Secure Shell Working Group J. Galbraith
Internet-Draft VanDyke Software
Expires: October 7, 2005 O. Saarenmaa
F-Secure
T. Ylonen
S. Lehtinen
SSH Communications Security Corp
April 5, 2005
SSH File Transfer Protocol
draft-ietf-secsh-filexfer-08.txt
Status of this Memo
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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
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transfer protocol for use with the SSH2 protocol. This document
describes the file transfer protocol and its interface to the SSH2
protocol suite.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Use with the SSH Connection Protocol . . . . . . . . . . . . . 4
2.1 The Use of 'stderr' in the server . . . . . . . . . . . . 4
3. General Packet Format . . . . . . . . . . . . . . . . . . . . 5
3.1 Request Synchronization and Reordering . . . . . . . . . . 6
3.2 New data types defined by this document . . . . . . . . . 6
3.3 Packet Types . . . . . . . . . . . . . . . . . . . . . . . 7
4. Protocol Initialization . . . . . . . . . . . . . . . . . . . 8
4.1 Client Initialization . . . . . . . . . . . . . . . . . . 8
4.2 Server Initialization . . . . . . . . . . . . . . . . . . 9
4.3 Determining Server Newline Convention . . . . . . . . . . 9
4.4 Vendor Id . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5 Supported Features . . . . . . . . . . . . . . . . . . . . 10
4.6 Version re-negotiation . . . . . . . . . . . . . . . . . . 12
5. File Names . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6. File Attributes . . . . . . . . . . . . . . . . . . . . . . . 15
6.1 valid-attribute-flags . . . . . . . . . . . . . . . . . . 16
6.2 Type . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.3 Size . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.4 allocation-size . . . . . . . . . . . . . . . . . . . . . 18
6.5 Owner and Group . . . . . . . . . . . . . . . . . . . . . 18
6.6 Permissions . . . . . . . . . . . . . . . . . . . . . . . 19
6.7 Times . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.8 ACL . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.9 attrib-bits and attrib-bits-valid . . . . . . . . . . . . 21
6.10 text-hint . . . . . . . . . . . . . . . . . . . . . . . . 24
6.11 mime-type . . . . . . . . . . . . . . . . . . . . . . . . 24
6.12 link-count . . . . . . . . . . . . . . . . . . . . . . . . 24
6.13 untranslated-name . . . . . . . . . . . . . . . . . . . . 25
6.14 Extended Attributes . . . . . . . . . . . . . . . . . . . 25
7. Requests From the Client to the Server . . . . . . . . . . . . 25
7.1 Opening and Closing Files and Directories . . . . . . . . 25
7.1.1 Opening a File . . . . . . . . . . . . . . . . . . . . 26
7.1.2 Opening a Directory . . . . . . . . . . . . . . . . . 31
7.1.3 Closing Handles . . . . . . . . . . . . . . . . . . . 32
7.2 Reading and Writing . . . . . . . . . . . . . . . . . . . 32
7.2.1 Reading Files . . . . . . . . . . . . . . . . . . . . 32
7.2.2 Reading Directories . . . . . . . . . . . . . . . . . 33
7.2.3 Writing Files . . . . . . . . . . . . . . . . . . . . 33
7.3 Removing and Renaming Files . . . . . . . . . . . . . . . 34
7.4 Creating and Deleting Directories . . . . . . . . . . . . 35
7.5 Retrieving File Attributes . . . . . . . . . . . . . . . . 36
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7.6 Setting File Attributes . . . . . . . . . . . . . . . . . 37
7.7 Dealing with Links . . . . . . . . . . . . . . . . . . . . 38
7.8 Canonicalizing the Server-Side Path Name . . . . . . . . . 40
7.8.1 Best Practice for Dealing with Paths . . . . . . . . . 42
8. Responses from the Server to the Client . . . . . . . . . . . 42
8.1 Status Response . . . . . . . . . . . . . . . . . . . . . 43
8.2 Handle Response . . . . . . . . . . . . . . . . . . . . . 47
8.3 Data Response . . . . . . . . . . . . . . . . . . . . . . 47
8.4 Name Response . . . . . . . . . . . . . . . . . . . . . . 48
8.5 Attrs Response . . . . . . . . . . . . . . . . . . . . . . 49
9. Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . 49
9.1 File Hashing . . . . . . . . . . . . . . . . . . . . . . . 50
9.1.1 Checking File Contents: v5 extension . . . . . . . . . 51
9.1.2 Checking File Contents . . . . . . . . . . . . . . . . 52
9.2 Querying Available Space . . . . . . . . . . . . . . . . . 54
9.3 Querying User Home Directory . . . . . . . . . . . . . . . 55
10. Implementation Considerations . . . . . . . . . . . . . . . 55
11. Security Considerations . . . . . . . . . . . . . . . . . . 56
12. Changes from Previous Protocol Versions . . . . . . . . . . 57
12.1 Changes Between Versions 6 and 5 . . . . . . . . . . . . . 57
12.2 Changes Between Versions 5 and 4 . . . . . . . . . . . . . 58
12.3 Changes Between Versions 4 and 3 . . . . . . . . . . . . . 59
12.4 Changes Between Versions 3 and 2 . . . . . . . . . . . . . 59
12.5 Changes Between Versions 2 and 1 . . . . . . . . . . . . . 59
12.6 Changes Between Versions 1 and 0 . . . . . . . . . . . . . 59
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 60
13.1 Normative References . . . . . . . . . . . . . . . . . . . 60
13.2 Informative References . . . . . . . . . . . . . . . . . . 61
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 61
Intellectual Property and Copyright Statements . . . . . . . . 63
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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. 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".
2.1 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
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as part of the SFTP protocol at this early stage. However, the
errors or warnings MAY be sent as stderr text.
3. General Packet Format
All packets transmitted over the secure connection are of the
following format:
uint32 length
byte type
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.
'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
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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.
3.1 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.
A client MUST be prepared to recieve responses to multiple overlapped
requests out of order.
3.2 New data types defined by this document
This document defines these data types in addition to those defined
in [I-D.ietf-secsh-architecture].
int64
Represents a 64-bit signed integer. Stored as eight bytes in the
order of decreasing significance (network byte order).
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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.
3.3 Packet Types
The following values are defined for packet types.
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
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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 protocol version
(Section 4) MUST be incremented if the server is to send new packets
to the client (because the client has no way to respond indicating
that the packet isn't recognized.)
4. 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.
4.1 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 discontiguous 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.
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4.2 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.
'extensions' is 0 or more extension-pairs (Section 3.2).
Implementations MUST silently ignore any extensions whose names they
do not recognize.
4.3 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_TEXT file open flag (Section 7.1.1) 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.
4.4 Vendor Id
It is often necessary to detect the version of a the server so that
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bugs can be worked around. This extension allows the client to do
so. (It may also be sent by the client using an EXTENDED request.
string "vendor-id"
string vendor-structure
string vendor-name
string product-name
string product-version
uint64 product-build-number
vendor-name
Arbitrary name identifying the maker of the product.
product-name
Arbitrary name identifying the product.
product-name
Arbitrary string identifying the version of the product.
product-build-number
A build-number for the product, such that if a bug is fixed in
build-number 'x', it can be assumed that (barring regression in
the product) it is fixed in all build-numbers > 'x'.
4.5 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.
string "supported2"
string supported-structure
uint32 supported-attribute-mask
uint32 supported-attribute-bits
uint32 supported-open-flags
uint32 max-read-size
uint64 supported-open-block-masks
uint64 supported-block-masks
uint32 attrib-extension-count
string attrib-extension-names[attrib_extension-count]
uint32 extension-count
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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 6.1) 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 6.9) 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 7.1.1) flags field.
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.
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 specfied in Section 3.
supported-open-block-masks
Series of four-bit fields representing the support combinations of
SSH_FXF_ACCESS_BLOCK_READ, SSH_FXF_ACCESS_BLOCK_WRITE,
SSH_FXF_ACCESS_BLOCK_DELETE, and SSH_FXF_ACCESS_BLOCK_ADVISORY
(Section 7.1.1.3.)
The bits are interpreted in four-bit groups, beginning with the
least significant bit. The bit values are the values used in the
file open flags, shifted right so that BLOCK_READ is the least
significant bit.
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For example, a server that supported only the classic advisory
read lock (shared lock) and write lock (exclusive lock) would send
0b1011 1010, or 0xba.
supported-block-masks
Series of four-bit fields representing the supported combinations
of BLOCK_* flags for use with the SSH_FXF_BLOCK request, as
described above.
attrib-extension-count
Count of extension names in the attrib-extension array.
attrib-extension-names
Names of extensions that can be used in an ATTRS (Section 6.14)
structure.
extension-count
Count of extension names in the attrib-extension array.
extension-names
Names of extensions that can be used with the SSH_FXP_EXTEND
(Section 9) packet.
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.
4.6 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, for example, "3,6,7". Defined versions are: "2", "3", "4",
"5", "6". Any other version advertised by the server must follow the
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DNS extensibility naming convention outlined in [I-D.ietf-secsh-
architecture].
If the client and server have negotiated any version higher than
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"
uint32 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.
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.
The client SHOULD NOT select a version lower than was initially
negotiated; however, it is not forbidden to do so. One reason a
client might do so is to work around a buggy implementation.
5. 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 11) as
referring to the parent directory, and "." as referring to the
current directory.
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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 prefered encoding for filenames is UTF-8. This is consistant
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
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 reversable. 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.
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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 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.
6. File Attributes
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.
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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
6.1 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.
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:
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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.
6.2 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.
6.3 Size
The 'size' field specifies the number of bytes that can be read from
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.
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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.
6.4 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 6.9).
When present during file creation, the file SHOULD be created and the
specified number of bytes pre-allocated. If the pre-allocation
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.
6.5 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
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
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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.
6.6 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.
6.7 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.
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 attrbutes were changed. The
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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.
6.8 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 time:
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
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].
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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 6.5)
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@.
6.9 attrib-bits and attrib-bits-valid
These fields, taken together, reflect various attributes of the file
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
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comminicute only the bits it knows about without inadvertantly
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 4.5).
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
returns SSH_FX_FILE_ALREADY_EXISTS, it should prompt the user and
then retry the create specifying SSH_FXF_CREATE_TRUNCATE.
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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 7.1.1.3) 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.
SSH_FILEXFER_ATTR_FLAGS_SYNC
When the file is modified, the changes are written synchronously
to the disk.
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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.
6.10 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 hueristic 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 hueristic 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.
6.11 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.
6.12 link-count
This field contains the hard link count of the file. This attribute
MUST NOT be present during a setstat operation.
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6.13 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 6.9) bit in the
attrib-bits field.
6.14 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 4.5).
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.
7. Requests From the Client to the Server
Requests from the client to the server represent the various file
system operations.
7.1 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.
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
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strings.
The file handle strings MUST NOT be longer than 256 bytes.
7.1.1 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.)
7.1.1.1 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.
7.1.1.2 desired-access
The 'desired-access' field is a bitmask containing a combination of
values from the ace-mask flags (Section 6.8). 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 originaly requested.
In such cases, it is possible, and permissable in the protocol, that
the client could open a file requesting some limited access, and then
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.
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7.1.1.3 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_TEXT
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 4.3)
When a file is opened with the FXF_TEXT 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.
Clients MUST be prepared to handle SSH_FX_OP_UNSUPPORTED returned
for read or write operations that are not sequential.
SSH_FXF_ACCESS_BLOCK_READ
The server MUST guarantee that no other open has taken place which
blocked 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
combinatation 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 lock 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
combinatation 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.
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
information.
The 'attrs' field is ignored if an existing file is opened.
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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
7.1.2 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.
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).
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7.1.3 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.
7.2 Reading and Writing
7.2.1 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
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.
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offset
'offset' is the offset (in bytes) relative to the beginning of the
file that the read MUST start at. 'offset' is ignored if
SSH_FXF_TEXT 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 it's
'supported2' (Section 4.5) extension, then failure to return
'length' bytes indicates that EOF or an error occured.
7.2.2 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.
7.2.3 Writing Files
Writing to a file is achieved using the following message:
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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
'offset' is the offset (in bytes) relative to the beginning of the
file that the write MUST start at. 'offset' is ignored if
SSH_FXF_TEXT 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.
7.3 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.
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
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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_FXP_RENAME_OVERWRITE 0x00000001
SSH_FXP_RENAME_ATOMIC 0x00000002
SSH_FXP_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
requirements.
The server will respond to this request with a SSH_FXP_STATUS
message.
7.4 Creating and Deleting Directories
New directories can be created using the SSH_FXP_MKDIR request. It
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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.
7.5 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.
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:
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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.
7.6 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]
ATTRS attrs
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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.
7.7 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 hare or symbolic) on
the server.
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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 refering 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.
The SSH_FXP_LOCK 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.
byte SSH_FXP_BLOCK
uint32 request-id
string handle
uint64 offset
uint64 length
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uint32 uLockMask
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
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 7.1.1.3.
The SSH_FXP_UNLOCK removes a previously aquired 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' 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
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.
7.8 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:
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byte SSH_FXP_REALPATH
uint32 request-id
string original-path [UTF-8]
string compose-path [optional]
byte control-byte [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.
compose-path
A path which the client wishs 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.
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 seperate 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.
The server MUST take the 'original-path' and apply the 'compose-path'
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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.
7.8.1 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
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.
8. Responses from the Server to the Client
The server responds to the client using one of a few response
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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.
8.1 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.
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.
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.
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Error codes:
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_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 occured, 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 principle 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 specifed file was a directory in a context where a directory
cannot be used.
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SSH_FX_BYTE_RANGE_LOCK_CONFLICT
Some operating systems support locking a range of bytes within a
file. On such operating systems, it is possible for a SFTP
request to fail due to some other process owning a byte-range
lock, even though the SFTP protocol does not support byte-range
locks natively.
A read or write operation failed because another process's 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.
8.2 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.
8.3 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 7.2.1.)
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.)
8.4 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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8.5 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''.
9. 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. Vendor-specific extensions have
use the "name@domain" syntax, where domain is an internet domain
name of the vendor defining the request.
The IETF may also define extensions to the protocol. These
extension names will not have an '@' in them.
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
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extension-specific data from the server to the client. It is of the
following format:
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.
9.1 File Hashing
BEGIN: RFCEDITOR REMOVE BEFORE PUBLISHING
After some discussion of this at connectathon, I know of two uses for
this feature, neither one of which the feature is entirely suited
for:
o Checking that a file has been uploaded to the server correctly;
some portion of the customers wanting this feature want it in a
security sense, as part of proof the server has the file.
o Optimizing upload or download of the file; multiple hashes are
performed on small pieces of the file and the results are used to
determine what chunks of the file, if any, need to be transfered.
This is similar to the way rsync works.
I've seen both of these implemented.
For the first case, the extension has several drawbacks, including:
o A FIPS implementation can't ship md5.
o MD5's security is potential weaker than other options.
o Being hard-coded to MD5 makes in impossible to adapt to future
developments in the arena of MD5 compromises.
For the second case, the extension has these drawbacks:
o MD5 is expensive (relative to other options.)
o The extension must be sent potentially thousands of times to
retrieve the desired granularity of hashes.
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Therefore, for this draft, this section is marked experimental; I've
included a second proposed extension. Please post your thoughts on
the mailing list. (I did it this way just so I could get a draft out
that I and my active co-author are happy with.
In addition, implemenation experience has shown the quick check hash
to not be useful.
END: RFCEDITOR REMOVE BEFORE PUBLISHING
9.1.1 Checking File Contents: v5 extension
This extension allows a client to easily check if a file (or portion
thereof) that it already has matches what is on the server.
byte SSH_FXP_EXTENDED
uint32 request-id
string "md5-hash" / "md5-hash-handle"
string filename [UTF-8] / file-handle
uint64 start-offset
uint64 length
string quick-check-hash
filename
Used if "md5-hash" is specified; indicates the name of the file to
use. The hash will be of the file contents as it would appear on
the wire if the file were opened with no special flags.
file-handle
Used if "md5-hash-handle" is specified; specifies a file handle to
read the data from. The handle MUST be a file handle, and
ACE4_READ_DATA MUST have been included in the desired-access when
the file was opened.
If this file handle was opened in TEXT mode, the md5-hash must be
made of the data as it would be sent on the wire.
start-offset
The starting offset of the data to hash.
length
The length of data to include in the hash. If both start-offset
and length are zero, the entire file should be included.
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quick-check-hash
The hash over the first 2048 bytes of the data range as the client
knows it, or the entire range, if it is less than 2048 bytes.
This allows the server to quickly check if it is worth the
resources to hash a big file.
If this is a zero length string, the client does not have the
data, and is requesting the hash for reasons other than comparing
with a local file. The server MAY return SSH_FX_OP_UNSUPPORTED in
this case.
The response is either a SSH_FXP_STATUS packet, indicating an error,
or the following extended reply packet:
byte SSH_FXP_EXTENDED_REPLY
uint32 request-id
string "md5-hash"
string hash
If 'hash' is zero length, then the 'quick-check-hash' did not match,
and no hash operation was preformed. Otherwise, 'hash' contains the
hash of the entire data range (including the first 2048 bytes that
were included in the 'quick-check-hash'.)
9.1.2 Checking File Contents
This extension allows a client to easily check if a file (or portion
thereof) that it already has matches what is on the server.
byte SSH_FXP_EXTENDED
uint32 request-id
string "check-file"
string handle
string hash-algorithm-list
uint64 start-offset
uint64 length
uint32 block-size
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. If ACE4_READ_DATA MUST was not
included when the file was opened, the server MUST return
STATUS_PERMISSION_DENIED.
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If this file handle was opened in TEXT mode, the check must be
performed on the data as it would be sent on the wire.
hash-algorithm-list
A comma separated list of hash alogirthms the client is willing to
accept for this operation. The server MUST pick the first hash on
the list that it supports.
Currently supported algorithms are "md5", "sha1", "sha224",
"sha256", "sha384", "sha512", and "crc32". Additional algorithms
may be added by following the DNS extensibility naming convention
outlined in [I-D.ietf-secsh-architecture].
MD5 is described in [RFC1321]. SHA-1, SHA-224, SHA-256, SHA-384,
and SHA-512 are decribed in [FIPS-180-2]. crc32 is described in
[ISO.3309.1991], and is the same algorithm used in [RFC1510]
start-offset
The starting offset of the data to include in the hash.
length
The length of data to include in the hash. If length is zero, all
the data from start-offset to the end-of-file should be included.
block-size
An independant hash MUST be computed over every block in the file.
The size of blocks is specified by block-size. The block-size
MUST NOT be smaller than 256 bytes. If the block-size is 0, then
only one hash, over the entire range, MUST be made.
The response is either a SSH_FXP_STATUS packet, indicating an error,
or the following extended reply packet:
byte SSH_FXP_EXTENDED_REPLY
uint32 request-id
string "check-file"
string hash-algo-used
byte hash[n][block-count]
hash-algo-used
The hash algorithm that was actually used.
hash
The computed hashes. The hash algorithm used determines the size
of n. The number of block-size chunks of data in the file
determines block-count. The hashes are placed in the packet one
after another, with no decoration.
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Note that if the length of the range is not an even multiple of
block-size, the last hash will have been computed over only the
remainder of the range instead of a full block.
9.2 Querying Available Space
The following extension provides a way to discover the available
space for an arbitrary path.
byte SSH_FXP_EXTENDED
uint32 request-id
string "space-available"
string path [UTF-8]
path
'path' for which the available space should be reported. This
'path' is not required to be the mount point path, but MAY be a
directory or file contained within the mount.
The reply to the request is as follows:
byte SSH_FXP_EXTENDED_REPLY
uint32 request-id
uint64 bytes-on-device
uint64 unused-bytes-on-device
uint64 bytes-available-to-user
uint64 unused-bytes-available-to-user
uint32 bytes-per-allocation-unit
bytes-on-device
The total number of bytes on the device which stores 'path', both
used and unused, or 0 if unknown.
unused-bytes-on-device
The total number of unused bytes available on the device which
stores 'path', or 0 if unknown.
bytes-available-to-user
The total number of bytes, both used and unused, available to the
authenticated user on the device which stores 'path', or 0 if
unknown.
unused-bytes-available-to-user
The total number of unused bytes available to the authenticated
user on the device which stores 'path', or 0 if unknown.
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bytes-per-allocation-unit
The number of bytes in each allocation unit on the device, or in
other words, the minimum number of bytes that a file allocation
size can grow or shrink by. If the server does not know this
information, or the file-system in use does not use allocation
blocks, this value MUST be 0.
9.3 Querying User Home Directory
Many users are used to being able to type '~' as an alias for their
home directory, or ~username as an alias for another user's home
directory. To support this feature, a server MAY support following
extension.
byte SSH_FXP_EXTENDED
uint32 request-id
string "home-directory"
string username [UTF-8]
username
Username whose home directory path is being requested. An empty
string implies the current user.
The reply to the request is either a SSH_FXP_STATUS packet or the
following extended reply:
byte SSH_FXP_EXTENDED_REPLY
uint32 request-id
string "home-directory"
string absolute-pathname
absolute-pathname
Absolute pathname of the specified user's home directory, suitable
for use in operations such as REALPATH or OPEN.
10. 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
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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 3.1).)
Implementations MUST also be aware that read requests may not return
all the requested data, even if the data is available.
11. 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.
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 6.6) 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
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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.
12. Changes from Previous Protocol Versions
The SSH File Transfer Protocol has changed over time, before its
standardization. The following is a description of the incompatible
changes between different versions.
12.1 Changes Between Versions 6 and 5
o Add ability to negotiate version when client supports discontigous
ranges of protocol version.
o Add 'filename-charset' and the 'filename-translation-control'
extensions to allow better support of servers that can't reliably
translate to UTF-8.
o Add DIR_NOT_EMPTY, NOT_A_DIRECTORY, INVALID_FILENAME LINK_LOOP,
CANNOT_DELETE, INVALID_PARAMETER, FILE_IS_A_DIRECTORY,
BYTE_RANGE_LOCK_CONFLICT, BYTE_RANGE_LOCK_REFUSED, DELETE_PENDING,
and FILE_CORRUPT error codes.
o Added space-available extension.
o Added NOFOLLOW and DELETE_ON_CLOSE flag to open flags.
o Added allocation-size, text-hint, link-count, mime-type, and
untranslated-name fields to attrib structure. Add
ATTR_FLAGS_TRANSLATION_ERR to the attrib-bits.
o Add optional 'compose-path' and 'control-byte' to REALPATH; make
realpath's behaviour truly deterministic (i.e., MUST instead of
SHOULD.) Give clients the ability to compose path's without
understanding what is relative and what is absolute.
o Give SSH_FXP_DATA and SSH_FXP_NAME optional end-of-data-set flags,
which can help the client avoid a round trip during normal
operation.
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o Changed the SYMLINK packet to be LINK and give it the ability to
create hard links. Also change it's packet number because many
implementation implemented SYMLINK with the arguments reversed.
Hopefully the new argument names make it clear which way is which.
o Clarify who should apply umask to POSIX mode bits (the client, not
the server.)
o Specify behavior for otherwise valid packets with excess data and
unrecognized packet types.
o Add home directory extension.
o Remove "#define" from symbol definitions to shorten line and help
us pass idnits.
o Change "supported" extension to "supported2", remove desired-
access-bits, seperate attrib extension from SSH_FXP_EXTENDED
extensions.
o Add "vendor-id" extension.
o Add ctime and attrib-bits-valid to attrib structure.
o Unrecognized attrib extensions cause failure rather than ignored.
o Option 'end of data' flags on data ane names responses.
12.2 Changes Between Versions 5 and 4
Many of the changes between version 5 and version 4 are to better
support the changes in version 4, and to better specify error
conditions.
o Add "supported" extension to communicate features supported.
o Clarify error handling when client requests unsupported feature.
(For example, attempts to write an unsupported attribute.)
o Add attrib-bits field to the attribute structure, which specifies
a number of boolean attributes related to files and directories,
including advisory read-only and case-sensitivity bits.
o Clarify the actual bit values to be used for the permissions field
(since POSIX doesn't define values) and correct the value of
ATTR_PERMISSIONS flag.
o Some reordering of sections to attempt to get a better grouping of
related functionality.
o Open request explicitly specifies the access desired for the file.
o Add support for explicitly requesting file locking.
o Add support for better control of the rename operation.
o Add SSH_FX_NO_SPACE_ON_FILESYSTEM, SSH_FX_QUOTA_EXCEEDED, and
SSH_FX_UNKNOWN_PRINCIPLE error codes.
o Add support for error specific data. This is used by a new
SSH_FX_UNKNOWN_PRINCIPLE error to communicate which principles are
unknown.
o Add support for retrieving md5-hash of file contents.
o Update security section.
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12.3 Changes Between Versions 4 and 3
Many of the changes between version 4 and version 3 are to the
attribute structure to make it more flexible for non-unix platforms.
o Clarify the use of stderr by the server.
o Clarify handling of very large read requests by the server.
o Make all filenames UTF-8.
o Added 'newline' extension.
o Made time fields 64 bit, and optionally have nanosecond
resolution.
o Made file attribute owner and group strings so they can actually
be used on disparate systems.
o Added createtime field, and added separate flags for atime,
createtime, and mtime so they can be set separately.
o Split the file type out of the permissions field and into its own
field (which is always present.)
o Added acl attribute.
o Added SSH_FXF_TEXT file open flag.
o Added flags field to the get stat commands so that the client can
specifically request information the server might not normally
included for performance reasons.
o Removed the long filename from the names structure-- it can now be
built from information available in the attrs structure.
o Added reserved range of packet numbers for extensions.
o Added several additional error codes.
12.4 Changes Between Versions 3 and 2
o The SSH_FXP_READLINK and SSH_FXP_SYMLINK messages were added.
o The SSH_FXP_EXTENDED and SSH_FXP_EXTENDED_REPLY messages were
added.
o The SSH_FXP_STATUS message was changed to include fields 'error
message' and 'language tag'.
12.5 Changes Between Versions 2 and 1
o The SSH_FXP_RENAME message was added.
12.6 Changes Between Versions 1 and 0
o Implementation changes, no actual protocol changes.
13. References
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13.1 Normative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[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]
Lonvick, C., "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., "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.
[FIPS-180-2]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", Federal Information Processing
Standards Publication 180-2, August 2002.
[ISO.3309.1991]
International Organization for Standardization,
"Information Technology - Telecommunications and
information exchange between systems - High-level data
link control (HDLC) procedures - Frame structure",
ISO Standard 3309, June 1991.
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13.2 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.
[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., "SSH Authentication Protocol",
draft-ietf-secsh-userauth-27 (work in progress),
March 2005.
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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Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
Email: ylo@ssh.com
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
Email: sjl@ssh.com
Trademark notice
"ssh" is a registered trademark in the United States and/or other
countries.
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Acknowledgment
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