| Communication protocol | |
| Abbreviation | SFTP |
|---|---|
| Purpose | File transfer |
| Developer(s) | IETF SECSH working group |
| Introduction | 1997; 28 years ago (1997) |
| Based on | Secure Shell (SSH) |
| OSI layer | Application layer (7) |
| Port(s) | 22/TCP |
Incomputing, theSSH File Transfer Protocol, also known asSecure File Transfer Protocol (SFTP), is anetwork protocol that providesfile access,file transfer, andfile management over any reliabledata stream. It was designed by theInternet Engineering Task Force (IETF) as an extension of theSecure Shell protocol (SSH) version 2.0 to provide secure file transfer capabilities, and is seen as a replacement ofFile Transfer Protocol (FTP) due to superior security.[1] TheIETF 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 overTransport Layer Security (TLS) and transfer of management information inVPN applications.
This protocol assumes that it is run over asecure 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.
The official URI scheme issftp.[1]
Compared to theSCP protocol, which only allows file transfers, the SFTP protocol allows for a range of operations on remote files which make it more like a remotefile system protocol. An SFTPclient's extra capabilities include resuming interrupted transfers, directory listings, and remote file removal.[2] There is also support for all UNIX file types, including symbolic links.[3]
SFTP attempts to be more platform-independent than SCP; with SCP, for instance, the expansion ofwildcards specified by the client is up to the server, whereas SFTP's design avoids this problem. While SCP is most frequently implemented onUnix 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 notFTP run overSSH, but rather a new protocol designed from the ground up by theIETF SECSHworking group. It is sometimes confused withSimple File Transfer Protocol.[4]
The protocol itself does not provide authentication and security; it expects the underlying protocol to secure this. SFTP is most often used as subsystem ofSSH 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 commonFTP protocol.
The Internet Engineering Task Force (IETF) working group "Secsh" that was responsible for the development of theSecure 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.[5] 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 includefile access andfile management. Eventually, development stalled as some committee members began to view SFTP as afile system protocol, not just afile access orfile transfer protocol, which places it beyond the purview of the working group.[6] After a seven-year hiatus, in 2013 an attempt was made to restart work on SFTP using the version 3 draft as the baseline.[7]
Prior to the IETF's involvement, SFTP was a proprietary protocol ofSSH Communications Security, designed by Tatu Ylönen with assistance from Sami Lehtinen in 1997.[8] Differences between versions 0–2 and version 3 are enumerated upon insection 10 of draft-ietf-secsh-filexfer-02.
At the outset of the IETF Secure Shell File Transfer project, the Secsh group stated that its objective of SSH File Transfer Protocol was to provide a secure file transfer functionality over any reliable data stream, and to be the standard file transfer protocol for use with the SSH-2 protocol.
Drafts 00–02 of the IETF Internet Draft define successive revisions of version 3 of the SFTP protocol.
Drafts 03–04 of the IETF Internet Draft define version 4 of the protocol.
Draft 05 of the IETF Internet Draft defines version 5 of the protocol.
Drafts 06–13 of the IETF Internet Draft define successive revisions of version 6 of the protocol.
The SFTP protocol supports a generic way of indicating extended commands, along with a method of including them in version negotiation. An IANA registry is requested, but since the protocol never became an official standard, no such registry has been created.[3]
The termSFTP can also refer toSecure file transfer program, acommand-line program that implements theclient part of this protocol. As an example, the sftp program supplied withOpenSSH implements this.[10]
Some implementations of thescpprogram support both the SFTP and SCP protocols to perform file transfers, depending on what the server supports. The scp program supplied with OpenSSH 9.0 and higher defaults to using SFTP.[11]
SomeFTP server implementations implement the SFTP protocol; however, outside of dedicated file servers, SFTP protocol support is usually provided by anSSH server implementation, as it shares the default port of 22 with other SSH services. SFTP implementations may include an SSH protocol implementation to leverage integration of SSH connection details with preexisting FTP server access controls, where an alternative SSH server is tolerable or where alternative ports may be used. An SSH-2 server which supports subsystems may be leveraged to keep a uniform SSH implementation while enhancing access controls with third party software, at the cost of fine-grained integration with connection details, and SSH-1 compatibility.
It is difficult to control SFTP transfers on security devices at the network perimeter. There are standard tools for loggingFTP transactions, like TISgdev 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 such a tool are Shell Control Box from Balabit[12] and CryptoAuditor fromSSH Communications Security[13] (the original developer of the Secure Shell protocol) which provides functions such as SFTP transaction logging and logging of the actual data transmitted on the wire.