|Title||Feature negotiation mechanism for the File Transfer Protocol
Hethmon, R. Elz
Network Working Group P. Hethmon
Request for Comments: 2389 Hethmon Brothers
See Also: 959 R. Elz
Category: Standards Track University of Melbourne
Feature negotiation mechanism for the File Transfer Protocol
Status 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.
The File Transfer Protocol is, from time to time, extended with new
commands, or facilities. Implementations of the FTP protocol cannot
be assumed to all immediately implement all newly defined mechanisms.
This document provides a mechanism by which clients of the FTP
protocol can discover which new features are supported by a
particular FTP server.
Table of Contents
Abstract ................................................ 1
1 Introduction ............................................ 2
2 Document Conventions .................................... 2
2.1 Basic Tokens ............................................ 3
2.2 Server Replies .......................................... 3
3 Knowledge of Extra Capabilities - the FEAT Command ...... 3
3.1 Feature (FEAT) Command Syntax ........................... 4
3.2 FEAT Command Responses .................................. 4
3.3 Rationale for FEAT ...................................... 6
4 The OPTS Command ........................................ 6
5 Security Considerations ................................. 7
6 References .............................................. 8
Acknowledgements ........................................ 8
Editors' Addresses ...................................... 8
Full Copyright Statement ................................ 9
This document amends the File Transfer Protocol (FTP) . Two new
commands are added: "FEAT" and "OPTS".
These commands allow a client to discover which optional commands a
server supports, and how they are supported, and to select among
various options that any FTP command may support.
2. Document Conventions
This document makes use of the document conventions defined in BCP14
. That provides the interpretation of some capitalized words like
MUST, SHOULD, etc.
Terms defined in  will be used here as defined there. These
include ASCII, reply, server-FTP process, user-FTP process, server-
PI, user-PI, and user.
Syntax required is defined using the Augmented BNF defined in .
Some general ABNF definitions are required throughout the document,
those will be defined here. At first reading, it may be wise to
simply recall that these definitions exist here, and skip to the next
2.1. Basic Tokens
This document imports the definitions given in Appendix A of .
There definitions will be found for basic ABNF elements like ALPHA,
DIGIT, VCHAR, SP, etc. To that, the following terms are added for
use in this document.
TCHAR = VCHAR / SP / HTAB ; visible plus white space
The TCHAR type, and VCHAR from , give basic character types from
varying sub-sets of the ASCII character set for use in various
commands and responses.
error-response = error-code SP *TCHAR CRLF
error-code = ("4" / "5") 2DIGIT
Note that in ABNF, strings literals are case insensitive. That
convention is preserved in this document. However note that ALPHA,
in particular, is case sensitive, as are VCHAR and TCHAR.
2.2. Server Replies
Section 4.2 of  defines the format and meaning of replies by the
server-PI to FTP commands from the user-PI. Those reply conventions
are used here without change. Implementors should note that the ABNF
syntax (which was not used in ) in this document, and other FTP
related documents, sometimes shows replies using the one line format.
Unless otherwise explicitly stated, that is not intended to imply
that multi-line responses are not permitted. Implementors should
assume that, unless stated to the contrary, any reply to any FTP
command (including QUIT) may be of the multiline format described in
Throughout this document, replies will be identified by the three
digit code that is their first element. Thus the term "500 Reply"
means a reply from the server-PI using the three digit code "500".
3. Knowledge of Extra Capabilities - the FEAT Command
It is not to be expected that all servers will necessarily support
all of the new commands defined in all future amendments to the FTP
protocol. In order to permit clients to determine which new commands
are supported by a particular server, without trying each possible
command, one new command is added to the FTP command repertoire.
This command requests the server to list all extension commands, or
extended mechanisms, that it supports. That is, all defined and
specified commands and features not defined in , or this document,
must be included in the FEAT command output in the form specified in
the document that defines the extension.
User-FTP PIs must expect to see, in FEAT command responses, unknown
features listed. This is not an error, and simply indicates that the
server-FTP implementor has seen, and implemented, the specification
of a new feature that is unknown to the user-FTP.
3.1. Feature (FEAT) Command Syntax
feat = "Feat" CRLF
The FEAT command consists solely of the word "FEAT". It has no
parameters or arguments.
3.2. FEAT Command Responses
Where a server-FTP process does not support the FEAT command, it will
respond to the FEAT command with a 500 or 502 reply. This is simply
the normal "unrecognized command" reply that any unknown command
would elicit. Errors in the command syntax, such as giving
parameters, will result in a 501 reply.
Server-FTP processes that recognize the FEAT command, but implement
no extended features, and therefore have nothing to report, SHOULD
respond with the "no-features" 211 reply. However, as this case is
practically indistinguishable from a server-FTP that does not
recognize the FEAT command, a 500 or 502 reply MAY also be used. The
"no-features" reply MUST NOT use the multi-line response format,
exactly one response line is required and permitted.
Replies to the FEAT command MUST comply with the following syntax.
Text on the first line of the reply is free form, and not
interpreted, and has no practical use, as this text is not expected
to be revealed to end users. The syntax of other reply lines is
precisely defined, and if present, MUST be exactly as specified.
feat-response = error-response / no-features / feature-listing
no-features = "211" SP *TCHAR CRLF
feature-listing = "211-" *TCHAR CRLF
1*( SP feature CRLF )
"211 End" CRLF
feature = feature-label [ SP feature-parms ]
feature-label = 1*VCHAR
feature-parms = 1*TCHAR
Note that each feature line in the feature-listing begins with a
single space. That space is not optional, nor does it indicate
general white space. This space guarantees that the feature line can
never be misinterpreted as the end of the feature-listing, but is
required even where there is no possibility of ambiguity.
Each extension supported must be listed on a separate line to
facilitate the possible inclusion of parameters supported by each
extension command. The feature-label to be used in the response to
the FEAT command will be specified as each new feature is added to
the FTP command set. Often it will be the name of a new command
added, however this is not required. In fact it is not required that
a new feature actually add a new command. Any parameters included
are to be specified with the definition of the command concerned.
That specification shall also specify how any parameters present are
to be interpreted.
The feature-label and feature-parms are nominally case sensitive,
however the definitions of specific labels and parameters specify the
precise interpretation, and it is to be expected that those
definitions will usually specify the label and parameters in a case
independent manner. Where this is done, implementations are
recommended to use upper case letters when transmitting the feature
The FEAT command itself is not included in the list of features
supported, support for the FEAT command is indicated by return of a
reply other than a 500 or 502 reply.
A typical example reply to the FEAT command might be a multiline
reply of the form:
S> 211-Extensions supported:
S> MLST size*;create;modify*;perm;media-type
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
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 , 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
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
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.
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>
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"
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.
 Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)",
STD 9, RFC 959, October 1985.
 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
 Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
This protocol extension was developed in the FTPEXT Working Group of
the IETF, and the members of that group are all acknowledged as its
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