Network Working Group                                        A. Hathcock
Internet-Draft                                                 J. Merkel
Intended Status: Informational                        Alt-N Technologies
Expires: October 1, 2007                                   April 1, 2008


              The Minger Email Address Verification Protocol
                       draft-hathcock-minger-03.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes the Minger protocol.  Minger is a protocol
   which allows a mail handling entity to query a remote service and
   ask the question "do you accept mail for this email address?" It
   includes security in the form of a hashed shared secret but can also
   be used anonymously if desired.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].



Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  The problem  . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Existing solutions . . . . . . . . . . . . . . . . . . .  3
     1.3.  The Minger solution  . . . . . . . . . . . . . . . . . .  4
   2.  The Minger protocol  . . . . . . . . . . . . . . . . . . . .  4
     2.1  The Minger query process  . . . . . . . . . . . . . . . .  4
     2.2  Description of query elements . . . . . . . . . . . . . .  5
   3.  Minger responses . . . . . . . . . . . . . . . . . . . . . .  5
     3.1  Description of response elements  . . . . . . . . . . . .  5
     3.2  Example responses . . . . . . . . . . . . . . . . . . . .  6
   4.  Anonymous mode . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  7
   7.  Informative References . . . . . . . . . . . . . . . . . . .  7
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  8
   Intellectual Property and Copyright Statements . . . . . . . . .  9


































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. Introduction

1.1 The problem

   It is common for elements within a typical email handling topology
   to be unaware of whether individual local-parts are valid for the
   mail it accepts.  For example, so-called "edge" servers which provide
   security oriented services for downstream mail handling elements
   often do not have an exhaustive listing of all valid local-parts for
   a given domain.  Thus, they are sometimes forced to accept and process
   messages which might otherwise be rejected as "user unknown".
   Similarly, entities offering "backup MX" mail services are rarely
   privy to a complete local-part listing and are therefore forced to
   accept messages which might otherwise be rejected.  Finally, even
   within a common administrative framework of several locally maintained
   and controlled SMTP servers in a load balanced configuration, it is
   not always possible for all servers to access a common local-part
   database.

1.2 Existing solutions

   The need to determine whether an email address contains a valid local
   part has lead to the use of at least two existing mechanisms - Finger
   [RFC1288] and SMTP "call-back" / "call-forward".

   Finger [RFC1288] describes a protocol for the exchange of user
   information.  In theory, Finger could be used to determine whether an
   account exists by careful examination of the results of a Finger
   query.  However, Finger suffers from a lack of security which makes
   its modern day use problematic when coupled with the user level
   detail it can provide.  Also, Finger requires the use of TCP rather
   than UDP which seems ill suited to a simple verification scheme.

   SMTP "call-back" and "call-forward" are terms describing a widespread
   practice whereby SMTP servers place an incoming SMTP session on hold
   while they attempt to use an outbound SMTP session to determine
   whether or not a given email address is valid.  The theory behind this
   is as follows:  if an SMTP server responds positively to an SMTP RCPT
   or MAIL command [RFC2821] with a given email address then this
   potentially means that the address local part is valid.  One problem
   with such a scheme is the lack of efficiency inherent in the need to
   tear-up and tear-down an SMTP session over TCP.  Also, because these
   types of SMTP sessions are not purposed to deliver mail, they
   typically drop connection after the RCPT command is processed.  This






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   leads to a large number of SMTP sessions which appear in logs to have
   simply failed for no reason.  This leads to situations in which SMTP
   transaction logs can no longer distinguish legitimate network errors
   from "call-back"/"call-forward" traffic.

   SMTP includes a VRFY command which can be used to determine whether
   an email address exits.  VRFY is not in wide-spread use and suffers
   from the same inefficiency concerns described in the discussion on
   SMTP "call-back".  Additionally, a particular SMTP agent serving a
   domain that has several is not necessarily authoritative making a
   VRFY request to it potentially unreliable.

1.3 The Minger solution

   What's needed is a protocol which is secure, has little overhead, and
   can be easily invoked to determine whether a given email address is
   valid or not.  Minger achieves these goals using a shared secret for
   security and UDP to lower overhead.

. The Minger protocol

   Minger is a UDP protocol that operates on port 4069.

   Syntax descriptions use the form described in Augmented Backus-Naur
   Form for Syntax Specifications (ABNF) [RFC4234].

2.1 The Minger query process

   A Minger client constructs a query string as described below and
   transmits it over UDP to a Minger server.  The format of the query
   is as follows:

  ABNF:

   query-string = mailbox [SP "d=" digest] [SP tag-list]

   digest = base64                   ; digest for security
                                     ; base64 defined in [RFC1734]

   digest-text = shared-secret ":" mailbox ; input text for digest

   mailbox = Local-part "@" Domain   ; as defined in [RFC2821]

   shared-secret = 1*50(VCHAR)       ; password credential

   tag-list = tag-spec 0*(SP tag-spec) ; tag/value list

   tag-spec = tag-name "=" tag-value


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    tag-name = 1 * (any ALPHA other than 'd' / DIGIT / "_")

    tag-value = 1 * (ALPHA / DIGIT / "_")


2.2 Description of query elements

   mailbox

      This is the email address for which verification of
      existence is desired.

   digest

     This is the base64 encoding of the MD5 [RFC1321] hash of
     digest-text.  Digest-text is constructed, the MD5 hash of that
     is computed, and that result is base64 encoded.

   tag-list

     Tag-list is provided so that future capability might be added
     in an easy way.

. Minger responses

   Minger servers return a response string of the following form:

  ABNF:

   response-string = mailbox status

   mailbox = Local-part "@" Domain   ; as defined in [RFC2821]

   status = %x30-35                  ; single digit result code
                                     ; from 0 - 5

3.1 Description of response elements

   mailbox

      This is the email address for which verification of
      existence is desired.

   status

      The following status codes are defined:

      0 - invalid request (for example, malformed query string)


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      1 - access denied (for example, query from unauthorized IP)
      2 - bad or missing credentials (returned when anonymous
          mode is disabled and no credentials were provided in the
          query string or when the credentials themselves are
          invalid)
      3 - email address does not exist
      4 - email address exists but can not receive mail (for example,
          the account associated with the email address has exceeded
          local storage constraints or it is otherwise disabled due
          to local policy)
      5 - email address exists and is active (able to receive mail)

3.2 Example responses

   Minger response returned when the queried email address does
   not exist:

      arvel@example.com 3

   Minger response returned for invalid credentials:

      arvel@example.com 2

   Minger response returned when the queried email address exists:

      arvel@example.com 5


. Anonymous mode

   Minger clients MAY attempt anonymous queries; that is, queries which
   do not contain a shared secret digest within the query string. Minger
   servers MAY be configured to refuse anonymous queries.  If so, they
   MUST respond with a status of "2".
















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. Security Considerations

   Minger is a protocol which is used to determine whether a given
   email address is valid or not.  If a particular email
   infrastructure does not wish to advertise the email addresses that
   it services then this protocol should not be employed.

   If a shared secret is employed to secure Minger from anonymous use
   that shared secret should be at least 128 bits.


. IANA Considerations

   IANA has assigned tcp & upd port 4069 for Minger.


. Informative References

   [RFC1288]  Zimmerman, D., "The Finger User Information Protocol",
              RFC 1288, December 1991.

   [RFC1734]  Myers, J., "POP3 Authentication Command", RFC 1734,
              December 1994.

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

   [RFC4234]  Crocker, D., Ed. And P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

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

   [RFC1321]  Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
              MIT Laboratory for Computer Science and RSA Data Security,
              Inc., April 1992.













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Appendix A.  Acknowledgements

   We wish to thank the members of the MDaemon Beta Community
   (md-beta-subscribe@altn.com) for their ideas and help and Paul
   Hoffman for his valuable feedback.

Authors' Addresses

   Arvel Hathcock
   Alt-N Technologies
   http://www.altn.com

   Email: arvel.hathcock@altn.com


   Jonathan Merkel
   Alt-N Technologies
   http://www.altn.com

   Email: jon.merkel@altn.com






























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