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Network Working Group                                          C. NewmanRequest for Comments: 4468                              Sun MicrosystemsUpdates:3463                                                   May 2006Category: Standards TrackMessage Submission BURL ExtensionStatus 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.Newman                      Standards Track                     [Page 1]

RFC 4468           Message Submission BURL Extension            May 2006Table of Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................23. BURL Submission Extension .......................................33.1. SMTP Submission Extension Registration .....................33.2. BURL Transaction ...........................................33.3. The BURL IMAP Options ......................................43.4. Examples ...................................................53.5. Formal Syntax ..............................................64. 8-Bit and Binary ................................................75. Updates toRFC 3463 .............................................76. Response Codes ..................................................77. IANA Considerations .............................................98. Security Considerations .........................................99. References .....................................................119.1. Normative References ......................................119.2. Informative References ....................................12Appendix A.  Acknowledgements .....................................131.  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 inAppendix B of   RFC 4234.Newman                      Standards Track                     [Page 2]

RFC 4468           Message Submission BURL Extension            May 20063.  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://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 inSection 6.1.1 of RFC 3501, and the security   considerations inSection 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 submitNewman                      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   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:<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=Newman                      Standards Track                     [Page 5]

RFC 4468           Message Submission BURL Extension            May 2006   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.   Some examples of 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 failed3.5.  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 20064.  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 toRFC 3463   SMTP or Submit servers that advertise ENHANCEDSTATUSCODES [RFC2034]   use enhanced status codes defined inRFC 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.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      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.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 inSection 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 20069.  References9.1.  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.Newman                      Standards Track                    [Page 11]

RFC 4468           Message Submission BURL Extension            May 20069.2.  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)",RFC4120, 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 2006Appendix 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: chris.newman@sun.comNewman                      Standards Track                    [Page 13]

RFC 4468           Message Submission BURL Extension            May 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Newman                      Standards Track                    [Page 14]