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Versions: 00 01 02 03 04 05 06                                          
Network Working Group                                        A. Hathcock
Internet-Draft                                                 J. Merkel
Intended Status: Informational                        Alt-N Technologies
Expires: September 6, 2007                                 March 6, 2007

              The Minger Email Address Verification Protocol

Status of this Memo

   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 becomes
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   This Internet-Draft will expire on September 6, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes the Minger protocol.  Minger is a protocol
   for determining whether an email address exists and, optionally,
   retrieving some information about the user of that address.  It
   includes security in the form of a username/hashed password but can
   also be used anonymously if desired.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 solution . . . . . . . . . . . . . . . . . . . . . .  3
   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  . . . . . . . . . . . .  6
     3.2  Example responses . . . . . . . . . . . . . . . . . . . .  6
   4.  Anonymous mode . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  8
   7.  Informative References . . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . 11

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1. 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 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

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.  For example, it is possible for
   attackers to obtain information about the users of an email system
   which they can then sell or use as targets for spam and viruses.
   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.

   SMTP includes a VRFY command which can be used to determine whether
   an email address exits.  It is routinely disabled for the same
   reasons described above in the discussion on Finger.

1.3 The solution

   What's needed is a UDP based protocol which is secure, has little
   overhead, and can be easily invoked to determine whether a given
   email address is valid or not.  Minger fulfills this need.

2. 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 comprised of either two or
   four elements and transmits it over UDP to a Minger server.  The
   format of the query is as follows:


   Query-string = id SP mailbox [SP credentials]

   id = 1*50(VCHAR)                  ; used to match a query to a
                                     ; response

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

   credentials = username SP digest  ; authentication credentials

   username = 1*50(VCHAR)            ; username credential

   password = 1*50(VCHAR)            ; password credential

   digest-text = username ":" password ; input text for digest

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

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2.2 Description of query elements


      This is an identifier assigned by the program that generates the
      query.  This same value will be echoed back in the response
      returned by the Minger server and can therefore be used to match
      a response to the proper query.


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


      These values are pre-arranged elements determined and
      configured in advance so that Minger servers provide service only
      to authorized clients.  When not provided, Minger is operating in
      anonymous mode.


     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.

3. Minger responses

   Minger servers return responses in a simple XML format.  The XML
   format returned by the Minger server has certain required elements
   but can include additional optional elements as desired by particular


   <!DOCTYPE MingerResponse [
   <!ELEMENT MingerResponse (name?, email?)>
   <!ATTLIST MingerResponse
        id CDATA #REQUIRED
        status DIGIT #REQUIRED>
   <!ELEMENT name CDATA>
   <!ELEMENT email CDATA>

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3.1 Description of response elements


      Queries submit an identifier.  That value is copied into the
      id field within responses.  This allows clients to match up
      responses to the proper queries.


      The following status codes are defined:

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

   name and email

      Optional full name and email address associated with the query
      result.  The value returned in the "email" element might differ
      from the email address used in the actual query itself.  For
      example, if the query specifies an email address alias then the
      minger result might contain the actual email address in the
      "email" element.

  Note: Minger servers MAY supply one or more additional XML elements
      to provide additional data not specified by this document.

3.2 Example responses

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

      <MingerResponse id="12345" status="3"/>

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   Minger response returned for invalid credentials:

      <MingerResponse id="543" status="2"/>

   Minger response returned when the queried email address exists:

      <MingerResponse id="ab12fg" status="5"/>

   Minger response returning optional name and email elements:

      <MingerResponse id="lkj234" status="5">
      <name>Arvel Hathcock</name>

4. Anonymous mode

   Minger clients MAY attempt anonymous queries; that is, queries which
   do not contain authentication credentials within the query string.
   Minger servers SHOULD respond to anonymous queries in the same way
   they respond to authenticated queries.  However, Minger servers MAY
   be configured to refuse anonymous queries.  If so, they MUST respond
   with a status of "2".  Additionally, Minger servers MAY respond to
   anonymous queries with a sub-set or none of any optional user data
   that may otherwise be provided.

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5. Security Considerations

   Minger is used to obtain information about the validity of an email
   address.  It may also be used to retrieve additional implementation
   specific data about the user of an email address.  Minger also
   supports an anonymous mode concept in which use of authentication
   credentials is not required.  Extreme care must therefore be taken
   to ensure that sensitive data is not transmitted unless appropriate.
   To reduce the likelihood of abuse, Minger servers should require
   authentication and secure access with IP-based ACLs.

   With using authentication credentials, the original password is
   safe because only a hash is sent.  However, since the hash does
   not depend on the message, it is subject to replay abuse.

6. IANA Considerations

   IANA has assigned tcp & upd port 4069 for Minger.

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7. 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.

   [W3C-XML]  Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
              "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
              REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.

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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.

Authors' Addresses

   Arvel Hathcock
   Alt-N Technologies

   Email: arvel.hathcock@altn.com

   Jonathan Merkel
   Alt-N Technologies

   Email: jon.merkel@altn.com

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