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MARID                                                         D. Crocker
Internet-Draft                               Brandenburg InternetWorking
Expires: January 12, 2005                                      J. Leslie
                                                                 D. Otis
                                            Mail Abuse Prevention System
                                                           July 14, 2004

                      Client SMTP Validation (CSV)

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on January 12, 2005.

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. Widespread abuse of the email
   system has led operators to demand accountability for the email their
   receiving SMTP servers are being asked to process. Client SMTP
   Validation (CSV) provides an economical service that permits a
   receiving SMTP server to decide whether a sending SMTP client is
   likely to produce well-behaved traffic, or at least to decide whether

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   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 practice of service
   providers that accredit the networks from which sending systems are

Table of Contents

   1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Service Goal . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1   Assessing Authorization  . . . . . . . . . . . . . . . . .  6
     4.2   Assessing Accreditation  . . . . . . . . . . . . . . . . .  8
   5.  Client SMTP Validation Details . . . . . . . . . . . . . . . .  8
     5.1   Assessing Authorization  . . . . . . . . . . . . . . . . .  8
     5.2   Assessing Accreditation  . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.1   Normative References . . . . . . . . . . . . . . . . . . . . 10
   7.2   Informative References . . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
   A.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   B.  Host Name Authentication . . . . . . . . . . . . . . . . . . . 12
     B.1   DNS-based Mapping  . . . . . . . . . . . . . . . . . . . . 13
     B.2   Reverse DNS  . . . . . . . . . . . . . . . . . . . . . . . 13
     B.3   Forward DNS Lookup . . . . . . . . . . . . . . . . . . . . 14
       Intellectual Property and Copyright Statements . . . . . . . . 15

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1.  Overview

   CSV considers two questions at the start of each SMTP session:

   o  Does a domain's management authorize this MTA to be sending email?

   o  Do independent accreditation services consider that domain's
      policies and practices sufficient for controlling email abuse?

   To validate an SMTP session from an unknown sending SMTP client using
   CSV, the receiving SMTP server:

   1.  Obtains the remote IP address of the TCP connection.

   2.  Extracts the domain name from the EHLO command sent by the SMTP

   3.  Queries a chosen Accreditation Service for the EHLO domain name
       (see Domain Name Accreditation (DNA) [ID-CSVDNA])

   4.  Queries DNS for a SRV record under the EHLO domain name (see
       Client SMTP Authorization (CSA) [ID-CSVCSA])

   5.  Checks the SRV reply for flags returned, and checks for a match
       in the list of returned IP addresses

   6.  Determines the level of trust to give to the sending SMTP client,
       based on the results of (3) and (5)

   If the level of trust is high enough, process all email from that
   session in the traditional manner, delivering or forwarding without
   the need for further validation.

   If the level of trust is too low, return an error showing the reason
   for not trusting the sending SMTP client.

   If the level of trust is in between, document the result in a header
   in each email delivered or forwarded, and/or perform additional

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

   Discussion:  The venue for discussing this proposal is the IETF MARID
      working group[1].

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2.  Background

   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 decides whether to accept
   the message traffic that is being sent to it, trusting that delivery
   of that traffic's messages 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
   transmitted by the sending SMTP client are likely to be well-behaved,
   or at least to decide whether the client is sufficiently accountable
   for its actions.

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

      What is the "name" of the client to be trusted? How is it

      CSV uses the domain name supplied by a client in the SMTP HELO/

      Is the client MTA legitimately associated with that name? Can we
      prove that the client is who it purports to be?

      By finding the sending SMTP client's actual IP address, in the
      list of IP addresses returned by a DNS Address query on the EHLO
      domain-name, CSV satisfies the minimal authentication needs of
      this task.

      Is the remote host permitted to act as a sending SMTP client? Has
      the domain management authorized it to perform this function?

      CSV specifies a DNS-based record that states whether an associated
      host has permission to operate as a client MTA.

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      What is the trust that is to be extended to the entity that
      authorized the sending SMTP client? Does the receiving SMTP server
      have a basis for deciding that the entity providing authorization
      for the client MTA can, itself, be trusted to make accountable

      CSV defines a DNS record that permits domains to announce the
      accreditation services in which they are listed. It also defines a
      separate record by which accreditation services publish their
      assessments of sending domains.

   An implementation well might combine some of these steps. However it
   is important to consider them independently, in order to ensure that
   they are specified in a valid manner, or at least that the
   constraints of the proposal are clear for each of these conceptual
   functions. This specification is based on validation of the EHLO
   domain name. The 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 mechanisms.

   Address-based Accreditation:
      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 that the operator of the receiving SMTP server
      deems worthy to assess other sites.

      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 continuing
      TCP session 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 persistent 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,

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   CSV makes its assessment before messages cross the Internet, thereby
   saving bandwidth and reducing the impact of a distributed denial of
   service attack.

3.  Service Goal

   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

   o  A standardized method of documenting authorization to operate as a
      sending SMTP client.

   o  A standardized method of referencing accreditation services.

   o  A standardized method of querying an accreditation service.

4.  Requirements

4.1  Assessing Authorization

   For a receiving SMTP server to determine whether a host has
   authorization to act as a sending SMTP client, it is necessary to
   identify the host and verify its association with that identity.
   Given that, a DNS query on the name can return an explicit

   Identification The means of identifying a remote host or service
      anchor=""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 DNS administrator for a domain will maintain
      record(s) listing one or more IP addresses associated with that

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      name, even though reverse-DNS records (not controlled by the same
      DNS administrator) may give conflicting information. The
      forward-DNS is considered the valid authority for CSV purposes.
      Therefore, authentication of a domain name's reference to a
      particular IP Address requires an explicit authentication step.

   Authentication If the sending SMTP client of a 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
      organization's local network.

      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.

      A number of strong authentication mechanisms are possible, but
      none has yet attained widespread adoption among MTAs with no prior
      relationship. CSV specifies a weaker authentication scheme that
      meets the modest requirements for this service. Stronger methods
      can be supported later, if necessary. However they must be tied to
      a domain name and must not require any prior relationship.

   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 email
      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 authorization of other hosts that might act as email
      relays. 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
      accountable domain management can announce its authorization of a
      particular host to operate a particular service. CSV defines such
      a mechanism for sending SMTP clients.

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4.2  Assessing Accreditation

   This portion of CSV determines accreditations for the sending SMTP
   client or for the administration under which it operates. The basis
   for deciding that an authorizing agency is, itself, to be trusted can
   be highly varied. Often, well-established practices are not that
   well-understood. This makes it difficult to predict what methods of
   accreditation will be most appropriate and successful for Internet
   mail. 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 standard means for:

   o  referencing different accreditation services, and

   o  querying a service to obtain information about the domain it is

5.  Client SMTP Validation Details

   CSV defines a mechanism for session-time, domain-based validation of
   a sending SMTP client. 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 name.

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

   Validation has two separate phases: assessing authorization and
   assessing accreditation. The first is performed between the receiving
   SMTP server and the sending SMTP client. The second is performed
   between the receiving SMTP server and one or more accrediting

5.1  Assessing Authorization

   This phase provides a means for a network administrator to publicly
   state what hosts are authorized by it to act as client MTAs. Absent
   such a statement of authority, it is possible that the client is a
   rogue or compromised host.

   The assessment requires three steps:

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   Identification:  The sending SMTP client host is identified by a
      Domain Name. The domain name serves as a unique,
      topologically-independent, persistent identifier that is
      registered in the Domain Name Service.

      A sending SMTP client MUST supply a published domain name as the
      parameter to an SMTP HELO or EHLO. The sending SMTP client MAY
      issue multiple EHLO's over the course of a session, such as for
      isolating email flows for accreditation, with different domain
      names to represent different users on the client system. If an
      EHLO is issued, the entire CSV process MUST be restarted without
      needing to make a new connection.

      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 it is 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
      seeking to send email, upon which CSV assessments are then made.

   Authentication:  There is no universal, strong method to authenticate
      that a host is correctly identifying itself. For most email
      transport purposes, it will be sufficient to show that the EHLO
      domain name forward-resolves to the IP address of the sending SMTP

      The response to a CSV authentication query usually includes the
      list of associated IP addresses in the Additional Information
      section. Formally, this additional information is the same as
      would be obtained from additional queries for that information. A
      server includes it in the CSV query for efficiency, to avoid
      additional DNS queries.

      If the list is returned and the actual IP address of the sending
      SMTP client is in it, the receiving SMTP server SHOULD consider
      the EHLO domain name to be authenticated. Conversely, if the list
      is returned and the actual IP address is not in it, the assertion
      of the EHLO domain name SHOULD be considered incorrect, and result
      in an error being returned.

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

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

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5.2  Assessing Accreditation

   The CSV authorization phase provides a basis for trusting that the
   sending SMTP client is under the control of a domain's management;
   but this says nothing about the policies and practices of that
   management. Separate accreditation services are needed for that. It
   is expected that there will be numerous services that provide
   accreditation. CSV is intended to support use of any accreditation
   service that gains credibility among operators of SMTP servers.

   An initial set of capabilities for specifying CSV-related
   accreditation services is specified in Domain Name Accreditation
   (DNA) [ID-CSVDNA]. Sending SMTP clients SHOULD publish CSV records
   referring to accreditation services in which they are listed.
   Accreditation services MUST publish DNA-conformant records.

6.  Security Considerations

   CSV defines a security mechanism. The nature of the security
   requirements for CSV are significantly different from typical,
   "strong" methods required for most Internet security functions.

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

7.  References

7.1  Normative References

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

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

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

   [RFC2554]  Myers, J., "SMTP Service Extension for Authentication",
              RFC 2554, March 1999.

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

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

7.2  Informative References

              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.


   [1]  <http://ietf.org/html.charters/marid-charter.html>

Authors' Addresses

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Drive
   Sunnyvale, CA  94086

   Phone: +1.408.246.8253
   EMail: dcrocker@brandenburg.com

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   John Leslie
   10 Souhegan Street
   Milford, NH  03055

   Phone: +1.603.673.6132
   EMail: john@jlc.net

   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.

   Review comments and suggestions, on previous versions of CSV, have
   been made by: Tony Finch, Carl Hutzler, Meng Weng Wong, Greg Connor.

Appendix B.  Host Name Authentication

   The routing infrastructure of the Internet distinguishes hosts by
   their topological attachment, noted as its IP Address. Because IP
   Addresses change periodically and users prefer references that can be
   mnemonic, hosts on the Internet generally have one or more Domain
   Names (DNS) [RFC1035] assigned to them. A Domain Name is globally
   unique. The core function of the DNS is to map from a name supplied
   by the user, to an IP Address associated with that name. Internet
   protocols often permit a host to identify itself with its domain

   But what if a host is programmed incorrectly, or even maliciously? We
   need a way to authenticate that a host is reporting its name
   correctly. Establishing this authentication is separate from
   determining its authorization to perform any particular service.
   Until the relationship is authenticated, we cannot apply policies
   associated with the name.

   A number of methods for authenticating the relationship between the
   host and its reported name might be used. The current CSV

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   specification supports authentication through Domain Name Service
   mappings between a domain name and an IP Address. Other equally valid
   methods are possible. However none has yet proved practical for
   authenticating a client to a server, without prior arrangement
   between them.

B.1  DNS-based Mapping

   The Domain Name System has a common mapping mechanism that can be
   used in a variety of ways, based on the schema for assigning names
   and the types of data listed under those names. The two most popular
   schemas are forward mapping and Reverse-DNS. Forward looks up a
   "regular" domain name and receives information about it, such as a
   list of IP Addresses associated with that name. Reverse DNS starts
   with an IP Address and maps it to a pointer to a "regular" domain

   Often when contacted by a remote host, a host uses a reverse-DNS
   query to get the name of the remote host. This can be followed by a
   forward-DNS query to see if the name reported by the reverse-DNS
   query matches an IP address reported by the forward-DNS query. If so,
   this is generally considered an authentication of the relationship of
   the name to the host. This method is often used by receiving SMTP
   servers to decide whether to trust the sending SMTP client.

   Closing the circle in this manner permits verifying both that the
   domain assigning the name and the service provider assigning IP
   addresses agree that this is the appropriate name for that remote
   host. Although this process has known limitations, it is considered
   sufficient for many basic uses.

   Use of an IP Address returned by the DNS is sufficient for
   CSV-related authentication requirements. However it MUST NOT be
   considered a strong form of authentication for allowing otherwise
   privileged access. The use of this mechanism is to aid selection of
   accreditation services, such as whether to query using the domain
   name or the client address. Other measures may be taken intended to
   limit exposure to unknown clients but are beyond the scope of this

B.2  Reverse DNS

   Reverse DNS can be used by itself to associate a domain name with an
   IP address. It indicates that the entity responsible for allocating
   that block of IP addresses has designated an IP address to be used by
   the domain name. Unfortunately, the reverse-IP branch of the DNS has
   a long history of being poorly maintained, and often does not match
   the forward-DNS information even when the relationship of host to

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   name is genuine.

   Reverse DNS by itself SHOULD NOT be considered sufficient

B.3  Forward DNS Lookup

   An isolated forward lookup is sufficient for simple sending SMTP
   client authentication, if an IP Address returned for that name
   matches the IP Address reported by the underlying IP service for that
   remote host. This indicates that the domain in question currently
   designates that IP Address as an IP address entitled to respond for
   that domain name.

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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
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   The IETF invites any interested party to bring to its attention any
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   this standard. Please address the information to the IETF at

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2004). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


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