ErrMsg: "Invalid port command" AS400 Client FTP to NT 3.51 SP2

• Microsoft Windows NT Workstation versions 3.51
• Microsoft Windows NT Server versions 3.51

SYMPTOMS

IBM AS400 FTP client fails with the error "Invalid port command" when you try to ftp to Windows NT version 3.51 Service Pack 2 server.

CAUSE

Windows NT 3.51 Server Service Pack 2 has added a new feature that rejects a client request for a TCP port number that is less than 1024. When this feature is implemented, an FTP session from an AS/400 client to the Windows NT 3.51 server may fail due to this added security feature."

Ports numbers up to 1024 are reserved and used by well known applications. Port numbers above 1024 can be used arbitrarily by the user. When an ftp session is initiated from AS400 client, it might request for a TCP port number less than 1024 when a command is issued. This request is rejected by the Windows NT server and the client gets the above error. If more commands are issued from the client, it increments the TCP port number requested for every command issued and eventually when it reaches 1024, the request is accepted by the server.

STATUS

This is by design.