Secure Shell Working Group J. Galbraith Internet-Draft VanDyke Software Expires:
April 16,June 18, 2003 T. Ylonen S. Lehtinen SSH Communications Security Corp October 16,December 18, 2002 SSH File Transfer Protocol draft-ietf-secsh-filexfer-03.txtdraft-ietf-secsh-filexfer-04.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. 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 16,June 18, 2003. Copyright Notice Copyright (C) The Internet Society (2002). 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3 2. Use with the SSH Connection Protocol . . . . . . . . . . . 4 3. General Packet Format . . . . . . . . . . . . . . . . . . 5 3.1 The use of stderr in the server . . . . . . . . . . . . . 6 4. Protocol Initialization . . . . . . . . . . . . . . . . . 78 4.1 Client Initialization . . . . . . . . . . . . . . . . . . 78 4.2 Server Initialization . . . . . . . . . . . . . . . . . . 78 4.3 Determining Server Newline Convention . . . . . . . . . . 89 5. File Attributes . . . . . . . . . . . . . . . . . . . . . 910 5.1 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 910 5.2 Type . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 5.3 Size . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011 5.4 Owner and Group . . . . . . . . . . . . . . . . . . . . . 1011 5.5 Permissions . . . . . . . . . . . . . . . . . . . . . . . 1112 5.6 Times . . . . . . . . . . . . . . . . . . . . . . . . . . 1112 5.7 ACL . . . . . . . . . . . . . . . . . . . . . . . . . . . 1112 5.8 Extended attributes . . . . . . . . . . . . . . . . . . . 1214 6. Requests From the Client to the Server . . . . . . . . . . 1315 6.1 Request Synchronization and Reordering . . . . . . . . . . 1315 6.2 File Names . . . . . . . . . . . . . . . . . . . . . . . . 1416 6.3 Opening, Creating, and Closing Files . . . . . . . . . . . 1416 6.4 Reading and Writing . . . . . . . . . . . . . . . . . . . 1719 6.5 Removing and Renaming Files . . . . . . . . . . . . . . . 1820 6.6 Creating and Deleting Directories . . . . . . . . . . . . 1921 6.7 Scanning Directories . . . . . . . . . . . . . . . . . . . 1921 6.8 Retrieving File Attributes . . . . . . . . . . . . . . . . 2022 6.9 Setting File Attributes . . . . . . . . . . . . . . . . . 2123 6.10 Dealing with Symbolic links . . . . . . . . . . . . . . . 2224 6.11 Canonicalizing the Server-Side Path Name . . . . . . . . . 2325 6.11.1 Best practice for dealing with paths . . . . . . . . . . . 2325 7. Responses from the Server to the Client . . . . . . . . . 2426 8. Vendor-Specific Extensions . . . . . . . . . . . . . . . . 2830 9. Security Considerations . . . . . . . . . . . . . . . . . 2931 10. Changes from previous protocol versions . . . . . . . . . 3032 10.1 Changes between versions 4 and 3 . . . . . . . . . . . . . 3032 10.2 Changes between versions 3 and 2 . . . . . . . . . . . . . 3133 10.3 Changes between versions 2 and 1 . . . . . . . . . . . . . 3133 10.4 Changes between versions 1 and 0 . . . . . . . . . . . . . 3133 11. Trademark Issues . . . . . . . . . . . . . . . . . . . . . 3234 References . . . . . . . . . . . . . . . . . . . . . . . . 3335 Authors' Addresses . . . . . . . . . . . . . . . . . . . . 33 Full35 Intellectual Property and Copyright Statement . . . . . . .Statements . . . . . . . . . . 3537 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 . 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.  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  and transfer of management information in VPN applications. 2. Use with the SSH Connection Protocol When used with the SSH2 Protocol suite, this protocol is intended to be used from the SSH Connection Protocol  as a subsystem, as described in section ``Starting a Shell or a Command''. The subsystem name used with this protocol is "sftp". 3. General Packet Format All packets transmitted over the secure connection are of the following format: uint32 length byte type byte[length - 1] data payload That is, they are just data preceded by 32-bit length and 8-bit type fields. The `length' is the length of the data area, and does not include the `length' field itself. The format and interpretation of the data area depends on the packet type. All packet descriptions below only specify the packet type and the data that goes into the data field. Thus, they should be prefixed by the `length' and `type' fields. The maximum size of a packet is in 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. 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 RESERVED_FOR_EXTENSIONS 210-255 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. 3.1 The use of stderr in the server Packets are sent and received on stdout and stdin. Data sent on stderr by the server SHOULD be considered debug or supplemental error information, and MAY be displayed to the user. For example, during initialization, there is no client request active, so errors or warning information cannot be sent to the client as part of the SFTP protocol at this early stage. However, the errors or warnings MAY be sent as stderr text. 4. 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 particular version of the protocol. The version number of the protocol specified in this document is 4. The version number should be incremented for each incompatible revision of this protocol. 4.1 Client Initialization The SSH_FXP_INIT packet (from client to server) has the following data: uint32 version Version 3 of this protocol allowed clients to include extensions in the SSH_FXP_INIT packet; however, this can cause interoperability problems with version 1 and version 2 servers because the client must send this packet before knowing the servers version. In this version of the protocol, clients MUST use the SSH_FXP_EXTENDED packet to send extensions to the server after version exchange has completed. Clients MUST NOT include extensions in the version packet. This will prevent interoperability problems with older servers 4.2 Server Initialization The SSH_FXP_VERSION packet (from server to client) has the following data: uint32 version <extension data> 'version' is the lower of the protocol version supported by the server and the version number received from the client. The extension data 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. 4.3 Determining Server Newline Convention In order to correctly process text files in a cross platform compatible way, the newline convention must be converted from that of the server to that of the client, or, during an upload, from that of the client to that of the server. Versions 3 and prior of this protocol made no provisions for processing text files. Many clients implemented some sort of conversion algorithm, but without either a 'canonical' on the wire format or knowledge of the servers newline convention, correct conversion was not always possible. Starting with Version 4, the SSH_FXF_TEXT file open flag (Section 6.3) makes it possible to request that the server translate a file to a 'canonical' on the wire format. This format uses \r\n as the line separator. Servers for systems using multiple newline characters (for example, Mac OS X or VMS) or systems using counted records, MUST translate to 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 "\r" or "\n" or "\r\n") 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 servers newline separator. In particular, if the newline sequence being used is the canonical "\r\n" sequence, a lone \r or a lone \n SHOULD be written through without change. 5. File Attributes A new compound data type 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. 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 byte type always present uint64 size present only if flag SSH_FILEXFER_ATTR_SIZESIZE string owner present only if flag SSH_FILEXFER_ATTR_OWNERGROUPOWNERGROUP string group present only if flag SSH_FILEXFER_ATTR_OWNERGROUPOWNERGROUP uint32 permissions present only if flag SSH_FILEXFER_ATTR_PERMISSIONS uint32PERMISSIONS uint64 atime present only if flag SSH_FILEXFER_ATTR_ACCESSTIMEACCESSTIME uint32 atime_nseconds present only if flag SUBSECOND_TIMES uint64 createtime present only if flag SSH_FILEXFER_ATTR_CREATETIMECREATETIME uint32 createtime_nseconds present only if flag SUBSECOND_TIMES uint64 mtime present only if flag SSH_FILEXFER_ATTR_MODIFYTIMEMODIFYTIME uint32 mtime_nseconds present only if flag SUBSECOND_TIMES string acl present only if flag SSH_FILEXFER_ATTR_ACLACL uint32 extended_count present only if flag SSH_FILEXFER_ATTR_EXTENDEDEXTENDED string extended_type string extended_data ... more extended data (extended_type - extended_data pairs), so that number of pairs equals extended_count 5.1 Flags 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 flags bits are defined to have the following values: #define SSH_FILEXFER_ATTR_SIZE 0x00000001 #define SSH_FILEXFER_ATTR_PERMISSIONS 0x000000040x00000040 #define SSH_FILEXFER_ATTR_ACCESSTIME 0x00000008 #define SSH_FILEXFER_ATTR_CREATETIME 0x00000010 #define SSH_FILEXFER_ATTR_MODIFYTIME 0x00000020 #define SSH_FILEXFER_ATTR_ACL 0x00000040 #define SSH_FILEXFER_ATTR_OWNERGROUP 0x00000080 #define SSH_FILEXFER_ATTR_SUBSECOND_TIMES 0x00000100 #define SSH_FILEXFER_ATTR_EXTENDED 0x80000000 In previous versions of this protocol flags value 0x00000002 was SSH_FILEXFER_ATTR_UIDGID. This value is now unused, and OWNERGROUP was given a new value in order to ease implementation burden. 0x00000002 MUST NOT appear in the mask. Some future version of this protocol may reuse flag 0x00000002. 5.2 Type The type field is always present. The following types are defined: #define SSH_FILEXFER_TYPE_REGULAR 1 #define SSH_FILEXFER_TYPE_DIRECTORY 2 #define SSH_FILEXFER_TYPE_SYMLINK 3 #define SSH_FILEXFER_TYPE_SPECIAL 4 #define SSH_FILEXFER_TYPE_UNKNOWN 5 On a POSIX system, these values would be derived from the permission field. 5.3 Size The `size' field specifies the size of the file on disk, in bytes. If it is present during file creation, it should be considered a hint as to the files eventual size. Files opened with the SSH_FXF_TEXT flag may have a size that is greater or less than the value of the size field. 5.4 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.  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 with the attribute value. Even though the attribute value can not be translated, it may still be useful. In the case of a client, the attribute string may be used for local display of ownership. 5.5 Permissions The `permissions' field contains a bit mask of file permissions as defined by POSIX . 5.6 Times The 'atime', 'createtime', and 'mtime' contain the access, creation, and modification times of the files, respectively. They 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. 5.7 ACL The 'ACL' field contains an ACL similar to that defined in section 5.9 of NFS version 4 Protocol . 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 : const ACE4_ACCESS_ALLOWED_ACE_TYPE = 0x00000000; const ACE4_ACCESS_DENIED_ACE_TYPE = 0x00000001; const ACE4_SYSTEM_AUDIT_ACE_TYPE = 0x00000002; const ACE4_SYSTEM_ALARM_ACE_TYPE = 0x00000003; ace-flag is a combination of the following flag values. See NFS Version 4 Protocol  section 5.9.2: const ACE4_FILE_INHERIT_ACE = 0x00000001; const ACE4_DIRECTORY_INHERIT_ACE = 0x00000002; const ACE4_NO_PROPAGATE_INHERIT_ACE = 0x00000004; const ACE4_INHERIT_ONLY_ACE = 0x00000008; const ACE4_SUCCESSFUL_ACCESS_ACE_FLAG = 0x00000010; const ACE4_FAILED_ACCESS_ACE_FLAG = 0x00000020; const ACE4_IDENTIFIER_GROUP = 0x00000040; ace-mask is any combination of the following flags (taken from NFS Version 4 Protocol  section 5.9.3: const ACE4_READ_DATA = 0x00000001; const ACE4_LIST_DIRECTORY = 0x00000001; const ACE4_WRITE_DATA = 0x00000002; const ACE4_ADD_FILE = 0x00000002; const ACE4_APPEND_DATA = 0x00000004; const ACE4_ADD_SUBDIRECTORY = 0x00000004; const ACE4_READ_NAMED_ATTRS = 0x00000008; const ACE4_WRITE_NAMED_ATTRS = 0x00000010; const ACE4_EXECUTE = 0x00000020; const ACE4_DELETE_CHILD = 0x00000040; const ACE4_READ_ATTRIBUTES = 0x00000080; const ACE4_WRITE_ATTRIBUTES = 0x00000100; const ACE4_DELETE = 0x00010000; const ACE4_READ_ACL = 0x00020000; const ACE4_WRITE_ACL = 0x00040000; const ACE4_WRITE_OWNER = 0x00080000; const ACE4_SYNCHRONIZE = 0x00100000; who is a UTF-8 string of the form described in 'Owner and Group' (Section 5.4)form described in 'Owner and Group' (Section 5.4) Also, as per '5.9.4 ACE who'  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 "@" and should appear in the form "xxxx@" (note: no domain name after the "@"). For example: ANONYMOUS@. 5.8 Extended attributes The SSH_FILEXFER_ATTR_EXTENDED flag provides a general extension mechanism for vendor-specific extensions. If the flag is specified, then the `extended_count' field is present. It specifies the number of extended_type-extended_data pairs that follow. Each of these pairs specifies an extended attribute. For each of the attributes, the extended_type field should be a string of the format "name@domain", where "domain" is a valid, registered domain name and "name" identifies the method. The IETF may later standardize certain names that deviate from this format (e.g., that do not contain the "@" sign). The interpretation of `extended_data' depends on the type. Implementations SHOULD ignore extended data fields that they do not understand. Additional fields can be added to the attributes by either defining additional bits to the flags field to indicate their presence, or by defining extended attributes for them. The extended attributes mechanism is recommended for most purposes; additional flags bits should only be defined by an IETF standards action that also increments the protocol version number. The use of such new fields MUST be negotiated by the version number in the protocol exchange. It is a protocol error if a packet with unsupported protocol bits is received. 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 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. 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. In order to comply with IETF Policy on Character Sets and Languages , all filenames are to be encoded in UTF-8. 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. 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 [UTF-8] uint32 pflags ATTRS attrs The `id' field is the request identifier as for all requests. The `filename' field specifies the file name. See Section ``File Names'' for more information. The `pflags' field is a bitmask. The following bits have been defined. #define SSH_FXF_READ 0x00000001 #define SSH_FXF_WRITE 0x00000002 #define SSH_FXF_APPEND 0x00000004 #define SSH_FXF_CREAT 0x00000008 #define SSH_FXF_TRUNC 0x00000010 #define SSH_FXF_EXCL 0x00000020 #define SSH_FXF_TEXT 0x00000040 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. The offset parameter to write will be ignored. 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. 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 both the read and write function are ignored. Servers MUST correctly process multiple parallel reads and writes correctly in this mode. Naturally, it is permissible for them to do this by serializing the requests. It would not be possible for a client to reliably detect a server that does not implement parallel writes in time to prevent damage. Clients SHOULD use the SSH_FXF_APPEND flag to append data to a text file rather then using write with a calculated offset. To support seeks on text file 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. 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. The response to this message will be either SSH_FXP_HANDLE (if the operation is successful) or SSH_FXP_STATUS (if the operation fails). A file is closed by using the SSH_FXP_CLOSE request. Its data field has the following format: uint32 id string handle where `id' is the request identifier, and `handle' is a handle 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 thefollowing format:message: byte SSH_FXP_READ 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 normally guaranteed that this will read the specified number of bytes, or up to end of file. However, if the read length is very long, the server may truncate it if it doesn't support packets of that length. See General Packet Format (Section 3). 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 thefollowing format:message: byte SSH_FXP_WRITE uint32 id string handle uint64 offset string data where `id' is a request identifier, `handle' is a file handle returned by SSH_FXP_OPEN, `offset' is the offset (in bytes) from the beginning of the file where to start writing, and `data' is the data to be written. The write will extend the file if writing beyond the end of the file. It is legal to write way beyond the end of the file; the semantics are to write zeroes from the end of the file to the specified offset and then the data. On most operating systems, such writes do not allocate disk space but instead leave "holes" in the file. The server responds to a write request with a SSH_FXP_STATUS message. 6.5 Removing and Renaming Files Files can be removed using the SSH_FXP_REMOVE message. It has the following format: uint32 id string filename [UTF-8] 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 [UTF-8] string newpath [UTF-8] 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. 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 [UTF-8] ATTRS attrs where `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: uint32 id string path [UTF-8] where `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. Errors may be returned from this operation for various reasons, including, but not limited to, the path does not exist, the path does not refer to a directory object, the directory is not empty, or the user has insufficient access or permission to perform the requested operation. 6.7 Scanning Directories The files in a directory can be listed using the SSH_FXP_OPENDIR and SSH_FXP_READDIR requests. Each SSH_FXP_READDIR request returns one or more file names with full file attributes for each file. The client should call SSH_FXP_READDIR repeatedly until it has found the file it is looking for or until the server responds with a SSH_FXP_STATUS message indicating an error (normally SSH_FX_EOF if there are no more files in the directory). The client should then close the handle using the SSH_FXP_CLOSE request. The SSH_FXP_OPENDIR opens a directory for reading. It has the following format: uint32 id string path [UTF-8] 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. 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. 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 [UTF-8] uint32 flags where `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). Its format is as follows: uint32 id string handle uint32 flags 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 [UTF-8] ATTRS attrs where `id' is the request identifier, `path' specifies the file system object (e.g. file or directory) whose attributes are to be modified, and `attrs' specifies the modifications to be made to its attributes. Attributes are discussed in more detail in Section ``File Attributes''. An error will be returned if the specified file system object does not exist or the user does not have sufficient rights to modify the specified attributes. The server responds to this request with a SSH_FXP_STATUS message. The SSH_FXP_FSETSTAT request modifies the attributes of a file which is already open. It has the following format: uint32 id string handle ATTRS attrs where `id' is the request identifier, `handle' (MUST be returned by SSH_FXP_OPEN) identifies the file whose attributes are to be modified, and `attrs' specifies the modifications to be made to its attributes. Attributes are discussed in more detail in Section ``File Attributes''. The server will respond to this request with SSH_FXP_STATUS. 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 [UTF-8] 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 [UTF-8] string targetpath [UTF-8] 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 [UTF-8] 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 the name in canonical form and a dummy attributes value. If an error occurs, the server may also respond with SSH_FXP_STATUS. 6.11.1 Best practice for dealing with paths 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 use 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. 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 "c:/x/y/z/..". As a second example, if the client wishes to open the file "x.txt" in the current directory, and server has returned "dka100:/x/y/z" as the canonical path of the directory, the client SHOULD open "dka100:/x/y/ z/x.txt" 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. 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 #define SSH_FX_INVALID_HANDLE 9 #define SSH_FX_NO_SUCH_PATH 10 #define SSH_FX_FILE_ALREADY_EXISTS 11 #define SSH_FX_WRITE_PROTECT 12 #define SSH_FX_NO_MEDIA 13 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 does not exist. 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). 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). 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 can not be completed because there is no media available in the drive. 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 [UTF-8] 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), and `attrs' is the attributes of the file as described in Section ``File Attributes''. The SSH_FXP_ATTRS response has the following format: uint32 id ATTRS attrs where `id' is the request identifier, and `attrs' is the returned file attributes as described in Section ``File Attributes''. 8. Vendor-Specific Extensions The SSH_FXP_EXTENDED request provides a generic extension mechanism for adding vendor-specific commands. The request has the following format: uint32 id string extended-request ... any request-specific data ... where `id' is the request identifier, and `extended-request' is a string 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 ... There is a range of packet types reserved for use by extensions. In order to avoid collision, extensions that turn on the use of 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 additional 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. 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 . Care must be taken in the server implementation to check the validity of received file handle strings. The server should not rely on them directly; it MUST check the validity of each handle before relying on it. 10. Changes from previous protocol versions The 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 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 resultion. 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 it's 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. o Change the way version negotiate works slightly. Previously, if the client version were higher than the server version, the server was supposed to 'echo back' the clients version. The server now sends it's own version and the lower of the two is considered to be the one in use.10.2 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'. 10.3 Changes between versions 2 and 1 o The SSH_FXP_RENAME message was added. 10.4 Changes between versions 1 and 0 o Implementation changes, no actual protocol changes. 11. Trademark Issues "ssh" is a registered trademark of SSH Communications Security Corp in the United States and/or other countries. References  Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January 1999.  Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame, C., Eisler, M. and D. Noveck, "NFS version 4 Protocol", RFC 3010, December 2000.  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.  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Protocol Architecture", draft-ietf-secsh- architecture-13draft-ietf-secsh-architecture-13 (work in progress), September 2002.  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Protocol Transport Protocol", draft-ietf-secsh- transport-15draft-ietf-secsh-transport-15 (work in progress), September 2002.  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Connection Protocol", draft-ietf-secsh-connect-16 (work in progress), September 2002.  Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S. Lehtinen, "SSH Authentication Protocol", draft-ietf-secsh- userauth-16draft-ietf-secsh-userauth-16 (work in progress), September 2002. Authors' Addresses Joseph Galbraith VanDyke Software 4848 Tramway Ridge Blvd Suite 101 Albuquerque, NM 87111 US Phone: +1 505 332 5700 EMail: firstname.lastname@example.org Tatu Ylonen SSH Communications Security Corp Fredrikinkatu 42 HELSINKI FIN-00100 Finland EMail: email@example.com Sami Lehtinen SSH Communications Security Corp Fredrikinkatu 42 HELSINKI FIN-00100 Finland EMail: firstname.lastname@example.org Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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