File Transfer Protocol (FTP)

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.[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 and were not very user-friendly, despite implementing standard commands and syntax. GUI (graphical user interface) clients, which are much easier to learn and use, have been developed for many of the popular desktop operating systems in use today.

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

Protocol overview

The protocol is specified in RFC 959, which is summarized below.

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, opened by the server from its port 20 to a client port (specified in the negotiation dialog) 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 (i.e., ) or needed parameters (i.e., ).[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 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 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 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.
Compressed mode: Data is compressed using a single algorith

Security

The original FTP specification has many security concerns. In May 1999, the following flaws were addressed:
Bounce Attacks
Spoof Attacks
Brute Force Attacks
Sniffing
Username Protection
Port Stealing
This has no encrytion tool

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.m (usually Run-length encoding).

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. As most internet users these days have ready access to FTP, this procedure is no longer in everyday use.

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 layer gateways 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 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.

http://en.wikipedia.org/wiki/File_Transfer_Protocol


One response to “File Transfer Protocol (FTP)”

  1. Howard Kroymann May 21st, 2013 at 9:39 pm
    | Reply

    I use plain old Windows 7 FTP to upload files and have discovered that it will not function with today’s mifi devices. I gather that I need to run in Passive mode and Windows FTP does not support this. Would passive-ftp substitute for Windows FTP? Does it understand the commands we have in the scripts we use with Windows FTP

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