7Network Working Group C. Newman
8Request for Comments: 4468 Sun Microsystems
10Category: Standards Track
13 Message Submission BURL Extension
17 This document specifies an Internet standards track protocol for the
18 Internet community, and requests discussion and suggestions for
19 improvements. Please refer to the current edition of the "Internet
20 Official Protocol Standards" (STD 1) for the standardization state
21 and status of this protocol. Distribution of this memo is unlimited.
25 Copyright (C) The Internet Society (2006).
29 The submission profile of Simple Mail Transfer Protocol (SMTP)
30 provides a standard way for an email client to submit a complete
31 message for delivery. This specification extends the submission
32 profile by adding a new BURL command that can be used to fetch
33 submission data from an Internet Message Access Protocol (IMAP)
34 server. This permits a mail client to inject content from an IMAP
35 server into the SMTP infrastructure without downloading it to the
36 client and uploading it back to the server.
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60RFC 4468 Message Submission BURL Extension May 2006
65 1. Introduction ....................................................2
66 2. Conventions Used in This Document ...............................2
67 3. BURL Submission Extension .......................................3
68 3.1. SMTP Submission Extension Registration .....................3
69 3.2. BURL Transaction ...........................................3
70 3.3. The BURL IMAP Options ......................................4
71 3.4. Examples ...................................................5
72 3.5. Formal Syntax ..............................................6
73 4. 8-Bit and Binary ................................................7
74 5. Updates to RFC 3463 .............................................7
75 6. Response Codes ..................................................7
76 7. IANA Considerations .............................................9
77 8. Security Considerations .........................................9
78 9. References .....................................................11
79 9.1. Normative References ......................................11
80 9.2. Informative References ....................................12
81 Appendix A. Acknowledgements .....................................13
85 This specification defines an extension to the standard Message
86 Submission [RFC4409] protocol to permit data to be fetched from an
87 IMAP server at message submission time. This MAY be used in
88 conjunction with the CHUNKING [RFC3030] mechanism so that chunks of
89 the message can come from an external IMAP server. This provides the
90 ability to forward an email message without first downloading it to
932. Conventions Used in This Document
95 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
96 in this document are to be interpreted as defined in "Key words for
97 use in RFCs to Indicate Requirement Levels" [RFC2119].
99 The formal syntax uses the Augmented Backus-Naur Form (ABNF)
100 [RFC4234] notation including the core rules defined in Appendix B of
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116RFC 4468 Message Submission BURL Extension May 2006
1193. BURL Submission Extension
121 This section defines the BURL submission extension.
1233.1. SMTP Submission Extension Registration
125 1. The name of this submission extension is "BURL". This extends
126 the Message Submission protocol on port 587 and MUST NOT be
127 advertised by a regular SMTP [RFC2821] server on port 25 that
128 acts as a relay for incoming mail from other SMTP relays.
130 2. The EHLO keyword value associated with the extension is "BURL".
132 3. The BURL EHLO keyword will have zero or more arguments. The only
133 argument defined at this time is the "imap" argument, which MUST
134 be present in order to use IMAP URLs with BURL. Clients MUST
135 ignore other arguments after the BURL EHLO keyword unless they
136 are defined by a subsequent IETF standards track specification.
137 The arguments that appear after the BURL EHLO keyword may change
138 subsequent to the use of SMTP AUTH [RFC2554], so a server that
139 advertises BURL with no arguments prior to authentication
140 indicates that BURL is supported but authentication is required
143 4. This extension adds the BURL SMTP verb. This verb is used as a
144 replacement for the DATA command and is only permitted during a
145 mail transaction after at least one successful RCPT TO.
149 A simple BURL transaction will consist of MAIL FROM, one or more RCPT
150 TO headers, and a BURL command with the "LAST" tag. The BURL command
151 will include an IMAP URL pointing to a fully formed message ready for
152 injection into the SMTP infrastructure. If PIPELINING [RFC2920] is
153 advertised, the client MAY send the entire transaction in one round
154 trip. If no valid RCPT TO address is supplied, the BURL command will
155 simply fail, and no resolution of the BURL URL argument will be
156 performed. If at least one valid RCPT TO address is supplied, then
157 the BURL URL argument will be resolved before the server responds to
160 A more sophisticated BURL transaction MAY occur when the server also
161 advertises CHUNKING [RFC3030]. In this case, the BURL and BDAT
162 commands may be interleaved until one of them terminates the
163 transaction with the "LAST" argument. If PIPELINING [RFC2920] is
164 also advertised, then the client may pipeline the entire transaction
165 in one round-trip. However, it MUST wait for the results of the
166 "LAST" BDAT or BURL command prior to initiating a new transaction.
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172RFC 4468 Message Submission BURL Extension May 2006
175 The BURL command directs the server to fetch the data object to which
176 the URL refers and include it in the message. If the URL fetch
177 fails, the server will fail the entire transaction.
1793.3. The BURL IMAP Options
181 When "imap" is present in the space-separated list of arguments
182 following the BURL EHLO keyword, it indicates that the BURL command
183 supports the URLAUTH [RFC4467] extended form of IMAP URLs [RFC2192]
184 and that the submit server is configured with the necessary
185 credentials to resolve "urlauth=submit+" IMAP URLs for the submit
188 Subsequent to a successful SMTP AUTH command, the submission server
189 MAY indicate a prearranged trust relationship with a specific IMAP
190 server by including a BURL EHLO keyword argument of the form
191 "imap://imap.example.com". In this case, the submission server will
192 permit a regular IMAP URL referring to messages or parts of messages
193 on imap.example.com that the user who authenticated to the submit
194 server can access. Note that this form does not imply that the
195 submit server supports URLAUTH URLs; the submit server must advertise
196 both "imap" and "imap://imap.example.com" to indicate support for
197 both extended and non-extended URL forms.
199 When the submit server connects to the IMAP server, it acts as an
200 IMAP client and thus is subject to both the mandatory-to-implement
201 IMAP capabilities in Section 6.1.1 of RFC 3501, and the security
202 considerations in Section 11 of RFC 3501. Specifically, this
203 requires that the submit server implement a configuration that uses
204 STARTTLS followed by SASL PLAIN [SASL-PLAIN] to authenticate to the
207 When the submit server resolves a URLAUTH IMAP URL, it uses submit
208 server credentials when authenticating to the IMAP server. The
209 authentication identity and password used for submit credentials MUST
210 be configurable. The string "submit" is suggested as a default value
211 for the authentication identity, with no default for the password.
212 Typically, the authorization identity is empty in this case; thus the
213 IMAP server will derive the authorization identity from the
214 authentication identity. If the IMAP URL uses the "submit+" access
215 identifier prefix, the submit server MUST refuse the BURL command
216 unless the userid in the URL's <access> token matches the submit
217 client's authorization identity.
219 When the submit server resolves a regular IMAP URL, it uses the
220 submit client's authorization identity when authenticating to the
221 IMAP server. If both the submit client and the submit server's
222 embedded IMAP client use SASL PLAIN (or the equivalent), the submit
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228RFC 4468 Message Submission BURL Extension May 2006
231 server SHOULD forward the client's credentials if and only if the
232 submit server knows that the IMAP server is in the same
233 administrative domain. If the submit server supports SASL mechanisms
234 other than PLAIN, it MUST implement a configuration in which the
235 submit server's embedded IMAP client uses STARTTLS and SASL PLAIN
236 with the submit server's authentication identity and password (for
237 the respective IMAP server) and the submit client's authorization
242 In examples, "C:" and "S:" indicate lines sent by the client and
243 server, respectively. If a single "C:" or "S:" label applies to
244 multiple lines, then the line breaks between those lines are for
245 editorial clarity only and are not part of the actual protocol
248 Two successful submissions (without and with pipelining) follow:
250 <SSL/TLS encryption layer negotiated>
251 C: EHLO potter.example.com
252 S: 250-owlry.example.com
257 S: 250 ENHANCEDSTATUSCODES
258 C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=
259 S: 235 2.7.0 PLAIN authentication successful.
260 C: MAIL FROM:<harry@gryffindor.example.com>
261 S: 250 2.5.0 Address Ok.
262 C: RCPT TO:<ron@gryffindor.example.com>
263 S: 250 2.1.5 ron@gryffindor.example.com OK.
264 C: BURL imap://harry@gryffindor.example.com/outbox
265 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
266 :internal:91354a473744909de610943775f92038 LAST
269 <SSL/TLS encryption layer negotiated>
270 C: EHLO potter.example.com
271 S: 250-owlry.example.com
277 S: 250 ENHANCEDSTATUSCODES
278 C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=
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284RFC 4468 Message Submission BURL Extension May 2006
287 C: MAIL FROM:<harry@gryffindor.example.com>
288 C: RCPT TO:<ron@gryffindor.example.com>
289 C: BURL imap://harry@gryffindor.example.com/outbox
290 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
291 :internal:91354a473744909de610943775f92038 LAST
292 S: 235 2.7.0 PLAIN authentication successful.
293 S: 250 2.5.0 Address Ok.
294 S: 250 2.1.5 ron@gryffindor.example.com OK.
297 Note that PIPELINING of the AUTH command is only permitted if the
298 selected mechanism can be completed in one round trip, a client
299 initial response is provided, and no SASL security layer is
300 negotiated. This is possible for PLAIN and EXTERNAL, but not for
301 most other SASL mechanisms.
303 Some examples of failure cases:
305 C: MAIL FROM:<harry@gryffindor.example.com>
306 C: RCPT TO:<malfoy@slitherin.example.com>
307 C: BURL imap://harry@gryffindor.example.com/outbox
308 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
309 :internal:91354a473744909de610943775f92038 LAST
310 S: 250 2.5.0 Address Ok.
311 S: 550 5.7.1 Relaying not allowed: malfoy@slitherin.example.com
312 S: 554 5.5.0 No recipients have been specified.
314 C: MAIL FROM:<harry@gryffindor.example.com>
315 C: RCPT TO:<ron@gryffindor.example.com>
316 C: BURL imap://harry@gryffindor.example.com/outbox
317 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
318 :internal:71354a473744909de610943775f92038 LAST
319 S: 250 2.5.0 Address Ok.
320 S: 250 2.1.5 ron@gryffindor.example.com OK.
321 S: 554 5.7.0 IMAP URL authorization failed
325 The following syntax specification inherits ABNF [RFC4234] and
326 Uniform Resource Identifiers [RFC3986].
328 burl-param = "imap" / ("imap://" authority)
329 ; parameter to BURL EHLO keyword
331 burl-cmd = "BURL" SP absolute-URI [SP end-marker] CRLF
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340RFC 4468 Message Submission BURL Extension May 2006
345 A submit server that advertises BURL MUST also advertise 8BITMIME
346 [RFC1652] and perform the down conversion described in that
347 specification on the resulting complete message if 8-bit data is
348 received with the BURL command and passed to a 7-bit server. If the
349 URL argument to BURL refers to binary data, then the submit server
350 MAY refuse the command or down convert as described in Binary SMTP
353 The Submit server MAY refuse to accept a BURL command or combination
354 of BURL and BDAT commands that result in un-encoded 8-bit data in
355 mail or MIME [RFC2045] headers. Alternatively, the server MAY accept
356 such data and down convert to MIME header encoding [RFC2047].
3585. Updates to RFC 3463
360 SMTP or Submit servers that advertise ENHANCEDSTATUSCODES [RFC2034]
361 use enhanced status codes defined in RFC 3463 [RFC3463]. The BURL
362 extension introduces new error cases that that RFC did not consider.
363 The following additional enhanced status codes are defined by this
366 X.6.6 Message content not available
368 The message content could not be fetched from a remote system.
369 This may be useful as a permanent or persistent temporary
372 X.7.8 Trust relationship required
374 The submission server requires a configured trust relationship
375 with a third-party server in order to access the message content.
379 This section includes example response codes to the BURL command.
380 Other text may be used with the same response codes. This list is
381 not exhaustive, and BURL clients MUST tolerate any valid SMTP
382 response code. Most of these examples include the appropriate
383 enhanced status code [RFC3463].
385 554 5.5.0 No recipients have been specified
387 This response code occurs when BURL is used (for example, with
388 PIPELINING) and all RCPT TOs failed.
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396RFC 4468 Message Submission BURL Extension May 2006
399 503 5.5.0 Valid RCPT TO required before BURL
401 This response code is an alternative to the previous one when BURL
402 is used (for example, with PIPELINING) and all RCPT TOs failed.
404 554 5.6.3 Conversion required but not supported
406 This response code occurs when the URL points to binary data and
407 the implementation does not support down conversion to base64.
408 This can also be used if the URL points to message data with 8-bit
409 content in headers and the server does not down convert such
412 554 5.3.4 Message too big for system
414 The message (subsequent to URL resolution) is larger than the
415 per-message size limit for this server.
417 554 5.7.8 URL resolution requires trust relationship
419 The submit server does not have a trust relationship with the IMAP
420 server specified in the URL argument to BURL.
422 552 5.2.2 Mailbox full
424 The recipient is local, the submit server supports direct
425 delivery, and the recipient has exceeded his quota and any grace
426 period for delivery attempts.
428 554 5.6.6 IMAP URL resolution failed
430 The IMAP URLFETCH command returned an error or no data.
432 250 2.5.0 Waiting for additional BURL or BDAT commands
434 A BURL command without the "LAST" modifier was sent. The URL for
435 this BURL command was successfully resolved, but the content will
436 not necessarily be committed to persistent storage until the rest
437 of the message content is collected. For example, a Unix server
438 may have written the content to a queue file buffer, but may not
439 yet have performed an fsync() operation. If the server loses
440 power, the content can still be lost.
442 451 4.4.1 IMAP server unavailable
444 The connection to the IMAP server to resolve the URL failed.
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452RFC 4468 Message Submission BURL Extension May 2006
457 The URL was successfully resolved, and the complete message data
458 has been committed to persistent storage.
460 250 2.6.4 MIME header conversion with loss performed
462 The URL pointed to message data that included mail or MIME headers
463 with 8-bit data. This data was converted to MIME header encoding
464 [RFC2047], but the submit server may not have correctly guessed
465 the unlabeled character set.
4677. IANA Considerations
469 The "BURL" SMTP extension as described in Section 3 has been
470 registered. This registration has been marked for use by message
471 submission [RFC4409] only in the registry.
4738. Security Considerations
475 Modern SMTP submission servers often include content-based security
476 and denial-of-service defense mechanisms such as virus filtering,
477 size limits, server-generated signatures, spam filtering, etc.
478 Implementations of BURL should fetch the URL content prior to
479 application of such content-based mechanisms in order to preserve
482 Clients that generate unsolicited bulk email or email with viruses
483 could use this mechanism to compensate for a slow link between the
484 client and submit server. In particular, this mechanism would make
485 it feasible for a programmable cell phone or other device on a slow
486 link to become a significant source of unsolicited bulk email and/or
487 viruses. This makes it more important for submit server vendors
488 implementing BURL to have auditing and/or defenses against such
489 denial-of-service attacks including mandatory authentication, logging
490 that associates unique client identifiers with mail transactions,
491 limits on reuse of the same IMAP URL, rate limits, recipient count
492 limits, and content filters.
494 Transfer of the URLAUTH [RFC4467] form of IMAP URLs in the clear can
495 expose the authorization token to network eavesdroppers.
496 Implementations that support such URLs can address this issue by
497 using a strong confidentiality protection mechanism. For example,
498 the SMTP STARTTLS [RFC3207] and the IMAP STARTTLS [RFC3501]
499 extensions, in combination with a configuration setting that requires
500 their use with such IMAP URLs, would address this concern.
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508RFC 4468 Message Submission BURL Extension May 2006
511 Use of a prearranged trust relationship between a submit server and a
512 specific IMAP server introduces security considerations. A
513 compromise of the submit server should not automatically compromise
514 all accounts on the IMAP server, so trust relationships involving
515 super-user proxy credentials are strongly discouraged. A system that
516 requires the submit server to authenticate to the IMAP server with
517 submit credentials and subsequently requires a URLAUTH URL to fetch
518 any content addresses this concern. A trusted third party model for
519 proxy credentials (such as that provided by Kerberos 5 [RFC4120])
522 When a client uses SMTP STARTTLS to send a BURL command that
523 references non-public information, there is a user expectation that
524 the entire message content will be treated confidentially. To
525 address this expectation, the message submission server SHOULD use
526 STARTTLS or a mechanism providing equivalent data confidentiality
527 when fetching the content referenced by that URL.
529 A legitimate user of a submit server may try to compromise other
530 accounts on the server by providing an IMAP URLAUTH URL that points
531 to a server under that user's control that is designed to undermine
532 the security of the submit server. For this reason, the IMAP client
533 code that the submit server uses must be robust with respect to
534 arbitrary input sizes (including large IMAP literals) and arbitrary
535 delays from the IMAP server. Requiring a prearranged trust
536 relationship between a submit server and the IMAP server also
537 addresses this concern.
539 An authorized user of the submit server could set up a fraudulent
540 IMAP server and pass a URL for that server to the submit server. The
541 submit server might then contact the fraudulent IMAP server to
542 authenticate with submit credentials and fetch content. There are
543 several ways to mitigate this potential attack. A submit server that
544 only uses submit credentials with a fixed set of trusted IMAP servers
545 will not be vulnerable to exposure of those credentials. A submit
546 server can treat the IMAP server as untrusted and include defenses
547 for buffer overflows, denial-of-service slowdowns, and other
548 potential attacks. Finally, because authentication is required to
549 use BURL, it is possible to keep a secure audit trail and use that to
550 detect and punish the offending party.
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564RFC 4468 Message Submission BURL Extension May 2006
5699.1. Normative References
571 [RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
572 Crocker, "SMTP Service Extension for
573 8bit-MIMEtransport", RFC 1652, July 1994.
575 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
576 Requirement Levels", BCP 14, RFC 2119, March 1997.
578 [RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192,
581 [RFC2554] Myers, J., "SMTP Service Extension for Authentication",
582 RFC 2554, March 1999.
584 [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
587 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP
588 over Transport Layer Security", RFC 3207,
591 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
592 VERSION 4rev1", RFC 3501, March 2003.
594 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
595 "Uniform Resource Identifier (URI): Generic Syntax",
596 STD 66, RFC 3986, January 2005.
598 [RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
599 Specifications: ABNF", RFC 4234, October 2005.
601 [RFC4409] Gellens, R. and J. Klensin, "Message Submission for
602 Mail", RFC 4409, April 2006.
604 [RFC4467] Crispin, M., "Internet Message Access Protocol (IMAP) -
605 URLAUTH Extension", RFC 4467, May 2006.
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620RFC 4468 Message Submission BURL Extension May 2006
6239.2. Informative References
625 [RFC2034] Freed, N., "SMTP Service Extension for Returning
626 Enhanced Error Codes", RFC 2034, October 1996.
628 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet
629 Mail Extensions (MIME) Part One: Format of Internet
630 Message Bodies", RFC 2045, November 1996.
632 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
633 Extensions) Part Three: Message Header Extensions for
634 Non-ASCII Text", RFC 2047, November 1996.
636 [RFC2920] Freed, N., "SMTP Service Extension for Command
637 Pipelining", STD 60, RFC 2920, September 2000.
639 [RFC3030] Vaudreuil, G., "SMTP Service Extensions for
640 Transmission of Large and Binary MIME Messages",
641 RFC 3030, December 2000.
643 [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
644 RFC 3463, January 2003.
646 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
647 Kerberos Network Authentication Service (V5)", RFC
650 [SASL-PLAIN] Zeilenga, K., "The Plain SASL Mechanism", Work in
651 Progress, March 2005.
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676RFC 4468 Message Submission BURL Extension May 2006
679Appendix A. Acknowledgements
681 This document is a product of the lemonade WG. Many thanks are due
682 to all the participants of that working group for their input. Mark
683 Crispin was instrumental in the conception of this mechanism. Thanks
684 to Randall Gellens, Alexey Melnikov, Sam Hartman, Ned Freed, Dave
685 Cridland, Peter Coates, and Mark Crispin for review comments on the
686 document. Thanks to the RFC Editor for correcting the author's
687 grammar mistakes. Thanks to Ted Hardie, Randall Gellens, Mark
688 Crispin, Pete Resnick, and Greg Vaudreuil for extremely interesting
689 debates comparing this proposal and alternatives. Thanks to the
690 lemonade WG chairs Eric Burger and Glenn Parsons for concluding the
691 debate at the correct time and making sure this document got
698 3401 Centrelake Dr., Suite 410
702 EMail: chris.newman@sun.com
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735Full Copyright Statement
737 Copyright (C) The Internet Society (2006).
739 This document is subject to the rights, licenses and restrictions
740 contained in BCP 78, and except as set forth therein, the authors
741 retain all their rights.
743 This document and the information contained herein are provided on an
744 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
745 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
746 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
747 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
748 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
749 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
753 The IETF takes no position regarding the validity or scope of any
754 Intellectual Property Rights or other rights that might be claimed to
755 pertain to the implementation or use of the technology described in
756 this document or the extent to which any license under such rights
757 might or might not be available; nor does it represent that it has
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762 Copies of IPR disclosures made to the IETF Secretariat and any
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764 attempt made to obtain a general license or permission for the use of
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767 http://www.ietf.org/ipr.
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786Newman Standards Track [Page 14]