CS 596 Client-Server Programming
FTP Protocol
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San Diego State University -- This page last updated April 11, 1996
Contents of FTP Protocol Lecture
- FTP Protocol
- Data Transfer Issues
- Protocol Commands
- ACCESS CONTROL COMMANDS
- TRANSFER PARAMETER COMMANDS
- FTP SERVICE COMMANDS
- Establishing Data Connection
Reference:
- File Transfer Protocol. J. Postel, J.K. Reynolds. Oct-01-1985, rtf
959
Port Numbers
- 20 default data
- 21 for control
Terms
These definitions are taken directly from rfc 959
control connection
- The communication path between the USER-PI and SERVER-PI for the exchange
of commands and replies.
-
- This connection follows the Telnet Protocol.
data connection
- A full duplex connection over which data is transferred, in a specified
mode and type.
-
- The data transferred may be a part of a file, an entire file or a number of
files.
-
- The path may be between a server-DTP and a user-DTP, or between two
server-DTPs.
data port
- The passive data transfer process "listens" on the data port for a
connection from the active transfer process in order to open the data
connection.
DTP
- The data transfer process establishes and manages the data connection.
-
- The DTP can be passive or active
PI
- The protocol interpreter.
-
- The user and server sides of the protocol have distinct roles implemented
in a user-PI and a server-PI.
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.
user
- A person or a process on behalf of a person wishing to obtain file transfer
service.
Motivation for Two Connections
Data is represented differently on different machines
- ASCII verses EBCDIC
-
- Word sizes can differ
- 32 bits
- 36 bits
-
- Byte order can be different
-
- End-of-line character(s) can differ
Binary data of various formats
Basic Client-Server Interaction
Client sends command to server
Server always returns a reply
Command and reply are on command connection
Commands are not case sensitive
reply consists of:
- three digit code <sp> text <telnet end of line>
- three digit code indicates status to program
-
- text for humans
Multi-line replies
- three digit code-text <telnet end of line>
- second line
- etc.
- three digit code <sp> whatever
No data is sent on the command connection between client and server
Details from rfc 959
Replies to File Transfer Protocol commands are devised to ensure the
synchronization of requests and actions in the process of file transfer, and to
guarantee that the user process always knows the state of the Server. Every
command must generate at least one reply, although there may be more than one;
in the latter case, the multiple replies must be easily distinguished. In
addition, some commands occur in sequential groups, such as USER, PASS and
ACCT, or RNFR and RNTO. The replies show the existence of an intermediate
state if all preceding commands have been successful. A failure at any point in
the sequence necessitates the repetition of the entire sequence from the
beginning.
An FTP reply consists of a three digit number (transmitted as three
alphanumeric characters) followed by some text. The number is intended for use
by automata to determine what state to enter next; the text is intended for the
human user. It is intended that the three digits contain enough encoded
information that the user-process (the User-PI) will not need to examine the
text and may either discard it or pass it on to the user, as appropriate. In
particular, the text may be server-dependent, so there are likely to be varying
texts for each reply code.
A reply is defined to contain the 3-digit code, followed by Space <SP>,
followed by one line of text (where some maximum line length has been
specified), and terminated by the Telnet end-of-line code. There will be cases
however, where the text is longer than a single line. In these cases the
complete text must be bracketed so the User-process knows when it may stop
reading the reply (i.e. stop processing input on the control connection) and go
do other things. This requires a special format on the first line to indicate
that more than one line is coming, and another on the last line to designate it
as the last. At least one of these must contain the appropriate reply code to
indicate the state of the transaction. To satisfy all factions, it was decided
that both the first and last line codes should be the same.
Thus the format for multi-line replies is that the first line will begin with
the exact required reply code, followed immediately by a Hyphen, "-" (also
known as Minus), followed by text. The last line will begin with the same
code, followed immediately by Space <SP>, optionally some text, and the
Telnet end-of-line code.
For example:
- 123-First line
- Second line
- 234 A line beginning with numbers
- 123 The last line
FTP Protocol Commands
USER <SP> <username> <CRLF>
PASS <SP> <password> <CRLF>
ACCT <SP> <account-information> <CRLF>
CWD <SP> <pathname> <CRLF>
CDUP <CRLF>
SMNT <SP> <pathname> <CRLF>
QUIT <CRLF>
REIN <CRLF>
PORT <SP> <host-port> <CRLF>
PASV <CRLF>
TYPE <SP> <type-code> <CRLF>
STRU <SP> <structure-code> <CRLF>
MODE <SP> <mode-code> <CRLF>
RETR <SP> <pathname> <CRLF>
STOR <SP> <pathname> <CRLF>
STOU <CRLF>
APPE <SP> <pathname> <CRLF>
ALLO <SP> <decimal-integer>
[<SP> R <SP> <decimal-integer>] <CRLF>
REST <SP> <marker> <CRLF>
RNFR <SP> <pathname> <CRLF>
RNTO <SP> <pathname> <CRLF>
ABOR <CRLF>
DELE <SP> <pathname> <CRLF>
RMD <SP> <pathname> <CRLF>
MKD <SP> <pathname> <CRLF>
PWD <CRLF>
LIST [<SP> <pathname>] <CRLF>
NLST [<SP> <pathname>] <CRLF>
SITE <SP> <string> <CRLF>
SYST <CRLF>
STAT [<SP> <pathname>] <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
Note following descriptions are from the rfc 595
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.
CHANGE WORKING DIRECTORY (CWD)
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.
REPRESENTATION TYPE (TYPE)
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).
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:
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)
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)
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.
ABORT (ABOR)
This command tells the server to abort the previous FTP service command and
any associated transfer of data.
DELETE (DELE)
REMOVE DIRECTORY (RMD)
MAKE DIRECTORY (MKD)
PRINT WORKING DIRECTORY (PWD)
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 [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.
Sample FTP Protocol Session
rohan 13-> telnet saturn 21
Trying 130.191.229.1...
Connected to saturn.sdsu.edu.
Escape character is '^]'.
220 saturn FTP server (SunOS 4.1) ready.
user whitney
331 Password required for whitney.
pass getRealMan;
230 User whitney logged in.
ls
500 'LS': command not understood.
list
425 Can't build data connection: Connection refused.
cd ..
500 'CD ..': command not understood.
cwd ..
250 CWD command successful.
Sample Multi-Line Response
help
214-The following commands are recognized (* =>'s unimplemented).
USER PORT RETR MSND* ALLO DELE SITE* XMKD CDUP
PASS PASV STOR MSOM* REST* CWD STAT* RMD XCUP
ACCT* TYPE APPE MSAM* RNFR XCWD HELP XRMD STOU
REIN* STRU MLFL* MRSQ* RNTO LIST NOOP PWD
QUIT MODE MAIL* MRCP* ABOR NLST MKD XPWD
214 Direct comments to bugs@Sun.COM.
Sample FTP Client-User Interaction
rohan 14-> ftp saturn
Connected to saturn.sdsu.edu.
220 saturn FTP server (SunOS 4.1) ready.
Name (saturn:whitney):<return>
331 Password required for whitney.
Password:
230 User whitney logged in.
ftp>
ls
200 PORT command successful.
150 ASCII data connection for /bin/ls (130.191.143.100,36027) (0 bytes).
...
mbox
paragon
public_html
smalltalk
226 ASCII Transfer complete.
585 bytes received in 0.14 seconds (4.2 Kbytes/s)
ftp>
cd ..
250 CWD command successful.
FTP Client Commands
ftp> help
Commands may be abbreviated. Commands are:
! cr macdef proxy send
$ delete mdelete sendport status
account debug mdir put struct
append dir mget pwd sunique
ascii disconnect mkdir quit tenex
bell form mls quote trace
binary get mode recv type
bye glob mput remotehelp user
case hash nmap rename verbose
cd help ntrans reset ?
cdup lcd open rmdir
close ls prompt runique
Server Default Data Port: 20
Client Default Data Port: same port used for command connection
Client can request server use different port for server data connection with
PASV command
Client can tell server to use different port for client data connection via
PORT command
User agent "listens" on data port
Server connects to user agents data port, starts data transmission
No error checking is done by FTP
Transmission Modes
Stream Mode
Passes the data with little or no processing
If sending files send raw bytes, EOF is indicated by closing connection
If sending records, EOR and EOF are indicated by two bytes of control
information
- Escape byte is all ones
-
- EOR is 1
-
- EOF is 2
-
- If data contains escape byte, repeat the escape byte
Block Mode
Data is sent in blocks
Allows for marking data for restarts of data transmission
Can mark original data as "suspect"
Compressed Mode
Does some simple data compression
