FTP Command and Extension Registry

Abstract

Every version of the FTP specification has added a few new commands, with the early ones summarized in RFC 959. RFC 2389 established a mechanism for specifying and negotiating FTP extensions. The number of extensions, both those supported by the mechanism and some that are not, continues to increase. An IANA registry of FTP Command and Feature names is established to reduce the likelihood of conflict of names and the consequent ambiguity. This specification establishes that registry.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5797.

Copyright Notice

Copyright © 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Registry Definition .............................................2
      2.1. Registry Name ..............................................2
      2.2. Registry Format ............................................2
      2.3. Criteria for Registration ..................................4
      2.4. Base FTP Commands ..........................................5
      2.5. Obsolete Commands ..........................................5
   3. Initial Contents of Registry ....................................6
   4. Acknowledgments .................................................8
   5. IANA Considerations .............................................9
   6. Security Considerations .........................................9
   7. References ......................................................9
      7.1. Normative References .......................................9
      7.2. Informative References .....................................9

1. Introduction

Every version of the FTP specification has added a few new commands, with the early ones summarized in RFC 959 [RFC0959]. RFC 2389 [RFC2389] established a mechanism for specifying and negotiating extensions to the FTP protocol specified in RFC 959, by means of "FEAT Strings" identifying extensions supported by the FTP server, and sent in response to a "FEAT" command. The number of extensions continues to grow, not all of them supported by FEAT. An IANA registry is established to reduce the likelihood of a conflict of names and the consequent ambiguity and to encourage the sharing of information. This specification establishes that registry.

2. Registry Definition

2.1. Registry Name

The name of this registry is "FTP Commands and Extensions".

2.2. Registry Format

As specified in this RFC, IANA has established a registry for FTP commands and extensions. Registration requests and registry entries should include the following:

Command Name - The FTP command, either new or modified, used in the

extension or with which the extension is used.

Following the long-standing practice to capitalize command names in specification documents for FTP, the command names are entered in all uppercase. For extensions amending the operation of a command, a plus sign ("+") is appended to the command name. However, if an extension affects the overall command parameter handling and/or transaction processing, instead of being bound to one command (or a small number of commands), the string "-N/A-" is entered.

It is intended to have the registry entries ordered by ascending ASCII collation order of this column (including the "+" suffix if present).

Extension name - The name of the extension.

FTP extensions predating RFC 2389 [RFC2389], and some extensions published after it, did not specify a keyword to identify the extension in a FEAT response. Some later specifications established FEAT strings with the respective command names as their keywords. In order to provide for keywords for future specifications in such cases, this document establishes 'placeholder' keywords to reserve reasonable feature names for future standardization. Similarly, placeholder keywords are used for the basic FTP commands specified in RFC 959 [RFC0959] and those of its predecessors that are still in use. These placeholder keywords are placed in the registry for convenience; it is not intended that they be returned in FEAT responses. To compensate for this idiosyncrasy, the column in the registry is entitled "FEAT Code", and to clearly distinguish between the two cases, defined FEAT keywords codes are listed in all uppercase, whereas placeholder keywords (henceforth called "pseudo FEAT codes") are listed in lowercase. Future specifications are allowed to "upgrade" a placeholder to a true keyword unless it is specifically declared 'immutable' below, but otherwise IANA maintains uniqueness of feature names (FEAT codes) based on case- insensitive comparison.

Description - A brief description of the extension and, where

appropriate, the command.

FEAT String - (optional in registration requests to IANA)

The string expected to be included in the response to the FEAT command [RFC2389] if the extension is supported.

In many cases, the FEAT string required to identify an extension only consists of the "FEAT Code", making this item redundant. Therefore, this item should only be specified if it is intended to register a FEAT string that contains mandatory elements other than the "FEAT Code" itself.

Due to space restrictions, and to allow registrants to provide additional information, IANA should present these registration items (if given) in numbered footnotes to the table, not in an additional table column.

Command Type - The type (or 'kind') of the command.

Section 4.1 of RFC 959 [RFC0959] introduced a subdivision of FTP commands into three types: Access control, transfer Parameter {setting}, and Service {execution}. For clarity, and as a service to the user of the registry, this subdivision is extended to all registered FTP commands, using the characteristic initial of the type, 'a', 'p', or 's', respectively, filed in the registry column titled "type"; combinations are allowed, e.g., 'p/s'.

Conformance Requirements - The support expectation for the command.

RFC 959 specifies mandatory-to-implement commands and optional commands. This classification is carried over to all registered commands, using a column titled "conf" carrying a single character -- either 'm' or 'o', for "mandatory" and "optional", respectively. Similarly, obsoleted or historic entries are left in the registry to avoid conflicts with deployed implementations, and these entries are marked with 'h' (for "historic"). Beyond the initial registrations, Standards Action [RFC5226] is needed to register new "mandatory" entries or to move such entries to "historic".

Reference - A reference to an RFC or other definition of the

extension and/or to implementations supporting it (see the next section).

2.3. Criteria for Registration

This registry is primarily intended to avoid conflicting uses of the same extension names and command keywords for different purposes, not to demonstrate that an extension is somehow "approved". The "Expert Review" method will be used, but the designated expert is expected to check only that at least one of the two criteria that follow are met.

1. The extension is documented in a permanent and readily available public specification (this is the same as the "Specification Required" registration policy defined in RFC 5226 [RFC5226]).

2. The extension is actually implemented in FTP client and server systems that are generally available (not necessarily either free or unencumbered, but available).

For an extension or command to be marked "mandatory" ('m' in the "conf" column), an IETF Standards Track specification is required. An IESG Standards Action is allowed to direct IANA to change the Conformance Requirements listed for any entry.

2.4. Base FTP Commands

The following commands are part of the base FTP specification [RFC0959] and are listed in the registry with the immutable pseudo FEAT code "base".

Mandatory commands:

      ABOR, ACCT, ALLO, APPE, CWD, DELE, HELP, LIST, MODE, NLST, NOOP,
      PASS, PASV, PORT, QUIT, REIN, REST, RETR, RNFR, RNTO, SITE, STAT,
      STOR, STRU, TYPE, USER

Optional commands:

      CDUP, MKD, PWD, RMD, SMNT, STOU, SYST

Note: STD 3 [RFC1123] clarified and updated the status and implementation requirements of these standard FTP commands, and it contains important complementary information for the following commands:

      LIST, NLST, PASV, REST, SITE, STOU

2.5. Obsolete Commands

The following commands were specified as experimental in an extension to an early version of the FTP specification [RFC0775] but later deprecated by RFC 1123 [RFC1123], because Standard FTP [RFC0959] specifies their standard successors. They are listed in the registry with the immutable pseudo FEAT code "hist".

      XCUP, XCWD, XMKD, XPWD, XRMD
   
   Implementation note:  Deployed FTP clients still make use of the
      deprecated commands and most FTP servers support them as aliases
      for the standard commands.

The following commands were specified as part of the "FOOBAR" IPng effort in RFC 1545 [RFC1545] and, later, RFC 1639 [RFC1639] and are now obsolete. They are listed in the registry with the immutable pseudo FEAT code "hist".

      LPRT, LPSV

3. Initial Contents of Registry

As a service to users of the registry and the authors of existing specifications, all FTP commands and features published in RFCs after STD 3 [RFC1123] and up to the time of this writing were included in the registry upon creation.

The following pseudo FEAT codes have been assigned, according to Section 2:

      base - FTP standard commands [RFC0959]
      hist - Historic experimental commands [RFC0775], [RFC1639]
      secu - FTP Security Extensions [RFC2228]
      feat - FTP Feature Negotiation [RFC2389]
      nat6 - FTP Extensions for NAT/IPv6 [RFC2428]
   
   +-------+------+-------------------+------+------+------------------+
   | cmd   | FEAT | description       | type | conf | RFC#s/References |
   |       | Code |                   |      |      | and Notes        |
   +-------+------+-------------------+------+------+------------------+
   | ABOR  | base | Abort             | s    | m    | 959              |
   | ACCT  | base | Account           | a    | m    | 959              |
   | ADAT  | secu | Authentication/   | a    | o    | 2228, 2773, 4217 |
   |       |      | Security Data     |      |      |                  |
   | ALLO  | base | Allocate          | s    | m    | 959              |
   | APPE  | base | Append (with      | s    | m    | 959              |
   |       |      | create)           |      |      |                  |
   | AUTH  | secu | Authentication/   | a    | o    | 2228             |
   |       |      | Security          |      |      |                  |
   |       |      | Mechanism         |      |      |                  |
   | AUTH+ | AUTH | Authentication/   | a    | o    | 2773, 4217 #2    |
   |       |      | Security          |      |      |                  |
   |       |      | Mechanism         |      |      |                  |
   | CCC   | secu | Clear Command     | a    | o    | 2228             |
   |       |      | Channel           |      |      |                  |
   | CDUP  | base | Change to Parent  | a    | o    | 959              |
   |       |      | Directory         |      |      |                  |
   | CONF  | secu | Confidentiality   | a    | o    | 2228             |
   |       |      | Protected Command |      |      |                  |
   | CWD   | base | Change Working    | a    | m    | 959              |
   |       |      | Directory         |      |      |                  |
   | DELE  | base | Delete File       | s    | m    | 959              |
   | ENC   | secu | Privacy Protected | a    | o    | 2228, 2773, 4217 |
   |       |      | Command           |      |      |                  |
   | EPRT  | nat6 | Extended Port     | p    | o    | 2428             |
   | EPSV  | nat6 | Extended Passive  | p    | o    | 2428             |
   |       |      | Mode              |      |      |                  |
   | FEAT  | feat | Feature           | a    | m #1 | 2389             |
   |       |      | Negotiation       |      |      |                  |
   | HELP  | base | Help              | s    | m    | 959              |
   | LANG  | UTF8 | Language (for     | p    | o    | 2640             |
   |       |      | Server Messages)  |      |      |                  |
   | LIST  | base | List              | s    | m    | 959, 1123        |
   | LPRT  | hist | Data Port         | p    | h    | 1545, 1639       |
   |       |      | {FOOBAR}          |      |      |                  |
   | LPSV  | hist | Passive Mode      | p    | h    | 1545, 1639       |
   |       |      | {FOOBAR}          |      |      |                  |
   | MDTM  | MDTM | File Modification | s    | o    | 3659             |
   |       |      | Time              |      |      |                  |
   | MIC   | secu | Integrity         | a    | o    | 2228, 2773, 4217 |
   |       |      | Protected Command |      |      |                  |
   | MKD   | base | Make Directory    | s    | o    | 959              |
   | MLSD  | MLST | List Directory    | s    | o    | 3659             |
   |       |      | (for machine)     |      |      |                  |
   | MLST  | MLST | List Single       | s    | o    | 3659             |
   |       |      | Object            |      |      |                  |
   | MODE  | base | Transfer Mode     | p    | m    | 959              |
   | NLST  | base | Name List         | s    | m    | 959, 1123        |
   | NOOP  | base | No-Op             | s    | m    | 959              |
   | OPTS  | feat | Options           | p    | m #1 | 2389             |
   | PASS  | base | Password          | a    | m    | 959              |
   | PASV  | base | Passive Mode      | p    | m    | 959, 1123        |
   | PBSZ  | secu | Protection Buffer | p    | o    | 2228             |
   |       |      | Size              |      |      |                  |
   | PBSZ+ | PBSZ | Protection Buffer | p    | o    | 4217             |
   |       |      | Size              |      |      |                  |
   | PORT  | base | Data Port         | p    | m    | 959              |
   | PROT  | secu | Data Channel      | p    | o    | 2228             |
   |       |      | Protection Level  |      |      |                  |
   | PROT+ | PROT | Data Channel      | p    | o    | 4217             |
   |       |      | Protection Level  |      |      |                  |
   | PWD   | base | Print Directory   | s    | o    | 959              |
   | QUIT  | base | Logout            | a    | m    | 959              |
   | REIN  | base | Reinitialize      | a    | m    | 959              |
   | REST  | base | Restart           | s/p  | m    | 959, 1123        |
   | REST+ | REST | Restart (for      | s/p  | m    | 3659 #3          |
   |       |      | STREAM mode)      |      |      |                  |
   | RETR  | base | Retrieve          | s    | m    | 959              |
   | RMD   | base | Remove Directory  | s    | o    | 959              |
   | RNFR  | base | Rename From       | s/p  | m    | 959              |
   | RNTO  | base | Rename From       | s    | m    | 959              |
   | SITE  | base | Site Parameters   | s    | m    | 959, 1123        |
   | SIZE  | SIZE | File Size         | s    | o    | 3659             |
   | SMNT  | base | Structure Mount   | a    | o    | 959              |
   | STAT  | base | Status            | s    | m    | 959              |
   | STOR  | base | Store             | s    | m    | 959              |
   | STOU  | base | Store Unique      | a    | o    | 959, 1123        |
   | STRU  | base | File Structure    | p    | m    | 959              |
   | SYST  | base | System            | s    | o    | 959              |
   | TYPE  | base | Representation    | p    | m    | 959 #4           |
   |       |      | Type              |      |      |                  |
   | USER  | base | User Name         | a    | m    | 959              |
   | XCUP  | hist | {precursor for    | s    | h    | 775, 1123        |
   |       |      | CDUP}             |      |      |                  |
   | XCWD  | hist | {precursor for    | s    | h    | 775, 1123        |
   |       |      | CWD}              |      |      |                  |
   | XMKD  | hist | {precursor for    | s    | h    | 775, 1123        |
   |       |      | MKD}              |      |      |                  |
   | XPWD  | hist | {precursor for    | s    | h    | 775, 1123        |
   |       |      | PWD}              |      |      |                  |
   | XRMD  | hist | {precursor for    | s    | h    | 775, 1123        |
   |       |      | RMD}              |      |      |                  |
   | -N/A- | TVFS | Trivial Virtual   | p    | o    | 3659             |
   |       |      | File Store        |      |      |                  |
   +-------+------+-------------------+------+------+------------------+

Table 1

   Notes:
   #1 While an IETF Standards Action would be required to make the FEAT
      mechanism [RFC2389] mandatory, implementation of that extension
      mechanism is clearly required in conjunction with any extension or
      feature that depends on it.
   #2 FEAT String for [RFC4217]: AUTH TLS
      FEAT String for [RFC2773]: AUTH KEA-SKIPJACK
   #3 FEAT String: REST STREAM
   #4 FEAT String: TYPE {semicolon-separated list of supported types}

4. Acknowledgments

Any work to update or extend FTP depends on the base specification in RFC 959. The contributions of its editors, Jon Postel and Joyce Reynolds, are gratefully acknowledged. The option-negotiation mechanism specified in RFC 2389 (and the accumulation of features that followed it) made this registry relevant; the authors of those documents are acknowledged as well.

Barry Leiba and Alexey Melnikov made several suggestions about earlier versions of this document, most of which have been incorporated.

Anthony Bryan spotted a few typographical errors.

Scott Bradner suggested a clarification to the "Expert Review" text.

The authors appreciate the comments and support for this work received from FTP implementers and many IETF participants. Comments from the IESG helped to shape this document and registry to improve its utility.

5. IANA Considerations

IANA has established the registry described in Section 2 using the initial content specified in Section 3 and including the body of Sections 2.4 and 2.5 as explanatory text in the preface of the registry.

New entries should be added to this registry when extensions to FTP are approved or defined in RFCs or when extensions that are already in use and well-documented are identified. In other words, the requirement for registration is a slightly relaxed version of "Specification Required" [RFC5226] with Expert Review. See Section 2.3 for specifics and exceptions.

6. Security Considerations

The creation of this registry provides improved documentation and protection against interoperability problems. It introduces no new security issues.

7. References

7.1. Normative References

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.
   
   [RFC2389]  Hethmon, P. and R. Elz, "Feature negotiation mechanism for
              the File Transfer Protocol", RFC 2389, August 1998.
   
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

7.2. Informative References

   [RFC0775]  Mankins, D., Franklin, D., and A. Owen, "Directory
              oriented FTP commands", RFC 775, December 1980.
   
   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.
   
   [RFC1545]  Piscitello, D., "FTP Operation Over Big Address Records
              (FOOBAR)", RFC 1545, November 1993.
   
   [RFC1639]  Piscitello, D., "FTP Operation Over Big Address Records
              (FOOBAR)", RFC 1639, June 1994.
   
   [RFC2228]  Horowitz, M., "FTP Security Extensions", RFC 2228,
              October 1997.
   
   [RFC2428]  Allman, M., Ostermann, S., and C. Metz, "FTP Extensions
              for IPv6 and NATs", RFC 2428, September 1998.
   
   [RFC2773]  Housley, R. and P. Yee, "Encryption using KEA and
              SKIPJACK", RFC 2773, February 2000.
   
   [RFC4217]  Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
              October 2005.

Authors' Addresses

   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140
   USA
   
   Phone: +1 617 245 1457
   EMail: john+ietf@jck.com
   
   Alfred Hoenes
   TR-Sys
   Gerlinger Str. 12
   Ditzingen  D-71254
   Germany
   
   EMail: ah@TR-Sys.de