DNS-Based Authentication of Named Entities (DANE)               T. Finch
Internet-Draft                                   University of Cambridge
Intended status: Standards Track                               M. Miller
Expires: June 16, August 15, 2014                                    P. Saint-Andre                             Cisco Systems, Inc.
                                                       December 13, 2013
                                                          P. Saint-Andre
                                                                    &yet
                                                       February 11, 2014

  Using DNS-Based Authentication of Named Entities (DANE) TLSA records
                        with SRV and MX records.
                         draft-ietf-dane-srv-03
                         draft-ietf-dane-srv-04

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
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   This Internet-Draft will expire on June 16, August 15, 2014.

Copyright Notice

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   document authors.  All rights reserved.

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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
   6.  Guidance for Application Protocols  . . . . . . . . . . . . .   6
   7.  Guidance for Server Operators . . . . . . . . . . . . . . . .   6   7
   8.  Internationalization Considerations . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7   8
     10.1.  Mixed Security Status  . . . . . . . . . . . . . . . . .   7   8
     10.2.  A Service Domain Trusts its Servers  . . . . . . . . . .   7   8
     10.3.  Certificate Subject Name Matching  . . . . . . . . . . .   8
     10.4.  Deliberate Omissions
   11. Acknowledgements  . . . . . . . . . . . . . . . . . .   8
   11. Acknowledgements . . . .   9
   12. References  . . . . . . . . . . . . . . . . . .   8
   12. References . . . . . . .   9
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.1.  Normative
     12.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Mail Example . . .   9
     12.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A. B.  XMPP Example . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix B. C.  Rationale  . . . . . . . . . . . . . . . . . . . . .  10  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  12

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"
   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:

   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 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).

4.2.  TLSA Queries

   This sub-section applies to each server host name individually,
   provided

   If the SRV response was insecure or indeterminate, the client MUST
   NOT perform any TLSA queries.  If the SRV response is secure
   according to DNSSEC validation. validation, the client performs a TLSA query for
   each SRV target as describes in this section.

   For each SRV target host name, if the response to the address (A or
   AAAA) query is insecure or indeterminate, the client MUST NOT perform
   a TLSA query for that target; the TLSA a query will most likely fail.

   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

   If a usable TLSA record with Certificate Usage "3" matches the TLS
   server's certificate, or public key for the certificate, all other
   validation and verification checks MAY be ignored (e.g., reference
   identifier, key usage, expiration, issuance, etc.).

   Otherwise, 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 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 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 conform to support this specification, server software MUST
   implement the TLS Server Name Indication extension (TLS SNI)
   [RFC6066] (or its functional equivalent in 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 relevant application
   protocol) server's certificate.  Except for selecting Certificate Usage
   "3", the appropriate certificate.

   A server that supports TLS and is certificate authenticated by the target of an SRV record TLSA record(s) MUST
   have contain
   a TLS certificate reference identifier that authenticates the SRV query domain (i.e. matches:

   o  the service domain, or domain name (the "source domain" in [RFC6125] terms).  This is
   necessary for clients that cannot perform DNSSEC validation.  This
   certificate MUST be the default that terms,
      which is presented if the client does
   not use TLS SNI or its functional equivalent.

   In order to support this specification, SRV query domain); and/or

   o  the server SHOULD also have a
   certificate that authenticates host name (the "derived domain" in [RFC6125] terms,
      which is the SRV target domain (e.g., the mail
   server hostname).  This target).

   Servers that support multiple service domains (i.e., multi-tenant)
   can be done using a multi-name certificate or
   by using the client's TLS SNI implement Server Name Identifier (TLS SNI) [RFC6066] 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 determine which certificate to authenticate its host name;
   they are also unlikely to support SNI.  This means offer.
   Clients that servers do not support this specification will indicate a
   preference for
   old the service domain name, while clients need a different default certificate from servers that
   are support
   this specification will indicate the targets of SRV records.  If server host name.  However, the
   server does not have a determines what 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 present in the SRV targets.  (The latter
   avoids TLS handshake;
   e.g., the need for additional certificates.) presented certificate might only authenticate the server
   host name.

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

   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 checks are not necessary if the matching TLSA record is of
   Certificate Usage "3".  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, which in
   turn authenticates and the certificate.  However TLSA record provides a binding to a
   certificate, this latter step can be
   indirect, indirect via a chain of
   certificates.  A usage=0  For example, a Certificate 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  Therefore, clients need to
   check whether the server's certificate matches one of the server host name, expected
   reference identifiers 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.) expects.

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, Viktor Dukhovni,
   Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, Hector
   Santos, Jonas Schneider, and Alessandro Vesely for helpful
   suggestions.

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.

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 "SMTP security via
              opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-05
              (work in progress),
              November 2013. 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-04 draft-ietf-xmpp-dna-05 (work in progress),
              October 2013.
              February 2014.

Appendix A.  Mail 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.           RRSIG  A ...

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

   ; 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.  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-clientexample.com.  RRSIG   SRV ...

   ; XMPP 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 ...

   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 C.  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.

   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

   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
   &yet

   Email: psaintan@cisco.com ietf@stpeter.im