THE INFORMATION IN THIS ARTICLE APPLIES TO:
FTP client hangs on the list command
Internet Security and Acceleration (ISA) Server maintains secondary connections for secure network address translation (NAT) clients in Kernel mode, which can improve data throughput for protocols that use secondary connections. Secondary connections for secure NAT clients are only supported if an application filter that can process the protocol is installed on ISA Server.
For example, File Transfer Protocol (FTP) uses the secondary connection over port 20 to transfer data. Because the primary connection is enabled only if the requirements
for all applicable rules are met, and the secondary connection is established only after the primary connection is established, there is no need to inspect the traffic for this connection.
For information on correcting this problem, view the following articles in Microsoft’s knowledge base: