Network Working Group T. Ylonen Internet-Draft S. Lehtinen Expires: April 1, 2002 SSH Communications Security Corp October 2001 SSH File Transfer Protocol draft-ietf-secsh-filexfer-02.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 1, 2002. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract The SSH File Transfer Protocol provides secure file transfer functionality over any reliable data stream. It is the standard file transfer protocol for use with the SSH2 protocol. This document describes the file transfer protocol and its interface to the SSH2 protocol suite. Ylonen & Lehtinen Expires April 1, 2002 [Page 1] Internet-Draft SSH File Transfer Protocol October 2001 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Use with the SSH Connection Protocol . . . . . . . . . . . . 4 3. General Packet Format . . . . . . . . . . . . . . . . . . . 5 4. Protocol Initialization . . . . . . . . . . . . . . . . . . 7 5. File Attributes . . . . . . . . . . . . . . . . . . . . . . 8 6. Requests From the Client to the Server . . . . . . . . . . . 10 6.1 Request Synchronization and Reordering . . . . . . . . . . . 10 6.2 File Names . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3 Opening, Creating, and Closing Files . . . . . . . . . . . . 11 6.4 Reading and Writing . . . . . . . . . . . . . . . . . . . . 13 6.5 Removing and Renaming Files . . . . . . . . . . . . . . . . 14 6.6 Creating and Deleting Directories . . . . . . . . . . . . . 15 6.7 Scanning Directories . . . . . . . . . . . . . . . . . . . . 15 6.8 Retrieving File Attributes . . . . . . . . . . . . . . . . . 16 6.9 Setting File Attributes . . . . . . . . . . . . . . . . . . 17 6.10 Dealing with Symbolic links . . . . . . . . . . . . . . . . 18 6.11 Canonicalizing the Server-Side Path Name . . . . . . . . . . 18 7. Responses from the Server to the Client . . . . . . . . . . 20 8. Vendor-Specific Extensions . . . . . . . . . . . . . . . . . 24 9. Security Considerations . . . . . . . . . . . . . . . . . . 25 10. Changes from previous protocol versions . . . . . . . . . . 26 10.1 Changes between versions 3 and 2 . . . . . . . . . . . . . . 26 10.2 Changes between versions 2 and 1 . . . . . . . . . . . . . . 26 10.3 Changes between versions 1 and 0 . . . . . . . . . . . . . . 26 11. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . 27 References . . . . . . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 28 Full Copyright Statement . . . . . . . . . . . . . . . . . . 29 Ylonen & Lehtinen Expires April 1, 2002 [Page 2] Internet-Draft SSH File Transfer Protocol October 2001 1. Introduction This protocol provides secure file transfer (and more generally file system access) functionality over a reliable data stream, such as a channel in the SSH2 protocol [3]. This protocol is designed so that it could be used to implement a secure remote file system service, as well as a secure file transfer service. This protocol assumes that it runs over a secure channel, and that the server has already authenticated the user at the client end, and that the identity of the client user is externally available to the server implementation. In general, this protocol follows a simple request-response model. Each request and response contains a sequence number and multiple requests may be pending simultaneously. There are a relatively large number of different request messages, but a small number of possible response messages. Each request has one or more response messages that may be returned in result (e.g., a read either returns data or reports error status). The packet format descriptions in this specification follow the notation presented in the secsh architecture draft.[3]. 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 RFC 2246 [1] and transfer of management information in VPN applications. Ylonen & Lehtinen Expires April 1, 2002 [Page 3] Internet-Draft SSH File Transfer Protocol October 2001 2. Use with the SSH Connection Protocol When used with the SSH2 Protocol suite, this protocol is intended to be used from the SSH Connection Protocol [5] as a subsystem, as described in section ``Starting a Shell or a Command''. The subsystem name used with this protocol is "sftp". Ylonen & Lehtinen Expires April 1, 2002 [Page 4] Internet-Draft SSH File Transfer Protocol October 2001 3. General Packet Format All packets transmitted over the secure connection are of the following format: uint32 length byte type byte[length - 1] data payload That is, they are just data preceded by 32-bit length and 8-bit type fields. The `length' is the length of the data area, and does not include the `length' field itself. The format and interpretation of the data area depends on the packet type. All packet descriptions below only specify the packet type and the data that goes into the data field. Thus, they should be prefixed by the `length' and `type' fields. The maximum size of a packet is in practice determined by the client (the maximum size of read or write requests that it sends, plus a few bytes of packet overhead). All servers SHOULD support packets of at least 34000 bytes (where the packet size refers to the full length, including the header above). This should allow for reads and writes of at most 32768 bytes. There is no limit on the number of outstanding (non-acknowledged) requests that the client may send to the server. In practice this is limited by the buffering available on the data stream and the queuing performed by the server. If the server's queues are full, it should not read any more data from the stream, and flow control will prevent the client from sending more requests. Note, however, that while there is no restriction on the protocol level, the client's API may provide a limit in order to prevent infinite queuing of outgoing requests at the client. Ylonen & Lehtinen Expires April 1, 2002 [Page 5] Internet-Draft SSH File Transfer Protocol October 2001 The following values are defined for packet types. #define SSH_FXP_INIT 1 #define SSH_FXP_VERSION 2 #define SSH_FXP_OPEN 3 #define SSH_FXP_CLOSE 4 #define SSH_FXP_READ 5 #define SSH_FXP_WRITE 6 #define SSH_FXP_LSTAT 7 #define SSH_FXP_FSTAT 8 #define SSH_FXP_SETSTAT 9 #define SSH_FXP_FSETSTAT 10 #define SSH_FXP_OPENDIR 11 #define SSH_FXP_READDIR 12 #define SSH_FXP_REMOVE 13 #define SSH_FXP_MKDIR 14 #define SSH_FXP_RMDIR 15 #define SSH_FXP_REALPATH 16 #define SSH_FXP_STAT 17 #define SSH_FXP_RENAME 18 #define SSH_FXP_READLINK 19 #define SSH_FXP_SYMLINK 20 #define SSH_FXP_STATUS 101 #define SSH_FXP_HANDLE 102 #define SSH_FXP_DATA 103 #define SSH_FXP_NAME 104 #define SSH_FXP_ATTRS 105 #define SSH_FXP_EXTENDED 200 #define SSH_FXP_EXTENDED_REPLY 201 Additional packet types should only be defined if the protocol version number (see Section ``Protocol Initialization'') is incremented, and their use MUST be negotiated using the version number. However, the SSH_FXP_EXTENDED and SSH_FXP_EXTENDED_REPLY packets can be used to implement vendor-specific extensions. See Section ``Vendor-Specific-Extensions'' for more details. Ylonen & Lehtinen Expires April 1, 2002 [Page 6] Internet-Draft SSH File Transfer Protocol October 2001 4. Protocol Initialization When the file transfer protocol starts, it first sends a SSH_FXP_INIT (including its version number) packet to the server. The server responds with a SSH_FXP_VERSION packet, supplying the lowest of its own and the client's version number. Both parties should from then on adhere to particular version of the protocol. The SSH_FXP_INIT packet (from client to server) has the following data: uint32 version <extension data> The SSH_FXP_VERSION packet (from server to client) has the following data: uint32 version <extension data> The version number of the protocol specified in this document is 3. The version number should be incremented for each incompatible revision of this protocol. The extension data in the above packets may be empty, or may be a sequence of string extension_name string extension_data pairs (both strings MUST always be present if one is, but the `extension_data' string may be of zero length). If present, these strings indicate extensions to the baseline protocol. The `extension_name' field(s) identify the name of the extension. The name should be of the form "name@domain", where the domain is the DNS domain name of the organization defining the extension. Additional names that are not of this format may be defined later by the IETF. Implementations MUST silently ignore any extensions whose name they do not recognize. Ylonen & Lehtinen Expires April 1, 2002 [Page 7] Internet-Draft SSH File Transfer Protocol October 2001 5. File Attributes A new compound data type is defined for encoding file attributes. It is basically just a combination of elementary types, but is defined once because of the non-trivial description of the fields and to ensure maintainability. The same encoding is used both when returning file attributes from the server and when sending file attributes to the server. When sending it to the server, the flags field specifies which attributes are included, and the server will use default values for the remaining attributes (or will not modify the values of remaining attributes). When receiving attributes from the server, the flags specify which attributes are included in the returned data. The server normally returns all attributes it knows about. uint32 flags uint64 size present only if flag SSH_FILEXFER_ATTR_SIZE uint32 uid present only if flag SSH_FILEXFER_ATTR_UIDGID uint32 gid present only if flag SSH_FILEXFER_ATTR_UIDGID uint32 permissions present only if flag SSH_FILEXFER_ATTR_PERMISSIONS uint32 atime present only if flag SSH_FILEXFER_ACMODTIME uint32 mtime present only if flag SSH_FILEXFER_ACMODTIME uint32 extended_count present only if flag SSH_FILEXFER_ATTR_EXTENDED string extended_type string extended_data ... more extended data (extended_type - extended_data pairs), so that number of pairs equals extended_count The `flags' specify which of the fields are present. Those fields for which the corresponding flag is not set are not present (not included in the packet). New flags can only be added by incrementing the protocol version number (or by using the extension mechanism described below). The `size' field specifies the size of the file in bytes. The `uid' and `gid' fields contain numeric Unix-like user and group identifiers, respectively. The `permissions' field contains a bit mask of file permissions as defined by posix [1]. The `atime' and `mtime' contain the access and modification times of the files, respectively. They are represented as seconds from Jan 1, 1970 in UTC. The SSH_FILEXFER_ATTR_EXTENDED flag provides a general extension Ylonen & Lehtinen Expires April 1, 2002 [Page 8] Internet-Draft SSH File Transfer Protocol October 2001 mechanism for vendor-specific extensions. If the flag is specified, then the `extended_count' field is present. It specifies the number of extended_type-extended_data pairs that follow. Each of these pairs specifies an extended attribute. For each of the attributes, the extended_type field should be a string of the format "name@domain", where "domain" is a valid, registered domain name and "name" identifies the method. The IETF may later standardize certain names that deviate from this format (e.g., that do not contain the "@" sign). The interpretation of `extended_data' depends on the type. Implementations SHOULD ignore extended data fields that they do not understand. Additional fields can be added to the attributes by either defining additional bits to the flags field to indicate their presence, or by defining extended attributes for them. The extended attributes mechanism is recommended for most purposes; additional flags bits should only be defined by an IETF standards action that also increments the protocol version number. The use of such new fields MUST be negotiated by the version number in the protocol exchange. It is a protocol error if a packet with unsupported protocol bits is received. The flags bits are defined to have the following values: #define SSH_FILEXFER_ATTR_SIZE 0x00000001 #define SSH_FILEXFER_ATTR_UIDGID 0x00000002 #define SSH_FILEXFER_ATTR_PERMISSIONS 0x00000004 #define SSH_FILEXFER_ATTR_ACMODTIME 0x00000008 #define SSH_FILEXFER_ATTR_EXTENDED 0x80000000 Ylonen & Lehtinen Expires April 1, 2002 [Page 9] Internet-Draft SSH File Transfer Protocol October 2001 6. Requests From the Client to the Server Requests from the client to the server represent the various file system operations. Each request begins with an `id' field, which is a 32-bit identifier identifying the request (selected by the client). The same identifier will be returned in the response to the request. One possible implementation of it is a monotonically increasing request sequence number (modulo 2^32). Many operations in the protocol operate on open files. The SSH_FXP_OPEN request can return a file handle (which is an opaque variable-length string) which may be used to access the file later (e.g. in a read operation). The client MUST NOT send requests the server with bogus or closed handles. However, the server MUST perform adequate checks on the handle in order to avoid security risks due to fabricated handles. This design allows either stateful and stateless server implementation, as well as an implementation which caches state between requests but may also flush it. The contents of the file handle string are entirely up to the server and its design. The client should not modify or attempt to interpret the file handle strings. The file handle strings MUST NOT be longer than 256 bytes. 6.1 Request Synchronization and Reordering The protocol and implementations MUST process requests relating to the same file in the order in which they are received. 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. Ylonen & Lehtinen Expires April 1, 2002 [Page 10] Internet-Draft SSH File Transfer Protocol October 2001 6.2 File Names This protocol represents file names as strings. File names are assumed to use the slash ('/') character as a directory separator. File names starting with a slash are "absolute", and are relative to the root of the file system. Names starting with any other character are relative to the user's default directory (home directory). Note that identifying the user is assumed to take place outside of this protocol. Servers SHOULD interpret a path name component ".." as referring to the parent directory, and "." as referring to the current directory. If the server implementation limits access to certain parts of the file system, it must be extra careful in parsing file names when enforcing such restrictions. There have been numerous reported security bugs where a ".." in a path name has allowed access outside the intended area. An empty path name is valid, and it refers to the user's default directory (usually the user's home directory). Otherwise, no syntax is defined for file names by this specification. Clients should not make any other assumptions; however, they can splice path name components returned by SSH_FXP_READDIR together using a slash ('/') as the separator, and that will work as expected. It is understood that the lack of well-defined semantics for file names may cause interoperability problems between clients and servers using radically different operating systems. However, this approach is known to work acceptably with most systems, and alternative approaches that e.g. treat file names as sequences of structured components are quite complicated. 6.3 Opening, Creating, and Closing Files Files are opened and created using the SSH_FXP_OPEN message, whose data part is as follows: uint32 id string filename uint32 pflags ATTRS attrs The `id' field is the request identifier as for all requests. The `filename' field specifies the file name. See Section ``File Names'' for more information. Ylonen & Lehtinen Expires April 1, 2002 [Page 11] Internet-Draft SSH File Transfer Protocol October 2001 The `pflags' field is a bitmask. The following bits have been defined. #define SSH_FXF_READ 0x00000001 #define SSH_FXF_WRITE 0x00000002 #define SSH_FXF_APPEND 0x00000004 #define SSH_FXF_CREAT 0x00000008 #define SSH_FXF_TRUNC 0x00000010 #define SSH_FXF_EXCL 0x00000020 These have the following meanings: SSH_FXF_READ Open the file for reading. SSH_FXF_WRITE Open the file for writing. If both this and SSH_FXF_READ are specified, the file is opened for both reading and writing. SSH_FXF_APPEND Force all writes to append data at the end of the file. SSH_FXF_CREAT If this flag is specified, then a new file will be created if one does not already exist (if O_TRUNC is specified, the new file will be truncated to zero length if it previously exists). SSH_FXF_TRUNC Forces an existing file with the same name to be truncated to zero length when creating a file by specifying SSH_FXF_CREAT. SSH_FXF_CREAT MUST also be specified if this flag is used. SSH_FXF_EXCL Causes the request to fail if the named file already exists. SSH_FXF_CREAT MUST also be specified if this flag is used. The `attrs' field specifies the initial attributes for the file. Default values will be used for those attributes that are not specified. See Section ``File Attributes'' for more information. Regardless the server operating system, the file will always be opened in "binary" mode (i.e., no translations between different character sets and newline encodings). The response to this message will be either SSH_FXP_HANDLE (if the operation is successful) or SSH_FXP_STATUS (if the operation fails). Ylonen & Lehtinen Expires April 1, 2002 [Page 12] Internet-Draft SSH File Transfer Protocol October 2001 A file is closed by using the SSH_FXP_CLOSE request. Its data field has the following format: uint32 id string handle where `id' is the request identifier, and `handle' is a handle previously returned in the response to SSH_FXP_OPEN or SSH_FXP_OPENDIR. The handle becomes invalid immediately after this request has been sent. The response to this request will be a SSH_FXP_STATUS message. One should note that on some server platforms even a close can fail. This can happen e.g. if the server operating system caches writes, and an error occurs while flushing cached writes during the close. 6.4 Reading and Writing Once a file has been opened, it can be read using the SSH_FXP_READ message, which has the following format: uint32 id string handle uint64 offset uint32 len where `id' is the request identifier, `handle' is an open file handle returned by SSH_FXP_OPEN, `offset' is the offset (in bytes) relative to the beginning of the file from where to start reading, and `len' is the maximum number of bytes to read. In response to this request, the server will read as many bytes as it can from the file (up to `len'), and return them in a SSH_FXP_DATA message. If an error occurs or EOF is encountered before reading any data, the server will respond with SSH_FXP_STATUS. For normal disk files, it is guaranteed that this will read the specified number of bytes, or up to end of file. For e.g. device files this may return fewer bytes than requested. Writing to a file is achieved using the SSH_FXP_WRITE message, which has the following format: uint32 id string handle uint64 offset string data where `id' is a request identifier, `handle' is a file handle Ylonen & Lehtinen Expires April 1, 2002 [Page 13] Internet-Draft SSH File Transfer Protocol October 2001 returned by SSH_FXP_OPEN, `offset' is the offset (in bytes) from the beginning of the file where to start writing, and `data' is the data to be written. The write will extend the file if writing beyond the end of the file. It is legal to write way beyond the end of the file; the semantics are to write zeroes from the end of the file to the specified offset and then the data. On most operating systems, such writes do not allocate disk space but instead leave "holes" in the file. The server responds to a write request with a SSH_FXP_STATUS message. 6.5 Removing and Renaming Files Files can be removed using the SSH_FXP_REMOVE message. It has the following format: uint32 id string filename where `id' is the request identifier and `filename' is the name of the file to be removed. See Section ``File Names'' for more information. This request cannot be used to remove directories. The server will respond to this request with a SSH_FXP_STATUS message. Files (and directories) can be renamed using the SSH_FXP_RENAME message. Its data is as follows: uint32 id string oldpath string newpath where `id' is the request identifier, `oldpath' is the name of an existing file or directory, and `newpath' is the new name for the file or directory. It is an error if there already exists a file with the name specified by newpath. The server may also fail rename requests in other situations, for example if `oldpath' and `newpath' point to different file systems on the server. The server will respond to this request with a SSH_FXP_STATUS message. Ylonen & Lehtinen Expires April 1, 2002 [Page 14] Internet-Draft SSH File Transfer Protocol October 2001 6.6 Creating and Deleting Directories New directories can be created using the SSH_FXP_MKDIR request. It has the following format: uint32 id string path ATTRS attrs where `id' is the request identifier, `path' and `attrs' specifies the modifications to be made to its attributes. See Section ``File Names'' for more information on file names. Attributes are discussed in more detail in Section ``File Attributes''. specifies the directory to be created. An error will be returned if a file or directory with the specified path already exists. The server will respond to this request with a SSH_FXP_STATUS message. Directories can be removed using the SSH_FXP_RMDIR request, which has the following format: uint32 id string path where `id' is the request identifier, and `path' specifies the directory to be removed. See Section ``File Names'' for more information on file names. An error will be returned if no directory with the specified path exists, or if the specified directory is not empty, or if the path specified a file system object other than a directory. The server responds to this request with a SSH_FXP_STATUS message. 6.7 Scanning Directories The files in a directory can be listed using the SSH_FXP_OPENDIR and SSH_FXP_READDIR requests. Each SSH_FXP_READDIR request returns one or more file names with full file attributes for each file. The client should call SSH_FXP_READDIR repeatedly until it has found the file it is looking for or until the server responds with a SSH_FXP_STATUS message indicating an error (normally SSH_FX_EOF if there are no more files in the directory). The client should then close the handle using the SSH_FXP_CLOSE request. Ylonen & Lehtinen Expires April 1, 2002 [Page 15] Internet-Draft SSH File Transfer Protocol October 2001 The SSH_FXP_OPENDIR opens a directory for reading. It has the following format: uint32 id string path where `id' is the request identifier and `path' is the path name of the directory to be listed (without any trailing slash). See Section ``File Names'' for more information on file names. This will return an error if the path does not specify a directory or if the directory is not readable. The server will respond to this request with either a SSH_FXP_HANDLE or a SSH_FXP_STATUS message. Once the directory has been successfully opened, files (and directories) contained in it can be listed using SSH_FXP_READDIR requests. These are of the format uint32 id string handle where `id' is the request identifier, and `handle' is a handle returned by SSH_FXP_OPENDIR. (It is a protocol error to attempt to use an ordinary file handle returned by SSH_FXP_OPEN.) The server responds to this request with either a SSH_FXP_NAME or a SSH_FXP_STATUS message. One or more names may be returned at a time. Full status information is returned for each name in order to speed up typical directory listings. When the client no longer wishes to read more names from the directory, it SHOULD call SSH_FXP_CLOSE for the handle. The handle should be closed regardless of whether an error has occurred or not. 6.8 Retrieving File Attributes Very often, file attributes are automatically returned by SSH_FXP_READDIR. However, sometimes there is need to specifically retrieve the attributes for a named file. This can be done using the SSH_FXP_STAT, SSH_FXP_LSTAT and SSH_FXP_FSTAT requests. SSH_FXP_STAT and SSH_FXP_LSTAT only differ in that SSH_FXP_STAT follows symbolic links on the server, whereas SSH_FXP_LSTAT does not follow symbolic links. Both have the same format: uint32 id string path where `id' is the request identifier, and `path' specifies the file Ylonen & Lehtinen Expires April 1, 2002 [Page 16] Internet-Draft SSH File Transfer Protocol October 2001 system object for which status is to be returned. The server responds to this request with either SSH_FXP_ATTRS or SSH_FXP_STATUS. SSH_FXP_FSTAT differs from the others in that it returns status information for an open file (identified by the file handle). Its format is as follows: uint32 id string handle where `id' is the request identifier and `handle' is a file handle returned by SSH_FXP_OPEN. The server responds to this request with SSH_FXP_ATTRS or SSH_FXP_STATUS. 6.9 Setting File Attributes File attributes may be modified using the SSH_FXP_SETSTAT and SSH_FXP_FSETSTAT requests. These requests are used for operations such as changing the ownership, permissions or access times, as well as for truncating a file. The SSH_FXP_SETSTAT request is of the following format: uint32 id string path ATTRS attrs where `id' is the request identifier, `path' specifies the file system object (e.g. file or directory) whose attributes are to be modified, and `attrs' specifies the modifications to be made to its attributes. Attributes are discussed in more detail in Section ``File Attributes''. An error will be returned if the specified file system object does not exist or the user does not have sufficient rights to modify the specified attributes. The server responds to this request with a SSH_FXP_STATUS message. The SSH_FXP_FSETSTAT request modifies the attributes of a file which is already open. It has the following format: uint32 id string handle ATTRS attrs where `id' is the request identifier, `handle' (MUST be returned by SSH_FXP_OPEN) identifies the file whose attributes are to be modified, and `attrs' specifies the modifications to be made to its Ylonen & Lehtinen Expires April 1, 2002 [Page 17] Internet-Draft SSH File Transfer Protocol October 2001 attributes. Attributes are discussed in more detail in Section ``File Attributes''. The server will respond to this request with SSH_FXP_STATUS. 6.10 Dealing with Symbolic links The SSH_FXP_READLINK request may be used to read the target of a symbolic link. It would have a data part as follows: uint32 id string path where `id' is the request identifier and `path' specifies the path name of the symlink to be read. The server will respond with a SSH_FXP_NAME packet containing only one name and a dummy attributes value. The name in the returned packet contains the target of the link. If an error occurs, the server may respond with SSH_FXP_STATUS. The SSH_FXP_SYMLINK request will create a symbolic link on the server. It is of the following format uint32 id string linkpath string targetpath where `id' is the request identifier, `linkpath' specifies the path name of the symlink to be created and `targetpath' specifies the target of the symlink. The server shall respond with a SSH_FXP_STATUS indicating either success (SSH_FX_OK) or an error condition. 6.11 Canonicalizing the Server-Side Path Name The SSH_FXP_REALPATH request can be used to have the server canonicalize any given path name to an absolute path. This is useful for converting path names containing ".." components or relative pathnames without a leading slash into absolute paths. The format of the request is as follows: uint32 id string path where `id' is the request identifier and `path' specifies the path name to be canonicalized. The server will respond with a SSH_FXP_NAME packet containing only one name and a dummy attributes value. The name is the returned packet will be in canonical form. Ylonen & Lehtinen Expires April 1, 2002 [Page 18] Internet-Draft SSH File Transfer Protocol October 2001 If an error occurs, the server may also respond with SSH_FXP_STATUS. Ylonen & Lehtinen Expires April 1, 2002 [Page 19] Internet-Draft SSH File Transfer Protocol October 2001 7. Responses from the Server to the Client The server responds to the client using one of a few response packets. All requests can return a SSH_FXP_STATUS response upon failure. When the operation is successful, any of the responses may be returned (depending on the operation). If no data needs to be returned to the client, the SSH_FXP_STATUS response with SSH_FX_OK status is appropriate. Otherwise, the SSH_FXP_HANDLE message is used to return a file handle (for SSH_FXP_OPEN and SSH_FXP_OPENDIR requests), SSH_FXP_DATA is used to return data from SSH_FXP_READ, SSH_FXP_NAME is used to return one or more file names from a SSH_FXP_READDIR or SSH_FXP_REALPATH request, and SSH_FXP_ATTRS is used to return file attributes from SSH_FXP_STAT, SSH_FXP_LSTAT, and SSH_FXP_FSTAT requests. Exactly one response will be returned for each request. Each response packet contains a request identifier which can be used to match each response with the corresponding request. Note that it is legal to have several requests outstanding simultaneously, and the server is allowed to send responses to them in a different order from the order in which the requests were sent (the result of their execution, however, is guaranteed to be as if they had been processed one at a time in the order in which the requests were sent). Response packets are of the same general format as request packets. Each response packet begins with the request identifier. The format of the data portion of the SSH_FXP_STATUS response is as follows: uint32 id uint32 error/status code string error message (ISO-10646 UTF-8 [RFC-2279]) string language tag (as defined in [RFC-1766]) where `id' is the request identifier, and `error/status code' indicates the result of the requested operation. The value SSH_FX_OK indicates success, and all other values indicate failure. Ylonen & Lehtinen Expires April 1, 2002 [Page 20] Internet-Draft SSH File Transfer Protocol October 2001 Currently, the following values are defined (other values may be defined by future versions of this protocol): #define SSH_FX_OK 0 #define SSH_FX_EOF 1 #define SSH_FX_NO_SUCH_FILE 2 #define SSH_FX_PERMISSION_DENIED 3 #define SSH_FX_FAILURE 4 #define SSH_FX_BAD_MESSAGE 5 #define SSH_FX_NO_CONNECTION 6 #define SSH_FX_CONNECTION_LOST 7 #define SSH_FX_OP_UNSUPPORTED 8 SSH_FX_OK Indicates successful completion of the operation. SSH_FX_EOF indicates end-of-file condition; for SSH_FX_READ it means that no more data is available in the file, and for SSH_FX_READDIR it indicates that no more files are contained in the directory. SSH_FX_NO_SUCH_FILE is returned when a reference is made to a file which should exist but doesn't. SSH_FX_PERMISSION_DENIED is returned when the authenticated user does not have sufficient permissions to perform the operation. SSH_FX_FAILURE is a generic catch-all error message; it should be returned if an error occurs for which there is no more specific error code defined. SSH_FX_BAD_MESSAGE may be returned if a badly formatted packet or protocol incompatibility is detected. SSH_FX_NO_CONNECTION is a pseudo-error which indicates that the client has no connection to the server (it can only be generated locally by the client, and MUST NOT be returned by servers). SSH_FX_CONNECTION_LOST is a pseudo-error which indicates that the connection to the server has been lost (it can only be generated locally by the client, and MUST NOT be returned by servers). Ylonen & Lehtinen Expires April 1, 2002 [Page 21] Internet-Draft SSH File Transfer Protocol October 2001 SSH_FX_OP_UNSUPPORTED indicates that an attempt was made to perform an operation which is not supported for the server (it may be generated locally by the client if e.g. the version number exchange indicates that a required feature is not supported by the server, or it may be returned by the server if the server does not implement an operation). The SSH_FXP_HANDLE response has the following format: uint32 id string handle where `id' is the request identifier, and `handle' is an arbitrary string that identifies an open file or directory on the server. The handle is opaque to the client; the client MUST NOT attempt to interpret or modify it in any way. The length of the handle string MUST NOT exceed 256 data bytes. The SSH_FXP_DATA response has the following format: uint32 id string data where `id' is the request identifier, and `data' is an arbitrary byte string containing the requested data. The data string may be at most the number of bytes requested in a SSH_FXP_READ request, but may also be shorter if end of file is reached or if the read is from something other than a regular file. The SSH_FXP_NAME response has the following format: uint32 id uint32 count repeats count times: string filename string longname ATTRS attrs where `id' is the request identifier, `count' is the number of names returned in this response, and the remaining fields repeat `count' times (so that all three fields are first included for the first file, then for the second file, etc). In the repeated part, `filename' is a file name being returned (for SSH_FXP_READDIR, it will be a relative name within the directory, without any path components; for SSH_FXP_REALPATH it will be an absolute path name), `longname' is an expanded format for the file name, similar to what is returned by "ls -l" on Unix systems, and `attrs' is the attributes Ylonen & Lehtinen Expires April 1, 2002 [Page 22] Internet-Draft SSH File Transfer Protocol October 2001 of the file as described in Section ``File Attributes''. The format of the `longname' field is unspecified by this protocol. It MUST be suitable for use in the output of a directory listing command (in fact, the recommended operation for a directory listing command is to simply display this data). However, clients SHOULD NOT attempt to parse the longname field for file attributes; they SHOULD use the attrs field instead. The recommended format for the longname field is as follows: -rwxr-xr-x 1 mjos staff 348911 Mar 25 14:29 t-filexfer 1234567890 123 12345678 12345678 12345678 123456789012 Here, the first line is sample output, and the second field indicates widths of the various fields. Fields are separated by spaces. The first field lists file permissions for user, group, and others; the second field is link count; the third field is the name of the user who owns the file; the fourth field is the name of the group that owns the file; the fifth field is the size of the file in bytes; the sixth field (which actually may contain spaces, but is fixed to 12 characters) is the file modification time, and the seventh field is the file name. Each field is specified to be a minimum of certain number of character positions (indicated by the second line above), but may also be longer if the data does not fit in the specified length. The SSH_FXP_ATTRS response has the following format: uint32 id ATTRS attrs where `id' is the request identifier, and `attrs' is the returned file attributes as described in Section ``File Attributes''. Ylonen & Lehtinen Expires April 1, 2002 [Page 23] Internet-Draft SSH File Transfer Protocol October 2001 8. Vendor-Specific Extensions The SSH_FXP_EXTENDED request provides a generic extension mechanism for adding vendor-specific commands. The request has the following format: uint32 id string extended-request ... any request-specific data ... where `id' is the request identifier, and `extended-request' is a string of the format "name@domain", where domain is an internet domain name of the vendor defining the request. The rest of the request is completely vendor-specific, and servers should only attempt to interpret it if they recognize the `extended-request' name. The server may respond to such requests using any of the response packets defined in Section ``Responses from the Server to the Client''. Additionally, the server may also respond with a SSH_FXP_EXTENDED_REPLY packet, as defined below. If the server does not recognize the `extended-request' name, then the server MUST respond with SSH_FXP_STATUS with error/status set to SSH_FX_OP_UNSUPPORTED. The SSH_FXP_EXTENDED_REPLY packet can be used to carry arbitrary extension-specific data from the server to the client. It is of the following format: uint32 id ... any request-specific data ... Ylonen & Lehtinen Expires April 1, 2002 [Page 24] Internet-Draft SSH File Transfer Protocol October 2001 9. Security Considerations This protocol assumes that it is run over a secure channel and that the endpoints of the channel have been authenticated. Thus, this protocol assumes that it is externally protected from network-level attacks. This protocol provides file system access to arbitrary files on the server (only constrained by the server implementation). It is the responsibility of the server implementation to enforce any access controls that may be required to limit the access allowed for any particular user (the user being authenticated externally to this protocol, typically using the SSH User Authentication Protocol [6]. Care must be taken in the server implementation to check the validity of received file handle strings. The server should not rely on them directly; it MUST check the validity of each handle before relying on it. Ylonen & Lehtinen Expires April 1, 2002 [Page 25] Internet-Draft SSH File Transfer Protocol October 2001 10. Changes from previous protocol versions The SSH File Transfer Protocol has changed over time, before it's standardization. The following is a description of the incompatible changes between different versions. 10.1 Changes between versions 3 and 2 o The SSH_FXP_READLINK and SSH_FXP_SYMLINK messages were added. o The SSH_FXP_EXTENDED and SSH_FXP_EXTENDED_REPLY messages were added. o The SSH_FXP_STATUS message was changed to include fields `error message' and `language tag'. 10.2 Changes between versions 2 and 1 o The SSH_FXP_RENAME message was added. 10.3 Changes between versions 1 and 0 o Implementation changes, no actual protocol changes. Ylonen & Lehtinen Expires April 1, 2002 [Page 26] Internet-Draft SSH File Transfer Protocol October 2001 11. Trademark Issues "ssh" is a registered trademark of SSH Communications Security Corp in the United States and/or other countries. Ylonen & Lehtinen Expires April 1, 2002 [Page 27] Internet-Draft SSH File Transfer Protocol October 2001 References [1] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [2] 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. [3] Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Protocol Architecture", draft-ietf-secsh- architecture-09 (work in progress), July 2001. [4] Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Protocol Transport Protocol", draft-ietf-secsh- architecture-09 (work in progress), July 2001. [5] Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Connection Protocol", draft-ietf-secsh-connect-11 (work in progress), July 2001. [6] Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Authentication Protocol", draft-ietf-secsh- userauth-11 (work in progress), July 2001. Authors' Addresses 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 Ylonen & Lehtinen Expires April 1, 2002 [Page 28] Internet-Draft SSH File Transfer Protocol October 2001 Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Ylonen & Lehtinen Expires April 1, 2002 [Page 29]