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:
<!DOCTYPE MingerResponse [
<!ELEMENT MingerResponse (name?, email?)>
<!ATTLIST MingerResponse
id CDATA #REQUIRED
status DIGIT #REQUIRED>
<!ELEMENT name CDATA>
<!ELEMENT email CDATA>
]>
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:
<MingerResponse id="12345" status="3"/>
Hathcock Expires September 26, 2007 [Page 6]
Internet-Draft draft-hathcock-minger-01.txt March 2007
Minger response returned for invalid credentials:
<MingerResponse id="543" status="2"/>
Minger response returned when the queried email address exists:
<MingerResponse id="ab12fg" status="5"/>
Minger response returning optional name and email elements:
<MingerResponse id="lkj234" status="5">
<name>Arvel Hathcock</name>
<email>arvel@altn.com</email>
</MingerResponse>
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, <http://www.w3.org/TR/REC-xml>.
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).
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.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. 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 11]