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Versions: 00 01 02                                                      
marid                                                            D. Otis
Internet-Draft                              Mail Abuse Prevention System
Expires: August 21, 2005                                      D. Crocker
                                             Brandenburg InternetWorking
                                                               J. Leslie
                                                       February 20, 2005

                    Client SMTP Authorization (CSA)

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 21, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   Internet operation has typically required no public mechanism for
   announcing restriction or permission of particular hosts to operate
   clients or servers for particular services on behalf of particular
   domains.  What is missing is an open, interoperable means by which a

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   trusted agency can announce authorization for a host to operate a
   service.  The current specification supports this capability for
   sending SMTP clients.  Specifically, is a sending SMTP client
   permitted to act as a client MTA? Has a separate authority given it
   permission to perform this service? Client SMTP Authorization (CSA)
   specifies a DNS-based record that states whether an associated host
   has permission to operate as a client MTA.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Definitions  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Model  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.   Mechanism  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.   Client SMTP Authorization SRV Record . . . . . . . . . . . .   5
   6.   Publishing CSA Records . . . . . . . . . . . . . . . . . . .   8
   7.   Using CSA Records  . . . . . . . . . . . . . . . . . . . . .   9
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  10
   9.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  10
   10.  Working Group Evaluation . . . . . . . . . . . . . . . . . .  10
   11.  References . . . . . . . . . . . . . . . . . . . . . . . . .  11
   11.1   References - Normative . . . . . . . . . . . . . . . . . .  11
   11.2   References - Informative . . . . . . . . . . . . . . . . .  12
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  12
   A.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  13
        Intellectual Property and Copyright Statements . . . . . . .  14

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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 turned into unwilling participants in large
   networks of hostile MTAs that send spam and worms, and contribute to
   denial of service attacks.  Enhancing the Internet mail transfer
   service to deal with these issues requires identification,
   authentication, authorization and accreditation capabilities about
   the sending SMTP client, as per [ID-CSV].  The current specification
   addresses the requirement for explicit authorization.

   It is important to distinguish this security function from
   authentication.  Authentication establishes that a name is being used
   legitimately.  Authorization establishes that the name is permitted
   to perform a particular service.  The relationship between these two
   functions is that once a client of an exchange is authenticated, then
   it is possible to query the permission of that client to perform
   specific services.

   This specification defines a mechanism to permit session-time
   verification that a connecting SMTP client is authorized to request
   service as a mail transfer client.  The mechanism uses a DNS SRV
   [RFC2782] record as a basis for verifying that the associated domain
   name is authorized to act as an SMTP client.  The mechanism is small,
   simple and useful.  Separate mechanisms provide the means of
   authenticating that the domain name is associated with the connecting
   host, and accrediting the agency that is authorizing the sending
   host's operation as an SMTP client.

   Use of the mechanism specified here MAY also satisfy the
   authentication requirement.  This can occur as a side-effect of the
   DNS server response optimization that returns IP Address mappings in
   the Additional Information portion of a response.

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

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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3.  Model

   The SMTP [RFC2821], [RFC0821] protocol permits a client to declare
   its affiliation, by asserting a domain name in the HELO or EHLO

   The current proposal has a receiving SMTP server take the domain name
   associated with an SMTP client and do a forward query of the DNS.
   The returned DNS information indicates whether that domain name is
   authorized by the domain administrator to be an SMTP client.

   For efficiency, the DNS response MAY also return authentication
   information, as per [ID-CSV].  However the authentication
   functionality is outside the scope of this specification.

4.  Mechanism

   The receiving SMTP server's authorization procedure is:

   1.  Obtain a domain name that is associated with the sending SMTP

   2.  Perform a DNS lookup of:
       QNAME = _client._smtp.<name>
       QCLASS = IN
       QTYPE = SRV
       where <name> is associated with the host attempting to obtain
       service as an SMTP client.

   3.  If there is no SRV RR matching this QNAME, the CSA information is
       Unknown; otherwise at least one CSA record exists.

   4.  If there is a matching QNAME:

          Target addresses MAY be returned in the Additional Data
          section, or a query for address records of the target name may
          be needed to determine the associated address(es).  This MAY
          be used to satisfy the authentication function specified in
          Certified Server Validation [ID-CSV].

          Examine Priority, Weight, and Port, to assess whether the
          client address is authorized as an SMTP client.

   Weight equal to 2 indicates that any client with a valid claim to
   that EHLO string is authorized to send email.  When Weight equals 2,
   the receiving SMTP server SHOULD check whether the source IP address
   of the connection is contained in a response (whether returned as
   Additional Info by the SRV query or returned by a separate address

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   lookup).  If it is not, then the sending SMTP client is NOT
   authorized when failing the address check.

   When Weight equals 1, the sending SMTP client is NOT authorized,
   regardless of whether its IP address is included in the response.

   When Weight equals 3, the sending SMTP client may or may not be
   authorized, whether or not its IP address is included in the response
   (but the EHLO name is authorized, if the receiving SMTP server can
   find some other way to authenticate its right to use that EHLO name).

   If the sending SMTP client[ID-CSV] is both authenticated and
   authorized (with Weight equal 2 and the IP address matching), CSA
   processing is successful, and the receiving SMTP server can treat
   messages arriving in this SMTP session as authorized by the EHLO
   domain administrator.

   Otherwise, caution is required.  The receiving SMTP server might:

      - Generate an SMTP session error, as suggested below.

      - Mark the message, to indicate that it failed validation.

      - Place the message into a special queue, for separate handling.

   For the Unknown case, in which there is no SRV RR, the receiving SMTP
   server's local policy MAY test whether the domain name, from the
   HELO/EHLO announcement, is part of a domain that makes an EXPLICIT
   assertion, as described in Section 5.

   When a CSV related session error is generated, a 550 error code
   SHOULD be used and enough information SHOULD be provided in the reply
   text to facilitate debugging of the sending system.

5.  Client SMTP Authorization SRV Record

   The SRV CSA Record has the following contents:




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      Domain name asserted in SMTP EHLO announcements.

      (These first three fields become the QNAME _client._smtp.Name.)

      Standard DNS meaning [RFC1035].

      Standard DNS meaning [RFC1035].  SRV-CSA records are only defined
      for the IN Class.

      The intended use of [RFC2782] SRV records was to aid discovery and
      selection of servers by prospective clients.  Implementing this
      client authentication mechanism for the server, the Priority,
      Weight, and Port fields are no longer used for either discovery or
      selection.  Thus only one SRV-CSA record is needed and these three
      fields are assigned different meanings.  Priority defines the
      revision level of this mechanism starting at 1.

      Weight is a group of bit-fields, as follows:

   |   Bit Value  | Meaning                                            |
   |       1      | Ignore Target: The domain name in the Target field |
   |              | is a placeholder, and any IP addresses it resolves |
   |              | to MUST NOT be used for authentication.            |
   |       2      | Authorized: Any host with a valid claim to this    |
   |              | name is authorized to send mail.                   |
   |       -      | Other bit values are reserved for expansion and    |
   |              | must be set to zero.                               |

   The resulting unsigned integer values for weight are:

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   | Summed Value | Meaning                                            |
   |       0      | Should not be used, but MAY be interpreted as the  |
   |              | summed value 1.                                    |
   |       1      | No mail should be coming from clients with this    |
   |              | name.                                              |
   |       2      | Clients with this name are authorized to send      |
   |              | mail.                                              |
   |       3      | Clients with this name are authorized to send      |
   |              | mail, but IP addresses associated with the Target  |
   |              | field MUST NOT be used for authentication.         |

      This field allows the domain administrator to declare assertions
      which apply to all names within the domain, including those names
      not present in the DNS.  At present, only one assertion in the
      Port field is defined, as follows:

   |   Assertion  | Meaning                                            |
   |   Bit Value  |                                                    |
   |       1      | Explicit: All authorized names have specific       |
   |              | CSV-CSA records.                                   |
   |       -      | Other bit values are reserved for expansion and    |
   |              | must be set to zero. This range of values should   |
   |              | be ignored by the recipient when their function is |
   |              | unknown.                                           |

      Domain administrators MAY assert the "Explicit" bit when they have
      identified all authorized sending SMTP clients within their domain
      and published specific CSA SRV records for them; that is, all
      positive authorizations within the domain are explicitly
      advertised in DNS.

      This enables receiving SMTP servers to reject SMTP sessions with
      no specific CSV-CSA record if the HELO string is within a domain
      that asserts explicit authorization.

      This assertion greatly simplifies the task of specifying a large
      class of subdomains which will never legitimately be used as EHLO
      strings, and makes it practical for large organizations to
      indicate that individuals should not be using the subdomains
      assigned to them as EHLO strings.  It also deals with invalid EHLO
      strings that do not appear in the DNS.

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      A domain name (typically the same as the EHLO domain) that
      resolves to the correct list of IP addresses.  If this record is
      defined with the "Ignore Target" bit value, this field should be
      set to the Name portion of the QNAME, rather than the "."
      mentioned in [RFC2782], as a means to prevent excessive traffic on
      root DNS servers by errant implementations.

6.  Publishing CSA Records

   If a domain administrator declares an assertion about all names
   within a domain, the appropriate bit MUST be set in the Port field of
   the CSV-CSA record at the root of the domain for which the assertion
   applies, and MAY be repeated at subdomains of that domain.  The
   Explicit bit applies to a domain and all its subdomains.  If it is
   repeated in a subdomain it has no effect on the semantics, but it
   might cause a search to stop sooner.

   Domain administrators SHOULD publish records with such assertions in
   the port field at a level no deeper than sixth-level domains, such as


   since receivers are expected to search no deeper than that, and will
   most likely not find records published for seventh-level or deeper.
   (Receivers will, of course, still query for the weight field at the
   exact level of the EHLO string.)

   Although a conceptual framework might list the accreditation step as
   logically following the authorization step, these steps MAY run in
   parallel.  Thus, those responsible for maintaining CSV DNS records
   should make allowance for the fact that the response of the
   accreditation service (which depends only on the EHLO string or the
   client address) is likely to arrive at the receiving MTA before the
   response to the DNS SRV query detailed here.  As a result, the
   receiving SMTP server may not follow-up partial or truncated UDP
   responses for expediency.  Regardless of what is specified, this
   receiving SMTP server may decide to refuse the client if their chosen
   accreditation service returns "Unknown".  The following
   recommendations explain how to ensure that the complete list of IP
   addresses reaches the receiving SMTP server in the response to its
   SRV query.

   Currently UDP has a limit of 512 octets.  Replies requiring more than
   512 octets may create UDP fragmentation and, depending upon the
   connection and handling, in addition to a higher rate of packet loss,
   may also cause truncated or partial replies.  Furthermore, delivery

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   and resolver handling of truncated and partial responses varies,
   leading to additional delays and queries.  Domain administrators are
   strongly advised to keep DNS replies below 512 octets for these

   In some cases, domains advertising SRV records will benefit by
   reassigning some EHLO strings so as to limit the number of IP
   addresses to be reported in SRV responses.  Owing to the efficient
   nature of the SRV record, the mechanism discussed here calls for a
   single DNS query per SMTP session (not counting an out-of-band
   accreditation query), which is substantially less network traffic
   than per-message methods.

   To help ensure complete answers are obtained from cached records, TTL
   values of the SRV-CSA and related address records should be the same.
   Beware some DNS server implementations consider the SOA TTL as a
   default rather than a minimum.

7.  Using CSA Records

   A receiving SMTP server MAY discover domain assertion information
   (after finding no record for the specific domain in the EHLO string)
   by searching for CSV-CSA records in parent nodes of the EHLO string,
   within the DNS hierarchy.  Such a search MUST NOT query a top-level
   domain (such as COM, NET, or UK), and SHOULD NOT query deeper than a
   sixth-level domain.  Receiving SMTP servers SHOULD ensure that they
   query a server which caches negative results to avoid useless traffic
   to the root servers.

   Receiving SMTP servers MAY maintain and/or query a database which
   saves domain-names for which a record has been found with the
   "Explicit" bit set, and MAY reject or otherwise flag sessions for
   which the "Explicit" assertion applies but no specific CSV-CSA record
   is found.

   With a complete response to an SRV-CSA query, SMTP server is able to
   employ Right Hand Side Black List (RHSBL) services based upon the
   domain name rather than address alone and as well as the
   accreditation services detailed in [ID-CSVDNA].  These domain-based
   services will not suffer from the same outdated-record problems as
   the IP-Address-based services widely used at the time of this
   writing.  Also, of course, domain-based services will be able to
   accredit those domains which must periodically change their IP
   address.  Reliance on the HELO/EHLO response allows isolation of
   domains which may share common address space as with virtual hosting
   or allow detection of domains for which there is insufficient history
   which may invoke a go-slow approach as example.

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8.  Security Considerations

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

   The proposal also relies on security of the underlying IP network and
   on the integrity of DNS data.  It performs a basic authentication of
   the peer MTA, based on domain name registration of the peer's IP
   Address.  As such, the mechanism provides a basic building block to a
   larger repertoire of email security services.

   There is no way a site can keep its hosts from being referenced as
   servers.  This could lead to denial of service.

   With SRV, DNS spoofers can supply false addresses.  Because this
   vulnerability exists already with names and addresses, this is not a
   new vulnerability, merely a slightly extended one.  However, as
   SRV-CSA records are used in an authorization context, the DNS servers
   can be protected by DNSSEC [RFC3008] should this vulnerability become

9.  IANA Considerations

   The tokens "_client" as _Service and "_smtp" as _Proto labels needs
   to be registered as used with DNS SRV records [RFC2782].

10.  Working Group Evaluation

   This section contains responses to the issues put forward by the
   MARID working group chairs.

   1.  Amount of change in software components
       DNS administration, servers and clients MUST support SRV queries.
       Client MTA's MUST put their registered domain name in EHLO
       Server MTA's MUST implement the validation procedure described in
       this specification.

   2.  Configuration complexity
       Requires registering each IP Address of an authorized Client MTA,
       whenever the set of Addresses changes.  No other configuration is

   3.  Current use cases that will no longer be viable
       All current use cases will still be viable.  This mechanism is
       only enabled by the explicit presence of the defined SRV record
       for the domain name in the EHLO announcement.

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   4.  Needed infrastructure changes
       Explicit registration of Client MTAs.
       Considerations for use in both IPv4 and IPv6
       Validation mechanism is based on IP Addresses and requires the
       usual query and handling of address types that will be
       encountered from the IP module and the DNS.

11.  References

11.1  References - Normative

   [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.

              RFC 1034, November 1987.

   [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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
              2671, August 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

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   [RFC3008]  Wellington, B., "Domain Name System Security (DNSSEC)
              Signing Authority", RFC 3008, November 2000.

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

11.2  References - Informative

   [ID-CSV]   Crocker, D., Otis, D. and J. Leslie, "Certified Server
              Validation (CSV)", February 2005.

              Leslie, J., Crocker, D. and D. Otis, "Domain Name
              Accreditation (DNA)", February 2005.

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

Authors' Addresses

   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

   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

Appendix A.  Acknowledgements

   John Levine, Tony Finch, and Sam Silberman provided helpful comments.

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Intellectual Property Statement

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Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2005).  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.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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