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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP General File Transfer Protocols (FTP and TFTP)
                     9  File Transfer Protocol (FTP)
                          9  FTP Commands and Replies

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FTP Internal Protocol Commands and Command Groups
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FTP User Interface and User Commands
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FTP Replies, Reply Code Format and Important Reply Codes
(Page 2 of 5)

Reply Code Structure and Digit Interpretation

To make reply codes even more useful, the codes are not just assigned in a linear or random order, but a special encoding scheme is used. Each code has three digits that each communicate a particular type of information and to categorize replies. A code can be considered to be of the form “xyz”, where “x” is the first digit, “y” the second and “z” the third.

Key Concept: Each command sent by the FTP client results in a reply sent by the FTP server. FTP replies consist of a three-digit numeric reply code, along with a line of descriptive text. The reply code serves to standardize FTP replies, both so they can be interpreted by client software, and so experienced users can see at a glance what the results were of a command. The reply code is structured so that the first two digits indicate the type of reply and to what category it belongs.


First Reply Code Digit (“x”)

The first digit of the reply code indicates the success or failure of the command in general terms, whether a successful command is complete or incomplete, and whether an unsuccessful one should be tried again or not. Table 228 shows the possible values.


Table 228: FTP Reply Code Format: First Digit Interpretation

Reply Code Format

Meaning

Description

1yz

Positive
Preliminary Reply

An initial response indicating that the command has been accepted and processing of it is still in progress. The user should expect another reply before a new command may be sent.

2yz

Positive Completion Reply

The command has been successfully processed and completed.

3yz

Positive Intermediate Reply

The command was accepted, but processing of it has been delayed, pending receipt of additional information. This type of reply is used in the middle of command sequences. For example, it is used as part of the authentication sequence after receiving a USER command but before the matching PASS command is sent.

4yz

Transient Negative Completion Reply

The command was not accepted and no action was taken, but the error is temporary and the command may be tried again. This is used for errors that may be a result of temporary glitches or conditions that may change; for example, a file being “busy” due to another resource accessing it at the time a request was made for it.

5yz

Permanent Negative Completion Reply

The command was not accepted and no action was taken. Trying the same command again is likely to result in another error. For example, a request for a file that is not found on the server would fall into this category, or sending an invalid command like “BUGU”. J


Second Reply Code Digit (“y”)

The second digit is used to categorize messages into functional groups. These groups are shown in Table 229.


Table 229: FTP Reply Code Format: Second Digit Interpretation

Reply Code Format

Meaning

Description

x0z

Syntax

Syntax errors or miscellaneous messages.

x1z

Information

Replies to requests for information, such as status requests.

x2z

Connections

Replies related to the control connection or data connection.

x3z

Authentication and Accounting

Replies related to login procedures and accounting.

x4z

Unspecified

Not defined.

x5z

File System

Replies related to the server's file system.


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.)


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FTP Internal Protocol Commands and Command Groups
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FTP User Interface and User Commands
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