Internet Draft: On-Demand Mail Relay R. Gellens Document: draft-gellens-on-demand-03.txt QUALCOMM, Inc. Expires: 11 September 1998 11 March, 1998 On-Demand Mail Relay Status of this Memo: This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." To learn the current status of any Internet Draft, please check the "1id-abstracts.txt" listing contained in the Internet Drafts shadow directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). A version of this draft document is intended for submission to the RFC editor as a Proposed Standard for the Internet Community. Discussion and suggestions for improvement are requested. Please send comments to the IETF Disconnected SMTP mailing list, <ietf-disconn-smtp@imc.org>. To subscribe, send a message containing SUBSCRIBE to <ietf-disconn-smtp-request@imc.org>. Copyright Notice Copyright (C) The Internet Society 1998. All Rights Reserved. Introduction With the spread of low-cost computer systems and Internet connectivity, the demand for local mail servers had been rising. Many people now want to operate a mail server on a system which has only an intermittent connection to a service provider. If the system has a static IP address, the [ESMTP] [ETRN] command can be used. However, systems with dynamic IP addresses (which are very common with low-cost connections) have no good solution. Gellens Expires September 1998 [Page 1]
Internet Draft On-Demand Mail Relay March 1998 This memo proposes a new service, On-Demand Mail Relay, which is a profile of [ESMTP], providing for a secure, extensible, easy to implement approach to the problem. 1. 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]. 2. Description On-Demand Mail Relay is a restricted profile of SMTP which runs on port xxxx. 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 [ESMTP] proceeds. The provider has an On-Demand Mail Relay process listening for connections on port xxxx. 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. 3. 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: Gellens Expires September 1998 [Page 2]
Internet Draft On-Demand Mail Relay March 1998 --------------------------- ! initial state ! --------------------------- ! ! QUIT AUTH ! ! ! V ! ----------------------- ! ! authenticated state ! ! ----------------------- ! ! ! ! QUIT ATRN ! ! ! ! ! V ! ! ------------------ ! ! ! reversed state ! ! ! ------------------ ! ! ! ! ! QUIT ! ! ! V V V --------------------- ! termination ! --------------------- 3.1. 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. 3.1.1. EHLO The EHLO command is the same as in [ESMTP]. The response must include AUTH and ATRN. 3.1.2. 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. Gellens Expires September 1998 [Page 3]
Internet Draft On-Demand Mail Relay March 1998 3.1.3. QUIT The QUIT command is the same as in [SMTP]. 3.2. Authenticated State The authenticated state is entered after a successful AUTH command. Two commands are valid in the authenticated state: ATRN and QUIT. 3.2.1. 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 503 should be used for this. The timeout for this command MUST be at least 15 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. If the authentication used by the customer does not provide access to any of the domains specified in ATRN, the provider MUST NOT send mail for those 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 also rejects the ATRN with 450. If [SMTP-CODES] is used according to [CODES-EXTENSION], the provider MUST NOT distinguish between these cases. 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. [ABNF] for ATRN: atrn ::= "ATRN" [domain *("," domain)] domain ::= sub-domain 1*("." sub-domain) sub-domain ::= (ALPHA / DIGIT) *(ldh-str) ldh-str ::= *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) 3.3. 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. At this point normal [ESMTP] commands are used. Typically the provider sends MAIL FROM immediately following the success response to ATRN. Gellens Expires September 1998 [Page 4]
Internet Draft On-Demand Mail Relay March 1998 3.4. Other Commands The provider SHOULD reject all commands other than EHLO, AUTH, ATRN, and QUIT with response code 502. 4. Example On-Demand Mail Relay Session: P: 220 ISP.NET on-demand mail relay server ready C: EHLO foobar.net P: 250-AUTH CRAM-MD5 Kerberos-v5 P: 250 ATRN C: AUTH CRAM-MD5 P: 334 MTg5Ni42OTcxNzA5NTJASVNQLkNPTQo= C: Zm9vYmFyLm5ldCBiOTEzYTYwMmM3ZWRhN2E0OTViNGU2ZTczMzRkMzg5MAo= P: 235 now authenticated as foobar.net C: ATRN foobar.net, vanity.com P: 250 OK now reversing the connection P: MAIL FROM: <Lester.Tester@dot.foo.org> C: 250 OK P: RCPT TO: <l.eva.msg@vanity.com> C: 250 OK, recipient accepted ... P: QUIT C: 221 foobar.net closing connection 5. 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, a TCP wrapper or firewall could be used to block access to port xxxx 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 3.1.2 and 3.2.1 for additional security details. 6. Acknowledgments This draft has been developed in part based on comments and discussions which took place on and off the IETF-disconn-smtp mailing list. Gellens Expires September 1998 [Page 5]
Internet Draft On-Demand Mail Relay March 1998 7. References [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd., November 1997. <ftp://ftp.isi.edu/in-notes/rfc2234.txt> [AUTH] Myers, J., "SMTP Service Extension for Authentication", (work in progress), <ftp://ftp.isi.edu/internet-drafts/draft-myers-smtp-auth-11.txt> [CODES-EXTENSION] Freed, N., "SMTP Service Extension for Returning Enhanced Error Codes", RFC 2034, October 1996, <ftp://ds.internic.net/rfc/rfc2034.txt> [CRAM] Klensin, J., Catoe, R., Krumviede, P. "IMAP/POP AUTHorize Extension for Simple Challenge/Response", RFC 2195, September 1997, <ftp://ds.internic.net/rfc/rfc2195.txt> [ESMTP] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extensions", RFC 1869, STD 10, November 1995, <ftp://ds.internic.net/rfc/rfc1869.txt> [ETRN] De Winter, J., "SMTP Service Extension for Remote Message Queue Starting", RFC 1985, August 1996, <ftp://ds.internic.net/rfc/rfc1985.txt> [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997, <ftp://ds.internic.net/rfc/rfc2119.txt> [SASL] Myers, J., "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997, <ftp://ds.internic.net/rfc/rfc2222.txt> [SMTP-CODES] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893, January 1996, <ftp://ds.internic.net/rfc/rfc1893.txt> [SMTP] J. Postel, "Simple Mail Transfer Protocol", RFC 821, STD 10, August 1982, <ftp://ds.internic.net/rfc/rfc821.txt> 8. Full Copyright Statement Copyright (C) The Internet Society 1998. 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 implmentation 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 Gellens Expires September 1998 [Page 6]
Internet Draft On-Demand Mail Relay March 1998 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. 9. Author's Address Randall Gellens +1.619.651.5115 Qualcomm, Inc. +1.619.651.5334 (fax) 6455 Lusk Blvd. Randy@Qualcomm.Com San Diego, CA 92121-2779 U.S.A. Gellens Expires September 1998 [Page 7]