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Why is my connection being immediately closed?

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by (120 points)

Binding FTP server to port 21 (explicit mode)...
Binding FTP server to port 990 (implicit mode)...
Starting...
08:09:26.725 Info Server: Starting server.
08:09:26.749 Info Server: Listening for connections at 0.0.0.0:21.
08:09:26.758 Info Server: Listening for connections at 0.0.0.0:990.
08:09:26.765 Info Server: Server started.
FTP server has started and is ready to accept connections.
Log level changed to Debug.
08:09:57.435 Debug Server: Accepted FTP connection from 192.168.0.141:49165.
08:09:57.796 Info Server: FtpSession 1: Started on connection from 192.168.0.141:49165.
08:09:57.816 Debug Info: Starting session.
08:09:57.826 Debug Info: Control connection TLS mode is 'Explicit'.
08:09:57.827 Debug Info: Control connection protection mode is 'Optional'.
08:09:57.837 Debug Info: Data connection protection mode is 'Optional'.
08:09:57.872 Debug Response: 220 Rebex FTP Server ready.
08:09:57.950 Debug Info: Session started.
08:09:58.029 Debug Command: USER Cryptobox
08:09:58.075 Debug Response: 331 Password required for 'Cryptobox'.
08:09:58.096 Debug Command: PASS **********
08:09:58.130 Info Server: FtpSession 1: Authentication for 'Cryptobox' succeeded.
08:09:58.229 Debug FTP: Getting item info on '/': success.
08:09:58.232 Debug Response: 230 User 'Cryptobox' logged in.
08:09:58.244 Debug Command: TYPE A
08:09:58.262 Debug Response: 200 TYPE set to A.
08:09:58.375 Debug Command: PWD
08:09:58.379 Debug Response: 257 "/" is current directory.
08:09:58.388 Debug Command: PASV
08:09:58.441 Debug Info: PASV::binding.
08:09:58.447 Debug Info: PASV::listening(192.168.0.117:56825).
08:09:58.452 Debug Response: 227 Entering Passive Mode (192,168,0,117,221,249)
08:09:58.502 Debug Info: Established data connection to 192.168.0.141:49166 in passive mode.
08:09:58.519 Debug Command: TYPE I
08:09:58.519 Debug Response: 200 TYPE set to I.
08:09:58.531 Debug Command: LIST -l
08:09:58.576 Debug FTP: Getting item info on '/-l': not found.
08:09:58.576 Debug Response: 550 No such directory.
08:09:58.594 Debug Info: Discarding data connection.
08:09:58.684 Debug Info: Closing data connection.
08:09:58.685 Debug Command: QUIT
08:09:58.700 Debug Response: 221 Closing session.
08:09:58.730 Debug Info: Closing session.
08:09:58.755 Debug Info: Shutting down control connection.
08:09:58.764 Debug Info: Awaiting shutdown confirmation from remote end.
08:09:58.785 Debug Info: Shutdown confirmed by remote end.
08:09:58.789 Debug Info: Closing control connection.
08:09:58.812 Debug Info: Session closed.
08:09:58.817 Info Server: FtpSession 1: Closed connection from 192.168.0.141:49165.

Applies to: Rebex FTP/SSL

1 Answer

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by (75.4k points)

The connection is closed by the client explicitly after the initial directory listing fails. So, the question is why the initial directory listing fails?

The client issued command LIST -l which is not supported by Rebex FTP server. The supported format of the LIST command is (as defined in RFC 959):

LIST [<SP> <pathname>] <CRLF>

It seems that the client is trying to pass a -l option to the LIST command, which results to interpreting the -l as a <pathname> argument. Since the -l directory does not exist on the server the 550 No such directory response is sent to the client.

The -l option is a non-standard extension for the LIST command.

The currently possible solution is to disable sending -l option at the client side. However, we will consider to add support for -l and -a options, which seems to be used by some FTP clients. Please leave a comment here, if you want to receive preview version in case the support is added.

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