File Transfer Protocol


The File Transfer Protocol is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data connections between the client and the server. FTP users may authenticate themselves with a plain-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that protects the username and password, and encrypts the content, FTP is often [|secured] with SSL/TLS or replaced with SSH File Transfer Protocol.
The first FTP client applications were command-line programs developed before operating systems had graphical user interfaces, and are still shipped with most Windows, Unix, and Linux operating systems. Many dedicated FTP clients and automation utilities have since been developed for desktops, servers, mobile devices, and hardware, and FTP has been incorporated into productivity applications such as HTML editors and file managers.
An FTP client used to be commonly integrated in web browsers, where file servers are browsed with the URI prefix "ftp://". In 2021, FTP support was dropped by Google Chrome and Firefox, two major web browser vendors, due to it being superseded by the more secure SFTP and FTPS; although neither of them have implemented the newer protocols.

History of FTP servers

The original specification for the File Transfer Protocol was written by Abhay Bhushan and published as on 16 April 1971. Until 1980, FTP ran on NCP, the predecessor of TCP/IP. The protocol was later replaced by a TCP/IP version, and , the current specification. Several proposed standards amend, for example enables Firewall-Friendly FTP, proposes security extensions, adds support for IPv6 and defines a new type of passive mode.

Protocol overview

Communication and data transfer

FTP may run in active or passive mode, which determines how the data connection is established.
  • In active mode, the client starts listening for incoming data connections from the server on port M. It sends the FTP command PORT M to inform the server on which port it is listening. The server then initiates a data channel to the client from its port 20, the FTP server data port.
  • In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used. In this mode, the client uses the control connection to send a PASV command to the server and then receives a server IP address and server port number from the server, which the client then uses to open a data connection from an arbitrary client port to the server IP address and server port number received.
Both modes were updated in September 1998 to support IPv6. Further changes were introduced to the passive mode at that time, updating it to extended passive mode.
The server responds over the control connection with three-digit status codes in ASCII with an optional text message. For example, "200" means that the last command was successful. The numbers represent the code for the response and the optional text represents a human-readable explanation or request. An ongoing transfer of file data over the data connection can be aborted using an interrupt message sent over the control connection.
FTP needs two ports because it was originally designed to operate on top of Network Control Protocol, which was a simplex protocol that utilized two port addresses, establishing two connections, for two-way communications. An odd and an even port were reserved for each application layer application or protocol. The standardization of TCP and UDP reduced the need for the use of two simplex ports for each application down to one duplex port, but the FTP protocol was never altered to only use one port, and continued using two for backwards compatibility.

NAT and firewall traversal

FTP normally transfers data by having the server connect back to the client, after the PORT command is sent by the client. This is problematic for both NATs and firewalls, which do not allow connections from the Internet towards internal hosts. For NATs, an additional complication is that the representation of the IP addresses and port number in the PORT command refer to the internal host's IP address and port, rather than the public IP address and port of the NAT.
There are two approaches to solve this problem. One is that the FTP client and FTP server use the PASV command, which causes the data connection to be established from the FTP client to the server. This is widely used by modern FTP clients. Another approach is for the NAT to alter the values of the PORT command, using an application-level gateway for this purpose.

Data types

While transferring data over the network, five data types are defined:
  • ASCII : Used for text. Data is converted, if needed, from the sending host's character representation to "8-bit ASCII" before transmission, and to the receiving host's character representation, including newlines. As a consequence, this mode is inappropriate for files that contain data other than ASCII.
  • Image : The sending machine sends each file byte by byte, and the recipient stores the bytestream as it receives it..
  • EBCDIC : Used for plain text between hosts using the EBCDIC character set.
  • Local : Designed to support file transfer between machines which do not use 8-bit bytes, e.g. 36-bit systems such as DEC PDP-10s. For example, "TYPE L 9" would be used to transfer data in 9-bit bytes, or "TYPE L 36" to transfer 36-bit words. Most contemporary FTP clients/servers only support L 8, which is equivalent to I.
  • Unicode text files using UTF-8 : defined in an expired Internet Draft which never became an RFC, though it has been implemented by several FTP clients/servers.
Note these data types are commonly called "modes", although ambiguously that word is also used to refer to active-vs-passive communication mode, and the modes set by the FTP protocol MODE command.
For text files, three different format control options are provided, to control how the file would be printed:
These formats were mainly relevant to line printers; most contemporary FTP clients/servers only support the default format control of N.

File structures

File organization is specified using the STRU command. The following file structures are defined in section 3.1.1 of RFC959:
  • F or FILE structure. Files are viewed as an arbitrary sequence of bytes, characters or words. This is the usual file structure on Unix systems and other systems such as CP/M, MS-DOS and Microsoft Windows.
  • R or RECORD structure. Files are viewed as divided into records, which may be fixed or variable length. This file organization is common on mainframe and midrange systems, such as MVS, VM/CMS, OS/400 and VMS, which support record-oriented filesystems.
  • P or PAGE structure. Files are divided into pages, which may either contain data or metadata; each page may also have a header giving various attributes. This file structure was specifically designed for TENEX systems, and is generally not supported on other platforms. RFC1123 section 4.1.2.3 recommends that this structure not be implemented.
Most contemporary FTP clients and servers only support STRU F. STRU R is still in use in mainframe and minicomputer file transfer applications.

Data transfer modes

Data transfer can be done in any of three modes:
  • Stream mode : Data is sent as a continuous stream, relieving FTP from doing any processing. Rather, all processing is left up to TCP. No End-of-file indicator is needed, unless the data is divided into records.
  • Block mode : Designed primarily for transferring record-oriented files, although can also be used to transfer stream-oriented text files. FTP puts each record of data into several blocks and then passes it on to TCP.
  • Compressed mode : Extends MODE B with data compression using run-length encoding.
Most contemporary FTP clients and servers do not implement MODE B or MODE C; FTP clients and servers for mainframe and minicomputer operating systems are the exception to that.
Some FTP software also implements a DEFLATE-based compressed mode, sometimes called "Mode Z" after the command that enables it. This mode was described in an Internet Draft, but not standardized.
GridFTP defines additional modes, MODE E and MODE X, as extensions of MODE B.

Additional commands

More recent implementations of FTP support the Modify Fact: Modification Time command, which allows a client to adjust that file attribute remotely, enabling the preservation of that attribute when uploading files.
To retrieve a remote file timestamp, there's MDTM command. Some servers support nonstandard syntax of the MDTM command with two arguments, that works the same way as ''MFMT''

Login

FTP login uses normal username and password scheme for granting access. The username is sent to the server using the USER command, and the password is sent using the PASS command. This sequence is unencrypted "on the wire", so may be vulnerable to a network sniffing attack. If the information provided by the client is accepted by the server, the server will send a greeting to the client and the session will commence. If the server supports it, users may log in without providing login credentials, but the same server may authorize only limited access for such sessions.

Anonymous FTP

A host that provides an FTP service may provide anonymous FTP access. Users typically log into the service with an 'anonymous' account when prompted for user name. Although users are commonly asked to send their email address instead of a password, no verification is actually performed on the supplied data. Many FTP hosts whose purpose is to provide software updates will allow anonymous logins.