Editing File Transfer Protocol

{{Short description|Standard protocol for transferring files over TCP/IP networks}}
{{Redirect|FTP}}
{{Use dmy dates|date=August 2016}}
{{Infobox networking protocol
|       title = File Transfer Protocol
|     purpose = [[File transfer]]
|   developer = [[Abhay Bhushan]] for RFC 114
|        date = {{Start date and age|1971|4|16}}
|    osilayer = [[Application layer]]
|       ports = 21 for control, 20 for data transfer
|        rfcs = RFC 959
}}
{{IPstack}}
{{Internet history timeline}}

The '''File Transfer Protocol''' ('''FTP''') is a standard [[communication protocol]] used for the transfer of [[computer file]]s 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.<ref name="for">{{cite book |last=Forouzan |first=B.A. |year=2000 |title=TCP/IP: Protocol Suite |edition=1st |location=New Delhi, India |publisher=Tata McGraw-Hill Publishing Company Limited}}</ref> FTP users may authenticate themselves with a [[plaintext|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 [[#Security|secured]] with [[Transport Layer Security|SSL/TLS]] ([[FTPS]]) or replaced with [[SSH File Transfer Protocol]] (SFTP).

The first FTP client applications were [[Command-line interface|command-line programs]] developed before [[operating system]]s had [[graphical user interface]]s, and are still shipped with most [[Windows]], [[Unix]], and [[Linux]] operating systems.<ref name="tcpip">{{cite web |last=Kozierok |first=Charles M. |year=2005 |title=The TCP/IP Guide v3.0 |url=http://www.tcpipguide.com/free/t_FTPOverviewHistoryandStandards.htm |publisher=Tcpipguide.com}}</ref><ref name="net+"/> Many dedicated FTP [[client (computing)|client]]s and automation utilities have since been developed for [[desktop computer|desktops]], servers, mobile devices, and hardware, and FTP has been incorporated into productivity applications such as [[HTML editor]]s and [[file managers]].

An FTP client used to be commonly integrated in [[web browser]]s, where file servers are browsed with the [[Uniform Resource Identifier|URI]] prefix "<code>{{Cite FTP |url=ftp:// |server=FTP server |url-status=dead |title=FTP link }}</code>". In 2021, FTP support was dropped by [[Google Chrome]] and [[Firefox]],<ref name=":0">{{Cite web|last=Vonau|first=Manuel|date=2021-07-07|title=Firefox follows in Chrome's footsteps and drops FTP support (APK Download)|url=https://www.androidpolice.com/2021/07/14/firefox-90-fully-removes-ftp-support-and-reorganizes-some-settings-apk-download/|access-date=2021-07-12|website=Android Police|language=en-US}}</ref><ref name="chromestatus">{{Cite web|title=Remove FTP support - Chrome Platform Status|url=https://chromestatus.com/feature/6246151319715840|access-date=2021-09-02|website=www.chromestatus.com}}</ref>  two major web browser vendors, due to it being superseded by the more secure [[SSH File Transfer Protocol|SFTP]] and [[FTPS]]; although neither of them have implemented the newer protocols.<ref>{{Cite web |last=by |first=Written |date=2020-03-23 |title=Firefox is dropping FTP support |url=https://news.sophos.com/en-us/2020/03/23/firefox-is-dropping-ftp-support/ |access-date=2023-10-13 |website=Sophos News |language=en-US}}</ref><ref>{{Cite web |last=Edwards |first=Benj |date=2022-07-14 |title=Chrome and Firefox Killed FTP Support: Here's an Easy Alternative |url=https://www.howtogeek.com/744569/chrome-and-firefox-killed-ftp-support-heres-an-easy-alternative/ |access-date=2023-10-13 |website=How-To Geek |language=en}}</ref>

==History of FTP servers==
The original specification for the File Transfer Protocol was written by [[Abhay Bhushan]] and published as {{IETF RFC|114}} on 16 April 1971. Until 1980, FTP ran on [[Network Control Protocol (ARPANET)|NCP]], the predecessor of [[Internet protocol suite|TCP/IP]].<ref name="tcpip" /> The protocol was later replaced by a TCP/IP version, {{IETF RFC|765}} (June 1980) and {{IETF RFC|959}} (October 1985), the current specification. Several proposed standards amend {{IETF RFC|959}}, for example {{IETF RFC|1579}} (February 1994) enables Firewall-Friendly FTP (passive mode), {{IETF RFC|2228}} (June 1997) proposes security extensions, {{IETF RFC|2428}} (September 1998) adds support for [[IPv6]] and defines a new type of passive mode.<ref name="clark">{{cite book |last=Clark |first=M.P. |year=2003 |title=Data Networks IP and the Internet |edition=1st |location=West Sussex, England |publisher=John Wiley & Sons Ltd.}}</ref>

==Protocol overview== 

===Communication and data transfer===
[[File:Passive FTP Verbindung.svg|thumb|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.<ref name="slack">{{cite web |title=Active FTP vs. Passive FTP, a Definitive Explanation |url=http://slacksite.com/other/ftp.html |publisher=Slacksite.com}}</ref> (This sense of "mode" is different from that of the MODE command in the FTP protocol.)
* 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 (computing)|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,<ref name="slack"/> 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.<ref name="postel">{{IETF RFC|959}} (Standard) File Transfer Protocol (FTP). Postel, J. & Reynolds, J. (October 1985).</ref>
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''.<ref>{{IETF RFC|2428}} (Proposed Standard) Extensions for IPv6, NAT, and Extended Passive Mode. Allman, M. & Metz, C. & Ostermann, S. (September 1998).</ref>

The server responds over the control connection with [[List of FTP server return codes|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. &lt;Need account for storing file&gt;).<ref name="for" /> 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 (one for sending and one for receiving) because it was originally designed to operate on top of [[Network Control Protocol (ARPANET)|Network Control Protocol]] (NCP), which was a [[Simplex communication|simplex protocol]] that utilized two [[List of TCP and UDP port numbers|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,<ref name="stevens1994v1">{{cite book | last = Stevens | first =  W. Richard | title = TCP/IP Illustrated Volume I | location = Reading, Massachusetts, USA | publisher=  Addison-Wesley Publishing Company | year = 1994 | isbn = 0-201-63346-9 | volume = 1 }}</ref>{{Rp|15}} 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 [[Network address translation|NATs]] and firewalls, which do not allow connections from the Internet towards internal hosts.<ref name="ncftp">{{cite web |last=Gleason |first=Mike |year=2005 |title=The File Transfer Protocol and Your Firewall/NAT |url=https://www.ncftp.com/ncftpd/doc/misc/ftp_and_firewalls.html |publisher=Ncftp.com}}</ref> 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.<ref name="ncftp" /> 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.<ref name="ncftp" />
[[File:FTP model.png|thumb|A model chart of how FTP works]]

===Data types===
While transferring data over the network, five data types are defined:<ref name="tcpip" /><ref name="net+" /><ref name="clark" />
* [[ASCII]] (TYPE A): Used for text. Data is converted, if needed, from the sending host's character representation to [[Extended ASCII|"8-bit ASCII"]] before transmission, and (again, if necessary) to the receiving host's character representation, including [[newline]]s. As a consequence, this mode is inappropriate for files that contain data other than ASCII.
* Image (TYPE I, commonly called [[Binary data|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]] (TYPE E): Used for plain text between hosts using the EBCDIC character set.
* Local (TYPE L ''n''): Designed to support file transfer between machines which do not use 8-bit bytes, e.g. [[36-bit computing|36-bit systems]] such as DEC [[PDP-10]]s. 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]] (TYPE U): defined in an expired [[Internet Draft]]<ref>{{Cite IETF|last=Klensin|first=John|title=FTP TYPE Extension for Internationalized Text|draft=draft-klensin-ftpext-typeu-00|access-date=2020-06-09}}</ref> 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 (see above), and the modes set by the FTP protocol MODE command (see below).

For text files (TYPE A and TYPE E), three different format control options are provided, to control how the file would be printed:
* Non-print (TYPE A N and TYPE E N) – the file does not contain any carriage control characters intended for a printer
* [[Telnet]] (TYPE A T and TYPE E T) – the file contains Telnet (or in other words, ASCII C0) carriage control characters (CR, LF, etc)
* [[ASA carriage control characters|ASA]] (TYPE A A and TYPE E A) – the file contains ASA carriage control characters
These formats were mainly relevant to [[line printer]]s; 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 (stream-oriented). 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. (Section 3.1.1.1)
* '''R''' or RECORD structure (record-oriented). 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 filesystem]]s.
* '''P''' or PAGE structure (page-oriented). 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 (operating system)|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:<ref name="for" /><ref name="tcpip" />
* Stream mode (MODE S): Data is sent as a continuous stream, relieving FTP from doing any processing. Rather, all processing is left up to [[Transmission Control Protocol|TCP]]. No End-of-file indicator is needed, unless the data is divided into [[Record (computer science)|records]].
* Block mode (MODE B): Designed primarily for transferring record-oriented files (STRU R), although can also be used to transfer stream-oriented (STRU F) text files. FTP puts each record (or line) of data into several blocks (block header, byte count, and data field) and then passes it on to TCP.<ref name="clark" />
* Compressed mode (MODE C): 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.<ref>{{cite IETF |title=Deflate transmission mode for FTP |draft=draft-preston-ftpext-deflate-03 |first=J. |last=Preston |date=January 2005 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=27 January 2016}}</ref>

[[GridFTP]] defines additional modes, MODE E<ref>{{Cite web|last=Allcock|first=W.|date=April 2003|title=GridFTP: Protocol Extensions to FTP for the Grid|url=https://ogf.org/documents/GFD.20.pdf}}</ref> and MODE X,<ref>{{Cite web|last=Mandrichenko|first=I.|date=May 4, 2005|title=GridFTP v2 Protocol Description|url=https://ogf.org/documents/GFD.47.pdf}}</ref> as extensions of MODE B.

===Additional commands===
More recent implementations of FTP support the ''Modify Fact: Modification Time'' (MFMT) command, which allows a client to adjust that [[file attribute]] remotely, enabling the preservation of that attribute when uploading files.<ref>{{cite web |title=MFMT FTP command |url=https://support.solarwinds.com/SuccessCenter/s/article/MFMT-FTP-command |website=support.solarwinds.com |date=2018-10-11}}</ref><ref>{{cite web |title=FTP Commands: DSIZ, MFCT, MFMT, AVBL, PASS, XPWD, XMKD {{!}} Serv-U |url=https://www.serv-u.com/resources/tutorial/dsiz-mfct-mfmt-avbl-pass-xpwd-xmkd-ftp-command |website=www.serv-u.com |language=en}}</ref>

To retrieve a remote file timestamp, there's ''MDTM'' command. Some servers (and clients) support nonstandard syntax of the ''MDTM'' command with two arguments, that works the same way as ''MFMT''<ref>{{cite web |title=MDTM FTP command |url=https://support.solarwinds.com/SuccessCenter/s/article/MDTM-FTP-command |website=support.solarwinds.com |date=2018-10-11}}</ref>

==Login==
[[File:1st-ftp-southpole-1994.jpg|thumb|A computer at [[Amundsen–Scott South Pole Station]] logging into an FTP server and transferring a file, in 1994]]
FTP login uses normal username and password scheme for granting access.<ref name="tcpip" /> The username is sent to the server using the USER command, and the password is sent using the PASS command.<ref name="tcpip" /> This sequence is unencrypted "on the wire", so may be vulnerable to a network [[sniffing attack]].<ref>{{cite web|last1=Prince|first1=Brian|title=Should Organizations Retire FTP for Security?|url=https://www.securityweek.com/should-organizations-retire-ftp-security/|website=Security Week|date=24 January 2012 |access-date=14 September 2017}}</ref> 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.<ref name="tcpip" /> 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.<ref name="tcpip" />

===Anonymous FTP===
A host that provides an FTP service may provide [[Anonymity|anonymous]] FTP access.<ref name="tcpip" /> 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,<ref name="net+">{{cite book |last=Dean |first=Tamara |title=Network+ Guide to Networks |publisher=Delmar |year=2010 |pages=168–171}}</ref> no verification is actually performed on the supplied data.<ref name="rfc16">{{IETF RFC|1635}} (Informational) How to Use Anonymous FTP. P. & Emtage, A. & Marine, A. (May 1994).</ref> Many FTP hosts whose purpose is to provide software updates will allow anonymous logins.<ref name="net+" />

==Software support==
[[File:FileZilla.png|thumb|[[FileZilla]] client running on Windows, one of the best known FTP client software]]

=== File managers ===
Many file managers tend to have FTP access implemented, such as [[File Explorer]] (formerly Windows Explorer) on [[Microsoft Windows]]. This client is only recommended for small file transfers from a server, due to limitations compared to dedicated client software.<ref>[https://helpdesk.egnyte.com/hc/en-us/articles/201637914-FTP-Access-through-Windows-Explorer FTP Access through Windows Explorer]</ref> It does not support [[SSH File Transfer Protocol|SFTP]].<ref>{{Cite web |title=CSC373/406: SSH [2011/03/27-29] |url=https://fpl.cs.depaul.edu/jriely/373/lectures/class-01-014.html |access-date=2023-10-13 |website=fpl.cs.depaul.edu}}</ref>

Both the native file managers for [[KDE]] on Linux ([[Dolphin (file manager)|Dolphin]] and [[Konqueror]]) support FTP as well as SFTP.<ref>{{Cite web |title=FTP |url=https://docs.kde.org/stable5/en/konqueror/konqueror/ftp.html |access-date=2023-10-13 |website=docs.kde.org}}</ref><ref>{{Cite web |last=Cohen |first=Brent |date=2023-07-26 |title=How To Connect to FTP/SFTP in Dolphin {{!}} DeviceTests |url=https://devicetests.com/connect-ftp-sftp-dolphin |access-date=2023-10-13 |language=en-US}}</ref>
[[File:Primitive FTPd screenshot.png|left|thumb|198x198px|Primitive FTPd on Android, actively running an FTP and SFTP server]]
On [[Android (operating system)|Android]], the My Files file manager on [[Samsung Galaxy]] has a built-in FTP and [[SSH File Transfer Protocol|SFTP]] client.<ref>{{Cite web |last=Staff |first=Moyens |date=2022-02-28 |title=Samsung My Files vs Google Files: Which File Manager is Better on Galaxy Phones |url=https://uk.moyens.net/android/samsung-my-files-vs-google-files-which-file-manager-is-better-on-galaxy-phones/ |access-date=2023-10-13 |website=Moyens I/O |language=en-GB}}</ref>

===Web browser===
For a long time, most common [[web browser]]s were able to retrieve files hosted on FTP servers, although not all of them had support for protocol extensions such as [[FTPS]].<ref name="net+" /><ref name="matthews">{{cite book |last=Matthews |first=J. |year=2005 |title=Computer Networking: Internet Protocols in Action |edition=1st |location=Danvers, MA |publisher=John Wiley & Sons Inc.}}</ref> 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. 

Google Chrome removed FTP support entirely in Chrome 88, also affecting other [[Chromium]]-based browsers such as [[Microsoft Edge]].<ref>{{cite web|url=https://www.omgubuntu.co.uk/2021/01/linux-release-roundup-chrome-lightworks-more|date=2021-01-26|access-date=2021-01-30|website=omgubuntu.co.uk|title=Linux Release Roundup: GParted, Lightworks, Google Chrome + More|first1=Joey|last1=Sneddon}}</ref> Firefox 88 disabled FTP support by default, with Firefox 90 dropping support entirely.<ref>{{cite web|date=2021-04-19|title=See what's new in Firefox: 88.0 Firefox Release|url=https://www.mozilla.org/en-US/firefox/88.0/releasenotes/|access-date=2021-04-20|website=mozilla.org}}</ref><ref name=":0" />

[[FireFTP]] is a discontinued browser extension that was designed as a full-featured FTP client to be run within [[Firefox]], but when Firefox dropped support for FTP the extension developer recommended using [[Waterfox]].<ref>{{Cite web |title=FireFTP - The Free FTP Client for Waterfox |url=https://fireftp.net/ |url-status=dead |archive-url=https://web.archive.org/web/20220301212656/https://fireftp.net/ |archive-date=2022-03-01 |website=FireFTP.net}}</ref> Some browsers, such as the text-based [[Lynx (web browser)|Lynx]], still support FTP.<ref>{{Cite web |title=URL Schemes Supported in Lynx |url=https://lynx.invisible-island.net/lynx_help/lynx_url_support.html |access-date=2023-07-06 |website=Lynx website}}</ref>

====Syntax====
FTP URL syntax is described in {{IETF RFC|1738}}, taking the form: <code>{{cite web |url=ftp://[user[:password |title=FTP link |work=user:password }}]@]host[:port]/[url-path]</code> (the bracketed parts are optional).

For example, the URL <nowiki>{{Cite FTP |url=ftp://public.ftp-servers.example.com/mydirectory/myfile.txt |server=public.ftp-servers.example.com |url-status=dead |title=FTP link }}</nowiki> represents the file ''myfile.txt'' from the directory ''mydirectory'' on the server ''public.ftp-servers.example.com'' as an FTP resource. The URL <nowiki>{{Cite FTP |url=ftp://user001:secretpassword@private.ftp-servers.example.com/mydirectory/myfile.txt |server=private.ftp-servers.example.com |url-status=dead |title=FTP link }}</nowiki> 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]]<ref>{{cite web |url=https://support.mozilla.org/en-US/kb/Accessing%20FTP%20servers#w_ftp-servers-that-require-a-username-and-password |title=Accessing FTP servers &#124; How to &#124; Firefox Help |publisher=Support.mozilla.com |date=2012-09-05 |access-date=2013-01-16}}</ref> and [[Internet Explorer]]<ref>{{Cite web |url=https://support.microsoft.com/en-us/kb/135975 |title=How to Enter FTP Site Password in Internet Explorer |access-date=13 February 2020 |archive-date=2 July 2015 |archive-url=https://web.archive.org/web/20150702005840/https://support.microsoft.com/en-us/kb/135975 |url-status=dead }} Written for IE versions 6 and earlier. Might work with newer versions.</ref>). 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 user.<ref>{{cite web |url=https://jkorpela.fi/ftpurl.html |title=FTP URLs |author=Jukka “Yucca” Korpela |publisher="IT and communication" (jkorpela.fi) |date=1997-09-18 |access-date=2020-01-26}}</ref>

===Download manager===
Most common [[download manager]]s can receive files hosted on FTP servers, while some of them also give the interface to retrieve the files hosted on FTP servers. [[DownloadStudio]] allows not only download a file from FTP server but also view the list of files on a FTP server.<ref>{{cite web |url=http://www.conceiva.com/products/downloadstudio/features.asp |title=DownloadStudio - Internet Download Manager And Download Accelerator - Features |author= |publisher=Conceiva |date= |access-date=2021-10-19}}</ref>

===Other===
[[LibreOffice]] declared its FTP support deprecated from 7.4 release, this was later removed in 24.2 release.<ref>{{Cite web |url=https://wiki.documentfoundation.org/ReleaseNotes/7.4 |title=LibreOffice 7.4: Release Notes |publisher=The Document Foundation's Wiki |accessdate=2022-09-10 }}</ref><ref>{{Cite web |url=https://wiki.documentfoundation.org/ReleaseNotes/24.2 |title=ReleaseNotes/24.2 |publisher=The Document Foundation's Wiki |accessdate=2024-03-24 }}</ref>

==Security==
FTP was not designed to be a secure protocol, and has many security weaknesses.<ref name="nurdle">{{cite web |title=Securing FTP using SSH |url=https://nurdletech.com/linux-notes/ftp/ssh.html |publisher=Nurdletech.com}}</ref> In May 1999, the authors of {{IETF RFC|2577}} listed a vulnerability to the following problems:
* [[Brute-force attack]]
* [[FTP bounce attack]]
* [[Packet capture]]
* Port stealing (guessing the next open port and usurping a legitimate connection)
* [[Spoofing attack]]
* Username enumeration
* [[Denial-of-service attack|DoS or DDoS]]

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 ([[Packet analyzer|sniffing]]) on the network.<ref name="tcpip" /><ref name="nurdle" /> This problem is common to many of the Internet Protocol specifications (such as [[SMTP]], [[Telnet]], [[Post Office Protocol|POP]] and [[Internet Message Access Protocol|IMAP]]) that were designed prior to the creation of encryption mechanisms such as [[Transport Layer Security|TLS]] or SSL.<ref name="clark" />

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 network]] (VPN).

===FTP over SSH=== <!-- This section is linked from [[OpenSSH]] -->
FTP over SSH is the practice of tunneling a normal FTP session over a Secure Shell connection.<ref name="nurdle"/> Because FTP uses multiple [[Transmission Control Protocol|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 forwarding]]s for FTP data channels. Software packages that support this mode include:
* Tectia ConnectSecure (Win/Linux/Unix)<ref>{{cite web|url=https://www.ssh.com/manuals/mft-events-product/63/ssh-solutions-your-business-components.html|title=Components of the Information Assurance Platform (section Tectia ConnectSecure)|website=ssh.com|archive-url=https://web.archive.org/web/20200731160323/https://www.ssh.com/manuals/mft-events-product/63/ssh-solutions-your-business-components.html|archive-date=2020-07-31}}</ref> of [[SSH Communications Security]]'s software suite

FTP over SSH should not be confused with [[SSH File Transfer Protocol]] (SFTP).

==Derivatives==

===FTPS===
{{Main article|FTPS}}
Explicit 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 {{IETF 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.

===SSH 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, though some FTP client software offers support for the SSH file transfer protocol as well.

===Trivial File Transfer Protocol ===
{{Main article|Trivial File Transfer Protocol}}
Trivial 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 [[Network booting|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 {{IETF RFC|1350}}.

===Simple File Transfer Protocol===
<!--Simple File Transfer Protocol redirects here-->
Simple File Transfer Protocol (the first protocol abbreviated SFTP), as defined by {{IETF 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 [[Internet Engineering Task Force|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 file|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'').

==FTP commands==
{{Main article|List of FTP commands}}

==FTP reply codes==
{{main article|List of FTP server return codes}}
Below is a summary of [[List of FTP server return codes|FTP reply codes]] that may be returned by an FTP [[Server (computing)|server]]. These codes have been standardized in {{IETF 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&nbsp;— success, failure, or to indicate an error or incomplete reply:
* 2yz&nbsp;&ndash; Success reply
* 4yz or 5yz &ndash; Failure reply
* 1yz or 3yz &ndash; Error or Incomplete reply

The second digit defines the kind of error:
* x0z &ndash; Syntax. These replies refer to syntax errors.
* x1z &ndash; Information. Replies to requests for information.
* x2z &ndash; Connections. Replies referring to the control and data connections.
* x3z &ndash; Authentication and accounting. Replies for the login process and accounting procedures.
* x4z &ndash; Not defined.
* x5z &ndash; File system. These replies relay status codes from the server file system.

The third digit of the reply code is used to provide additional detail for each of the categories defined by the second digit.

==See also==
{{columns-list|colwidth=20em|
* [[Comparison of FTP client software]]
* [[Comparison of FTP server software packages]]
* [[Comparison of file transfer protocols]]
* [[Curl-loader]] – FTP/S loading/testing open-source software
* [[DTXT]]
* [[File eXchange Protocol]] (FXP)
* [[File Service Protocol]] (FSP)
* [[FTAM]]
* [[FTPFS]]
* [[List of FTP commands]]
* [[List of FTP server return codes]]
* [[Managed File Transfer]]
* [[OBject EXchange|OBEX]]
* [[Shared file access]]
* [[TCP Wrapper]]
}}

==References==
{{Reflist}}

==Further reading==
* {{IETF RFC|697}} – CWD Command of FTP. July 1975.
* {{IETF RFC|959}} – (Standard) File Transfer Protocol (FTP). J. Postel, J. Reynolds. October 1985.
* {{IETF RFC|1579}} – (Informational) Firewall-Friendly FTP. February 1994.
* {{IETF RFC|1635}} – (Informational) How to Use Anonymous FTP. May 1994.
* {{IETF RFC|1639}} – FTP Operation Over Big Address Records (FOOBAR). June 1994.
* {{IETF RFC|1738}} – Uniform Resource Locators (URL). December 1994.
* {{IETF RFC|2228}} – (Proposed Standard) FTP Security Extensions. October 1997.
* {{IETF RFC|2389}} – (Proposed Standard) Feature negotiation mechanism for the File Transfer Protocol. August 1998.
* {{IETF RFC|2428}} – (Proposed Standard) Extensions for IPv6, NAT, and Extended passive mode. September 1998.
* {{IETF RFC|2577}} – (Informational) FTP Security Considerations. May 1999.
* {{IETF RFC|2640}} – (Proposed Standard) Internationalization of the File Transfer Protocol. July 1999.
* {{IETF RFC|3659}} – (Proposed Standard) Extensions to FTP. P. Hethmon. March 2007.
* {{IETF RFC|5797}} – (Proposed Standard) FTP Command and Extension Registry. March 2010.
* {{IETF RFC|7151}} – (Proposed Standard) File Transfer Protocol HOST Command for Virtual Hosts.  March 2014.
* [https://www.iana.org/assignments/ftp-commands-extensions/ftp-commands-extensions.xhtml IANA FTP Commands and Extensions registry] &ndash; The official registry of FTP Commands and Extensions

== External links ==
* {{Wikibooks-inline|Communication Networks/File Transfer Protocol}}
* [//servertest.online/ftp FTP Server Online Tester] Authentication, encryption, mode and connectivity.
* Anonymous FTP Servers by Country Code [[Top-level domain|TLD]] (2012): {{Cite web|url=https://www.jumpjet.info/Offbeat-Internet/Public/FTP/url.htm|title=Offbeat Internet - Public Access - FTP|website=www.jumpjet.info|access-date=2020-01-16|date=2012|archive-url=https://web.archive.org/web/20230328113445/https://www.jumpjet.info/Offbeat-Internet/Public/FTP/url.htm|archive-date=2023-03-28|url-status=dead}}

{{URI scheme}}
{{Authority control}}

[[Category:File Transfer Protocol| ]]
[[Category:Application layer protocols]]
[[Category:Clear text protocols]]
[[Category:Computer-related introductions in 1971]]
[[Category:History of the Internet]]
[[Category:Internet Standards]]
[[Category:Network file transfer protocols]]
[[Category:OS/2 commands]]
[[Category:Unix network-related software]]
[[Category:File sharing]]