example.net. TLSA ...
699 _9143._tcp.imap.example.net. RRSIG TLSA ...
701 Mail messages received for addresses at example.com are retrieved via
702 IMAP at imap.example.net. Connections to imap.example.net port 9143
703 that use STARTTLS will get a server certificate that authenticates
704 the name imap.example.net.
709 _xmpp-client._tcp.example.com. SRV 1 0 5222 im.example.net.
710 _xmpp-client._tcp.example.com. RRSIG SRV ...
712 ; target server hostname
713 im.example.net. A 192.0.2.3
714 im.example.net. RRSIG A ...
716 im.example.net. AAAA 2001:db8:212:8::e:4
717 im.example.net. RRSIG AAAA ...
719 ; TLSA resource record
720 _5222._tcp.im.example.net. TLSA ...
721 _5222._tcp.im.example.net. RRSIG TLSA ...
723 XMPP sessions for addresses at example.com are established at
724 im.example.net. Connections to im.example.net port 5222 that use
725 STARTTLS will get a server certificate that authenticates the name
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732RFC 7673 TLSA and SRV October 2015
737 The long-term goal of this specification is to settle on TLS
738 certificates that verify the target server hostname rather than the
739 service domain name, since this is more convenient for servers
740 hosting multiple domains (so-called "multi-tenanted environments")
741 and scales up more easily to larger numbers of service domain names.
743 There are a number of other reasons for doing it this way:
745 o The certificate is part of the server configuration, so it makes
746 sense to associate it with the target server hostname rather than
747 the service domain name.
749 o In the absence of TLS SNI, if the certificate identifies the
750 target server hostname, then it does not need to list all the
751 possible service domain names.
753 o When the server certificate is replaced, it is much easier if
754 there is one part of the DNS that needs updating to match, instead
755 of an unbounded number of hosted service domain names.
757 o The same TLSA records work with this specification, and with
758 direct connections to the connection endpoint in the style of
761 o Some application protocols, such as SMTP, allow a client to
762 perform transactions with multiple service domain names in the
763 same connection. It is not, in general, feasible for the client
764 to specify the service domain name using TLS SNI when the
765 connection is established, and the server might not be able to
766 present a certificate that authenticates all possible service
767 domain names. See [RFC7672] for details.
769 o It is common for SMTP servers to act in multiple roles -- for
770 example, as outgoing relays or as incoming MX servers, depending
771 on the client identity. It is simpler if the server can present
772 the same certificate regardless of the role in which it is to act.
773 Sometimes the server does not know its role until the client has
774 authenticated, which usually occurs after TLS has been
775 established. See [RFC7672] for details.
786Finch, et al. Standards Track [Page 14]
788RFC 7673 TLSA and SRV October 2015
791 This specification does not provide an option to put TLSA records
792 under the service domain name, because that would add complexity
793 without providing any benefit; security protocols are best kept
794 simple. As described above, there are real-world cases where
795 authenticating the service domain name cannot be made to work, so
796 there would be complicated criteria regarding when service domain
797 name TLSA records might be used and when they cannot. This is all
798 avoided by putting the TLSA records under the target server hostname.
800 The disadvantage is that clients that do not complete DNSSEC
801 validation must, according to [RFC6125] rules, check the server
802 certificate against the service domain name, since they have no other
803 way to authenticate the server. This means that SNI support or its
804 functional equivalent is necessary for backward compatibility.
808 Thanks to Mark Andrews for arguing that authenticating the target
809 server hostname is the right thing, and that we ought to rely on
810 DNSSEC to secure the SRV lookup. Thanks to Stephane Bortzmeyer,
811 James Cloos, Viktor Dukhovni, Ned Freed, Olafur Gudmundsson, Paul
812 Hoffman, Phil Pennock, Hector Santos, Jonas Schneider, and Alessandro
813 Vesely for helpful suggestions.
815 Carl Wallace completed an insightful review on behalf of the Security
818 Ben Campbell, Brian Haberman, and Alvaro Retana provided helpful
819 feedback during IESG review.
821 The authors gratefully acknowledge the assistance of Olafur
822 Gudmundsson and Warren Kumari as the working group chairs and Stephen
823 Farrell as the sponsoring Area Director.
825 Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
826 employing him during his work on earlier draft versions of this
842Finch, et al. Standards Track [Page 15]
844RFC 7673 TLSA and SRV October 2015
850 University of Cambridge Information Services
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867 Email: mamille2@cisco.com
873 Email: peter@andyet.com
874 URI: https://andyet.com/
898Finch, et al. Standards Track [Page 16]