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Network Working Group                          T. Ylonen and S. Lehtinen
INTERNET-DRAFT                               SSH Communications Security
draft-ietf-secsh-filexfer-01.txt                           2 March, 2001
Expires: 2 September, 2001



                  Secure Shell File Transfer Protocol

Status of This Memo

This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups.  Note that
other groups may also distribute working documents as
Internet-Drafts.

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

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

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



















T. Ylonen and S. Lehtinen                                       [page 1]

INTERNET-DRAFT                                            2 March, 2001

Table of Contents

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
2.  Use with the Secure Shell Connection Protocol   . . . . . . . . .  3
3.  General Packet Format   . . . . . . . . . . . . . . . . . . . . .  3
4.  Protocol Initialization   . . . . . . . . . . . . . . . . . . . .  4
5.  File Attributes   . . . . . . . . . . . . . . . . . . . . . . . .  5
6.  Responses from the Server to the Client   . . . . . . . . . . . .  6
7.  Requests From the Client to the Server  . . . . . . . . . . . . .  9
  7.1.  Request Synchronization and Reordering  . . . . . . . . . . . 10
  7.2.  File Names  . . . . . . . . . . . . . . . . . . . . . . . . . 10
  7.3.  Opening, Creating, and Closing Files  . . . . . . . . . . . . 11
  7.4.  Reading and Writing   . . . . . . . . . . . . . . . . . . . . 12
  7.5.  Removing and Renaming Files   . . . . . . . . . . . . . . . . 13
  7.6.  Creating and Deleting Directories   . . . . . . . . . . . . . 14
  7.7.  Scanning Directories  . . . . . . . . . . . . . . . . . . . . 14
  7.8.  Retrieving File Attributes  . . . . . . . . . . . . . . . . . 15
  7.9.  Setting File Attributes   . . . . . . . . . . . . . . . . . . 16
  7.10.  Dealing with Symbolic links  . . . . . . . . . . . . . . . . 16
  7.11.  Canonicalizing the Server-Side Path Name   . . . . . . . . . 17
8.  Vendor-Specific Extensions  . . . . . . . . . . . . . . . . . . . 17
9.  Security Considerations   . . . . . . . . . . . . . . . . . . . . 18
10.  Changes from previous protocol versions  . . . . . . . . . . . . 18
  10.1.  Changes between versions 3 and 2   . . . . . . . . . . . . . 18
  10.2.  Changes between versions 2 and 1   . . . . . . . . . . . . . 18
  10.3.  Changes between versions 1 and 0   . . . . . . . . . . . . . 18
11.  Trademark Issues   . . . . . . . . . . . . . . . . . . . . . . . 18
12.  References   . . . . . . . . . . . . . . . . . . . . . . . . . . 19
13.  Authors' Addresses   . . . . . . . . . . . . . . . . . . . . . . 19



1.  Introduction

This protocol provides secure file transfer (and more generally file
system access) functionality over a reliable data stream, such as a
channel in the Secure Shell Remote Login Protocol [SECSH-ARCH].

This protocol 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, and that the
server has already authenticated the user at the client end, and that
the identity of the client user is externally available to the server
implementation.

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).


T. Ylonen and S. Lehtinen                                       [page 2]

INTERNET-DRAFT                                            2 March, 2001

The packet format descriptions in this specification follow the notation
presented in [SECSH-ARCH].

Even though this protocol is described in the context of the Secure
Shell Remote Login Protocol, this protocol is general and independent of
the rest of the Secure Shell protocol suite.  It could be used in a
number of different applications, such as secure file transfer over TLS
[RFC-2246] and transfer of management information in VPN applications.

2.  Use with the Secure Shell Connection Protocol

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

3.  General Packet Format

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

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

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

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

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

There is no limit on the number of outstanding (non-acknowledged)
requests that the client may send to the server.  In practise 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 queueing of outgoing requests at the
client.

The following values are defined for packet types.



T. Ylonen and S. Lehtinen                                       [page 3]

INTERNET-DRAFT                                            2 March, 2001

  #define SSH_FXP_INIT                1
  #define SSH_FXP_VERSION             2
  #define SSH_FXP_OPEN                3
  #define SSH_FXP_CLOSE               4
  #define SSH_FXP_READ                5
  #define SSH_FXP_WRITE               6
  #define SSH_FXP_LSTAT               7
  #define SSH_FXP_FSTAT               8
  #define SSH_FXP_SETSTAT             9
  #define SSH_FXP_FSETSTAT           10
  #define SSH_FXP_OPENDIR            11
  #define SSH_FXP_READDIR            12
  #define SSH_FXP_REMOVE             13
  #define SSH_FXP_MKDIR              14
  #define SSH_FXP_RMDIR              15
  #define SSH_FXP_REALPATH           16
  #define SSH_FXP_STAT               17
  #define SSH_FXP_RENAME             18
  #define SSH_FXP_READLINK           19
  #define SSH_FXP_SYMLINK            20
  #define SSH_FXP_STATUS            101
  #define SSH_FXP_HANDLE            102
  #define SSH_FXP_DATA              103
  #define SSH_FXP_NAME              104
  #define SSH_FXP_ATTRS             105
  #define SSH_FXP_EXTENDED          200
  #define SSH_FXP_EXTENDED_REPLY    201

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

4.  Protocol Initialization

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

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

  uint32 version
  <extension data>

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

  uint32 version
  <extension data>


T. Ylonen and S. Lehtinen                                       [page 4]

INTERNET-DRAFT                                            2 March, 2001

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

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

  string extension_name
  string extension_data

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

5.  File Attributes

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

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

  uint32   flags
  uint64   size           present only if flag SSH_FILEXFER_ATTR_SIZE
  uint32   uid            present only if flag SSH_FILEXFER_ATTR_UIDGID
  uint32   gid            present only if flag SSH_FILEXFER_ATTR_UIDGID
  uint32   permissions    present only if flag
                          SSH_FILEXFER_ATTR_PERMISSIONS
  uint32   atime          present only if flag SSH_FILEXFER_ACMODTIME
  uint32   mtime          present only if flag SSH_FILEXFER_ACMODTIME
  uint32   extended_count present only if flag
                          SSH_FILEXFER_ATTR_EXTENDED
  string   extended_type
  string   extended_data
  ...      more extended data (extended_type - extended_data pairs),
           so that number of pairs equals extended_count

The `flags' specify which of the fields are present.  Those fields for
which the corresponding flag is not set are not present (not included in
the packet).  New flags can only be added by incrementing the protocol


T. Ylonen and S. Lehtinen                                       [page 5]

INTERNET-DRAFT                                            2 March, 2001

version number (or by using the extension mechanism described below).

The `size' field specifies the size of the file in bytes.

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

The `permissions' field contains a bit mask of file permissions as
defined by [POSIX].

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

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

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

The flags bits are defined to have the following values:

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

6.  Responses from the Server to the Client

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


T. Ylonen and S. Lehtinen                                       [page 6]

INTERNET-DRAFT                                            2 March, 2001

used to return one or more file names from a SSH_FXP_READDIR or
SSH_FXP_REALPATH request, and SSH_FXP_ATTRS is used to return file
attributes from SSH_FXP_STAT, SSH_FXP_LSTAT, and SSH_FXP_FSTAT requests.

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

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

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

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

where `id' is the request identifier, and `error/status code' indicates
the result of the requested operation.  The value SSH_FX_OK indicates
success, and all other values indicate failure.  Currently, the follow-
ing values are defined (other values may be defined by future versions
of this protocol):

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

   SSH_FX_OK
      Indicates successful completion of the operation.

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

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

   SSH_FX_PERMISSION_DENIED


T. Ylonen and S. Lehtinen                                       [page 7]

INTERNET-DRAFT                                            2 March, 2001

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

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

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

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

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

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

The SSH_FXP_HANDLE response has the following format:

  uint32     id
  string     handle

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

The SSH_FXP_DATA response has the following format:

   uint32     id
   string     data

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

The SSH_FXP_NAME response has the following format:


T. Ylonen and S. Lehtinen                                       [page 8]

INTERNET-DRAFT                                            2 March, 2001

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

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

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

The recommended format for the longname field is as follows:

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

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

The SSH_FXP_ATTRS response has the following format:

  uint32     id
  ATTRS      attrs

where `id' is the request identifier, and `attrs' is the returned file
attributes as described in Section ``File Attributes''.

7.  Requests From the Client to the Server

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


T. Ylonen and S. Lehtinen                                       [page 9]

INTERNET-DRAFT                                            2 March, 2001

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

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

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

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

7.1.  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 the time.

7.2.  File Names

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

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

Servers SHOULD interpret a path name component ".." as referring to the
parent directory, and "." as referring to the current directory.  If the
server implementation limits access to certain parts of the file system,


T. Ylonen and S. Lehtinen                                      [page 10]

INTERNET-DRAFT                                            2 March, 2001

it must be extra careful in parsing file names when enforcing such
restrictions.  There have been numerous reported security bugs where a
".." in a path name has allowed access outside the intended area.

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

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

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

7.3.  Opening, Creating, and Closing Files

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

  uint32        id
  string        filename
  uint32        pflags
  ATTRS         attrs

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

The `filename' field specifies the file name.  See Section ``File
Names'' for more information.

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

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

These have the following meanings:

   SSH_FXF_READ
      Open the file for reading.

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

   SSH_FXF_APPEND


T. Ylonen and S. Lehtinen                                      [page 11]

INTERNET-DRAFT                                            2 March, 2001

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

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

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

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

The `attrs' field specifies the initial attributes for the file.
Default values will be used for those attributes that are not specified.
See Section ``File Attributes'' for more information.

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

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

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

  uint32     id
  string     handle

where `id' is the request identifier, and `handle' is a handle previ-
ously returned in the response to SSH_FXP_OPEN or SSH_FXP_OPENDIR.  The
handle becomes invalid immediately after this request has been sent.

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

7.4.  Reading and Writing

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

  uint32     id
  string     handle
  uint64     offset
  uint32     len

where `id' is the request identifier, `handle' is an open file handle


T. Ylonen and S. Lehtinen                                      [page 12]

INTERNET-DRAFT                                            2 March, 2001

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

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

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

  uint32     id
  string     handle
  uint64     offset
  string     data

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

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

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

7.5.  Removing and Renaming Files

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

  uint32     id
  string     filename

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

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

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

  uint32     id
  string     oldpath
  string     newpath



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

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

7.6.  Creating and Deleting Directories

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

  uint32     id
  string     path
  ATTRS      attrs

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

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

  uint32     id
  string     path

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

7.7.  Scanning Directories

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

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



T. Ylonen and S. Lehtinen                                      [page 14]

INTERNET-DRAFT                                            2 March, 2001

  uint32     id
  string     path

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

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

  uint32     id
  string     handle

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

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

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

7.8.  Retrieving File Attributes

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

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

  uint32     id
  string     path

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

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

  uint32     id


T. Ylonen and S. Lehtinen                                      [page 15]

INTERNET-DRAFT                                            2 March, 2001

  string     handle

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

7.9.  Setting File Attributes

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

The SSH_FXP_SETSTAT request is of the following format:

  uint32     id
  string     path
  ATTRS      attrs

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

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

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

  uint32     id
  string     handle
  ATTRS      attrs

where `id' is the request identifier, `handle' (MUST be returned by
SSH_FXP_OPEN) identifies the file whose attributes are to be modified,
and `attrs' specifies the modifications to be made to its attributes.
Attributes are discussed in more detail in Section ``File Attributes''.
The server will respond to this request with SSH_FXP_STATUS.

7.10.  Dealing with Symbolic links

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

  uint32     id
  string     path

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



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INTERNET-DRAFT                                            2 March, 2001

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

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

  uint32     id
  string     linkpath
  string     targetpath

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

7.11.  Canonicalizing the Server-Side Path Name

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

  uint32     id
  string     path

where `id' is the request identifier and `path' specifies the path name
to be canonicalized.  The server will respond with a SSH_FXP_NAME packet
containing only one name and a dummy attributes value.  The name is the
returned packet will be in canonical form.  If an error occurs, the
server may also respond with SSH_FXP_STATUS.

8.  Vendor-Specific Extensions

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

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

where `id' is the request identifier, and `extended-request' is a string
of the format "name@domain", where domain is an internet domain name of
the vendor defining the request.  The rest of the request is completely
vendor-specific, and servers should only attempt to interpret it if they
recognize the `extended-request' name.

The server may respond to such requests using any of the response
packets defined in 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


T. Ylonen and S. Lehtinen                                      [page 17]

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`extended-request' name, then the server MUST respond with
SSH_FXP_STATUS with error/status set to SSH_FX_OP_UNSUPPORTED.

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

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

9.  Security Considerations

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

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

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

10.  Changes from previous protocol versions

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

10.1.  Changes between versions 3 and 2

o  The SSH_FXP_READLINK and SSH_FXP_SYMLINK mesages were added.

o  The SSH_FXP_EXTENDED and SSH_FXP_EXTENDED_REPLY messages were added.

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

10.2.  Changes between versions 2 and 1

o  The SSH_FXP_RENAME message was added.

10.3.  Changes between versions 1 and 0

o  Implementation changes, no actual protocol changes.




T. Ylonen and S. Lehtinen                                      [page 18]

INTERNET-DRAFT                                            2 March, 2001

11.  Trademark Issues

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

12.  References

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

[POSIX] ISO/IEC Std 9945-1, ANSI/IEEE Std 1003.1 Information technology
-- Portable Operating System Interface (POSIX)-Part 1: System
Application Program Interface (API) [C Language], July 1996.

[SECSH-ARCH] Ylonen, T., et al: "Secure Shell Protocol Architecture",
Internet-Draft, draft-ietf-secsh-architecture-08.txt

[SECSH-TRANSPORT] Ylonen, T., et al: "Secure Shell Transport Protocol",
Internet-Draft, draft-ietf-secsh-transport-10.txt

[SECSH-USERAUTH] Ylonen, T., et al: "Secure Shell Authentication
Protocol", Internet-Draft, draft-ietf-secsh-userauth-10.txt

[SECSH-CONNECT] Ylonen, T., et al: "Secure Shell Connection Protocol",
Internet-Draft, draft-ietf-secsh-connect-10.txt

13.  Authors' Addresses

    Tatu Ylonen
    SSH Communications Security Corp
    Fredrikinkatu 42
    FIN-00100 HELSINKI
    Finland
    E-mail: ylo@ssh.com

    Sami Lehtinen
    SSH Communications Security Corp
    Fredrikinkatu 42
    FIN-00100 HELSINKI
    Finland
    E-mail: sjl@ssh.com













T. Ylonen and S. Lehtinen                                      [page 19]


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