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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).


 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.

Newman Standards Track [Page 1] 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.

Newman Standards Track [Page 2] 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.

Newman Standards Track [Page 3] RFC 4468 Message Submission BURL Extension May 2006

 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://".  In this case, the submission server will
 permit a regular IMAP URL referring to messages or parts of messages
 on 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://" 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

Newman Standards Track [Page 4] RFC 4468 Message Submission BURL Extension May 2006

 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

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
 Two successful submissions (without and with pipelining) follow:
 <SSL/TLS encryption layer negotiated>
 S: 250-8BITMIME
 S: 250-BURL imap
 S: 250-DSN
 S: 235 2.7.0 PLAIN authentication successful.
 S: 250 2.5.0 Address Ok.
 C: RCPT TO:<>
 S: 250 2.1.5 OK.
 C: BURL imap://
         :internal:91354a473744909de610943775f92038 LAST
 S: 250 2.5.0 Ok.
 <SSL/TLS encryption layer negotiated>
 S: 250-8BITMIME
 S: 250-BURL imap
 S: 250-DSN

Newman Standards Track [Page 5] RFC 4468 Message Submission BURL Extension May 2006

 C: RCPT TO:<>
 C: BURL imap://
         :internal:91354a473744909de610943775f92038 LAST
 S: 235 2.7.0 PLAIN authentication successful.
 S: 250 2.5.0 Address Ok.
 S: 250 2.1.5 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: RCPT TO:<>
 C: BURL imap://
         :internal:91354a473744909de610943775f92038 LAST
 S: 250 2.5.0 Address Ok.
 S: 550 5.7.1 Relaying not allowed:
 S: 554 5.5.0 No recipients have been specified.
 C: RCPT TO:<>
 C: BURL imap://
         :internal:71354a473744909de610943775f92038 LAST
 S: 250 2.5.0 Address Ok.
 S: 250 2.1.5 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"

Newman Standards Track [Page 6] RFC 4468 Message Submission BURL Extension May 2006

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
 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
 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
 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.

Newman Standards Track [Page 7] RFC 4468 Message Submission BURL Extension May 2006

 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
 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.

Newman Standards Track [Page 8] RFC 4468 Message Submission BURL Extension May 2006

 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.

Newman Standards Track [Page 9] RFC 4468 Message Submission BURL Extension May 2006

 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.

Newman Standards Track [Page 10] RFC 4468 Message Submission BURL Extension May 2006

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.
               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.

Newman Standards Track [Page 11] RFC 4468 Message Submission BURL Extension May 2006

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.

Newman Standards Track [Page 12] RFC 4468 Message Submission BURL Extension May 2006

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

Author's Address

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

Newman Standards Track [Page 13] RFC 4468 Message Submission BURL Extension May 2006

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.
 This document and the information contained herein are provided on an

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Newman Standards Track [Page 14]

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