rfc4468.txt

广泛使用的邮件服务器！同时


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.





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   250 2.5.0 Ok.

      The URL was successfully resolved, and the complete message data
      has been committed to persistent storage.

   250 2.6.4 MIME header conversion with loss performed

      The URL pointed to message data that included mail or MIME headers
      with 8-bit data.  This data was converted to MIME header encoding
      [RFC2047], but the submit server may not have correctly guessed
      the unlabeled character set.

7.  IANA Considerations

   The "BURL" SMTP extension as described in Section 3 has been
   registered.  This registration has been marked for use by message
   submission [RFC4409] only in the registry.

8.  Security Considerations

   Modern SMTP submission servers often include content-based security
   and denial-of-service defense mechanisms such as virus filtering,
   size limits, server-generated signatures, spam filtering, etc.
   Implementations of BURL should fetch the URL content prior to
   application of such content-based mechanisms in order to preserve
   their function.

   Clients that generate unsolicited bulk email or email with viruses
   could use this mechanism to compensate for a slow link between the
   client and submit server.  In particular, this mechanism would make
   it feasible for a programmable cell phone or other device on a slow
   link to become a significant source of unsolicited bulk email and/or
   viruses.  This makes it more important for submit server vendors
   implementing BURL to have auditing and/or defenses against such
   denial-of-service attacks including mandatory authentication, logging
   that associates unique client identifiers with mail transactions,
   limits on reuse of the same IMAP URL, rate limits, recipient count
   limits, and content filters.

   Transfer of the URLAUTH [RFC4467] form of IMAP URLs in the clear can
   expose the authorization token to network eavesdroppers.
   Implementations that support such URLs can address this issue by
   using a strong confidentiality protection mechanism.  For example,
   the SMTP STARTTLS [RFC3207] and the IMAP STARTTLS [RFC3501]
   extensions, in combination with a configuration setting that requires
   their use with such IMAP URLs, would address this concern.





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   Use of a prearranged trust relationship between a submit server and a
   specific IMAP server introduces security considerations.  A
   compromise of the submit server should not automatically compromise
   all accounts on the IMAP server, so trust relationships involving
   super-user proxy credentials are strongly discouraged.  A system that
   requires the submit server to authenticate to the IMAP server with
   submit credentials and subsequently requires a URLAUTH URL to fetch
   any content addresses this concern.  A trusted third party model for
   proxy credentials (such as that provided by Kerberos 5 [RFC4120])
   would also suffice.

   When a client uses SMTP STARTTLS to send a BURL command that
   references non-public information, there is a user expectation that
   the entire message content will be treated confidentially.  To
   address this expectation, the message submission server SHOULD use
   STARTTLS or a mechanism providing equivalent data confidentiality
   when fetching the content referenced by that URL.

   A legitimate user of a submit server may try to compromise other
   accounts on the server by providing an IMAP URLAUTH URL that points
   to a server under that user's control that is designed to undermine
   the security of the submit server.  For this reason, the IMAP client
   code that the submit server uses must be robust with respect to
   arbitrary input sizes (including large IMAP literals) and arbitrary
   delays from the IMAP server.  Requiring a prearranged trust
   relationship between a submit server and the IMAP server also
   addresses this concern.

   An authorized user of the submit server could set up a fraudulent
   IMAP server and pass a URL for that server to the submit server.  The
   submit server might then contact the fraudulent IMAP server to
   authenticate with submit credentials and fetch content.  There are
   several ways to mitigate this potential attack.  A submit server that
   only uses submit credentials with a fixed set of trusted IMAP servers
   will not be vulnerable to exposure of those credentials.  A submit
   server can treat the IMAP server as untrusted and include defenses
   for buffer overflows, denial-of-service slowdowns, and other
   potential attacks.  Finally, because authentication is required to
   use BURL, it is possible to keep a secure audit trail and use that to
   detect and punish the offending party.











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9.  References

9.1.  Normative References

   [RFC1652]     Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
                 Crocker, "SMTP Service Extension for
                 8bit-MIMEtransport", RFC 1652, July 1994.

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2192]     Newman, C., "IMAP URL Scheme", RFC 2192,
                 September 1997.

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

   [RFC2821]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
                 April 2001.

   [RFC3207]     Hoffman, P., "SMTP Service Extension for Secure SMTP
                 over Transport Layer Security", RFC 3207,
                 February 2002.

   [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.












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RFC 4468           Message Submission BURL Extension            May 2006


9.2.  Informative References

   [RFC2034]     Freed, N., "SMTP Service Extension for Returning
                 Enhanced Error Codes", RFC 2034, October 1996.

   [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.

   [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail
                 Extensions) Part Three: Message Header Extensions for
                 Non-ASCII Text", RFC 2047, November 1996.

   [RFC2920]     Freed, N., "SMTP Service Extension for Command
                 Pipelining", STD 60, RFC 2920, September 2000.

   [RFC3030]     Vaudreuil, G., "SMTP Service Extensions for
                 Transmission of Large and Binary MIME Messages",
                 RFC 3030, December 2000.

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

   [RFC4120]     Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
                 Kerberos Network Authentication Service (V5)", RFC
                 4120, July 2005.

   [SASL-PLAIN]  Zeilenga, K., "The Plain SASL Mechanism", Work in
                 Progress, March 2005.






















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Appendix A.  Acknowledgements

   This document is a product of the lemonade WG.  Many thanks are due
   to all the participants of that working group for their input.  Mark
   Crispin was instrumental in the conception of this mechanism.  Thanks
   to Randall Gellens, Alexey Melnikov, Sam Hartman, Ned Freed, Dave
   Cridland, Peter Coates, and Mark Crispin for review comments on the
   document.  Thanks to the RFC Editor for correcting the author's
   grammar mistakes.  Thanks to Ted Hardie, Randall Gellens, Mark
   Crispin, Pete Resnick, and Greg Vaudreuil for extremely interesting
   debates comparing this proposal and alternatives.  Thanks to the
   lemonade WG chairs Eric Burger and Glenn Parsons for concluding the
   debate at the correct time and making sure this document got
   completed.

Author's Address

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

   EMail: [e-mail]



























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Full Copyright Statement

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