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Message Submission BURL Extension
RFC 4468

Document Type RFC - Proposed Standard (May 2006) Errata
Updated by RFC 5248
Updates RFC 3463
Author Chris Newman
Last updated 2020-01-21
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Ted Hardie
Send notices to (None)
RFC 4468
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.

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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://".  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

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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

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>
   C: EHLO
   S: 250-8BITMIME
   S: 250-BURL imap
   S: 250-AUTH PLAIN
   S: 250-DSN
   C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=
   S: 235 2.7.0 PLAIN authentication successful.
   C: MAIL FROM:<>
   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>
   C: EHLO
   S: 250-8BITMIME
   S: 250-BURL imap
   S: 250-AUTH PLAIN
   S: 250-DSN
   C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=

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   C: MAIL FROM:<>
   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: MAIL FROM:<>
   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: MAIL FROM:<>
   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"

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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

   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.

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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

   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.

                 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

Author's Address

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


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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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