Posted by arlene

In the FTP protocol, every command must be followed by a reply from the server. In some cases, more than one reply will be sent to the client. The actual reply is a three-digit number, but it is transmitted as text characters. Following this number is usually some variable-length text. The numerical value is used by the program, and the text is intended for the user of the FTP client. Because some of the text is configurable, you can expect to see different text for the same numerical reply from one implementation to another. For example, you can usually specify the text that is displayed on the client when the user first logs in to your server.

To reply to the client, the three characters representing the numeric reply code are sent, followed by a minus sign (—) or space character and then the reply text. A simple convention is used for multiline text messages. The first line contains the three-letter numerical code followed by the — character and then the text. The last line replaces the — character with the space character. By matching up the two three-digit codes, the client can determine the beginning and ending of a particular multiline message.

Reply Codes

Each of the actual digits that make up the reply code is significant. If you’ve ever wondered why all those text lines start with numbers, you’ll find their meaning in this section. The text displayed will vary from one vendor to another, but the codes should still be implemented for the same general reply condition.

The first digit indicates success or failure:

• A value of 1 indicates a Positive Preliminary reply, which means that the function requested by the client has been started.
• A value of 2 is the Positive Completion reply, which means that the requested function was successfully performed.
• A value of 3 is the Positive Intermediate reply, indicating that the command was received by the server but has not been executed. The server might be waiting for further information.
• A value of 4 indicates a Transient Negative Completion reply. This means that a temporary error situation has prevented the function requested by the client from being performed.
• A value of 5 in the first position is the Permanent Negative Completion reply. This indicates that the requested action was not performed. Unlike the Transient message code, the user is not encouraged to retry the command.

The second digit is used to place message types into groups:

• A value of 0 refers to a syntax error.
• A value of 1 is an informational message.
• A value of 2 indicates that the reply refers to either the control or the data connection.
• A value of 3 is used for replies regarding authentication and accounting, such as during the initial logon process.
• A value of 4 is unspecified at this time.
• A value of 5 means that the reply refers to the status of the file server’s file system as it relates to the user request.

The third digit in the response code is used to further divide the replies based on the categories indicated by the second digit.

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