"FTP" redirects here. For other uses, see FTP (disambiguation). This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (March 2010) Internet Protocol Suite Application Layer BGP · DHCP · DNS · FTP · HTTP · IMAP · IRC · LDAP · MGCP · NNTP · NTP · POP · RIP · RPC · RTP · SIP · SMTP · SNMP · SSH · Telnet · TLS/SSL · XMPP · (more) Transport Layer TCP · UDP · DCCP · SCTP · RSVP · ECN · (more) Internet Layer IP (IPv4, IPv6) · ICMP · ICMPv6 · IGMP · IPsec · (more) Link Layer ARP/InARP · NDP · OSPF · Tunnels (L2TP) · PPP · Media Access Control (Ethernet, DSL, ISDN, FDDI) · (more) v · d · e File Transfer Protocol (FTP) is a standard network protocol used to copy a file from one host to another over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server.1 FTP users may authenticate themselves using a clear-text sign-in protocol but can connect anonymously if the server is configured to allow it. The first FTP client applications were interactive command-line tools, implementing standard commands and syntax. Graphical user interface clients have since been developed for many of the popular desktop operating systems in use today. Contents 1 History 2 Protocol overview 3 Security 4 Anonymous FTP 5 Remote FTP or FTPmail 6 Web browser support 7 NAT and firewall traversal 8 Secure FTP 8.1 FTPS (explicit) 8.2 FTPS (implicit) 8.3 SFTP 8.4 FTP over SSH (not SFTP) 9 List of FTP commands 10 See also 11 References 12 Further reading 13 External links // History

The original specification for the File Transfer Protocol was written by Abhay Bhushan and 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 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 TCP connection to the server's port 21. This connection, called the control connection, remains open for the duration of the session, with a second connection, called the data connection, either opened by the server from its port 20 to a negotiated client port (active mode) or opened by the client from an arbitrary port to a negotiated server port (passive mode) as required to transfer file data. The control connection is used for session administration (i.e., commands, identification, passwords)4 exchanged between the client and server using a telnet-like protocol. For example "RETR filename" would transfer the specified file from the server to the client. Due to this two-port structure, FTP is considered an out-of-band, as opposed to an in-band protocol such as HTTP.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 (e.g., <OK>) or needed parameters (e.g., <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 or passive mode, which determine how the data connection is established. In active mode, the client sends the server the IP address and port number on which the client will listen, and the server initiates the TCP connection. 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 sends a PASV command to the server and receives an IP address and port number in return. The client uses these to open the data connection to the server.3 Both modes were updated in September 1998 to add support for IPv6. Other changes were made to passive mode at that time, making it extended passive mode.5

While transferring data over the network, four data representations can be used2: ASCII mode: used for text. Data is converted, if needed, from the sending host's character representation to "8-bit ASCII" before transmission, and (again, if necessary) to the receiving host's character representation. As a consequence, this mode is inappropriate for files that contain data other than plain text. Image mode (commonly called Binary mode): the sending machine sends each file byte for byte, and the recipient stores the bytestream as it receives it. (Image mode support has been recommended for all implementations of FTP). EBCDIC mode: use for plain text between hosts using the EBCDIC character set. This mode is otherwise like ASCII mode. Local mode: Allows two computers with identical setups to send data in a proprietary format without the need to convert it to ASCII For text files, different format control and record structure options are provided. These features were designed to facilitate files containing Telnet or ASA formatting. Data transfer can be done in any of three modes1: Stream mode: Data is sent as a continuous stream, relieving FTP from doing any processing. Rather, all processing is left up to TCP. No End-of-file indicator is needed, unless the data is divided into records. Block mode: FTP breaks the data into several blocks (block header, byte count, and data field) and then passes it on to TCP.2 Compressed mode: Data is compressed using a single algorithm (usually run-length encoding). Security FTP was not designed to be a secure protocol—especially by today's standards—and has many security weaknesses. In May 1999, the authors of RFC 2577 enumerated the following flaws:6 Bounce attacks Spoof attacks Brute force attacks Packet capture (sniffing) Username protection Port stealing FTP was not designed to encrypt its traffic; all transmissions are in clear text, and user names, passwords, commands and data can be easily read by anyone able to perform packet capture (sniffing) on the network. This problem is common to many Internet Protocol specifications (such as SMTP, Telnet, POP and IMAP) designed prior to the creation of encryption mechanisms such as TLS or SSL.2 A common solution to this problem is use of the "secure", TLS-protected versions of the insecure protocols (e.g. FTPS for FTP, TelnetS for Telnet, etc.) or selection of a different, more secure protocol that can handle the job, such as the SFTP/SCP tools included with most implementations of the Secure Shell protocol. Anonymous FTP

A host that provides an FTP service may additionally provide anonymous 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 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. The service was used when some users' only internet access was via email through gateways such as a BBS or online service. As most internet users these days have ready access to FTP, this procedure is no longer in everyday use. Web browser support Most common web browsers can retrieve files hosted on FTP servers, although they may not support protocol extensions such as FTPS.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. A full-featured FTP client can be run within Firefox in the form of an extension called FireFTP [1] FTP URL 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 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 the representation of the IP addresses and port number in the PORT command refer to the internal host's IP address and port, rather than the public IP address and port of the NAT. There are two approaches to this problem. One is that the FTP client and FTP server use the PASV command, which causes the data connection to be established from the FTP client to the server. This is widely used by modern FTP clients. Another approach is for the NAT to alter the values of the PORT command, using an application-level gateway for this purpose. Secure FTP There are several methods of securely transferring files that have been called "Secure FTP" at one point or another. FTPS (explicit) Explicit FTPS is an extension to the FTP standard that allows clients to request that the FTP session 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. The latest definition of this protocol is RFC 4217. FTPS (implicit) Implicit FTPS is deprecated standard for FTP that required the use of a SSL or TLS connection. It was specified to use different ports than plain FTP. SFTP SFTP, the "SSH File Transfer Protocol," is not related to FTP except that it also transfers files and has a similar command set for users. FTP over SSH (not SFTP) FTP over SSH (not SFTP) refers to the practice of tunneling a normal FTP session over an SSH 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 will set up new TCP connections (data channels), which bypass the SSH connection, and thus have no confidentiality, integrity protection, 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's software suite, the GPL 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). Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol. List of FTP commands Below is a list of FTP commands that may be sent to an FTP server, including all commands that are standardized in RFC 959 by the IETF. All commands below are RFC 959 based unless stated otherwise. Note that most command-line FTP clients present their own set of commands to users. For example, GET is the common user command to download a file instead of the raw command RETR. Command RFC Description ABOR Abort an active file transfer. ACCT Account information. ADAT RFC 2228 Authentication/Security Data ALLO Allocate sufficient disk space to receive a file. APPE Append. AUTH RFC 2228 Authentication/Security Mechanism CCC RFC 2228 Clear Command Channel CDUP Change to Parent Directory. CONF RFC 2228 Confidentiality Protection Command CWD Change working directory. DELE Delete file. ENC RFC 2228 Privacy Protected Channel EPRT RFC 2428 Specifies an extended address and port to which the server should connect. EPSV RFC 2428 Enter extended passive mode. FEAT RFC 2389 Get the feature list implemented by the server. LANG RFC 2640 Language Negotiation LIST Returns information of a file or directory if specified, else information of the current working directory is returned. LPRT RFC 1639 Specifies a long address and port to which the server should connect. LPSV RFC 1639 Enter long passive mode. MDTM RFC 3659 Return the last-modified time of a specified file. MIC RFC 2228 Integrity Protected Command MKD Make directory. MLSD RFC 3659 Lists the contents of a directory if a directory is named. MLST RFC 3659 Provides data about exactly the object named on its command line, and no others. MODE Sets the transfer mode (Stream, Block, or Compressed). NLST Returns a list of file names in a specified directory. NOOP No operation (dummy packet; used mostly on keepalives). OPTS RFC 2389 Select options for a feature. PASS Authentication password. PASV Enter passive mode. PBSZ RFC 2228 Protection Buffer Size PORT Specifies an address and port to which the server should connect. PROT RFC 2228 Data Channel Protection Level. PWD Print working directory. Returns the current directory of the host. QUIT Disconnect. REIN Re initializes the connection. REST Restart transfer from the specified point. RETR Transfer a copy of the file RMD Remove a directory. RNFR Rename from. RNTO Rename to. SITE Sends site specific commands to remote server. SIZE RFC 3659 Return the size of a file. SMNT Mount file structure. STAT Returns the current status. STOR Accept the data and to store the data as a file at the server site STOU Store file uniquely. STRU Set file transfer structure. SYST Return system type. TYPE Sets the transfer mode (ASCII/Binary). USER Authentication username. List of FTP server return codes - in response to commands from a client, the FTP server returns reply codes See also Comparison of FTP client software Comparison of FTP server software Curl-loader - FTP/S loading/testing open-source SW File eXchange Protocol (FXP) File Service Protocol (FSP) FTAM FTPFS List of file transfer protocols List of FTP server return codes Managed File Transfer OBEX Shared file access TCP Wrapper References ^ 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 Clark, M.P. (2003). Data Networks IP and the Internet. 1st ed. West Sussex, England: John Wiley & Sons Ltd. ^ a b Postel, J., & Reynolds. J. (October 1985). RFC 959. In The Internet Engineering Task Force. ^ a b Kurose, J.F. & Ross, K.W. (2010). Computer Networking. 5th ed. Boston, MA: Pearson Education, Inc. ^ Allman, M. & Metz, C. & Ostermann, S. (September 1998). RFC 2428. In The Internet Engineering Task Force. ^ Allman, M. & Ostermann, S. (May 1999). RFC 2577. In The Internet Engineering Task Force. Retrieved from http://www.ietf.org/rfc/rfc2577.txt ^ 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 ^ Matthews, J. (2005). Computer Networking: Internet Protocols in Action. 1st ed. Danvers, MA: John Wiley & Sons Inc. ^ 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. RFC 697 - CWD Command of FTP RFC 959 - File Transfer Protocol (FTP) RFC 1639 - FTP Operation Over Big Address Records (FOOBAR) RFC 2228 - FTP Security Extensions RFC 2389 - Feature negotiation mechanism for the File Transfer Protocol RFC 2428 - FTP Extensions for IPv6 and NATs RFC 2640 - Internationalization of the File Transfer Protocol RFC 3659 - Extensions to FTP RFC 5797 - FTP Command and Extension Registry External links Wikibooks has a book on the topic of Communication Networks/File Transfer Protocol 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 IANA FTP Commands and Extensions registry - The official registry of FTP Commands and Extensions Raw FTP command list Basic FTP simulation v · d · eURI scheme Official aaa · aaas · acap · cap · cid · crid · data · dav · dict · dns · fax · file · ftp · geo · go · gopher · h323 · http · https · im · imap · info · ldap · mailto · mid · news · nfs · nntp · pop · pres · rtsp · sip · sips · snmp · tag · tel · telnet · urn · wais · xmpp Unofficial about · afp · aim · apt · bolo · bzr · callto · coffee · cvs · daap · ed2k · feed · fish · gg · git · gizmoproject · iax2 · irc · ircs · itms · ldaps · magnet · mms · msnim · postal2  · rsync · secondlife · skype · spotify · ssh · svn · sftp · smb · sms · steam · view-source · vzochat · webcal · winamp · wyciwyg · xfire · ymsgr Protocol list