rfc2645.txt

用C#开发实现SMTP相关技术,能接收到带附件的邮件服务功能.

Network Working Group                                        R. Gellens
Request for Comments: 2645                                     Qualcomm
Category: Standards Track                                   August 1999


                      ON-DEMAND MAIL RELAY (ODMR)
                    SMTP with Dynamic IP Addresses

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Table of Contents

    1.  Abstract . . . . . . . . . . . . . . . . . . . . . . . . . .  1
    2.  Conventions Used in this Document . . . . . . . . . . . . . . 2
    3.  Comments . . . . . . . . . . . . . . . . . . . . . . . . . .  2
    4.  Description . . . . . . . . . . . . . . . . . . . . . . . . . 2
    5.  States . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
      5.1.  Initial State . . . . . . . . . . . . . . . . . . . . . . 4
        5.1.1.  EHLO . . . . . . . . . . . . . . . . . . . . . . . .  4
        5.1.2.  AUTH  . . . . . . . . . . . . . . . . . . . . . . . . 4
        5.1.3.  QUIT . . . . . . . . . . . . . . . . . . . . . . . .  4
      5.2.  Authenticated State . . . . . . . . . . . . . . . . . . . 4
        5.2.1.  ATRN (Authenticated TURN)  . . . . . . . . . . . . .  4
      5.3.  Reversed State  . . . . . . . . . . . . . . . . . . . . . 5
      5.4.  Other Commands . . . . . . . . . . . . . . . . . . . . .  6
    6.  Example On-Demand Mail Relay Session: . . . . . . . . . . . . 6
    7.  Response Codes . . . . . . . . . . . . . . . . . . . . . . .  6
    8.  Security Considerations . . . . . . . . . . . . . . . . . . . 6
    9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  7
   10.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 7
   11.  Author's Address   . . . . . . . . . . . . . . . . . . . . .  8
   12.  Full Copyright Statement  . . . . . . . . . . . . . . . . . . 9

1.  Abstract

   With the spread of low-cost computer systems and Internet
   connectivity, the demand for local mail servers has been rising.
   Many people now want to operate a mail server on a system which has



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RFC 2645                  On-Demand Mail Relay               August 1999


   only an intermittent connection to a service provider.  If the system
   has a static IP address, the ESMTP ETRN command [ETRN] can be used.
   However, systems with dynamic IP addresses (which are very common
   with low-cost connections) have no widely-deployed solution.

   This memo proposes a new service, On-Demand Mail Relay (ODMR), which
   is a profile of SMTP [SMTP, ESMTP], providing for a secure,
   extensible, easy to implement approach to the problem.

2.  Conventions Used in this Document

   Because the client and server roles reverse during the session, to
   avoid confusion, the terms "customer" and "provider" will be used in
   place of "client" and "server", although of course this protocol may
   be useful in cases other than commercial service providers and
   customers.

   In examples, "P:" is used to indicate lines sent by the provider, and
   "C:" indicates those sent by the customer.  Line breaks within a
   command are for editorial purposes only.

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in [KEYWORDS].

   Examples use 'example.net' as the provider, and 'example.org' and '
   example.com' as the customers.

3.  Comments

   Private comments should be sent to the author.  Public comments may
   be sent to the IETF Disconnected SMTP mailing list,
   <[e-mail]>.  To subscribe, send a message to
   <[e-mail]> containing the word SUBSCRIBE as
   the body.

4.  Description

   On-Demand Mail Relay is a restricted profile of SMTP [SMTP, ESMTP].
   Port 366 is reserved for On-Demand Mail Relay.  The initial client
   and server roles are short-lived, as the point is to allow the
   intermittently-connected host to request mail held for it by a
   service provider.

   The customer initiates a connection to the provider, authenticates,
   and requests its mail.  The roles of client and server then reverse,
   and normal SMTP [SMTP, ESMTP] proceeds.





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RFC 2645                  On-Demand Mail Relay               August 1999


   The provider has an On-Demand Mail Relay process listening for
   connections on the ODMR port.  This process does not need to be a
   full SMTP server.  It does need to be an SMTP client with access to
   the outgoing mail queues, and as a server implement the EHLO, AUTH,
   ATRN, and QUIT commands.

   An MTA normally has a mail client component which processes the
   outgoing mail queues, attempting to send mail for particular domains,
   based on time or event (such as new mail being placed in the queue,
   or receipt of an ETRN command by the SMTP server component).  The
   On-Demand Mail Relay service processes the outgoing queue not on a
   timer or new mail creation, but on request.

   The provider side has normal SMTP server responsibilities [SMTP],
   including generation of delivery failure notices, etc. as needed.

5.  States

   The On-Demand Mail Relay service has three states: an initial state,
   an authenticated state, and a reversed state.  The state progression
   is illustrated in the following diagram:

   ---------------------------
   !      initial state      !
   ---------------------------
     !             !
   QUIT           AUTH
     !             !
     !             V
     !      -----------------------
     !      ! authenticated state !
     !      -----------------------
     !       !            !
     !      QUIT         ATRN
     !       !            !
     !       !            V
     !       !      ------------------
     !       !      ! reversed state !
     !       !      ------------------
     !       !         !
     !       !        QUIT
     !       !         !
     V       V         V
     ---------------------
     !    termination    !
     ---------------------





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RFC 2645                  On-Demand Mail Relay               August 1999


   (Note that in the reversed state, commands are sent by the provider,
   not the customer.)

5.1.  Initial State

   In the initial state, the provider is the server and the customer is
   the client.  Three commands are valid:  EHLO, AUTH, and QUIT.

5.1.1.  EHLO

   The EHLO command is the same as in [ESMTP].  The response MUST
   include AUTH and ATRN.

5.1.2.  AUTH

   The AUTH command is specified in [AUTH].  The AUTH command uses a
   [SASL] mechanism to authenticate the session.  The session is not
   considered authenticated until a success response to AUTH has been
   sent.

   For interoperability, implementations MUST support the CRAM-MD5
   mechanism [CRAM].  Other SASL mechanisms may be supported.  A site
   MAY disable CRAM-MD5 support if it uses more secure methods.  The
   EXTERNAL mechanism [SASL] might be useful in some cases, for example,
   if the provider has already authenticated the client, such as during
   a PPP connection.

5.1.3.  QUIT

   The QUIT command is the same as in [SMTP].

5.2.  Authenticated State

   The authenticated state is entered after a successful AUTH command.
   Two commands are valid in the authenticated state:  ATRN and QUIT.

5.2.1.  ATRN (Authenticated TURN)

   Unlike the TURN command in [SMTP], the ATRN command optionally takes
   one or more domains as a parameter.  The ATRN command MUST be
   rejected if the session has not been authenticated.  Response code
   530 [AUTH] is used for this.

   The timeout for this command MUST be at least 10 minutes to allow the
   provider time to process its mail queue.

   An ATRN command sent with no domains is equivalent to an ATRN command
   specifying all domains to which the customer has access.



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RFC 2645                  On-Demand Mail Relay               August 1999


   If the authentication used by the customer does not provide access to
   all of the domains specified in ATRN, the provider MUST NOT send mail
   for any domains to the customer; the provider MUST reject the ATRN
   command with a 450 code.

   If the customer does have access to all of the specified domains, but
   none of them have any queued mail, the provider normally rejects the
   ATRN command with response code 453.  The provider MAY instead issue
   a 250 success code, and after the roles are reversed, send a QUIT
   following the EHLO.

   The provider MAY also reject the ATRN command with a 450 response to
   indicate refusal to accept multiple requests issued within a
   particular time interval.

   If the customer has access to all of the specified domains and mail
   exists in at least one of them, the provider issues a 250 success
   code.

   If the server is unable to verify access to the requested domains
   (for example, a mapping database is temporarily unavailable),
   response code 451 is sent.

      [ABNF] for ATRN: