Using DNS-Based Authentication of Named Entities (DANE) TLSA Records with SRV Records
RFC 7673

Internet Engineering Task Force (IETF) T. Finch
Request for Comments: 7673 University of Cambridge
Category: Standards Track M. Miller
ISSN: 2070-1721 Cisco Systems, Inc.
 P. Saint-Andre
 &yet
 October 2015
 Using DNS-Based Authentication of Named Entities (DANE)
 TLSA Records with SRV Records
Abstract
 The DNS-Based Authentication of Named Entities (DANE) specification
 (RFC 6698) describes how to use TLSA resource records secured by
 DNSSEC (RFC 4033) to associate a server's connection endpoint with
 its Transport Layer Security (TLS) certificate (thus enabling
 administrators of domain names to specify the keys used in that
 domain's TLS servers). However, application protocols that use SRV
 records (RFC 2782) to indirectly name the target server connection
 endpoints for a service domain name cannot apply the rules from RFC
 6698. Therefore, this document provides guidelines that enable such
 protocols to locate and use TLSA records.
Status of This Memo
 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7673.
Finch, et al. Standards Track [Page 1]
RFC 7673 TLSA and SRV October 2015
Copyright Notice
 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors. All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document. Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document. Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
Table of Contents
 1. Introduction ....................................................3
 2. Terminology .....................................................4
 3. DNS Checks ......................................................4
 3.1. SRV Query ..................................................4
 3.2. Address Queries ............................................5
 3.3. TLSA Queries ...............................................6
 3.4. Impact on TLS Usage ........................................6
 4. TLS Checks ......................................................7
 4.1. SRV Records Only ...........................................7
 4.2. TLSA Records ...............................................8
 5. Guidance for Protocol Authors ...................................8
 6. Guidance for Server Operators ...................................8
 7. Guidance for Application Developers .............................9
 8. Internationalization Considerations .............................9
 9. Security Considerations ........................................10
 9.1. Mixed Security Status .....................................10
 9.2. Certificate Subject Name Matching .........................10
 10. References ....................................................11
 10.1. Normative References .....................................11
 10.2. Informative References ...................................12
 Appendix A. Examples ..............................................13
 A.1. IMAP .......................................................13
 A.2. XMPP .......................................................13
 Appendix B. Rationale .............................................14
 Acknowledgements ..................................................15
 Authors' Addresses ................................................16
Finch, et al. Standards Track [Page 2]
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1. Introduction
 The base DNS-Based Authentication of Named Entities (DANE)
 specification [RFC6698] describes how to use TLSA resource records
 secured by DNSSEC [RFC4033] to associate a target server's connection
 endpoint with its Transport Layer Security (TLS) certificate (thus
 enabling administrators of domain names to specify the keys used in
 that domain's TLS servers). Some application protocols locate
 connection endpoints indirectly via SRV records [RFC2782]. As a
 result of this indirection, the rules specified in [RFC6698] cannot
 be directly applied to such application protocols. (Rules for SMTP
 [RFC5321], which uses MX resource records instead of SRV records, are
 described in [RFC7672].)
 This document describes how to use DANE TLSA records with SRV
 records. To summarize:
 o We rely on DNSSEC to secure SRV records that map the desired
 service, transport protocol, and service domain name to the
 corresponding target server connection endpoints (i.e., the target
 server hostnames and port numbers returned in the SRV records for
 that service type).
 o Although in accordance with [RFC2782] a service domain name can
 advertise a number of SRV records (some of which might map to
 connection endpoints that do not support TLS), the intent of this
 specification is for a client to securely discover connection
 endpoints that support TLS.
 o The TLSA records for each connection endpoint are located using
 the transport protocol, port number, and hostname for the target
 server (not the service domain name).
 o When DNSSEC-validated TLSA records are published for a given
 connection endpoint, clients always use TLS when connecting (even
 if the connection endpoint supports cleartext communication).
 o If there is at least one usable TLSA record for a given connection
 endpoint, the connection endpoint's TLS certificate or public key
 needs to match at least one of those usable TLSA records.
 o If there are no usable TLSA records for a given connection
 endpoint, the target server hostname is used as one of the
 acceptable reference identifiers, as described in [RFC6125].
 Other reference identifiers might arise through CNAME expansion of
 either the service domain name or target server hostname, as
 detailed in [RFC7671].
Finch, et al. Standards Track [Page 3]
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 o If there are no usable TLSA records for any connection endpoint
 (and thus the client cannot securely discover a connection
 endpoint that supports TLS), the client's behavior is a matter for
 the application protocol or client implementation; this might
 involve a fallback to non-DANE behavior using the public key
 infrastructure [RFC5280].
2. Terminology
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this memo are to be interpreted as described in
 [RFC2119].
 This document uses the definitions for "secure", "insecure", "bogus",
 and "indeterminate" from Section 4.3 of [RFC4035]. This document
 uses the acronyms from [RFC7218] for the values of TLSA fields where
 appropriate.
 Additionally, this document uses the following terms:
 connection endpoint: A tuple of a fully qualified DNS hostname,
 transport protocol, and port number that a client uses to
 establish a connection to the target server.
 service domain name: The fully qualified DNS domain name that
 identifies an application service; corresponds to the term "source
 domain" from [RFC6125].
 This document uses the term "target server hostname" in place of the
 term "derived domain" from the so-called CertID specification
 [RFC6125].
3. DNS Checks
3.1. SRV Query
 When the client makes an SRV query, a successful result will
 typically be a list of one or more SRV records (or possibly a chain
 of CNAME/DNAME aliases leading to such a list).
 NOTE: Implementers need to be aware that unsuccessful results can
 occur because of various DNS-related errors; guidance on avoiding
 downgrade attacks can be found in Section 2.1 of [RFC7672].
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 For this specification to apply, the entire chain of DNS RRset(s)
 returned MUST be "secure" according to DNSSEC validation (Section 5
 of [RFC4035]). In the case where the answer is obtained via a chain
 of CNAME and/or DNAME aliases, the whole chain of CNAME and DNAME
 RRsets MUST also be secure.
 If the SRV lookup fails because the RRset is "bogus" (or the lookup
 fails for reasons other than no records), the client MUST abort its
 attempt to connect to the desired service. If the lookup result is
 "insecure" (or no SRV records exist), this protocol does not apply
 and the client SHOULD fall back to its non-DNSSEC, non-DANE (and
 possibly non-SRV) behavior.
 When the lookup returns a "secure" RRset (possibly via a chain of
 "secure" CNAME/DNAME records), the client now has an authentic list
 of target server connection endpoints with weight and priority
 values. It performs server ordering and selection using the weight
 and priority values without regard to the presence or absence of
 DNSSEC or TLSA records. It also takes note of the DNSSEC validation
 status of the SRV response for use when checking certificate names
 (see Section 4). The client can then proceed to making address
 queries on the target server hostnames as described in the following
 section.
3.2. Address Queries
 For each SRV target server connection endpoint, the client makes
 A and/or AAAA queries, performs DNSSEC validation on the address
 (A or AAAA) response, and continues as follows, based on the results:
 o If a returned RRSet is "secure", the client MUST perform a TLSA
 query for that target server connection endpoint, as described in
 the next section.
 o If no returned RRsets are "secure", the client MUST NOT perform a
 TLSA query for that target server connection endpoint; the TLSA
 query will most likely fail or produce spurious results.
 o If the address record lookup fails (a validation status of either
 "bogus" or "indeterminate"), the client MUST NOT connect to this
 connection endpoint; instead, it uses the next most appropriate
 SRV target. This helps prevent downgrade attacks.
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3.3. TLSA Queries
 The client SHALL construct the TLSA query name as described in
 Section 3 of [RFC6698], based on the fields from the SRV record: the
 port number from the SRV RDATA, the transport protocol from the SRV
 query name, and the TLSA base domain from the SRV target server
 hostname.
 For example, the following SRV record for IMAP (see [RFC6186])
 _imap._tcp.example.com. 86400 IN SRV 10 0 9143 imap.example.net.
 leads to the TLSA query shown below:
 _9143._tcp.imap.example.net. IN TLSA ?
3.4. Impact on TLS Usage
 The client SHALL determine if the TLSA records returned in the
 previous step are usable according to Section 4.1 of [RFC6698]. This
 affects the use of TLS as follows:
 o If the TLSA response is "secure" and usable, then the client MUST
 use TLS when connecting to the target server. The TLSA records
 are used when validating the server's certificate as described in
 Section 4.
 o If the TLSA response is "bogus" or "indeterminate" (or the lookup
 fails for reasons other than no records), then the client MUST NOT
 connect to the target server (the client can still use other SRV
 targets).
 o If the TLSA response is "insecure" (or no TLSA records exist),
 then the client SHALL proceed as if the target server had no TLSA
 records. It MAY connect to the target server with or without TLS,
 subject to the policies of the application protocol or client
 implementation.
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4. TLS Checks
 When connecting to a server, the client MUST use TLS if the responses
 to the SRV and TLSA queries were "secure" as described above. The
 rules described in the next two sections -- Section 4.2 for cases
 where there is at least one usable TLSA record, and Section 4.1
 otherwise -- apply to such secure responses.
4.1. SRV Records Only
 If the client received zero usable TLSA certificate associations, it
 SHALL validate the server's TLS certificate using the normal PKIX
 rules [RFC5280] or protocol-specific rules (e.g., following
 [RFC6125]) without further input from the TLSA records. In this
 case, the client uses the information in the server certificate and
 the DNSSEC validation status of the SRV query in its authentication
 checks. It SHOULD use the Server Name Indication extension (TLS SNI)
 [RFC6066] or its functional equivalent in the relevant application
 protocol (e.g., in the Extensible Messaging and Presence Protocol
 (XMPP) [RFC6120], this is the 'to' address of the initial stream
 header). The preferred name SHALL be chosen as follows, and the
 client SHALL verify the identity asserted by the server's certificate
 according to Section 6 of [RFC6125], using a list of reference
 identifiers constructed as follows (note again that in RFC 6125 the
 terms "source domain" and "derived domain" refer to the same things
 as "service domain name" and "target server hostname" in this
 document). The examples below assume a service domain name of
 "im.example.com" and a target server hostname of
 "xmpp23.hosting.example.net".
 SRV is insecure: The reference identifiers SHALL include the service
 domain name and MUST NOT include the SRV target server hostname
 (e.g., include "im.example.com" but not
 "xmpp23.hosting.example.net"). The service domain name is the
 preferred name for TLS SNI or its equivalent.
 SRV is secure: The reference identifiers SHALL include both the
 service domain name and the SRV target server hostname (e.g.,
 include both "im.example.com" and "xmpp23.hosting.example.net").
 The service domain name is still the preferred name for TLS SNI or
 its equivalent (this reduces code complexity and the possibility
 of interoperability problems).
 In the latter case, the client will accept either identity to ensure
 compatibility with servers that support this specification as well as
 servers that do not support this specification.
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4.2. TLSA Records
 If the client received one or more usable TLSA certificate
 associations, it SHALL process them as described in Section 2.1 of
 [RFC6698].
 If the TLS server's certificate -- or the public key of the server's
 certificate -- matches a usable TLSA record with certificate usage
 DANE-EE, the client MUST ignore validation checks from [RFC5280] and
 reference identifier checks from [RFC6125]. The information in such
 a TLSA record supersedes the non-key information in the certificate.
5. Guidance for Protocol Authors
 This document describes how to use DANE with application protocols in
 which target servers are discovered via SRV records. Although this
 document attempts to provide generic guidance applying to all such
 protocols, additional documents for particular application protocols
 could cover related topics, such as:
 o Fallback logic in the event that a client is unable to connect
 securely to a target server by following the procedures defined in
 this document.
 o How clients ought to behave if (1) they do not support SRV lookups
 or (2) they do support SRV lookups and encounter service domain
 names that do not offer SRV records.
 o Whether or not the application protocol has a functional
 equivalent for TLS SNI that is preferred within that protocol.
 o The use of SRV records with additional discovery technologies,
 such as the use of both SRV records and NAPTR records [RFC3403]
 for transport selection in the Session Initiation Protocol (SIP).
 For example, [XMPP-DNA] covers such topics for XMPP.
6. Guidance for Server Operators
 To conform to this specification, the published SRV records and
 subsequent address (A and AAAA) records MUST be secured with DNSSEC.
 There SHOULD also be at least one TLSA record published that
 authenticates the server's certificate.
 When using TLSA records with certificate usage DANE-EE, it is not
 necessary for the deployed certificate to contain an identifier for
 either the source domain or target server hostname. However,
 operators need to be aware that servers relying solely on validation
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 using certificate usage DANE-EE TLSA records might prevent clients
 that do not support this specification from successfully connecting
 with TLS.
 For TLSA records with certificate usage types other than DANE-EE, the
 certificate(s) MUST contain an identifier that matches:
 o the service domain name (the "source domain" in [RFC6125] terms,
 which is the SRV query domain), and/or
 o the target server hostname (the "derived domain" in [RFC6125]
 terms, which is the SRV target hostname).
 Servers that support multiple service domain names (i.e., so-called
 "multi-tenanted environments") can implement TLS SNI [RFC6066] or its
 functional equivalent to determine which certificate to offer.
 Clients that do not support this specification will indicate a
 preference for the service domain name, while clients that support
 this specification will indicate the target server hostname.
 However, the server determines what certificate to present in the TLS
 handshake; e.g., the presented certificate might only authenticate
 the target server hostname.
7. Guidance for Application Developers
 Developers of application clients that depend on DANE-SRV often would
 like to prepare as quickly as possible for making a connection to the
 intended service, thus reducing the wait time for end users. To make
 this optimization possible, a DNS library might perform the address
 queries and TLSA queries in parallel. (Because a TLSA record can be
 ignored if it turns out that the address record on which it depends
 is not secure, performing the TLSA queries in parallel with the
 address queries is not harmful from a security perspective and can
 yield some operational benefits.)
8. Internationalization Considerations
 If any of the DNS queries are for an internationalized domain name,
 then they need to use the A-label form [RFC5890].
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9. Security Considerations
9.1. Mixed Security Status
 We do not specify that all of the target server connection endpoints
 for a service domain name need to be consistent in whether they have
 or do not have TLSA records. This is so that partial or incremental
 deployment does not break the service. Different levels of
 deployment are likely if a service domain name has a third-party
 fallback server, for example.
 The SRV sorting rules are unchanged; in particular, they have not
 been altered in order to prioritize secure connection endpoints over
 insecure connection endpoints. If a site wants to be secure, it
 needs to deploy this protocol completely; a partial deployment is not
 secure, and we make no special effort to support it.
9.2. Certificate Subject Name Matching
 Section 4 of the TLSA specification [RFC6698] leaves the details of
 checking names in certificates to higher-level application protocols,
 though it suggests the use of [RFC6125].
 Name checks are not necessary if the matching TLSA record is of
 certificate usage DANE-EE. Because such a record identifies the
 specific certificate (or public key of the certificate), additional
 checks are superfluous and potentially conflicting.
 Otherwise, while DNSSEC provides a secure binding between the server
 name and the TLSA record, and the TLSA record provides a binding to a
 certificate, this latter step can be indirect via a chain of
 certificates. For example, a certificate usage PKIX-TA TLSA record
 only authenticates the Certification Authority (CA) that issued the
 certificate, and third parties can obtain certificates from the same
 CA. Therefore, clients need to check to see whether or not the
 server's certificate matches one of the expected reference
 identifiers to ensure that the certificate was issued by the CA to
 the server the client expects (naturally, this is in addition to
 standard certificate-related checks as specified in [RFC5280],
 including but not limited to certificate syntax, certificate
 extensions such as name constraints and extended key usage, and
 handling of certification paths).
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10. References
10.1. Normative References
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
 specifying the location of services (DNS SRV)", RFC 2782,
 DOI 10.17487/RFC2782, February 2000,
 <http://www.rfc-editor.org/info/rfc2782>.
 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
 Rose, "DNS Security Introduction and Requirements",
 RFC 4033, DOI 10.17487/RFC4033, March 2005,
 <http://www.rfc-editor.org/info/rfc4033>.
 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
 Rose, "Protocol Modifications for the DNS Security
 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
 <http://www.rfc-editor.org/info/rfc4035>.
 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
 Housley, R., and W. Polk, "Internet X.509 Public Key
 Infrastructure Certificate and Certificate Revocation List
 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
 <http://www.rfc-editor.org/info/rfc5280>.
 [RFC5890] Klensin, J., "Internationalized Domain Names for
 Applications (IDNA): Definitions and Document Framework",
 RFC 5890, DOI 10.17487/RFC5890, August 2010,
 <http://www.rfc-editor.org/info/rfc5890>.
 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
 Extensions: Extension Definitions", RFC 6066,
 DOI 10.17487/RFC6066, January 2011,
 <http://www.rfc-editor.org/info/rfc6066>.
 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
 Verification of Domain-Based Application Service Identity
 within Internet Public Key Infrastructure Using X.509
 (PKIX) Certificates in the Context of Transport Layer
 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
 March 2011, <http://www.rfc-editor.org/info/rfc6125>.
Finch, et al. Standards Track [Page 11]
RFC 7673 TLSA and SRV October 2015
 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
 of Named Entities (DANE) Transport Layer Security (TLS)
 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
 August 2012, <http://www.rfc-editor.org/info/rfc6698>.
 [RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
 Conversations about DNS-Based Authentication of Named
 Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218,
 April 2014, <http://www.rfc-editor.org/info/rfc7218>.
 [RFC7671] Dukhovni, V. and W. Hardaker, "The DNS-Based
 Authentication of Named Entities (DANE) Protocol: Updates
 and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
 October 2015, <http://www.rfc-editor.org/info/rfc7671>.
 [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
 Opportunistic DNS-Based Authentication of Named Entities
 (DANE) Transport Layer Security (TLS)", RFC 7672,
 DOI 10.17487/RFC7672, October 2015,
 <http://www.rfc-editor.org/info/rfc7672>.
10.2. Informative References
 [RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
 Part Three: The Domain Name System (DNS) Database",
 RFC 3403, DOI 10.17487/RFC3403, October 2002,
 <http://www.rfc-editor.org/info/rfc3403>.
 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
 DOI 10.17487/RFC5321, October 2008,
 <http://www.rfc-editor.org/info/rfc5321>.
 [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
 Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
 March 2011, <http://www.rfc-editor.org/info/rfc6120>.
 [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
 Submission/Access Services", RFC 6186,
 DOI 10.17487/RFC6186, March 2011,
 <http://www.rfc-editor.org/info/rfc6186>.
 [XMPP-DNA] Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name
 Associations (DNA) in the Extensible Messaging and
 Presence Protocol (XMPP)", Work in Progress,
 draft-ietf-xmpp-dna-11, September 2015.
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Appendix A. Examples
 In the following, most of the DNS resource data is elided for
 simplicity.
A.1. IMAP
 ; mail domain
 _imap._tcp.example.com. SRV 10 0 9143 imap.example.net.
 example.com. RRSIG SRV ...
 ; target server hostname
 imap.example.net. A 192.0.2.1
 imap.example.net. RRSIG A ...
 imap.example.net. AAAA 2001:db8:212:8::e:1
 imap.example.net. RRSIG ...
 ; TLSA resource record
 _9143._tcp.imap.example.net. TLSA ...
 _9143._tcp.imap.example.net. RRSIG TLSA ...
 Mail messages received for addresses at example.com are retrieved via
 IMAP at imap.example.net. Connections to imap.example.net port 9143
 that use STARTTLS will get a server certificate that authenticates
 the name imap.example.net.
A.2. XMPP
 ; XMPP domain
 _xmpp-client._tcp.example.com. SRV 1 0 5222 im.example.net.
 _xmpp-client._tcp.example.com. RRSIG SRV ...
 ; target server hostname
 im.example.net. A 192.0.2.3
 im.example.net. RRSIG A ...
 im.example.net. AAAA 2001:db8:212:8::e:4
 im.example.net. RRSIG AAAA ...
 ; TLSA resource record
 _5222._tcp.im.example.net. TLSA ...
 _5222._tcp.im.example.net. RRSIG TLSA ...
 XMPP sessions for addresses at example.com are established at
 im.example.net. Connections to im.example.net port 5222 that use
 STARTTLS will get a server certificate that authenticates the name
 im.example.net.
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Appendix B. Rationale
 The long-term goal of this specification is to settle on TLS
 certificates that verify the target server hostname rather than the
 service domain name, since this is more convenient for servers
 hosting multiple domains (so-called "multi-tenanted environments")
 and scales up more easily to larger numbers of service domain names.
 There are a number of other reasons for doing it this way:
 o The certificate is part of the server configuration, so it makes
 sense to associate it with the target server hostname rather than
 the service domain name.
 o In the absence of TLS SNI, if the certificate identifies the
 target server hostname, then it does not need to list all the
 possible service domain names.
 o When the server certificate is replaced, it is much easier if
 there is one part of the DNS that needs updating to match, instead
 of an unbounded number of hosted service domain names.
 o The same TLSA records work with this specification, and with
 direct connections to the connection endpoint in the style of
 [RFC6698].
 o Some application protocols, such as SMTP, allow a client to
 perform transactions with multiple service domain names in the
 same connection. It is not, in general, feasible for the client
 to specify the service domain name using TLS SNI when the
 connection is established, and the server might not be able to
 present a certificate that authenticates all possible service
 domain names. See [RFC7672] for details.
 o It is common for SMTP servers to act in multiple roles -- for
 example, as outgoing relays or as incoming MX servers, depending
 on the client identity. It is simpler if the server can present
 the same certificate regardless of the role in which it is to act.
 Sometimes the server does not know its role until the client has
 authenticated, which usually occurs after TLS has been
 established. See [RFC7672] for details.
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 This specification does not provide an option to put TLSA records
 under the service domain name, because that would add complexity
 without providing any benefit; security protocols are best kept
 simple. As described above, there are real-world cases where
 authenticating the service domain name cannot be made to work, so
 there would be complicated criteria regarding when service domain
 name TLSA records might be used and when they cannot. This is all
 avoided by putting the TLSA records under the target server hostname.
 The disadvantage is that clients that do not complete DNSSEC
 validation must, according to [RFC6125] rules, check the server
 certificate against the service domain name, since they have no other
 way to authenticate the server. This means that SNI support or its
 functional equivalent is necessary for backward compatibility.
Acknowledgements
 Thanks to Mark Andrews for arguing that authenticating the target
 server hostname is the right thing, and that we ought to rely on
 DNSSEC to secure the SRV lookup. Thanks to Stephane Bortzmeyer,
 James Cloos, Viktor Dukhovni, Ned Freed, Olafur Gudmundsson, Paul
 Hoffman, Phil Pennock, Hector Santos, Jonas Schneider, and Alessandro
 Vesely for helpful suggestions.
 Carl Wallace completed an insightful review on behalf of the Security
 Directorate.
 Ben Campbell, Brian Haberman, and Alvaro Retana provided helpful
 feedback during IESG review.
 The authors gratefully acknowledge the assistance of Olafur
 Gudmundsson and Warren Kumari as the working group chairs and Stephen
 Farrell as the sponsoring Area Director.
 Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
 employing him during his work on earlier draft versions of this
 document.
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Authors' Addresses
 Tony Finch
 University of Cambridge Information Services
 Roger Needham Building
 7 JJ Thomson Avenue
 Cambridge CB3 0RB
 United Kingdom
 Phone: +44 797 040 1426
 Email: dot@dotat.at
 URI: http://dotat.at/
 Matthew Miller
 Cisco Systems, Inc.
 1899 Wynkoop Street, Suite 600
 Denver, CO 80202
 United States
 Email: mamille2@cisco.com
 Peter Saint-Andre
 &yet
 Email: peter@andyet.com
 URI: https://andyet.com/
Finch, et al. Standards Track [Page 16]