DNS-Based Authentication of Named Entities (DANE)               T. FinchInternet-Draft                                   University of CambridgeIntended status: Standards Track                               M. MillerExpires: October 25, 2015                            Cisco Systems, Inc.                                                          P. Saint-Andre                                                                    &yet                                                          April 23, 2015  Using DNS-Based Authentication of Named Entities (DANE) TLSA Records                            with SRV Records                         draft-ietf-dane-srv-14Abstract   The 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 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 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 Internet-Draft is submitted in full conformance with the   provisions of BCP 78 and BCP 79.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF).  Note that other groups may also distribute   working documents as Internet-Drafts.  The list of current Internet-   Drafts is at http://datatracker.ietf.org/drafts/current/.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on October 25, 2015.Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.Finch, et al.           Expires October 25, 2015                [Page 1]Internet-Draft                TLSA and SRV                    April 2015   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  . . . . . . . . . . . . . . . . . . . . . . . .   2   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4   3.  DNS Checks  . . . . . . . . . . . . . . . . . . . . . . . . .   4     3.1.  SRV Query . . . . . . . . . . . . . . . . . . . . . . . .   4     3.2.  Address Queries . . . . . . . . . . . . . . . . . . . . .   5     3.3.  TLSA Queries  . . . . . . . . . . . . . . . . . . . . . .   5     3.4.  Impact on TLS Usage . . . . . . . . . . . . . . . . . . .   6   4.  TLS Checks  . . . . . . . . . . . . . . . . . . . . . . . . .   6     4.1.  SRV Records Only  . . . . . . . . . . . . . . . . . . . .   6     4.2.  TLSA Records  . . . . . . . . . . . . . . . . . . . . . .   7   5.  Guidance for Protocol Authors . . . . . . . . . . . . . . . .   7   6.  Guidance for Server Operators . . . . . . . . . . . . . . . .   8   7.  Guidance for Application Developers . . . . . . . . . . . . .   9   8.  Internationalization Considerations . . . . . . . . . . . . .   9   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9   10. Security Considerations . . . . . . . . . . . . . . . . . . .   9     10.1.  Mixed Security Status  . . . . . . . . . . . . . . . . .   9     10.2.  Certificate Subject Name Matching  . . . . . . . . . . .   9   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10     11.1.  Normative References . . . . . . . . . . . . . . . . . .  10     11.2.  Informative References . . . . . . . . . . . . . . . . .  11   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  11     A.1.  IMAP  . . . . . . . . . . . . . . . . . . . . . . . . . .  12     A.2.  XMPP  . . . . . . . . . . . . . . . . . . . . . . . . . .  12   Appendix B.  Rationale  . . . . . . . . . . . . . . . . . . . . .  13   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  14   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  141.  Introduction   The base DANE specification [RFC6698] describes how to use TLSA   resource records secured by DNSSEC [RFC4033] to associate a target   server's connection endpoint with its 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 thisFinch, et al.           Expires October 25, 2015                [Page 2]Internet-Draft                TLSA and SRV                    April 2015   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 [I-D.ietf-dane-smtp-with-dane].)   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 to the      corresponding target server connection endpoints (i.e., the target      server host names and port numbers returned in the SRV records for      that service type).   o  Although in accordance with [RFC2782] a service domain 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 host name for the target      server (not the service domain).   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 host name 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 or target server host name, as detailed      in [I-D.ietf-dane-ops].   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].Finch, et al.           Expires October 25, 2015                [Page 3]Internet-Draft                TLSA and SRV                    April 20152.  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 draft uses the definitions for "secure", "insecure", "bogus",   and "indeterminate" from Section 4.3 of [RFC4035].  This draft 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 host name,      transport protocol, and port number that a client uses to      establish a connection to the target server.   service domain:  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 host name" in place of the   term "derived domain" from the CertID specification [RFC6125].3.  DNS Checks3.1.  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      [I-D.ietf-dane-smtp-with-dane].   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 applyFinch, et al.           Expires October 25, 2015                [Page 4]Internet-Draft                TLSA and SRV                    April 2015   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 host names as described in the following   section.3.2.  Address Queries   For each SRV target server connnection 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 (this 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 mitigates against downgrade attacks.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 host   name.   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 ?Finch, et al.           Expires October 25, 2015                [Page 5]Internet-Draft                TLSA and SRV                    April 20153.4.  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.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 apply to such secure   responses; Section 4.2 where there is at least one usable TLSA   record, and Section 4.1 otherwise.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 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" to refer   to the same things as "service domain" and "target server host name"   in this document).  The examples below assume a service domain ofFinch, et al.           Expires October 25, 2015                [Page 6]Internet-Draft                TLSA and SRV                    April 2015   "im.example.com" and a target server host name of   "xmpp23.hosting.example.net".   SRV is insecure:  The reference identifiers SHALL include the service      domain and MUST NOT include the SRV target server host name (e.g.,      include "im.example.com" but not "xmpp23.hosting.example.net").      The service domain is the preferred name for TLS SNI or its      equivalent.   SRV is secure:  The reference identifiers SHALL include both the      service domain and the SRV target server host name (e.g., include      both "im.example.com" and "xmpp23.hosting.example.net").  The      target server host name is the preferred name for TLS SNI or its      equivalent.   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.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 they do not support SRV lookups, or      if clients that support SRV lookups encounter service domains that      do not offer SRV records.Finch, et al.           Expires October 25, 2015                [Page 7]Internet-Draft                TLSA and SRV                    April 2015   o  Whether the application protocol has a functional equivalent for      TLS SNI that is preferred within that protocol.   o  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, [I-D.ietf-xmpp-dna] covers such topics for the   Extensible Messaging and Presence Protocol (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 host name.  However,   operators need to be aware that servers relying solely on validation   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 host name (the "derived domain" in [RFC6125]      terms, which is the SRV target host name).   Servers that support multiple service domains (i.e., so-called   "multi-tenanted environments") can implement the Transport Layer   Security Server Name Indication (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 host name.  However, the server   determines what certificate to present in the TLS handshake; e.g.,   the presented certificate might only authenticate the target server   host name.Finch, et al.           Expires October 25, 2015                [Page 8]Internet-Draft                TLSA and SRV                    April 20157.  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 SRV   queries, 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 SRV queries and 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].9.  IANA Considerations   No IANA action is required.10.  Security Considerations10.1.  Mixed Security Status   We do not specify that all of the target server connection endpoints   for a service domain 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 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.10.2.  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.Finch, et al.           Expires October 25, 2015                [Page 9]Internet-Draft                TLSA and SRV                    April 2015   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 CA that issued the certificate, and third   parties can obtain certificates from the same CA.  Therefore, clients   need to check whether 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).11.  References11.1.  Normative References   [I-D.ietf-dane-ops]              Dukhovni, V. and W. Hardaker, "Updates to and Operational              Guidance for the DANE Protocol", draft-ietf-dane-ops-07              (work in progress), October 2014.   [I-D.ietf-dane-smtp-with-dane]              Dukhovni, V. and W. Hardaker, "SMTP security via              opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-15              (work in progress), March 2015.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119, March 1997.   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for              specifying the location of services (DNS SRV)", RFC 2782,              February 2000.   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "DNS Security Introduction and Requirements", RFC              4033, March 2005.   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.              Rose, "Protocol Modifications for the DNS Security              Extensions", RFC 4035, March 2005.   [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, May 2008.Finch, et al.           Expires October 25, 2015               [Page 10]Internet-Draft                TLSA and SRV                    April 2015   [RFC5890]  Klensin, J., "Internationalized Domain Names for              Applications (IDNA): Definitions and Document Framework",              RFC 5890, August 2010.   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:              Extension Definitions", RFC 6066, January 2011.   [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, March 2011.   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication              of Named Entities (DANE) Transport Layer Security (TLS)              Protocol: TLSA", RFC 6698, August 2012.   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify              Conversations about DNS-Based Authentication of Named              Entities (DANE)", RFC 7218, April 2014.11.2.  Informative References   [I-D.ietf-xmpp-dna]              Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name              Associations (DNA) in the Extensible Messaging and              Presence Protocol (XMPP)", draft-ietf-xmpp-dna-10 (work in              progress), March 2015.   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)              Part Three: The Domain Name System (DNS) Database", RFC              3403, October 2002.   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,              October 2008.   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence              Protocol (XMPP): Core", RFC 6120, March 2011.   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email              Submission/Access Services", RFC 6186, March 2011.Appendix A.  Examples   In the following, most of the DNS resource data is elided for   simplicity.Finch, et al.           Expires October 25, 2015               [Page 11]Internet-Draft                TLSA and SRV                    April 2015A.1.  IMAP   ; mail domain   _imap._tcp.example.com.   SRV 10 0 9143 imap.example.net.   example.com.              RRSIG   SRV ...   ; target server host name   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 host name   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.Finch, et al.           Expires October 25, 2015               [Page 12]Internet-Draft                TLSA and SRV                    April 2015Appendix B.  Rationale   The long-term goal of this specification is to settle on TLS   certificates that verify the target server host name rather than the   service domain, 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 domains.   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 server host name rather than the      service domain.   o  In the absence of TLS SNI, if the certificate identifies the      target server host name then it does not need to list all the      possible service domains.   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 domains.   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 domains in the same      connection.  It is not in general feasible for the client to      specify the service domain using TLS SNI when the connection is      established, and the server might not be able to present a      certificate that authenticates all possible service domains.  See      [I-D.ietf-dane-smtp-with-dane] 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 [I-D.ietf-dane-smtp-with-dane] for details.   This specification does not provide an option to put TLSA records   under the service domain because that would add complexity without   providing any benefit, and security protocols are best kept simple.   As described above, there are real-world cases where authenticating   the service domain cannot be made to work, so there would be   complicated criteria for when service domain TLSA records might beFinch, et al.           Expires October 25, 2015               [Page 13]Internet-Draft                TLSA and SRV                    April 2015   used and when they cannot.  This is all avoided by putting the TLSA   records under the target server host name.   The disadvantage is that clients which do not complete DNSSEC   validation must, according to [RFC6125] rules, check the server   certificate against the service domain, since they have no other way   to authenticate the server.  This means that SNI support or its   functional equivalent is necessary for backward compatibility.Appendix C.  Acknowledgements   Thanks to Mark Andrews for arguing that authenticating the target   server host name 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 versions of this document.Authors' Addresses   Tony Finch   University of Cambridge Computing Service   New Museums Site   Pembroke Street   Cambridge  CB2 3QH   ENGLAND   Phone: +44 797 040 1426   Email: dot@dotat.at   URI:   http://dotat.at/Finch, et al.           Expires October 25, 2015               [Page 14]Internet-Draft                TLSA and SRV                    April 2015   Matthew Miller   Cisco Systems, Inc.   1899 Wynkoop Street, Suite 600   Denver, CO  80202   USA   Email: mamille2@cisco.com   Peter Saint-Andre   &yet   Email: peter@andyet.com   URI:   https://andyet.com/Finch, et al.           Expires October 25, 2015               [Page 15]