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 draft-hathcock-minger-00.txt 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 aware will be disclosed, in accordance with Section 6 of BCP 79. 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 6, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document describes the Minger protocol. Minger is a protocol for determining whether an email address exists and, optionally, retrieving some basic information about the user of that address. It includes security in the form of a username/password combination but can also be used anonymously if desired. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Hathcock Expires September 6, 2007 [Page 1] Internet-Draft Minger March 2007 Table of Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 The problem . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Existing solutions . . . . . . . . . . . . . . . . . . . . 3 1.2.1 Finger . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2.2 SMTP "call-back" / "call-forward" . . . . . . . . . . . . 3 1.3 The solution . . . . . . . . . . . . . . . . . . . . . . . 4 2. The Minger protocol . . . . . . . . . . . . . . . . . . . 4 2.1 The Minger query process . . . . . . . . . . . . . . . . . 4 2.2 Minger responses . . . . . . . . . . . . . . . . . . . . . 5 2.2.1 Example responses . . . . . . . . . . . . . . . . . . . . 5 3. Anonymous mode . . . . . . . . . . . . . . . . . . . . . . 6 4. Security Considerations . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . 7 6. Informative References . . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 10 Hathcock Expires September 6, 2007 [Page 2] Internet-Draft Minger March 2007 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 database. 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". 1.2.1 Finger 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. 1.2.2 SMTP "call-back" / "call-forward" These terms are used to describe 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 leads to a large number of SMTP sessions which appear in logs to have simply failed for no reason. Hathcock Expires September 6, 2007 [Page 3] Internet-Draft Minger March 2007 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 49152. Editor's note: The authors have applied to IANA for a registered port. Until then, implementations will test using the above private port. 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: ABNF: Query string = id SP mailbox [SP username SP password] id = 1 * 50(VCHAR) ; used to match a query to a ; response mailbox = Local-part "@" Domain ; as defined in RFC2821 username = 1 * 50(VCHAR) ; optional username for security password = 1 * 50(VCHAR) ; optional password for security id - This is a randomly generated value which Minger clients include in each query. This same value will be echoed back in the response returned by the Minger server and can therefore be used to match responses with the proper query. mailbox - This is the email address for which verification of existence is desired. username and password - 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. Hathcock Expires September 6, 2007 [Page 4] Internet-Draft Minger March 2007 2.2 Minger responses Minger servers return responses in a simple XML format. Despite the overhead of including XML tags within the limited space available with UDP, XML allows for very easy parsing by the receiving client and the data returned would rarely approach the UDP space limit. The XML format returned by the Minger server has certain required elements but can include other elements as desired by particular implementations. ABNF: Response = "" minger-data "" Minger-data = "" id "" (status user-data / error) id = 1 * 50(VCHAR) ; id of the query being responded to status = "" ("Active" / "Not found" / "Disabled" / 1*(ALPHA / DIGIT)) "" error = "" 1 * (ALPHA / DIGIT) "" user-data = *("<" tag-name ">" 1 * (VCHAR) "") tag-name = 1 * (VCHAR) Minger servers MUST support "status" values of "active", "not found", and "disabled". Minger servers MAY return additional XML nodes containing data not defined in this specification. 2.2.1 Example responses A. Minger response when email address "not found" (returned when the queried email address does not exist): 12345 not found Hathcock Expires September 6, 2007 [Page 5] Internet-Draft Minger March 2007 B. Minger response for error conditions (in this case, invalid credentials): 54321 bad username or password C. Minger response for "active" email addresses (returned when the queried email address exists and is ready to receive mail): abc123def Active D. Minger response returning optional extra data: gfs54ad4fs Active Arvel Hathcock arvel@altn.com 3. Anonymous mode Minger clients MAY attempt anonymous queries; that is, queries which do not contain a username or password 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 an error as described above. Additionally, Minger responses to anonymous queries may contain a sub-set or none of the optional extra XML data that would otherwise be present. However, any response must meet the minimums required by this specification. Hathcock Expires September 6, 2007 [Page 6] Internet-Draft Minger March 2007 4. Security Considerations Minger is used to obtain information about the validity of an email address. It can also be used to retrieve implementation specific "extra" data about the user of an email address. Minger include a username/password concept to prevent unauthorized use. However, it also supports an anonymous mode in which use of these credentials may not be required. It's conceivable that the use of anonymous mode or the compromise of authentication credentials could lead to the undesired provision of information which could then be used for nefarious purposes. Care must be taken to secure the credentials used by Minger and to police the provision of information when using anonymous mode. 5. IANA Considerations Minger requires allocation of a Registered Port by IANA. Hathcock Expires September 6, 2007 [Page 7] Internet-Draft Minger March 2007 6. Informative References [RFC1288] Zimmerman, D., "The Finger User Information Protocol", RFC 1288, December 1991. [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. Hathcock Expires September 6, 2007 [Page 8] Internet-Draft Minger March 2007 Appendix A. Acknowledgements We wish to thank the members of the MDaemon Beta Community (subscribe-md-beta@altn.com) for their ideas and help. Authors' Addresses Arvel Hathcock Alt-N Technologies http://www.altn.com Email: arvel.hathcock@altn.com Jonathan Merkel Alt-N Technologies http://www.altn.com Email: jon.merkel@altn.com Hathcock Expires September 6, 2007 [Page 9] Internet-Draft Minger March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Hathcock Expires September 6, 2007 [Page 10]