Network Working Group T. Ylonen and S. Lehtinen
INTERNET-DRAFT SSH Communications Security
draft-ietf-secsh-filexfer-00.txt 9 January, 2001
Expires: 9 July, 2001
SSH 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.
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Abstract
The SSH File Transfer Protocol provides secure file transfer functional-
ity 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Use with the SSH Connection Protocol . . . . . . . . . . . . . . 3
3. General Packet Format . . . . . . . . . . . . . . . . . . . . . 3
4. Protocol Initialization . . . . . . . . . . . . . . . . . . . . 4
5. File Attributes . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Requests From the Client to the Server . . . . . . . . . . . . . 6
6.1. Request Synchronization and Reordering . . . . . . . . . . . 7
6.2. File Names . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.3. Opening, Creating, and Closing Files . . . . . . . . . . . . 8
6.4. Reading and Writing . . . . . . . . . . . . . . . . . . . . 9
6.5. Removing and Renaming Files . . . . . . . . . . . . . . . . 10
6.6. Creating and Deleting Directories . . . . . . . . . . . . . 10
6.7. Scanning Directories . . . . . . . . . . . . . . . . . . . . 11
6.8. Retrieving File Attributes . . . . . . . . . . . . . . . . . 12
6.9. Setting File Attributes . . . . . . . . . . . . . . . . . . 12
6.10. Canonicalizing the Server-Side Path Name . . . . . . . . . 13
7. Responses from the Server to the Client . . . . . . . . . . . . 13
8. Vendor-Specific Extensions . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 17
10. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
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 SSH2 protocol [SSH-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).
The packet format descriptions in this specification follow the notation
presented in [SSH-ARCH].
Even though this protocol is described in the context of the SSH2
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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 [RFC-2246] and
transfer of management information in VPN applications.
2. Use with the SSH Connection Protocol
When used with the SSH2 Protocol suite, this protocol is intended to be
used from the SSH Connection Protocol as a subsystem, as described in
[SSH-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.
#define SSH_FXP_INIT 1
#define SSH_FXP_VERSION 2
#define SSH_FXP_OPEN 3
#define SSH_FXP_CLOSE 4
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#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_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>
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
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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
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
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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.
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. Requests From the Client to the Server
Requests from the client to the server represent the various file system
operations. Each request begins with an `id' field, which is a 32-bit
identifier identifying the request (selected by the client). The same
identifier will be returned in the response to the request. One
possible implementation of it is a monotonically increasing request
sequence number (modulo 2^32).
Many operations in the protocol operate on open files. The SSH_FXP_OPEN
request can return a file handle (which is an opaque variable-length
string) which may be used to access the file later (e.g. in a read
operation). The client MUST NOT send requests the server with bogus or
closed handles. However, the server MUST perform adequate checks on the
handle in order to avoid security risks due to fabricated handles.
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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.
6.1. Request Synchronization and Reordering
The protocol and implementations MUST process requests relating to the
same file in the order in which they are received. 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.
6.2. File Names
This protocol represents file names as strings. File names are assumed
to use the slash ('/') character as a directory separator.
File names starting with a slash are "absolute", and are relative to the
root of the file system. Names starting with any other character are
relative to the user's default directory (home directory). Note that
identifying the user is assumed to take place outside of this protocol.
Servers SHOULD interpret a path name component ".." as referring to the
parent directory, and "." as referring to the current directory. If the
server implementation limits access to certain parts of the file system,
it must be extra careful in parsing file names when enforcing such
restrictions. There have been numerous reported security bugs where a
".." in a path name has allowed access outside the intended area.
An empty path name is valid, and it refers to the user's default
directory (usually the user's home directory).
Otherwise, no syntax is defined for file names by this specification.
Clients should not make any other assumptions; however, they can splice
path name components returned by SSH_FXP_READDIR together using a slash
('/') as the separator, and that will work as expected.
It is understood that the lack of well-defined semantics for file names
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may cause interoperability problems between clients and servers using
radically different operating systems. However, this approach is known
to work acceptably with most systems, and alternative approaches that
e.g. treat file names as sequences of structured components are quite
complicated.
6.3. Opening, Creating, and Closing Files
Files are opened and created using the SSH_FXP_OPEN message, whose data
part is as follows:
uint32 id
string filename
uint32 pflags
ATTRS attrs
The `id' field is the request identifier as for all requests.
The `filename' field specifies the file name. See Section ``File
Names'' for more information.
The `pflags' field is a bitmask. The following bits have been defined.
#define SSH_FXF_READ 0x00000001
#define SSH_FXF_WRITE 0x00000002
#define SSH_FXF_APPEND 0x00000004
#define SSH_FXF_CREAT 0x00000008
#define SSH_FXF_TRUNC 0x00000010
#define SSH_FXF_EXCL 0x00000020
These have the following meanings:
SSH_FXF_READ
Open the file for reading.
SSH_FXF_WRITE
Open the file for writing. If both this and SSH_FXF_READ are
specified, the file is opened for both reading and writing.
SSH_FXF_APPEND
Force all writes to append data at the end of the file.
SSH_FXF_CREAT
If this flag is specified, then a new file will be created if one
does not 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.
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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.
6.4. Reading and Writing
Once a file has been opened, it can be read using the SSH_FXP_READ
message, which has the following format:
uint32 id
string handle
uint64 offset
uint32 len
where `id' is the request identifier, `handle' is an open file handle
returned by SSH_FXP_OPEN, `offset' is the offset (in bytes) relative to
the beginning of the file from where to start reading, and `len' is the
maximum number of bytes to read.
In response to this request, the server will read as many bytes as it
can from the file (up to `len'), and return them in a SSH_FXP_DATA
message. If an error occurs or EOF is encountered before reading any
data, the server will respond with SSH_FXP_STATUS. For normal disk
files, it is guaranteed that this will read the specified number of
bytes, or up to end of file. For e.g. device files this may return
fewer bytes than requested.
Writing to a file is achieved using the SSH_FXP_WRITE message, which has
the following format:
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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.
6.5. Removing and Renaming Files
Files can be removed using the SSH_FXP_REMOVE message. It has the
following format:
uint32 id
string filename
where `id' is the request identifier and `filename' is the name of the
file to be removed. See Section ``File Names'' for more information.
This request cannot be used to remove directories.
The server will respond to this request with a SSH_FXP_STATUS message.
Files (and directories) can be renamed using the SSH_FXP_RENAME message.
Its data is as follows:
uint32 id
string oldpath
string newpath
where `id' is the request identifier, `oldpath' is the name of an 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.
6.6. Creating and Deleting Directories
New directories can be created using the SSH_FXP_MKDIR request. It has
the following format:
uint32 id
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string path
where `id' is the request identifier and `path' specifies the directory
to be created. See Section ``File Names'' for more information on file
names. An error will be returned if a file or directory with the speci-
fied path already exists. The server will respond to this request with
a SSH_FXP_STATUS message.
Directories can be removed using the SSH_FXP_RMDIR request, which has
the following format:
uint32 id
string path
where `id' is the request identifier, and `path' specifies the directory
to be removed. See Section ``File Names'' for more information on file
names. An error will be returned if no directory with the specified
path exists, or if the specified directory is not empty, or if the path
specified a file system object other than a directory. The server
responds to this request with a SSH_FXP_STATUS message.
6.7. Scanning Directories
The files in a directory can be listed using the SSH_FXP_OPENDIR and
SSH_FXP_READDIR requests. Each SSH_FXP_READDIR request returns one or
more file names with full file attributes for each file. The client
should call SSH_FXP_READDIR repeatedly until it has found the file it is
looking for or until the server responds with a SSH_FXP_STATUS message
indicating an error (normally SSH_FX_EOF if there are no more files in
the directory). The client should then close the handle using the
SSH_FXP_CLOSE request.
The SSH_FXP_OPENDIR opens a directory for reading. It has the following
format:
uint32 id
string path
where `id' is the request identifier and `path' is the path name of the
directory to be listed (without any trailing slash). See 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
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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.
6.8. Retrieving File Attributes
Very often, file attributes are automatically returned by
SSH_FXP_READDIR. However, sometimes there is need to specifically
retrieve the attributes for a named file. This can be done using the
SSH_FXP_STAT, SSH_FXP_LSTAT and SSH_FXP_FSTAT requests.
SSH_FXP_STAT and SSH_FXP_LSTAT only differ in that SSH_FXP_STAT follows
symbolic links on the server, whereas SSH_FXP_LSTAT does not follow
symbolic links. Both have the same format:
uint32 id
string path
where `id' is the request identifier, and `path' 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
string handle
where `id' is the request identifier and `handle' is a file handle
returned by SSH_FXP_OPEN. The server responds to this request with
SSH_FXP_ATTRS or SSH_FXP_STATUS.
6.9. Setting File Attributes
File attributes may be modified using the SSH_FXP_SETSTAT and
SSH_FXP_FSETSTAT requests. These requests are used for operations such
as changing the ownership, permissions or access times, as well as for
truncating a file.
The SSH_FXP_SETSTAT request is of the following format:
uint32 id
string path
ATTRS attrs
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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.
6.10. 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.
7. Responses from the Server to the Client
The server responds to the client using one of a few response packets.
All requests can return a SSH_FXP_STATUS response upon failure. When
the operation is successful, any of the responses may be returned
(depending on the operation). If no data needs to be returned to the
client, the SSH_FXP_STATUS response with SSH_FX_OK status is
appropriate. Otherwise, the SSH_FXP_HANDLE message is used to return a
file handle (for SSH_FXP_OPEN and SSH_FXP_OPENDIR requests),
SSH_FXP_DATA is used to return data from SSH_FXP_READ, SSH_FXP_NAME is
used to return one or more file names from a SSH_FXP_READDIR or
SSH_FXP_REALPATH request, and SSH_FXP_ATTRS is used to return file
attributes from SSH_FXP_STAT, SSH_FXP_LSTAT, and SSH_FXP_FSTAT requests.
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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
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
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.
T. Ylonen and S. Lehtinen [page 14]
INTERNET-DRAFT 9 January, 2001
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:
uint32 id
uint32 count
repeats count times:
string filename
string longname
ATTRS attrs
T. Ylonen and S. Lehtinen [page 15]
INTERNET-DRAFT 9 January, 2001
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 file 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''.
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
T. Ylonen and S. Lehtinen [page 16]
INTERNET-DRAFT 9 January, 2001
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
`extended-request' name, then the server MUST respond with
SSH_FXP_STATUS with error/status set to SSH_FX_OP_UNSUPPORTED.
The SSH_FXP_EXTENDED_REPLY packet can be used to carry arbitrary
extension-specific data from the server to the client. It is of the
following format:
uint32 id
... any request-specific data ...
9. Security Considerations
This protocol assumes that it is run over a secure channel and that the
endpoints of the channel have been authenticated. Thus, this protocol
assumes that it is externally protected from network-level attacks.
This protocol provides file system access to arbitrary files on the
server (only constrained by the server implementation). It is the
responsibility of the server implementation to enforce any access
controls that may be required to limit the access allowed for any
particular user (the user being authenticated externally to this
protocol, typically using the SSH User Authentication Protocol [SSH-
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. Trademark Issues
SSH is a registered trademark and Secure Shell is a trademark of SSH
Communications Security Corp. SSH Communications Security Corp permits
the use of these trademarks as the name of this standard and protocol,
and permits their use to describe that a product conforms to this
standard, provided that the following acknowledgement is included where
the trademarks are used: ``SSH is a registered trademark and Secure
Shell is a trademark of SSH Communications Security Corp
(www.ssh.com)''. These trademarks may not be used as part of a product
name or in otherwise confusing manner without prior written permission
of SSH Communications Security Corp.
11. References
T. Ylonen and S. Lehtinen [page 17]
INTERNET-DRAFT 9 January, 2001
[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.
[SSH-ARCH] Ylonen, T., et al: "SSH Protocol Architecture", Internet-
Draft, draft-ietf-secsh-architecture-07.txt
[SSH-TRANSPORT] Ylonen, T., et al: "SSH Transport Protocol", Internet-
Draft, draft-ietf-secsh-transport-09.txt
[SSH-USERAUTH] Ylonen, T., et al: "SSH Authentication Protocol",
Internet-Draft, draft-ietf-secsh-userauth-09.txt
[SSH-CONNECT] Ylonen, T., et al: "SSH Connection Protocol", Internet-
Draft, draft-ietf-secsh-connect-09.txt
12. 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 18]