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Versions: (draft-twine-ftpmd5) 00 01 02 03 04 05 06 07 08 draft-ietf-ftpext2-hash

Network Working Group                                           A. Bryan
Internet-Draft                                                  T. Kosse
Intended status: Standards Track                             D. Stenberg
Expires: April 28, 2011                                 October 25, 2010


      File Transfer Protocol HASH Command for Cryptographic Hashes
                        draft-bryan-ftp-hash-08

Abstract

   The File Transfer Protocol does not offer any method to verify the
   integrity of a transferred file, nor can two files be compared
   against each other without actually transferring them first.
   Cryptographic hashes are a possible solution to this problem.  In the
   past, several attempts have been made to add commands to obtain
   checksums and hashes, however none have been formally specified,
   leading to non-interoperability and confusion.  To solve these
   issues, this document specifies a new FTP command to be used by
   clients to request cryptographic hashes of files.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 28, 2011.

Copyright Notice

   Copyright (c) 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



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Document Conventions . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Basic Tokens . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Server Replies . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The HASH Command (HASH)  . . . . . . . . . . . . . . . . . . .  5
     3.1.  FEAT Command Response for HASH Command . . . . . . . . . .  6
     3.2.  OPTS Parameters for HASH . . . . . . . . . . . . . . . . .  6
     3.3.  User-PI usage of HASH  . . . . . . . . . . . . . . . . . .  7
     3.4.  HASH Command Errors  . . . . . . . . . . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   5.  Implementation Requirements  . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 11
   Appendix B.  List of Non-standard Cryptographic Hash or
                Checksum Commands and Implementations . . . . . . . . 11
   Appendix C.  Document History  . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15






















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1.  Introduction

   The File Transfer Protocol [RFC0959] does not offer any method to
   verify the integrity of a transferred file, nor can two files be
   compared against each other without actually transferring them first.
   Cryptographic hashes are a possible solution to this problem.  In the
   past, several attempts have been made to add commands to obtain
   checksums and hashes, however none have been formally specified,
   leading to non-interoperability and confusion.  (See Appendix B for
   more information).  To solve these issues, this document specifies a
   new FTP command to be used by clients to request cryptographic hashes
   of files.  HTTP has a similar feature named Instance Digests
   [RFC3230] which allows a client to request the cryptographic hash of
   a file.

   [[ Discussion of this draft should take place on ftpext@ietf.org (or
   apps-discuss@ietf.org if necessary). ]]

1.1.  Example

   Example of HASH client request:

      C> HASH filename.ext

   HASH server response with Positive Completion code, the currently
   selected HASH algorithm, the requested hash of the file, and the
   pathname of the file:

      S> 213 SHA-1 80bc95fd391772fa61c91ed68567f0980bb45fd9 filename.ext


2.  Document Conventions

   This specification describes conformance of File Transfer Protocol
   Extension for cryptographic hashes.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119], as
   scoped to those conformance targets.

   This document also uses notation defined in STD 9, [RFC0959].  In
   particular, the terms or commands "reply", "user", "file", "FTP
   commands", "user-PI" (user protocol interpreter), "server-FTP
   process", "server-PI", "mode", "type", "STOR", "RETR", and "ASCII",
   are all used here as defined there.  The term "pathname" is used as
   defined in Section 2.2 of [RFC3659].




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   In the examples of FTP dialogs presented in this document, lines that
   begin "C> " were sent over the control connection from the user-PI to
   the server-PI, and lines that begin "S> " were sent over the control
   connection from the server-PI to the user-PI.  In all cases, the
   prefixes shown above, including the one space, have been added for
   the purposes of this document, and are not a part of the data
   exchanged between client and server.

   Syntax required is defined using the Augmented BNF defined in
   [RFC5234].

2.1.  Basic Tokens

   This document imports the core definitions given in Appendix B of
   [RFC5234].  There definitions will be found for basic ABNF elements
   like ALPHA, DIGIT, SP, etc.  To that, the following term is added for
   use in this document.


      TCHAR = VCHAR / SP / HTAB    ; visible plus white space

   The VCHAR (from [RFC5234]) and TCHAR rules give basic character types
   from varying sub-sets of the ASCII character set for use in various
   commands and responses.

   Note that in ABNF, string literals are case insensitive.  That
   convention is preserved in this document, and implies that FTP
   commands and parameters that are added by this specification have
   values that can be represented in any case.  That is, "HASH" is the
   same as "hash", "Hash", "HaSh", etc., and "ftp.example.com" is the
   same as "Ftp.Example.Com", "fTp.eXample.cOm", etc.

2.2.  Server Replies

   Section 4.2 of [RFC0959] 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.


      error-response = error-code SP *TCHAR CRLF
      error-code     = ("4" / "5") 2DIGIT

   Implementers should note that the ABNF syntax (which was not used in
   [RFC0959]) used 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.  Implementers should assume that, unless
   stated to the contrary, any reply to any FTP command (including QUIT)



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   can be of the multi-line format described in [RFC0959].

   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.  The HASH Command (HASH)

   A new command "HASH" is added to the FTP command set to allow the
   client to request the cryptographic hash of a file from a server-FTP
   process.

   The syntax for the HASH command is:


      hash-command = "HASH" SP <pathname>

   As with all FTP commands, the "HASH" command word is case
   independent, and MAY be specified in any character case desired.

   The HASH command keyword MUST be followed by a single space (ASCII
   32) followed by the pathname.

   The pathname argument should reference the same file as other file
   based commands such as STOR or RETR which the same argument would
   reference.  The pathname argument MUST represent a file path, not a
   directory path.

   The text returned in response to the HASH command MUST be:


      hash-response = hash-ok / error-response
      hash-ok       = "213" SP hashname SP 1*HEXDIGIT SP <pathname> CRLF

   All hash values MUST be encoded in lowercase hexadecimal format.

   The HASH command uses the currently selected hash algorithm.  The
   currently selected hash algorithm can be determined with FEAT or OPTS
   HASH, and changed with OPTS HASH.

   The HASH command is meant to be used for files transmitted in Image
   type mode (TYPE I) and Stream transfer mode (MODE S).  The returned
   hash MUST be calculated as if a client were to download the full file
   using TYPE I and MODE S and were to calculate the hash on the
   received octet data.  In other words, if a client were to download a
   full file using TYPE I and MODE S and were to calculate the hash on
   the received octet data, it would be identical to the hash returned



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   by HASH.

3.1.  FEAT Command Response for HASH Command

   When replying to the FEAT command [RFC2389], a server-FTP process
   that supports the HASH command MUST include a feature line indicating
   that the HASH command is supported, along with a list of all
   supported hash algorithms in a semicolon separated list.  The hash
   algorithm that is currently selected MUST be marked with an asterisk.
   The order of hash algorithms is insignificant.  This command word is
   case insensitive, and MAY be sent in any mixture of upper or lower
   case, however it SHOULD be sent in upper case.  That is, the response
   SHOULD be:


      C> FEAT
      S> 211-Extensions supported:
      S>  ...
      S>  HASH SHA-256;SHA-512;SHA-1*;MD5
      S>  ...
      S> 211 END

   The ellipses indicate place holders where other features may be
   included, and are not required.  The one-space indentation of the
   feature lines is mandatory [RFC2389].

   The IANA registry named "Hash Function Textual Names" defines values
   for hash algorithms.  Hash names SHOULD be presented in uppercase,
   but comparisons should be case-insensitive, e.g.  MD5, md5, Md5 are
   all the same.


      hash-feat = SP "HASH" SP hashlist CRLF
      hashlist  = 1*( hashname ["*"] ";" )
      hashname  = 1*( hchar )
      hchar     = ALPHA / DIGIT / "-" / "_" / "/" / "." / ","

3.2.  OPTS Parameters for HASH

   To query the current hash algorithm and to change it, the OPTS
   command as defined in [RFC2389] is used with HASH as the first
   argument.

   If no second argument is passed, OPTS HASH simply returns the
   currently selected hash algorithm.






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      C> OPTS HASH
      S> 200 SHA-1

   To change the algorithm, a valid hash algorithm MUST be given as
   second argument.  A list of valid hash algorithms is available via
   the FEAT command.  If the command is successful, all future calls to
   HASH until the next successful OPTS HASH command or until the session
   is reinitialized (REIN) will use the selected hash algorithm.


      C> OPTS HASH SHA-512
      S> 200 SHA-512

   Requesting unknown or unsupported algorithms produces an error
   response.


      C> OPTS HASH CRC-37
      S> 501 Unknown algorithm, current selection not changed

   The syntax for OPTS HASH:


      hashopts-cmd      = "OPTS HASH" [ SP hashname ] CRLF
      hashopts-response = hashopts-ok / error-response
      hashopts-ok       = "200" SP hashname CRLF

3.3.  User-PI usage of HASH

   The user-PI issues the FEAT command to query the server-PI about
   which algorithm is currently selected.  This also reveals the other
   algorithms that the server supports.  In this example, the SHA-1
   algorithm is currently selected, as indicated by the asterisk.


      C> FEAT
      S> 211-Extensions supported:
      S>  ...
      S>  HASH SHA-256;SHA-512;SHA-1*;MD5
      S>  ...
      S> 211 END

   OPTS HASH is an alternative method for the user-PI to query the
   server-PI about which algorithm is currently selected.







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      C> OPTS HASH
      S> 200 SHA-1

   In this example, we wish to select SHA-256, a different algorithm.


      C> OPTS HASH SHA-256
      S> 200 SHA-256

   The user-PI requests the cryptographic hash of a file with HASH
   command.  Server-PI replies with the Positive Completion code, the
   currently selected HASH algorithm, the requested hash of the file,
   and the pathname of the file.


      C> HASH filename.ext
      S> 213 SHA-256 f0ad929cd259957e160ea442eb80986b5f... filename.ext

   Client downloads file.  Client hashes the downloaded file and
   compares its hash to the hash obtained from the server.  The HASH
   command could also be used to verify that an uploaded file has the
   same hash as the local file.

3.4.  HASH Command Errors

   The server-PI SHOULD reply with a 450 reply if the server is busy,
   e.g. already hashing other files yet inviting the client to retry in
   the future.

   The server-PI SHOULD reply with a 500 reply if the HASH command is
   unrecognized or unimplemented.

   The server-PI SHOULD reply with a 501 reply to the OPTS HASH command
   if the user-PI has requested an unknown or unsupported algorithm.

   The server-PI SHOULD reply with a 550 reply if the HASH command is
   used on a file that can not be found.

   The server-PI SHOULD reply with a 551 reply if the server-PI can not
   calculate the hash of a file because it is unable to deliver the file
   with TYPE I and MODE S.

   The server-PI SHOULD reply with a 552 reply if the user is not
   allowed to use the HASH command.

   The server-PI SHOULD reply with a 553 reply if the user requests the
   HASH of a directory, which is not allowed.




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   The server-PI SHOULD reply with a 556 reply if the HASH command is
   used on a file that cannot be processed for policy reasons.  (For
   example, if the file size exceeds the server's hashing policy.)


4.  IANA Considerations

   This new command is added to the "FTP Commands and Extensions"
   registry created by [RFC5797].

   Command Name: HASH

   Description: Cryptographic Hash of a file

   FEAT String: HASH

   Command Type: Service execution

   Conformance Requirements: Optional

   Reference: This specification


5.  Implementation Requirements

   All conforming implementations MUST at least support the SHA-1
   algorithm [FIPS-180-3].  Implementations SHOULD NOT make any
   algorithm the default that is known to be weaker than SHA-1.  Support
   for any additional algorithms is OPTIONAL.


6.  Security Considerations

   The server MUST only allow the HASH command to be processable for
   files which the logged in user has a right to access.

   Implementing the HASH command may impose a considerable load on the
   server, which could lead to denial-of-service attacks.  Servers have,
   however, implemented this for many years, without significant
   reported difficulties.  On an affected server a malicious user could,
   for example, continuously send HASH commands over multiple
   connections and thus consume most of the FTP server's resources,
   leaving little room for other operations.  To mitigate this risk, a
   server MAY cache the calculated hashes so that the hash of a file is
   only calculated once even if multiple hash requests are sent for that
   file, provided it updates or invalidates the cached hash when the
   content of the corresponding file changes.  A server may refuse to
   process a HASH command for many reasons, one of which may be a



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   suspected denial-of-service attack.  A client MUST be able to
   understand that refusal to process HASH commands may be transient (if
   indicated by a 4yz response) and MAY be honoured later if the server
   so decides.  A client MUST allow that a HASH command might take a
   reasonably long time to complete.

   Server operators might wish to allow the HASH command but restrict
   its use to certain files, for example, if the file size exceeds the
   server's hashing policy.  A client MUST be able to understand that
   refusal to process HASH commands may be permanent (if indicated by a
   556 response) and will not be honoured later.

   In addition, the HASH command can be used to draw conclusions about
   the contents of a file.  If the hash of a file on some server matches
   the hash of some known file, then both files are likely identical.
   By uploading a file, running HASH against it and running HASH against
   another file location, the client could infer some filesystem
   deployment information (e.g. that there is a logical link between a
   pair of directories in the tree).  This is probably not an issue if
   the user has access to both branches of the directory tree, there is
   the possibility that this information is exposable.  To prevent this
   scenario it suffices to limit use of the HASH command to users who
   would already be able to download the file.

   This mechanism simply allows the FTP protocol to expose HASH values
   of files, using the currently chosen mechanism, accessible to the
   server by the client.  The suitability or otherwise of a specific
   hash algorithm for a specific purpose is an implementation decision.


7.  References

7.1.  Normative References

   [FIPS-180-3]
              National Institute of Standards and Technology (NIST),
              "Secure Hash Standard (SHS)", FIPS PUB 180-3,
              October 2008.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 0959, October 1985.

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

   [RFC2389]  Hethmon, P. and R. Elz, "Feature negotiation mechanism for
              the File Transfer Protocol", RFC 2389, August 1998.




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   [RFC3659]  Hethmon, P., "Extensions to FTP", RFC 3659, March 2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

7.2.  Informative References

   [RFC3230]  Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
              RFC 3230, January 2002.

   [RFC5797]  Klensin, J. and A. Hoenes, "FTP Command and Extension
              Registry", RFC 5797, March 2010.

   [draft-twine-ftpmd5]
              Twine, J., "The MD5 and MMD5 FTP Command Extensions",
              draft-twine-ftpmd5-00 (work in progress), May 2002.


Appendix A.  Acknowledgements and Contributors

   Thanks to John C. Klensin, Alfred Hoenes, James Twine, Robert
   McMurray, Mathias Berchtold, Tatsuhiro Tsujikawa, and Paul Ford-
   Hutchinson.

   Portions of [RFC3659] were wholly reused in this document.


Appendix B.  List of Non-standard Cryptographic Hash or Checksum
             Commands and Implementations

   [[ to be removed by the RFC editor before publication as an RFC. ]]

   A number of similar checksum or hash commands exist, but are not
   formally specified, leading to non-interoperability and confusion.
   The commands, any specifications, and relevant details:

   o  CKSM: GridFTP v2 Protocol Description
      http://www.ogf.org/documents/GFD.47.pdf Usage: OPTS CKSM
      <algorithm> CRLF.  Supports ADLER32, MD5, CRC32.
   o  MD5/MMD5: Expired Internet Draft [draft-twine-ftpmd5] from 2002.
      Usage: MD5 <filepath> Algorithm specific command.  Response codes:
      251 positive completion, 500 Command Not Recognized, 502 Command
      Not Implemented, 504 Command Not Implemented for the Specified
      Argument.
   o  SITE CHECKSUM: Usage: SITE check_login SP CHECKSUM SP pathname
      CRLF.  Supports CRC32 and MD5.





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   o  SITE SHOHASH: Usage: site shohash [filename].  Supports MD5.
      Response codes: 200 positive completion.
   o  XCRC: By GlobalSCAPE in 2001. http://help.globalscape.com/help/
      secureserver2/File_Integrity_Checking.htm Usage: XCRC <filename>
      SP EP.  SP is starting point and EP is ending point in bytes and
      are optional parameters.  Algorithm specific command.  Response
      codes: 250 positive completion, 450 Requested file action not
      taken.  (File is busy), 550 Requested action not taken.  (File not
      found, no read permission, SP or EP not correct).
   o  XMD5: XMD5 <filename> SP EP.  Similar to XCRC.  Algorithm specific
      command.
   o  XSHA, XSHA1, XSHA256, XSHA512: Usage similar to XCRC, although
      SP/EP usage unknown.  Algorithm specific commands.

   An incomplete list of FTP clients and servers that have implemented
   these commands:

   o  Akamai NetStorage (supports SITE CHKHSH/SITE SHOHASH) p17-18
      http://pigdogslow.dyndns.org/NetStorage_UserGuide.pdf
   o  Apache Ftp Server (supports MD5/MMD5 from draft-twine-ftpmd5)
      http://cwiki.apache.org/FTPSERVER/documentation.html
   o  Backup4all Pro (supports XCRC)
   o  Backup to FTP (supports XCRC)
   o  BlackMoon FTP Server (supports XCRC)
      http://www.blackmoonftpserver.com/portal/readmore/features.html
   o  C.P.A. Secure (supports XCRC)
      http://www.cpasecure.com/CPASecureVsSecureFTP.html
   o  Cerberus FTP server (supports XCRC, XMD5, XSHA1, XSHA256, XSHA512)
      http://www.softpedia.com/progChangelog/
      Cerberus-FTP-Server-Changelog-1904.html
   o  Core FTP Pro (supports XCRC)
   o  Cross FTP Server (supports MD5/MMD5)
   o  FileCOPA FTP Server (supports XCRC, XMD5, XSHA1)
      http://www.filecopa-ftpserver.com/features.html
   o  File Watchdogs FTP Server (supports XCRC, XMD5, XSHA1, XSHA256,
      XSHA512)
      http://www.filewatchdogs.com/ftpsitehosting/help/15559.htm
   o  FireFTP (supports XMD5, XSHA1)
      http://fireftp.mozdev.org/features.html
   o  FTP Daemon (supports SITE CHECKMETHOD/SITE CHECKSUM)
      http://www.pro-bono-publico.de/projects/ftpd.html
   o  FTP Voyager (supports XCRC) http://www.ftpvoyager.com/XCRC.asp
   o  Gene6 FTP Server
      http://www.g6ftpserver.com/en/information#features
   o  GlobalSCAPE's Secure FTP Server / EFT Server / CuteFTP clients
      (supports XCRC)





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   o  Globus FTP client / Globus Toolkit(supports CKSM) http://
      www.globus.org/toolkit/releasenotes/3.2.0/gridftp_notes.html
   o  GoldenGate FTP (Ftp Full Java Server) (supports XCRC, XMD5, XSHA1)
   o  IceWarp FTP Server http://www.icewarp.com/products/ftp_server/
   o  ICS FTP client (supports XCRC, XMD5)
      http://www.magsys.co.uk/delphi/magics.asp
   o  ioFTPD (supports XCRC)
   o  JAFS (supports XCRC and MD5)
      http://www.sbbi.net/site/jafs/features.html
   o  Kellerman FTP (supports XCRC)
      http://sharptoolbox.com/tools/kellerman-ftp
   o  Limagito FTP server (supports XCRC, XMD5, XSHA1)
      http://www.limagito.com/file-mover-features.html
   o  MOVEit DMZ (supports XSHA1)
   o  Nofeel FTP server (supports XCRC, XMD5, XSHA1)
      http://www.nftpserver.com/history.php
   o  Null FTP (supports XCRC, XMD5, XSHA)
      http://www.sharewareconnection.com/null-ftp-client-pro.htm
   o  Orenosv FTP Client (supports XCRC, XMD5)
      http://www.orenosv.com/orenosv/ftpcli_en.html
   o  ProFTPD module mod_digest (supports XCRC, XMD5, XSHA1, SHA256)
      http://www.smartftp.com/oss/proftpd/mod_digest.html
   o  PSFTPd Secure FTP Server (supports XCRC, XMD5, XSHA)
      http://www.psftp.de/psftpd_fo.php
   o  Quick 'n Easy FTP Server (supports XCRC) http://
      www.pablosoftwaresolutions.com/html/
      quick__n_easy_ftp_server_pro.html
   o  RaidenFTPD32 FTP server (supports XCRC, XMD5)
   o  Robo-FTP Server (supports XCRC, XMD5, XSHA1)
      http://kb.robo-ftp.com/change_log/show/61
   o  SyncBackPro and SyncBackSE (supports XCRC)
      http://www.2brightsparks.com/syncback/sbpro-changes.html
   o  Secure FTP Factory (supports XCRC)
   o  Serv-U FTP Server (supports XCRC) http://www.serv-u.com/help/
      serv_u_help/additional_ftp_commands_supported_by_serv_u.htm
   o  SmartFTP client (supports XCRC, XMD5, XSHA, CKSM)
      http://www.smartftp.com/features/
   o  Starksoft Ftp Component for .NET / Mono (supports XCRC, XMD5,
      XSHA1) http://www.starksoft.com/prod_ftp.html
   o  Titan FTP Server (supports XCRC)
   o  Turbo FTP (supports XCRC)
   o  WISE-FTP (supports XCRC) http://www.wise-ftp.com/news/
   o  WS_FTP client / server (supports XSHA1, server also XMD5, XSHA1,
      XSHA256, XSHA512) http://ipswitchft.custhelp.com/app/answers/
      detail/a_id/671/kw/xmd5/r_id/166/sno/1
   o  wuftpd (supports SITE CHECKMETHOD/SITE CHECKSUM)





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   o  wzdFTPd (supports XCRC, XMD5)
      http://www.wzdftpd.net/wiki/index.php/Commands
   o  Zalman FTP Client (supports XCRC)
      http://www.zalmansoftware.com/download.html
   o  zFTPServer


Appendix C.  Document History

   [[ to be removed by the RFC editor before publication as an RFC. ]]

   Known issues concerning this draft:
   o  Section needs improvement: which files should a server process a
      HASH command for a logged in user?

   -08 : October 25, 2010.
   o  New server reply 556: Servers that allow HASH but restrict its use
      to certain files.

   -07 : August 5, 2010.
   o  Clarify that HASH is only for files, not directories.

   -06 : July 9, 2010.
   o  Change server reply format.

   -05 : June 29, 2010.
   o  Add Basic Tokens and Server Replies subsections from RFC 3659.

   -04 : June 11, 2010.
   o  User-PI usage and command errors sections updated.

   -03 : May 21, 2010.
   o  List of non-standard checksum and hash commands and their
      implementations.

   -02 : April 16, 2010.
   o  Error codes section.

   -01 : April 7, 2010.
   o  Changing HASH algorithm with OPTS.
   o  Reference RFC 5797 and add IANA Considerations section.
   o  Informative Reference to expired Internet Draft
      (draft-twine-ftpmd5) which attempted to address this issue (it
      only supported one hash, MD5).

   -00 : October 19, 2009.





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   o  Initial draft.


Authors' Addresses

   Anthony Bryan
   Pompano Beach, FL
   USA

   Email: anthonybryan@gmail.com
   URI:   http://www.metalinker.org


   Tim Kosse

   Email: tim.kosse@filezilla-project.org
   URI:   http://filezilla-project.org/


   Daniel Stenberg

   Email: daniel@haxx.se
   URI:   http://www.haxx.se/




























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