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Troubleshooting FTP Error 425

This article was previously published under Q129395
This article has been archived. It is offered "as is" and will no longer be updated.
FTP error 425 may be returned to an FTP client if the server is unableto open a socket for data transfer. This article explains the error and howto troubleshoot it.
When using FTP to do directory listings, file transfers, etc. from aserver, one session is maintained as a control connection. A separateconnection is established from the server back to the client for eachtransfer of data. This is commonly referred to as a "back connection". Forexample, when an ftp user logs onto an ftp server and types DIR, thefollowing 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 toopen the connection back to the client. The most likely cause for this isan FTP client application that is attempting to re-use a socket that isstill in the TCP TIME-WAIT state due to an earlier connection that hasn'tbeen timed out yet. Per RFC793, Windows NT 3.5 systems leave oldconnections in the TIME-WAIT state for 2MSL (240 seconds). Duringthis 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 becauseit 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 reusethe same socket twice within four minutes (2MSL or 240 seconds).

To verify whether this is the cause of an FTP error 425, a network tracingtool can be used. More useful background information and definition ofsome of the terms used in this article may be found in RFC793.
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Article ID: 129395 - Last Review: 12/04/2015 11:18:39 - Revision: 4.1

Microsoft Windows NT Workstation 3.5, Microsoft Windows NT Server 3.5

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