MARID J. Levine
Internet-Draft Taughannock Networks
Intended status: Informational D. Crocker
Expires: July 13, 2007 Brandenburg InternetWorking
S. Silberman
Openwave
T. Finch
University of Cambridge
January 9, 2007
Bounce Address Tag Validation (BATV)
draft-levine-smtp-batv-00
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Copyright Notice
Copyright (C) The Internet Society (2007).
Abstract
The envelope of Internet mail contains an RFC2821.MailFrom command,
which may supply an address to be used as the recipient of
transmission and delivery notices about the original message.
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Existing Internet mail permits unauthorized use of addresses in the
MailFrom command, causing notices to be sent to unwitting and
unwilling recipients. Bounce Address Tag Validation (BATV) defines
an extensible mechanism for validating the MailFrom address. It also
defines an initial use of that mechanism which requires no
administrative overhead and no global implementation.
This document is a revision of draft-levine-mass-batv-02.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Meta-Syntax . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Tagging Schemes . . . . . . . . . . . . . . . . . . . . . 4
2.3. Beyond BATV . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Operation . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Local-Part Meta-Syntax . . . . . . . . . . . . . . . . . . . . 7
4. Simple Private Signature (prvs) . . . . . . . . . . . . . . . 8
4.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Operation . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Interoperability . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. References - Normative . . . . . . . . . . . . . . . . . . 12
7.2. References - Informative . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Appendix B. IANA Considerations . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
The envelope for Internet Mail may contain an address that is
designated to receive transmission-related notifications. It is
specified in the RFC2821.MailFrom command. The field is set by the
RFC2822.Sender, acting as an agent of the message author specified in
RFC2822.From. However no portion of the MailFrom address is required
to have any similarity to any portion of the From or Sender
addresses, and valid usage scenarios do call for the MailFrom address
to have no visible relationship to the From or Sender values.
Further, existing Internet mail permits unauthorized use of addresses
in the MailFrom command, which results in having notices sent to
unwitting and unwilling recipients. Therefore, the challenge is to
distinguish legitimate uses from these unauthorized uses and to do
this with a mechanism that incurs modest administration, operations
and performance costs.
Bounce Address Tag Validation (BATV) defines a framework for
mechanisms that validate the value in this command. Multiple
validation methods are envisioned. So BATV defines a common
syntactic framework that enhances the local-part field of the
MailFrom address. An initial, specific validation scheme is also
defined; it requires no administrative overhead and no global
implementation.
The <local-part> of an Internet mail address is a globally opaque
string. Hence, the specified modification to the local-part can be
deployed in a manner that is entirely transparent to the public
Internet mail service, except for mail system components within the
scope of the MailFrom domain, and then only for components that
process the MailFrom address local-part. The result permits the
MailFrom target domain to distinguish notification message addresses
that are valid from those that are not. Enhancements would permit
processing agents that are along the original message's transfer path
to determine whether the MailFrom adress is likely to be valid. This
assessment could aid in deciding whether to send a bounce message,
thereby reducing the Internet mail infrastructure cost for
transmitting notification messages in response to addresses used
without permission. It might even be used to detect invalid
messages, thereby reducing Internet mail infrastructure cost for
original messages.
Terminology: Terminology conforms to [I-D.email-arch]
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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]
2. Model
BATV defines a method for tagging information to be included in the
<local-part> of the RFC2821.MailFrom address. This permits encoding
information that authenticates the MailFrom. Because the information
is placed in MailFrom, rather than in an RFC2822 header, it sometimes
is not as publicly visible as an RFC2822 header. Tagging the
MailFrom address rather than any of the RFC2822 addresses avoids
problems arising from rewriting message headers that may be visible
to recipients, and enables the validation process to operate within
an SMTP session, before the contents of a message are transferred.
2.1. Meta-Syntax
BATV tagging is based on a meta-syntax that defines a field-oriented
structure for an address local-part. It permits use of a variety of
address authentication methods, while supporting remote extraction of
the core portion of the local-part, without having to understand the
semantics of any particular scheme.
NOTE: BATV is for the purpose of detecting invalid RFC2821.MailFrom
addresses. Any BATV-related modifications that are made to the
original MailFrom MUST preserve the result of returning valid
bounces to the address originally specified in that MailFrom.
The meta-syntax for MailFrom local-part is defined in Section 3.
2.2. Tagging Schemes
BATV permits alternative schemes. To ensure interoperability among
independent participants, other specifications adopting the meta-
syntax conventions MUST define and register with IANA a unique, case
insensitive <tag-type> element, to identify the specific mechanism
that is being used for MailFrom validation.
Private Tagging: If MailFrom validity assessment is performed only
within the scope of the domain referenced in the MailFrom address,
then its semantic scope is private (closed), encompassing only
that domain and the one that generated the validity information.
To the rest of the Internet, the tag information is opaque, like a
cookie. In these situations, the closed system is free to use any
tagging scheme it deems helpful, although a standard format aids
other systems that wish to avoid re-tagging addresses that are
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already tagged, or to strip off the tag for compatibility with
legacy systems that key on the MailFrom address of incoming mail.
A simple scheme for this is defined in Section 4.
Public Tagging: Using a public-key approach, for signing the
MailFrom's local-part permits intermediaries which process the
envelope to validate that address. For example, an intermediary
that otherwise might create a bounce message would be able to
decide that the MailFrom address use is not valid, so they might
decide to terminate bounce processing. Such a scheme might use
the BATV meta-syntax in the following way:
pub3=<crypted>=<loc-core>@example.com
If the creator of a bounce could make this assessment, all of
earlier intermedite MTAs also could. Hence, every MTA would be
able to assess whether a message has an unauthorized
RFC2821.MailFrom.
Unfortunately, none of the multiple existing public key services
has yet gained wide adoption. Therefore, this specification is
not able to provide a single method for public MailFrom validity
checking.
2.3. Beyond BATV
BATV defines a framework that retains the original local-part of the
MailFrom address, within the BATV-encoded form. This permits
external inspection of the original local-part, such as for analyzing
its use with respect to particular RFC2822.From addresses.
Enhancements that go beyond the open information of BATV might
replace the original local-part with some form of translation.
Examples of such schemes could include:
Alias: The original RFC2821.MailFrom local-part could be replaced
with an alternative local-part. The meta-syntax provides a way of
flagging the difference between the new local-part and the
original.
Opaque Pointer: This could be used to consult a database with
records of mail sent and bounces received.
Challenge Response: The receiver could make a DNS-query for
instructions about processing the RFC2821.MailFrom bounce address.
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2.4. Operation
The basic methods for creating and interpreting BATV-encoded MailFrom
addresses are very simple.
2.4.1. Tag Creation
The RFC2821.MailFrom address is specified by the RFC2822.Sender.
This makes the MailFrom address an end-user string, created by the
oMUA or MSA. However it is entirely reasonable to have an outbound
MTA, under administrative control of the Sender's domain, perform the
necessary signing. What is significant is that this requires a
change to only two modules, one in the outbound sequence and one in
the corresponding inbound sequence. The change is transparent to all
other systems components that transmit the message.
NOTE: If a MailFrom local-part already conforms to the meta-syntax
then the string SHOULD be left unchanged, so as not to break
forwarding.
NOTE: An MTA MUST ONLY tag addresses in domains whose inbound MTAs
can validate the tags. In particular, when an MTA is relaying a
message, on behalf of another Administrative Management Domain
(ADMD), it must not tag the MailFrom address, even if the original
ADMD did not add a tag. In all cases, the MTA must only tag
addresses for which it has access to the signing key that
corresponds to the validation key used by the inbound MTA for the
address' domain.
2.4.2. Tag Interpretation:
Addresses that contain BATV tags can be interpreted for two different
purposes: bounce address validation and bounce delivery.
Address validation: An MTA MAY validate a BATV-encoded MailFrom
address. This requires the MTA to be able to process the specific
BATV validation scheme that is specified by the <tag-type> field.
If the address is determined to be invalid, the MTA SHOULD process
the address as having a permanent failure, for example by
returning a 550 response to the SMTP command containing the
address.
The MTA MAY also require that the use of the address is
appropriate, for example that the message is a bounce as indicated
by a null RFC2821.MailFrom; other heuristically determined
contexts MAY also be appropriate. For example, messages with
MailFroms beginning with "mailer-daemon@" are in practice almost
always bounces. Use of a BATV address in inappropriate contexts
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SHOULD cause a permanent failure as above.
Bounce delivery: When an MTA within the specified address delivery
domain's administration receives a delivery notification directed
to a BATV-encoded address, the MTA SHOULD validate that address
when that message has a null MailFrom. A receiving server MAY
also perform heuristic selection of other incoming mail, such as
ones that have a MailFrom starting with "mailer-daemon@". If it
determines that the use is not valid, it SHOULD reject the message
during the mail transfer connection, such as with SMTP.
If the BATV address passes these checks the message SHOULD then be
delivered to the original RFC2821.MailFrom address. This original
MailFrom address will be recovered as a side-effect of validating
the BATV address.
3. Local-Part Meta-Syntax
A meta-syntax for the <local-part> of an address creates a public
convention for partitioning an address' local-part field (left-hand
side) into sub-fields of attributes associated with the <addr-spec>
that was the original local-part.
A standardized meta-syntax for local-part permits attributes to be
present in the address, without requiring that public processing of
the address have any understanding of the attributes' semantics. The
semantics of <local-part> are strictly local to the domain
administering the <local-part> field. This separation between global
semantics, versus local, has been a powerful benefit to Internet
mail. It affords considerable operational flexibility. The meta-
syntax permits public information in an address to be richer, while
maintaining the local/global separation.
The generic element syntax for the structured fields defined for a
BATV <local-part> is:
local-part = tag-type "=" tag-val "=" loc-core
tag-type = 1*( DIGIT / ALPHA / "-" )
; specific, registered validation scheme
loc-core = {original local-part value}
tag-val = 1*( DIGIT / ALPHA / "-" )
; the validation data
This syntax is chosen so that software that needs, for legacy
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compatibility reasons, to recover the original bounce address can do
so by checking for the presence of the tag-type, and if it is
present, discarding the local-part up through the second equal sign.
4. Simple Private Signature (prvs)
This scheme signs the original MailFrom by using a simple shared-key
to add a hash of the address and some time-based randomizing
information.
4.1. Syntax
This scheme is identified as:
tag-type = "prvs"
; simple private signature
tag-val = K DDD SSSSSS
K = 1DIGIT
; key number, to allow key rotation
DDD = 3DIGIT
; day number, low three digits of
; the number of days since 1970
; when the address will expire
SSSSSS = 6HEXDIG
; hex of the first three bytes of the
; SHA-1 HMAC of <hash-source> and a key
hash-source = K DDD <orig-mailfrom>
orig-mailfrom = {original RFC2821.MailFrom address}
4.2. Operation
4.2.1. Signature Creation
PRVS creates a package around an existing <local-part>, comprising
the PRVS label and the signature hash on the left. The hash is
extremely simple and not very robust, because the requirements for
BATV do not entail strong protection. The mechanism provides very
weak protection against replay, in order to keep the effort to create
or validate the signature small.
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4.2.2. Signature Checking
Checking of private signatures is only performed within the domain
specified in the MailFrom header. The first component that processes
the MailFrom's local-part must be able to interpret the meta-syntax.
It MAY also perform validation.
The scheme described here permits algorithmic validation. It does
not require maintaining a database of information about recently sent
messages.
The DDD part of the <tag-val> allows a domain to limit the lifetime
of PRVS addresses to give very basic protection against replay
attacks. If the expiry time has passed the address SHOULD be
considered invalid even if the HMAC is OK. The address lifetime
SHOULD be 7 days, to allow for long delivery delays before a bounce
occurs. Since it is valid and often useful for a single message to
provoke multiple bounces, it is specifically not a goal of BATV to
prevent them.
5. Interoperability
BATV seeks to retrofit a standardized syntactic structure onto the
<local-part> of an RFC2821.MailFrom email address. Although it is
based on an existing, standard structure, it will be used in new
environments. Because this field has previously been opaque to these
environments, it is likely to create some usage problems with some
existing services. Problems are most likely in some services that
operate in the scope of the delivery stage of processing, rather than
in intermediaries between independent user services. In particular
serious problems are likely to be with third-party services that
constrain local-part beyond the Internet standards. Hence they
restrict interoperability, even without concern for BATV.
As an example, such systems incorrectly identify the sender of the
message by using the MailFrom address, rather than the RFC2822.Sender
address. Examples are listed below. Further, they require that this
address be the same for all future postings from the RFC2822.From
address. Problems arise because messages authored by a particular
RFC2822.From address are like to vary the associated MailFrom address
over time, particularly when BATV encoding is used.
Such systems SHOULD fix the underlying problem, at a minimum by using
the RFC2822.Sender address to identify the sender. However, note
that a Internet mail does not require that the value of the Sender
address be constant for a From address, and there are many,
legitimate reasons that it varies.
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Some systems MAY continue to require correlation between MailFrom and
From. For example the system might operate on the envelope before
the message data has been transmitted, so software might strip off
the meta-syntax to recover the <loc-core> which can then be used as
the MailFrom address's original <local-part>. For such validation
processing this altered address MUST NOT be used for further mail-
delivery processing. Rather the MailFrom string MUST be preserved as
it was received.
The benefit of a standardized meta-syntax for adding validation
attributes is that it permits such mechanisms to detect the
"attribute" portions of the local-part and extract only the core
portion, without having to understand any of the details of the
attributes.
The known and likely set of problem third-parties are:
Greylisters: A correct BATV implementation will only result in
routine delays in this case. However the result of BATV tagging
MUST be a constant local-part, for a given message, and not (say)
created at delivery time such that each retry gets a different
validation string, which would prevent it from ever getting
through to a greylisting site.
Mailing Lists: BATV will cause problems with some mailing lists
that identify posters by their bounce address. The list will not
recognize the identical MailFrom addresses, because it will
interpret the differing BATV attributes as part of the address.
These services will either reject postings or pass them all to the
moderator.
Challenge-Response Systems: The problem with these is similar to
the that with mailing lists, but the challenged user will have to
take special action for every message recipient that auto-sorts
mail by bounce address.
Sorting and Duplicate Detection: Any system that sorts by bounce
address (MailFrom) will interpret the addresses as different, even
though they are not. This may include whitelisting services.
BATV requires that the sending and receiving mail software for a
domain share the secret key used to create the signature. Usually
this is easy to arrange, by creating the signature in a domain's
outgoing mail relay and checking it in the inbound MX if both are run
by the same management. But it is not necessary for a domain's
inbound and outbound relays to be under the same management; for
example it is fairly common for incoming mail for a small business
domain to be received by an MTA run by a hosting company, while the
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outbound mail is sent through the ISP that provides the connection to
the company's office. In this case, it may be necessary to sign the
outgoing mail in the individual senders' MUAs, to check the signature
in the individual recipients' MUAs, or both.
6. Security Considerations
This entire document pertains to the security of email's asynchronous
error handling (bounce notification) mechanism, by describing a way
to detect valid and invalid bounce addresses. This document does not
directly provide a mechanism for authenticating RFC2821.MailFrom
addresses at intermediate. The ability to perform validation across
the entire transfer sequence is possible if a standardized public key
scheme is defined.
The PRVS scheme described here provides minimal protection of the
RFC2821.Mailfrom against forgery, with detection done at the target
(delivery) domain. The scheme does not attempt to protect against a
replay attack in which a valid, signed MailFrom is used but the
message contents are replaced. The same will be true for any other
BATV scheme that does not include some link with the message data;
however such protection is only reliable for the recipient of the
original message, because the integrity of the link will often be
broken when the original message data is mangled into the bounce.
There are two common forms of email address forgery: guessing (e.g.
attaching common <local-part>s to a domain) and harvesting (e.g. from
the web or usenet). Cryptographic BATV schemes make guessing attacks
unfeasibly difficult; however these are relatively minor compared to
replay attacks, which deserve closer attention.
MailFrom addresses are not usually exposed in the places from which
addresses are usually harvested. Many mailing list systems archive
messages sent to a list on the web; however they usually replace the
original MailFrom address with one that refers to the mailing list
manager. So this case is generally not a problem, although there are
exceptions. There are other instances of systems that archive email
publicly without altering the MailFrom address, such as bug tracking
systems; these are a problem.
A proportion of forgeries are caused by mass mailing viruses. Unlike
spammers, these have access to private email stores and are therefore
more likely to be able to find and replay BATV addresses. For that
matter, they can generate MailFrom addresses that are entirely valid.
The PRVS scheme includes a modest protection against replay attacks,
by virtue of its using an expiry time, which prevents very old
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addresses from being used by attackers. It does not prevent replay
attacks of young addresses.
7. References
7.1. References - Normative
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. References - Informative
[I-D.email-arch]
Crocker, D., "Internet Mail Architecture", May 2004.
Appendix A. Acknowledgements
This specification was greatly improved by the extensive
participation of John Leslie and Douglas Otis, in early design
discussions.
Appendix B. IANA Considerations
It may be desirable to establish a registry of BATV tagging schemes
and tag types.
Authors' Addresses
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Email: standards@taugh.com
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Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.com
Sam Silberman
Openwave
Email: sam_silberman@openwave.com
Tony Finch
University of Cambridge
Cambridge CB2 1TN
UK
Email: dot@dotat.at
URI: http://dotat.at
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