File Transfer Protocol (FTP) is a standard network protocol used
for the transfer of computer files between a client and server on a
FTP is built on a client-server model architecture and uses separate
control and data connections between the client and the server. FTP
users may authenticate themselves with 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 protects the username and password, and encrypts the
content, FTP is often secured with SSL/TLS (FTPS). SSH
Protocol (SFTP) is sometimes also used instead; it is technologically
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 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 web page editors.
1 History of FTP server
2 Protocol overview
2.1 Communication and data transfer
2.3 Anonymous FTP
2.4 NAT and firewall traversal
2.5 Differences from HTTP
Web browser support
4.1 FTP over SSH
File Transfer Protocol
File Transfer Protocol
File Transfer Protocol
6 FTP commands
7 FTP reply codes
8 FTP Servers
9 See also
11 Further reading
12 External links
History of FTP server
The original specification for the
File Transfer Protocol was written
Abhay Bhushan and published as RFC 114 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, RFC 765 (June 1980) and
RFC 959 (October 1985), the current specification. Several
proposed standards amend RFC 959, for example RFC 1579
(February 1994) enables Firewall-Friendly FTP (passive mode),
RFC 2228 (June 1997) proposes security extensions, RFC 2428
(September 1998) adds support for
IPv6 and defines a new type of
Communication and data transfer
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 both cases, the client creates a TCP
control connection from a random, usually an unprivileged, port N to
the FTP server command port 21.
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" (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.
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 by byte, and the recipient stores the bytestream as it
receives it. (Image mode support has been recommended for all
implementations of FTP).
EBCDIC mode: Used for plain text between hosts using the EBCDIC
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
Telnet or ASA.
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
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 simple algorithm (usually
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.
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.
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 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.
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
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.
Differences from HTTP
HTTP essentially fixes the bugs in FTP that made it inconvenient to
use for many small ephemeral transfers as are typical in web pages.
FTP has a stateful control connection which maintains a current
working directory and other flags, and each transfer requires a
secondary connection through which the data are transferred. In
"passive" mode this secondary connection is from client to server,
whereas in the default "active" mode this connection is from server to
client. This apparent role reversal when in active mode, and random
port numbers for all transfers, is why firewalls and NAT gateways have
such a hard time with FTP. HTTP is stateless and multiplexes control
and data over a single connection from client to server on well-known
port numbers, which trivially passes through NAT gateways and is
simple for firewalls to manage.
Setting up an FTP control connection is quite slow due to the
round-trip delays of sending all of the required commands and awaiting
responses, so it is customary to bring up a control connection and
hold it open for multiple file transfers rather than drop and
re-establish the session afresh each time. In contrast, HTTP
originally dropped the connection after each transfer because doing so
was so cheap. While HTTP has subsequently gained the ability to reuse
the TCP connection for multiple transfers, the conceptual model is
still of independent requests rather than a session.
When FTP is transferring over the data connection, the control
connection is idle. If the transfer takes too long, the firewall or
NAT may decide that the control connection is dead and stop tracking
it, effectively breaking the connection and confusing the download.
The single HTTP connection is only idle between requests and it is
normal and expected for such connections to be dropped after a
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.
URL syntax is described in RFC 1738, taking the form:
ftp://[user[:password]@]host[:port]/url-path (the bracketed parts are
For example, the URL
the file myfile.txt from the directory mydirectory on the server
public.ftp-servers.example.com as an FTP resource. The URL
adds a specification of the username and password that must be used to
access this resource.
More details on specifying a username and password may be found in the
browsers' documentation (e.g., Firefox and
By default, most web browsers use passive (PASV) mode, which more
easily traverses end-user firewalls.
Some variation has existed in how different browsers treat path
resolution in cases where there is a non-root home directory for a
FTP was not designed to be a secure protocol, and has many security
weaknesses. In May 1999, the authors of RFC 2577 listed a
vulnerability to the following problems:
Brute force attack
FTP bounce attack
Port stealing (guessing the next open port and usurping a legitimate
FTP does not encrypt its traffic; all transmissions are in clear text,
and usernames, passwords, commands and data can be 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.
Common solutions to this problem include:
Using the secure versions of the insecure protocols, e.g., FTPS
instead of FTP and TelnetS instead of Telnet.
Using a different, more secure protocol that can handle the job, e.g.
SSH File Transfer Protocol or Secure Copy Protocol.
Using a secure tunnel such as
Secure Shell (SSH) or virtual private
FTP over SSH
FTP over SSH is the practice of tunneling a normal FTP session over a
Secure Shell 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 sets
up new TCP connections (data channels) and thus have no
confidentiality or integrity protection.
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
Tectia ConnectSecure (Win/Linux/Unix) of SSH Communications
Security's software suite
Main article: FTPS
FTPS is an extension to the FTP standard that allows clients
to request FTP sessions to 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 RFC 4217. Implicit
FTPS is an outdated standard for
FTP that required the use of a SSL or TLS connection. It was specified
to use different ports than plain FTP.
File Transfer Protocol
Main article: SSH
File Transfer Protocol
The SSH file transfer protocol (chronologically the second of the two
protocols abbreviated SFTP) transfers files and has a similar command
set for users, but uses the
Secure Shell protocol (SSH) to transfer
files. Unlike FTP, it encrypts both commands and data, preventing
passwords and sensitive information from being transmitted openly over
the network. It cannot interoperate with FTP software.
File Transfer Protocol
Main article: Trivial
File Transfer Protocol
File Transfer Protocol (TFTP) is a simple, lock-step FTP that
allows a client to get a file from or put a file onto a remote host.
One of its primary uses is in the early stages of booting from a local
area network, because TFTP is very simple to implement. TFTP lacks
security and most of the advanced features offered by more robust file
transfer protocols such as
File Transfer Protocol. TFTP was first
standardized in 1981 and the current specification for the protocol
can be found in RFC 1350.
File Transfer Protocol
File Transfer Protocol (the first protocol abbreviated SFTP),
as defined by RFC 913, was proposed as an (unsecured) file
transfer protocol with a level of complexity intermediate between TFTP
and FTP. It was never widely accepted on the Internet, and is now
assigned Historic status by the IETF. It runs through port 115, and
often receives the initialism of SFTP. It has a command set of 11
commands and support three types of data transmission: ASCII, binary
and continuous. For systems with a word size that is a multiple of 8
bits, the implementation of binary and continuous is the same. The
protocol also supports login with user ID and password, hierarchical
folders and file management (including rename, delete, upload,
download, download with overwrite, and download with append).
Main article: List of FTP commands
FTP reply codes
Main article: List of FTP server return codes
Below is a summary of FTP reply codes that may be returned by an FTP
server. These codes have been standardized in RFC 959 by the
IETF. The reply code is a three-digit value. The first digit is used
to indicate one of three possible outcomes — success, failure,
or to indicate an error or incomplete reply:
2yz – Success reply
4yz or 5yz – Failure reply
1yz or 3yz – Error or Incomplete reply
The second digit defines the kind of error:
x0z – Syntax. These replies refer to syntax errors.
x1z – Information. Replies to requests for information.
x2z – Connections. Replies referring to the control and data
x3z – Authentication and accounting. Replies for the login process
and accounting procedures.
x4z – Not defined.
File system. These replies relay status codes from the server
The third digit of the reply code is used to provide additional detail
for each of the categories defined by the second digit.
Some popular open source FTP server implementations are:
FileZilla Server (Windows)
CrushFTP (Mac, Win, Linux)
WingFTP (Mac, Win)
Comparison of FTP client software
Comparison of FTP server software
Comparison of file transfer protocols
Curl-loader – FTP/S loading/testing open-source software
File eXchange Protocol (FXP)
File Service Protocol (FSP)
List of FTP commands
List of FTP server return codes
List of FTP server software
Shared file access
^ a b c Forouzan, B.A. (2000). TCP/IP: Protocol Suite (1st ed.). New
Delhi, India: Tata McGraw-Hill Publishing Company Limited.
^ a b c d e f g h i j Kozierok, Charles M. (2005). "The TCP/IP Guide
^ a b c d e Dean, Tamara (2010). Network+ Guide to Networks. Delmar.
^ a b c d Clark, M.P. (2003). Data Networks IP and the
ed.). West Sussex, England: John Wiley & Sons Ltd.
^ a b "Active FTP vs. Passive FTP, a Definitive Explanation".
Slacksite.com. Archived from the original on 4 May 2011.
^ Parker, Don (September 2005). "Understanding the FTP Protocol".
^ RFC 959 (Standard)
File Transfer Protocol (FTP). Postel, J.
& Reynolds, J. (October 1985).
^ RFC 2428 (Proposed Standard) Extensions for IPv6, NAT, and
Extended Passive Mode. Allman, M. & Metz, C. & Ostermann, S.
^ Preston, J. (January 2005). Deflate transmission mode for FTP. IETF.
Retrieved 27 January 2016.
^ Prince, Brian. "Should Organizations Retire FTP for Security?".
Security Week. Security Week. Retrieved 14 September 2017.
^ RFC 1635 (Informational) How to Use Anonymous FTP. P. &
Emtage, A. & Marine, A. (May 1994).
^ a b c Gleason, Mike (2005). "The
File Transfer Protocol and Your
^ Matthews, J. (2005). Computer Networking:
Internet Protocols in
Action (1st ed.). Danvers, MA: John Wiley & Sons Inc.
^ "Accessing FTP servers How to
Support.mozilla.com. 2012-09-05. Retrieved 2013-01-16.
^ "How to Enter FTP Site Password in
Support.microsoft.com. 2011-09-23. Retrieved 2015-03-28. Written
for IE versions 6 and earlier. Might work with newer versions.
^ Jukka “Yucca” Korpela (1997-09-18). "FTP URLs". "IT and
communication" (www.cs.tut.fi/~jkorpela/). Retrieved 2016-01-06.
^ a b c "Securing FTP using SSH". Nurdletech.com.
^ "Access using SSH keys & PCI DSS compliance". ssh.com.
RFC 697 – CWD Command of FTP. July 1975.
RFC 959 – (Standard)
File Transfer Protocol (FTP). J. Postel,
J. Reynolds. October 1985.
RFC 1579 – (Informational) Firewall-Friendly FTP. February
RFC 1635 – (Informational) How to Use Anonymous FTP. May 1994.
RFC 1639 – FTP Operation Over Big Address Records (FOOBAR).
RFC 1738 – Uniform Resource Locators (URL). December 1994.
RFC 2228 – (Proposed Standard) FTP Security Extensions. October
RFC 2389 – (Proposed Standard) Feature negotiation mechanism
File Transfer Protocol. August 1998.
RFC 2428 – (Proposed Standard) Extensions for IPv6, NAT, and
Extended passive mode. September 1998.
RFC 2577 – (Informational) FTP Security Considerations. May
RFC 2640 – (Proposed Standard) Internationalization of the File
Transfer Protocol. July 1999.
RFC 3659 – (Proposed Standard) Extensions to FTP. P. Hethmon.
RFC 5797 – (Proposed Standard) FTP Command and Extension
Registry. March 2010.
RFC 7151 - (Proposed Standard)
File Transfer Protocol HOST
Command for Virtual Hosts. March 2014.
IANA FTP Commands and Extensions registry – The official registry of
FTP Commands and Extensions
File Transfer Protocol at Wikibooks
FTP Server Online Tester Authentication, encryption, mode and
Uniform Resource Identifier (URI) schemes
sip / sips
ws / wss
irc / irc6 / ircs