The term file transfer protocol (FTP) refers to a process that involves the transfer of files between devices over a network. The process works when one party allows another to send or receive files over the internet. Originally used as a way for users to communicate and exchange information between two physical devices, it is now commonly used to store files in the cloud, which is usually a secure location that is held remotely. Show FTP may be used by a business or individual to transfer files from one computer system to another or by websites to upload or download files from their servers.
File transfer protocol allows individuals and businesses to share electronic files with others without having to be in the same space. This can be done using an FTP client or through the cloud. Regardless of the option, both parties require a working internet connection. Most web browsers come with FTP clients that enable users to transfer files from their computer to a server and vice versa. Some users may want to use a third-party FTP client because many of them offer extra features. Examples of FTP clients that are free to download include FileZilla Client, FTP Voyager, WinSCP, CoffeeCup Free FTP, and Core FTP. Many people have used FTP before without even realizing it. If you have ever downloaded a file from a web page, you've used FTP. The first step is to log in, which may occur automatically or by manually inputting a username and password. FTP will also require you to access an FTP server through a specific port number. Once you access the FTP server through your FTP client, you can now transfer files. Not all public FTP servers require you to sign in because some servers enable you to access them anonymously. As noted above, FTP was originally developed as a way to send and receive files between two physical computers. But with changes in technology, users can execute file transfers through the cloud. Using the cloud allows transfers to be done conveniently and safely (which could protect individuals and companies from data breaches), and with relatively low cost.
The term FTP client refers to the software that allows you to transfer files to another party. File transfer protocol is one of many different protocols that dictate how computers and computing systems behave on the internet. Other such protocols include the:
FTP enables computers on the internet to transfer files back and forth. As such, it is an essential tool for those building and maintaining websites today. Individual FTP clients provide different features that allow users to modify the way they upload and download files. For instance, if you use FileZilla, the program lets you set bandwidth limits for files. This enables you to control upload and download speeds, which can be helpful if you manage multiple file transfers at once. Other features you may want to look for in an FTP client include public-key authentication, the ability to set file compression levels, or tools that enable you to search a server using file masks. FTP software is relatively straightforward to set up. FileZilla is a free, downloadable FTP client. Type in the address of the server you wish to access, the port, and the password for accessing the server. Once access has been granted, the user's files on their local system as well as the accessed server will be visible. The user can download files from the server to the local system, or upload files from the local system to the server. They can also make changes to files on the server, as long as they have the proper authorization to do so. The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files 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.[1] 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) or replaced with SSH File Transfer Protocol (SFTP).
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.[2][3] Many dedicated 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 HTML editors and file managers.
An FTP client used to be commonly integrated in web browsers, where file servers are browsed with the URI prefix "ftp://". Throughout 2021, the two major web browser vendors removed this ability. Support for the FTP protocol was first disabled in Google Chrome 88 in January 2021,[4] followed by Firefox 88.0 in April 2021.[5] In July 2021, Firefox 90 dropped FTP entirely,[6] and Google followed suit in October 2021, removing FTP entirely in Google Chrome 95.[7] The original specification for the File Transfer Protocol was written by Abhay Bhushan and published as RFC 114 on 16 April 1971. Until 1980, FTP ran on NCP, the predecessor of TCP/IP.[2] 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 passive mode.[8] FTP may run in active or passive mode, which determines how the data connection is established.[9] (This sense of "mode" is different from that of the MODE command in the FTP protocol.)
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.[11] 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. FTP needs two ports (one for sending and one for receiving) because it was originally designed to operate on top of Network Control Protocol (NCP), which was a simplex protocol that utilized two 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,[12]: 15 but the FTP protocol was never altered to only use one port, and continued using two for backwards compatibility. NAT and firewall traversalFTP 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.[13] 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.[13] 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.[13] Data typesWhile transferring data over the network, four data types are defined:[2][3][8]
An expired Internet Draft defined a TYPE U for transferring Unicode text files using UTF-8;[14] although the draft never became an RFC, 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:
These formats were mainly relevant to line printers; most contemporary FTP clients/servers only support the default format control of N. File structuresFile organization is specified using the STRU command. The following file structures are defined in section 3.1.1 of RFC959:
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 modesData transfer can be done in any of three modes:[1][2]
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.[15] GridFTP defines additional modes, MODE E[16] and MODE X,[17] as extensions of MODE B. Additional commandsMore 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.[18][19] 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[20] FTP login uses normal username and password scheme for granting access.[2] The username is sent to the server using the USER command, and the password is sent using the PASS command.[2] This sequence is unencrypted "on the wire", so may be vulnerable to a network sniffing attack.[21] 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.[2] 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.[2] Anonymous FTPA host that provides an FTP service may provide anonymous FTP access.[2] 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,[3] no verification is actually performed on the supplied data.[22] Many FTP hosts whose purpose is to provide software updates will allow anonymous logins.[3] 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 time-out. Most common web browsers can retrieve files hosted on FTP servers, although they may not support protocol extensions such as FTPS.[3][23] 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. FireFTP is a browser extension designed as a full-featured FTP client, it could be run within Firefox in the past, but it's now recommend working with Waterfox. Google Chrome removed FTP support entirely in Chrome 88.[24] As of 2019, Mozilla was discussing proposals, including only removing support for old FTP implementations that are no longer in use to simplify their code.[25][7] In April, 2021, Mozilla released Firefox 88.0 which disabled FTP support by default.[26] In July 2021, Firefox 90 dropped FTP support entirely.[6] SyntaxFTP URL syntax is described in RFC 1738, taking the form: ftp://[user[:password]@]host[:port]/url-path (the bracketed parts are optional). For example, the URL ftp://public.ftp-servers.example.com/mydirectory/myfile.txt represents the file myfile.txt from the directory mydirectory on the server public.ftp-servers.example.com as an FTP resource. The URL ftp://user001:/mydirectory/myfile.txt 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[27] and Internet Explorer[28]). 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.[29] Download managerMost common download managers 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.[30] FTP was not designed to be a secure protocol, and has many security weaknesses.[31] In May 1999, the authors of RFC 2577 listed a vulnerability to the following problems:
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.[2][31] 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.[8] Common solutions to this problem include:
FTP over SSHFTP over SSH is the practice of tunneling a normal FTP session over a Secure Shell connection.[31] 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 include:
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 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 ProtocolThe 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 ProtocolTrivial 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. Simple File Transfer ProtocolSimple 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). 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:
The second digit defines the kind of error:
The third digit of the reply code is used to provide additional detail for each of the categories defined by the second digit.
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