I have a problem regarding the ftp reply code 550 (Requested action not
taken:  file unavailable e.g. file busy) and 226 (Closing data connection;
requested file action successful. For example, file transfer or file abort.)
for the NLST command.
I have a third party ftp client application, which checks if the filename of
the file which shall be written to the ftp target folder already exist. This
is done to prevent overwriting existing files.

The check is done by "ls filename.xxx"
If the file is not existing on the server, some server are sending the reply
code 226 and the IIS sends 550. As soon as the client application receives
the code 226, the control channel will be closes from client site. If the
reply code 550 is send, the client starts with the file transmission.

I checked the related RFC's 542 & 614. So far as I could check, the RFC 614
is the latest RFC regarding ftp reply codes. This RFC is mention that only
following reply codes are valid -->

LIST, NLST
125, 150
226, 250
425, 426, 451
450
500, 501, 502, 421, 530

However, the older RFC 542 is mention the reply code 550.

Know my statements for confirmation -->

1) According my understanding, a FTP server must not send reply code 550 as
an answer to NLST.
2) A client Application must not quit the control channel, if the reply code
226 is received.


Please send me your thoughts

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