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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                               T. Finch
Entities (DANE)                                  University of Cambridge
Internet-Draft                                         February 25, 2013
Intended status: Standards Track
Expires: August 29, 2013


  Using DNS-Based Authentication of Named Entities (DANE) TLSA records
                        with SRV and MX records.
                         draft-ietf-dane-srv-02

Abstract

   The 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 with DNSSEC.  Some
   application protocols can use SRV records [RFC2782] 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.
   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 August 29, 2013.

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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Relation between SRV and MX records  . . . . . . . . . . . . .  3
   3.  DNS checks for TLSA and SRV records  . . . . . . . . . . . . .  4
     3.1.  SRV query  . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  TLSA queries . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  TLS checks for TLSA and SRV records  . . . . . . . . . . . . .  5
   5.  Guidance for application protocols . . . . . . . . . . . . . .  6
   6.  Guidance for server operators  . . . . . . . . . . . . . . . .  6
   7.  Security considerations  . . . . . . . . . . . . . . . . . . .  7
     7.1.  Mixed security status  . . . . . . . . . . . . . . . . . .  7
     7.2.  A service domain trusts its servers  . . . . . . . . . . .  7
     7.3.  Certificate subject name matching  . . . . . . . . . . . .  8
     7.4.  Deliberate omissions . . . . . . . . . . . . . . . . . . .  8
   8.  Internationalization Considerations  . . . . . . . . . . . . .  8
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     11.1. Normative References . . . . . . . . . . . . . . . . . . .  9
     11.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Example . . . . . . . . . . . . . . . . . . . . . . . 10
   Appendix B.  Rationale . . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11



















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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, but
   instead use a service domain.  The domain's servers are located
   indirectly via SRV records [RFC2782] (or MX records in the case of
   SMTP [RFC5321]).  When they do not use host names [RFC6698] does not
   direcly apply to these protocols.

   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 result of the
      SRV or MX query.

   o  The TLSA records are located using the SRV port, protocol, and
      target host name fields.

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

   o  The server's certificate is expected to authenticate the server
      host name, rather than the service domain.

   Separate documents give the details that are specific to particular
   application protocols.  For examples, see [I-D.ietf-dane-smtp] and
   [I-D.ietf-dane-mua].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   memo are to be interpreted as described in [RFC2119].


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




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   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

   For example this MX record is treated as if it were the following SRV
   record:

   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].


3.  DNS checks for TLSA and SRV records

3.1.  SRV query

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

   For this specification to take effect, 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
   behaviour.  (This corresponds to the AD bit being unset.)

   If any of the responses is "bogus" according to DNSSEC validation the



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   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 Section 4).

3.2.  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: port from
   the SRV RDATA, protocol from the SRV query name, and the TLSA base
   domain is the SRV target host name.

   For example this SRV record leads to the following TLSA query:

   _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 in section Section 4.

   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.)


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.  If the
   client received zero usable TLSA certificate associations, it SHALL
   validate the server's TLS certificate using the normal PKIX rules



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   [RFC5280] 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] with the
   preferred name chosen as follows.  It SHALL verify the identity
   asserted by the server's certificate according to [RFC6125] section
   6, using a list of reference identifiers constructed as follows.

   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.

   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.

   (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.)


5.  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: (This section is
   currently sketchy.)

   o  SRV fallback logic?  In the event of bogus replies etc.

   o  Compatibility with non-SRV clients.


6.  Guidance for server operators

   In order to support this specification, server software MUST
   implement the TLS Server Name Indication extension (TLS SNI)
   [RFC6066] for selecting the appropriate certificate.

   A server that supports TLS and is the target of a 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



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   not use TLS SNI.

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


7.  Security considerations

7.1.  Mixed security status

   We do not specify that clients check that 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 fall-back 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.

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






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7.3.  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.

   So this specification says that clients check that 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.

7.4.  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.)


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.  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



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   Schneider, and Alessandro Vesely for helpful suggestions.


11.  References

11.1.  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.

   [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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11.2.  Informative References

   [I-D.ietf-dane-smtp]
              Finch, T., "Secure SMTP using DNS-Based Authentication of
              Named Entities (DANE) TLSA records.", draft-ietf-dane-smtp
              (work in progress), March 2013.

   [I-D.ietf-dane-mua]
              Finch, T., "Using DNS-Based Authentication of Named
              Entities (DANE) with POP, IMAP, and message submission.",
              draft-ietf-dane-mua (work in progress), March 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 and scales up more easily to larger numbers of service
   domains.

   There are a number of other reasons for doing it this way:





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   o  The certificate is part of the server configuration, so it makes
      sense to associate it with the server 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.

   o  It is common for SMTP servers to act in multiple roles, 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 Server Name Indication
   support is necessary for backwards compatibility.







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Author's Address

   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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