Summary

We have a FTP server (running in active mode). We have an FTP client which is connecting to the server, carrying out a task and then closing the connection. The FTP Client can perform this operation on multiple threads.

Problem

We have a situation where customers are experiencing occasional failures to carry out operations on a FTP connection. The actual connection has been made to the server but when the server attempts to return data on the data port if fails.

Observations

We have a simple test FTP client which is running two separate threads. Each thread is performing a recursive listing of files from a root directory. With the firewall running on the client machine the hang happens within a few minutes. If the firewall is turned off on the client machine, the test application seems to run correctly. This does point to a potential firewall issue. However, with the firewall on we can list files on our company FTP server without any issues.

If the simple test FTP client runs a single thread then we do not experience any problems whether or not the firewall is turned on.

We have another simple test FTP client which was running 4 threads (with each opening a new FTP connection, doing a directory listing and closing the FTP connection as fast as possible) overnight with the firewall turned off. With the firewall turned on it fails in a short space of time. The confusing thing is that if the test FTP client and the FTP server are run on the same machine the failure occurs even though the firewall is turned off. This means that the problem may not be firewall related.

Any help with what this could be would be much appreciated.

Thanks Ian