The Nexsure Exchange Manager is a utility used to transfer ACORD AL3 and Microsoft® Excel® files via FTP from the local PC to the Nexsure server for processing. This is an ActiveX control which will be downloaded and installed on the user’s PC. This utility uses Passive FTP when transferring files to the Nexsure server.
FTP is a TCP based service. It utilizes two ports, a command port, and a data port. Traditionally these are port 21 for the command port and port 20 for the data port. However the ports that are utilized depend on the FTP mode.
In Active FTP mode, the client connects from a random unprivileged port (N > 1024) to the FTP server's command port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. The server will then connect back to the client's specified data port from its local data port, which is port 20.
Active FTP Ports used:
Command = client > 1024 -> server 21
Data = client > 1024 <-server 20
Passive FTP makes the FTP client establish all connections to the FTP server, as opposed to the web host supplying the return port. Firewalls typically allow passive FTP connections without requiring additional configuration information.
In passive mode FTP the client initiates both connections to the server, solving the problem of firewalls filtering the incoming data port connection to the client from the server. When opening an FTP connection, the client opens two random unprivileged ports locally (N > 1024 and N+1). The first port contacts the server on port 21, but instead of then issuing a PORT command and allowing the server to connect back to its data port, the client will issue the PASV command. The result is that the server then opens a random unprivileged port (P > 1024) and sends the PORT P command back to the client. The client then initiates the connection from port N+1 to port P on the server to transfer data.
Passive FTP Ports used:
Command = client > 1024 -> server 21
Data = client > 1024 -> server 1024