Network Working Group A. Hathcock Internet-Draft J. Merkel Intended Status: Informational Alt-N Technologies Expires: February 3, 2010 August 3, 2009 The Minger Email Address Verification Protocol draft-hathcock-minger-06.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 February 3, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document describes the Minger protocol. Minger is a protocol which allows a mail handling entity to query a remote service and ask the question "do you accept mail for this email address?" It includes security in the form of a hashed shared secret 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 Minger solution . . . . . . . . . . . . . . . . . . 4 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 . . . . . . . . . . . . 5 3.2 Example responses . . . . . . . . . . . . . . . . . . . . 6 4. Anonymous mode . . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7 7. Informative References . . . . . . . . . . . . . . . . . . . 7 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . . 9 Hathcock Expires February 3, 2010 [Page 2] Internet-Draft draft-hathcock-minger-06.txt August 2008 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 and process 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 often decide 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-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 when coupled with the user level detail it can provide. Also, Finger requires the use of TCP rather than UDP which seems ill suited to a simple verification scheme. SMTP "call-forward" is a term 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 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 Hathcock Expires February 3, 2010 [Page 3] Internet-Draft draft-hathcock-minger-06.txt August 2008 sessions which appear in logs to have simply failed for no reason. This leads to situations in which SMTP transaction logs can no longer distinguish legitimate network errors from "call-forward" traffic. SMTP includes a VRFY command which can be used to determine whether an email address exists. VRFY is not in wide-spread use and suffers from the same inefficiency concerns described in the discussion on SMTP "call-forward". Additionally, SMTP agents providing mail services to a domain are often not authoritative making VRFY requests potentially unreliable. LDAP could be used to determine whether an email address exists. However, LDAP is overly complex to configure and maintain. 1.3 The Minger solution What's needed is a protocol which is secure, has little overhead, and can be easily invoked to determine whether a given email address is valid or not. Minger achieves these goals using a shared secret for security and UDP to lower overhead. 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 as described below and transmits it over UDP to a Minger server. The format of the query is as follows: ABNF: query-string = mailbox [SP %x64 "=" digest] [SP tag-list] digest = base64 ; digest for security ; base64 defined in [RFC5034] digest-text = shared-secret ":" mailbox ; input text for digest mailbox = Local-part "@" Domain ; as defined in [RFC2821] shared-secret = 1*50(VCHAR) ; password credential Hathcock Expires February 3, 2010 [Page 4] Internet-Draft draft-hathcock-minger-06.txt August 2008 tag-list = tag-spec *(SP tag-spec) ; tag/value list tag-spec = tag-name "=" tag-value tag-name = 1 * ( ALPHA / DIGIT / "_") ; except 'd' tag-value = 1 * (ALPHA / DIGIT / "_") 2.2 Description of query elements mailbox This is the email address for which verification of existence is desired. 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. tag-list Tag-list is provided so that future capability might be added in an easy way. Tag-names are case-sensitive and MUST NOT be used more than once. 3. Minger responses Minger servers return a response string of the following form: ABNF: response-string = mailbox status mailbox = Local-part "@" Domain ; as defined in [RFC2821] status = %x30-35 ; single digit result code ; from 0 - 5 3.1 Description of response elements mailbox This is the email address for which verification of existence is desired. Hathcock Expires February 3, 2010 [Page 5] Internet-Draft draft-hathcock-minger-06.txt August 2008 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) 3.2 Example responses Minger response returned when the queried email address does not exist: arvel@example.com 3 Minger response returned for invalid credentials: arvel@example.com 2 Minger response returned when the queried email address exists: arvel@example.com 5 4. Anonymous mode Minger clients MAY attempt anonymous queries; that is, queries which do not contain a shared secret digest within the query string. Minger servers MAY be configured to refuse anonymous queries. If so, they MUST respond with a status of "2". Hathcock Expires February 3, 2010 [Page 6] Internet-Draft draft-hathcock-minger-06.txt August 2008 5. Security Considerations Minger is a protocol which is used to determine whether a given email address is valid or not. If a particular email infrastructure does not wish to advertise the email addresses that it services then this protocol should not be employed. If a shared secret is employed to secure Minger from anonymous use that shared secret should be at least 128 bits. 6. IANA Considerations IANA has assigned tcp & upd port 4069 for Minger. 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. Hathcock Expires February 3, 2010 [Page 7] Internet-Draft draft-hathcock-minger-06.txt August 2008 Appendix A. Acknowledgements We wish to thank the members of the MDaemon Beta Community for their ideas and help and Paul Hoffman for his valuable feedback. 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 February 3, 2010 [Page 8] Internet-Draft draft-hathcock-minger-06.txt August 2008 Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Hathcock Expires February 3, 2010 [Page 9]