rfc2389.txt

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   reply other than a 500 or 502 reply.

   A typical example reply to the FEAT command might be a multiline
   reply of the form:

        C> feat
        S> 211-Extensions supported:
        S>  MLST size*;create;modify*;perm;media-type
        S>  SIZE
        S>  COMPRESSION
        S>  MDTM
        S> 211 END

   The particular extensions shown here are simply examples of what may
   be defined in other places, no particular meaning should be
   attributed to them.  Recall also, that the feature names returned are
   not command names, as such, but simply indications that the server
   possesses some attribute or other.

   The order in which the features are returned is of no importance,
   server-FTP processes are not required to implement any particular
   order, or even to consistently return the same order when the command
   is repeated.




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   FTP implementations which support FEAT MUST include in the response
   to the FEAT command all properly documented FTP extensions beyond
   those commands and mechanisms described in RFC959 [1], including any
   which existed before the existence of FEAT.  That is, when a client
   receives a FEAT response from an FTP server, it can assume that the
   only extensions the server supports are those that are listed in the
   FEAT response.

   User-FTP processes should, however, be aware that there have been
   several FTP extensions developed, and in widespread use, prior to the
   adoption of this document and the FEAT command.  The effect of this
   is that an error response to the FEAT command does not necessarily
   imply that those extensions are not supported by the server-FTP
   process.  User-PIs should test for such extensions individually if an
   error response has been received to the FEAT command.

3.3. Rationale for FEAT

   While not absolutely necessary, a standard mechanism for the server-
   PI to inform the user-PI of any features and extensions supported
   will help reduce unnecessary traffic between the user-PI and server-
   PI as more extensions may be introduced in the future.  If no
   mechanism existed for this, a user-FTP process would have to try each
   extension in turn resulting in a series of exchanges between the
   user-PI and server-PI.  Apart from being possibly wasteful, this
   procedure may not always be possible, as issuing of a command just to
   determine if it is supported or not may have some effect that is not
   desired.

4. The OPTS Command

   The OPTS (options) command allows a user-PI to specify the desired
   behavior of a server-FTP process when another FTP command (the target
   command) is later issued.  The exact behavior, and syntax, will vary
   with the target command indicated, and will be specified with the
   definition of that command.  Where no OPTS behavior is defined for a
   particular command there are no options available for that command.

   Request Syntax:
        opts             = opts-cmd SP command-name
                               [ SP command-options ] CRLF
        opts-cmd         = "opts"
        command-name     = <any FTP command which allows option setting>
        command-options  = <format specified by individual FTP command>







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   Response Syntax:
        opts-response    = opts-good / opts-bad
        opts-good        = "200" SP response-message CRLF
        opts-bad         = "451" SP response-message CRLF /
                           "501" SP response-message CRLF
        response-message = *TCHAR

   An "opts-good" response (200 reply) MUST be sent when the command-
   name specified in the OPTS command is recognized, and the command-
   options, if any, are recognized, and appropriate.  An "opts-bad"
   response is sent in other cases.  A 501 reply is appropriate for any
   permanent error.  That is, for any case where simply repeating the
   command at some later time, without other changes of state, will also
   be an error.  A 451 reply should be sent where some temporary
   condition at the server, not related to the state of communications
   between user and server, prevents the command being accepted when
   issued, but where if repeated at some later time, a changed
   environment for the server-FTP process may permit the command to
   succeed.  If the OPTS command itself is not recognized, a 500 or 502
   reply will, of course, result.

   The OPTS command MUST be implemented whenever the FEAT command is
   implemented.  Because of that, there is no indication in the list of
   features returned by FEAT to indicate that the OPTS command itself is
   supported.  Neither the FEAT command, nor the OPTS command, have any
   optional functionality, thus there are no "OPTS FEAT" or "OPTS OPTS"
   commands.

5. Security Considerations

   No significant new security issues, not already present in the FTP
   protocol, are believed to have been created by this extension.
   However, this extension does provide a mechanism by which users can
   determine the capabilities of an FTP server, and from which
   additional information may be able to be deduced.  While the same
   basic information could be obtained by probing the server for the
   various commands, if the FEAT command were not provided, that method
   may reveal an attacker by logging the attempts to access various
   extension commands.  This possibility is not considered a serious
   enough threat to be worthy of any remedial action.

   The security of any additional features that might be reported by the
   FEAT command, and manipulated by the OPTS command, should be
   addressed where those features are defined.







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RFC 2389             Feature negotiation mechanism           August 1998


6. References

   [1]  Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)",
        STD 9, RFC 959, October 1985.

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

   [3]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

Acknowledgements

   This protocol extension was developed in the FTPEXT Working Group of
   the IETF, and the members of that group are all acknowledged as its
   creators.

Editors' Addresses

   Paul Hethmon
   Hethmon Brothers
   2305 Chukar Road
   Knoxville, TN 37923 USA

   Phone: [phone]
   Email: [e-mail]


   Robert Elz
   University of Melbourne
   Department of Computer Science
   Parkville, Vic   3052
   Australia

   Email: [e-mail]
















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Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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