



Secure Shell Working Group                                  J. Galbraith
Internet-Draft                                          VanDyke Software
Expires: December 3, 2005                                   O. Saarenmaa
                                                                F-Secure
                                                               June 2005


                       SSH File Transfer Protocol
                    draft-ietf-secsh-filexfer-10.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

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.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Use with the SSH Connection Protocol . . . . . . . . . . . . .  4
     3.1.  The Use of 'stderr' in the server  . . . . . . . . . . . .  5
   4.  General Packet Format  . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Request Synchronization and Reordering . . . . . . . . . .  6
     4.2.  New data types defined by this document  . . . . . . . . .  7
     4.3.  Packet Types . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Protocol Initialization  . . . . . . . . . . . . . . . . . . .  9
     5.1.  Client Initialization  . . . . . . . . . . . . . . . . . .  9
     5.2.  Server Initialization  . . . . . . . . . . . . . . . . . .  9
     5.3.  Determining Server Newline Convention  . . . . . . . . . . 10
     5.4.  Supported Features . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Version re-negotiation . . . . . . . . . . . . . . . . . . 13
   6.  File Names . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  File Attributes  . . . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  valid-attribute-flags  . . . . . . . . . . . . . . . . . . 16
     7.2.  Type . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.3.  Size . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.4.  allocation-size  . . . . . . . . . . . . . . . . . . . . . 18
     7.5.  Owner and Group  . . . . . . . . . . . . . . . . . . . . . 18
     7.6.  Permissions  . . . . . . . . . . . . . . . . . . . . . . . 19
     7.7.  Times  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.8.  ACL  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.9.  attrib-bits and attrib-bits-valid  . . . . . . . . . . . . 21
     7.10. text-hint  . . . . . . . . . . . . . . . . . . . . . . . . 24
     7.11. mime-type  . . . . . . . . . . . . . . . . . . . . . . . . 24
     7.12. link-count . . . . . . . . . . . . . . . . . . . . . . . . 25
     7.13. untranslated-name  . . . . . . . . . . . . . . . . . . . . 25
     7.14. Extended Attributes  . . . . . . . . . . . . . . . . . . . 25
   8.  Requests From the Client to the Server . . . . . . . . . . . . 25
     8.1.  Opening and Closing Files and Directories  . . . . . . . . 25
       8.1.1.  Opening a File . . . . . . . . . . . . . . . . . . . . 26
       8.1.2.  Opening a Directory  . . . . . . . . . . . . . . . . . 31
       8.1.3.  Closing Handles  . . . . . . . . . . . . . . . . . . . 32
     8.2.  Reading and Writing  . . . . . . . . . . . . . . . . . . . 32
       8.2.1.  Reading Files  . . . . . . . . . . . . . . . . . . . . 32
       8.2.2.  Reading Directories  . . . . . . . . . . . . . . . . . 33
       8.2.3.  Writing Files  . . . . . . . . . . . . . . . . . . . . 34
     8.3.  Removing and Renaming Files  . . . . . . . . . . . . . . . 34
     8.4.  Creating and Deleting Directories  . . . . . . . . . . . . 36
     8.5.  Retrieving File Attributes . . . . . . . . . . . . . . . . 36
     8.6.  Setting File Attributes  . . . . . . . . . . . . . . . . . 37
     8.7.  Dealing with Links . . . . . . . . . . . . . . . . . . . . 38
     8.8.  Byte-range locks . . . . . . . . . . . . . . . . . . . . . 39
     8.9.  Canonicalizing the Server-Side Path Name . . . . . . . . . 40



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       8.9.1.  Best Practice for Dealing with Paths . . . . . . . . . 42
   9.  Responses from the Server to the Client  . . . . . . . . . . . 43
     9.1.  Status Response  . . . . . . . . . . . . . . . . . . . . . 43
     9.2.  Handle Response  . . . . . . . . . . . . . . . . . . . . . 48
     9.3.  Data Response  . . . . . . . . . . . . . . . . . . . . . . 48
     9.4.  Name Response  . . . . . . . . . . . . . . . . . . . . . . 49
     9.5.  Attrs Response . . . . . . . . . . . . . . . . . . . . . . 50
   10. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . 50
   11. Implementation Considerations  . . . . . . . . . . . . . . . . 51
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 51
   13. Changes from Previous Protocol Versions  . . . . . . . . . . . 53
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 53
     14.2. Informative References . . . . . . . . . . . . . . . . . . 53
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 55
   Intellectual Property and Copyright Statements . . . . . . . . . . 56



































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1.  Introduction

   This 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 [I-D.ietf-secsh-architecture], and 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 [I-D.ietf-secsh-architecture].

   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 [RFC2246] and
   transfer of management information in VPN applications.


2.  Acknowledgements

   This document owes it's initial creation and protocol design to Tatu
   Ylonen and Sami Lehtinen of SSH Communications Security Corp.

   We express our gratitude to them for their initial work on this
   protocol.


3.  Use with the SSH Connection Protocol

   When used with the SSH2 Protocol suite, this protocol is intended to
   be used as a subsystem as described in [I-D.ietf-secsh-connect] in
   the section "Starting a Shell or a Command".  The subsystem name used
   with this protocol is "sftp".






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3.1.  The Use of 'stderr' in the server

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

   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.


4.  General Packet Format

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

       uint32           length
       byte             type
       uint32           request-id
           ... type specific fields ...

   'length'
      The length of the entire packet, excluding the length field
      itself, such that, for example, for a packet type containing no
      type specific fields, the length field would be 5, and 9 bytes of
      data would be sent on the wire.  (This is the packet format used
      in [I-D.ietf-secsh-transport].)

      All packet descriptions in this document omit the length field for
      brevity; the length field MUST be included in any case.

      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.

   'type'
      The type code for the packet.






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   'request-id'
      Each request from the client contains a 'request-id' field.  Each
      response from the server includes that same 'request-id' from the
      request that the server is responding to.  One possible
      implementation is for the client to us a monotonically increasing
      request sequence number (modulo 2^32).  There is, however, no
      particular requirement the 'request-id' fields be unique.

      There are two packets, INIT and VERSION, which do not use the
      request-id.
      Packet descriptions in this document will contain the 'request-id'
      field, but will not redefine it.


   Implementations MUST ignore excess data at the end of an otherwise
   valid packet.  Implementations MUST respond to unrecognized packet
   types with an SSH_FX_OP_UNSUPPORTED error.  This will allow the
   protocol to be extended in a backwards compatible way as needed.

   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.

4.1.  Request Synchronization and Reordering

   The protocol and implementations MUST process requests relating to
   the same file in the order in which they are received.  In other
   words, 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.  However, there are no ordering
   restrictions on the server for processing requests from two different
   file transfer connections.  The server may interleave and parallelize
   them at will.

   There are no restrictions on the order in which responses to
   outstanding requests are delivered to the client, except that the
   server must ensure fairness in the sense that processing of no
   request will be indefinitely delayed even if the client is sending
   other requests so that there are multiple outstanding requests all



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   the time.

   A client MUST be prepared to receive responses to multiple overlapped
   requests out of order.

4.2.  New data types defined by this document

   This document defines these data types in addition to those defined
   in [I-D.ietf-secsh-architecture].

   uint16
      Represents a 16-bit unsigned integer.  Stored as 2 bytes in the
      order of decreasing significance (network byte order).

   int64
      Represents a 64-bit signed integer.  Stored using two's
      complement, as eight bytes in the order of decreasing significance
      (network byte order).

   extension-pair

       string extension-name
       string extension-data

      'extension-name' is the name of a protocol extension.  Extensions
      not defined by IETF CONSENSUS MUST follow the the DNS
      extensibility naming convention outlined in [I-D.ietf-secsh-
      architecture].

      'extension-data' is any data specific to the extension, and MAY be
      zero length if the extension has no data.


4.3.  Packet Types

   The following values are defined for packet types.















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       SSH_FXP_INIT                1
       SSH_FXP_VERSION             2
       SSH_FXP_OPEN                3
       SSH_FXP_CLOSE               4
       SSH_FXP_READ                5
       SSH_FXP_WRITE               6
       SSH_FXP_LSTAT               7
       SSH_FXP_FSTAT               8
       SSH_FXP_SETSTAT             9
       SSH_FXP_FSETSTAT           10
       SSH_FXP_OPENDIR            11
       SSH_FXP_READDIR            12
       SSH_FXP_REMOVE             13
       SSH_FXP_MKDIR              14
       SSH_FXP_RMDIR              15
       SSH_FXP_REALPATH           16
       SSH_FXP_STAT               17
       SSH_FXP_RENAME             18
       SSH_FXP_READLINK           19
       SSH_FXP_LINK               21
       SSH_FXP_BLOCK              22
       SSH_FXP_UNBLOCK            23

       SSH_FXP_STATUS            101
       SSH_FXP_HANDLE            102
       SSH_FXP_DATA              103
       SSH_FXP_NAME              104
       SSH_FXP_ATTRS             105

       SSH_FXP_EXTENDED          200
       SSH_FXP_EXTENDED_REPLY    201

   SSH_FXP_EXTENDED and SSH_FXP_EXTENDED_REPLY packets can be used to
   implement extensions, which can be vendor specific.  See Section
   ''Extensions'' for more details.

   Values 210-255 are reserved for use in conjunction with these
   extensions.  The SSH_FXP_EXTENDED packet can be used to negotiate the
   meaning of these reserved types.  It is suggested that the actual
   value to be used also be negotiated, since this will prevent
   collision among multiple uncoordinated extensions.

   The server MUST respond with SSH_FXP_STATUS(SSH_FX_OP_UNSUPPORTED) if
   it receives a packet it does not recognize.

   The use of additional packet types in the non-extension range MUST be
   introduced through IETF consensus.  New packet types to be sent from
   the client to the server MAY be introduced without changing the



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   protocol version (Section 5).  Because the client has no way to
   respond to unrecognized packet types, new packet types to be sent
   from the server to the client the client MUST not used unless the
   protocol version is changed or the client has negotiated to received
   them.  This negotiation MAY be explicit, through the use of
   extensions, or MAY be implicit, by the client itself using a packet
   type not defined above.


5.  Protocol Initialization

   When the file transfer protocol starts, the client 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 that particular version of the
   protocol.

   The version number of the protocol specified in this document is 6.
   The version number should be incremented for each incompatible
   revision of this protocol.

   Note that these two packets DO NOT contain a request id.  These are
   the only such packets in the protocol.

5.1.  Client Initialization

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

       uint32 version

   'version' is the version number of the client.  If the client wishes
   to interoperate with servers that support discontinuous version
   numbers it SHOULD send '3', and then use the 'version-select'
   extension (see below.)  Otherwise, this value is '6' for this version
   of the protocol.

5.2.  Server Initialization

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

       uint32 version
       extension-pair extensions[0..n]

   'version' is the lower of the protocol version supported by the
   server and the version number received from the client.



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   'extensions' is 0 or more extension-pairs (Section 4.2).
   Implementations MUST silently ignore any extensions whose names they
   do not recognize.

5.3.  Determining Server Newline Convention

   In order to correctly process text files in a cross platform
   compatible way, newline sequences must be converted between client
   and server conventions.

   The SSH_FXF_ACCESS_TEXT_MODE file open flag (Section 8.1.1) makes it
   possible to request that the server translate a file to a 'canonical'
   wire format.  This format uses CRLF as the line separator.

   Servers for systems using other conventions MUST translate to and
   from the canonical form.

   However, to ease the burden of implementation on servers that use a
   single, simple, separator sequence the following extension allows the
   canonical format to be changed.

       string "newline"
       string new-canonical-separator (usually CR or LF or CRLF)

   All clients MUST support this extension.

   When processing text files, clients SHOULD NOT translate any
   character or sequence that is not an exact match of the server's
   newline separator.

   In particular, if the newline sequence being used is the canonical
   CRLF sequence, a lone CR or a lone LF SHOULD be written through
   without change.

5.4.  Supported Features

   The sftp protocol has grown to be very rich, and now supports a
   number of features that may not be available on all servers.

   When a server receives a request for a feature it cannot support, it
   MUST return a SSH_FX_OP_UNSUPPORTED status code, unless otherwise
   specified.  The following extension facilitates clients being able to
   use the maximum available feature set, and yet not be overly burdened
   by dealing with SSH_FX_OP_UNSUPPORTED status codes.  All servers MUST
   include as part of their version packet.






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       string "supported2"
       string supported-structure
           uint32 supported-attribute-mask
           uint32 supported-attribute-bits
           uint32 supported-open-flags
           uint32 supported-access-mask
           uint32 max-read-size
           uint16 supported-open-block-vector
           uint16 supported-block-vector
           uint32 attrib-extension-count
           string attrib-extension-names[attrib_extension-count]
           uint32 extension-count
           string extension-names[extension-count]

   Note that the name "supported2" is used here to avoid conflict with
   the slightly different "supported" extension that was previously
   used.
   supported-attribute-mask
      This mask MAY by applied to the 'File Attributes' valid-attribute-
      flags field (Section 7.1) to ensure that no unsupported attributes
      are present during a operation which writes attributes.

   supported-attribute-bits
      This mask MAY by applied to the 'File Attributes' attrib-bits
      field (Section 7.9) to ensure that no unsupported attrib-bits are
      present during a operation which writes attributes.

   supported-open-flags
      The supported-open-flags mask MAY be applied to the SSH_FXP_OPEN
      (Section 8.1.1) flags field.

   supported-access-mask
      This mask may be applied to the ace-mask field of an ACL.

      This mask SHOULD NOT be applied to the desired-access field of the
      SSH_FXP_OPEN (Section 8.1.1) request.  Doing so will simply result
      in not requesting the access required by the client.  In this
      case, the server is responsible for translating the clients
      requested access to a mode it supports that is sufficient to grant
      all access requested by the client.

   max-read-size
      This is the maximum read size that the server guarantees to
      complete.  For example, certain embedded server implementations
      complete only the first 4K of a read, even if there is additional
      data to be read from the file.





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      If the server specifies a non-zero value for max-read-size, it
      MUST return the requested number of bytes for reads that are less
      than or equal to the value, unless it encounters EOF or an ERROR.

      The server MAY use this value to express that it is willing to
      handle very large read requests, in excess of the standard 34000
      bytes specified in Section 4.

   supported-open-block-vector
   supported-block-vector
      16-bit masks specifying which combinations of blocking flags are
      supported.  Each bit corresponds to one combination of the
      SSH_FXF_ACCESS_BLOCK_READ, SSH_FXF_ACCESS_BLOCK_WRITE,
      SSH_FXF_ACCESS_BLOCK_DELETE, and SSH_FXF_ACCESS_BLOCK_ADVISORY
      bits from Section 7.1.1.3, with a set bit corresponding to a
      supported combination and a clear bit an unsupported combination.
      The index of a bit, bit zero being the least significant bit,
      viewed as a four-bit number, corresponds to a combination of flag
      bits, shifted right so that BLOCK_READ is the least significant
      bit.  The combination `no blocking flags' MUST be supported, so
      the low bit will always be set.

      For example, a server supporting only the classic advisory read
      (shared) and write (exclusive) locks would set the bits
      corresponding to READ+WRITE+ADVISORY, 0b1011, and WRITE+ADVISORY,
      0b1010, plus the always-set bit 0b0000, giving a mask value of
      0b0000110000000001, or 0x0c01; a server supporting no locking at
      all would set only bit zero, giving 0x0001.

      'supported-open-block-masks' applies to the SSH_FXP_OPEN
      (Section 8.1.1) flags field. 'supported-block-masks' applies to
      the SSH_FXF_BLOCK request.

   attrib-extension-count
      Count of extension names in the attrib-extension-names array.

   attrib-extension-names
      Names of extensions that can be used in an ATTRS (Section 7.14)
      structure.

   extension-count
      Count of extension names in the extension-names array.

   extension-names
      Names of extensions that can be used with the SSH_FXP_EXTEND
      (Section 10) packet.





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   Naturally, if a given attribute field, attribute mask bit, open flag,
   or extension is required for correct operation, the client MUST
   either not allow the bit to be masked off, or MUST fail the operation
   gracefully without sending the request to the server.

   The client MAY send requests that are not supported by the server;
   however, it is not normally expected to be productive to do so.  The
   client SHOULD apply the mask even to attrib structures received from
   the server.  The server MAY include attributes or attrib-bits that
   are not included in the mask.  Such attributes or attrib-bits are
   effectively read-only.

5.5.  Version re-negotiation

   If the server supports other versions than what was negotiated, it
   may wish to send the 'versions' extension to inform the client of
   this fact.  The client may then optionally choose to use one of the
   other versions supported.

       string "versions"
       string comma-separated-versions

   'comma-separated-versions' is a string of comma separated version
   numbers.  Defined versions are: "2", "3", "4", "5", "6".  Any other
   version advertised by the server must follow the DNS extensibility
   naming convention outlined in [I-D.ietf-secsh-architecture].

   For example: "2,3,6,private@example.com".

   If the client and server have negotiated a a version greater than or
   equal to version '3' (the version at which SSH_FXP_EXTENDED was
   introduced) in the initial VERSION/INIT exchange, the client may
   select a new version to use from the list the server provided using
   the following SSH_FXP_EXTENDED request.

       string "version-select"
       string version-from-list

   If the 'version-from-list' is one of the versions on the servers
   list, the server MUST respond with SSH_FX_OK.  If the server did not
   send the "versions" extension, or the version-from-list was not
   included, the server MAY send a status response describing the
   failure, but MUST then close the channel without processing any
   further requests.

   The 'version-select' MUST be the first request from the client to the
   server; if it is not, the server MUST fail the request and close the
   channel.



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   Although this request does take a full round trip, no client need
   wait for the response before continuing, because any valid request
   MUST succeed, and any invalid request results in a channel close.
   Since the request is the first request, it is not possible for the
   server to have already sent responses conforming to the old version.

   Typically, the client SHOULD NOT down-grade the protocol version
   using this extension; however, it is not forbidden to do so.  One
   reason a client might do so is to work around a buggy implementation.


6.  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 ".."  (Section 12) as
   referring to the parent directory, and "." as referring to the
   current directory.

   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.

   The preferred encoding for filenames is UTF-8.  This is consistent
   with IETF Policy on Character Sets and Languages [RFC2277] and it is
   further supposed that the server is more likely to support any local
   character set and be able to convert it to UTF-8.

   However, because the server does not always know the encoding of
   filenames, it is not always possible for the server to preform a



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   valid translation to UTF-8.  When an invalid translation to UTF-8 is
   preformed, it becomes impossible to manipulate the file, because the
   translation is not reversible.  Therefore, the following extensions
   are provided in order to make it possible for the server to
   communicate it's abilities to the client, and to allow the client to
   control whether the server attempts the conversion.

   A server MAY include the following extension with it's version
   packet.

       string "filename-charset"
       string charset-name

   A server that can always provide a valid UTF-8 translation for
   filenames SHOULD NOT send this extension.  Otherwise, the server
   SHOULD send this extension and include the encoding most likely to be
   used for filenames.  This value will most likely be derived from the
   LC_CTYPE on most unix-like systems.

   A server that does not send this extension MUST send all filenames
   encoded in UTF-8.  All clients MUST support UTF-8 filenames.

   If the server included the 'filename-charset' extension with its
   VERSION packet, a client MAY send the following extension to turn off
   server translation to UTF-8.

       string  "filename-translation-control"
       bool    do-translate

   If the client does not send this extension, the server MUST continue
   to attempt translation to UTF-8.  When a client sends this extension,
   the server MUST enable filename translation if 'do-translate' is
   true, or disable filename translation if it is false.

   The server MUST respond with a STATUS response; if the server sent a
   'filename-charset' extension, the status MUST be SUCCESS.  Otherwise,
   the status MUST be SSH_FX_OP_UNSUPPORTED.

   When UTF-8 is sent, the shortest valid UTF-8 encoding of the UNICODE
   data MUST be used.  The server is responsible for converting the
   UNICODE data to whatever canonical form it requires.  For example, if
   the server requires that precomposed characters always be used, the
   server MUST NOT assume the filename as sent by the client has this
   attribute, but must do this normalization itself.


7.  File Attributes




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   A new compound data type, 'ATTRS', is defined for encoding file
   attributes.  The same encoding is used both when returning file
   attributes from the server and when sending file attributes to the
   server.

       uint32   valid-attribute-flags
       byte     type                   always present
       uint64   size                   if flag SIZE
       uint64   allocation-size        if flag ALLOCATION_SIZE
       string   owner                  if flag OWNERGROUP
       string   group                  if flag OWNERGROUP
       uint32   permissions            if flag PERMISSIONS
       int64    atime                  if flag ACCESSTIME
       uint32   atime-nseconds            if flag SUBSECOND_TIMES
       int64    createtime             if flag CREATETIME
       uint32   createtime-nseconds       if flag SUBSECOND_TIMES
       int64    mtime                  if flag MODIFYTIME
       uint32   mtime-nseconds            if flag SUBSECOND_TIMES
       int64    ctime                  if flag CTIME
       uint32   ctime-nseconds            if flag SUBSECOND_TIMES
       string   acl                    if flag ACL
       uint32   attrib-bits            if flag BITS
       uint32   attrib-bits-valid      if flag BITS
       byte     text-hint              if flag TEXT_HINT
       string   mime-type              if flag MIME_TYPE
       uint32   link-count             if flag LINK_COUNT
       string   untranslated-name      if flag UNTRANSLATED_NAME
       uint32   extended-count         if flag EXTENDED
       extended-pair extensions

7.1.  valid-attribute-flags

   The 'valid-attribute-flags' specifies which of the fields are
   present.  Those fields for which the corresponding flag is not set
   are not present (not included in the packet).

   The server generally includes all attributes it knows about; however,
   it may exclude attributes that are overly expensive to retrieve
   unless the client explicitly requests them.

   When writing attributes, the server SHOULD NOT modify attributes that
   are not present in the structure.  However, if necessary, the server
   MAY use a default value for an absent attribute.

   In general, unless otherwise specified, if a server cannot support
   writing an attribute requested, it must fail the setstat operation.
   In this case, none of the attributes SHOULD be changed.




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   New fields can only be added by incrementing the protocol version
   number (or by using the extension mechanism described below).

   The following values are defined:

       SSH_FILEXFER_ATTR_SIZE              0x00000001
       SSH_FILEXFER_ATTR_PERMISSIONS       0x00000004
       SSH_FILEXFER_ATTR_ACCESSTIME        0x00000008
       SSH_FILEXFER_ATTR_CREATETIME        0x00000010
       SSH_FILEXFER_ATTR_MODIFYTIME        0x00000020
       SSH_FILEXFER_ATTR_ACL               0x00000040
       SSH_FILEXFER_ATTR_OWNERGROUP        0x00000080
       SSH_FILEXFER_ATTR_SUBSECOND_TIMES   0x00000100
       SSH_FILEXFER_ATTR_BITS              0x00000200
       SSH_FILEXFER_ATTR_ALLOCATION_SIZE   0x00000400
       SSH_FILEXFER_ATTR_TEXT_HINT         0x00000800
       SSH_FILEXFER_ATTR_MIME_TYPE         0x00001000
       SSH_FILEXFER_ATTR_LINK_COUNT        0x00002000
       SSH_FILEXFER_ATTR_UNTRANSLATED_NAME 0x00004000
       SSH_FILEXFER_ATTR_CTIME             0x00008000
       SSH_FILEXFER_ATTR_EXTENDED          0x80000000

   0x00000002 was used in a previous version of this protocol.  It is
   now a reserved value and MUST NOT appear in the mask.  Some future
   version of this protocol may reuse this value.

7.2.  Type

   The type field is always present.  The following types are defined:

       SSH_FILEXFER_TYPE_REGULAR          1
       SSH_FILEXFER_TYPE_DIRECTORY        2
       SSH_FILEXFER_TYPE_SYMLINK          3
       SSH_FILEXFER_TYPE_SPECIAL          4
       SSH_FILEXFER_TYPE_UNKNOWN          5
       SSH_FILEXFER_TYPE_SOCKET           6
       SSH_FILEXFER_TYPE_CHAR_DEVICE      7
       SSH_FILEXFER_TYPE_BLOCK_DEVICE     8
       SSH_FILEXFER_TYPE_FIFO             9

   On a POSIX system, these values would be derived from the mode field
   of the stat structure.  SPECIAL should be used for files that are of
   a known type which cannot be expressed in the protocol.  UNKNOWN
   should be used if the type is not known.

7.3.  Size

   The 'size' field specifies the number of bytes that can be read from



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   the file, or in other words, the location of the end-of-file.  This
   attribute MUST NOT be present during file creation.

   If this field is present during a setstat operation, the file MUST be
   extended or truncated to the specified size.

   Files opened with the SSH_FXF_ACCESS_TEXT flag may have a size that
   is greater or less than the value of the size field.  The server MAY
   fail setstat operations specifying size for files opened with the
   SSH_FXF_ACCESS_TEXT flag.

7.4.  allocation-size

   The 'allocation-size' field specifies the number of bytes that the
   file consumes on disk.  This field MAY be less than the 'size' field
   if the file is 'sparse' (Section 7.9).

   When present during file creation, the file SHOULD be created and the
   specified number of bytes preallocated.  If the preallocation fails,
   the file should be removed (if it was created) and an error returned.

   If this field is present during a setstat operation, the file SHOULD
   be extended or truncated to the specified size.  The 'size' of the
   file may be affected by this operation.  If the operation succeeds,
   the 'size' should be the minimum of the 'size' before the operation
   and the new 'allocation-size'.

   Querying the 'allocation-size' after setting it MUST return a value
   that is greater-than or equal to the value set, but it MAY not return
   the precise value set.

   If both 'size' and 'allocation-size' are set during a setstat
   operation, and 'allocation-size' is less than 'size', the server MUST
   return SSH_FX_INVALID_PARAMETER.

7.5.  Owner and Group

   The 'owner' and 'group' fields are represented as UTF-8 strings; this
   is the form used by NFS v4.  See NFS version 4 Protocol [RFC3010].
   The following text is selected quotations from section 5.6.

   To avoid a representation that is tied to a particular underlying
   implementation at the client or server, the use of UTF-8 strings has
   been chosen.  The string should be of the form "user@dns_domain".
   This will allow for a client and server that do not use the same
   local representation the ability to translate to a common syntax that
   can be interpreted by both.  In the case where there is no
   translation available to the client or server, the attribute value



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   must be constructed without the "@".  Therefore, the absence of the @
   from the owner or owner_group attribute signifies that no translation
   was available and the receiver of the attribute should not place any
   special meaning on the attribute value.  Even though the attribute
   value cannot be translated, it may still be useful.  In the case of a
   client, the attribute string may be used for local display of
   ownership.

   user@localhost represents a user in the context of the server.

   If either the owner or group field is zero length, the field should
   be considered absent, and no change should be made to that specific
   field during a modification operation.

7.6.  Permissions

   The 'permissions' field contains a bit mask specifying file
   permissions.  These permissions correspond to the st_mode field of
   the stat structure defined by POSIX [IEEE.1003-1.1996].

   This protocol uses the following values for the symbols declared in
   the POSIX standard.

       S_IRUSR  0000400 (octal)
       S_IWUSR  0000200
       S_IXUSR  0000100
       S_IRGRP  0000040
       S_IWGRP  0000020
       S_IXGRP  0000010
       S_IROTH  0000004
       S_IWOTH  0000002
       S_IXOTH  0000001
       S_ISUID  0004000
       S_ISGID  0002000
       S_ISVTX  0001000

   Implementations MUST NOT send bits that are not defined.

   The server SHOULD NOT apply a 'umask' to the mode bits; but should
   set the mode bits as specified by the client.  The client MUST apply
   an appropriate 'umask' to the mode bits before sending them.

7.7.  Times

   The 'atime' field contains the last access time of the file.  Many
   operating systems either don't have this field, only optionally
   maintain it, or maintain it with less resolution than other fields.




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   The 'mtime' contains the last time the file was written.

   'createtime' contains the creation time of the file.

   'ctime' contains the last time the file attributes were changed.  The
   exact meaning of this field depends on the server.

   All times are represented as seconds from Jan 1, 1970 in UTC.  A
   negative value indicates number of seconds before Jan 1, 1970.  In
   both cases, if the SSH_FILEXFER_ATTR_SUBSECOND_TIMES flag is set, the
   nseconds field is to be added to the seconds field for the final time
   representation.  For example, if the time to be represented is one-
   half second before 0 hour January 1, 1970, the seconds field would
   have a value of negative one (-1) and the nseconds fields would have
   a value of one-half second (500000000).  Values greater than
   999,999,999 for nseconds are considered invalid.

7.8.  ACL

   The 'ACL' field contains an ACL similar to that defined in section
   5.9 of NFS version 4 Protocol [RFC3010].

       uint32   ace-count

       repeated ace-count times:
       uint32   ace-type
       uint32   ace-flag
       uint32   ace-mask
       string   who [UTF-8]

   ace-type is one of the following four values (taken from NFS Version
   4 Protocol [RFC3010]:

       ACE4_ACCESS_ALLOWED_ACE_TYPE 0x00000000
       ACE4_ACCESS_DENIED_ACE_TYPE  0x00000001
       ACE4_SYSTEM_AUDIT_ACE_TYPE   0x00000002
       ACE4_SYSTEM_ALARM_ACE_TYPE   0x00000003

   ace-flag is a combination of the following flag values.  See NFS
   Version 4 Protocol [RFC3010] section 5.9.2:

       ACE4_FILE_INHERIT_ACE           0x00000001
       ACE4_DIRECTORY_INHERIT_ACE      0x00000002
       ACE4_NO_PROPAGATE_INHERIT_ACE   0x00000004
       ACE4_INHERIT_ONLY_ACE           0x00000008
       ACE4_SUCCESSFUL_ACCESS_ACE_FLAG 0x00000010
       ACE4_FAILED_ACCESS_ACE_FLAG     0x00000020
       ACE4_IDENTIFIER_GROUP           0x00000040



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   ace-mask is any combination of the following flags (taken from
   [RFC3010], section 5.9.3.  The semantic meaning of these flags is
   also given in [RFC3010].

       ACE4_READ_DATA         0x00000001
       ACE4_LIST_DIRECTORY    0x00000001
       ACE4_WRITE_DATA        0x00000002
       ACE4_ADD_FILE          0x00000002
       ACE4_APPEND_DATA       0x00000004
       ACE4_ADD_SUBDIRECTORY  0x00000004
       ACE4_READ_NAMED_ATTRS  0x00000008
       ACE4_WRITE_NAMED_ATTRS 0x00000010
       ACE4_EXECUTE           0x00000020
       ACE4_DELETE_CHILD      0x00000040
       ACE4_READ_ATTRIBUTES   0x00000080
       ACE4_WRITE_ATTRIBUTES  0x00000100
       ACE4_DELETE            0x00010000
       ACE4_READ_ACL          0x00020000
       ACE4_WRITE_ACL         0x00040000
       ACE4_WRITE_OWNER       0x00080000
       ACE4_SYNCHRONIZE       0x00100000

   who is a UTF-8 string of the form described in 'Owner and Group'
   (Section 7.5)

   Also, as per '5.9.4 ACE who' [RFC3010] there are several identifiers
   that need to be understood universally.  Some of these identifiers
   cannot be understood when an client access the server, but have
   meaning when a local process accesses the file.  The ability to
   display and modify these permissions is permitted over SFTP.

      OWNER The owner of the file.
      GROUP The group associated with the file.
      EVERYONE The world.
      INTERACTIVE Accessed from an interactive terminal.
      NETWORK Accessed via the network.
      DIALUP Accessed as a dialup user to the server.
      BATCH Accessed from a batch job.
      ANONYMOUS Accessed without any authentication.
      AUTHENTICATED Any authenticated user (opposite of ANONYMOUS).
      SERVICE Access from a system service.

   To avoid conflict, these special identifiers are distinguish by an
   appended "@".  For example: ANONYMOUS@.

7.9.  attrib-bits and attrib-bits-valid

   These fields, taken together, reflect various attributes of the file



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   or directory, on the server.

   Bits not set in 'attrib-bits-valid' MUST be ignored in the 'attrib-
   bits' field.  This allows both the server and the client to
   communicate only the bits it knows about without inadvertently
   twiddling bits they don't understand.

   The following attrib-bits are defined:

       SSH_FILEXFER_ATTR_FLAGS_READONLY         0x00000001
       SSH_FILEXFER_ATTR_FLAGS_SYSTEM           0x00000002
       SSH_FILEXFER_ATTR_FLAGS_HIDDEN           0x00000004
       SSH_FILEXFER_ATTR_FLAGS_CASE_INSENSITIVE 0x00000008
       SSH_FILEXFER_ATTR_FLAGS_ARCHIVE          0x00000010
       SSH_FILEXFER_ATTR_FLAGS_ENCRYPTED        0x00000020
       SSH_FILEXFER_ATTR_FLAGS_COMPRESSED       0x00000040
       SSH_FILEXFER_ATTR_FLAGS_SPARSE           0x00000080
       SSH_FILEXFER_ATTR_FLAGS_APPEND_ONLY      0x00000100
       SSH_FILEXFER_ATTR_FLAGS_IMMUTABLE        0x00000200
       SSH_FILEXFER_ATTR_FLAGS_SYNC             0x00000400
       SSH_FILEXFER_ATTR_FLAGS_TRANSLATION_ERR  0x00000800

   SSH_FILEXFER_ATTR_FLAGS_READONLY
      Advisory, read-only bit.  This bit is not part of the access
      control information on the file, but is rather an advisory field
      indicating that the file should not be written.

   SSH_FILEXFER_ATTR_FLAGS_SYSTEM
      The file is part of the operating system.

   SSH_FILEXFER_ATTR_FLAGS_HIDDEN
      File SHOULD NOT be shown to user unless specifically requested.
      For example, most UNIX systems SHOULD set this bit if the filename
      begins with a 'period'.  This bit may be read-only (Section 5.4).
      Most UNIX systems will not allow this to be changed.

   SSH_FILEXFER_ATTR_FLAGS_CASE_INSENSITIVE
      This attribute applies only to directories.  This attribute is
      always read-only, and cannot be modified.  This attribute means
      that files and directory names in this directory should be
      compared without regard to case.

      It is recommended that where possible, the server's filesystem be
      allowed to do comparisons.  For example, if a client wished to
      prompt a user before overwriting a file, it should not compare the
      new name with the previously retrieved list of names in the
      directory.  Rather, it should first try to create the new file by
      specifying SSH_FXF_CREATE_NEW flag.  Then, if this fails and



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      returns SSH_FX_FILE_ALREADY_EXISTS, it should prompt the user and
      then retry the create specifying SSH_FXF_CREATE_TRUNCATE.

      Unless otherwise specified, filenames are assumed to be case
      sensitive.

   SSH_FILEXFER_ATTR_FLAGS_ARCHIVE
      The file should be included in backup / archive operations.

   SSH_FILEXFER_ATTR_FLAGS_ENCRYPTED
      The file is stored on disk using file-system level transparent
      encryption.  This flag does not affect the file data on the wire
      (for either READ or WRITE requests.)

   SSH_FILEXFER_ATTR_FLAGS_COMPRESSED
      The file is stored on disk using file-system level transparent
      compression.  This flag does not affect the file data on the wire.

   SSH_FILEXFER_ATTR_FLAGS_SPARSE
      The file is a sparse file; this means that file blocks that have
      not been explicitly written are not stored on disk.  For example,
      if a client writes a buffer at 10 M from the beginning of the
      file, the blocks between the previous EOF marker and the 10 M
      offset would not consume physical disk space.

      Some servers may store all files as sparse files, in which case
      this bit will be unconditionally set.  Other servers may not have
      a mechanism for determining if the file is sparse, and so the file
      MAY be stored sparse even if this flag is not set.

   SSH_FILEXFER_ATTR_FLAGS_APPEND_ONLY
      Opening the file without either the SSH_FXF_ACCESS_APPEND_DATA or
      the SSH_FXF_ACCESS_APPEND_DATA_ATOMIC flag (Section 8.1.1.3) MUST
      result in an SSH_FX_INVALID_PARAMETER error.

   SSH_FILEXFER_ATTR_FLAGS_IMMUTABLE
      The file cannot be deleted or renamed, no hard link can be created
      to this file, and no data can be written to the file.

      This bit implies a stronger level of protection than
      SSH_FILEXFER_ATTR_FLAGS_READONLY, the file permission mask or
      ACLs.  Typically even the superuser cannot write to immutable
      files, and only the superuser can set or remove the bit.








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   SSH_FILEXFER_ATTR_FLAGS_SYNC
      When the file is modified, the changes are written synchronously
      to the disk.

   SSH_FILEXFER_ATTR_FLAGS_TRANSLATION_ERR
      The server MAY include this bit in a directory listing or realpath
      response.  It indicates there was a failure in the translation to
      UTF-8.  If this flag is included, the server SHOULD also include
      the UNTRANSLATED_NAME attribute.


7.10.  text-hint

   The value is one of the following enumerations, and indicates what
   the server knows about the content of the file.

       SSH_FILEXFER_ATTR_KNOWN_TEXT        0x00
       SSH_FILEXFER_ATTR_GUESSED_TEXT      0x01
       SSH_FILEXFER_ATTR_KNOWN_BINARY      0x02
       SSH_FILEXFER_ATTR_GUESSED_BINARY    0x03

   SSH_FILEXFER_ATTR_KNOWN_TEXT
      The server knows the file is a text file, and should be opened
      using the SSH_FXF_ACCESS_TEXT_MODE flag.

   SSH_FILEXFER_ATTR_GUESSED_TEXT
      The server has applied a heuristic or other mechanism and believes
      that the file should be opened with the SSH_FXF_ACCESS_TEXT_MODE
      flag.

   SSH_FILEXFER_ATTR_KNOWN_BINARY
      The server knows the file has binary content.

   SSH_FILEXFER_ATTR_GUESSED_BINARY
      The server has applied a heuristic or other mechanism and believes
      has binary content, and should not be opened with the
      SSH_FXF_ACCESS_TEXT_MODE flag.


   This flag MUST NOT be present during either a setstat or a fsetstat
   operation.

7.11.  mime-type

   The 'mime-type' field contains the mime-type [RFC1521] string.  Most
   servers will not know this information and should not set the bit in
   their supported-attribute-mask.




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7.12.  link-count

   This field contains the hard link count of the file.  This attribute
   MUST NOT be present during a setstat operation.

7.13.  untranslated-name

   This field contains the name before filename translation was attempt.
   It MUST NOT be included unless the server also set the
   SSH_FILEXFER_ATTR_FLAGS_TRANSLATION_ERR (Section 7.9) bit in the
   attrib-bits field.

7.14.  Extended Attributes

   The SSH_FILEXFER_ATTR_EXTENDED flag provides a general extension
   mechanism for the attrib structure.  If the flag is specified, then
   the 'extended_count' field is present.  It specifies the number of
   'extension-pair' items that follow.  Each of these items specifies an
   extended attribute.  Implementations MUST return SSH_FX_UNSUPPORTED
   if there are any unrecognized extensions.  Clients can avoid sending
   unsupported extensions by examining the attrib-extension-names of the
   "supported2" extension attrib-extension-names (Section 5.4).

   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 be defined only 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.


8.  Requests From the Client to the Server

   Requests from the client to the server represent the various file
   system operations.

8.1.  Opening and Closing Files and Directories

   Many operations in the protocol operate on open files.  The
   SSH_FXP_OPEN and SSH_FXP_OPENDIR requests return a handle (which is
   an opaque, variable-length string) which may be used to access the
   file or directory later.  The client MUST NOT send requests to 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.



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   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.

8.1.1.  Opening a File

   Files are opened and created using the SSH_FXP_OPEN message.

       byte   SSH_FXP_OPEN
       uint32 request-id
       string filename [UTF-8]
       uint32 desired-access
       uint32 flags
       ATTRS  attrs

   The response to this message will be either SSH_FXP_HANDLE (if the
   operation is successful) or SSH_FXP_STATUS (if the operation fails.)

8.1.1.1.  filename

   The 'filename' field specifies the file name.  See Section ''File
   Names'' for more information.  If 'filename' is a directory file, the
   server MUST return an SSH_FX_FILE_IS_A_DIRECTORY error.

8.1.1.2.  desired-access

   The 'desired-access' field is a bitmask containing a combination of
   values from the ace-mask flags (Section 7.8).  Note that again, the
   meaning of these flags is given in [RFC3010].

   The server MUST be prepared to translate the SFTP access flags into
   its local equivalents.  If the server cannot grant the access
   desired, it MUST return SSH_FX_PERMISSION_DENIED.

   The server MAY open the file with greater access than requested if
   the user has such access and the server implementation requires it.
   For example, a server that does not distinguish between
   READ_ATTRIBUTE and READ_DATA will have to request full 'read' access
   to the file when the client only requested READ_ATTRIBUTE, resulting
   in greater access than the client originally requested.

   In such cases, it is possible, and permissible in the protocol, that
   the client could open a file requesting some limited access, and then



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   access the file in a way not permitted by that limited access and the
   server would permit such action.  However, the server MUST NOT ever
   grant access to the file that the client does not actually have the
   rights to.

8.1.1.3.  flags

   The 'flags' field controls various aspects of the operation,
   including whether or not the file is created and the kind of locking
   desired.

   The following 'flags' are defined:

           SSH_FXF_ACCESS_DISPOSITION         = 0x00000007
               SSH_FXF_CREATE_NEW             = 0x00000000
               SSH_FXF_CREATE_TRUNCATE        = 0x00000001
               SSH_FXF_OPEN_EXISTING          = 0x00000002
               SSH_FXF_OPEN_OR_CREATE         = 0x00000003
               SSH_FXF_TRUNCATE_EXISTING      = 0x00000004
           SSH_FXF_ACCESS_APPEND_DATA         = 0x00000008
           SSH_FXF_ACCESS_APPEND_DATA_ATOMIC  = 0x00000010
           SSH_FXF_ACCESS_TEXT_MODE           = 0x00000020
           SSH_FXF_ACCESS_BLOCK_READ          = 0x00000040
           SSH_FXF_ACCESS_BLOCK_WRITE         = 0x00000080
           SSH_FXF_ACCESS_BLOCK_DELETE        = 0x00000100
           SSH_FXF_ACCESS_BLOCK_ADVISORY      = 0x00000200
           SSH_FXF_ACCESS_NOFOLLOW            = 0x00000400
           SSH_FXF_ACCESS_DELETE_ON_CLOSE     = 0x00000800

   SSH_FXF_ACCESS_DISPOSITION
      Disposition is a 3 bit field that controls how the file is opened.
      The server MUST support these bits.  Any one of the following
      enumeration is allowed:

      SSH_FXF_CREATE_NEW
         A new file is created; if the file already exists, the server
         MUST return status SSH_FX_FILE_ALREADY_EXISTS.

      SSH_FXF_CREATE_TRUNCATE
         A new file is created; if the file already exists, it is opened
         and truncated.

      SSH_FXF_OPEN_EXISTING
         An existing file is opened.  If the file does not exist, the
         server MUST return SSH_FX_NO_SUCH_FILE.  If a directory in the
         path does not exist, the server SHOULD return
         SSH_FX_NO_SUCH_PATH.  It is also acceptable if the server
         returns SSH_FX_NO_SUCH_FILE in this case.



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      SSH_FXF_OPEN_OR_CREATE
         If the file exists, it is opened.  If the file does not exist,
         it is created.

      SSH_FXF_TRUNCATE_EXISTING
         An existing file is opened and truncated.  If the file does not
         exist, the server MUST return the same error codes as defined
         for SSH_FXF_OPEN_EXISTING.

   SSH_FXF_ACCESS_APPEND_DATA
      Data is always written at the end of the file.  The offset field
      of the SSH_FXP_WRITE requests are ignored.

      Data is not required to be appended atomically.  This means that
      if multiple writers attempt to append data simultaneously, data
      from the first may be lost.  However, data MAY be appended
      atomically.

   SSH_FXF_ACCESS_APPEND_DATA_ATOMIC
      Data is always written at the end of the file.  The offset field
      of the SSH_FXP_WRITE requests are ignored.

      Data MUST be written atomically so that there is no chance that
      multiple appenders can collide and result in data being lost.

      If both append flags are specified, the server SHOULD use atomic
      append if it is available, but SHOULD use non-atomic appends
      otherwise.  The server SHOULD NOT fail the request in this case.

   SSH_FXF_ACCESS_TEXT_MODE
      Indicates that the server should treat the file as text and
      convert it to the canonical newline convention in use.  (See
      Determining Server Newline Convention.  (Section 5.3)

      When a file is opened with this flag, the offset field in the read
      and write functions is ignored.

      Servers MUST process multiple, parallel reads and writes correctly
      in this mode.  Naturally, it is permissible for them to do this by
      serializing the requests.

      Clients SHOULD use the SSH_FXF_ACCESS_APPEND_DATA flag to append
      data to a text file rather then using write with a calculated
      offset.







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      To support seeks on text files the following SSH_FXP_EXTENDED
      packet is defined.


               string "text-seek"
               string file-handle
               uint64 line-number

      line-number is the index of the line number to seek to, where byte
      0 in the file is line number 0, and the byte directly following
      the first newline sequence in the file is line number 1 and so on.

      The response to a "text-seek" request is an SSH_FXP_STATUS
      message.

      An attempt to seek past the end-of-file should result in a
      SSH_FX_EOF status.

      Servers SHOULD support at least one "text-seek" in order to
      support resume.  However, a client MUST be prepared to receive
      SSH_FX_OP_UNSUPPORTED when attempting a "text-seek" operation.
      The client can then try a fall-back strategy, if it has one.

   SSH_FXF_ACCESS_BLOCK_READ
      The server MUST guarantee that no other handle has been opened
      with ACE4_READ_DATA access, and that no other handle will be
      opened with ACE4_READ_DATA access until the client closes the
      handle.  (This MUST apply both to other clients and to other
      processes on the server.)

      If there is a conflicting lock the server MUST return
      SSH_FX_LOCK_CONFLICT.  If the server cannot make the locking
      guarantee, it MUST return SSH_FX_OP_UNSUPPORTED.

      Other handles MAY be opened for ACE4_WRITE_DATA or any other
      combination of accesses, as long as ACE4_READ_DATA is not included
      in the mask.

   SSH_FXF_ACCESS_BLOCK_WRITE
      The server MUST guarantee that no other handle has been opened
      with ACE4_WRITE_DATA or ACE4_APPEND_DATA access, and that no other
      handle will be opened with ACE4_WRITE_DATA or ACE4_APPEND_DATA
      access until the client closes the handle.  (This MUST apply both
      to other clients and to other processes on the server.)







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      If there is a conflicting lock the server MUST return
      SSH_FX_LOCK_CONFLICT.  If the server cannot make the locking
      guarantee, it MUST return SSH_FX_OP_UNSUPPORTED.

      Other handles MAY be opened for ACE4_READ_DATA or any other
      combination of accesses, as long as neither ACE4_WRITE_DATA nor
      ACE4_APPEND_DATA are included in the mask.

   SSH_FXF_ACCESS_BLOCK_DELETE
      The server MUST guarantee that no other handle has been opened
      with ACE4_DELETE access, opened with the
      SSH_FXF_ACCESS_DELETE_ON_CLOSE flag set, and that no other handle
      will be opened with ACE4_DELETE access or with the
      SSH_FXF_ACCESS_DELETE_ON_CLOSE flag set, and that the file itself
      is not deleted in any other way until the client closes the
      handle.

      If there is a conflicting lock the server MUST return
      SSH_FX_LOCK_CONFLICT.  If the server cannot make the locking
      guarantee, it MUST return SSH_FX_OP_UNSUPPORTED.

   SSH_FXF_ACCESS_BLOCK_ADVISORY
      If this bit is set, the above BLOCK modes are advisory.  In
      advisory mode, only other accesses that specify a BLOCK mode need
      be considered when determining whether the BLOCK can be granted,
      and the server need not prevent I/O operations that violate the
      block mode.

      The server MAY perform mandatory locking even if the
      BLOCK_ADVISORY bit is set.

   SSH_FXF_ACCESS_NOFOLLOW
      If the final component of the path is a symlink, then the open
      MUST fail, and the error SSH_FX_LINK_LOOP MUST be returned.

   SSH_FXF_ACCESS_DELETE_ON_CLOSE
      The file should be deleted when the last handle to it is closed.
      (The last handle may not be an sftp-handle.)  This MAY be emulated
      by a server if the OS doesn't support it by deleting the file when
      this handle is closed.

      It is implementation specific whether the directory entry is
      removed immediately or when the handle is closed.


   The 'attrs' field specifies the initial attributes for the file.
   Default values MUST be supplied by the server for those attributes
   that are not specified.  See Section ''File Attributes'' for more



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   information.

   The 'attrs' field is ignored if an existing file is opened.

   The following table is provided to assist in mapping POSIX semantics
   to equivalent SFTP file open parameters:
   O_RDONLY
      desired-access = READ_DATA|READ_ATTRIBUTES

   O_WRONLY
      desired-access = WRITE_DATA|WRITE_ATTRIBUTES

   O_RDWR
      desired-access = READ_DATA|READ_ATTRIBUTES|WRITE_DATA|
      WRITE_ATTRIBUTES

   O_APPEND
      desired-access = WRITE_DATA|WRITE_ATTRIBUTES|APPEND_DATA
      flags = SSH_FXF_ACCESS_APPEND_DATA and or
      SSH_FXF_ACCESS_APPEND_DATA_ATOMIC

   O_CREAT
      flags = SSH_FXF_OPEN_OR_CREATE

   O_TRUNC
      flags = SSH_FXF_TRUNCATE_EXISTING

   O_TRUNC|O_CREATE
      flags = SSH_FXF_CREATE_TRUNCATE

8.1.2.  Opening a Directory

   To enumerate a directory, the client first obtains a handle and then
   issues directory read requests.  When enumeration is complete, the
   handle MUST be closed.

       byte   SSH_FXP_OPENDIR
       uint32 request-id
       string path [UTF-8]

   path
      The 'path' field is the path name of the directory to be listed
      (without any trailing slash).  See Section 'File Names' for more
      information on file names.







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      If 'path' does not refer to a directory, the server MUST return
      SSH_FX_NOT_A_DIRECTORY.


   The response to this message will be either SSH_FXP_HANDLE (if the
   operation is successful) or SSH_FXP_STATUS (if the operation fails).

8.1.3.  Closing Handles

   A handle is closed using the following request.

       byte   SSH_FXP_CLOSE
       uint32 request-id
       string handle

   handle
      '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.  Note
   that on some server platforms even a close can fail.  For example, if
   the server operating system caches writes, and an error occurs while
   flushing cached writes, the close operation may fail.

   Note that the handle is invalid regardless of the SSH_FXP_STATUS
   result.  There is no way for the client to recover a handle that
   fails to close.  The client MUST release all resources associated
   with the handle regardless of the status.  The server SHOULD take
   whatever steps it can to recover from a close failure and to ensure
   that all resources associated with the handle on the server are
   correctly released.

8.2.  Reading and Writing

8.2.1.  Reading Files

   The following request can be used to read file data:

       byte   SSH_FXP_READ
       uint32 request-id
       string handle
       uint64 offset
       uint32 length






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   handle
      'handle' is an open file handle returned by SSH_FXP_OPEN.  If
      'handle' is not a handle returned by SSH_FXP_OPEN, the server MUST
      return SSH_FX_INVALID_HANDLE.

   offset
      The offset (in bytes) relative to the beginning of the file that
      the read MUST start at.  This field is ignored if
      SSH_FXF_ACCESS_TEXT_MODE was specified during the open.

   length
      'length' is the maximum number of bytes to read.

      The server MUST not respond with more data than is specified by
      the 'length' parameter.  However, the server MAY respond with less
      data if EOF is reached, an error is encountered, or the servers
      internal buffers can not handle such a large request.

      If the server specified a non-zero 'max-read-size' in its
      'supported2' (Section 5.4) extension, then failure to return
      'length' bytes indicates that EOF or an error occurred.


8.2.2.  Reading Directories

   In order to retrieve a directory listing, the client issues one or
   more SSH_FXP_READDIR requests.  In order to obtain a complete
   directory listing, the client MUST issue repeated SSH_FXP_READDIR
   requests until the server responds with an SSH_FXP_STATUS message.

       byte   SSH_FXP_READDIR
       uint32 request-id
       string handle

   handle
      'handle' is a handle returned by SSH_FXP_OPENDIR.  If 'handle' is
      an ordinary file handle returned by SSH_FXP_OPEN, the server MUST
      return SSH_FX_INVALID_HANDLE.


   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.

   If there are no more names available to be read, the server MUST
   respond with a SSH_FXP_STATUS message with error code of SSH_FX_EOF.




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8.2.3.  Writing Files

   Writing to a file is achieved using the following message:

       byte   SSH_FXP_WRITE
       uint32 request-id
       string handle
       uint64 offset
       string data

   handle
      'handle' is an open file handle returned by SSH_FXP_OPEN.  If
      'handle' is not a handle returned by SSH_FXP_OPEN, the server MUST
      return SSH_FX_INVALID_HANDLE.

   offset
      The offset (in bytes) relative to the beginning of the file that
      the write MUST start at.  This field is ignored if
      SSH_FXF_ACCESS_TEXT_MODE was specified during the open.

      The write will extend the file if writing beyond the end of the
      file.  It is legal to write to an offset that extends beyond the
      end of the file; the semantics are to write the byte value 0x00
      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 create a sparse file.

   data
      The data to write to the file.

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

8.3.  Removing and Renaming Files

   The following request can be used to remove a file:

       byte   SSH_FXP_REMOVE
       uint32 request-id
       string filename [UTF-8]

   filename
      'filename' is the name of the file to be removed.  See Section
      'File Names' for more information.








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      This request cannot be used to remove directories.  The server
      MUST return SSH_FX_FILE_IS_A_DIRECTORY in this case.


   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.

       byte   SSH_FXP_RENAME
       uint32 request-id
       string oldpath [UTF-8]
       string newpath [UTF-8]
       uint32 flags

   where 'request-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.

   'flags' is 0 or a combination of:

       SSH_FXF_RENAME_OVERWRITE  0x00000001
       SSH_FXF_RENAME_ATOMIC     0x00000002
       SSH_FXF_RENAME_NATIVE     0x00000004

   If flags does not include SSH_FXP_RENAME_OVERWRITE, and there already
   exists a file with the name specified by newpath, the server MUST
   respond with SSH_FX_FILE_ALREADY_EXISTS.

   If flags includes SSH_FXP_RENAME_ATOMIC, and the destination file
   already exists, it is replaced in an atomic fashion.  I.e., there is
   no observable instant in time where the name does not refer to either
   the old or the new file.  SSH_FXP_RENAME_ATOMIC implies
   SSH_FXP_RENAME_OVERWRITE.

   If flags includes SSH_FXP_RENAME_ATOMIC and the server cannot replace
   the destination in an atomic fashion, then the server MUST respond
   with SSH_FX_OP_UNSUPPORTED.

   Because some servers cannot provide atomic rename, clients should
   only specify atomic rename if correct operation requires it.  If
   SSH_FXP_RENAME_OVERWRITE is specified, the server MAY perform an
   atomic rename even if it is not requested.

   If flags includes SSH_FXP_RENAME_NATIVE, the server is free to do the
   rename operation in whatever fashion it deems appropriate.  Other
   flag values are considered hints as to desired behavior, but not



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   requirements.

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

8.4.  Creating and Deleting Directories

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

       byte   SSH_FXP_MKDIR
       uint32 request-id
       string path [UTF-8]
       ATTRS  attrs

   where 'request-id' is the request identifier.

   'path' specifies the directory to be created.  See Section ''File
   Names'' for more information on file names.

   'attrs' specifies the attributes that should be applied to it upon
   creation.  Attributes are discussed in more detail in Section ''File
   Attributes''.

   The server will respond to this request with a SSH_FXP_STATUS
   message.  If a file or directory with the specified path already
   exists, an error will be returned.

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

       byte   SSH_FXP_RMDIR
       uint32 request-id
       string path [UTF-8]

   where 'request-id' is the request identifier, and 'path' specifies
   the directory to be removed.  See Section ''File Names'' for more
   information on file names.

   The server responds to this request with a SSH_FXP_STATUS message.

8.5.  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.




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   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:

       byte   SSH_FXP_STAT or SSH_FXP_LSTAT
       uint32 request-id
       string path [UTF-8]
       uint32 flags

   where 'request-id' is the request identifier, and 'path' specifies
   the file system object for which status is to be returned.  The
   server responds to this request with either SSH_FXP_ATTRS or
   SSH_FXP_STATUS.

   The flags field specify the attribute flags in which the client has
   particular interest.  This is a hint to the server.  For example,
   because retrieving owner / group and acl information can be an
   expensive operation under some operating systems, the server may
   choose not to retrieve this information unless the client expresses a
   specific interest in it.

   The client has no guarantee the server will provide all the fields
   that it has expressed an interest in.

   SSH_FXP_FSTAT differs from the others in that it returns status
   information for an open file (identified by the file handle).

       byte   SSH_FXP_FSTAT
       uint32 request-id
       string handle
       uint32 flags

   handle
      'handle' is an open file handle from either SSH_FXP_OPEN or
      SSH_FXP_OPENDIR.


   The server responds to this request with SSH_FXP_ATTRS or
   SSH_FXP_STATUS.

8.6.  Setting File Attributes

   File attributes may be modified using the SSH_FXP_SETSTAT and
   SSH_FXP_FSETSTAT requests.

       byte   SSH_FXP_SETSTAT
       uint32 request-id
       string path [UTF-8]



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       ATTRS  attrs


       byte   SSH_FXP_FSETSTAT
       uint32 request-id
       string handle
       ATTRS  attrs

   path
      The file system object (e.g. file or directory) whose attributes
      are to be modified.  If this object does not exist, or the user
      does not have sufficient access to write the attributes, the
      request MUST fail.

   handle
      'handle' is an open file handle from either SSH_FXP_OPEN or
      SSH_FXP_OPENDIR.  If the handle was not opened with sufficient
      access to write the requested attributes, the request MUST fail.

   attrs
      Specifies the modified attributes to be applied.  Attributes are
      discussed in more detail in Section ''File Attributes''.

   The server will respond with a SSH_FXP_STATUS message.

   Because some systems must use separate system calls to set various
   attributes, it is possible that a failure response will be returned,
   but yet some of the attributes may be have been successfully
   modified.  If possible, servers SHOULD avoid this situation; however,
   clients MUST be aware that this is possible.

8.7.  Dealing with Links

   The SSH_FXP_READLINK request reads the target of a symbolic link.

       byte   SSH_FXP_READLINK
       uint32 request-id
       string path [UTF-8]

   where 'request-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_LINK request creates a link (either hard or symbolic) on



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   the server.

       byte   SSH_FXP_LINK
       uint32 request-id
       string new-link-path [UTF-8]
       string existing-path [UTF-8]
       bool   sym-link

   new-link-path
      Specifies the path name of the new link to create.

   existing-path
      Specifies the path of an existing file system object to which the
      new-link-path will refer.

   sym-link
      Specifies that the link should be a symbolic link, or a special
      file that redirects file system parsing to the resulting path.  It
      is generally possible to create symbolic links across device
      boundaries; however, it is not required that a server support
      this.

      If 'sym-link' is false, the link should be a hard link, or a
      second directory entry referring to the same file or directory
      object.  It is generally not possible to create hard links across
      devices.


   The server shall respond with a SSH_FXP_STATUS.  Clients should be
   aware that some servers may return SSH_FX_OP_UNSUPPORTED for either
   the hard-link, sym-link, or both operations.

8.8.  Byte-range locks

   SSH_FXP_BLOCK creates a byte-range lock on the file specified by the
   handle.  The lock can be either mandatory (meaning that the server
   enforces that no other process or client can perform operations in
   violation of the lock) or advisory (meaning that no other process can
   obtain a conflicting lock, but the server does not enforce that no
   operation violates the lock.

   A server MAY implement an advisory lock in a mandatory fashion; in
   other words, the server MAY enforce that no operation violates the
   lock even when operating in advisory mode.

   The result is a SSH_FXP_STATUS return.





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       byte   SSH_FXP_BLOCK
       uint32 request-id
       string handle
       uint64 offset
       uint64 length
       uint32 uLockMask

   handle
      'handle' is a handle returned by SSH_FXP_OPEN or SSH_FXP_OPENDIR.
      Note that some server MAY return SSH_FX_OP_UNSUPPORTED if the
      handle is a directory handle.

   offset
      Beginning of the byte-range to lock.

   length
      Number of bytes in the range to lock.  The special value 0 means
      lock from 'offset' to the end of the file.

   uLockMask
      A bit mask of SSH_FXF_ACCESS_BLOCK_* values; the meanings are
      described in Section 8.1.1.3.

   SSH_FXP_UNBLOCK removes a previously acquired byte-range lock on the
   specified handle.

   The result is a SSH_FXP_STATUS return.

       byte   SSH_FXP_UNBLOCK
       uint32 request-id
       string handle
       uint64 offset
       uint64 length

   handle
      'handle' on which a SSH_FXP_BLOCK request has previously been
      issued.

   offset
      Beginning of the byte-range to lock.

   length
      Number of bytes in the range to lock.  The special value 0 means
      lock from 'offset' to the end of the file.

8.9.  Canonicalizing the Server-Side Path Name

   The SSH_FXP_REALPATH request can be used to have the server



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   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:

       byte   SSH_FXP_REALPATH
       uint32 request-id
       string original-path [UTF-8]
       byte   control-byte [optional]
       string compose-path[0..n] [optional]

   original-path
      The first component of the path which the client wishes resolved
      into a absolute canonical path.  This may be the entire path.

   control-byte

       SSH_FXP_REALPATH_NO_CHECK    0x00000001
       SSH_FXP_REALPATH_STAT_IF     0x00000002
       SSH_FXP_REALPATH_STAT_ALWAYS 0x00000003

      This field is optional, and if it is not present in the packet, it
      is assumed to be SSH_FXP_REALPATH_NO_CHECK.

      If SSH_FXP_REALPATH_NO_CHECK is specified, the server MUST NOT
      fail the request if the path does not exist, is hidden, or the
      user does not have access to the path or some component thereof.
      However, the path MAY NOT be completely resolved to it's component
      form.  For example, symlinks may not be followed in this case.
      The server MAY fail the request if the path is not syntactically
      valid, or for other reasons.

      If SSH_FXP_REALPATH_STAT_IF is specified, the server MUST stat the
      path if it exists and is accessible to the client.  However, if
      the path does not exist, isn't visible, or isn't accessible, the
      server MUST NOT fail the request.  If the stat failed, the file
      type will be SSH_FILEXFER_TYPE_UNKNOWN.  If the client needs to
      distinguish between files that are actually
      SSH_FILEXFER_TYPE_UNKNOWN and paths that don't exist, it will have
      to issue a separate stat command in this case.

      If SSH_FXP_REALPATH_STAT_ALWAYS is specified the server MUST stat
      the path.  If the stat operation fails, the server MUST fail the
      request.







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   compose-path
      A path which the client wishes the server to compose with the
      original path to form the new path.  This field is optional, and
      if it is not present in the packet, it is assumed to be a zero
      length string.

      The client may specify multiple 'compose-path' elements, in which
      case the server should build the resultant path up by applying
      each compose path to the accumulated result until all 'compose-
      path' elements have been applied.


   The server MUST take the 'original-path' and apply the 'compose-path'
   as a modification to it. 'compose-path' MAY be relative to 'original-
   path' or may be an absolute path, in which case 'original-path' will
   be discarded.  The 'compose-path' MAY be zero length.

   The server will respond with a SSH_FXP_NAME packet containing the
   canonical form of the composite path.  If SSH_FXP_REALPATH_NO_CHECK
   is specified, the attributes are dummy values.

8.9.1.  Best Practice for Dealing with Paths

   BEGIN: RFCEDITOR REMOVE BEFORE PUBLISHING

   Previous to this version, clients typically composed new paths
   themselves and then called both realpath and stat on the resulting
   path to get its canonical name and see if it really existed and was a
   directory.

   This required clients to assume certain things about how a relative
   vs. realpath looked.  The new realpath allows clients to no longer
   make those assumptions and to remove one round trip from the process
   and get deterministic behavior from all servers.

   END: RFCEDITOR REMOVE BEFORE PUBLISHING

   The client SHOULD treat the results of SSH_FXP_REALPATH as a
   canonical absolute path, even if the path does not appear to be
   absolute.  A client that uses REALPATH(".", "") and treats the result
   as absolute, even if there is no leading slash, will continue to
   function correctly, even when talking to a Windows NT or VMS style
   system, where absolute paths may not begin with a slash.

   The client SHOULD also use SSH_FXP_REALPATH call to compose paths so
   that it does not need to know when a path is absolute or relative.

   For example, if the client wishes to change directory up, and the



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   server has returned "c:/x/y/z" from REALPATH, the client SHOULD use
   REALPATH("c:/x/y/z", "..", SSH_FXP_REALPATH_STAT_ALWAYS)

   As a second example, if the client wishes transfer local file "a" to
   remote file "/b/d/e", and server has returned "dka100:/x/y/z" as the
   canonical path of the current directory, the client SHOULD send
   REALPATH("dka100:/x/y/z", "/b/d/e", SSH_FXP_REALPATH_STAT_IF).  This
   call will determine the correct path to use for the open request and
   whether the /b/d/e represents a directory.


9.  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, and no data needs to be
   returned, the SSH_FXP_STATUS response with SSH_FX_OK status is
   appropriate.

   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.

9.1.  Status Response

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

       byte   SSH_FXP_STATUS
       uint32 request-id
       uint32 error/status code
       string error message (ISO-10646 UTF-8 [RFC-2279])
       string language tag (as defined in [RFC-1766])
         error-specific data

   request-id
      The 'request-id' specified by the client in the request the server
      is responding to.





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   error/status code
      Machine readable status code indicating the result of the request.
      Error code values are defined below.  The value SSH_FX_OK
      indicates success, and all other values indicate failure.

      Implementations MUST be prepared to receive unexpected error codes
      and handle them sensibly (such as by treating them as equivalent
      to SSH_FX_FAILURE).  Future protocol revisions will add additional
      error codes without changing the version number.

   error message
      Human readable description of the error.

   language tag
      'language tag' specifies the language the error is in.

   error-specific data
      The error-specific data may be empty, or may contain additional
      information about the error.  For error codes that send error-
      specific data, the format of the data is defined below.

   Error codes:





























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       SSH_FX_OK                            0
       SSH_FX_EOF                           1
       SSH_FX_NO_SUCH_FILE                  2
       SSH_FX_PERMISSION_DENIED             3
       SSH_FX_FAILURE                       4
       SSH_FX_BAD_MESSAGE                   5
       SSH_FX_NO_CONNECTION                 6
       SSH_FX_CONNECTION_LOST               7
       SSH_FX_OP_UNSUPPORTED                8
       SSH_FX_INVALID_HANDLE                9
       SSH_FX_NO_SUCH_PATH                  10
       SSH_FX_FILE_ALREADY_EXISTS           11
       SSH_FX_WRITE_PROTECT                 12
       SSH_FX_NO_MEDIA                      13
       SSH_FX_NO_SPACE_ON_FILESYSTEM        14
       SSH_FX_QUOTA_EXCEEDED                15
       SSH_FX_UNKNOWN_PRINCIPAL             16
       SSH_FX_LOCK_CONFLICT                 17
       SSH_FX_DIR_NOT_EMPTY                 18
       SSH_FX_NOT_A_DIRECTORY               19
       SSH_FX_INVALID_FILENAME              20
       SSH_FX_LINK_LOOP                     21
       SSH_FX_CANNOT_DELETE                 22
       SSH_FX_INVALID_PARAMETER             23
       SSH_FX_FILE_IS_A_DIRECTORY           24
       SSH_FX_BYTE_RANGE_LOCK_CONFLICT      25
       SSH_FX_BYTE_RANGE_LOCK_REFUSED       26
       SSH_FX_DELETE_PENDING                27
       SSH_FX_FILE_CORRUPT                  28
       SSH_FX_OWNER_INVALID                 29
       SSH_FX_GROUP_INVALID                 30

   SSH_FX_OK
      Indicates successful completion of the operation.

   SSH_FX_EOF
      An attempt to read past the end-of-file was made; or, there are no
      more directory entries to return.

   SSH_FX_NO_SUCH_FILE
      A reference was made to a file which does not exist.

   SSH_FX_PERMISSION_DENIED
      The user does not have sufficient permissions to perform the
      operation.






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   SSH_FX_FAILURE
      An error occurred, but no specific error code exists to describe
      the failure.

      This error message SHOULD always have meaningful text in the the
      'error message' field.

   SSH_FX_BAD_MESSAGE
      A badly formatted packet or other SFTP protocol incompatibility
      was detected.

   SSH_FX_NO_CONNECTION
      There is no connection to the server.  This error MAY be used
      locally, but MUST NOT be return by a server.

   SSH_FX_CONNECTION_LOST
      The connection to the server was lost.  This error MAY be used
      locally, but MUST NOT be return by a server.

   SSH_FX_OP_UNSUPPORTED
      An attempted operation could not be completed by the server
      because the server does not support the operation.

      This error 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.

   SSH_FX_INVALID_HANDLE
      The handle value was invalid.

   SSH_FX_NO_SUCH_PATH
      The file path does not exist or is invalid.

   SSH_FX_FILE_ALREADY_EXISTS
      The file already exists.

   SSH_FX_WRITE_PROTECT
      The file is on read-only media, or the media is write protected.

   SSH_FX_NO_MEDIA
      The requested operation cannot be completed because there is no
      media available in the drive.








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   SSH_FX_NO_SPACE_ON_FILESYSTEM
      The requested operation cannot be completed because there is no
      free space on the filesystem.

   SSH_FX_QUOTA_EXCEEDED
      The operation cannot be completed because it would exceed the
      user's storage quota.

   SSH_FX_UNKNOWN_PRINCIPAL
      A principal referenced by the request (either the 'owner',
      'group', or 'who' field of an ACL), was unknown.  The error
      specific data contains the problematic names.  The format is one
      or more:

           string unknown-name

      Each string contains the name of a principal that was unknown.

   SSH_FX_LOCK_CONFLICT
      The file could not be opened because it is locked by another
      process.

   SSH_FX_DIR_NOT_EMPTY
      The directory is not empty.

   SSH_FX_NOT_A_DIRECTORY
      The specified file is not a directory.

   SSH_FX_INVALID_FILENAME
      The filename is not valid.

   SSH_FX_LINK_LOOP
      Too many symbolic links encountered.

   SSH_FX_CANNOT_DELETE
      The file cannot be deleted.  One possible reason is that the
      advisory READONLY attribute-bit is set.

   SSH_FX_INVALID_PARAMETER
      On of the parameters was out of range, or the parameters specified
      cannot be used together.

   SSH_FX_FILE_IS_A_DIRECTORY
      The specified file was a directory in a context where a directory
      cannot be used.






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   SSH_FX_BYTE_RANGE_LOCK_CONFLICT
      A read or write operation failed because another process's
      mandatory byte-range lock overlaps with the request.

   SSH_FX_BYTE_RANGE_LOCK_REFUSED
      A request for a byte range lock was refused.

   SSH_FX_DELETE_PENDING
      An operation was attempted on a file for which a delete operation
      is pending.

   SSH_FX_FILE_CORRUPT
      The file is corrupt; an filesystem integrity check should be run.

   SSH_FX_OWNER_INVALID
      The principal specified can not be assigned as an owner of a file.

   SSH_FX_GROUP_INVALID
      The principal specified can not be assigned as the primary group
      of a file.


9.2.  Handle Response

   The SSH_FXP_HANDLE response has the following format:

       byte   SSH_FXP_HANDLE
       uint32 request-id
       string handle

   'handle'
      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.


9.3.  Data Response

   The SSH_FXP_DATA response has the following format:

       byte   SSH_FXP_DATA
       uint32 request-id
       string data
       bool   end-of-file [optional]






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   data
      '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.  (See
      Section 8.2.1.)

   end-of-file
      This field is optional.  If it is present in the packet, it MUST
      be true, and it indicates that EOF was reached during this read.
      This can help the client avoid a round trip to determine whether a
      short read was normal (due to EOF) or some other problem (limited
      server buffer for example.)


9.4.  Name Response

   The SSH_FXP_NAME response has the following format:

       byte   SSH_FXP_NAME
       uint32 request-id
       uint32 count
       repeats count times:
           string     filename [UTF-8]
           ATTRS      attrs
       bool end-of-list [optional]

   count
      The number of names returned in this response, and the 'filename'
      and 'attrs' field repeat 'count' times.

   filename
      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.)

   attrs
      The attributes of the file as described in Section ''File
      Attributes''.

   end-of-list
      This field is optional.  If present in the packet, it MUST be
      true, and indicates that there are no more entries to be read.
      This can save the client a round trip to determine if there are
      more entries to be read.







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9.5.  Attrs Response

   The SSH_FXP_ATTRS response has the following format:

       byte   SSH_FXP_ATTRS
       uint32 request-id
       ATTRS  attrs

   attrs
      The returned file attributes as described in Section ''File
      Attributes''.



10.  Extensions

   The SSH_FXP_EXTENDED request provides a generic extension mechanism
   for adding additional commands.

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

   request-id
      Identifier to be returned from the server with the response.

   extended-request
      A string naming the extension, following the the DNS extensibility
      naming convention outlined in [I-D.ietf-secsh-architecture], or
      defined by IETF consensus.

   request-specific data
      The rest of the request is defined by the extension; servers
      SHOULD NOT attempt to interpret it if they do not recognize the
      'extended-request' name.

   The server may respond to such requests using any of the response
   packets defined in Section ''Responses from the Server to the
   Client''.  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:



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

   There is a range of packet types reserved for use by extensions.  In
   order to avoid collision, extensions that that use additional packet
   types should determine those numbers dynamically.

   The suggested way of doing this is have an extension request from the
   client to the server that enables the extension; the extension
   response from the server to the client would specify the actual type
   values to use, in addition to any other data.

   Extension authors should be mindful of the limited range of packet
   types available (there are only 45 values available) and avoid
   requiring a new packet type where possible.


11.  Implementation Considerations

   In order for this protocol to perform well, especially over high
   latency networks, multiple read and write requests should be queued
   to the server.

   The data size of requests should match the maximum packet size for
   the next layer up in the protocol chain.

   When implemented over ssh, the best performance should be achieved
   when the data size matches the channel's max packet, and the channel
   window is a multiple of the channel packet size.

   Implementations MUST be aware that requests do not have to be
   satisfied in the order issued.  (See Request Synchronization and
   Reordering (Section 4.1).)

   Implementations MUST also be aware that read requests may not return
   all the requested data, even if the data is available.


12.  Security Considerations

   It is assumed that both ends of the connection have been
   authenticated and that the connection has privacy and integrity
   features.  Such security issues are left to the underlying transport
   protocol, except to note that if this is not the case, an attacker
   could manipulate files on the server at will and thus wholly
   compromise the server.




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   This protocol provides file system access to arbitrary files on the
   server (constrained only 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 [I-D.ietf-secsh-userauth].

   Extreme care must be used when interpreting file handle strings.  In
   particular, care must be taken that a file handle string is valid in
   the context of a given 'file-share' session.  For example, the 'file-
   share' server daemon may have files which it has opened for its own
   purposes, and the client must not be able to access these files by
   specifying an arbitrary file handle string.

   The permission field of the attrib structure (Section 7.6) may
   include the SUID, SGID, and SVTX (sticky) bits.  Clients should use
   extreme caution when setting these bits on either remote or local
   files.  (I.e., just because a file was SUID on the remote system does
   not necessarily imply that it should be SUID on the local system.)

   Filesystems often contain entries for objects that are not files at
   all, but are rather devices.  For example, it may be possible to
   access serial ports, tape devices, or named pipes using this
   protocol.  Servers should exercise caution when granting access to
   such resources.  In addition to the dangers inherent in allowing
   access to such a device, some devices may be 'slow', and could cause
   denial of service by causing the server to block for a long period of
   time while I/O is performed to such a device.

   Servers should take care that file-system quotas are respected for
   users.  In addition, implementations should be aware that attacks may
   be possible, or facilitated, by filling a filesystem.  For example,
   filling the filesystem where event logging and auditing occurs may,
   at best, cause the system to crash, or at worst, allow the attacker
   to take untraceable actions in the future.

   Servers should take care that filenames are in their appropriate
   canonical form, and to ensure that filenames not in canonical form
   cannot be used to bypass access checks or controls.

   If the server implementation limits access to certain parts of the
   file system, extra care must be taken in parsing file names which
   contain the '..' path element, and when following symbolic links,
   shortcuts, or other filesystem objects which might transpose the path
   to refer to an object outside of the restricted area.  There have
   been numerous reported security bugs where a ".." in a path name has
   allowed access outside the intended area.




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13.  Changes from Previous Protocol Versions

   RFC EDITOR: PLEASE REMOVE ENTIRE SECTION BEFORE PUBLISHING

   Please refer to the following web page for pervious versions of the
   protocol:

   http://tools.ietf.org/wg/secsh/draft-ietf-secsh-filexfer/

   RFC EDITOR: END PLEASE REMOVE ENTIRE SECTION BEFORE PUBLISHING


14.  References

14.1.  Normative References

   [RFC3010]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "NFS version 4
              Protocol", RFC 3010, December 2000.

   [I-D.ietf-secsh-architecture]
              Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
              draft-ietf-secsh-architecture-22 (work in progress),
              March 2005.

   [I-D.ietf-secsh-transport]
              Lonvick, C., "SSH Transport Layer Protocol",
              draft-ietf-secsh-transport-24 (work in progress),
              March 2005.

   [I-D.ietf-secsh-connect]
              Lonvick, C. and T. Ylonen, "SSH Connection Protocol",
              draft-ietf-secsh-connect-25 (work in progress),
              March 2005.

   [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.

14.2.  Informative References

   [RFC1521]  Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
              Mail Extensions) Part One: Mechanisms for Specifying and
              Describing the Format of Internet Message Bodies",
              RFC 1521, September 1993.




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   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [I-D.ietf-secsh-userauth]
              Lonvick, C. and T. Ylonen, "SSH Authentication Protocol",
              draft-ietf-secsh-userauth-27 (work in progress),
              March 2005.

Trademark notice

   "ssh" is a registered trademark in the United States and/or other
   countries.




































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Authors' Addresses

   Joseph Galbraith
   VanDyke Software
   4848 Tramway Ridge Blvd
   Suite 101
   Albuquerque, NM  87111
   US

   Phone: +1 505 332 5700
   Email: galb-list@vandyke.com


   Oskari Saarenmaa
   F-Secure
   Tammasaarenkatu 7
   Helsinki  00180
   FI

   Email: oskari.saarenmaa@f-secure.com































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Intellectual Property Statement

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Disclaimer of Validity

   This document and the information contained herein are provided on an
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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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