File Transfer Protocol

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File Transfer Protocol (FTP) is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. It is often used to upload web pages and other documents from a private development machine to a public web-hosting server. FTP is built on a client-server architecture and uses separate control and data connections between the client and the server.[1] FTP users may authenticate themselves using 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.

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,[2][3] including general web design programs like Microsoft Expression Web, and specialist FTP clients such as CuteFTP.

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[edit] Protocol overview

[edit] Differences from HTTP

FTP operates on the application layer of the OSI model, and is used to transfer files using TCP/IP.[3] To do so, an FTP server has to be running and waiting for incoming requests.[3] The client computer is then able to communicate with the server on port 21.[3][4] This connection, called the control connection,[5] remains open for the duration of the session. A second connection, called the data connection,[2][5] can either be opened by the server from its port 20 to a negotiated client port (active mode), or by the client from an arbitrary port to a negotiated server port (passive mode) as required to transfer file data.[2][4] The control connection is used for session administration, for example commands, identification and passwords exchanged between the client and the server using a telnet-like protocol.[6] 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 protocol, as opposed to an in-band protocol such as HTTP.[6]

[edit] Communication and data transfer

The protocol is specified in RFC 959,[2] which is summarized here.[7]

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.

Illustration of starting a passive connection using port 21

FTP may run in active or passive mode, which determines how the data connection is established.[5] In active mode, the client sends the server an IP address and port number and then waits until the server initiates the TCP connection.[4] 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 from the server,[4][5] which the client then uses to open a data connection to the server.[7] 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.[8]

While transferring data over the network, four data representations can be used:[2][3][9]

  • 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][2]

  • 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.[9]
  • Compressed mode: Data is compressed using a single algorithm (usually run-length encoding).

[edit] Login

FTP login utilizes a 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] 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 server may authorize only limited access for such sessions.[2]

[edit] Anonymous FTP

A host that provides an FTP service may provide anonymous FTP access.[2] 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,[3] no verification is actually performed on the supplied data.[10] Many FTP hosts whose purpose is to provide software updates will provide anonymous logins.[3]

[edit] 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.[11] 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 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.[11] 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.[11]

[edit] FTPmail

Main article: FTPmail

Where FTP access is restricted, an FTPmail service can be used to circumvent the problem. An e-mail containing the FTP commands to be performed is sent to an FTPmail server, which parses the incoming e-mail, executes the requested FTP commands and sends back an e-mail with any downloaded files as attachments. This service is less flexible than an FTP client, as it is not possible to view directories interactively or to issue any modify commands. There can also be problems with large file attachments not getting through mail servers. The service was used in the days when some users' only internet access was via e-mail 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 widespread use.

[edit] Web browser support

Most common web browsers can retrieve files hosted on FTP servers, although they may not support protocol extensions such as FTPS.[3][12] 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. A full-featured FTP client can be run within Firefox in the form of an extension called FireFTP

[edit] Syntax

FTP URL syntax is described in RFC1738,[13] taking the form: ftp://[<user>[:<password>]@]<host>[:<port>]/<url-path>[13] (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 username and password may be found in the browsers' documentation, such as, for example, Firefox [14] and Internet Explorer.[15] By default, most web browsers use passive (PASV) mode, which more easily traverses end-user firewalls.

[edit] Security

FTP was not designed to be a secure protocol—especially by today's standards—and has many security weaknesses.[16] In May 1999, the authors of RFC 2577 listed a vulnerability to the following problems:[17]

FTP is not able to encrypt its traffic; all transmissions are in clear text, and usernames, passwords, commands and data can be easily read by anyone able to perform packet capture (sniffing) on the network.[2][16] 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.[9] A common solution to this problem is to use the "secure", TLS-protected versions of the insecure protocols (e.g. FTPS for FTP, TelnetS for Telnet, etc.) or 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.

[edit] Secure FTP

There are several methods of securely transferring files that have been called "Secure FTP" at one point or another.

[edit] FTPS

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. This protocol extension is defined in the proposed standard: RFC 4217. Implicit FTPS is a deprecated standard for FTP that required the use of a SSL or TLS connection. It was specified to use different ports than plain FTP.

[edit] 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. SFTP, or secure FTP, is a program that uses Secure Shell (SSH) to transfer files. Unlike standard FTP, it encrypts both commands and data, preventing passwords and sensitive information from being transmitted openly over the network. It is functionally similar to FTP, but because it uses a different protocol, you can't use a standard FTP client to talk to an SFTP server, nor can you connect to an FTP server with a client that supports only SFTP.

[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.[16] 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 or integrity protection, etc.

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 are:

FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as 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] 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 RFC 697 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
HELP Help
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 RFC 3659 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
XCUP RFC 775 Change to the parent of the current working directory
XMKD RFC 775 Make a directory
XPWD RFC 775 Print the current working directory
XRCP RFC 743
XRMD RFC 775 Remove the directory
XRSQ RFC 743
XSEM RFC 737 Send, mail if cannot
XSEN RFC 737 Send to terminal

[edit] FTP reply codes

Main article: List of FTP server return codes

Below is a summary of the reply codes that may be returned by an FTP server. These codes have been standardized in RFC 959 by the IETF. As stated earlier in this article, 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:

  • 2xx – Success reply
  • 4xx or 5xx – Failure Reply
  • 1xx or 3xx – Error or Incomplete reply

The second digit defines the kind of error:

  • x0z – Syntax. These replies refer to syntax errors.
  • x1z – Information. Replies to requests for information.
  • x2z – Connections. Replies referring to the control and data connections.
  • x3z – Authentication and accounting. Replies for the login process and accounting procedures.
  • x4z – Not defined.
  • x5z – File system. These replies relay status codes from the server file system.

The third digit of the reply code is used to provide additional detail for each of the categories defined by the second digit.

[edit] See also

[edit] 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 e f g h i j k l Kozierok, Charles M. (2005). The TCP/IP Guide v3.0, Retrieved From: http://www.tcpipguide.com/free/t_FTPOverviewHistoryandStandards.htm
  3. ^ a b c d e f g h Dean, Tamara (2010). Network+ Guide to Networks. Delmar. pp. 168–171. 
  4. ^ a b c d Parker, Don, (September 2005). Understanding the FTP Protocol. Retrieved from http://www.windowsnetworking.com/articles_tutorials/understanding-ftp-protocol.html
  5. ^ a b c d Active FTP vs. Passive FTP, a Definitive Explanation. Retrieved from http://slacksite.com/other/ftp.html
  6. ^ a b Kurose, J.F. & Ross, K.W. (2010). Computer Networking. 5th ed. Boston, MA: Pearson Education, Inc.
  7. ^ a b Postel, J., & Reynolds. J. (October 1985). RFC 959. In The Internet Engineering Task Force.
  8. ^ Allman, M. & Metz, C. & Ostermann, S. (September 1998). RFC 2428. In The Internet Engineering Task Force.
  9. ^ a b c Clark, M.P. (2003). Data Networks IP and the Internet. 1st ed. West Sussex, England: John Wiley & Sons Ltd!
  10. ^ 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
  11. ^ a b c Gleason, Mike, (2005), The File Transfer Protocol and Your Firewall/NAT. Retrieved from http://www.ncftp.com/ncftpd/doc/misc/ftp_and_firewalls.html
  12. ^ Matthews, J. (2005). Computer Networking: Internet Protocols in Action. 1st ed. Danvers, MA: John Wiley & Sons Inc.
  13. ^ 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
  14. ^ http://support.mozilla.com/en-US/kb/Accessing+FTP+servers#FTP_servers_that_require_a_username_and_password
  15. ^ http://support.microsoft.com/kb/135975
  16. ^ a b c Securing FTP using SSH. Retrieved from http://www.nurdletech.com/ftp.html
  17. ^ Allman, M. & Ostermann, S. (May 1999). RFC 2577. In The Internet Engineering Task Force. Retrieved from http://www.ietf.org/rfc/rfc2577.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.
  • RFC 697 - CWD Command of FTP
  • RFC 1639 - FTP Operation Over Big Address Records (FOOBAR)
  • RFC 5797 - FTP Command and Extension Registry

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