File Transfer Protocol (FTP) is a standard network protocol In computing, a protocol is a set of rules which is used by computers to communicate with each other across a network. A protocol is a convention or standard that controls or enables the connection, communication, and data transfer between computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax, used to copy a file from one host to another over a TCP/IP The Internet Protocol Suite is the set of communications protocols used for the Internet and other similar networks. It is named from two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in this standard. Today's IP networking-based network, such as the Internet The Internet is a global system of interconnected computer networks that use the standard Internet Protocol Suite to serve billions of users worldwide. It is a network of networks that consists of millions of private, public, academic, business, and government networks of local to global scope that are linked by a broad array of electronic and. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications, which solves the problem of different end host configurations (i.e., Operating System, file names).^[1] FTP is used with user-based password authentication or with anonymous user access.

Applications were originally interactive command-line tools with a standardized command syntax, but graphical user interfaces have been developed for all desktop operating systems in use today.PORTS FTP (Data) 20 FTP (Control) 21

History

The original specification for the File Transfer Protocol was published as RFC 114 on 16 April 1971 and later replaced by RFC 765 (June 1980) and RFC 959 (October 1985), the current specification. Several proposed standards amend RFC 959, for example RFC 2228 (June 1997) proposes security extensions and RFC 2428 (September 1998) adds support for IPv6 Internet Protocol version 6 is an Internet Protocol version which is designed to succeed IPv4, the first implementation which is still in dominant use currently[update]. It is an Internet Layer protocol for packet-switched internetworks. The main driving force for the redesign of Internet Protocol is the foreseeable IPv4 address exhaustion. IPv6 and defines a new type of passive mode. ^[2]

Protocol overview

The protocol is specified in RFC 959, which is summarized below.^[3]

A client makes a connection to the server on TCP The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite (the other being Internet Protocol, or IP), so the entire suite is commonly referred to as TCP/IP. Whereas IP handles lower-level transmissions from computer to computer as a message makes its way port 21. This connection, called the control connection, remains open for the duration of the session, with a second connection, called the data connection, on port 20 opened as required to transfer file data. The control connection is used to send administrative data (i.e., commands, identification, passwords).^[4] Commands are sent by the client over the control connection in ASCII and terminated by a carriage return and line feed. For example "RETR filename" would transfer the specified file from the server to the client. Due to this two port structure, FTP is considered out-of-band, as opposed to an in-band protocol such as HTTP The Hypertext Transfer Protocol is an Application Layer protocol for distributed, collaborative, hypermedia information systems^[4].

The server responds on 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 number and the optional text represent explanations (i.e., <OK>) or needed parameters (i.e., <Need account for storing file>)^[1]. A file transfer in progress over the data connection can be aborted using an interrupt message sent over the control connection.

FTP can be run in active mode or passive mode, which control how the second connection is opened. In active mode the client sends the server the IP address port number that the client will use for the data connection, and the server opens the connection. Passive mode was devised for use where the client is behind a firewall and unable to accept incoming TCP connections. The server sends the client an IP address and port number and the client opens the connection to the server.^[3] Both modes were updated in September 1998 to add support for IPv6 Internet Protocol version 6 is an Internet Protocol version which is designed to succeed IPv4, the first implementation which is still in dominant use currently[update]. It is an Internet Layer protocol for packet-switched internetworks. The main driving force for the redesign of Internet Protocol is the foreseeable IPv4 address exhaustion. IPv6 and made some other changes to passive mode, making it extended passive mode^[5].

While transferring data over the network, four data representations can be used^[2]:

• ASCII The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text. Most modern character-encoding schemes are based on ASCII, though they support many more characters than did 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 numeric data in binary, floating point In computing, floating point describes a system for representing numbers that would be too large or too small to be represented as integers. Numbers are in general represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16. The typical number that can be or binary coded decimal In computing and electronic systems, binary-coded decimal (sometimes called natural binary-coded decimal, NBCD) or, in its most common modern implementation, packed decimal, is an encoding for decimal numbers in which each digit is represented by its own binary sequence. Its main virtue is that it allows easy conversion to decimal digits for form.
• Image mode (commonly called Binary A binary file is a computer file which may contain any type of data, encoded in binary form for computer storage and processing purposes; for example, computer document files containing formatted text. Many binary file formats contain parts that can be interpreted as text; binary files that contain only textual data—without, for example, any mode): the sending machine sends each file byte The byte is a unit of digital information in computing and telecommunications. It is an ordered collection of bits, in which each bit denotes the binary value of 1 or 0. Historically, a byte was the number of bits (typically 5, 6, 7, 8, 9, or 16) used to encode a single character of text in a computer and it is for this reason the basic for byte and as such the recipient stores the bytestream In computer science, a byte stream is a bit stream, in which data bits are grouped into units, called bytes as it receives it. (Image mode support has been recommended for all implementations of FTP).
• EBCDIC Extended Binary Coded Decimal Interchange Code is an 8-bit character encoding (code page) used on IBM mainframe operating systems such as z/OS, OS/390, VM and VSE, as well as IBM midrange computer operating systems such as OS/400 and i5/OS (see also Binary Coded Decimal). It is also employed on various non-IBM platforms such as Fujitsu-Siemens' BS2 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 TELNET is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communications facility via a virtual terminal connection. User data is interspersed in-band with TELNET control information in an 8-bit byte oriented data connection over the Transmission Control Protocol 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 In computer science, a record is one of the simplest data structures, consisting of two or more values or variables stored in consecutive memory positions; so that each component (called a field or member of the record) can be accessed by applying different offsets to the starting address.
• Block mode: FTP breaks the data into several blocks (block header, byte count, and data field) and then passes it on to TCP.^[2]
• Compressed mode: Data is compressed using a single algorithm (usually Run-length encoding Run-length encoding is a very simple form of data compression in which runs of data (that is, sequences in which the same data value occurs in many consecutive data elements) are stored as a single data value and count, rather than as the original run. This is most useful on data that contains many such runs: for example, relatively simple graphic).

Security

The original FTP specification has many security concerns. In May 1999, the following flaws were addressed^[6]:

• Bounce Attacks
• Spoof Attacks In the context of network security, a spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage
• Brute Force Attacks In cryptography, a brute force attack is a strategy used to break the encryption of data. It involves traversing the search space of possible keys until the correct key is found
• Sniffing
• Username Protection
• Port Stealing

FTP has no encryption tools meaning all transmissions are in clear text; user names, passwords, FTP commands and transferred files can be read by anyone sniffing on the network. This is a problem common to many Internet protocol specifications written prior to the creation of SSL Transport Layer Security and its predecessor, Secure Socket Layer (SSL), are cryptographic protocols that provide security for communications over networks such as the Internet. TLS and SSL encrypt the segments of network connections at the Transport Layer end-to-end, such as HTTP The Hypertext Transfer Protocol is an Application Layer protocol for distributed, collaborative, hypermedia information systems, SMTP Simple Mail Transfer Protocol is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined in RFC 821 (STD 15) (1982), and last updated by RFC 5321 (2008) which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today. SMTP is specified for and Telnet TELNET is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communications facility via a virtual terminal connection. User data is interspersed in-band with TELNET control information in an 8-bit byte oriented data connection over the Transmission Control Protocol^[2]. The common solution to this problem is to use either SFTP In computing, the SSH File Transfer Protocol is a network protocol that provides file access, file transfer, and file management functionality over any reliable data stream. It was designed by the Internet Engineering Task Force (IETF) as an extension of the Secure Shell protocol (SSH) version 2.0 to provide secure file transfer capability, but is (SSH File Transfer Protocol), or FTPS FTPS should not be confused with the SSH File Transfer Protocol , an incompatible secure file transfer subsystem for the Secure Shell (SSH) protocol. It is also different from Secure FTP, the practice of tunneling FTP through an SSH connection (FTP over SSL), which adds SSL or TLS Transport Layer Security and its predecessor, Secure Socket Layer (SSL), are cryptographic protocols that provide security for communications over networks such as the Internet. TLS and SSL encrypt the segments of network connections at the Transport Layer end-to-end encryption In cryptography, encryption is the process of transforming information using an algorithm (called cipher) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information (in cryptography, referred to as ciphertext). In many contexts, the word encryption to FTP as specified in RFC 4217.

Anonymous FTP

A host that provides an FTP service may additionally provide anonymous Anonymity is derived from the Greek word ἀνωνυμία, anonymia, meaning "without a name" or "namelessness". In colloquial use, anonymous typically refers to a person, and often means that the personal identity, or personally identifiable information of that person is not known FTP access. Users typically log into the service with an 'anonymous' account when prompted for user name. Although users are commonly asked to send their email Electronic mail, commonly called email or e-mail, is a method of exchanging digital messages across the Internet or other computer networks. Email systems are based on a store-and-forward model in which email server computer systems accept, forward, deliver and store messages on behalf of users, who only need to connect to the email infrastructure, address in lieu of a password, no verification is actually performed on the supplied data^[7]; examples of anonymous FTP servers can be found here.

Remote FTP or FTPmail

Where FTP access is restricted, a remote FTP (or FTPmail) service can be used to circumvent the problem. An e-mail containing the FTP commands to be performed is sent to a remote FTP server, which is a mail server that parses the incoming e-mail, executes the FTP commands, and sends back an e-mail with any downloaded files as an attachment. Obviously this is less flexible than an FTP client, as it is not possible to view directories interactively or to modify commands, and there can also be problems with large file attachments in the response not getting through mail servers. As most internet users these days have ready access to FTP, this procedure is no longer in everyday use.

Web browser support

Most recent web browsers A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content. Hyperlinks present in resources enable users to easily navigate their browsers to can retrieve files hosted on FTP servers, although they may not support protocol extensions such as FTPS FTPS should not be confused with the SSH File Transfer Protocol , an incompatible secure file transfer subsystem for the Secure Shell (SSH) protocol. It is also different from Secure FTP, the practice of tunneling FTP through an SSH connection^[8]. When an FTP—rather than HTTP—URL is supplied, the accessible contents of the remote server is presented in a manner similar to that used for other Web content.

FTP URL In computing, a Uniform Resource Locator is a Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI,. The best-known example of a URL is the " syntax is described in RFC1738^[9], taking the form:

ftp://[<user>[:<password>]@]<host>[:<port>]/<url-path>^[9]

(The bracketed parts are optional.) For example:

ftp://public.ftp-servers.example.com/mydirectory/myfile.txt

or:

ftp://user001:secretpassword@private.ftp-servers.example.com/mydirectory/myfile.txt

More details on specifying a user name 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.

NAT traversal

The representation of the IP addresses and port numbers in the PORT command and PASV reply poses a challenge to FTP in traversing Network address translators In computer networking, network address translation is the process of modifying network address information in datagram (IP) packet headers while in transit across a traffic routing device for the purpose of remapping a given address space into another (NAT). The NAT device must alter these values, so that they contain the IP address of the NAT-ed client, and a port chosen by the NAT device for the data connection. The new address and port will probably differ in length in their decimal representation from the original address and port. Such translation is not usually performed in most NAT devices, but special application layer gateways In the context of computer networking, an application-level gateway consists of a security component that augments a firewall or NAT employed in a computer network. It allows customized NAT traversal filters to be plugged into the gateway to support address and port translation for certain application layer "control/data" protocols such exist for this purpose.

FTP over SSH (not SFTP)

FTP over SSH (not SFTP) refers to the practice of tunneling a normal FTP session over an SSH Secure Shell or SSH is a network protocol that allows data to be exchanged using a secure channel between two networked devices. Used primarily on GNU/Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for Telnet and other insecure remote shells, which send information, notably passwords, in plaintext, connection.

Because FTP uses multiple TCP The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite (the other being Internet Protocol, or IP), so the entire suite is commonly referred to as TCP/IP. Whereas IP handles lower-level transmissions from computer to computer as a message makes its way 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 Confidentiality is an ethical principle associated with several professions . In ethics, and (in some places) in law and alternative forms of legal dispute resolution such as mediation, some types of communication between a person and one of these professionals are "privileged" and may not be discussed or divulged to third parties. In, integrity protection Data integrity is a term used in computer science and telecommunications that can mean ensuring data is "whole" or complete, the condition in which data is identically maintained during any operation , the preservation of data for their intended use, or, relative to specified operations, the a priori expectation of data quality. Put, etc.

Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, and monitor and rewrite FTP control channel messages and autonomously open new forwardings for FTP data channels. Version 3 of SSH Communications Security SSH Communications Security is a Finnish company that is based in Helsinki and was founded by Tatu Ylönen in 1995. It is known as the original developer of the Secure Shell protocol and it currently has about 80 employees. In 2006 its revenue numbered 9,5 million euros while the loss was 0,2 million. The company's CEO is Jari Mielonen. The's software suite, the GPL The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU project licensed FONC, and Co:Z FTPSSH Proxy are three software packages that support this mode.

FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as with SSL/TLS (FTPS FTPS should not be confused with the SSH File Transfer Protocol , an incompatible secure file transfer subsystem for the Secure Shell (SSH) protocol. It is also different from Secure FTP, the practice of tunneling FTP through an SSH connection). Other methods of transferring files using SSH that are not related to FTP include SFTP In computing, the SSH File Transfer Protocol is a network protocol that provides file access, file transfer, and file management functionality over any reliable data stream. It was designed by the Internet Engineering Task Force (IETF) as an extension of the Secure Shell protocol (SSH) version 2.0 to provide secure file transfer capability, but is and SCP Secure Copy or SCP is a means of securely transferring computer files between a local and a remote host or between two remote hosts, using the Secure Shell protocol; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol.

See also

• Comparison of FTP client software The following tables compare general and technical information for a number of FTP clients, and related clients that use other file transfer protocols. Please see the individual products' articles for further information. This article is not all-inclusive or necessarily up-to-date. Unless otherwise specified in footnotes, comparisons are based on
• Comparison of FTP server software
• Curl-loader curl-loader is an open-source software performance testing tool written in the C programming language. It is capable of simulating application behavior of hundreds of thousands of HTTP/HTTPS and FTP/FTPS clients, each with its own source IP-address. In contrast to other tools curl-loader is using real C-written client protocol stacks, namely, HTTP - FTP/S loading/testing open-source SW
• File eXchange Protocol File eXchange Protocol and (FXSP) is a method of data transfer which uses the FTP protocol to transfer data from one remote server to another (inter-server) without routing this data through the client's connection. Conventional FTP involves a single server and a single client; all data transmission is done between these two. In the FXP session, a (FXP)
• FTAM FTAM, ISO standard 8571, is an OSI Application layer protocol for File Transfer Access and Management
• FTPFS FTPFS refers to file systems that support access to a File Transfer Protocol server through standard file system application programming interfaces (APIs)
• List of file transfer protocols
• List of FTP commands Below is a list of FTP commands that may be sent to an FTP host, including all commands that are standardized in RFC 959 by the IETF. All commands below are RFC 959 based unless stated otherwise. These commands differ in use between clients. For example, GET is used instead of RETR, but most clients parse this into the proper command. In this, GET
• List of FTP server return codes
• Managed File Transfer Managed file transfer refers to software solutions that facilitate the secure transfer of data from one computer to another through a network (e.g., the Internet). MFT solutions are often built using the FTP network protocol[citation needed]. However, the term specifically describes solutions that remedy the disadvantages associated with FTP
• OBEX
• Shared file access
• TCP Wrapper

References

1. ^ ^{a b c} Forouzan, B.A. (2000). TCP/IP: Protocol Suite. 1st ed. New Delhi, India: Tata McGraw-Hill Publishing Company Limited.
2. ^ ^{a b c d} Clark, M.P. (2003). Data Networks IP and the Internet. 1st ed. West Sussex, England: John Wiley & Sons Ltd.
3. ^ ^{a b} Postel, J., & Reynolds. J. (October 1985). RFC 959. In The Internet Engineering Task Force. Retrieved from http://www.ietf.org/rfc/rfc0959.txt
4. ^ ^{a b} Kurose, J.F. & Ross, K.W. (2010). Computer Networking. 5th ed. Boston, MA: Pearson Education, Inc.
5. ^ Allman, M. & Metz, C. & Ostermann, S. (September 1998). RFC 2428. In The Internet Engineering Task Force. Retrieved from http://www.ietf.org/rfc/rfc2428.txt
6. ^ Allman, M. & Ostermann, S. (May 1999). RFC 2577. In The Internet Engineering Task Force. Retrieved from http://www.ietf.org/rfc/rfc2577.txt
7. ^ Deutsch, P. & Emtage, A. & Marine, A. (May 1994). RFC 1635. In The Internet Engineering Task Force. Retrieved from http://www.ietf.org/rfc/rfc1635.txt
8. ^ Matthews, J. (2005). Computer Networking: Internet Protocols in Action. 1st ed. Danvers, MA: John Wiley & Sons Inc.
9. ^ ^{a b} Berners-Lee, T. & Masinter, L. & McCahill, M. (December 1994). RFC 1738. In The Internet Engineering Task Force. Retrieved from http://www.ietf.org/rfc/rfc1738.txt

Further reading

• RFC 959 – (Standard) File Transfer Protocol (FTP). J. Postel, J. Reynolds. October 1985.
• RFC 1579 – (Informational) Firewall-Friendly FTP.
• RFC 2228 – (Proposed Standard) FTP Security Extensions.
• RFC 2389 – (Proposed Standard) Feature negotiation mechanism for the File Transfer Protocol. August 1998.
• RFC 2428 – (Proposed Standard) Extensions for IPv6, NAT, and Extended passive mode. September 1998.
• RFC 2640 – (Proposed Standard) Internationalization of the File Transfer Protocol.
• RFC 3659 – (Proposed Standard) Extensions to FTP. P.Hethmon. March 2007.
• RFC 5797 – (Proposed Standard) FTP Command and Extension Registry. March 2010.

External links

• FTP Reviewed — a review of the protocol notably from a security standpoint, pintday.org
• Raw FTP command list, nsftools.com
• FTP Sequence Diagram (in PDF format), eventhelix.com
• FTP Server Connectivity Test, infobyip.com

Categories: Clear text protocols | Network file transfer protocols | FTP | Servers | Internet history | Application layer protocols | Internet protocols | Internet standards | Unix network-related software

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