File Transfer Protocol
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File Transfer Protocol (FTP) is a standard network protocol used to copy a file from one host to another over a TCP/IP-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 applications, which solves the problem of different end host configurations (i.e., Operating Systems, 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.
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[edit] 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 and defines a new type of passive mode. [2]
[edit] Protocol overview
The protocol is specified in RFC 959, which is summarized below.[3]
A client makes a connection to the server on TCP 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[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 and 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 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 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 or binary coded decimal form.
- Image mode (commonly called Binary mode): the sending machine sends each file byte for byte and as such 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 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.
- 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).
[edit] Security
The original FTP specification has many security concerns. In May 1999, the following flaws were addressed[6]:
- Bounce Attacks
- Spoof Attacks
- Brute Force Attacks
- 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, such as HTTP, SMTP and Telnet[2]. The common solution to this problem is to use either SFTP (SSH File Transfer Protocol), or FTPS (FTP over SSL), which adds SSL or TLS encryption to FTP as specified in RFC 4217.
[edit] 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.
[edit] 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.
[edit] Web browser support
Most recent 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. Firefox has a full-featured FTP client 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.
[edit] 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 (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 exist for this purpose.
[edit] 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.
[edit] 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 commands
- List of FTP server return codes
- Managed File Transfer
- OBEX
- Shared file access
- TCP Wrapper
[edit] 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. Retrieved from http://www.ietf.org/rfc/rfc0959.txt
- ^ 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. Retrieved from http://www.ietf.org/rfc/rfc2428.txt
- ^ 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
[edit] 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.
[edit] External links
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Wikibooks has a book on the topic of |
- 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
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