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Versions: 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 7673

DNS-Based Authentication of Named Entities (DANE)               T. Finch
Internet-Draft                                   University of Cambridge
Intended status: Standards Track                               M. Miller
Expires: 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-14

Abstract

   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.





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   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  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  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 this



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   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].






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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 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 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
      [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 apply



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   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 ?



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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.

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 of



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   "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.




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   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.







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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 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 Considerations

10.1.  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.



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   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.  References

11.1.  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.




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   [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.





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A.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.




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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 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
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   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/








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   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/





































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