Charles M. Kozierok The TCP-IP Guide

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Third Reply Code Digit (“z”)

The last reply code digit indicates a specific type of message within each of the functional

groups described by the second digit. The third digit allows each functional group to have

10 different reply codes for each reply type given by the first code digit (preliminary

success, transient failure and so on.)

Combining Digit Values to Make Specific Reply Codes

These “x”, “y” and “z” digit meanings are combined to make specific reply codes. For

example, consider reply code “530”, diagrammed in Figure 292. The first digit tells you that

this is a permanent negative reply; the second indicates that it is related to login or

accounting. (It is in fact an error message received when a login fails.)

Using “encoded” reply codes allows the code itself to immediately communicate infor-

mation, and provides a way of keeping different types of responses organized. The idea

described above was adapted for use by several other application protocols, including

SMTP for e-mail, NNTP for network news and HTTP for the World Wide Web.

x4z Unspecified Not defined.

x5z File System Replies related to the server's file system.

Figure 292: FTP Reply Code Format

This diagram shows how the three-digit FTP reply code format is interpreted. In this example, reply code 530,

the “5” indicates a permanent error, the “3” specifies that the error is related to authentication or accounting,

and the “0” is the specific error type. A similar method is used for reply codes in many other TCP/IP appli-

cation protocols, including SMTP and HTTP.

Table 230: FTP Reply Code Format: Second Digit Interpretation (Page 2 of 2)

Code

Format

Meaning Description

Permanent Negative

Completion Reply

(Permanent Error)

3 0

Authentication and

Accounting Group

Error #0 Within

Functional Group

Common FTP Reply Codes

Table 231 contains a list of some of the more common FTP reply codes taken from RFC

959. They are shown in numerical order, along with the reply text given as typical in that

document, and additional descriptive information from myself where needed:

Table 231: FTP Reply Codes (Page 1 of 2)

Code

Typical Reply Text Description

110 Restart marker reply.

Used as part of the marker restart feature when trans-

ferring in block mode.

120 Service ready in “nnn” minutes.

“nnn” indicates the number of minutes until the service will

be available.

125

Data connection already open;

transfer starting.

150

File status okay; about to open

data connection.

200 Command okay.

Sometimes the text indicates the name of the command

that was successful.

202

Command not implemented, or

superfluous at this site.

211

System status, or system help

reply.

Will contain system-specific status or help information.

212 Directory status.

213 File status.

214 Help message.

Includes help information of use to a human user of this

server.

215 “NAME” system type.

“NAME” is the name of a type of operating system. Often

sent as a reply to the SYST command.

220 Service ready for new user.

Sent when the command channel is established before the

USER command is sent.

221

Service closing control

connection.

A “goodbye” message sent when the session is closed.

225

Data connection open; no transfer

in progress.

226 Closing data connection. Sent after a successful file transfer or a file abort.

227

Entering Passive Mode

(h1,h2,h3,h4,p1,p2).

Sent in reply to the PASV command, indicates the IP

address and port to use for the data connection.

230 User logged in, proceed.

Sent after successful USER and PASS authentication.

Systems often include additional “greeting” or other infor-

mation with this code after a login.

250

Requested file action okay,

completed.

The text description will provide more details about what

was successfully done, such as confirming a change of

directory or deleted file.

257 “PATHNAME” created. “PATHNAME” is replaced by the path created.

331 User name okay, need password.

Intermediate result after sending USER but before sending

PASS.

332 Need account for login.

350

Requested file action pending

further information.

421

Service not available, closing

control connection.

Sometimes sent if the FTP server is in the process of

shutting down.

425 Can't open data connection.

426

Connection closed; transfer

aborted.

450

Requested file action not taken.

File unavailable.

The file is not available; for example, it may be locked by

another user. Contrast to reply code 550.

451

Requested action aborted: local

error in processing.

452

Requested action not taken.

Insufficient storage space in

system.

The file system is full.

500

Syntax error, command

unrecognized.

Bad or excessively long command line was sent.

501

Syntax error in parameters or

arguments.

502 Command not implemented.

503 Bad sequence of commands.

504

Command not implemented for

that parameter.

530 Not logged in.

Sent if authentication fails due to a bad user name or

incorrect password.

550

Requested action not taken. File

unavailable.

File was not found or user does not have access to it. This

error code may be sent in reply to any file transfer

command if the user has not successfully logged in yet.

Contrast to reply code 450.

551

Requested action aborted: page

type unknown.

552

Requested file action aborted.

Exceeded storage allocation.

553

Requested action not taken. File

name not allowed.

Table 231: FTP Reply Codes (Page 2 of 2)

Code

Typical Reply Text Description

FTP Multiple-Line Text Replies

It is possible for a reply to contain more than one line of text. In this case, each line starts

with the reply code, and all lines but the last have a hyphen between the reply code and the

reply text, to indicate that the reply continues. The last line has a space between the reply

code and reply text, just like a single-line reply. This facility is often used to provide

additional response information after a user logs in, via the 230 reply code. Table 232

contains an example.

As mentioned before, the actual text string for each reply code is implementation-specific.

You can sometimes find some rather humorous text strings associated with some of these

error messages. For example, I tried some commands using the FreeBSD FTP client on

one of my Internet accounts. If you try to send or receive a file before you are logged in, it

doesn't return an error like “Requested action not taken. File unavailable”. It tells me “Login

first, then I might let you do that.”. Laugh… what an attitude these computers have today! ☺

FTP User Interface and User Commands

The FTP protocol command set provides a rich, complete set of instructions for imple-

menting the File Transfer Protocol. A human user could employ those commands to

perform file transfer functions directly with an FTP server. But to do this requires that the

user have an intimate knowledge of how FTP works. The user must know exactly which

commands to send at which time, and in what order.

FTP User Interface Benefits

Memorizing internal FTP commands might be a reasonable assignment for an internet-

working expert, but not for a typical TCP/IP application user. For this reason, the FTP

protocol defines an additional protocol component as part of the User-FTP Process: the

FTP user interface. It provides three main benefits to the FTP user:

☯ User Friendliness: The FTP user interface presents FTP to the human user in a way

that is easier and simpler to use than issuing protocol commands. Instead of requiring

the knowledge of all those four-letter codes, the user interface can allow functions to

be performed with more intuitive human-language commands. For example, we can

say “get” a file instead of knowing to use the command RETR.

Table 232: FTP Multiple-Line Text Reply Example

230-Welcome user to FTP server jabberwockynocky.

230-

230-You are user #17 of 100 simultaneous users allowed.

230-

230-Please see the file "faq.txt" for help using this server.

230-

230 Logged in.

☯ Customization: The command used to perform a particular function can be

customized based on common parlance in the networking industry, without requiring

changes to be made to the FTP protocol itself. For example, the “image” transfer mode

is now also commonly called “binary” mode, so a user command called “binary” has

been created to set this mode.

☯ Detail Abstraction and Command Sequence Simplification: A single user

command can be made to issue multiple FTP protocol commands, hiding internal FTP

details and making the protocol easier to use. In particular, commands that are related

to the maintenance of the connection and other “overhead” issues that users don't

want to deal with can be automated. For example, an FTP client normally issues a

PASV or PORT command prior to each data transfer. The user interface can take care

of issuing this command automatically prior to a RETR or STOR command when a

user tells FTP to get or send a file.

Command-Line and Graphical FTP Interfaces

Traditionally, FTP clients have used a command-line interface. In this familiar arrangement,

an FTP client is invoked and the user is automatically asked for a user name and password

to establish an FTP session. Then, the user is presented with a command prompt, at which

they can type various FTP commands to perform different functions. Text responses from

the server are displayed to the user to indicate the results of various commands. Normally,

the internal protocol commands (such as PASV and STOR) sent by the client are

suppressed to avoid “screen clutter” but can be enabled in a debug mode.

Command-line utilities are efficient but some folks don't care for them. They are rather “old

school” in the context of modern graphical operating systems and applications. Thus, there

are also now many FTP clients that are graphical in nature. They allow actions to be

performed arguably even more quickly, by clicking on buttons instead of typing commands.

Some allow files to be transferred by “dragging and dropping” from a local file system

display to one on a remote server. These make FTP even easier to use.

Key Concept: The FTP user interface is the component on the FTP client that acts

as an intermediary between the human user and the FTP software. The existence of

the user interface allows FTP to be used in a friendly manner without requiring

knowledge of FTP’s internal protocol commands. Most FTP software uses either a

command-line interface that understands English-like user commands, or a graphical

interface, where mouse clicks and other graphical operations are translated into FTP

commands.

Typical FTP User Commands

To discover the specific commands supported by an FTP client, consult its documentation.

In a command-line client, you can enter the command “?” to get a list of supported

commands. Table 233 shows some of the common commands encountered in typical FTP

command-line clients, along with the typical parameters they require.

Table 233: FTP User Commands (Page 1 of 2)

User Command Description

account <account-name> Sends the ACCT command to access a particular account on the server.

append <file-name> Appends data to a file using APPE.

ascii Sets the ASCII data type for subsequent transfers.

binary

Sets the Image data type for subsequent transfers. Same as the image

command.

bye Terminates FTP session and exits the FTP client (same as exit and quit).

cd <directory-path> Change remote server working directory (using CWD protocol command.)

cdup Go to parent of current working directory.

chmod <file-name> On UNIX systems, change file permissions of a file.

close Closes a particular FTP session but user stays at FTP command line.

debug Sets debug mode.

delete <file-name> Deletes a file on the FTP server.

dir [<optional-file-

specification>]

Lists contents of current working directory (or files matching the

specification).

exit Another synonym for bye and quit.

form <format> Set transfer format.

ftp <ftp-server> Open session to FTP server.

get <file-name> [<dest-file-

name>]

Get a file. If the “<dest-file-name>” parameter is specified, it is used for the

name of the file retrieved; otherwise, the source file name is used.

help [<optional-command-

name>]

Displays FTP client help information. Same as “?”.

image Set Image data type, like the binary command.

ls [<optional-file-

specification>]

Lists contents of current working directory (or files matching the specifi-

cation). Same as dir.

mget <file-specification> Gets multiple files from the server.

mkdir <directory-name> Create directory on remote server.

mode <transfer-mode> Set file transfer mode.

mput <file-specification> Sends (“puts”) multiple files to the server.

msend <file-specification> Same as mput.

open <ftp-server> Open session to FTP server (same as ftp).

passive Turns passive transfer mode on and off.

Note how many of these commands are actually synonyms, such as bye, exit and quit.

Similarly, one can use the command “type ascii” to set the ASCII data type, or just use the

ascii command. This is all done for the convenience of the user and is one of the benefits of

having a flexible user interface that is distinct from the FTP protocol command set.

Finally, an alternative way of using FTP is through the specification of an FTP Uniform

Resource Locator (URL). While FTP is at its heart an interactive system, FTP URLs allow

simple functions, such as retrieving a single file, to be done quickly and easily. They also

allow FTP file references to be integrated with hypertext (World Wide Web) documents. See

the discussion of URL schemes for more on how FTP uses URLs.

put <file-name> [<dest-file-

name>]

Sends a file to the server. If the “<dest-file-name>” parameter is specified,

it is used as the name for the file on the destination host; otherwise, the

source file name is used.

pwd Prints current working directory.

quit Terminates FTP session and exits FTP client (same as bye and exit.)

recv <file-name> [<dest-file-

name>]

Receives file (same as get). If the “<dest-file-name>” parameter is

specified, it is used for the name of the file retrieved; otherwise, the source

file name is used.

rename <old-file-name>

<new-file-name>

Renames a file.

rhelp Displays remote help information, obtained using FTP HELP command.

rmdir <directory-name> Removes a directory.

send <file-name> [<dest-file-

name>]

Sends a file (same as put). If the “<dest-file-name>” parameter is specified,

it is used as the name for the file on the destination host; otherwise, the

source file name is used.

site Sends a site-specific command to the server.

size <file-name> Show size of remote file.

status Displays current session status.

struct <structure-type> Sets file structure.

system Shows the server's operating system type.

type <data-type> Sets data type for transfers.

user <user-name> Log in to server as a new user. Server will prompt for password.

? [<optional-command-

name>]

Displays FTP client help information. Same as help.

Table 233: FTP User Commands (Page 2 of 2)

User Command Description

FTP Sample User and Internal Command Dialog

The previous two sections have described how FTP operates, and shown the commands

and reply codes used to implement communication between an FTP user and an FTP

server. Having now seen all the details of how FTP works, let's tie everything together by

looking at a sample FTP session between an FTP client and server, to see FTP commands

and replies “in action”.

In this example, I will invoke FTP from a client to retrieve a text file from an FTP server, then

delete the file from the server and the directory that contained it. In the process, I will issue

some additional commands to illustrate more of how FTP works. I will enable “debug” mode

in the FTP client so that for each user command we can see the actual FTP protocol

commands generated.

Table 234 shows the sample FTP dialog, slightly simplified. The first column contains

commands entered by the user (that's me) on the FTP client. The second shows the actual

protocol command(s) sent to the FTP server in highlighted text, and the reply returned from

the server to the client in plain text. The third contains descriptive comments.

Table 234: Sample FTP Session (Page 1 of 3)

User

Command

FTP Protocol Command / FTP Server

Comments

ftp -d

pcguide.com

Connected to pcguide.com.

220 ftp199.pair.com NcFTPd Server

(licensed copy) ready.

Name (pcguide.com:ixl):

This is the command to start up FTP. The

“-d” enables debug mode. In this initial

step the TCP control connection is made,

and the server replies with a 220 reply

code indicating that it is ready for user

identification. The FTP client automati-

cally prompts for the user name.

ixl

USER ixl

331 User ixl okay, need password.

I use “ixl” for user names commonly. Here,

the FTP client sends the user name and

the server responds back asking for the

password.

****

PASS XXXX

230-You are user #1 of 300 simultaneous

users allowed.

230-

230-Welcome to (<system name>)

230-

230 Logged in.

SYST

215 UNIX Type: L8

Remote system type is UNIX.

Using binary mode to transfer files.

I enter my password, which is sent to the

FTP server, and a bunch of stuff happens.

☺ The server authenticates me and sends

back a 230 message. This tells me the

with additional information.

The FTP client then automatically sends a

SYST command to tell me what type of

system the server is using, in this case

UNIX. The client also tells me that binary

mode has been selected by default; this is

often the default when doing FTP from

UNIX to UNIX (as I am doing here) since

there is no need for ASCII mode when

moving text files between similar systems.

pwd

PWD

257 “/usr/home/ixl” is cwd.

I check the current working directory

(“cwd”), which the server tells me is my

own home directory on this system.

cd ftptest

CWD ftptest

550 No such directory.

I try to go to a directory called “ftptest” but

that was the wrong name, so I get a 550

error for my trouble. (I wasn't trying to do

this, I forgot the directory name but

figured I might as well show it to you

anyway! ☺)

cd ftpdemo

CWD ftpdemo

250 “/usr/home/ixl/ftpdemo” is new

cwd.

I got the name right this time and the

server confirms the new working directory.

dir

PASV

227 Entering Passive Mode

(ip1,ip2,ip3,ip4,193,224)

LIST

150 Data connection accepted from

ip5.ip6.ip7.ip8:4279; transfer

starting.

-rw-r—r— 1 ixl users 16 May 22 17:47

testfile.txt

226 Listing completed.

I request a list of files from the server. The

FTP client automatically issues a PASV

command and the server responds with a

port number and IP address for it to use (I

have not shown the IP here for security

reasons.) The directory listing is then

transferred from the server to the client.

asc

TYPE A

200 Type okay.

I set ASCII mode. Don't really need to, as

I said above, but what the heck. Note that

this client allowed me to abbreviate the

ascii command as “asc”.

get

testfile.tx

PASV

227 Entering Passive Mode

(ip1,ip2,ip3,ip4,193,226)

RETR testfile.txt

150 Data connection accepted from

ip5.ip6.ip7.ip8:4283; transfer

starting for testfile.txt (16 bytes).

226 Transfer completed.

17 bytes received in 0.10 seconds (0.17

KB/s)

I get the file in this demo directory using a

get command. The server accepts the

PASV command and sends the file. It

initially sends a 150 reply as the transfer

starts (initial positive reply) then sends

226 when it is done.

Note that the port numbers used here are

different (for both server and client) than

they were for the directory listing I did

earlier.

del

testfile.tx

DELE testfile.txt

250 Deleted.

I delete the original file on the server.

Table 234: Sample FTP Session (Page 2 of 3)

User

Command

FTP Protocol Command / FTP Server

Comments

cdup

CDUP

250 "/usr/home/ixl" is new cwd.

I go up to the parent directory.

rmdir

ftpdemo

RMD ftpdemo

250 Directory removed.

I remove the directory that the file was in.

quit

QUIT

221 Goodbye.

I end the FTP session. The quit command

also automatically closes the FTP client

and returns me to the UNIX shell.

Table 234: Sample FTP Session (Page 3 of 3)

User

Command

FTP Protocol Command / FTP Server

Comments