SSH File Transfer Protocol

In computing, the SSH File Transfer Protocol, also known as Secure File Transfer Protocol (SFTP), is a network protocol that provides file access, file transfer, and file management 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 capabilities, and is seen as a replacement of File Transfer Protocol (FTP) due to superior security. 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.

Compared to the SCP protocol, which only allows file transfers, the SFTP protocol allows for a range of operations on remote files which make it more like a remote file system protocol. An SFTP client's extra capabilities include resuming interrupted transfers, directory listings, and remote file removal. There is also support for all UNIX file types, including symbolic links.

SFTP attempts to be more platform-independent than SCP; with SCP, for instance, 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. In SFTP, the file transfer can be easily terminated without terminating a session like other mechanisms do.

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. It is possible, however, to run it over SSH-1 (and some implementations support this), or other data streams. However, running an SFTP server over SSH-1 is not platform-independent, as SSH-1 does not support the concept of subsystems. An SFTP client connecting to an SSH-1 server must be aware of the path to the SFTP server binary on the server side.

Uploaded files may be associated with their basic attributes, such as time stamps. This is an advantage over the common FTP protocol.

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.