Message Submission BURL Extension
draft-ietf-lemonade-burl-04

Network Working Group                                          C. Newman
Internet-Draft                                          Sun Microsystems
Updates: 3463 (if approved)                            November 10, 2005
Expires: May 14, 2006

                   Message Submission BURL Extension
                    draft-ietf-lemonade-burl-04.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

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 which 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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Table of Contents

   1.  Conventions Used in this Document  . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  BURL Submission Extension  . . . . . . . . . . . . . . . . . .  3
     3.1   SMTP Submission Extension Registration . . . . . . . . . .  3
     3.2   BURL Transaction . . . . . . . . . . . . . . . . . . . . .  4
     3.3   The BURL IMAP Option . . . . . . . . . . . . . . . . . . .  4
     3.4   Examples . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.5   Formal Syntax  . . . . . . . . . . . . . . . . . . . . . .  7
   4.  8-bit and Binary . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Updates to RFC 3463  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Response Codes . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1   Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2   Informative References . . . . . . . . . . . . . . . . . . 12
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
       Intellectual Property and Copyright Statements . . . . . . . . 14

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

2.  Introduction

   This specification defines an extension to the standard Message
   Submission [I-D.gellens-submit-bis] 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.

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 which appear after the BURL EHLO keyword may change
       subsequent to the use of SMTP AUTH [RFC2554], so a server which
       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.

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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.

   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 Option

   When "imap" is present in the space-separated list of arguments
   following the BURL EHLO keyword, that indicates the BURL command
   supports the URLAUTH [I-D.ietf-lemonade-urlauth] extended form of
   IMAP URLs [RFC2192] and 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 pre-arranged 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 which the user who authenticated to the submit
   server can access.  Note that this form does not imply 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

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   considerations in section 11 of RFC 3501.  Specifically, this
   requires that the submit server implement a configuration that uses
   STARTTLS followed by SASL PLAIN [I-D.ietf-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.

   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
   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:

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   <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:<harry@gryffindor.example.com>
   S: 250 2.5.0 Address Ok.
   C: RCPT TO:<ron@gryffindor.example.com>
   S: 250 2.1.5 ron@gryffindor.example.com OK.
   C: BURL imap://harry@gryffindor.example.com/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=
   C: MAIL FROM:<harry@gryffindor.example.com>
   C: RCPT TO:<ron@gryffindor.example.com>
   C: BURL imap://harry@gryffindor.example.com/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 ron@gryffindor.example.com 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.

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   Some example failure cases:

   C: MAIL FROM:<harry@gryffindor.example.com>
   C: RCPT TO:<malfoy@slitherin.example.com>
   C: BURL imap://harry@gryffindor.example.com/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: malfoy@slitherin.example.com
   S: 554 5.5.0 No recipients have been specified.

   C: MAIL FROM:<harry@gryffindor.example.com>
   C: RCPT TO:<ron@gryffindor.example.com>
   C: BURL imap://harry@gryffindor.example.com/outbox
           ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
           :internal:71354a473744909de610943775f92038 LAST
   S: 250 2.5.0 Address Ok.
   S: 250 2.1.5 ron@gryffindor.example.com 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"

4.  8-bit and Binary

   A submit server which 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 which 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].

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5.  Updates to RFC 3463

   SMTP or Submit servers which advertise ENHANCEDSTATUSCODES [RFC2034]
   use enhanced status codes defined in RFC 3463 [RFC3463].  The BURL
   extension introduces new error cases which 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.

   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.

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   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 FETCHURL command returned an error or no data.

   354 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 not yet
      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.

   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 which 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.

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7.  IANA Considerations

   When this is published as an RFC, the "BURL" SMTP extension as
   described in Section 3 will be registered.  This registration will be
   marked for use by message submission [I-D.gellens-submit-bis] 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 which 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
   which associates unique client identifiers with mail transactions,
   limits on re-use of the same IMAP URL, rate limits, recipient count
   limits and content filters.

   Transfer of the URLAUTH [I-D.ietf-lemonade-urlauth] form of IMAP URLs
   in the clear can expose the authorization token to network
   eavesdroppers.  Implementations which 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
   which requires their use with such IMAP URLs would address this
   concern.

   Use of a pre-arranged 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
   which 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 Kerberos5
   [RFC1510] would also suffice.

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   When a client uses SMTP STARTTLS to send a BURL command which
   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 which points
   to a server under that user's control which is designed to undermine
   the security of the submit server.  For this reason, the IMAP client
   code which 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 pre-arranged 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
   which 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.  And 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.

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.

   [RFC2222]  Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997.

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

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   [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.

   [I-D.ietf-lemonade-urlauth]
              Crispin, M., "Internet Message Access Protocol (IMAP) -
              URLAUTH Extension", draft-ietf-lemonade-urlauth-08 (work
              in progress), October 2005.

   [I-D.gellens-submit-bis]
              Gellens, R. and J. Klensin, "Message Submission for Mail",
              draft-gellens-submit-bis-02 (work in progress),
              April 2005.

9.2  Informative References

   [RFC1510]  Kohl, J. and B. Neuman, "The Kerberos Network
              Authentication Service (V5)", RFC 1510, September 1993.

   [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.

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

   [I-D.ietf-sasl-plain]
              Zeilenga, K., "The Plain SASL Mechanism",
              draft-ietf-sasl-plain-08 (work in progress), March 2005.

Author's Address

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

   Email: chris.newman@sun.com

Appendix A.  Acknowledgments

   This document is a product of the lemonade WG.  Many thanks are due
   to the all 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 and Mark
   Crispin for review comments on the document.  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.

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Newman                    Expires May 14, 2006                 [Page 14]