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File Transfer Protocol (FTP) is a standard network protocol used to exchange and manipulate files over a TCP/IP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications. FTP is used with user-based password authentication or with anonymous user access.
Applications were originally interactive command-line tools with a standardized command syntax, but graphical user interfaces have been developed for all desktop operating systems in use today.
The original specification of the File Transfer Protocol was published as RFC 114 on 16 April 1971 and later replaced by RFC 765 (June 1980) and RFC 959 (October 1985), the current specification. Several proposed standards amend RFC 959, for example RFC 2228 (June 1997) proposes security extensions and RFC 2428 (September 1998) adds support for IPv6 and defines a new type of passive mode.
As outlined by its RFC, FTP is used to:
A client makes a connection to the server using TCP port 21. This connection, called the control connection, remains open for the duration of the session, with a second connection on port 20 opened as required to transfer file data. Commands are sent by the client over the control connection in ASCII and terminated by a carriage return and line feed. For example "RETR filename" would transfer the specified file from the server to the client.
The server responds on the control connection with three digit status codes in ASCII with an optional text message, for example "200" (or "200 OK.") means that the last command was successful. A file transfer in progress over the data connection can be aborted using an interrupt message sent over the control connection.
FTP can be run in active mode or passive mode, which control how the second connection is opened. In active mode the client sends the server the IP address port number that the client will use for the data connection, and the server opens the connection. Passive mode was devised for use where the client is behind a firewall and unable to accept incoming TCP connections. The server sends the client an IP address and port number and the client opens the connection to the server. Both modes were updated in September 1998 to add support for IPv6 and made some other changes to passive mode, making it extended passive mode.
While transferring data over the network, four data representations can be used, of which only the first two are in common use:
For text files, different format control and record structure options are provided. These features were designed to facilitate files containing Telnet or ASA formatting and are rarely needed any longer.
Data transfer can be done in any of three modes.
Of the above methods, the default of "stream" is invariably used now.
The original FTP specification is an inherently unsecure method of transferring files because there is no method specified for transferring data in an encrypted fashion. This means that under most network configurations, user names, passwords, FTP commands and transferred files can be captured by anyone on the same network using a packet sniffer. This is a problem common to many Internet protocol specifications written prior to the creation of SSL, such as HTTP, SMTP and Telnet. The common solution to this problem is to use either SFTP (SSH File Transfer Protocol), or FTPS (FTP over SSL), which adds SSL or TLS encryption to FTP as specified in RFC 4217.
A host that provides an FTP service may additionally provide anonymous FTP access. Users typically login to the service with an 'anonymous' account when prompted for user name. Although users are commonly asked to send their email address in lieu of a password, little to no verification is actually performed on the supplied data, an example for this is the FTP server of the National Center for Biotechnology Information
As modern FTP clients typically hide the anonymous login process from the user, the ftp client will supply dummy data as the password (since the user's email address may not be known to the application).
Where FTP access is restricted, a remote FTP (or FTPmail) service can be used to circumvent the problem. An e-mail containing the FTP commands to be performed is sent to a remote FTP server, which is a mail server that parses the incoming e-mail, executes the FTP commands, and sends back an e-mail with any downloaded files as an attachment. Obviously this is less flexible than an ftp client, as it is not possible to view directories interactively or to modify commands, and there can also be problems with large file attachments in the response not getting through mail servers. As most internet users these days have ready access to FTP, this procedure is no longer in everyday use.
Most recent web browsers and file managers can connect to FTP servers, although they may lack the support for protocol extensions such as FTPS. This allows manipulation of remote files over FTP through an interface similar to that used for local files. This is done via an FTP URL, which takes the form
ftp(s)://<ftpserveraddress> (e.g., ftp://ftp.gimp.org/)
A password can optionally be given in the URL, e.g.:
Most web-browsers require the use of passive mode FTP, which not all FTP servers are capable of handling. Some browsers, such as Mozilla Firefox and Netscape, allow only the downloading of files, but offer no way to upload files to the server.
The representation of the IP addresses and port numbers in the PORT command and PASV reply poses a challenge to FTP in traversing Network address translators (NAT). The NAT device must alter these values, so that they contain the IP address of the NAT-ed client, and a port chosen by the NAT device for the data connection. The new address and port will probably differ in length in their decimal representation from the original address and port. Such translation is not usually performed in most NAT devices, but special application layer gateways exist for this purpose.
FTP over SSH (not SFTP) refers to the practice of tunneling a normal FTP session over an SSH connection.
Because FTP uses multiple TCP connections (unusual for a TCP/IP protocol that is still in use), it is particularly difficult to tunnel over SSH. With many SSH clients, attempting to set up a tunnel for the control channel (the initial client-to-server connection on port 21) will protect only that channel; when data is transferred, the FTP software at either end will set up new TCP connections (data channels) which will bypass the SSH connection, and thus have no confidentiality, integrity protection, etc.
Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, and monitor and rewrite FTP control channel messages and autonomously open new forwardings for FTP data channels. Version 3 of SSH Communications Security's software suite, the GPL licensed FONC, and Co:Z FTPSSH Proxy are three software packages that support this mode.
FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as with SSL/TLS (FTPS). Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol.