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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

Gellens Standards Track [Page 1] 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,
 <ietf-disconn-smtp@imc.org>.  To subscribe, send a message to
 <ietf-disconn-smtp-request@imc.org> 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.

Gellens Standards Track [Page 2] 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:
  1. ————————–

! initial state !

  1. ————————–

! !

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

Gellens Standards Track [Page 3] 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.

Gellens Standards Track [Page 4] 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:
    atrn          = "ATRN" [SP domain *("," domain)]
    domain        = sub-domain 1*("." sub-domain)
    sub-domain    = (ALPHA / DIGIT) *(ldh-str)
    ldh-str       = *(ALPHA / DIGIT / "-") (ALPHA / DIGIT)

5.3. Reversed State

 After the provider has sent a success reply to the ATRN command, the
 roles reverse, and the customer becomes the server, and the provider
 becomes the client.
 After receiving the success response to ATRN, the customer sends a
 standard SMTP initial greeting line.  At this point normal SMTP
 [SMTP, ESMTP] commands are used.  Typically the provider sends EHLO
 after seeing the customer's greeting, to be followed by MAIL FROM and
 so on.

Gellens Standards Track [Page 5] RFC 2645 On-Demand Mail Relay August 1999

5.4. Other Commands

 The provider MAY reject all commands other than EHLO, AUTH, ATRN, and
 QUIT with response code 502.

6. Example On-Demand Mail Relay Session

    P:  220 EXAMPLE.NET on-demand mail relay server ready
    C:  EHLO example.org
    P:  250-EXAMPLE.NET
    P:  250-AUTH CRAM-MD5 EXTERNAL
    P:  250 ATRN
    C:  AUTH CRAM-MD5
    P:  334 MTg5Ni42OTcxNzA5NTJASVNQLkNPTQo=
    C:  Zm9vYmFyLm5ldCBiOTEzYTYwMmM3ZWRhN2E0OTViNGU2ZTczMzRkMzg5MAo=
    P:  235 now authenticated as example.org
    C:  ATRN example.org,example.com
    P:  250 OK now reversing the connection
    C:  220 example.org ready to receive email
    P:  EHLO EXAMPLE.NET
    C:  250-example.org
    C:  250 SIZE
    P:  MAIL FROM: <Lester.Tester@dot.foo.bar>
    C:  250 OK
    P:  RCPT TO: <l.eva.msg@example.com>
    C:  250 OK, recipient accepted
    ...
    P:  QUIT
    C:  221 example.org closing connection

7. Response Codes

 The response codes used in this document are:
 250  Requested mail action okay, completed
 450  ATRN request refused
 451  Unable to process ATRN request now
 453  You have no mail
 502  Command not implemented
 530  Authentication required [AUTH]

8. Security Considerations

 Because access to the On-Demand Mail Relay server is only useful with
 a prior arrangement between the parties (so the provider is the
 target of MX records for the customer's domains and thus has mail to
 relay), it may be useful for the provider to restrict access to the
 On-Demand Mail Relay port.  For example, the ODMR server could be

Gellens Standards Track [Page 6] RFC 2645 On-Demand Mail Relay August 1999

 configurable, or a TCP wrapper or firewall could be used, to block
 access to port 366 except within the provider's network.  This might
 be useful when the provider is the customer's ISP.  Use of such
 mechanisms does not reduce the need for the AUTH command, however,
 but can increase the security it provides.
 Use of SASL in the AUTH command allows for substitution of more
 secure authentication mechanisms in the future.
 See sections 5.1.2 and 5.2.1 for additional security details.

9. Acknowledgments

 This memo has been developed in part based on comments and
 discussions which took place on and off the IETF-disconn-smtp mailing
 list.  Special thanks to Chris Newman and Ned Freed for their
 comments.

10. References

 [ABNF]      Crocker, D. and P. Overell,  "Augmented BNF for Syntax
             Specifications: ABNF", RFC 2234, November 1997.
 [AUTH]      Myers, J., "SMTP Service Extension for Authentication",
             RFC 2554, March 1999.
 [CRAM]      Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP
             AUTHorize Extension for Simple Challenge/Response", RFC
             2195, September 1997.
 [ESMTP]     Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
             Crocker, "SMTP Service Extensions", RFC 1869, November
             1995.
 [ETRN]      De Winter, J., "SMTP Service Extension for Remote Message
             Queue Starting", RFC 1985, August 1996.
 [KEYWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [SASL]      Myers, J., "Simple Authentication and Security Layer
             (SASL)", RFC 2222, October 1997.
 [SMTP]      Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
             821, August 1982.

Gellens Standards Track [Page 7] RFC 2645 On-Demand Mail Relay August 1999

11. Author's Address

 Randall Gellens
 QUALCOMM Incorporated
 5775 Morehouse Dr.
 San Diego, CA  92121-2779
 U.S.A.
 Phone: +1.619.651.5115
 EMail: randy@qualcomm.com

Gellens Standards Track [Page 8] RFC 2645 On-Demand Mail Relay August 1999

12. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Gellens Standards Track [Page 9]

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