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Network Working Group                                        R. GellensRequest for Comments: 2645                                     QualcommCategory: Standards Track                                   August 1999ON-DEMAND MAIL RELAY (ODMR)SMTP with Dynamic IP AddressesStatus 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 Contents1.  Abstract . . . . . . . . . . . . . . . . . . . . . . . . . .12.  Conventions Used in this Document . . . . . . . . . . . . . .23.  Comments . . . . . . . . . . . . . . . . . . . . . . . . . .24.  Description . . . . . . . . . . . . . . . . . . . . . . . . .25.  States . . . . . . . . . . . . . . . . . . . . . . . . . . .35.1.  Initial State . . . . . . . . . . . . . . . . . . . . . .45.1.1.  EHLO . . . . . . . . . . . . . . . . . . . . . . . .45.1.2.  AUTH  . . . . . . . . . . . . . . . . . . . . . . . .45.1.3.  QUIT . . . . . . . . . . . . . . . . . . . . . . . .45.2.  Authenticated State . . . . . . . . . . . . . . . . . . .45.2.1.  ATRN (Authenticated TURN)  . . . . . . . . . . . . .45.3.  Reversed State  . . . . . . . . . . . . . . . . . . . . .55.4.  Other Commands . . . . . . . . . . . . . . . . . . . . .66.  Example On-Demand Mail Relay Session: . . . . . . . . . . . .67.  Response Codes . . . . . . . . . . . . . . . . . . . . . . .68.  Security Considerations . . . . . . . . . . . . . . . . . . .69.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .710.  References  . . . . . . . . . . . . . . . . . . . . . . . . .711.  Author's Address   . . . . . . . . . . . . . . . . . . . . .812.  Full Copyright Statement  . . . . . . . . . . . . . . . . . .91.  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 hasGellens                     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:   ---------------------------   !      initial state      !   ---------------------------     !             !   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 19995.4.  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 connection7.  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 beGellens                     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 sections5.1.2 and5.2.1 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",RFC2195, 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,RFC821, August 1982.Gellens                     Standards Track                     [Page 7]

RFC 2645                  On-Demand Mail Relay               August 199911.  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.comGellens                     Standards Track                     [Page 8]

RFC 2645                  On-Demand Mail Relay               August 199912.  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]