The first FTP client applications were interactive command-line tools, implementing standard commands and syntax. Graphical user interfaces have since been developed for many of the popular desktop operating systems in use today, including general web design programs like Microsoft Expression Web, and specialist FTP clients such as CuteFTP.
Communication and data transfer
The protocol is specified in RFC 959, which is summarized here.
The server responds over 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. The numbers represent the code for the response and the optional text represents a human-readable explanation or request (e.g. <Need account for storing file>).An ongoing transfer of file data over the data connection can be aborted using an interrupt message sent over the control connection.
Illustration of starting a passive connection using port 21
FTP may run in active or passive mode, which determines how the data connection is established. In active mode, the client creates a TCP control connection to the server and sends the server the client's IP address and an arbitrary client port number, and then waits until the server initiates the data connection over TCP to that client IP address and client port number. 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.
While transferring data over the network, four data representations can be used:
ASCII mode: used for text. Data is converted, if needed, from the sending host's character representation to "8-bit ASCII" before transmission, and (again, if necessary) to the receiving host's character representation. As a consequence, this mode is inappropriate for files that contain data other than plain text.
Image mode (commonly called Binary mode): the sending machine sends each file byte for byte, and the recipient stores the bytestream as it receives it. (Image mode support has been recommended for all implementations of FTP).
EBCDIC mode: use for plain text between hosts using the EBCDIC character set. This mode is otherwise like ASCII mode.
Local mode: Allows two computers with identical setups to send data in a proprietary format without the need to convert it to ASCII
For text files, different format control and record structure options are provided. These features were designed to facilitate files containing Telnet or ASA formatting.
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: FTP breaks the data into several blocks (block header, byte count, and data field) and then passes it on to TCP.
Compressed mode: Data is compressed using a single algorithm (usually run-length encoding).
FTP login utilizes a normal usernames 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.[ 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 server may authorize only limited access for such sessions.
A host that provides an FTP service may provide anonymous FTP access. Users typically log into the service with an 'anonymous' (lower-case and case-sensitive in some FTP servers) account when prompted for user name. Although users are commonly asked to send their email address in lieu of a password, no verification is actually performed on the supplied data. Many FTP hosts whose purpose is to provide software updates will provide anonymous logins.
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 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.
Web browser support
Most common web browsers can retrieve files hosted on FTP servers, although they may not support protocol extensions such as FTPS. When an FTP—rather than an HTTP—URL is supplied, the accessible contents on the remote server are presented in a manner that is similar to that used for other Web content. A full-featured FTP client can be run within Firefox in the form of an extension called FireFTP
FTP URL syntax is described in RFC1738, taking the form: ftp://[<user>[:<password>]@]<host>[:<port>]/<url-path> (The bracketed parts are optional.) For example:
More details on specifying a username and password may be found in the browsers' documentation, such as, for example, Firefox and Internet Explorer. By default, most web browsers use passive (PASV) mode, which more easily traverses end-user firewalls.
FTP was not designed to be a secure protocol—especially by today's standards—and has many security weaknesses. In May 1999, the authors of RFC 2577 listed a vulnerability to the following problems:
- Bounce attacks
- Spoof attacks
- Brute force attacks
- Packet capture (sniffing)
- Username protection
- Port stealing
FTP is not able to encrypt its traffic; all transmissions are in clear text, and usernames, passwords, commands and data can be easily read by anyone able to perform packet capture (sniffing) on the network. This problem is common to many of the Internet Protocol specifications (such as SMTP, Telnet, POP and IMAP) that were designed prior to the creation of encryption mechanisms such as TLS or SSL. A common solution to this problem is to use the "secure", TLS-protected versions of the insecure protocols (e.g. FTPS for FTP, TelnetS for Telnet, etc.) or a different, more secure protocol that can handle the job, such as the SFTP/SCP tools included with most implementations of the Secure Shell protocol.
There are several methods of securely transferring files that have been called "Secure FTP" at one point or another.
Explicit FTPS is an extension to the FTP standard that allows clients to request that the FTP session be encrypted. This is done by sending the "AUTH TLS" command. The server has the option of allowing or denying connections that do not request TLS. This protocol extension is defined in the proposed standard: RFC 4217. Implicit FTPS is a deprecated standard for FTP that required the use of a SSL or TLS connection. It was specified to use different ports than plain FTP.
SFTP, the "SSH File Transfer Protocol," is not related to FTP except that it also transfers files and has a similar command set for users. SFTP, or secure FTP, is a program that uses Secure Shell (SSH) to transfer files. Unlike standard FTP, it encrypts both commands and data, preventing passwords and sensitive information from being transmitted openly over the network. It is functionally similar to FTP, but because it uses a different protocol, you can't use a standard FTP client to talk to an SFTP server, nor can you connect to an FTP server with a client that supports only SFTP.
FTP over SSH (not SFTP)
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 bypass the SSH connection and thus have no confidentiality or integrity protection, etc.
Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, to monitor and rewrite FTP control channel messages and autonomously open new packet forwardings for FTP data channels. Software packages that support this mode are:
Tectia ConnectSecure (Win/Linux/Unix) of SSH Communications Security's software suite
Tectia Server for IBM z/OS of SSH Communications Security's software suite
FONC (the GPL licensed)
Co:Z FTPSSH Proxy
FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as 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.