Network Working Group C. Newman

Network Working Group C. Newman
Request for Comments: 4468 Sun Microsystems
Updates: 3463 May 2006
Category: Standards Track

Message Submission BURL Extension

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

The submission profile of Simple Mail Transfer Protocol (SMTP)
provides a standard way for an email client to submit a complete
message for delivery. This specification extends the submission
profile by adding a new BURL command that can be used to fetch
submission data from an Internet Message Access Protocol (IMAP)
server. This permits a mail client to inject content from an IMAP
server into the SMTP infrastructure without downloading it to the
client and uploading it back to the server.

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RFC 4468 Message Submission BURL Extension May 2006

Table of Contents

1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. BURL Submission Extension .......................................3
3.1. SMTP Submission Extension Registration .....................3
3.2. BURL Transaction ...........................................3
3.3. The BURL IMAP Options ......................................4
3.4. Examples ...................................................5
3.5. Formal Syntax ..............................................6
4. 8-Bit and Binary ................................................7
5. Updates to RFC 3463 .............................................7
6. Response Codes ..................................................7
7. IANA Considerations .............................................9
8. Security Considerations .........................................9
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................12
Appendix A. Acknowledgements .....................................13

1. Introduction

This specification defines an extension to the standard Message
Submission [RFC4409] protocol to permit data to be fetched from an
IMAP server at message submission time. This MAY be used in
conjunction with the CHUNKING [RFC3030] mechanism so that chunks of
the message can come from an external IMAP server. This provides the
ability to forward an email message without first downloading it to
the client.

2. Conventions Used in This Document

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in "Key words for
use in RFCs to Indicate Requirement Levels" [RFC2119].

The formal syntax uses the Augmented Backus-Naur Form (ABNF)
[RFC4234] notation including the core rules defined in Appendix B of
RFC 4234.

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RFC 4468 Message Submission BURL Extension May 2006

3. BURL Submission Extension

This section defines the BURL submission extension.

3.1. SMTP Submission Extension Registration

1. The name of this submission extension is "BURL". This extends
the Message Submission protocol on port 587 and MUST NOT be
advertised by a regular SMTP [RFC2821] server on port 25 that
acts as a relay for incoming mail from other SMTP relays.

2. The EHLO keyword value associated with the extension is "BURL".

3. The BURL EHLO keyword will have zero or more arguments. The only
argument defined at this time is the "imap" argument, which MUST
be present in order to use IMAP URLs with BURL. Clients MUST
ignore other arguments after the BURL EHLO keyword unless they
are defined by a subsequent IETF standards track specification.
The arguments that appear after the BURL EHLO keyword may change
subsequent to the use of SMTP AUTH [RFC2554], so a server that
advertises BURL with no arguments prior to authentication
indicates that BURL is supported but authentication is required
to use it.

4. This extension adds the BURL SMTP verb. This verb is used as a
replacement for the DATA command and is only permitted during a
mail transaction after at least one successful RCPT TO.

3.2. BURL Transaction

A simple BURL transaction will consist of MAIL FROM, one or more RCPT
TO headers, and a BURL command with the "LAST" tag. The BURL command
will include an IMAP URL pointing to a fully formed message ready for
injection into the SMTP infrastructure. If PIPELINING [RFC2920] is
advertised, the client MAY send the entire transaction in one round
trip. If no valid RCPT TO address is supplied, the BURL command will
simply fail, and no resolution of the BURL URL argument will be
performed. If at least one valid RCPT TO address is supplied, then
the BURL URL argument will be resolved before the server responds to
the command.

A more sophisticated BURL transaction MAY occur when the server also
advertises CHUNKING [RFC3030]. In this case, the BURL and BDAT
commands may be interleaved until one of them terminates the
transaction with the "LAST" argument. If PIPELINING [RFC2920] is
also advertised, then the client may pipeline the entire transaction
in one round-trip. However, it MUST wait for the results of the
"LAST" BDAT or BURL command prior to initiating a new transaction.

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The BURL command directs the server to fetch the data object to which
the URL refers and include it in the message. If the URL fetch
fails, the server will fail the entire transaction.

3.3. The BURL IMAP Options

When "imap" is present in the space-separated list of arguments
following the BURL EHLO keyword, it indicates that the BURL command
supports the URLAUTH [RFC4467] extended form of IMAP URLs [RFC2192]
and that the submit server is configured with the necessary
credentials to resolve "urlauth=submit+" IMAP URLs for the submit
server's domain.

Subsequent to a successful SMTP AUTH command, the submission server
MAY indicate a prearranged trust relationship with a specific IMAP
server by including a BURL EHLO keyword argument of the form
"imap://imap.example.com". In this case, the submission server will
permit a regular IMAP URL referring to messages or parts of messages
on imap.example.com that the user who authenticated to the submit
server can access. Note that this form does not imply that the
submit server supports URLAUTH URLs; the submit server must advertise
both "imap" and "imap://imap.example.com" to indicate support for
both extended and non-extended URL forms.

When the submit server connects to the IMAP server, it acts as an
IMAP client and thus is subject to both the mandatory-to-implement
IMAP capabilities in Section 6.1.1 of RFC 3501, and the security
considerations in Section 11 of RFC 3501. Specifically, this
requires that the submit server implement a configuration that uses
STARTTLS followed by SASL PLAIN [SASL-PLAIN] to authenticate to the
IMAP server.

When the submit server resolves a URLAUTH IMAP URL, it uses submit
server credentials when authenticating to the IMAP server. The
authentication identity and password used for submit credentials MUST
be configurable. The string "submit" is suggested as a default value
for the authentication identity, with no default for the password.
Typically, the authorization identity is empty in this case; thus the
IMAP server will derive the authorization identity from the
authentication identity. If the IMAP URL uses the "submit+" access
identifier prefix, the submit server MUST refuse the BURL command
unless the userid in the URL's <access> token matches the submit
client's authorization identity.

When the submit server resolves a regular IMAP URL, it uses the
submit client's authorization identity when authenticating to the
IMAP server. If both the submit client and the submit server's
embedded IMAP client use SASL PLAIN (or the equivalent), the submit

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server SHOULD forward the client's credentials if and only if the
submit server knows that the IMAP server is in the same
administrative domain. If the submit server supports SASL mechanisms
other than PLAIN, it MUST implement a configuration in which the
submit server's embedded IMAP client uses STARTTLS and SASL PLAIN
with the submit server's authentication identity and password (for
the respective IMAP server) and the submit client's authorization
identity.

3.4. Examples

In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.

Two successful submissions (without and with pipelining) follow:

<SSL/TLS encryption layer negotiated>
C: EHLO potter.example.com
S: 250-owlry.example.com
S: 250-8BITMIME
S: 250-BURL imap
S: 250-AUTH PLAIN
S: 250-DSN
S: 250 ENHANCEDSTATUSCODES
C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=
S: 235 2.7.0 PLAIN authentication successful.
C: MAIL FROM:<[email protected]>
S: 250 2.5.0 Address Ok.
C: RCPT TO:<[email protected]>
S: 250 2.1.5 [email protected] OK.
C: BURL imap://[email protected]/outbox
;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
:internal:91354a473744909de610943775f92038 LAST
S: 250 2.5.0 Ok.

<SSL/TLS encryption layer negotiated>
C: EHLO potter.example.com
S: 250-owlry.example.com
S: 250-8BITMIME
S: 250-PIPELINING
S: 250-BURL imap
S: 250-AUTH PLAIN
S: 250-DSN
S: 250 ENHANCEDSTATUSCODES
C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=

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RFC 4468 Message Submission BURL Extension May 2006

C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
C: BURL imap://[email protected]/outbox
;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
:internal:91354a473744909de610943775f92038 LAST
S: 235 2.7.0 PLAIN authentication successful.
S: 250 2.5.0 Address Ok.
S: 250 2.1.5 [email protected] OK.
S: 250 2.5.0 Ok.

Note that PIPELINING of the AUTH command is only permitted if the
selected mechanism can be completed in one round trip, a client
initial response is provided, and no SASL security layer is
negotiated. This is possible for PLAIN and EXTERNAL, but not for
most other SASL mechanisms.

Some examples of failure cases:

C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
C: BURL imap://[email protected]/outbox
;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
:internal:91354a473744909de610943775f92038 LAST
S: 250 2.5.0 Address Ok.
S: 550 5.7.1 Relaying not allowed: [email protected]
S: 554 5.5.0 No recipients have been specified.

C: MAIL FROM:<[email protected]>
C: RCPT TO:<[email protected]>
C: BURL imap://[email protected]/outbox
;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
:internal:71354a473744909de610943775f92038 LAST
S: 250 2.5.0 Address Ok.
S: 250 2.1.5 [email protected] OK.
S: 554 5.7.0 IMAP URL authorization failed

3.5. Formal Syntax

The following syntax specification inherits ABNF [RFC4234] and
Uniform Resource Identifiers [RFC3986].

burl-param = "imap" / ("imap://" authority)
; parameter to BURL EHLO keyword

burl-cmd = "BURL" SP absolute-URI [SP end-marker] CRLF

end-marker = "LAST"

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4. 8-Bit and Binary

A submit server that advertises BURL MUST also advertise 8BITMIME
[RFC1652] and perform the down conversion described in that
specification on the resulting complete message if 8-bit data is
received with the BURL command and passed to a 7-bit server. If the
URL argument to BURL refers to binary data, then the submit server
MAY refuse the command or down convert as described in Binary SMTP
[RFC3030].

The Submit server MAY refuse to accept a BURL command or combination
of BURL and BDAT commands that result in un-encoded 8-bit data in
mail or MIME [RFC2045] headers. Alternatively, the server MAY accept
such data and down convert to MIME header encoding [RFC2047].

5. Updates to RFC 3463

SMTP or Submit servers that advertise ENHANCEDSTATUSCODES [RFC2034]
use enhanced status codes defined in RFC 3463 [RFC3463]. The BURL
extension introduces new error cases that that RFC did not consider.
The following additional enhanced status codes are defined by this
specification:

X.6.6 Message content not available

The message content could not be fetched from a remote system.
This may be useful as a permanent or persistent temporary
notification.

X.7.8 Trust relationship required

The submission server requires a configured trust relationship
with a third-party server in order to access the message content.

6. Response Codes

This section includes example response codes to the BURL command.
Other text may be used with the same response codes. This list is
not exhaustive, and BURL clients MUST tolerate any valid SMTP
response code. Most of these examples include the appropriate
enhanced status code [RFC3463].

554 5.5.0 No recipients have been specified

This response code occurs when BURL is used (for example, with
PIPELINING) and all RCPT TOs failed.

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503 5.5.0 Valid RCPT TO required before BURL

This response code is an alternative to the previous one when BURL
is used (for example, with PIPELINING) and all RCPT TOs failed.

554 5.6.3 Conversion required but not supported

This response code occurs when the URL points to binary data and
the implementation does not support down conversion to base64.
This can also be used if the URL points to message data with 8-bit
content in headers and the server does not down convert such
content.

554 5.3.4 Message too big for system

The message (subsequent to URL resolution) is larger than the
per-message size limit for this server.

554 5.7.8 URL resolution requires trust relationship

The submit server does not have a trust relationship with the IMAP
server specified in the URL argument to BURL.

552 5.2.2 Mailbox full

The recipient is local, the submit server supports direct
delivery, and the recipient has exceeded his quota and any grace
period for delivery attempts.

554 5.6.6 IMAP URL resolution failed

The IMAP URLFETCH command returned an error or no data.

250 2.5.0 Waiting for additional BURL or BDAT commands

A BURL command without the "LAST" modifier was sent. The URL for
this BURL command was successfully resolved, but the content will
not necessarily be committed to persistent storage until the rest
of the message content is collected. For example, a Unix server
may have written the content to a queue file buffer, but may not
yet have performed an fsync() operation. If the server loses
power, the content can still be lost.

451 4.4.1 IMAP server unavailable

The connection to the IMAP server to resolve the URL failed.

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250 2.5.0 Ok.

The URL was successfully resolved, and the complete message data
has been committed to persistent storage.

250 2.6.4 MIME header conversion with loss performed

The URL pointed to message data that included mail or MIME headers
with 8-bit data. This data was converted to MIME header encoding
[RFC2047], but the submit server may not have correctly guessed
the unlabeled character set.

7. IANA Considerations

The "BURL" SMTP extension as described in Section 3 has been
registered. This registration has been marked for use by message
submission [RFC4409] only in the registry.

8. Security Considerations

Modern SMTP submission servers often include content-based security
and denial-of-service defense mechanisms such as virus filtering,
size limits, server-generated signatures, spam filtering, etc.
Implementations of BURL should fetch the URL content prior to
application of such content-based mechanisms in order to preserve
their function.

Clients that generate unsolicited bulk email or email with viruses
could use this mechanism to compensate for a slow link between the
client and submit server. In particular, this mechanism would make
it feasible for a programmable cell phone or other device on a slow
link to become a significant source of unsolicited bulk email and/or
viruses. This makes it more important for submit server vendors
implementing BURL to have auditing and/or defenses against such
denial-of-service attacks including mandatory authentication, logging
that associates unique client identifiers with mail transactions,
limits on reuse of the same IMAP URL, rate limits, recipient count
limits, and content filters.

Transfer of the URLAUTH [RFC4467] form of IMAP URLs in the clear can
expose the authorization token to network eavesdroppers.
Implementations that support such URLs can address this issue by
using a strong confidentiality protection mechanism. For example,
the SMTP STARTTLS [RFC3207] and the IMAP STARTTLS [RFC3501]
extensions, in combination with a configuration setting that requires
their use with such IMAP URLs, would address this concern.

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Use of a prearranged trust relationship between a submit server and a
specific IMAP server introduces security considerations. A
compromise of the submit server should not automatically compromise
all accounts on the IMAP server, so trust relationships involving
super-user proxy credentials are strongly discouraged. A system that
requires the submit server to authenticate to the IMAP server with
submit credentials and subsequently requires a URLAUTH URL to fetch
any content addresses this concern. A trusted third party model for
proxy credentials (such as that provided by Kerberos 5 [RFC4120])
would also suffice.

When a client uses SMTP STARTTLS to send a BURL command that
references non-public information, there is a user expectation that
the entire message content will be treated confidentially. To
address this expectation, the message submission server SHOULD use
STARTTLS or a mechanism providing equivalent data confidentiality
when fetching the content referenced by that URL.

A legitimate user of a submit server may try to compromise other
accounts on the server by providing an IMAP URLAUTH URL that points
to a server under that user's control that is designed to undermine
the security of the submit server. For this reason, the IMAP client
code that the submit server uses must be robust with respect to
arbitrary input sizes (including large IMAP literals) and arbitrary
delays from the IMAP server. Requiring a prearranged trust
relationship between a submit server and the IMAP server also
addresses this concern.

An authorized user of the submit server could set up a fraudulent
IMAP server and pass a URL for that server to the submit server. The
submit server might then contact the fraudulent IMAP server to
authenticate with submit credentials and fetch content. There are
several ways to mitigate this potential attack. A submit server that
only uses submit credentials with a fixed set of trusted IMAP servers
will not be vulnerable to exposure of those credentials. A submit
server can treat the IMAP server as untrusted and include defenses
for buffer overflows, denial-of-service slowdowns, and other
potential attacks. Finally, because authentication is required to
use BURL, it is possible to keep a secure audit trail and use that to
detect and punish the offending party.

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9. References

9.1. Normative References

[RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
Crocker, "SMTP Service Extension for
8bit-MIMEtransport", RFC 1652, July 1994.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192,
September 1997.

[RFC2554] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.

[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.

[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP
over Transport Layer Security", RFC 3207,
February 2002.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, RFC 3986, January 2005.

[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.

[RFC4409] Gellens, R. and J. Klensin, "Message Submission for
Mail", RFC 4409, April 2006.

[RFC4467] Crispin, M., "Internet Message Access Protocol (IMAP) -
URLAUTH Extension", RFC 4467, May 2006.

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9.2. Informative References

[RFC2034] Freed, N., "SMTP Service Extension for Returning
Enhanced Error Codes", RFC 2034, October 1996.

[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.

[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, November 1996.

[RFC2920] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, September 2000.

[RFC3030] Vaudreuil, G., "SMTP Service Extensions for
Transmission of Large and Binary MIME Messages",
RFC 3030, December 2000.

[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.

[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC
4120, July 2005.

[SASL-PLAIN] Zeilenga, K., "The Plain SASL Mechanism", Work in
Progress, March 2005.

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Appendix A. Acknowledgements

This document is a product of the lemonade WG. Many thanks are due
to all the participants of that working group for their input. Mark
Crispin was instrumental in the conception of this mechanism. Thanks
to Randall Gellens, Alexey Melnikov, Sam Hartman, Ned Freed, Dave
Cridland, Peter Coates, and Mark Crispin for review comments on the
document. Thanks to the RFC Editor for correcting the author's
grammar mistakes. Thanks to Ted Hardie, Randall Gellens, Mark
Crispin, Pete Resnick, and Greg Vaudreuil for extremely interesting
debates comparing this proposal and alternatives. Thanks to the
lemonade WG chairs Eric Burger and Glenn Parsons for concluding the
debate at the correct time and making sure this document got
completed.

Author's Address

Chris Newman
Sun Microsystems
3401 Centrelake Dr., Suite 410
Ontario, CA 91761
US

EMail: [email protected]

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Full Copyright Statement

Copyright (C) The Internet Society (2006).

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.

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