Network Working Group M. Peterson
D. Papenthien
Internet Draft Rhino Software, Inc.
Intended status: Streamlined FTP Command Extensions April 22, 2008
Expires: October 2008
Streamlined FTP Command Extensions
draft-peterson-streamlined-ftp-command-extensions-03.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document specifies FTP commands to download thumbnails of remote
images, remove entire directory trees, request the amount of storage
space available to the user, request the size of a remote directory
and its contents, and to specify an FTP host. The commands are
designed to reduce the number of server / client exchanges, provide
information that was not previously available, and to reduce
bandwidth requirements for some higher level operations.
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Conventions used in this document
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 [RFC2119].
This document uses previously established definitions for FTP related
terms as defined in [RFC959]. In particular, the terms "data
connection", "FTP command", "file", "pathname", "reply", "server-FTP
process", "user", and "user-FTP process" are used as defined. The
terms "server" and "client" are also used in place of "server-FTP
process" and "user-FTP process", respectively, for the sake of
brevity.
The syntax for responses to FTP commands used in this document is
explained using Augmented BNF as defined in [RFC5234]. Additional
tokens defined in [RFC3659] that extend ABNF are also used,
specifically the TCHAR token.
In command examples, "C>" and "S>" indicate lines sent by the client
and server, respectively.
Table of Contents
1. Introduction...................................................3
2. Remove Directory All (RMDA)....................................3
2.1. RMDA Syntax...............................................3
2.2. RMDA Error Responses......................................4
2.3. RMDA FEAT Response........................................4
2.4. RMDA Examples.............................................5
3. Directory Size (DSIZ)..........................................5
3.1. DSIZ Syntax...............................................6
3.2. DSIZ Error Responses......................................6
3.3. DSIZ FEAT Response........................................7
3.4. DSIZ Examples.............................................7
4. Available Octets (AVBL)........................................7
4.1. AVBL Syntax...............................................8
4.2. AVBL Error Responses......................................8
4.3. AVBL FEAT Response........................................9
4.4. AVBL Examples.............................................9
5. Change Host (HOST)............................................10
5.1. HOST Syntax..............................................10
5.2. HOST Error Responses.....................................10
5.3. HOST FEAT Response.......................................11
5.4. HOST Examples............................................11
6. Retrieve Thumbnail of Remote Image File (THMB)................12
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6.1. THMB Syntax..............................................12
6.2. THMB Error Responses.....................................13
6.3. THMB FEAT Response.......................................14
6.4. THMB Examples............................................14
7. Security Considerations.......................................15
8. IANA Considerations...........................................15
9. Conclusions...................................................15
10. Acknowledgments..............................................15
Authors' Addresses...............................................18
Intellectual Property Statement..................................18
Disclaimer of Validity...........................................19
1. Introduction
This document updates the File Transfer Protocol [RFC959]. It adds
five new commands: "RMDA", "DSIZ", "AVBL", "HOST", and "THMB". The
"HOST" command has been used in various forms for several years while
the other four commands are new. These commands have been designed
to streamline certain client and server communications, reduce the
amount of bandwidth required to perform various operations, and allow
a server-FTP process to operate multiple hosts on the same IP address
and port combination.
2. Remove Directory All (RMDA)
The FTP command REMOVE DIRECTORY ALL (RMDA) removes a directory from
the server and all of its contents including all files and
subdirectories. The RMDA command is considered analogous to
recursively deleting all files and directories contained in a given
remote directory (including the directory itself) one at a time.
The primary advantage to using this command is the elimination of the
additional commands usually required to perform the equivalent
action. For directories containing a large number of files and
subdirectories, using RMDA eliminates the overhead of querying for
subdirectory listings. The end result is a more responsive operation
for both the client and server.
Depending on the content being deleted (files and subdirectories)
this can be a very lengthy operation for the server. Servers that
implement this command are encouraged to do so in a way that does not
block operations for other client connections.
2.1. RMDA Syntax
The syntax of the RMDA command is:
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rmda = "RMDA" SP pathname CRLF
All FTP commands, including RMDA, are case insensitive. However, the
"pathname" variable provided with the command may be case sensitive
as dictated by the server's operating system.
The "pathname" specifies a remote directory that should be deleted
along with all of its contents.
rmda-response = "250" SP *TCHAR CRLF /
error-response
Responses should be the same as those for the RMD command. If
processing fails at any point during the operation, the specified
pathname is not to be removed. Any cached information about the
pathname is considered invalid when RMDA returns an error.
2.2. RMDA Error Responses
The RMDA command can return any error response listed for the RMD
command in [RFC959]. The most common of these responses occurs in
the event of an error while deleting a file or subdirectory, in which
case the server should return a 550 reply. A 550 reply should also
be sent if the provided path is a file instead of a directory.
In the event that a nested file or subdirectory cannot be deleted for
any reason, the parent directories of that file or subdirectory
should not be deleted. When the server encounters this situation, it
should attempt to delete all other files and subdirectories before
stopping or responding to the client. Once the server has attempted
to delete all files and subdirectories, the server MUST report
failure to the client if even a single file or subdirectory could not
be deleted.
2.3. RMDA FEAT Response
When replying to the FEAT command, a server-FTP process that supports
the RMDA command MUST include a line starting with the word "RMDA".
If descriptive text is optionally desired, a space MUST immediately
follow the "RMDA" word.
C> FEAT
S> 211- <any descriptive text>
S> ...
S> RMDA <any descriptive text>
S> ...
S> 211 End
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The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory per [RFC2389].
2.4. RMDA Examples
This example assumes that the server contains a directory named "A"
and that the client has the necessary access to delete "A" and its
contents. The client-server exchange for using RMDA to accomplish
this would be:
C> RMDA A
S> 250 RMDA command successful.
If deletion of the directory, any contained file, or subdirectory
fails the client-server exchange would be:
C> RMDA A
S> 550 A: Cannot delete directory.
If deletion fails due to inadequate permissions for the directory,
any contained file, or subdirectory for the connected client, the
client-server exchange would be:
C> RMDA A
S> 550 A: Permission denied.
3. Directory Size (DSIZ)
The FTP command DIRECTORY SIZE (DSIZ) returns the number of octets
(8-bit bytes) on the remote file system occupied by a given directory
and its contents. The parameter to this command MUST represent a
directory path, not a file path.
While a method already exists to retrieve similar information by
recursively issuing the SIZE command as defined in [RFC3659] and
tallying the results, the SIZE command returns the number of transfer
octets for a specified file, which means that it considers the
current data representation type in its calculated response. The
DSIZ command should not consider this in its response, instead
leaving it up to the underlying file system to determine the number
of octets occupied by the files.
In addition, issuing a single DSIZ command saves on the number of
required commands and the overhead associated with recursively
querying for the SIZE of each individual file. This results in a
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more responsive and conservative communication exchange between the
client and server.
Depending on the size of the path being queried and its location
relative to the server, this can be a very lengthy operation.
Servers that implement this command are encouraged to do so in a way
that does not block operations for other client connections.
3.1. DSIZ Syntax
The syntax of the DSIZ command is:
dsiz = "DSIZ" [SP pathname] CRLF
All FTP commands, including DSIZ, are case insensitive. However, the
"pathname" variable provided with the command may be case sensitive
as dictated by the server's operating system. If the "pathname"
variable is not present in the received DSIZ command, its value is
assumed to be the current directory (i.e., the path that would be
returned in response to a Print Working Directory (PWD) command).
The "pathname" specifies a remote directory that should be queried to
retrieve the total storage space occupied by its contents on the
server's file system. The "pathname" MUST be a valid directory path;
it cannot be a path to a file or a system device.
dsiz-response = "213" SP 1*DIGIT CRLF /
error-response
The 213 reply is formatted in such a way that the returned size value
can be machine parsed.
A successful response includes a numeric value indicating the number
of octets on the server's file system occupied by the provided
pathname and its contents.
3.2. DSIZ Error Responses
In the event that the pathname is not a directory, the server MUST
return a permanent 550 error reply. Where the command cannot be
parsed, a 500 or 501 reply SHOULD be sent. The client MUST NOT
assume that the presence of a 550 reply indicates that it cannot
access the directory or its contents. The server may generate this
error for other reasons, for example the overhead required for the
operation is too great.
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3.3. DSIZ FEAT Response
When replying to the FEAT command, a server-FTP process that supports
the DSIZ command MUST include a line starting with the word "DSIZ".
If descriptive text is optionally desired, a space MUST immediately
follow the "DSIZ" word.
C> FEAT
S> 211- <any descriptive text>
S> ...
S> DSIZ <any descriptive text>
S> ...
S> 211 End
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory per [RFC2389].
3.4. DSIZ Examples
Assuming the presence of directory "A" on the server and that the
client has the necessary permissions to access and list it, a typical
DSIZ client-server exchange would be:
C> DSIZ A
S> 213 123456
Where 123456 is the number of octets on the server's file system
occupied by the directory "A" and its contents.
Assuming directory "A" cannot be listed due to client permissions,
the client-server exchange would be:
C> DSIZ A
S> 550 A: Permission denied.
Assuming the client provides the parameter "file.fid" where
"file.fid" is an existing file, the client-server exchange would be:
C> DSIZ file.fid
S> 550 A: Is a file.
4. Available Octets (AVBL)
The FTP command AVAILABLE OCTETS (AVBL) can be used to retrieve the
number of octets available to receive uploads in a specified remote
directory. Many server-FTP processes have implemented proprietary
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methods for restricting the amount of space available, either to the
user as a whole or within a specified remote location. The AVBL
command offers an interface for retrieving this value in a way that
can be machine parsed.
4.1. AVBL Syntax
The syntax of the AVBL command is:
avbl = "AVBL" [SP pathname] CRLF
All FTP commands, including AVBL, are case insensitive. However, the
optional "pathname" variable provided with the command may be case
sensitive as dictated by the server's operating system. If the
"pathname" variable is not present in the received AVBL command, its
value is assumed to be the current directory (i.e., the path that
would be returned in response to a Print Working Directory (PWD)
command).
The "pathname" variable specifies the remote directory that the
client wants to know how many octets are available to the user for
uploads. The "pathname" MUST be a valid directory path; it cannot be
a path to a file or a system device.
avbl-response = "213" SP 1*DIGIT CRLF /
error-response
The 213 reply is formatted in such a way that the returned size can
be machine parsed.
A successful response includes a numeric value that may be the number
of octets available as dictated by the physical limitations of the
underlying storage mechanism. It may also be a limit imposed upon
"pathname" or even the user that issued the command. The exact
method for calculating this numeric value is up to the specific
server-FTP process implementation.
4.2. AVBL Error Responses
In the event that the pathname is not a directory, the server MUST
return a permanent 550 error reply. Where the command cannot be
parsed, a 500 or 501 reply should be sent. The client MUST NOT
assume that the presence of a 550 reply indicates that it cannot
access the directory or its contents. The server may generate this
error for other reasons, for example the overhead required for the
operation is too great.
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4.3. AVBL FEAT Response
When replying to the FEAT command, a server-FTP process that supports
the AVBL command MUST include a line starting with the word "AVBL".
If descriptive text is optionally desired, a space MUST immediately
follow the "AVBL" word.
C> FEAT
S> 211- <any descriptive text>
S> ...
S> AVBL <any descriptive text>
S> ...
S> 211 End
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory per [RFC2389].
4.4. AVBL Examples
A typical client-server exchange using the AVBL command may look like
this:
C> AVBL
S> 213 123456
Where 123456 is the number of octets the user has available for file
uploads. In this example, no pathname is provided with the AVBL
command. Therefore, the server has used the current working
directory when processing its response.
Assuming the presence of directory "A" on the server and that the
client has the necessary permissions to access and list it:
C> AVBL A
S> 213 123456
Assuming directory "A" cannot be listed due to client permissions,
the client-server exchange would be:
C> AVBL A
S> 550 A: Permission denied.
Assuming the client provides the parameter "file.fid" where
"file.fid" is an existing file, the client-server exchange would be:
C> AVBL file.fid
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S> 550 A: Is a file.
5. Change Host (HOST)
The FTP command CHANGE HOST (HOST) allows the client to identify the
name used to connect to the server. This command allows the server
to host many unique domains within the server-FTP process that make
use of the same IP address and port number combination, much like the
"Host" header field of the HTTP protocol as defined in [RFC2616]
allows a web server to do the same.
5.1. HOST Syntax
The syntax of the HOST command is:
host = "HOST" SP host_name CRLF
All FTP commands, including HOST, are case insensitive. The
specified "host_name" is also case insensitive to provide
compatibility with domain names, which are case insensitive as well
[RFC1035].
The "host_name" parameter can be any arbitrary text that uniquely
identifies the client's desired domain to the server-FTP process.
The most common and appropriate application of a "host_name" would be
to specify a Fully Qualified Domain Name (FQDN) as this meets the
criteria of being unique. However, by not restricting this parameter
to a FQDN, other usages become available such as one not defined at
the time of this document's writing or a proprietary naming
convention employed within a large internal network.
host-response = "220" SP *TCHAR CRLF /
error-response
The HOST command MUST be sent prior to issuing the USER command or
logging into the server. If the specified host is valid, the server
responds with the usual 220 welcome response, returning the connect
message specified for the host.
5.2. HOST Error Responses
If the parameter is invalid, the server MUST use the 504 reply code.
If the client session is already logged in (successfully issued USER
and PASS commands), the server MUST use the 530 reply code. If the
host_name is unrecognized by the server, the server MUST use the 530
reply code.
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If the server does not accept a host_name parameter, the server MUST
continue with login to the default host on the IP address and port.
5.3. HOST FEAT Response
When replying to the FEAT command, a server-FTP process that supports
the HOST command SHOULD include a line starting with the word "HOST".
If descriptive text is optionally desired, a space MUST immediately
follow the "HOST" word.
C> FEAT
S> 211- <any descriptive text>
S> ...
S> HOST <any descriptive text>
S> ...
S> 211 End
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory per [RFC2389].
NOTE: Some servers do not allow the FEAT command to be issued prior
to the occurrence of a successful login. Because the HOST command is
required prior to login, a FEAT response code for HOST is optional
depending on the server's implementation of FEAT.
5.4. HOST Examples
Assuming host "ftp.example.com" is defined for the server-FTP
process, the client-server exchange would be:
C> HOST ftp.example.com
S> 220 Welcome to ftp.example.com, proceed...
When the domain name (host) does not exist, the client-server
exchange would be:
C> HOST ftp.example.com
S> 550 Host not found.
When invalid syntax is specified, the client-server exchange would
be:
C> HOST ftp example com
S> 504 Invalid parameter.
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6. Retrieve Thumbnail of Remote Image File (THMB)
Many user-FTP processes have implemented a view of the remote
directory contents that allows for a size-reduced representation of
remote files recognized as image types to be displayed. This size-
reduced representation of the image is commonly referred to as a
thumbnail. This view of the remote file system usually requires a
large amount of resources, in the form of time and bandwidth, in
order to be properly represented.
The THMB FTP command allows the server-FTP process to locally
generate a thumbnail of a requested image type and transfer the
resulting image instead of the entire contents of the original image
file. This facilitates a faster and more efficient method of
fulfilling a user-FTP process's implementation of a "thumbnail view".
Depending on the size of the image file being processed and the CPU
resources available to the server-FTP process, this can be a lengthy
operation. Servers that implement this command are encouraged to do
so in a way that does not block operations for other client
connections.
6.1. THMB Syntax
The THMB FTP command initiates a file transfer of the generated
thumbnail image in much the same way that the Retrieve (RETR) FTP
command initiates a download of a remote file. As such, thumbnail
transfer occurs over a previously established data connection as
indicated in [RFC959]. When the THMB command is received, an initial
response is sent over the command connection, the thumbnail is
transferred over the data connection, the data connection is closed,
and the final response is sent over the command connection.
The syntax of the THMB command is:
thmb = "THMB" SP fmt SP 1*DIGIT SP 1*DIGIT SP pathname
CRLF
All FTP commands, including THMB, are case insensitive. However, the
"pathname" variable provided with the command may be case sensitive
as dictated by the server's operating system.
The "fmt" parameter indicates an image format recognized by the
server-FTP process that should be used to generate the thumbnail
image. Its value corresponds to the commonly accepted file extension
used for the requested image format. Image formats supported by the
server are listed in the THMB FEAT Response [6.3]. The requested
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thumbnail format does not necessarily conform to the existing format
of the image. For example, a client-FTP process may request a PNG
thumbnail of a JPEG image.
The two numbers following "fmt" are the requested maximum width and
maximum height in pixels, respectively, that the client is willing to
accept for the generated thumbnail image. The server SHOULD preserve
the original aspect ratio of the image to avoid distortion while
ensuring these maximum values are not exceeded by the generated
thumbnail image.
The "pathname" specifies a remote file, of a supported image type,
that should be sent to the client in thumbnail form according to the
other parameters.
The syntax of a positive response is:
Host-response = "150" SP *TCHAR (1*DIGIT 1*TCHAR) CRLF /
error-response
(1*DIGIT) represents the size in octets of the forthcoming file
transfer. The value within the parentheses MAY include descriptive
text, for example (123456 Bytes). By providing this information to
the client, the client can determine the required file size and
transfer status information.
The potential responses to the THMB command that should be expected
over the command channel correspond to the list of acceptable
responses to the RETR command as defined in [RFC959].
As the THMB command initiates a data transfer over a previously
negotiated data connection, the binary image data for the server
generated thumbnail image is transferred over this data connection.
Because image data is always binary in nature, the server-FTP process
will always send this data as if the current data representation type
is BINARY, regardless of the setting that was established through the
last received TYPE command. After the THMB command is fully
processed, the server-FTP process reverts back to using the
previously established data representation type.
6.2. THMB Error Responses
If an error is encountered while attempting to process or generate
the thumbnail image, the server MUST use the 550 reply code. If an
error is encountered while allocating resources for temporary storage
of the generated thumbnail, the server MUST use the 550 reply code.
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6.3. THMB FEAT Response
When replying to the FEAT command, a server-FTP process that supports
the THMB command MUST include a line starting with the word "THMB".
If descriptive text is optionally desired, a space MUST immediately
follow the required "fmt" parameter.
C> FEAT
S> 211- <any descriptive text>
S> ...
S> THMB <fmt> <any descriptive text>
S> ...
S> 211 End
The "fmt" variable represents the supported image format(s); multiple
format abbreviations can be given separated by the "|" character.
Each format abbreviation must be the media subtype name of an 'image'
media type that is IANA-registered under the provisions established
in [RFC4288] describing a unique file storage format and giving a
specific file name extension. Supporting servers must support at
least JPEG or PNG formats. An example THMB response to the FEAT
command might be:
THMB JPEG|GIF|TIFF|PNG
This response identifies JPEG, GIF, TIFF, and PNG formats as image
types supported by the server for thumbnail generation.
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory per [RFC2389].
6.4. THMB Examples
Assuming the existence of image file "widget.png" on the server and
that the client has all necessary permissions to access and transfer
the "widget.png" file, a request for a "widget.png" thumbnail might
look like this:
C> THMB PNG 80 80 widget.png
S> 150 Starting thumbnail transfer (1234 Bytes) for widget.png
S> 226 Transfer complete.
In between the 150 and 226 server responses, the thumbnail image has
been transferred over a negotiated data connection.
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If the client requests a thumbnail for an unsupported image type, the
exchange would be:
C> THMB PSP 80 80 widget.psp
S> 501 Syntax error in parameters or arguments.
7. Security Considerations
This document does not explicitly address security issues as they
pertain to the FTP protocol in general. The commands introduced in
this document do not affect the security of the FTP protocol nor do
they address any of the security considerations described in
[RFC2577].
Implementation of the RMDA, DSIZ, AVBL, and THMB commands can require
significant server resources to process. This knowledge could be
used by attackers in a denial of service attack. However, this issue
has been addressed before with others commands, such as SIZE, and is
traditionally mitigated by the server-FTP process's implementation of
the command.
The FTP server is encouraged to take specific care when implementing
the RMDA command to avoid bypassing permissions. While it may be
tempting to take advantage of a method made available by the platform
to delete an entire tree of the file system at once, doing so could
allow for the possibility of bypassing permissions that apply to
specific files. Consequently, the contents of the path being deleted
by RMDA must be individually evaluated.
8. IANA Considerations
None.
9. Conclusions
Inclusion of these commands both for servers and client can
significantly reduce bandwidth requirements for many operations as
well as improving the client-side experience by reducing the amount
of time needed to perform these operations via other methods.
10. Acknowledgments
We would like to thank Alfred Hoenes for reviewing this document and
providing thorough and thoughtful suggestions on how to make it
better.
This document was prepared using 2-Word-v2.0.template.dot.
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Funding for the RFC Editor function is currently provided by the
Internet Society.
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Normative References
[RFC959] Postel, J. and J. Reynolds, "File Transfer Protocol
(FTP)", STD 9, RFC 959, October 1985.
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[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.
[RFC2577] Allman, M. and S. Ostermann, "FTP Security
Considerations", RFC 2577, May 1999.
[RFC3659] Hethmon, P., "Extensions to FTP", RFC 3659, March 2007.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December
2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
Informative References
[RFC2616] Fielding, R., et al., "Hypertext Transfer Protocol -
HTTP/1.1", RFC 2616, June 1999.
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Authors' Addresses
Mark P. Peterson
Rhino Software, Inc.
P.O. Box 53
Helenville, WI 53137
U.S.A.
Phone: +1 (262) 560-9627
FAX: +1 (262) 560-9628
Email: mark.peterson@rhinosoft.com
Douglas J. Papenthien
Rhino Software, Inc.
P.O. Box 53
Helenville, WI 53137
U.S.A.
Phone: +1 (262) 560-9627
FAX: +1 (262) 560-9628
Email: douglas.papenthien@rhinosoft.com
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