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MARID                                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Expires: December 13, 2004                                     J. Leslie
                                                                 D. Otis
                                            Mail Abuse Prevention System
                                                           June 14, 2004

                      Client SMTP Validation (CSV)

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   Internet mail relies on exchanges between systems that have made no
   prior arrangement with each other.  For reasons of history and
   expediency, the current service provides a level of accountability
   for participating hosts that is no longer adequate.  This makes it
   impossible to vet MTAs or find recourse when their operation causes
   problems.  Client SMTP Validation (CSV) provides an economical

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   service that permits an SMTP server to decide whether messages sent
   by the client SMTP are likely to be well-behaved, or at least to
   decide whether the client is sufficiently accountable for its
   actions.  CSV provides a small, simple and useful improvement to
   Internet mail service accountability.  It builds upon the existing
   practise of service providers that accredit the networks from which
   sending systems are connecting.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1   Identification . . . . . . . . . . . . . . . . . . . . . .  5
     2.2   Authentication . . . . . . . . . . . . . . . . . . . . . .  5
     2.3   Authorization  . . . . . . . . . . . . . . . . . . . . . .  6
     2.4   Accreditation  . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Client SMTP Validation Procedures  . . . . . . . . . . . . . .  6
     3.1   Identification . . . . . . . . . . . . . . . . . . . . . .  7
     3.2   Authentication . . . . . . . . . . . . . . . . . . . . . .  7
     3.3   Authorization  . . . . . . . . . . . . . . . . . . . . . .  7
     3.4   Accreditation  . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.1   References - Normative . . . . . . . . . . . . . . . . . . .  8
   5.2   References - Informative . . . . . . . . . . . . . . . . . .  9
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  9
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   B.  Preventing SMTP-level Joe-Jobs of Well-Known Names . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 12

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1.  Introduction

   Internet mail suffers from the operation of hosts acting as mail
   transfer agents (MTA) without any meaningful cross-net
   accountability.  This makes it impossible to vet MTAs or find
   recourse when their operations cause problems.  Many of these hosts
   have been compromised and have been turned into unwilling
   participants in large networks of hostile MTAs that send spam and
   worms, and contribute to denial of service attacks.

   When a server MTA receives a connection, it must decide whether to
   accept the message traffic that is being sent to it, trusting that
   its delivery will not be problematic to the operation of the provider
   or their users.  How can it do this, when operating in the open
   Internet? Client SMTP Validation (CSV) defines a service that permits
   the receiving SMTP server to decide whether messages sent by the
   sending SMTP client are likely to be well-behaved, or at least to
   decide whether the client is sufficiently accountable for its

   The process of deciding on this trust of the client requires
   performing a series of conceptually discrete steps:

   Identification: What is the "name" of the client to be trusted? How
      is it referenced?
      CSV uses the domain name supplied by a client in the SMTP HELO/

   Authentication: Is the client MTA legitimately associated with that
      name? Can we prove that the client is who it purports to be?
      CSV documents a range of existing techniques that are appropriate
      for use with CSV.

   Authorization: Is the sending SMTP client permitted to act as a
      client MTA? Has a separate authority given it permission to
      perform this service?
      CSV specifies a DNS-based record that states whether an associated
      host has permission to operate as a client MTA.

   Accreditation: What is the trust that is to be extended to the entity
      that authorized the client MTA? Does the server MTA have a basis
      for deciding that the entity providing authorization for the
      client MTA can, itself, be trusted to make valid authorizations?
      CSV discusses existing techniques for accrediting authorizing
      systems.  It also defines a DNS record that permits such systems
      to announce the accreditation services in which they are listed.
      It defines another DNS record that permits accreditation services
      to publish their assessments of client MTAs.

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   A proposal or its implementation well might combine these steps.
   However it is important to consider them independently, in order to
   ensure that the proposal specifies that they are performed in a valid
   manner, or at least that the constraints of the proposal are clear
   for each of these conceptual functions.  This specification
   distinguishes each of these logical steps and defines their operation
   separately.  It is based on validation of the EHLO domain name.  The
   proposed mechanism is small, simple and useful.  In particular it
   permits detecting machines that are prohibited from acting as Client
   MTAs and those that are permitted.  The mechanism is designed to be
   useful between peer MTAs and only requires use of well-established

   Address-based Authentication Currently, service providers often
      maintain lists of remote networks that are known to be trustworthy
      or untrustworthy as sending SMTP clients.  Typically, these lists
      are based on the use of IP Addresses of the clients.  The IP
      Addresses serve as identifiers.  The list specifies positive or
      negative authorization, and the source of the list is an
      organization the service provider deems worthy to assess other

      When used in this way, IP Addresses are authenticated by relying
      on their use in the IP routing infrastructure.  Packets are routed
      to the specified IP Address, over the open Internet.  A repeated
      exchange using that IP Address is therefore presumed to be an
      interaction with the host legitimately associated with that IP

      Increased topological, transfer and access complexities on the
      Internet are making IP Addresses increasingly problematic for use
      as identifiers.  Instead they are viewed as appropriate only for
      the most transient task of delivering individual packets.

   CSV builds upon this popular model.  Besides the considerable benefit
   of having operational practice, the model can be extremely efficient.
   It permits the service provider to assess the source of an entire
   message stream, rather than having to evaluate each message.  Also,
   CSV makes its assessment before messages cross the Internet, thereby
   saving bandwidth and reducing the impact of a distributed denial of
   service attack.

   CSV verifies that a host is authorized to act as an SMTP client and
   that the client is likely to be operated acceptably.  CSV enhances
   current practice with:

   o  Identification by persistent domain name rather than transient IP

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   o  Alternative authentication techniques

   o  Explicit listing of client service authorization

   o  A standardized method of referencing accreditation services

   o  A standardized method of querying an accreditation service

   Terminology: Terminology conforms to [ID-email-arch].

   Discussion: The venue for discussing this proposal is the

   NOTE: The current draft makes reference to quite a few underlying
      services that need citations (ed.)

2.  Requirements

2.1  Identification

   The means of identifying a remote host or service requires uniqueness
   and is aided by persistence.  The identifier must not be ambiguous
   and its use is made far more efficient if it is stable over time.
   The two usual choices are IP Addresses and Domain Names.

   An IP Address typically refers to a single host and can change
   relatively frequently, as the host's connection to the Internet
   changes.  IP Addresses are reported by the Internet infrastructure
   and for simple security requirements, transactional use of an IP
   Address through the Internet's routing fabric is taken as validation
   of the Address.  Domain Names are longer-lived but require new
   administrative effort.  They can be used to refer to multiple hosts
   simultaneously.  The administrator of a Domain Name may list any IP
   Address they wish to associate with the name, independent of the
   administrator and the Domain Name having a valid relationship to that
   Address.  Therefore, authentication of a domain name's reference to a
   particular IP Address requires an explicit authentication step.

2.2  Authentication

   If the sending SMTP client of an connection can be authenticated,
   then it is possible to develop an accountability mechanism based on
   that authentication.  MUA-MSA exchanges have a substantial number of
   useful authentication mechanisms available.  These are often very
   strong, and involve significant prior arrangement.  The same holds
   true for MDA-MUA exchanges, and often for MSA-MTA and MTA-MDA
   exchanges, such as within an organizations local network.

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   What is missing is a useful means of authenticating MTA-MTA exchanges
   over the open Internet.  Prior arrangement between such a pair of
   MTAs is antithetical to the history and operation of Internet mail.
   Spontaneous communications are at the core of Internet design and
   operation.  So the challenge is to develop an authentication
   mechanism that permits the necessary amount of accountability,
   without imposing undue overhead or restrictions.

2.3  Authorization

   Internet operation has typically required no public mechanism for
   restricting or permitting particular hosts to operate clients or
   servers for particular services on behalf of particular domains.  The
   DNS MX record states where to route mail that is destined for a
   specific domain; this implies a degree of authorization for the host
   referenced in the MX.  However the record is really for routing and
   there is no equivalent means of specifying prohibition of other hosts
   that might act as intermediaries.  Similarly there is no means for
   checking the authorization of World Wide Web servers, DNS
   servers,telnet clients or other Internet applications.

   What is missing is an open, interoperable means by which a trusted
   agency can announce its authorization of a particular host to operate
   a particular service.

2.4  Accreditation

   In non-Internet environments the basis for deciding that an
   authorizing agency is, itself, to be trusted, is highly varied and
   often is not well-understood.  It is expected that this portion of an
   Internet mail validation service will therefore need to support be a
   variety of accreditation service styles.

   What is needed is a means of announcing performance of accreditation
   and a means of querying a service to obtain information about the
   host it is accrediting.

3.  Client SMTP Validation Procedures

   CSV defines a mechanism for session-time, domain-based validation of
   a peer, sending SMTP client MTA.  It is useful across the open
   Internet, between MTAs that have made no prior arrangement with each
   other.  Validation establishes that the operation of the MTA is
   authorized by an accredited administrator of the declared domain

   The validation requirements are modest, because the system does not
   seek to provide long-term vetting of the client host, nor does it

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   assess the actual content being exchanged.  Techniques that would be
   wholly inadequate for classic, strong authentication and validation
   can be entirely sufficient for CSV's needs.

3.1  Identification

   The sending SMTP client host is identified by a Domain Name, supplied
   by that host as the first parameter to an opening SMTP HELO or EHLO.
   The domain name serves as a unique, topologically-independent,
   persistent identifier that is registered in the Domain Name Service.
   Hence, parametric values associated with the domain can be obtained
   through a DNS query.

   For CSV, a sending SMTP client places the domain name into the
   <Domain> field specified for a SMTP HELO or EHLO [RFC2821].  The
   domain name is any name under which they are claiming authorization
   to act as a sending SMTP client.  A receiving SMTP server will
   extract this name and use it as the identification for the client
   claiming authorization.

3.2  Authentication

   There is no single, common means of authenticating a host's use of a
   domain name.  CSV participants SHOULD use one of the techniques
   described in Host Name Authentication (HNA) [ID-Marid-CSVHNA].  It
   describes techniques that are widely used and acceptable for the
   purposes of CSV.

   The document highlights the range of techniques and the differences
   in their administrative overhead and strength of authentication.
   However other techniques can be equally valid.

3.3  Authorization

   The purpose of authorization, in CSV, is to establish that an
   accountable authority has given permission for the sending SMTP
   client host to operate in that role.

   CSV participants SHOULD use the method defined in Client SMTP
   Authentication (CSA) [ID-Marid-CSVCSA].  It specifies a DNS record
   that is associated with the domain name offered by the sending SMTP
   client host.

3.4  Accreditation

   The utility of any service like CSV is entirely dependent upon the
   relevance, reliability and accuracy of the service that accredits the
   authorizing agent.  It is expected that there will be numerous

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   services that provide accreditation.  CSV is intended to support use
   of any service that gains credibility among operators of SMTP

   An initial set of capabilities for specifying CSV-related
   accreditation services is specified in Domain Name Accreditation
   [ID-Marid-CSVDNA].  CSV participants SHOULD use the methods defined

4.  Security Considerations

   This entire proposal pertains to security, namely authentication and
   authorization of peer MTAs.

   The proposal relies on the integrity and authenticity of DNS data.

5.  References

5.1  References - Normative

              Otis, D., Crocker, D. and J. Leslie, "sending SMTP client
              Authentication (CSA)", June 2004.

              Leslie, J., Crocker, D. and D. Otis, "Domain Name
              Accreditation (DNA)", June 2004.

              Crocker, D., Otis, D. and J. Leslie, "Host Name
              Authentication (HNA)", June 2004.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September

   [RFC0821]  Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
              821, August 1982.

   [RFC0822]  Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2782]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for

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              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April

5.2  References - Informative

              Brand, R. and L. Sherzer, "Designated Relays Inquiry
              Protocol (DRIP)", draft-brand-drip-02 (work in progress),
              October 2003.

              Crocker, D., "Internet Mail Architecture", May 2004.

Authors' Addresses

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Drive
   Sunnyvale, CA  94086

   Phone: +1.408.246.8253
   EMail: dcrocker@brandenburg.com

   John Leslie
   10 Souhegan Street
   Milford, NH  03055

   Phone: +1.603.673.6132
   EMail: john@jlc.net

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   Douglas Otis
   Mail Abuse Prevention System
   1737 North First Street, Suite 680
   San Jose, CA  94043

   Phone: +1.408.453.6277
   EMail: dotis@mail-abuse.org

Appendix A.  Acknowledgements

   This proposal is similar to DRIP [ID-brand-drip], however it uses a
   different DNS [RFC1035] record.

Appendix B.  Preventing SMTP-level Joe-Jobs of Well-Known Names

   CSV permits using alternative functional components.  This can result
   in significant variations of administration and use.  Some of these
   can require extended effort to configure and use.  However some very
   simple scenarios can be quite useful.

   For example, domain names of well-known services are often used
   fraudulently in email, known as joe-jobs.  If the well-known service
   sends all of its email from its own network, then it can use CSV to
   register all authorized sending SMTP clients and list all others
   sources of email under its domain name as invalid.

   Such a use requires the following:

   1.  Domain name owner lists a wildcard for its "root" domain name,
       with a CSV record saying that client SMTP sending is prohibited.

   2.  Domain name owner lists the specific host domain names, with
       appropriate CSV records to list the hosts as permitted to send

   3.  Domain name owner's client SMTP sending hosts include their
       domain names in SMTP HELO/EHLO commands.

   4.  Receiving SMTP servers uses a small, locaol whitelist, to perform
       accreditation of the domain administration for the well-known

   CSV will ensure that any mail-sending client that uses the well-known
   name in the SMTP HELO/EHLO command must do so validly.

   It is possible to detect fraudulent use that is not announced in the
   SMTP HELO/EHLO.  To accomplish this the receiver MAY consult the CSV

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   information at a later stage of processing.  For example:

   1.  In the message, record the IP Address of the sending SMTP client.

   2.  Upon detecting that the From field that has a domain name
       associated with a well-known service, perform a CSV check using
       that domain name and the recorded IP Address.

   If the Joe-job scheme is based on the direct content, rather than the
   From field, a deeper and stronger validation technique will be
   necessary, based on content-signing

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Intellectual Property Statement

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