The TCP/IP Guide - FTP Replies, Reply Code Format and Important Reply Codes

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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.

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

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.

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.)