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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Newman 3 Internet-Draft Sun Microsystems 4 Updates: 3463 (if approved) November 10, 2005 5 Expires: May 14, 2006 7 Message Submission BURL Extension 8 draft-ietf-lemonade-burl-04.txt 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on May 14, 2006. 35 Copyright Notice 37 Copyright (C) The Internet Society (2005). 39 Abstract 41 The submission profile of Simple Mail Transfer Protocol (SMTP) 42 provides a standard way for an email client to submit a complete 43 message for delivery. This specification extends the submission 44 profile by adding a new BURL command which can be used to fetch 45 submission data from an Internet Message Access Protocol (IMAP) 46 server. This permits a mail client to inject content from an IMAP 47 server into the SMTP infrastructure without downloading it to the 48 client and uploading it back to the server. 50 Table of Contents 52 1. Conventions Used in this Document . . . . . . . . . . . . . . 3 53 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 3. BURL Submission Extension . . . . . . . . . . . . . . . . . . 3 55 3.1 SMTP Submission Extension Registration . . . . . . . . . . 3 56 3.2 BURL Transaction . . . . . . . . . . . . . . . . . . . . . 4 57 3.3 The BURL IMAP Option . . . . . . . . . . . . . . . . . . . 4 58 3.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 5 59 3.5 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 7 60 4. 8-bit and Binary . . . . . . . . . . . . . . . . . . . . . . . 7 61 5. Updates to RFC 3463 . . . . . . . . . . . . . . . . . . . . . 8 62 6. Response Codes . . . . . . . . . . . . . . . . . . . . . . . . 8 63 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 64 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 65 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 66 9.1 Normative References . . . . . . . . . . . . . . . . . . . 11 67 9.2 Informative References . . . . . . . . . . . . . . . . . . 12 68 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13 69 A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 70 Intellectual Property and Copyright Statements . . . . . . . . 14 72 1. Conventions Used in this Document 74 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" 75 in this document are to be interpreted as defined in "Key words for 76 use in RFCs to Indicate Requirement Levels" [RFC2119]. 78 The formal syntax use the Augmented Backus-Naur Form (ABNF) [RFC4234] 79 notation including the core rules defined in Appendix B of RFC 4234. 81 2. Introduction 83 This specification defines an extension to the standard Message 84 Submission [I-D.gellens-submit-bis] protocol to permit data to be 85 fetched from an IMAP server at message submission time. This MAY be 86 used in conjunction with the CHUNKING [RFC3030] mechanism so that 87 chunks of the message can come from an external IMAP server. This 88 provides the ability to forward an email message without first 89 downloading it to the client. 91 3. BURL Submission Extension 93 This section defines the BURL submission extension. 95 3.1 SMTP Submission Extension Registration 97 1. The name of this submission extension is "BURL". This extends 98 the Message Submission protocol on port 587 and MUST NOT be 99 advertised by a regular SMTP [RFC2821] server on port 25 that 100 acts as a relay for incoming mail from other SMTP relays. 102 2. The EHLO keyword value associated with the extension is "BURL". 104 3. The BURL EHLO keyword will have zero or more arguments. The only 105 argument defined at this time is the "imap" argument, which MUST 106 be present in order to use IMAP URLs with BURL. Clients MUST 107 ignore other arguments after the BURL EHLO keyword unless they 108 are defined by a subsequent IETF standards track specification. 109 The arguments which appear after the BURL EHLO keyword may change 110 subsequent to the use of SMTP AUTH [RFC2554], so a server which 111 advertises BURL with no arguments prior to authentication 112 indicates that BURL is supported but authentication is required 113 to use it. 115 4. This extension adds the BURL SMTP verb. This verb is used as a 116 replacement for the DATA command and is only permitted during a 117 mail transaction after at least one successful RCPT TO. 119 3.2 BURL Transaction 121 A simple BURL transaction will consist of MAIL FROM, one or more RCPT 122 TO headers and a BURL command with the "LAST" tag. The BURL command 123 will include an IMAP URL pointing to a fully formed message ready for 124 injection into the SMTP infrastructure. If PIPELINING [RFC2920] is 125 advertised, the client MAY send the entire transaction in one round 126 trip. If no valid RCPT TO address is supplied, the BURL command will 127 simply fail and no resolution of the BURL URL argument will be 128 performed. If at least one valid RCPT TO address is supplied, then 129 the BURL URL argument will be resolved before the server responds to 130 the command. 132 A more sophisticated BURL transaction MAY occur when the server also 133 advertises CHUNKING [RFC3030]. In this case, the BURL and BDAT 134 commands may be interleaved until one of them terminates the 135 transaction with the "LAST" argument. If PIPELINING [RFC2920] is 136 also advertised, then the client may pipeline the entire transaction 137 in one round-trip. However, it MUST wait for the results of the 138 "LAST" BDAT or BURL command prior to initiating a new transaction. 140 The BURL command directs the server to fetch the data object to which 141 the URL refers and include it in the message. If the URL fetch 142 fails, the server will fail the entire transaction. 144 3.3 The BURL IMAP Option 146 When "imap" is present in the space-separated list of arguments 147 following the BURL EHLO keyword, that indicates the BURL command 148 supports the URLAUTH [I-D.ietf-lemonade-urlauth] extended form of 149 IMAP URLs [RFC2192] and the submit server is configured with the 150 necessary credentials to resolve "urlauth=submit+" IMAP URLs for the 151 submit server's domain. 153 Subsequent to a successful SMTP AUTH command, the submission server 154 MAY indicate a pre-arranged trust relationship with a specific IMAP 155 server by including a BURL EHLO keyword argument of the form 156 "imap://imap.example.com". In this case, the submission server will 157 permit a regular IMAP URL referring to messages or parts of messages 158 on imap.example.com which the user who authenticated to the submit 159 server can access. Note that this form does not imply the submit 160 server supports URLAUTH URLs, the submit server must advertise both 161 "imap" and "imap://imap.example.com" to indicate support for both 162 extended and non-extended URL forms. 164 When the submit server connects to the IMAP server, it acts as an 165 IMAP client; and thus is subject to both the mandatory-to-implement 166 IMAP capabilities in section 6.1.1 of RFC 3501, and the security 167 considerations in section 11 of RFC 3501. Specifically, this 168 requires that the submit server implement a configuration that uses 169 STARTTLS followed by SASL PLAIN [I-D.ietf-sasl-plain] to authenticate 170 to the IMAP server. 172 When the submit server resolves a URLAUTH IMAP URL, it uses submit 173 server credentials when authenticating to the IMAP server. The 174 authentication identity and password used for submit credentials MUST 175 be configurable. The string "submit" is suggested as a default value 176 for the authentication identity, with no default for the password. 177 Typically, the authorization identity is empty in this case; thus the 178 IMAP server will derive the authorization identity from the 179 authentication identity. 181 When the submit server resolves a regular IMAP URL, it uses the 182 submit client's authorization identity when authenticating to the 183 IMAP server. If both the submit client and the submit server's 184 embedded IMAP client use SASL PLAIN (or the equivalent), the submit 185 server SHOULD forward the client's credentials if and only if the 186 submit server knows that the IMAP server is in the same 187 administrative domain. If the submit server supports SASL mechanisms 188 other than PLAIN, it MUST implement a configuration in which the 189 submit server's embedded IMAP client uses STARTTLS and SASL PLAIN 190 with the submit server's authentication identity and password (for 191 the respective IMAP server) and the submit client's authorization 192 identity. 194 3.4 Examples 196 In examples, "C:" and "S:" indicate lines sent by the client and 197 server respectively. If a single "C:" or "S:" label applies to 198 multiple lines, then the line breaks between those lines are for 199 editorial clarity only and are not part of the actual protocol 200 exchange. 202 Two successful submissions (without and with pipelining) follow: 204 205 C: EHLO potter.example.com 206 S: 250-owlry.example.com 207 S: 250-8BITMIME 208 S: 250-BURL imap 209 S: 250-AUTH PLAIN 210 S: 250-DSN 211 S: 250 ENHANCEDSTATUSCODES 212 C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8= 213 S: 235 2.7.0 PLAIN authentication successful. 214 C: MAIL FROM: 215 S: 250 2.5.0 Address Ok. 216 C: RCPT TO: 217 S: 250 2.1.5 ron@gryffindor.example.com OK. 218 C: BURL imap://harry@gryffindor.example.com/outbox 219 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry 220 :internal:91354a473744909de610943775f92038 LAST 221 S: 250 2.5.0 Ok. 223 224 C: EHLO potter.example.com 225 S: 250-owlry.example.com 226 S: 250-8BITMIME 227 S: 250-PIPELINING 228 S: 250-BURL imap 229 S: 250-AUTH PLAIN 230 S: 250-DSN 231 S: 250 ENHANCEDSTATUSCODES 232 C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8= 233 C: MAIL FROM: 234 C: RCPT TO: 235 C: BURL imap://harry@gryffindor.example.com/outbox 236 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry 237 :internal:91354a473744909de610943775f92038 LAST 238 S: 235 2.7.0 PLAIN authentication successful. 239 S: 250 2.5.0 Address Ok. 240 S: 250 2.1.5 ron@gryffindor.example.com OK. 241 S: 250 2.5.0 Ok. 243 Note that PIPELINING of the AUTH command is only permitted if the 244 selected mechanism can be completed in one round trip, a client 245 initial response is provided, and no SASL security layer is 246 negotiated. This is possible for PLAIN and EXTERNAL, but not for 247 most other SASL mechanisms. 249 Some example failure cases: 251 C: MAIL FROM: 252 C: RCPT TO: 253 C: BURL imap://harry@gryffindor.example.com/outbox 254 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry 255 :internal:91354a473744909de610943775f92038 LAST 256 S: 250 2.5.0 Address Ok. 257 S: 550 5.7.1 Relaying not allowed: malfoy@slitherin.example.com 258 S: 554 5.5.0 No recipients have been specified. 260 C: MAIL FROM: 261 C: RCPT TO: 262 C: BURL imap://harry@gryffindor.example.com/outbox 263 ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry 264 :internal:71354a473744909de610943775f92038 LAST 265 S: 250 2.5.0 Address Ok. 266 S: 250 2.1.5 ron@gryffindor.example.com OK. 267 S: 554 5.7.0 IMAP URL authorization failed 269 3.5 Formal Syntax 271 The following syntax specification inherits ABNF [RFC4234] and 272 Uniform Resource Identifiers [RFC3986]. 274 burl-param = "imap" / ("imap://" authority) 275 ; parameter to BURL EHLO keyword 277 burl-cmd = "BURL" SP absolute-URI [SP end-marker] CRLF 279 end-marker = "LAST" 281 4. 8-bit and Binary 283 A submit server which advertises BURL MUST also advertise 8BITMIME 284 [RFC1652] and perform the down conversion described in that 285 specification on the resulting complete message if 8-bit data is 286 received with the BURL command and passed to a 7-bit server. If the 287 URL argument to BURL refers to binary data, then the submit server 288 MAY refuse the command or down convert as described in Binary SMTP 289 [RFC3030]. 291 The Submit server MAY refuse to accept a BURL command or combination 292 of BURL and BDAT commands which result in un-encoded 8-bit data in 293 mail or MIME [RFC2045] headers. Alternatively, the server MAY accept 294 such data and down convert to MIME header encoding [RFC2047]. 296 5. Updates to RFC 3463 298 SMTP or Submit servers which advertise ENHANCEDSTATUSCODES [RFC2034] 299 use enhanced status codes defined in RFC 3463 [RFC3463]. The BURL 300 extension introduces new error cases which that RFC did not consider. 301 The following additional enhanced status codes are defined by this 302 specification: 304 X.6.6 Message content not available 306 The message content could not be fetched from a remote system. 307 This may be useful as a permanent or persistent temporary 308 notification. 310 X.7.8 Trust relationship required 312 The submission server requires a configured trust relationship 313 with a third-party server in order to access the message content. 315 6. Response Codes 317 This section includes example response codes to the BURL command. 318 Other text may be used with the same response codes. This list is 319 not exhaustive and BURL clients MUST tolerate any valid SMTP response 320 code. Most of these examples include the appropriate enhanced status 321 code [RFC3463]. 323 554 5.5.0 No recipients have been specified 325 This response code occurs when BURL is used (for example, with 326 PIPELINING) and all RCPT TOs failed. 328 503 5.5.0 Valid RCPT TO required before BURL 330 This response code is an alternative to the previous one when BURL 331 is used (for example, with PIPELINING) and all RCPT TOs failed. 333 554 5.6.3 Conversion required but not supported 335 This response code occurs when the URL points to binary data and 336 the implementation does not support down conversion to base64. 337 This can also be used if the URL points to message data with 8-bit 338 content in headers and the server does not down convert such 339 content. 341 554 5.3.4 Message too big for system 343 The message (subsequent to URL resolution) is larger than the per- 344 message size limit for this server. 346 554 5.7.8 URL resolution requires trust relationship 348 The submit server does not have a trust relationship with the IMAP 349 server specified in the URL argument to BURL. 351 552 5.2.2 Mailbox full 353 The recipient is local, the submit server supports direct delivery 354 and the recipient has exceeded his quota and any grace period for 355 delivery attempts. 357 554 5.6.6 IMAP URL resolution failed 359 The IMAP FETCHURL command returned an error or no data. 361 354 Waiting for additional BURL or BDAT commands 363 A BURL command without the "LAST" modifier was sent. The URL for 364 this BURL command was successfully resolved, but the content will 365 not necessarily be committed to persistent storage until the rest 366 of the message content is collected. For example, a Unix server 367 may have written the content to a queue file buffer, but not yet 368 performed an fsync() operation. If the server loses power, the 369 content can still be lost. 371 451 4.4.1 IMAP server unavailable 373 The connection to the IMAP server to resolve the URL failed. 375 250 2.5.0 Ok. 377 The URL was successfully resolved and the complete message data 378 has been committed to persistent storage. 380 250 2.6.4 MIME header conversion with loss performed 382 The URL pointed to message data which included mail or MIME 383 headers with 8-bit data. This data was converted to MIME header 384 encoding [RFC2047] but the submit server may not have correctly 385 guessed the unlabeled character set. 387 7. IANA Considerations 389 When this is published as an RFC, the "BURL" SMTP extension as 390 described in Section 3 will be registered. This registration will be 391 marked for use by message submission [I-D.gellens-submit-bis] only in 392 the registry. 394 8. Security Considerations 396 Modern SMTP submission servers often include content-based security 397 and denial-of-service defense mechanisms such as virus filtering, 398 size limits, server-generated signatures, spam filtering, etc. 399 Implementations of BURL should fetch the URL content prior to 400 application of such content-based mechanisms in order to preserve 401 their function. 403 Clients which generate unsolicited bulk email or email with viruses 404 could use this mechanism to compensate for a slow link between the 405 client and submit server. In particular, this mechanism would make 406 it feasible for a programmable cell phone or other device on a slow 407 link to become a significant source of unsolicited bulk email and/or 408 viruses. This makes it more important for submit server vendors 409 implementing BURL to have auditing and/or defenses against such 410 denial-of-service attacks including mandatory authentication, logging 411 which associates unique client identifiers with mail transactions, 412 limits on re-use of the same IMAP URL, rate limits, recipient count 413 limits and content filters. 415 Transfer of the URLAUTH [I-D.ietf-lemonade-urlauth] form of IMAP URLs 416 in the clear can expose the authorization token to network 417 eavesdroppers. Implementations which support such URLs can address 418 this issue by using a strong confidentiality protection mechanism. 419 For example, the SMTP STARTTLS [RFC3207] and the IMAP STARTTLS 420 [RFC3501] extensions in combination with a configuration setting 421 which requires their use with such IMAP URLs would address this 422 concern. 424 Use of a pre-arranged trust relationship between a submit server and 425 a specific IMAP server introduces security considerations: a 426 compromise of the submit server should not automatically compromise 427 all accounts on the IMAP server so trust relationships involving 428 super-user proxy credentials are strongly discouraged. A system 429 which requires the submit server to authenticate to the IMAP server 430 with submit credentials and subsequently requires a URLAUTH URL to 431 fetch any content addresses this concern. A trusted third party 432 model for proxy credentials such as that provided by Kerberos5 433 [RFC1510] would also suffice. 435 When a client uses SMTP STARTTLS to send a BURL command which 436 references non-public information there is a user expectation that 437 the entire message content will be treated confidentially. To 438 address this expectation, the message submission server SHOULD use 439 STARTTLS or a mechanism providing equivalent data confidentiality 440 when fetching the content referenced by that URL. 442 A legitimate user of a submit server may try to compromise other 443 accounts on the server by providing an IMAP URLAUTH URL which points 444 to a server under that user's control which is designed to undermine 445 the security of the submit server. For this reason, the IMAP client 446 code which the submit server uses must be robust with respect to 447 arbitrary input sizes (including large IMAP literals) and arbitrary 448 delays from the IMAP server. Requiring a pre-arranged trust 449 relationship between a submit server and the IMAP server also 450 addresses this concern. 452 An authorized user of the submit server could set up a fraudulent 453 IMAP server and pass a URL for that server to the submit server. The 454 submit server might then contact the fraudulent IMAP server to 455 authenticate with submit credentials and fetch content. There are 456 several ways to mitigate this potential attack. A submit server 457 which only uses submit credentials with a fixed set of trusted IMAP 458 servers will not be vulnerable to exposure of those credentials. A 459 submit server can treat the IMAP server as untrusted and include 460 defenses for buffer overflows, denial-of-service slowdowns and other 461 potential attacks. And finally, because authentication is required 462 to use BURL, it is possible to keep a secure audit trail and use that 463 to detect and punish the offending party. 465 9. References 467 9.1 Normative References 469 [RFC1652] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. 470 Crocker, "SMTP Service Extension for 8bit-MIMEtransport", 471 RFC 1652, July 1994. 473 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 474 Requirement Levels", BCP 14, RFC 2119, March 1997. 476 [RFC2192] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997. 478 [RFC2222] Myers, J., "Simple Authentication and Security Layer 479 (SASL)", RFC 2222, October 1997. 481 [RFC2554] Myers, J., "SMTP Service Extension for Authentication", 482 RFC 2554, March 1999. 484 [RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, 485 April 2001. 487 [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over 488 Transport Layer Security", RFC 3207, February 2002. 490 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 491 4rev1", RFC 3501, March 2003. 493 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 494 Resource Identifier (URI): Generic Syntax", STD 66, 495 RFC 3986, January 2005. 497 [RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 498 Specifications: ABNF", RFC 4234, October 2005. 500 [I-D.ietf-lemonade-urlauth] 501 Crispin, M., "Internet Message Access Protocol (IMAP) - 502 URLAUTH Extension", draft-ietf-lemonade-urlauth-08 (work 503 in progress), October 2005. 505 [I-D.gellens-submit-bis] 506 Gellens, R. and J. Klensin, "Message Submission for Mail", 507 draft-gellens-submit-bis-02 (work in progress), 508 April 2005. 510 9.2 Informative References 512 [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network 513 Authentication Service (V5)", RFC 1510, September 1993. 515 [RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced 516 Error Codes", RFC 2034, October 1996. 518 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 519 Extensions (MIME) Part One: Format of Internet Message 520 Bodies", RFC 2045, November 1996. 522 [RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) 523 Part Three: Message Header Extensions for Non-ASCII Text", 524 RFC 2047, November 1996. 526 [RFC2920] Freed, N., "SMTP Service Extension for Command 527 Pipelining", STD 60, RFC 2920, September 2000. 529 [RFC3030] Vaudreuil, G., "SMTP Service Extensions for Transmission 530 of Large and Binary MIME Messages", RFC 3030, 531 December 2000. 533 [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", 534 RFC 3463, January 2003. 536 [I-D.ietf-sasl-plain] 537 Zeilenga, K., "The Plain SASL Mechanism", 538 draft-ietf-sasl-plain-08 (work in progress), March 2005. 540 Author's Address 542 Chris Newman 543 Sun Microsystems 544 3401 Centrelake Dr., Suite 410 545 Ontario, CA 91761 546 US 548 Email: chris.newman@sun.com 550 Appendix A. Acknowledgments 552 This document is a product of the lemonade WG. Many thanks are due 553 to the all participants of that working group for their input. Mark 554 Crispin was instrumental in the conception of this mechanism. Thanks 555 to Randall Gellens, Alexey Melnikov, Sam Hartman, Ned Freed and Mark 556 Crispin for review comments on the document. Thanks to Ted Hardie, 557 Randall Gellens, Mark Crispin, Pete Resnick and Greg Vaudreuil for 558 extremely interesting debates comparing this proposal and 559 alternatives. Thanks to the lemonade WG chairs Eric Burger and Glenn 560 Parsons for concluding the debate at the correct time and making sure 561 this document got completed. 563 Intellectual Property Statement 565 The IETF takes no position regarding the validity or scope of any 566 Intellectual Property Rights or other rights that might be claimed to 567 pertain to the implementation or use of the technology described in 568 this document or the extent to which any license under such rights 569 might or might not be available; nor does it represent that it has 570 made any independent effort to identify any such rights. 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