Using DNS-Based Authentication of Named Entities (DANE) TLSA records with SRV and MX records.
draft-ietf-dane-srv-00

DNS-Based Authentication of Named                               T. Finch
Entities (DANE)                                  University of Cambridge
Internet-Draft                                         February 18, 2013
Intended status: Standards Track
Expires: August 22, 2013

  Using DNS-Based Authentication of Named Entities (DANE) TLSA records
                        with SRV and MX records.
                         draft-ietf-dane-srv-00

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
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   This Internet-Draft will expire on August 22, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Using TLSA records with SRV and MX . . . . . . . . . . . . . .  3
     2.1.  MX records . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  SRV query  . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  TLSA queries . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Guidelines for application protocols . . . . . . . . . . . . .  5
   4.  Security considerations  . . . . . . . . . . . . . . . . . . .  5
     4.1.  Mixed security status  . . . . . . . . . . . . . . . . . .  5
     4.2.  A service domain trusts its servers  . . . . . . . . . . .  6
     4.3.  Certificate subject name matching  . . . . . . . . . . . .  6
     4.4.  Deliberate omissions . . . . . . . . . . . . . . . . . . .  6
   5.  Internationalization Considerations  . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Example . . . . . . . . . . . . . . . . . . . . . . .  8
   Appendix B.  Rationale . . . . . . . . . . . . . . . . . . . . . .  8
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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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 SRV or MX query.

   o  The TLSA records are located alongside the SRV target host names.

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

   o  The server's certificate is expected to match 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.  Using TLSA records with SRV and MX

2.1.  MX records

   For the purpose of this specification (to avoid cluttering the
   description with special cases) each MX record ([RFC5321] section 5)
   is treated 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

   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.

2.2.  SRV query

   When the client makes a SRV query, a successful result can 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 a (chain 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
   client MUST abort.  (This usually corresponds to a "server failure"
   response.)

   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

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   absence of DNSSEC or TLSA records.

2.3.  TLSA queries

   This sub-section applies to each server host name individually.

   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 ?

   o  A secure answer containing one or more TLSA records, in which case
      the client SHALL proceed as descrbed below.

   o  A bogus answer or other failure, which the client MUST treat as a
      temporary error.

   o  If there is no TLSA record or its DNSSEC validation state is
      insecure or indeterminate, this protocol has not been fully
      deployed.  The client SHOULD deliver to this server insecurely
      (which might be over unauthenticated TLS, as described in the
      introduction).

3.  Guidelines 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  How should the client react to a "bogus" DNSSEC status?

4.  Security considerations

4.1.  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.

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   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.

4.2.  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.

4.3.  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.

4.4.  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.)

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5.  Internationalization Considerations

   If any of the DNS queries are for an internationalized domain name,
   then they need to use the A-label form [RFC5890].

6.  IANA Considerations

   No IANA action is required.

7.  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.

8.  References

8.1.  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.

   [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.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity

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              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.

8.2.  Informative References

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

   [I-D.ietf-dane-mua]
              Finch, T., "DNS-Based Authentication of Named Entities
              (DANE) for 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

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   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:

   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 puttling 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

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   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.

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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