In computing, the SSH File Transfer Protocol (also Secure File Transfer Protocol, or SFTP) is a network protocol that provides file access, file transfer, and file management functionalities over any reliable data stream. It was designed by the Internet Engineering Task Force (IETF) as an extension of the Secure Shell protocol (SSH) version 2.0 to provide secure file transfer capability, but is also intended to be usable with other protocols. The IETF Internet Draft states that even though this protocol is described in the context of the SSH-2 protocol, it could be used in a number of different applications, such as secure file transfer over Transport Layer Security (TLS) and transfer of management information in VPN applications.
This protocol assumes that it is run over a secure channel, such as SSH, that the server has already authenticated the client, and that the identity of the client user is available to the protocol.
Capabilities
Compared to the SCP protocol, which only allows file transfers, the SFTP protocol allows for a range of operations on remote files – it is more like a remote file system protocol. An SFTP client's extra capabilities compared to an SCP client include resuming interrupted transfers, directory listings, and remote file removal.
SFTP attempts to be more platform-independent than SCP; for instance, with SCP, the expansion of wildcards specified by the client is up to the server, whereas SFTP's design avoids this problem. While SCP is most frequently implemented on Unix platforms, SFTP servers are commonly available on most platforms.
SFTP is not FTP run over SSH, but rather a new protocol designed from the ground up by the IETF SECSH working group. It is sometimes confused with Simple File Transfer Protocol.
The protocol itself does not provide authentication and security; it expects the underlying protocol to secure this. SFTP is most often used as subsystem of SSH protocol version 2 implementations, having been designed by the same working group. However, it is possible to run it over SSH-1 (and some implementations support this) or other data streams. Running an SFTP server over SSH-1 is not platform independent as SSH-1 does not support the concept of subsystems. An SFTP client willing to connect to an SSH-1 server needs to know the path to the SFTP server binary on the server side.
For uploads, the transferred files may be associated with their basic attributes, such as timestamps. This is an advantage over the common FTP protocol, which does not have provision for uploads to include the original date/timestamp attribute without help.
History and development
The Internet Engineering Task Force (IETF) working group "Secsh" that was responsible for the development of the Secure Shell version 2 protocol (RFC 4251) also attempted to draft an extension of that standard for secure file transfer functionality. Internet Drafts were created that successively revised the protocol into new versions. The software industry began to implement various versions of the protocol before the drafts were standardized. As development work progressed, the scope of the Secsh File Transfer project expanded to include file access and file management. Eventually, development stalled as some committee members began to view SFTP as a file system protocol, not just a file access or file transfer protocol, which places it beyond the purview of the working group. After a seven-year hiatus, in 2013 an attempt was made to restart work on SFTP using the version 3 draft as the baseline.
Software
SFTP client
The term SFTP can also refer to Secure file transfer program, a command-line program that implements the client part of this protocol. As an example, the sftp program supplied with OpenSSH implements this.
Some implementations of the scp program support both the SFTP and SCP protocols to perform file transfers, depending on what the server supports.
SFTP server
There are numerous SFTP server implementations both for UNIX, Windows and z/OS. The most widely known is perhaps OpenSSH, but there are also proprietary implementations. Typically the port used is 22. SFTP file transfer protocol is part of SSH protocol suite.
SFTP proxy
It is difficult to control SFTP transfers on security devices at the network perimeter. There are standard tools for logging FTP transactions, like TIS fwtk or SUSE FTP proxy, but SFTP is encrypted, rendering traditional proxies ineffective for controlling SFTP traffic.
There are some tools that implement man-in-the-middle for SSH which also feature SFTP control. Examples of these tools include Shell Control Box from Balabit and FileGate SFP from Presaris. These provide functions such as SFTP transaction logging and logging of the actual data transmitted on the wire.
