DNS-Based Authentication of Named Entities (DANE)               T. Finch
Internet-Draft                                   University of Cambridge
Intended status: Standards Track                               M. Miller
Expires: August 17, December 12, 2014                           Cisco Systems, Inc.
                                                          P. Saint-Andre
                                                                    &yet
                                                       February 13,
                                                           June 10, 2014

  Using DNS-Based Authentication of Named Entities (DANE) TLSA records Records
                            with SRV and MX records.
                         draft-ietf-dane-srv-05 Records
                         draft-ietf-dane-srv-06

Abstract

   The DANE specification (RFC 6698) describes how to use TLSA resource
   records in the DNS to associate a server's host name with its TLS
   certificate.  The
   certificate, where the association is secured with DNSSEC.  Some  However,
   application protocols that use SRV records (RFC 2782) to indirectly
   name the target server hosts host names for a service domain (SMTP uses MX records for cannot apply
   the
   same purpose).  This specification gives generic instructions for how
   these application rules from RFC 6698.  Therefore this document provides guidelines
   that enable such protocols to locate and use TLSA records when
   technologies such as SRV records are used.  Separate documents give
   the details that are specific to particular application protocols. 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 August 17, December 12, 2014.

Copyright Notice

   Copyright (c) 2014 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
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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Relation between SRV and MX records  DNS Checks  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  DNS Checks for TLSA and
     3.1.  SRV Records Query . . . . . . . . . . . . . . . . . .   4
     4.1.  SRV Query . . . . . .   4
     3.2.  Address Queries . . . . . . . . . . . . . . . . . . . . .   4
     4.2.
     3.3.  TLSA Queries  . . . . . . . . . . . . . . . . . . . . . .   5
   5.
     3.4.  Impact on TLS Usage . . . . . . . . . . . . . . . . . . .   5
   4.  TLS Checks for TLSA and  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  SRV Records Only  . . . . . . . . . . . . . . . . . . . .   5
     4.2.  TLSA Records  . . . . . . . . . . . . . . . . . . . . . .   6
   6.
   5.  Guidance for Application Protocols  . . . . . . . . . . . . .   7
   7.
   6.  Guidance for Server Operators . . . . . . . . . . . . . . . .   7
   8.
   7.  Internationalization Considerations . . . . . . . . . . . . .   8
   9.
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10.
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     10.1.
     9.1.  Mixed Security Status . . . . . . . . . . . . . . . . . .   8
     10.2.
     9.2.  A Service Domain Trusts its Servers . . . . . . . . . . .   8
     10.3.
     9.3.  Certificate Subject Name Matching . . . . . . . . . . . .   9
   11.
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   12.
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Mail Example  Examples . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix B.
     A.1.  IMAP  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.2.  XMPP Example  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Appendix C. B.  Rationale  . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The base DANE specification [RFC6698] describes how to use TLSA
   resource records in the DNS to associate a server's host name with
   its TLS certificate.  The certificate, where the association is secured using DNSSEC.
   That document "only relates to securely associating certificates for
   TLS and DTLS with host names" (see the last paragraph of section 1.2
   of [RFC6698]).

   Some application protocols do not use host names directly; instead,
   they use a service domain domain, and the relevant target server host names
   are located indirectly via SRV records [RFC2782], or MX records in the case of
   SMTP [RFC5321] (Note: in the "CertID" specification [RFC6125], the
   source domain and host name are referred to as the "source domain"
   and the "derived domain"). [RFC2782].  Because of this
   intermediate resolution step, the normal DANE rules specified in
   [RFC6698] do not directly
   apply cannot be applied to protocols that use SRV or MX records.
   (Rules for SMTP [RFC5321], which uses MX 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 and MX
   records.  To summarize:

   o  We rely on DNSSEC to secure the association between the service
      domain and the target server host names (i.e., the host names that
      are discovered by the SRV or MX query).

   o  The TLSA records are located using the port, protocol, and target
      server host name fields (not the service domain).

   o  Clients always use TLS when connecting to servers with TLSA
      records.

   o  Assuming that the association is secure, the server's certificate
      is expected to authenticate the target server host name, rather
      than the service domain.

   Separate documents give

   Note: The "CertID" specification [RFC6125] does not use the details that are specific terms
   "service domain" and "target server host name", but refers to particular
   application protocols, such as SMTP [I-D.ietf-dane-smtp-with-dane] the
   same entities with the terms "source domain" and XMPP [I-D.ietf-xmpp-dna]. "derived domain".

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 [RFC4035]. [RFC4033].  This draft uses the acronyms
   from [I-D.ietf-dane-registry-acronyms] [RFC7218] for the values of TLSA fields where appropriate.

3.  Relation between SRV and MX records

   For the purpose of this specification (to avoid cluttering the
   description with special cases) we treat each MX record ([RFC5321]
   section 5) as being equivalent  DNS Checks

   To expedite connection to an SRV record [RFC2782] with
   corresponding fields copied from the MX record and intended service, where possible the remaining
   fields having fixed values as follows:

   Table 1: SRV Fields and MX Equivalents

      +---------------+-----------------------------+
      | DNS SRV Field | Equivalent MX Value         |
      +---------------+-----------------------------+
      | Service       | smtp                        |
      +---------------+-----------------------------+
      | Proto         | tcp                         |
      +---------------+-----------------------------+
      | Name          | MX owner name (mail domain) |
      +---------------+-----------------------------+
      | TTL           | MX TTL                      |
      +---------------+-----------------------------+
      | Class         | MX Class                    |
      +---------------+-----------------------------+
      | Priority      | MX Priority                 |
      +---------------+-----------------------------+
      | Weight        | 0                           |
      +---------------+-----------------------------+
      | Port          | 25                          |
      +---------------+-----------------------------+
      | Target        | MX Target                   |
      +---------------+-----------------------------+

   Thus we can treat
   queries described in the following MX record as if it were the SRV
   record shown below:

      example.com.            86400 IN MX  10      mx.example.net.

      _smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net.

   Other details that are specific to SMTP are described sections SHOULD be performed in
   [I-D.ietf-dane-smtp-with-dane].

4.  DNS Checks
   parallel (this is similar to the "happy eyeballs" approach for TLSA IPv4
   and SRV Records

4.1. IPv6 connections described in [RFC6555]).

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 referring leading to such a list).

   For this specification to apply, all of these the entire DNS RRsets RRset that is
   returned MUST be "secure" according to DNSSSEC validation ([RFC4033]
   section 5).  In the case of aliases, the whole chain of CNAME and
   DNAME RRsets MUST be secure as well.  This corresponds to the AD bit
   being set in the response(s); see [RFC4035] section 3.2.3.

   If they are the the entire RRset is not all secure, this protocol has not been
   correctly deployed.  The client SHOULD fall back to its non-DNSSEC non-DNSSEC,
   non-DANE behavior (this corresponds to the AD bit being unset).

   If any of the responses are a particular response is "bogus" or "indeterminate" according to
   DNSSEC validation, the client MUST abort (This usually corresponds to
   a "server failure" response). ignore that target server host
   name.

   In the successful case, the client now has an authentic list of
   target server host names 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 5).

4.2.  TLSA Queries

   If the SRV response was insecure, the 4).
   The client MUST NOT perform any
   TLSA queries.  If the SRV response is "secure" according can now proceed to DNSSEC
   validation, the client performs a TLSA query for each SRV making address queries on the target
   server host names as described in this the next section.

3.2.  Address Queries

   For each SRV target server host name, the client makes A / AAAA
   queries, performs DNSSEC validation on the address (A, AAAA) response
   response, and continues as follows based on the results:

   o  if  If the response is "secure" and usable, the client MUST perform a
      TLSA query for that target server host name as described in the
      next section.

   o  If the response is "insecure", the client MUST NOT perform a TLSA
      query for that target; target server host name; the TLSA query will most
      likely fail.

   o  If the response is "bogus" or "indeterminate", the client MUST NOT
      connect to this host name; target server; instead it uses the next most
      appropriate SRV target.

3.3.  TLSA Queries

   The client SHALL construct the TLSA query name as described in
   [RFC6698] section 3, based on fields from the SRV record: the port
   from the SRV RDATA, the protocol from the SRV query name, and the
   TLSA base domain set to the SRV target server host name.

   For example, the following SRV record for IMAP (see [RFC6186]) leads
   to the TLSA query shown below:

   _imap._tcp.example.com. 86400 IN SRV 10 0 143 9143 imap.example.net.

   _143._tcp.imap.example.net.

   _9143._tcp.imap.example.net. IN TLSA ?

3.4.  Impact on TLS Usage

   The client SHALL determine if the TLSA record(s) returned in the
   previous step are usable according to section 4.1 of [RFC6698].  This
   affects SRV handling the use TLS as follows:

   o  If the TLSA response is "secure", "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
      under Section 5. 4.

   o  If the TLSA response is "insecure", then the client SHALL proceed
      as if
   this the target server has had no TLSA records.  It MAY connect to
      the target server with or without TLS. TLS, subject to the policies of
      the application protocol or client implementation.

   o  If the TLSA response is "bogus" or "indeterminate", then the
      client MUST NOT connect to this the target server (the client can still
      use other SRV targets).

5.

4.  TLS Checks for TLSA and SRV Records

   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.

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.  If the client received
   one or more usable TLSA certificate associations, it SHALL process
   them as described in [RFC6698] section 2.1.

   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 consider the server to be authenticated.
   Because the information in such a TLSA record supersedes the non-key
   information in the certificate, all other [RFC5280] and [RFC6125]
   authentication checks (e.g., reference identifier, key usage,
   expiration, issuance, etc.)  MUST be ignored or omitted.

   Otherwise,

   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 [RFC6125] section 6, 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" and
   "target server host name" in this document).  The examples below
   assume a service domain of "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. 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. 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 so that it
   is compatible to ensure
   compatibility with servers that do and support this specification as well as
   servers that do not support this
   specification.

6.  Guidance for Application Protocols

   Separate documents describe how 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 consider the server to be authenticated.
   Because the information in such a TLSA record supersedes the non-key
   information in the certificate, all other [RFC5280] and [RFC6125]
   authentication checks (e.g., reference identifier, key usage,
   expiration, issuance) MUST be ignored or omitted.

5.  Guidance for Application Protocols

   This document describes how to use DANE with application protocols in
   which target servers are discovered via SRV records.  Although this
   document attempts to apply this specification provide generic guidance applying to all such
   protocols, additional documents for particular application protocols.  Such documents ought to protocols
   could cover the
   following points: related topics, such as:

   o  Fallback logic in the event of bogus replies and that a client is unable to connect
      securely to a target server by following the like. procedures defined in
      this document.

   o  The use of TLS SNI  How clients ought to behave if they do not support SRV lookups, or its functional equivalent.

   o  Appropriate mappings for non-SRV technologies such as MX.

   o  Compatibility with
      if clients that support SRV lookups encounter service domains that
      do not support offer SRV lookups.

7. records.

   o  Whether the application protocol has a functional equivalent for
      TLS SNI that is preferred within that protocol.

   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, 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 does not need to contain any of the possible
   reference identifiers discussed below.  Indeed, none of the
   certificate's information is necessary an identifier for such certificates.
   either the source domain or target server host name.  However,
   servers that rely solely on validation using Certificate Usage "DANE-EE" "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 a reference 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).

   Servers that support multiple service domains (i.e., multi-tenant) so-called
   "multi-tenanted environments") can implement the Transport Layer
   Security Server Name Indicator 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.

8.

7.  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.

8.  IANA Considerations

   No IANA action is required.

10.

9.  Security Considerations

10.1.

9.1.  Mixed Security Status

   We do not specify that clients checking all of a service domain's
   target server host names are 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 and MX sorting rules are unchanged; in particular they have not been
   altered in order to prioritize secure servers over insecure servers.
   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.

9.2.  A Service Domain Trusts its Servers

   By signing their zone with DNSSEC, service domain operators
   implicitly instruct their clients to check their server TLSA records.
   This implies another point in the trust relationship between service
   domain holders and their server operators.  Most of the setup
   requirements for this protocol fall on the server operator:
   installing a TLS certificate with the correct name (where necessary),
   and publishing a TLSA record for that certificate.  If these are not
   correct then connections from TLSA-aware clients might fail.

10.3.

9.3.  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 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.

11.

10.  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 / MX lookup.  Thanks to James Cloos, Viktor
   Dukhovni, Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock,
   Hector Santos, Jonas Schneider, and Alessandro Vesely for helpful
   suggestions.

12.

11.  References

12.1.

11.1.  Normative References

   [I-D.ietf-dane-registry-acronyms]
              Gudmundsson, O., "Adding acronyms to simplify DANE
              conversations", draft-ietf-dane-registry-acronyms-03 (work
              in progress), January 2014.

   [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.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [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.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 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.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186, 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.

12.2.

   [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-dane-smtp-with-dane]
              Dukhovni, V. and W. Hardaker, "SMTP security via
              opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05
              (work in progress), February 2014.

   [I-D.ietf-xmpp-dna]
              Saint-Andre, P. and M. Miller, "Domain Name Associations
              (DNA) in the Extensible Messaging and Presence Protocol
              (XMPP)", draft-ietf-xmpp-dna-05 (work in progress),
              February 2014.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

Appendix A.  Mail Example  Examples

   In the following, most of the DNS resource data is elided for
   simplicity.

A.1.  IMAP

   ; mail domain
   example.com.              MX      1 mx.example.net.
   _imap._tcp.example.com.   SRV 10 0 9143 imap.example.net.
   example.com.              RRSIG   MX   SRV ...

   ; SMTP target server host name
   mx.example.net.
   imap.example.net.         A      192.0.2.1
   mx.example.net.
   imap.example.net.         RRSIG  A ...

   mx.example.net.

   imap.example.net.         AAAA   2001:db8:212:8::e:1
   mx.example.net.
   imap.example.net.         RRSIG  ...

   ; TLSA resource record
   _25._tcp.mx.example.net.
   _9143._tcp.imap.example.net.  TLSA   ...
   _25._tcp.mx.example.net.
   _9143._tcp.imap.example.net.  RRSIG  TLSA ...

   Mail messages submitted for addresses at example.com is delivered by SMTP are sent via
   IMAP to
   mx.example.net. imap.example.net.  Connections to mx.example.net imap.example.net port 25 9143
   that use STARTTLS will get a server certificate that authenticates
   the name
   mx.example.net.

Appendix B. imap.example.net.

A.2.  XMPP Example

   In the following, most of the DNS resource data is elided for
   simplicity.

   ; XMPP domain
   _xmpp-client.example.com. SRV     1 0 5222 im.example.net.
   _xmpp-client.example.com. RRSIG   SRV ...

   ; XMPP 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.

Appendix C. 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 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 host name 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 be
   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 do 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.

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/

   Matthew Miller
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Email: mamille2@cisco.com

   Peter Saint-Andre
   &yet
   P.O. Box 787
   Parker, CO  80134
   USA

   Email: ietf@stpeter.im peter@andyet.com