Internet DRAFT - draft-hathcock-minger
draft-hathcock-minger
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
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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
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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 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
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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
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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.
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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".
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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.
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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
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Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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