Secure inter-domain SMTP with TLS, DNSSEC and TLSA records.
draft-fanf-dane-smtp-01

DNS-Based Authentication of Named                               T. Finch
Entities (DANE)                                  University of Cambridge
Internet-Draft                                              May 29, 2012
Intended status: Standards Track
Expires: November 30, 2012

      Secure inter-domain SMTP with TLS, DNSSEC and TLSA records.
                        draft-fanf-dane-smtp-01

Abstract

   SMTP supports STARTTLS for inter-domain mail transfer, but it only
   provides very limited security because the server's certificate
   cannot be authenticated.  This memo specifies how TLSA records in the
   DNS can be used for proper MX target server authentication.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 30, 2012.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Details of SMTP with TLSA  . . . . . . . . . . . . . . . . . .  4
   4.  The Transmitted: header field  . . . . . . . . . . . . . . . .  6
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  "with" protocol types  . . . . . . . . . . . . . . . . . .  6
     5.2.  Permanent message header field registration  . . . . . . .  7
     5.3.  "dane" MTA-name-type . . . . . . . . . . . . . . . . . . .  7
   6.  Security considerations  . . . . . . . . . . . . . . . . . . .  7
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Rationale - choice of certificate identity  . . . . . 10
   Appendix B.  Change log  . . . . . . . . . . . . . . . . . . . . . 10
     B.1.  Changes in version -01 . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   The specification for SMTP over TLS [RFC3207] does not describe how
   to authenticate a server: which identity relating to the connection
   ought to be authenticated by the server's certificate.  In practice,
   most certificates presented by publicly-referenced SMTP servers
   either cannot be validated with respect to a well-known certification
   authority, or do not verify any identity expected by the client.

   As a result, inter-domain SMTP clients cannot require working server
   authentication if they want to successfully send mail using TLS.
   Therefore TLS currently provides only a limited amount of additional
   security for inter-domain SMTP.  Its encryption protects against on-
   path passive eavesdropping; but it does not protect against an active
   attack, since the client has no way to detect when an attacker is
   spoofing the server.

   This memo describes how to fix this using DNSSEC [RFC4033] and TLSA
   records [I-D.ietf-dane-protocol].

   We use DNSSEC to secure the association between a mail domain and its
   SMTP server host names.  Each server's TLS certificate authenticates
   its host name.

   As well as its normal function of providing an association between a
   domain name and a certificate, we are also using the existance of a
   TLSA record to signal to the client that it can expect a valid server
   certificate.

   The protocol described in this memo adds new security checks that can
   cause email delivery to be delayed when a security failure is
   detected.  We specify that clients treat a problems as a "temporary
   failure", causing the message to be queued for a later delivery
   attempt, in the hope that the attack (or configuration error) will
   have been dealt with.

2.  Terminology

   ADMD:  An ADministrative Management Domain, as described in the
      Internet Mail Architecture [RFC5598].

   SMTP server host name:  The target of a (possibly implicit) MX
      record.

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   Inter-domain SMTP:  SMTP between different ADMDs across the public
      Internet, where a client sends mail to a publicly-referenced SMTP
      server.

   Mail domain:  The part of an email address after the "@"; also the
      owner name of a (possibly implicit) MX record.

   MX resolution:  The algorithm for resolving a mail domain into a set
      of SMTP server hosts, described in [RFC5321] section 5.

   Publicly-referenced SMTP server:  An SMTP server which runs on port
      25 of an Internet host located using MX resolution.  (This term is
      from [RFC3207].)

   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].

3.  Details of SMTP with TLSA

   In the following we describe some additions to the usual MX
   resolution algorithm described in [RFC5321] section 5.  If there is
   any conflict between [RFC5321] and this memo, that is an error in
   this memo.

   The client SHALL look up the MX RRset for the mail domain.  There are
   three succesful results that yield a list of SMTP server host names:

   o  A list of one or more MX records;

   o  An implicit MX record, in lieu of an empty list of MX records;

   o  A CNAME to a successful result.

   If the lookup is not successful, the client SHALL proceed as usual.

   All of these DNS RRsets MUST be "secure" according to DNSSEC
   validation ([RFC4033] section 5).  In the case of an implicit MX
   record, there MUST be a secure denial of existence of an MX RRset for
   the mail domain.  In the case of a (chain of) CNAME RRs, all the
   CNAMEs MUST be secure as well as their ultimate target.

   If any of the responses is "bogus", the client MUST treat this as a
   temporary error.

   If these security requirements are not satisfied, this protocol does
   not take effect.  The client SHOULD fall back to insecure delivery

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   (which might be over unauthenticated TLS).

   The client now has an authentic list of SMTP server host names and
   priority values.  It processes this list as usual.

   The rest of this section applies to each SMTP server host name
   individually.

   When connecting to a server, the client SHALL look up its TLSA RRset
   as described in [I-D.ietf-dane-protocol] section 3.  That is, the
   TLSA RRset owner name SHALL be "_25._tcp.hostname" where "hostname"
   is the SMTP server host name.  The response can be one of the
   following (as listed in [I-D.ietf-dane-protocol] section 4.1):

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

   o  A bogus answer, which the client SHALL treat as a temporary error.

   o  In the other cases the client SHOULD deliver to this server
      insecurely (which might be over unauthenticated TLS).

   The client now has one or more TLSA records for the server it is
   connecting to.

   The client MUST ensure that the server offers the STARTTLS service
   extension [RFC3207] in its response to the client's EHLO command
   ([RFC5321] section 4.1.1.1).

   The client SHALL then issue the STARTTLS command which MUST be
   successful.  It then proceeds with TLS negotiation.

   The client SHALL validate the server's certificate as described in
   [I-D.ietf-dane-protocol] section 2.1.

   The client SHALL verify the server's identity as described in
   [RFC6125] section 6.  Its list of reference identifiers SHOULD
   include the SMTP server host name with type DNS-ID, and MAY include a
   second copy of the host name with type CN-ID.

   If any of these checks fail, the client MUST disconnect from the
   server and treat this as a temporary failure.

   The client can now proceed to deliver mail securely.

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4.  The Transmitted: header field

   The client MAY wish to insert a Transmitted: header field at the
   start of the message header just before transmitting the message.
   This records the result of the checks specified in the previous
   section.  This is a client-side counterpart to the Received: header
   field ([RFC5321] section 4.4) and has very similar syntax.  It SHOULD
   be treated as a trace field.

   The syntax of the Transmitted: header field is described using ABNF
   [RFC5234].  Non-terminal syntax rules not defined in this memo are
   defined in [RFC5321], or [RFC5322], or [RFC5234].

   Transmitted-line  = "Transmitted:" FWS To-domain By-domain
                                Opt-info [CFWS] ";" date-time CRLF

   To-domain         = "TO" FWS Extended-Domain

   A <Transmitted-line> SHALL include:

   o  A <To-domain> clause describing the SMTP server.  The <Domain>
      part of a <To-domain> SHALL be the same as the SMTP server host
      name.

   o  A <By-domain> clause identifying the SMTP client that added the
      header.  (If the client also acts as a server this is the same
      <By-domain> clause it would include in any Received: header fields
      it adds.)  This clause helps with recovery if the original order
      of a message header's fields has been lost.

   o  Various <Opt-info> clauses, which MUST include a <With> clause.
      The <Protocol> part of this clause is used to indicate whether the
      client successfully authenticated the server, using one of the
      types specified in Section 5.1.

   o  And a <date-time> to further help with disordering in case a
      message is transmitted by the same client more than once.

5.  IANA Considerations

5.1.  "with" protocol types

   The "with" protocol type registry includes a number of keywords that
   indicate the use of SMTP with or without TLS and/or AUTH [RFC3848].
   When these types appear in a Transmitted: header field "with" clause
   they indicate that the client did not authenticate the server as

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   described in Section 3.

   o  The new keyword "ESMTPT" indicates the use of ESMTP [RFC5321] with
      STARTTLS [RFC3207] when the client successfully authenticated the
      server.

   o  The new keyword "ESMTPTA" indicates the use of ESMTP [RFC5321]
      with STARTTLS [RFC3207] and AUTH [RFC4954] when the client
      successfully authenticated the server.

   These new keywords are not for use in Received: header fields since
   the server cannot tell whether or not the client authenticated it.

   There are no keywords corresponding to a client trying and failing to
   authenticate the server, since in this case no message transmission
   occurs.

5.2.  Permanent message header field registration

   Header field name:  Transmitted:

   Applicable protocol:  mail

   Status:  standard

   Change controller:  IETF

   Specification document  this memo

5.3.  "dane" MTA-name-type

   Delivery status notifications [RFC3464] can include a Remote-MTA
   field recording an SMTP server host name.  When this has been
   authenticated according to Section 3 the reporting MTA MAY use an
   MTA-type-name of "dane".

   a.  MTA-type-name: "dane"

   b.  Syntax: same as the "dns" MTA-type-name [RFC3461]

   c.  Translation into US-ASCII: none needed

6.  Security considerations

   This memo provides only conditional security.  It allows a server to
   publish in the DNS the details of how it can be authenticated.
   Clients that implement this protocol can use it to provide a strong

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   guarantee that they are sending mail to the correct place.

   There is no secure way for a server to tell if a client has
   authenticated it using this protocol.  This is a general limitation
   of TLS.  The Transmitted: header field records this information for
   tracing and debugging, not for security purposes.

   By signing their zone with DNSSEC, a mail domain owner implicitly
   instructs SMTP clients to check their SMTP server TLSA records.  This
   implies another point in the trust relationship between mail domain
   owner and smtp server operator.  Most of the setup requirements for
   this protocol fall on the SMTP server operator: installing a TLS
   certificate with the correct name, and publishing a TLSA record under
   that name.  If these are not correct then mail delivery from TLSA-
   aware clients might be delayed.

   We do not specify that clients check that all of a mail domain's SMTP
   server host names consistently have or do not have TLSA records.
   This is so that partial or incremental deployment does not break mail
   delivery.  Different levels of deployment are likely if a domain has
   a third-party backup MX, for example.

   We do not specify that clients check the DNSSEC state of the SMTP
   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.)

   This memo does not specify any changes to SMTP client authentication.
   Inter-domain SMTP client authentication remains extremely weak.

7.  Acknowledgements

   Thanks to Mark Andrews for arguing that authenticating the SMTP
   server host name is the right thing, and that we should rely on
   DNSSEC to secure the MX lookup.  Thanks to Ned Freed 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.

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   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, January 2003.

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

   [RFC3848]  Newman, C., "ESMTP and LMTP Transmission Types
              Registration", RFC 3848, July 2004.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

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

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [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.

   [I-D.ietf-dane-protocol]
              Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", draft-ietf-dane-protocol-21 (work in
              progress), May 2012.

8.2.  Informative References

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,

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              July 2009.

Appendix A.  Rationale - choice of certificate identity

   There are a number of reasons for the certificate to authenticate the
   SMTP server host name rather than the mail domain.

   SMTP allows a client to transfer mail to recipients at multiple
   domains in the same connection.  If the certificate identifies the
   host name then it does not need to list all the possible mail
   domains.

   It is not in general feasible for the server to select a mail domain
   certificate based on the recipient domains when the connection is
   established (using Server Name Indication, [RFC6066] section 3),
   because an SMTP client might not know all of the recipients when it
   establishes the connection.

   Outgoing SMTP relays and message submission servers handle mail for
   any domain, so in those cases the only sensible option is for the
   certificate to contain the host name.  It is more consistent for
   incoming MX server certificates to match.

   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 protocol does not provide an option for directly authenticating
   the mail 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
   mail domain cannot be made to work, so there are complicated criteria
   for when mail domain TLSA records might be used and when they cannot.
   This is all avoided by authenticating the SMTP server host name.

   Finally, this protocol only affects the logic in the SMTP client and
   requires no additional SMTP server functionality, such as support for
   the TLS Server Name Indication extension.

Appendix B.  Change log

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B.1.  Changes in version -01

   More about not to authenticate mail domains in the rationale

   Change DNS-ID requirement from MUST to SHOULD to follow RFC 6125

   Acknowledgments section

   Transmitted: header trace field.  Not sure if this is a good idea;
   feedback wanted.

   "dane" MTA-name-type for use in DSNs.  Even less sure if this is a
   good idea.

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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