FTP Commands
4.1.1. ACCESS CONTROL COMMANDS
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 typeout. 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.
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. See
Appendix II for further details.
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).
All data transfer parameters have default values, and the
commands specifying data transfer parameters are required only
if the default parameter values are to be changed. The default
value is the last specified value, or if no value has been
specified, the standard default value is as stated here. This
implies that the server must "remember" the applicable default
values. The commands may be in any order except that they must
precede the FTP service request. The following commands
specify data transfer parameters:
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.
The argument specifies the representation type as described
in the Section on Data Representation and Storage. Several
types take a second parameter. The first parameter is
denoted by a single Telnet character, as is the second
Format parameter for ASCII and EBCDIC; the second parameter
for local byte is a decimal integer to indicate Bytesize.
The parameters are separated by a <SP> (Space, ASCII code
32).
The following codes are assigned for type:
\ /
A - ASCII | | N - Non-print
|-><-| T - Telnet format effectors
E - EBCDIC| | C - Carriage Control (ASA)
/ \
I - Image
L <byte size> - Local byte Byte size
The default representation type is ASCII Non-print. If the
Format parameter is changed, and later just the first
argument is changed, Format then returns to the Non-print
default.
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.
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 conform 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:
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", as discussed in
the Section on FTP Commands, 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). See Appendix II.
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). See Appendix II.
PRINT WORKING DIRECTORY (PWD)
This command causes the name of the current working
directory to be returned in the reply. See Appendix II.
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.
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 [4].
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
is 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.
The File Transfer Protocol follows the specifications of the Telnet
protocol for all communications over the control connection. Since
the language used for Telnet communication may be a negotiated
option, all references in the next two sections will be to the
"Telnet language" and the corresponding "Telnet end-of-line code".
Currently, one may take these to mean NVT-ASCII and <CRLF>. No other
specifications of the Telnet protocol will be cited.
FTP commands are "Telnet strings" terminated by the "Telnet end of
line code". The command codes themselves are alphabetic characters
terminated by the character <SP> (Space) if parameters follow and
Telnet-EOL otherwise. The command codes and the semantics of
commands are described in this section; the detailed syntax of
commands is specified in the Section on Commands, the reply sequences
are discussed in the Section on Sequencing of Commands and Replies,
and scenarios illustrating the use of commands are provided in the
Section on Typical FTP Scenarios.
FTP commands may be partitioned as those specifying access-control
identifiers, data transfer parameters, or FTP service requests.
Certain commands (such as ABOR, STAT, QUIT) may be sent over the
control connection while a data transfer is in progress. Some
servers may not be able to monitor the control and data connections
simultaneously, in which case some special action will be necessary
to get the server's attention. The following ordered format is
tentatively recommended:
1. User system inserts the Telnet "Interrupt Process" (IP) signal
in the Telnet stream.
2. User system sends the Telnet "Synch" signal.
3. User system inserts the command (e.g., ABOR) in the Telnet
stream.
4. Server PI, after receiving "IP", scans the Telnet stream for
EXACTLY ONE FTP command.
(For other servers this may not be necessary but the actions listed
above should have no unusual effect.)