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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group A. Hathcock 2 Internet-Draft J. Merkel 3 Intended Status: Informational Alt-N Technologies 4 Expires: November 7, 2007 May 7, 2007 6 The Minger Email Address Verification Protocol 7 draft-hathcock-minger-02.txt 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on September 6, 2007. 34 Copyright Notice 36 Copyright (C) The IETF Trust (2007). 38 Abstract 40 This document describes the Minger protocol. Minger is a protocol 41 which allows a mail handling entity to query a remote service and 42 ask the question "do you accept mail for this email address?" It 43 includes security in the form of a hashed shared secret but can also 44 be used anonymously if desired. 46 Requirements Language 48 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 49 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 50 document are to be interpreted as described in [RFC2119]. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 55 1.1. The problem . . . . . . . . . . . . . . . . . . . . . . 3 56 1.2. Existing solutions . . . . . . . . . . . . . . . . . . . 3 57 1.3. The Minger solution . . . . . . . . . . . . . . . . . . 4 58 2. The Minger protocol . . . . . . . . . . . . . . . . . . . . 4 59 2.1 The Minger query process . . . . . . . . . . . . . . . . 4 60 2.2 Description of query elements . . . . . . . . . . . . . . 5 61 3. Minger responses . . . . . . . . . . . . . . . . . . . . . . 5 62 3.1 Description of response elements . . . . . . . . . . . . 5 63 3.2 Example responses . . . . . . . . . . . . . . . . . . . . 6 64 4. Anonymous mode . . . . . . . . . . . . . . . . . . . . . . . 6 65 5. Security Considerations . . . . . . . . . . . . . . . . . . 7 66 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7 67 7. Informative References . . . . . . . . . . . . . . . . . . . 7 68 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . 8 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 70 Intellectual Property and Copyright Statements . . . . . . . . . 9 72 1. Introduction 74 1.1 The problem 76 It is common for elements within a typical email handling topology 77 to be unaware of whether individual local-parts are valid for the 78 mail it accepts. For example, so-called "edge" servers which provide 79 security oriented services for downstream mail handling elements 80 often do not have an exhaustive listing of all valid local-parts for 81 a given domain. Thus, they are sometimes forced to accept and process 82 messages which might otherwise be rejected as "user unknown". 83 Similarly, entities offering "backup MX" mail services are rarely 84 privy to a complete local-part listing and are therefore forced to 85 accept messages which might otherwise be rejected. Finally, even 86 within a common administrative framework of several locally maintained 87 and controlled SMTP servers in a load balanced configuration, it is 88 not always possible for all servers to access a common local-part 89 database. 91 1.2 Existing solutions 93 The need to determine whether an email address contains a valid local 94 part has lead to the use of at least two existing mechanisms - Finger 95 [RFC1288] and SMTP "call-back" / "call-forward". 97 Finger [RFC1288] describes a protocol for the exchange of user 98 information. In theory, Finger could be used to determine whether an 99 account exists by careful examination of the results of a Finger 100 query. However, Finger suffers from a lack of security which makes 101 its modern day use problematic when coupled with the user level 102 detail it can provide. Also, Finger requires the use of TCP rather 103 than UDP which seems ill suited to a simple verification scheme. 105 SMTP "call-back" and "call-forward" are terms describing a widespread 106 practice whereby SMTP servers place an incoming SMTP session on hold 107 while they attempt to use an outbound SMTP session to determine 108 whether or not a given email address is valid. The theory behind this 109 is as follows: if an SMTP server responds positively to an SMTP RCPT 110 or MAIL command [RFC2821] with a given email address then this 111 potentially means that the address local part is valid. One problem 112 with such a scheme is the lack of efficiency inherent in the need to 113 tear-up and tear-down an SMTP session over TCP. Also, because these 114 types of SMTP sessions are not purposed to deliver mail, they 115 typically drop connection after the RCPT command is processed. This 116 leads to a large number of SMTP sessions which appear in logs to have 117 simply failed for no reason. This leads to situations in which SMTP 118 transaction logs can no longer distinguish legitimate network errors 119 from "call-back"/"call-forward" traffic. 121 SMTP includes a VRFY command which can be used to determine whether 122 an email address exits. VRFY is not in wide-spread use and suffers 123 from the same inefficiency concerns described in the discussion on 124 SMTP "call-back". Additionally, a particular SMTP agent serving a 125 domain that has several is not necessarily authoritative making a 126 VRFY request to it potentially unreliable. 128 1.3 The Minger solution 130 What's needed is a protocol which is secure, has little overhead, and 131 can be easily invoked to determine whether a given email address is 132 valid or not. Minger achieves these goals using a shared secret for 133 security and UDP to lower overhead. 135 2. The Minger protocol 137 Minger is a UDP protocol that operates on port 4069. 139 Syntax descriptions use the form described in Augmented Backus-Naur 140 Form for Syntax Specifications (ABNF) [RFC4234]. 142 2.1 The Minger query process 144 A Minger client constructs a query string as described below and 145 transmits it over UDP to a Minger server. The format of the query 146 is as follows: 148 ABNF: 150 query-string = mailbox [SP digest] 152 digest = base64 ; digest for security 153 ; base64 defined in [RFC1734] 155 digest-text = shared-secret ":" mailbox ; input text for digest 157 mailbox = Local-part "@" Domain ; as defined in [RFC2821] 159 shared-secret = 1*50(VCHAR) ; password credential 161 2.2 Description of query elements 163 mailbox 165 This is the email address for which verification of 166 existence is desired. 168 digest 170 This is the base64 encoding of the MD5 [RFC1321] hash of 171 digest-text. Digest-text is constructed, the MD5 hash of that 172 is computed, and that result is base64 encoded. 174 3. Minger responses 176 Minger servers return a response string of the following form: 178 ABNF: 180 response-string = mailbox status 182 mailbox = Local-part "@" Domain ; as defined in [RFC2821] 184 status = %x30-35 ; single digit result code 185 ; from 0 - 5 187 3.1 Description of response elements 189 mailbox 191 This is the email address for which verification of 192 existence is desired. 194 status 196 The following status codes are defined: 198 0 - invalid request (for example, malformed query string) 199 1 - access denied (for example, query from unauthorized IP) 200 2 - bad or missing credentials (returned when anonymous 201 mode is disabled and no credentials were provided in the 202 query string or when the credentials themselves are 203 invalid) 204 3 - email address does not exist 205 4 - email address exists but can not receive mail (for example, 206 the account associated with the email address has exceeded 207 local storage constraints or it is otherwise disabled due 208 to local policy) 209 5 - email address exists and is active (able to receive mail) 211 3.2 Example responses 213 Minger response returned when the queried email address does 214 not exist: 216 arvel@example.com 3 218 Minger response returned for invalid credentials: 220 arvel@example.com 2 222 Minger response returned when the queried email address exists: 224 arvel@example.com 5 226 4. Anonymous mode 228 Minger clients MAY attempt anonymous queries; that is, queries which 229 do not contain a shared secret digest within the query string. Minger 230 servers MAY be configured to refuse anonymous queries. If so, they 231 MUST respond with a status of "2". 233 5. Security Considerations 235 Minger is a protocol which is used to determine whether a given 236 email address is valid or not. If a particular email 237 infrastructure does not wish to advertise the email addresses that 238 it services then this protocol should not be employed. 240 If a shared secret is employed to secure Minger from anonymous use 241 that shared secret should be at least 128 bits. 243 6. IANA Considerations 245 IANA has assigned tcp & upd port 4069 for Minger. 247 7. Informative References 249 [RFC1288] Zimmerman, D., "The Finger User Information Protocol", 250 RFC 1288, December 1991. 252 [RFC1734] Myers, J., "POP3 Authentication Command", RFC 1734, 253 December 1994. 255 [RFC2821] Klensin, J., Editor, "Simple Mail Transfer Protocol", RFC 256 2821, March 2001. 258 [RFC4234] Crocker, D., Ed. And P. Overell, "Augmented BNF for Syntax 259 Specifications: ABNF", RFC 4234, October 2005. 261 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 262 Requirement Levels", BCP 14, RFC 2119, March 1997. 264 [RFC1321] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, 265 MIT Laboratory for Computer Science and RSA Data Security, 266 Inc., April 1992. 268 Appendix A. Acknowledgements 270 We wish to thank the members of the MDaemon Beta Community 271 (md-beta-subscribe@altn.com) for their ideas and help and Paul 272 Hoffman for his valuable feedback. 274 Authors' Addresses 276 Arvel Hathcock 277 Alt-N Technologies 278 http://www.altn.com 280 Email: arvel.hathcock@altn.com 282 Jonathan Merkel 283 Alt-N Technologies 284 http://www.altn.com 286 Email: jon.merkel@altn.com 288 Full Copyright Statement 290 Copyright (C) The IETF Trust (2007). 292 This document is subject to the rights, licenses and restrictions 293 contained in BCP 78, and except as set forth therein, the authors 294 retain all their rights. 296 This document and the information contained herein are provided on an 297 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 298 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 299 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 300 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 301 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 302 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 304 Intellectual Property 306 The IETF takes no position regarding the validity or scope of any 307 Intellectual Property Rights or other rights that might be claimed to 308 pertain to the implementation or use of the technology described in 309 this document or the extent to which any license under such rights 310 might or might not be available; nor does it represent that it has 311 made any independent effort to identify any such rights. 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