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
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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].
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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
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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.
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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.
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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 = "<MingerResponse>" minger-data "</MingerResponse>"
Minger-data = "<ID>" id "</ID>" (status user-data / error)
id = 1 * 50(VCHAR) ; id of the query being responded to
status = "<Status>" ("Active" / "Not found" / "Disabled" /
1*(ALPHA / DIGIT)) "</Status>"
error = "<Error>" 1 * (ALPHA / DIGIT) "</Error>"
user-data = *("<" tag-name ">" 1 * (VCHAR) "</" tag-name ">")
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):
<MingerResponse>
<ID>12345</ID>
<Status>not found</Status>
</MingerResponse>
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B. Minger response for error conditions (in this case, invalid
credentials):
<MingerResponse>
<ID>54321</ID>
<Error>bad username or password</Error>
</MingerResponse>
C. Minger response for "active" email addresses (returned when the
queried email address exists and is ready to receive mail):
<MingerResponse>
<ID>abc123def</ID>
<Status>Active</Status>
</MingerResponse>
D. Minger response returning optional extra data:
<MingerResponse>
<ID>gfs54ad4fs</ID>
<Status>Active</Status>
<FullName>Arvel Hathcock</FullName>
<Email>arvel@altn.com</Email>
</MingerResponse>
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.
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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.
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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.
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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
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