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Versions: 00

Network Working Group                                  S. Moonesamy, Ed.
Internet-Draft
Intended Status: Standards Track
Expires: January 13, 2014
                                                           July 12, 2013


                       SSH File Transfer Protocol
                   draft-moonesamy-secsh-filexfer-00

Abstract

   The SSH File Transfer Protocol provides secure file transfer
   functionality over any reliable data stream.  It is the standard file
   transfer protocol for use with the SSH2 protocol.  This document
   describes the file transfer protocol and its interface to the SSH2
   protocol suite.

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
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Copyright Notice

   Copyright (c) 2013 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



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   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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials,this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


































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Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1 Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4
   2. Use with the Secure Shell Connection Protocol . . . . . . . . .  4
     2.1.  The Use of 'stderr' in the server  . . . . . . . . . . . .  4
   3. General Packet Format . . . . . . . . . . . . . . . . . . . . .  5
   4. Protocol Initialization . . . . . . . . . . . . . . . . . . . .  6
   5. File Attributes . . . . . . . . . . . . . . . . . . . . . . . .  7
   6. Requests From the Client to the Server  . . . . . . . . . . . .  9
     6.1 Request Synchronization and Reordering . . . . . . . . . . .  9
     6.2 File Names . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.3 Opening, Creating, and Closing Files . . . . . . . . . . . . 10
     6.4 Reading and Writing  . . . . . . . . . . . . . . . . . . . . 12
     6.5 Removing and Renaming Files  . . . . . . . . . . . . . . . . 13
     6.6 Creating and Deleting Directories  . . . . . . . . . . . . . 14
     6.7 Scanning Directories . . . . . . . . . . . . . . . . . . . . 14
     6.8 Retrieving File Attributes . . . . . . . . . . . . . . . . . 15
     6.9 Setting File Attributes  . . . . . . . . . . . . . . . . . . 16
     6.10 Dealing with Symbolic links . . . . . . . . . . . . . . . . 17
     6.11 Canonicalizing the Server-Side Path Name  . . . . . . . . . 17
   7. Responses from the Server to the Client . . . . . . . . . . . . 18
   8. Vendor-Specific Extensions  . . . . . . . . . . . . . . . . . . 21
   9. Security Considerations . . . . . . . . . . . . . . . . . . . . 22
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   13.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 23
     13.1  Normative References . . . . . . . . . . . . . . . . . . . 23
     13.1  Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A: Changes from previous protocol versions  . . . . . . . 23
     A.1 Changes between versions 3 and 2 . . . . . . . . . . . . . . 23
     A.2 Changes between versions 2 and 1 . . . . . . . . . . . . . . 24
     A.3 Changes between versions 1 and 0 . . . . . . . . . . . . . . 24
   Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
















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1. Introduction

   The SSH File Transfer Protocol provides secure file transfer (and
   more generally file system access).  It is designed so that it could
   be used to implement a secure remote file system service as well as a
   secure file transfer service.

   This protocol assumes that it runs over a secure channel, such as a
   channel in [RFC4251], that the server has already authenticated the
   client, and that the identity of the client user is available to the
   protocol.

   In general, this protocol follows a simple request-response model.
   Each request and response contains a sequence number and multiple
   requests may be pending simultaneously.  There are a relatively large
   number of different request messages, but a small number of possible
   response messages.  Each request has one or more response messages
   that may be returned in result (e.g., a read either returns data or
   reports error status).

   The packet format descriptions in this specification follow the
   notation presented in [RFC4251].

   Even though this protocol is described in the context of the SSH2
   protocol, this protocol is general and independent of the rest of the
   SSH2 protocol suite.  It could be used in a number of different
   applications, such as secure file transfer over TLS [RFC5246] and
   transfer of management information in VPN applications.

1.1 Terminology

   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].

2. Use with the Secure Shell Connection Protocol

   When used with the Secure Shell Connection Protocol suite, this
   protocol is intended to be used from the SSH Connection Protocol
   [RFC4254] as a subsystem, as described in section ``Starting a Shell
   or a Command''.  The subsystem name used with this protocol is
   "sftp".

2.1.  The Use of 'stderr' in the server

   This protocol uses stdout and stdin to transmit binary protocol data.
    The "session" channel ([RFC4254]), which is used by the subsystem,
   also supports the use of stderr.



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   Data sent on stderr by the server SHOULD be considered free format
   debug or supplemental error information, and MAY be displayed to the
   user.

   For example, during initialization, there is no client request
   active, so errors or warning information cannot be sent to the client
   as part of the SFTP protocol at this early stage.  However, the
   errors or warnings MAY be sent as stderr text.

3. General Packet Format

    All packets transmitted over the secure connection are of the
   following format:

   uint32             length
   byte               type
   byte[length - 1]   data payload

   That is, they are just data preceded by 32-bit length and 8-bit type
   fields.  The "length" is the length of the data area, and does not
   include the "length" field itself.  The format and interpretation of
   the data area depends on the packet type.

   All packet descriptions below only specify the packet type and the
   data that goes into the data field.  Thus, they should be prefixed by
   the "length" and "type" fields.

   The maximum size of a packet is in practice determined by the client
   (the maximum size of read or write requests that it sends, plus a few
   bytes of packet overhead).  All servers SHOULD support packets of at
   least 34000 bytes (where the packet size refers to the full length,
   including the header above).  This should allow for reads and writes
   of at most 32768 bytes.

   There is no limit on the number of outstanding (non-acknowledged)
   requests that the client may send to the server.  In practice this is
   limited by the buffering available on the data stream and the queuing
   performed by the server.  If the server's queues are full, it should
   not read any more data from the stream, and flow control will prevent
   the client from sending more requests.  Note, however, that while
   there is no restriction on the protocol level, the client's API may
   provide a limit in order to prevent infinite queuing of outgoing
   requests at the client.

   The following values are defined for packet types.

        #define SSH_FXP_INIT                1
        #define SSH_FXP_VERSION             2



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        #define SSH_FXP_OPEN                3
        #define SSH_FXP_CLOSE               4
        #define SSH_FXP_READ                5
        #define SSH_FXP_WRITE               6
        #define SSH_FXP_LSTAT               7
        #define SSH_FXP_FSTAT               8
        #define SSH_FXP_SETSTAT             9
        #define SSH_FXP_FSETSTAT           10
        #define SSH_FXP_OPENDIR            11
        #define SSH_FXP_READDIR            12
        #define SSH_FXP_REMOVE             13
        #define SSH_FXP_MKDIR              14
        #define SSH_FXP_RMDIR              15
        #define SSH_FXP_REALPATH           16
        #define SSH_FXP_STAT               17
        #define SSH_FXP_RENAME             18
        #define SSH_FXP_READLINK           19
        #define SSH_FXP_SYMLINK            20
        #define SSH_FXP_STATUS            101
        #define SSH_FXP_HANDLE            102
        #define SSH_FXP_DATA              103
        #define SSH_FXP_NAME              104
        #define SSH_FXP_ATTRS             105
        #define SSH_FXP_EXTENDED          200
        #define SSH_FXP_EXTENDED_REPLY    201

    Additional packet types should only be defined if the protocol
    version number (see Section "Protocol Initialization") is
    incremented, and their use MUST be negotiated using the version
    number.  However, the SSH_FXP_EXTENDED and SSH_FXP_EXTENDED_REPLY
    packets can be used to implement vendor-specific extensions.  See
    Section "Vendor-Specific-Extensions" for more details.

4. Protocol Initialization

    When the file transfer protocol starts, it first sends a
    SSH_FXP_INIT (including its version number) packet to the server.
    The server responds with a SSH_FXP_VERSION packet, supplying the
    lowest of its own and the client's version number.  Both parties
    should from then on adhere to particular version of the protocol.

    The SSH_FXP_INIT packet (from client to server) has the following
    data:

        uint32 version
        <extension data>

    The SSH_FXP_VERSION packet (from server to client) has the following



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    data:

        uint32 version
        <extension data>

    The version number of the protocol specified in this document is 3.
    The version number should be incremented for each incompatible
    revision of this protocol.

    The extension data in the above packets may be empty, or may be a
    sequence of:

        string extension_name
        string extension_data

    pairs (both strings MUST always be present if one is, but the
    `extension_data' string may be of zero length).  If present, these
    strings indicate extensions to the baseline protocol.  The
    "extension_name" field(s) identify the name of the extension.  The
    name should be of the form "name@domain", where the domain is the
    DNS domain name of the organization defining the extension.
    Additional names that are not of this format may be defined later by
    the IETF. Implementations MUST silently ignore any extensions whose
    name they do not recognize.

5. File Attributes

    A new compound data type is defined for encoding file attributes.
    It is basically just a combination of elementary types, but is
    defined once because of the non-trivial description of the fields
    and to ensure maintainability.

    The same encoding is used both when returning file attributes from
    the server and when sending file attributes to the server.  When
    sending it to the server, the flags field specifies which attributes
    are included, and the server will use default values for the
    remaining attributes (or will not modify the values of remaining
    attributes).  When receiving attributes from the server, the flags
    specify which attributes are included in the returned data.  The
    server normally returns all attributes it knows about.

    uint32   flags
    uint64   size        present only if flag SSH_FILEXFER_ATTR_SIZE
    uint32   uid         present only if flag SSH_FILEXFER_ATTR_UIDGID
    uint32   gid         present only if flag SSH_FILEXFER_ATTR_UIDGID
    uint32   permissions present only if flag
                              SSH_FILEXFER_ATTR_PERMISSIONS
    uint32   atime          present only if flag SSH_FILEXFER_ACMODTIME



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    uint32   mtime          present only if flag SSH_FILEXFER_ACMODTIME
    uint32   extended_count present only if flag
                              SSH_FILEXFER_ATTR_EXTENDED
    string   extended_type
    string   extended_data
    ...      more extended data (extended_type - extended_data pairs),
    so that number of pairs equals extended_count

    The "flags" specify which of the fields are present.  Those fields
    for which the corresponding flag is not set are not present (not
    included in the packet).  New flags can only be added by
    incrementing the protocol version number (or by using the extension
    mechanism described below).

    The "size" field specifies the size of the file in bytes.

    The "uid" and "gid" fields contain numeric Unix-like user and group
    identifiers, respectively.

    The "permissions" field contains a bit mask of file permissions as
    defined by POSIX [IEEE.1003-1.1996].

    The "atime" and "mtime" contain the access and modification times of
    the files, respectively.  They are represented as seconds from Jan
    1, 1970 in UTC.

    The SSH_FILEXFER_ATTR_EXTENDED flag provides a general extension
    mechanism for vendor-specific extensions.  If the flag is specified,
    then the "extended_count" field is present.  It specifies the number
    of extended_type-extended_data pairs that follow.  Each of these
    pairs specifies an extended attribute.  For each of the attributes,
    the extended_type field should be a string of the format
    "name@domain", where "domain" is a valid, registered domain name and
    "name" identifies the method.  The interpretation of `extended_data'
    depends on the type.  Implementations SHOULD ignore extended data
    fields that they do not understand.

    Additional fields can be added to the attributes by either defining
    additional bits to the flags field to indicate their presence, or by
    defining extended attributes for them.  The extended attributes
    mechanism is recommended for most purposes; additional flags bits
    should only be defined by an IETF standards action that also
    increments the protocol version number.  The use of such new fields
    MUST be negotiated by the version number in the protocol exchange.
    It is a protocol error if a packet with unsupported protocol bits is
    received.

    The flags bits are defined to have the following values:



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    #define SSH_FILEXFER_ATTR_SIZE          0x00000001
    #define SSH_FILEXFER_ATTR_UIDGID        0x00000002
    #define SSH_FILEXFER_ATTR_PERMISSIONS   0x00000004
    #define SSH_FILEXFER_ATTR_ACMODTIME     0x00000008
    #define SSH_FILEXFER_ATTR_EXTENDED      0x80000000

6. Requests From the Client to the Server

    Requests from the client to the server represent the various file
    system operations.  Each request begins with an `id' field, which is
    a 32-bit identifier identifying the request (selected by the
    client). The same identifier will be returned in the response to the
    request. One possible implementation of it is a monotonically
    increasing request sequence number (modulo 2^32).

    Many operations in the protocol operate on open files.  The
    SSH_FXP_OPEN request can return a file handle (which is an opaque
    variable-length string) which may be used to access the file later
    (e.g.  in a read operation).  The client MUST NOT send requests the
    server with bogus or closed handles.  However, the server MUST
    perform adequate checks on the handle in order to avoid security
    risks due to fabricated handles.

    This design allows either stateful and stateless server
    implementation, as well as an implementation which caches state
    between requests but may also flush it.  The contents of the file
    handle string are entirely up to the server and its design.  The
    client should not modify or attempt to interpret the file handle
    strings.

    The file handle strings MUST NOT be longer than 256 bytes.

6.1 Request Synchronization and Reordering

    The protocol and implementations MUST process requests relating to
    the same file in the order in which they are received.  It is up to
    the server to ensure fairness in the sense that processing of
    request will not be indefinitely delayed even if the client is
    sending multiple requests.  If an application submits multiple
    requests to the server, the results in the responses will be the
    same as if it had sent the requests one at a time and waited for the
    response in each case.  For example, the server may process non-
    overlapping read/write requests to the same file in parallel, but
    overlapping reads and writes cannot be reordered or parallelized.
    There isn't any ordering restriction on the server for processing
    requests from two different file transfer connections.  The server
    may interleave and parallelize them at will.




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6.2 File Names

    This protocol represents file names as strings.  File names are
    assumed to use the slash ('/') character as a directory separator.

    File names starting with a slash are "absolute", and are relative to
    the root of the file system.  Names starting with any other
    character are relative to the user's default directory (home
    directory).  Note that identifying the user is assumed to take place
    outside of this protocol.

    Servers SHOULD interpret a path name component ".." as referring to
    the parent directory, and "." as referring to the current directory.
    If the server implementation limits access to certain parts of the
    file system, it must be extra careful in parsing file names when
    enforcing such restrictions.  There have been numerous reported
    security bugs where a ".." in a path name has allowed access outside
    the intended area.

    An empty path name is valid, and it refers to the user's default
    directory (usually the user's home directory).

    Otherwise, no syntax is defined for file names by this
    specification. Clients should not make any other assumptions;
    however, they can splice path name components returned by
    SSH_FXP_READDIR together using a slash ('/') as the separator, and
    that will work as expected.

    It is understood that the lack of well-defined semantics for file
    names may cause interoperability problems between clients and
    servers using radically different operating systems.  However, this
    approach is known to work acceptably with most systems, and
    alternative approaches that e.g.  treat file names as sequences of
    structured components are quite complicated.

6.3 Opening, Creating, and Closing Files

    Files are opened and created using the SSH_FXP_OPEN message, whose
    data part is as follows:

    uint32        id
    string        filename
    uint32        pflags
    ATTRS         attrs

    The "id" field is the request identifier as for all requests.

    The "filename" field specifies the file name.  See Section "File



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    Names" for more information.

    The "pflags" field is a bitmask.  The following bits have been
    defined.

    #define SSH_FXF_READ            0x00000001
    #define SSH_FXF_WRITE           0x00000002
    #define SSH_FXF_APPEND          0x00000004
    #define SSH_FXF_CREAT           0x00000008
    #define SSH_FXF_TRUNC           0x00000010
    #define SSH_FXF_EXCL            0x00000020

    These have the following meanings:

    SSH_FXF_READ
        Open the file for reading.

    SSH_FXF_WRITE
        Open the file for writing.  If both this and SSH_FXF_READ are
        specified, the file is opened for both reading and writing.

    SSH_FXF_APPEND
        Force all writes to append data at the end of the file.

    SSH_FXF_CREAT
        If this flag is specified, then a new file will be created if
        one does not already exist (if O_TRUNC is specified, the new
        file will be truncated to zero length if it previously exists).

    SSH_FXF_TRUNC
        Forces an existing file with the same name to be truncated to
        zero length when creating a file by specifying SSH_FXF_CREAT.
        SSH_FXF_CREAT MUST also be specified if this flag is used.

    SSH_FXF_EXCL

        Causes the request to fail if the named file already exists.
        SSH_FXF_CREAT MUST also be specified if this flag is used.

    The "attrs" field specifies the initial attributes for the file.
    Default values will be used for those attributes that are not
    specified (see the "File Attributes" section for more information).

    Regardless the server operating system, the file will always be
    opened in "binary" mode (i.e., no translations between different
    character sets and newline encodings).

    The response to this message will be either SSH_FXP_HANDLE (if the



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    operation is successful) or SSH_FXP_STATUS (if the operation fails).


    A file is closed by using the SSH_FXP_CLOSE request.  Its data field
    has the following format:

    uint32     id
    string     handle

    where "id" is the request identifier, and "handle" is a handle
    previously returned in the response to SSH_FXP_OPEN or
    SSH_FXP_OPENDIR.  The handle becomes invalid immediately after this
    request has been sent.

    The response to this request will be a SSH_FXP_STATUS message.  One
    should note that on some server platforms even a close can fail.
    This can happen e.g.  if the server operating system caches writes,
    and an error occurs while flushing cached writes during the close.

6.4 Reading and Writing

    Once a file has been opened, it can be read using the SSH_FXP_READ
    message, which has the following format:

    uint32     id
    string     handle
    uint64     offset
    uint32     len

    where "id" is the request identifier, `handle' is an open file
    handle returned by SSH_FXP_OPEN, "offset" is the offset (in bytes)
    relative to the beginning of the file from where to start reading,
    and `len' is the maximum number of bytes to read.

    In response to this request, the server will read as many bytes as
    it can from the file (up to "len"), and return them in a
    SSH_FXP_DATA message.  If an error occurs or EOF is encountered
    before reading any data, the server will respond with
    SSH_FXP_STATUS.  For normal disk files, it is guaranteed that this
    will read the specified number of bytes, or up to end of file.  For
    e.g.  device files this may return fewer bytes than requested.

    Writing to a file is achieved using the SSH_FXP_WRITE message, which
    has the following format:

    uint32     id
    string     handle
    uint64     offset



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    string     data

    where `id' is a request identifier, "handle" is a file handle
    returned by SSH_FXP_OPEN, "offset" is the offset (in bytes) from the
    beginning of the file where to start writing, and "data" is the data
    to be written.

    The write will extend the file if writing beyond the end of the
    file. It is legal to write way beyond the end of the file; the
    semantics are to write zeroes from the end of the file to the
    specified offset and then the data.  On most operating systems, such
    writes do not allocate disk space but instead leave "holes" in the
    file.

    The server responds to a write request with a SSH_FXP_STATUS
    message.

6.5 Removing and Renaming Files

    Files can be removed using the SSH_FXP_REMOVE message.  It has the
    following format:

    uint32     id
    string     filename

    where "id" is the request identifier and `filename' is the name of
    the file to be removed.  See Section "File Names" for more
    information.  This request cannot be used to remove directories.

    The server will respond to this request with a SSH_FXP_STATUS
    message.

    Files (and directories) can be renamed using the SSH_FXP_RENAME
    message.  Its data is as follows:

    uint32     id
    string     oldpath
    string     newpath

    where "id" is the request identifier, "oldpath" is the name of an
    existing file or directory, and "newpath" is the new name for the
    file or directory.  It is an error if there already exists a file
    with the name specified by newpath.  The server may also fail rename
    requests in other situations, for example if "oldpath" and "newpath"
    point to different file systems on the server.

    The server will respond to this request with a SSH_FXP_STATUS
    message.



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6.6 Creating and Deleting Directories

    New directories can be created using the SSH_FXP_MKDIR request.  It
    has the following format:

    uint32     id
    string     path
    ATTRS      attrs

    where "id" is the request identifier, "path" and "attrs" specifies
    the modifications to be made to its attributes.  See the "File
    Names" section for more information on file names.  Attributes are
    discussed in more detail in Section "File Attributes".  specifies
    the directory to be created.  An error will be returned if a file or
    directory with the specified path already exists.  The server will
    respond to this request with a SSH_FXP_STATUS message.

    Directories can be removed using the SSH_FXP_RMDIR request, which
    has the following format:

    uint32     id
    string     path

    where "id" is the request identifier, and "path" specifies the
    directory to be removed.  See Section "File Names" for more
    information on file names.  An error will be returned if no
    directory with the specified path exists, or if the specified
    directory is not empty, or if the path specified a file system
    object other than a directory.  The server responds to this request
    with a SSH_FXP_STATUS message.

6.7 Scanning Directories

    The files in a directory can be listed using the SSH_FXP_OPENDIR and
    SSH_FXP_READDIR requests.  Each SSH_FXP_READDIR request returns one
    or more file names with full file attributes for each file.  The
    client should call SSH_FXP_READDIR repeatedly until it has found the
    file it is looking for or until the server responds with a
    SSH_FXP_STATUS message indicating an error (normally SSH_FX_EOF if
    there are no more files in the directory).  The client should then
    close the handle using the SSH_FXP_CLOSE request.










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    The SSH_FXP_OPENDIR opens a directory for reading.  It has the
    following format:

    uint32     id
    string     path

    where "id" is the request identifier and "path" is the path name of
    the directory to be listed (without any trailing slash).  See the
    "File Names" section for more information on file names.  This will
    return an error if the path does not specify a directory or if the
    directory is not readable.  The server will respond to this request
    with either a SSH_FXP_HANDLE or a SSH_FXP_STATUS message.

    Once the directory has been successfully opened, files (and
    directories) contained in it can be listed using SSH_FXP_READDIR
    requests.  These are of the format

    uint32     id
    string     handle

    where "id" is the request identifier, and `handle' is a handle
    returned by SSH_FXP_OPENDIR.  (It is a protocol error to attempt to
    use an ordinary file handle returned by SSH_FXP_OPEN.)

    The server responds to this request with either a SSH_FXP_NAME or a
    SSH_FXP_STATUS message.  One or more names may be returned at a
    time. Full status information is returned for each name in order to
    speed up typical directory listings.

    When the client no longer wishes to read more names from the
    directory it SHOULD call SSH_FXP_CLOSE for the handle.  The handle
    should be closed regardless of whether an error has occurred or not.

6.8 Retrieving File Attributes

    Very often, file attributes are automatically returned by
    SSH_FXP_READDIR.  However, sometimes there is need to specifically
    retrieve the attributes for a named file.  This can be done using
    the SSH_FXP_STAT, SSH_FXP_LSTAT and SSH_FXP_FSTAT requests.

    SSH_FXP_STAT and SSH_FXP_LSTAT only differ in that SSH_FXP_STAT
    follows symbolic links on the server, whereas SSH_FXP_LSTAT does not
    follow symbolic links.  Both have the same format:

    uint32     id
    string     path

    where "id" is the request identifier, and "path" specifies the file



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    system object for which status is to be returned.  The server
    responds to this request with either SSH_FXP_ATTRS or
    SSH_FXP_STATUS.

    SSH_FXP_FSTAT differs from the others in that it returns status
    information for an open file (identified by the file handle).  Its
    format is as follows:

    uint32     id
    string     handle

    where `id' is the request identifier and "handle" is a file handle
    returned by SSH_FXP_OPEN.  The server responds to this request with
    SSH_FXP_ATTRS or SSH_FXP_STATUS.

6.9 Setting File Attributes

    File attributes may be modified using the SSH_FXP_SETSTAT and
    SSH_FXP_FSETSTAT requests.  These requests are used for operations
    such as changing the ownership, permissions or access times, as well
    as for truncating a file.

    The SSH_FXP_SETSTAT request is of the following format:

    uint32     id
    string     path
    ATTRS      attrs

    where "id" is the request identifier, "path" specifies the file
    system object (e.g.  file or directory) whose attributes are to be
    modified, and "attrs" specifies the modifications to be made to its
    attributes.  Attributes are discussed in more detail in Section
    "File Attributes".

    An error will be returned if the specified file system object does
    not exist or the user does not have sufficient rights to modify the
    specified attributes.  The server responds to this request with a
    SSH_FXP_STATUS message.

    The SSH_FXP_FSETSTAT request modifies the attributes of a file which
    is already open.  It has the following format:

    uint32     id
    string     handle
    ATTRS      attrs

    where "id" is the request identifier, "handle" (MUST be returned by
    SSH_FXP_OPEN) identifies the file whose attributes are to be



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    modified, and "attrs" specifies the modifications to be made to its
    attributes.  Attributes are discussed in more detail in Section
    "File Attributes".  The server will respond to this request with
    SSH_FXP_STATUS.

6.10 Dealing with Symbolic links

    The SSH_FXP_READLINK request may be used to read the target of a
    symbolic link.  It would have a data part as follows:

    uint32     id
    string     path

    where "id" is the request identifier and "path" specifies the path
    name of the symlink to be read.

    The server will respond with a SSH_FXP_NAME packet containing only
    one name and a dummy attributes value.  The name in the returned
    packet contains the target of the link.  If an error occurs, the
    server may respond with SSH_FXP_STATUS.

    The SSH_FXP_SYMLINK request will create a symbolic link on the
    server.  It is of the following format

    uint32     id
    string     linkpath
    string     targetpath

    where "id" is the request identifier, "linkpath" specifies the path
    name of the symlink to be created and "targetpath" specifies the
    target of the symlink.  The server shall respond with a
    SSH_FXP_STATUS indicating either success (SSH_FX_OK) or an error
    condition.

6.11 Canonicalizing the Server-Side Path Name

    The SSH_FXP_REALPATH request can be used to have the server
    canonicalize any given path name to an absolute path.  This is
    useful for converting path names containing ".." components or
    relative pathnames without a leading slash into absolute paths.  The
    format of the request is as follows:

    uint32     id
    string     path

    where "id" is the request identifier and "path" specifies the path
    name to be canonicalized.  The server will respond with a
    SSH_FXP_NAME packet containing only one name and a dummy attributes



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    value.  The name is the returned packet will be in canonical form.

    If an error occurs, the server may also respond with SSH_FXP_STATUS.

7. Responses from the Server to the Client

    The server responds to the client using one of a few response
    packets.  All requests can return a SSH_FXP_STATUS response upon
    failure.  When the operation is successful, any of the responses may
    be returned (depending on the operation).  If no data needs to be
    returned to the client, the SSH_FXP_STATUS response with SSH_FX_OK
    status is appropriate.  Otherwise, the SSH_FXP_HANDLE message is
    used to return a file handle (for SSH_FXP_OPEN and SSH_FXP_OPENDIR
    requests), SSH_FXP_DATA is used to return data from SSH_FXP_READ,
    SSH_FXP_NAME is used to return one or more file names from a
    SSH_FXP_READDIR or SSH_FXP_REALPATH request, and SSH_FXP_ATTRS is
    used to return file attributes from SSH_FXP_STAT, SSH_FXP_LSTAT, and
    SSH_FXP_FSTAT requests.

    Exactly one response will be returned for each request.  Each
    response packet contains a request identifier which can be used to
    match each response with the corresponding request.  Note that it is
    legal to have several requests outstanding simultaneously, and the
    server is allowed to send responses to them in a different order
    from the order in which the requests were sent (the result of their
    execution, however, is guaranteed to be as if they had been
    processed one at a time in the order in which the requests were
    sent).

    Response packets are of the same general format as request packets.
    Each response packet begins with the request identifier.

    The format of the data portion of the SSH_FXP_STATUS response is as
    follows:

    uint32     id
    uint32     error/status code
    string     error message (ISO-10646 UTF-8 [RFC3629])
    string     language tag (as defined in [RFC5646])

    where "id" is the request identifier, and "error/status code"
    indicates the result of the requested operation.  The value
    SSH_FX_OK indicates success, and all other values indicate failure.








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    Currently, the following values are defined (other values may be
    defined by future versions of this protocol):

    #define SSH_FX_OK                            0
    #define SSH_FX_EOF                           1
    #define SSH_FX_NO_SUCH_FILE                  2
    #define SSH_FX_PERMISSION_DENIED             3
    #define SSH_FX_FAILURE                       4
    #define SSH_FX_BAD_MESSAGE                   5
    #define SSH_FX_NO_CONNECTION                 6
    #define SSH_FX_CONNECTION_LOST               7
    #define SSH_FX_OP_UNSUPPORTED                8

    SSH_FX_OK
       Indicates successful completion of the operation.

    SSH_FX_EOF
       indicates end-of-file condition; for SSH_FX_READ it means that no
       more data is available in the file, and for SSH_FX_READDIR it
       indicates that no more files are contained in the directory.

    SSH_FX_NO_SUCH_FILE
       is returned when a reference is made to a file which should exist
       but doesn't.

    SSH_FX_PERMISSION_DENIED
       is returned when the authenticated user does not have sufficient
       permissions to perform the operation.

    SSH_FX_FAILURE
       is a generic catch-all error message; it should be returned if an
       error occurs for which there is no more specific error code
       defined.

    SSH_FX_BAD_MESSAGE
       may be returned if a badly formatted packet or protocol
       incompatibility is detected.

    SSH_FX_NO_CONNECTION
       is a pseudo-error which indicates that the client has no
       connection to the server (it can only be generated locally by the
       client, and MUST NOT be returned by servers).

    SSH_FX_CONNECTION_LOST
       is a pseudo-error which indicates that the connection to the
       server has been lost (it can only be generated locally by the
       client, and MUST NOT be returned by servers).




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    SSH_FX_OP_UNSUPPORTED
       indicates that an attempt was made to perform an operation which
       is not supported for the server (it may be generated locally by
       the client if e.g.  the version number exchange indicates that a
       required feature is not supported by the server, or it may be
       returned by the server if the server does not implement an
       operation).

    The SSH_FXP_HANDLE response has the following format:

    uint32     id
    string     handle

    where "id" is the request identifier, and `handle' is an arbitrary
    string that identifies an open file or directory on the server.  The
    handle is opaque to the client; the client MUST NOT attempt to
    interpret or modify it in any way.  The length of the handle string
    MUST NOT exceed 256 data bytes.

    The SSH_FXP_DATA response has the following format:

    uint32     id
    string     data

    where "id" is the request identifier, and `data' is an arbitrary
    byte string containing the requested data.  The data string may be
    at most the number of bytes requested in a SSH_FXP_READ request, but
    may also be shorter if end of file is reached or if the read is from
    something other than a regular file.

    The SSH_FXP_NAME response has the following format:

    uint32     id
    uint32     count
    repeats count times:
    string     filename
    string     longname
    ATTRS      attrs

    where "id" is the request identifier, "count" is the number of names
    returned in this response, and the remaining fields repeat "count"
    times (so that all three fields are first included for the first
    file, then for the second file, etc).  In the repeated part,
    "filename" is a file name being returned (for SSH_FXP_READDIR, it
    will be a relative name within the directory, without any path
    components; for SSH_FXP_REALPATH it will be an absolute path name),
    "longname" is an expanded format for the file name, similar to what
    is returned by "ls -l" on Unix systems, and "attrs" is the



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    attributes of the file as described in "File Attributes" section.

    The format of the "longname" field is unspecified by this protocol.
    It MUST be suitable for use in the output of a directory listing
    command (in fact, the recommended operation for a directory listing
    command is to simply display this data).  However, clients SHOULD
    NOT attempt to parse the longname field for file attributes; they
    SHOULD use the attrs field instead.

    The recommended format for the longname field is as follows:

    -rwxr-xr-x   1 mjos     staff      348911 Mar 25 14:29 t-filexfer
    1234567890 123 12345678 12345678 12345678 123456789012

    Here, the first line is sample output, and the second field
    indicates widths of the various fields.  Fields are separated by
    spaces.  The first field lists file permissions for user, group, and
    others; the second field is link count; the third field is the name
    of the user who owns the file; the fourth field is the name of the
    group that owns the file; the fifth field is the size of the file in
    bytes; the sixth field (which actually may contain spaces, but is
    fixed to 12 characters) is the file modification time, and the
    seventh field is the file name.  Each field is specified to be a
    minimum of certain number of character positions (indicated by the
    second line above), but may also be longer if the data does not fit
    in the specified length.

    The SSH_FXP_ATTRS response has the following format:

    uint32     id
    ATTRS      attrs

    where "id" is the request identifier, and "attrs" is the returned
    file attributes as described in the "File Attributes" section.

8. Vendor-Specific Extensions

    The SSH_FXP_EXTENDED request provides a generic extension mechanism
    for adding vendor-specific commands.  The request has the following
    format:

    uint32     id
    string     extended-request
    ... any request-specific data ...

    where "id" is the request identifier, and "extended-request" is a
    string of the format "name@domain", where domain is an internet
    domain name of the vendor defining the request.  The rest of the



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    request is completely vendor-specific, and servers should only
    attempt to interpret it if they recognize the "extended-request"
    name.

    The server may respond to such requests using any of the response
    packets defined in the "Responses from the Server to the Client"
    section.  Additionally, the server may also respond with a
    SSH_FXP_EXTENDED_REPLY packet, as defined below.  If the server does
    not recognize the "extended-request" name, then the server MUST
    respond with SSH_FXP_STATUS with error/status set to
    SSH_FX_OP_UNSUPPORTED.

    The SSH_FXP_EXTENDED_REPLY packet can be used to carry arbitrary
    extension-specific data from the server to the client.  It is of the
    following format:

    uint32     id
    ... any request-specific data ...

9. Security Considerations

    This protocol assumes that it is run over a secure channel and that
    the endpoints of the channel have been authenticated.  Thus, this
    protocol assumes that it is externally protected from network-level
    attacks.

    This protocol provides file system access to arbitrary files on the
    server (only constrained by the server implementation).  It is the
    responsibility of the server implementation to enforce any access
    controls that may be required to limit the access allowed for any
    particular user (the user being authenticated externally to this
    protocol, typically using the SSH User Authentication Protocol
    [RFC4252].

    Care must be taken in the server implementation to check the
    validity of received file handle strings.  The server should not
    rely on them directly; it MUST check the validity of each handle
    before relying on it.

11. IANA Considerations

    [RFC Editor: please remove this section]

12. Acknowledgements

    "ssh" is a registered trademark of SSH Communications Security Corp
    in the United States and/or other countries.




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    Most of the text used in this document was written by Tatu Ylonen of
    SSH Communications Security Corp.  The  protocol was designed by
    Tatu Ylonen and Sami Lehtinen of SSH Communications Security Corp.

13.  References

13.1  Normative References

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC4254]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, January 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5646]  Phillips, A., Ed., and M. Davis, Ed., "Tags for
              Identifying Languages", BCP 47, RFC 5646, September 2009.

   [IEEE.1003-1.1996] Institute of Electrical and Electronics Engineers
              "Information Technology - Portable Operating System
              Interface (POSIX) - Part 1: System Application Program
              Interface (API) [C Language]", IEEE Standard 1003.2, 1996.

13.1  Informative References

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, January 2006.


Appendix A: Changes from previous protocol versions

The SSH File Transfer Protocol has changed over time, before it's
standardization.  The following is a description of the incompatible
changes between different versions.

A.1 Changes between versions 3 and 2

   o  The SSH_FXP_READLINK and SSH_FXP_SYMLINK messages were added.

   o  The SSH_FXP_EXTENDED and SSH_FXP_EXTENDED_REPLY messages were



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added.

   o  The SSH_FXP_STATUS message was changed to include fields `error
message' and `language tag'.


A.2 Changes between versions 2 and 1

   o  The SSH_FXP_RENAME message was added.


A.3 Changes between versions 1 and 0

   o  Implementation changes, no actual protocol changes.

Appendix B

This specification has been implemented by OpenSSH.

Authors' Addresses


S. Moonesamy (editor)
76, Ylang Ylang Avenue
Quatre Bornes
Mauritius

Email: sm+ietf@elandsys.com























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