Troubleshooting FTP Error 425

MORE INFORMATION

When using FTP to do directory listings, file transfers, etc. from a server, one session is maintained as a control connection. A separate connection is established from the server back to the client for each transfer of data. This is commonly referred to as a "back connection". For example, when an ftp user logs onto an ftp server and types DIR, the following sequence of events occurs:

• The FTP client establishes a socket to listen on for the results of the remote DIR command.
• The FTP client sends a PORT command to the server, telling it what IP address and TCP port it is listening on for the "back connection. This command is sent over the control session that was established during the logon process.
• The FTP client sends a LIST command to the server, telling it to list all files in the current directory. Again, this is sent over the original control session.
• The FTP server executes the LIST command, and passes the results back to the client by establishing a new connection to the callback address specified by the previous PORT command.

If an FTP error 425 occurs, this indicates that the server was unable to open the connection back to the client. The most likely cause for this is an FTP client application that is attempting to re-use a socket that is still in the TCP TIME-WAIT state due to an earlier connection that hasn't been timed out yet. Per RFC793, Windows NT 3.5 systems leave old connections in the TIME-WAIT state for 2MSL (240 seconds). During this time they cannot be re-used.

The socket that cannot be used is defined by a "5-tuplet":

• The source IP address (that of the ftp server)
• The source port (the ftp server data port)
• The destination IP address (that of the ftp client)
• The destination port (specified by the client in the PORT command)
• The protocol (TCP/IP)

You should not encounter this problem with the Microsoft FTP client because it allows Windows Sockets to select a socket that is not in use. However, some third-party FTP-based applications may do their own port management. When this is done, care must be taken by the application not to reuse the same socket twice within four minutes (2MSL or 240 seconds).



To verify whether this is the cause of an FTP error 425, a network tracing tool can be used. More useful background information and definition of some of the terms used in this article may be found in RFC793.