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FTP Commands Overview

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FTP COMMANDS:

file

An ordered set of computer data (including programs), of arbitrary length, uniquely identified by a pathname.

mode

The mode in which data is to be transferred via the data connection.The mode defines the data format during transfer including EOR and EOF.

NVT

The Network Virtual Terminal as defined in the Telnet Protocol.

page

A file may be structured as a set of independent parts called pages.FTP supports the transmission of discontinuous files as independent indexed pages.

pathname

Pathname is defined to be the character string which must be input to a file system by a user in order to identify a file. Pathname normally contains device and/or directory names, and file name specification. FTP does not yet specify a standard pathname convention.  Each user must follow the file naming conventions of the file systems involved in the transfer.

PI

The protocol interpreter.  The user and server sides of the protocol have distinct roles implemented in a user-PI and a server-PI.

record

A sequential file may be structured as a number of contiguous parts called records.  Record structures are supported by FTP but a file need not have record structure.

reply

A reply is an acknowledgment (positive or negative) sent from server to user via the control connection in response to FTP commands.  The general form of a reply is a completion code (including error codes) followed by a text string.  The codes are for use by programs and the text is usually intended for human users.

server-DTP

The data transfer process, in its normal "active" state, establishes the data connection with the "listening" data port. It sets up parameters for transfer and storage, and transfers data on command from its PI.  The DTP can be placed in a "passive" state to listen for, rather than initiate a connection on the data port.

server-FTP process

A process or set of processes which perform the function of file transfer in cooperation with a user-FTP process and, possibly, another server.  The functions consist of a protocol interpreter (PI) and a data transfer process (DTP).

server-PI

The server protocol interpreter "listens" on Port L for a connection from a user-PI and establishes a control communication connection.  It receives standard FTP commands from the user-PI, sends replies, and governs the server-DTP.

type

The data representation type used for data transfer and storage.  Type implies certain transformations between the time of data storage and data transfer. 

user

A person or a process on behalf of a person wishing to obtain file transfer service.  The human user may interact directly with a server-FTP process, but use of a user-FTP process is preferred since the protocol design is weighted towards automata.

user-DTP

The data transfer process "listens" on the data port for a connection from a server-FTP process.  If two servers are transferring data between them, the user-DTP is inactive.

user-FTP process

A set of functions including a protocol interpreter, a data transfer process and a user interface which together perform the function of file transfer in cooperation with one or more server-FTP processes.  The user interface allows a local language to be used in the command-reply dialogue with the user.

user-PI

The user protocol interpreter initiates the control connection from its port U to the server-FTP process, initiates FTP commands, and governs the user-DTP if that process is part of the file transfer.

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The following commands specify access control identifiers (command codes are shown in parentheses).

USER NAME (USER)

The argument field is a Telnet string identifying the user.

The user identification is that which is required by the server for access to its file system.  This command will normally be the first command transmitted by the user after the control connections are made (some servers may require this).  Additional identification information in the form of a password and/or an account command may also be required by some servers. 

Servers may allow a new USER command to be entered at any point in order to change the access control and/or accounting information.  This has the effect of flushing any user, password, and account information already supplied and beginning the login sequence again.  All transfer parameters are unchanged and any file transfer in progress is completed under the old access control parameters

PASSWORD (PASS)

The argument field is a Telnet string specifying the user's password.  This command must be immediately preceded by the user name command, and, for some sites, completes the user's identification for access control. Since password information is quite sensitive, it is desirable in general to "mask" it or suppress type out. It appears that the server has no foolproof way to achieve this. It is therefore the responsibility of the user-FTP process to hide the sensitive password information.

ACCOUNT (ACCT)

The argument field is a Telnet string identifying the user's account.  The command is not necessarily related to the USER command, as some sites may require an account for login and others only for specific access, such as storing files. In the latter case the command may arrive at any time.

There are reply codes to differentiate these cases for the automation: when account information is required for login, the response to a successful PASSword command is reply code 332.  On the other hand, if account information is NOT required for login, the reply to a successful PASSword command is 230; and if the account information is needed for a command issued later in the  dialogue, the server should return a 332 or 532 reply depending on whether it stores (pending receipt of the ACCounT command) or discards the command, respectively.

CHANGE WORKING DIRECTORY (CWD)

This command allows the user to work with a different directory or dataset for file storage or retrieval without altering his login or accounting information.  Transfer parameters are similarly unchanged.  The argument is a pathname specifying a directory or other system dependent file group designator.

CHANGE TO PARENT DIRECTORY (CDUP)

This command is a special case of CWD, and is included to simplify the implementation of programs for transferring directory trees between operating systems having different syntaxes for naming the parent directory. The reply codes shall be identical to the reply codes of CWD.

STRUCTURE MOUNT (SMNT)

This command allows the user to mount a different file system data structure without altering his login or accounting information. Transfer parameters are similarly unchanged.  The argument is a pathname specifying a directory or other system dependent file group designator.

REINITIALIZE (REIN)

This command terminates a USER, flushing all I/O and account information, except to allow any transfer in progress to be completed. All parameters are reset to the default settings and the control connection is left open. This is identical to the state in which a user finds himself immediately after the control connection is opened. A USER command may be expected to follow.

LOGOUT (QUIT)

This command terminates a USER and if file transfer is not in progress, the server closes the control connection.  If file transfer is in progress, the connection will remain open for result response and the server will then close it.

If the user-process is transferring files for several USERs but does not wish to close and then reopen connections for each, then the REIN command should be used instead of QUIT. An unexpected close on the control connection will cause the server to take the effective action of an abort (ABOR) and a logout (QUIT).

 

TRANSFER PARAMETER COMMANDS

DATA PORT (PORT)

 

The argument is a HOST-PORT specification for the data port to be used in data connection.  There are defaults for both the user and server data ports, and under normal circumstances this command and its reply are not needed.  If this command is used, the argument is the concatenation of a 32-bit internet host address and a 16-bit TCP port address. This address information is broken into 8-bit fields and the value of each field is transmitted as a decimal number (in character string  representation). The fields are separated by commas.  A port command would be:

               PORT h1,h2,h3,h4,p1,p2

where h1 is the high order 8 bits of the internet host address.

PASSIVE (PASV)

This command requests the server-DTP to "listen" on a data port (which is not its default data port) and to wait for a connection rather than initiate one upon receipt of a transfer command. The response to this command includes the host and port address this server is listening on.

REPRESENTATION TYPE (TYPE)

The argument specifies the representation type

FILE STRUCTURE (STRU)

The argument is a single Telnet character code specifying file structure described in the Section on Data Representation and Storage.

The following codes are assigned for structure:

               F - File (no record structure)

               R - Record structure

               P - Page structure

The default structure is File.

TRANSFER MODE (MODE)

The argument is a single Telnet character code specifying the data transfer modes described in the Section on Transmission Modes.

The following codes are assigned for transfer modes:

               S - Stream

               B - Block

               C - Compressed

The default transfer mode is Stream.

FTP SERVICE COMMANDS:

The FTP service commands define the file transfer or the file system function requested by the user. The argument of an FTP service command will normally be a pathname.  The syntax of pathnames must confirm to server site conventions (with standard defaults applicable), and the language conventions of the control connection.  The suggested default handling is to use the last specified device, directory or file name, or the standard default defined for local users.  The commands may be in any order except that a "rename from" command must be followed by a "rename to" command and the restart command must be followed by the interrupted service command (e.g., STOR or RETR). The data, when transferred in response to FTP service commands, shall always be sent over the data connection, except for certain informative replies.  The following commands specify FTP service requests:

RETRIEVE (RETR)

This command causes the server-DTP to transfer a copy of the file, specified in the pathname, to the server- or user-DTP at the other end of the data connection.  The status and contents of the file at the server site shall be unaffected.

STORE (STOR)

This command causes the server-DTP to accept the data transferred via the data connection and to store the data as a file at the server site. If the file specified in the pathname exists at the server site, then its contents shall be replaced by the data being transferred. A new file is created at the server site if the file specified in the pathname does not already exist.

STORE UNIQUE (STOU)

 

This command behaves like STOR except that the resultant file is to be created in the current directory under a name unique to that directory.  The 250 Transfer Started response must include the name generated.

APPEND (with create) (APPE)

This command causes the server-DTP to accept the data transferred via the data connection and to store the data in  a file at the server site.  If the file specified in the pathname exists at the server site, then the data shall be appended to that file; otherwise the file specified in the pathname shall be created at the server site

ALLOCATE (ALLO)

This command may be required by some servers to reserve sufficient storage to accommodate the new file to be transferred.  The argument shall be a decimal integer representing the number of bytes (using the logical byte size) of storage to be reserved for the file.  For files sent with record or page structure a maximum record or page size (in logical bytes) might also be necessary; this is indicated by a decimal integer in a second argument field of the command. 

This second argument is optional, but when present should be separated from the first by the three Telnet characters <SP> R <SP>. 

This command shall be followed by a STORe or APPEnd command.The ALLO command should be treated as a NOOP (no operation) by those servers which do not require that the maximum size of the file be declared beforehand, and those servers interested in only the maximum record or page size should accept a dummy value in the first argument and ignore it.

RESTART (REST)

 

The argument field represents the server marker at which file transfer is to be restarted.  This command does not cause file transfer but skips over the file to the specified data checkpoint. This command shall be immediately followed by the appropriate FTP service command which shall cause file transfer to resume.

RENAME FROM (RNFR)

This command specifies the old pathname of the file which is to be renamed.  This command must be immediately followed by a "rename to" command specifying the new file pathname.

RENAME TO (RNTO)

This command specifies the new pathname of the file specified in the immediately preceding "rename from" command.Together the two commands cause a file to be renamed.

ABORT (ABOR)

This command tells the server to abort the previous FTP service command and any associated  transfer of data. The abort command may require "special action", to force recognition by the   server. No action is to be taken if the previous command has been completed (including data transfer).

The control connection is not to be closed by the server, but the data connection must be closed.

There are two cases for the server upon receipt of this command:

  • (1) the FTP service command was already completed, or
  • (2) the FTP service command is still in progress.

In the first case, the server closes the data connection (if it is open) and responds with a 226 reply, indicating that the abort command was successfully processed.

In the second case, the server aborts the FTP service in progress and closes the data connection, returning a 426 reply to indicate that the service request terminated abnormally.  The server then sends a 226 reply, indicating that the abort command was successfully processed.

DELETE (DELE)

This command causes the file specified in the pathname to be deleted at the server site.  If an extra level of protection is desired (such as the query, "Do you really wish to delete?"), it should be provided by the user-FTP process

REMOVE DIRECTORY (RMD)

This command causes the directory specified in the pathname to be removed as a directory (if the pathname is absolute) or as a subdirectory of the current working directory (if the pathname is relative). 

MAKE DIRECTORY (MKD)

This command causes the directory specified in the pathname to be created as a directory (if the pathname is absolute) or as a subdirectory of the current working directory (if the pathname is relative).

PRINT WORKING DIRECTORY (PWD)

This command causes the name of the current working directory to be returned in the reply. 

LIST (LIST)

This command causes a list to be sent from the server to the passive DTP.  If the pathname specifies a directory or other group of files, the server should transfer a list of files in the specified directory.  If the pathname specifies a file then the server should send current information on the file.  A null argument implies the user's current working or default directory.  The data transfer is over the data connection in type ASCII or type EBCDIC.  (The user must ensure that the TYPE is appropriately ASCII or EBCDIC).

Since the information on a file may vary widely from system to system, this information may be hard to use automatically in a program, but may be quite useful to a human user.

NAME LIST (NLST)

This command causes a directory listing to be sent from server to user site.  The pathname should specify a directory or other system-specific file group descriptor; a null argument implies the current directory.  The server will return a stream of names of files and no other information. The data will be transferred in ASCII or EBCDIC type over the data connection as valid pathname strings separated by <CRLF> or <NL>.  (Again the user must ensure that the TYPE is correct.)  This command is intended to return information that can be used by a program to further process the files automatically. For example, in the implementation of a "multiple get" function.

SITE PARAMETERS (SITE)

This command is used by the server to provide services specific to his system that are essential to file transfer but not sufficiently universal to be included as commands in the protocol.  The nature of these services and the specification of their syntax can be stated in a reply to the HELP SITE command.

SYSTEM (SYST)

This command is used to find out the type of operating system at the server.  The reply shall have as its first word one of the system names listed in the current version of the Assigned Numbers document.

STATUS (STAT)

This command shall cause a status response to be sent over the control connection in the form of a reply. The command may be sent during a file transfer (along with the Telnet IP and Synch signals--see the Section on FTP Commands) in which case the server will respond with the status of the operation in progress, or it may be sent between file transfers. In the latter case, the command may have an argument field. If the argument is a pathname, the command analogous to the "list" command except that data shall be transferred over the control connection.  If a partial pathname is given, the server may respond with a list of file names or attributes associated with that specification. If no argument is given, the server should return general status information  about the server FTP process.  This should include current values of all transfer parameters and the status of connections.

HELP (HELP)

 

This command shall cause the server to send helpful information regarding its implementation status over the control connection to the user.  The command may take an argument (e.g., any command name) and return more specific information as a response.  The reply is type 211 or 214. It is suggested that HELP be allowed before entering a USER command. The server may use this reply to specify site-dependent parameters, e.g., in response to HELP SITE.

NOOP (NOOP)

This command does not affect any parameters or previously entered commands. It specifies no action other than that the server send an OK reply

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