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Using DNS-Based Authentication of Named Entities (DANE) TLSA records with SRV and MX records.
draft-ietf-dane-srv-03

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This is an older version of an Internet-Draft that was ultimately published as RFC 7673.
Authors Tony Finch , Matthew A. Miller , Peter Saint-Andre
Last updated 2013-12-19
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draft-ietf-dane-srv-03
DNS-Based Authentication of Named Entities (DANE)               T. Finch
Internet-Draft                                   University of Cambridge
Intended status: Standards Track                               M. Miller
Expires: June 16, 2014                                    P. Saint-Andre
                                                     Cisco Systems, Inc.
                                                       December 13, 2013

  Using DNS-Based Authentication of Named Entities (DANE) TLSA records
                        with SRV and MX records.
                         draft-ietf-dane-srv-03

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 association is secured with DNSSEC.  Some
   application protocols use SRV records (RFC 2782) to indirectly name
   the server hosts for a service domain (SMTP uses MX records for the
   same purpose).  This specification gives generic instructions for how
   these application protocols 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.

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 June 16, 2014.

Copyright Notice

   Copyright (c) 2013 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

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   (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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Relation between SRV and MX records . . . . . . . . . . . . .   3
   4.  DNS Checks for TLSA and SRV Records . . . . . . . . . . . . .   4
     4.1.  SRV Query . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  TLSA Queries  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  TLS Checks for TLSA and SRV Records . . . . . . . . . . . . .   5
   6.  Guidance for Application Protocols  . . . . . . . . . . . . .   6
   7.  Guidance for Server Operators . . . . . . . . . . . . . . . .   6
   8.  Internationalization Considerations . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7
     10.1.  Mixed Security Status  . . . . . . . . . . . . . . . . .   7
     10.2.  A Service Domain Trusts its Servers  . . . . . . . . . .   7
     10.3.  Certificate Subject Name Matching  . . . . . . . . . . .   8
     10.4.  Deliberate Omissions . . . . . . . . . . . . . . . . . .   8
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Rationale  . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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 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 and the relevant 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"

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   and the "derived domain".)  Because of this intermediate resolution
   step, the normal DANE rules specified in [RFC6698] do not directly
   apply to protocols that use SRV or MX records.

   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
      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 the details that are specific to particular
   application protocols, such as SMTP [I-D.ietf-dane-smtp-with-dane]
   and XMPP [I-D.ietf-xmpp-dna].

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].

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 to an SRV record [RFC2782] with
   corresponding fields copied from the MX record and the remaining
   fields having fixed values as follows:

   Service -  smtp

   Proto -  tcp

   Name -  MX owner name (mail domain)

   TTL -  MX TTL

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   Class -  MX Class

   Priority -  MX Priority

   Weight -  0

   Port -  25

   Target -  MX Target

   Thus we can treat 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 in
   [I-D.ietf-dane-smtp-with-dane].

4.  DNS Checks for TLSA and SRV Records

4.1.  SRV Query

   When the client makes an SRV query, a successful result will be a
   list of one or more SRV records (or possibly a chain of CNAME / DNAME
   aliases referring to such a list).

   For this specification to apply, all of these DNS RRsets MUST be
   "secure" according to DNSSSEC validation ([RFC4033] section 5).  In
   the case of aliases, the whole chain MUST be secure as well as the
   ultimate target.  (This corresponds to the AD bit being set in the
   response(s) - see [RFC4035] section 3.2.3.)

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

   If any of the responses is "bogus" according to DNSSEC validation,
   the client MUST abort.  (This usually corresponds to a "server
   failure" response.)

   In the successful case, the client now has an authentic list of
   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 takes note of the DNSSEC validation status of the SRV response for
   use when checking certificate names (see Section 5).

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4.2.  TLSA Queries

   This sub-section applies to each server host name individually,
   provided the SRV response was secure according to DNSSEC validation.

   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 host name.

   For example, the following SRV record leads to the TLSA query shown
   below:

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

   _143._tcp.imap.example.net. IN TLSA ?

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

   If the TLSA response is "secure", the client MUST use TLS when
   connecting to the server.  The TLSA records are used when validating
   the server's certificate as described under Section 5.

   If the TLSA response is "insecure" or "indeterminate", the client
   SHALL proceed as if this server has no TLSA records.  It MAY connect
   to the server with or without TLS.

   If the TLSA response is "bogus", then the client MUST NOT connect to
   the corresponding server.  (The client can still use other SRV
   targets.)

5.  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.  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.

   The client uses the DNSSEC validation status of the SRV query in its
   server certificate identity checks.  (The TLSA validation status does
   not affect the server certificate identity checks.)  It SHALL use the
   Server Name Indication extension (TLS SNI) [RFC6066] or its
   functional equivalent in the relevant application protocol (e.g., in

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   XMPP [RFC6120] this is the 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, 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 host name" in this document.)

   SRV is insecure or indeterminate:  The reference identifiers SHALL
      include the service domain and MUST NOT include the SRV target
      host name.  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 host name.  The target 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 with servers that do and do not support this
   specification.)

6.  Guidance for Application Protocols

   Separate documents describe how to apply this specification to
   particular application protocols.  If you are writing such as
   document the following points ought to be covered:

   o  Fallback logic in the event of bogus replies and the like.

   o  Compatibility with clients that do not support SRV lookups.

7.  Guidance for Server Operators

   In order to support this specification, server software MUST
   implement the TLS Server Name Indication extension (TLS SNI)
   [RFC6066] (or its functional equivalent in the relevant application
   protocol) for selecting the appropriate certificate.

   A server that supports TLS and is the target of an SRV record MUST
   have a TLS certificate that authenticates the SRV query domain (i.e.
   the service domain, or "source domain" in [RFC6125] terms).  This is
   necessary for clients that cannot perform DNSSEC validation.  This
   certificate MUST be the default that is presented if the client does
   not use TLS SNI or its functional equivalent.

   In order to support this specification, the server SHOULD also have a
   certificate that authenticates the SRV target domain (e.g., the mail
   server hostname).  This can be done using a multi-name certificate or

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   by using the client's TLS SNI or its functional equivalent to select
   the appropriate certificate.  The server's TLSA record SHOULD
   correspond to this certificate.

   Note: In some application protocols, there are old non-SRV clients
   that expect a server's TLS certificate to authenticate its host name;
   they are also unlikely to support SNI.  This means that servers for
   old clients need a different default certificate from servers that
   are the targets of SRV records.  If the server does not have a
   certificate that authenticates all relevant names, it is necessary to
   segregate old and new clients.  This can be done by using different
   target hosts or non-standard ports in the SRV targets.  (The latter
   avoids the need for additional certificates.)

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 clients checking all of a service domain's
   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.  A Service Domain Trusts its Servers

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   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, and publishing a
   TLSA record under that name.  If these are not correct then
   connections from TLSA-aware clients might fail.

10.3.  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 checking might appear to be unnecessary, since DNSSEC provides a
   secure binding between the server name and the TLSA record, which in
   turn authenticates the certificate.  However this latter step can be
   indirect, via a chain of certificates.  A usage=0 TLSA record only
   authenticates the CA that issued the certificate, and third parties
   can obtain certificates from the same CA.

   Therefore this specification says that a client needs to check
   whether the server's certificate matches the server host name, to
   ensure that the certificate was issued by the CA to the server that
   the client is connecting to.  The client always performs this check
   regardless of the TLSA usage, to simplify implementation and so that
   this specification is less likely to need updating when new TLSA
   usages are added.

10.4.  Deliberate Omissions

   We do not specify that clients check the DNSSEC state of the server
   address records.  This is not necessary since the certificate checks
   ensure that the client has connected to the correct server.  (The
   address records will normally have the same security state as the
   TLSA records, but they can differ if there are CNAME or DNAME
   indirections.)

11.  Acknowledgements

   Thanks to Mark Andrews for arguing that authenticating the 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, Ned Freed, Olafur
   Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, Jonas
   Schneider, and Alessandro Vesely for helpful suggestions.

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12.  References

12.1.  Normative References

   [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.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

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12.2.  Informative References

   [I-D.ietf-dane-smtp-with-dane]
              Dukhovni, V. and W. Hardaker, "(DANE) TLSA records.",
              draft-ietf-dane-smtp-with-dane (work in progress),
              November 2013.

   [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-04 (work in progress),
              October 2013.

Appendix A.  Example

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

   ; mail domain
   example.com.              MX      1 mx.example.net.
   example.com.              RRSIG   MX ...

   ; SMTP server host name
   mx.example.net.           A      192.0.2.1
   mx.example.net.           AAAA   2001:db8:212:8::e:1

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

   Mail for addresses at example.com is delivered by SMTP to
   mx.example.net.  Connections to mx.example.net port 25 that use
   STARTTLS will get a server certificate that authenticates the name
   mx.example.net.

Appendix B.  Rationale

   The long-term goal of this specification is to settle on TLS
   certificates that verify the 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.

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   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.

   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.

   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 server host name.

   The disadvantage is that clients which do not do 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

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

   Email: psaintan@cisco.com

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