File Transfer Protocols (FTP)

Network protocol used to transfer files from one host to another host over
a TCP -based network, such as the Internet . It is often used to upload web
pages and other documents from a private development machine to a public
web-hosting server. FTP is built on a client-server architecture and uses
separate control and data connections between the client and the server.
FTP users may authenticate themselves using a clear-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 hides (encrypts) your username and password, as well as
encrypts the content, you can try using a client that uses SSH File
Transfer Protocol .
Protocol overview Differences from HTTP

FTP operates on the application layer of the OSI model , and is used to
transfer files using TCP/IP . To do so, an FTP server has to be running
and waiting for incoming requests. The client computer is then able to
communicate with the server on port 21. This connection, called the control
connection , remains open for the duration of the session. A second
connection, called the data connection , can either be opened by the server
from its port 20 to a negotiated client port ( active mode), or by the
client from an arbitrary port to a negotiated server port ( passive mode)
as required to transfer file data. The control connection is used for
session administration, for example commands, identification and passwords
exchanged between the client and the server using a telnet-like protocol.
For example "RETR filename " would transfer the specified file from the
server to the client. Due to this two-port structure, FTP is considered an
out-of-band protocol, as opposed to an in-band protocol such as HTTP .

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>). [1] 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: [1]

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 ).

Login

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.

Anonymous FTP

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.