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-01.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 information about the user of that address. It includes security in the form of a username/hashed password 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]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. The problem . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Existing solutions . . . . . . . . . . . . . . . . . . . 3 1.3. The solution . . . . . . . . . . . . . . . . . . . . . . 3 2. The Minger protocol . . . . . . . . . . . . . . . . . . . . 4 2.1 The Minger query process . . . . . . . . . . . . . . . . 4 2.2 Description of query elements . . . . . . . . . . . . . . 5 3. Minger responses . . . . . . . . . . . . . . . . . . . . . . 5 3.1 Description of response elements . . . . . . . . . . . . 6 3.2 Example responses . . . . . . . . . . . . . . . . . . . . 6 4. Anonymous mode . . . . . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 8 7. Informative References . . . . . . . . . . . . . . . . . . . 9 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . 11 Hathcock Expires September 26, 2007 [Page 2] Internet-Draft draft-hathcock-minger-01.txt 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". 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. SMTP "call-back" and "call-forward" are terms describing 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 Hathcock Expires September 26, 2007 [Page 3] Internet-Draft draft-hathcock-minger-01.txt March 2007 leads to a large number of SMTP sessions which appear in logs to have simply failed for no reason. SMTP includes a VRFY command which can be used to determine whether an email address exits. It is routinely disabled for the same reasons described above in the discussion on Finger. 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 4069. 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 credentials] id = 1*50(VCHAR) ; used to match a query to a ; response mailbox = Local-part "@" Domain ; as defined in [RFC2821] credentials = username SP digest ; authentication credentials username = 1*50(VCHAR) ; username credential password = 1*50(VCHAR) ; password credential digest-text = username ":" password ; input text for digest digest = base64 ; digest for security ; base64 defined in [RFC1734] Hathcock Expires September 26, 2007 [Page 4] Internet-Draft draft-hathcock-minger-01.txt March 2007 2.2 Description of query elements id This is an identifier assigned by the program that generates the query. This same value will be echoed back in the response returned by the Minger server and can therefore be used to match a response to the proper query. mailbox This is the email address for which verification of existence is desired. credentials 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. digest This is the base64 encoding of the MD5 [RFC1321] hash of Digest-text. Digest-text is constructed, the MD5 hash of that is computed, and that result is base64 encoded. 3. Minger responses Minger servers return responses in a simple XML format. The XML format returned by the Minger server has certain required elements but can include additional optional elements as desired by particular implementations. [W3C-XML] DTD: ]> Hathcock Expires September 26, 2007 [Page 5] Internet-Draft draft-hathcock-minger-01.txt March 2007 3.1 Description of response elements id Queries submit an identifier. That value is copied into the id field within responses. This allows clients to match up responses to the proper queries. status The following status codes are defined: 0 - invalid request (for example, malformed query string) 1 - access denied (for example, query from unauthorized IP) 2 - bad or missing credentials (returned when anonymous mode is disabled and no credentials were provided in the query string or when the credentials themselves are invalid) 3 - email address does not exist 4 - email address exists but can not receive mail (for example, the account associated with the email address has exceeded local storage constraints or it is otherwise disabled due to local policy) 5 - email address exists and is active (able to receive mail) name and email Optional full name and email address associated with the query result. The value returned in the "email" element might differ from the email address used in the actual query itself. For example, if the query specifies an email address alias then the minger result might contain the actual email address in the "email" element. Note: Minger servers MAY supply one or more additional XML elements to provide additional data not specified by this document. 3.2 Example responses Minger response returned when the queried email address does not exist: Hathcock Expires September 26, 2007 [Page 6] Internet-Draft draft-hathcock-minger-01.txt March 2007 Minger response returned for invalid credentials: Minger response returned when the queried email address exists: Minger response returning optional name and email elements: Arvel Hathcock arvel@altn.com 4. Anonymous mode Minger clients MAY attempt anonymous queries; that is, queries which do not contain authentication credentials 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 a status of "2". Additionally, Minger servers MAY respond to anonymous queries with a sub-set or none of any optional user data that may otherwise be provided. Hathcock Expires September 26, 2007 [Page 7] Internet-Draft draft-hathcock-minger-01.txt March 2007 5. Security Considerations Minger is used to obtain information about the validity of an email address. It may also be used to retrieve additional implementation specific data about the user of an email address. Minger also supports an anonymous mode concept in which use of authentication credentials is not required. Extreme care must therefore be taken to ensure that sensitive data is not transmitted unless appropriate. To reduce the likelihood of abuse, Minger servers should require authentication and secure access with IP-based ACLs. With using authentication credentials, the original password is safe because only a hash is sent. However, since the hash does not depend on the message, it is subject to replay abuse. 6. IANA Considerations IANA has assigned tcp & upd port 4069 for Minger. Hathcock Expires September 26, 2007 [Page 8] Internet-Draft draft-hathcock-minger-01.txt March 2007 7. Informative References [RFC1288] Zimmerman, D., "The Finger User Information Protocol", RFC 1288, December 1991. [RFC1734] Myers, J., "POP3 Authentication Command", RFC 1734, December 1994. [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. [RFC1321] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, MIT Laboratory for Computer Science and RSA Data Security, Inc., April 1992. [W3C-XML] Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler, "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-xml, October 2000, . Hathcock Expires September 26, 2007 [Page 9] Internet-Draft draft-hathcock-minger-01.txt March 2007 Appendix A. Acknowledgements We wish to thank the members of the MDaemon Beta Community (md-beta-subscribe@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 26, 2007 [Page 10] Internet-Draft draft-hathcock-minger-01.txt March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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