A 500 response code may be sent in response to any command that the server is unable to recognize. It is a permanent negative response, which means the client is discouraged from sending the command again since the server will respond with the same reply code. It usually means that the client has sent a command to the server that the server does not recognize. This may be due to an error in the spelling or formatting of the command itself or that the command is newer than the FTP implementation in place on the server or is a proprietary command of another server implementation.
Example response
500 Syntax error, command unrecognized.
A 501 response code may be sent in response to any command that requires or supports the optional use of a parameter or argument. It is a permanent negative response, which means the client is discouraged from sending the exact command and parameter(s) again since the server will respond with the same response code. It differs from the 500 response code in that the server recognizes the command present but is unable to take the requested action due to a syntax error in the parameter(s) present with the command. Sending the same command to the server with a corrected parameter may result in a different response code.
Example response
501 Syntax error in parameters or argument.
A 502 code may be sent in response to any FTP command that the server does not support. It is a permanent negative reply, which means the client is discouraged from sending the command again since the server will respond with the same reply code. The original FTP specification dictates a minimum implementation for all FTP servers with a list of required commands. Because of this, a 502 reply code should not be sent in response to a required command.
Example response
502 Command not implemented.
A 503 response code may be sent in response to any command that requires the successful processing of previous commands first. It is a permanent negative reply, which means the client is discouraged from immediately sending the command again since the server will respond with the same reply code. For example, a file rename requires a successful RNFR command before the RNTO command can be sent. Sending the RNTO command first will result in a 503 response code.
Example response
503 Bad sequence of commands.
Possible commands:
A 504 response code can be sent in response to any command using a parameter that is not supported by the server. It is a permanent negative response, which means the client is discouraged from sending the command again since the server will respond with the same response code. Issuing the same command with a different parameter may result in a different response code.
Example response
504 Command not implemented for that parameter.
Possible commands: