When the FTP server is behind a NAT, it needs to know it’s external IP address, so it can provide it to the client in a response to PASV command. And the same range has to be opened/routed on the firewall/NAT. Typically, the FTP server software has a configuration option to setup a range of the ports, the server will use. The firewall and NAT on the FTP server side have to be configured not only to allow/route the incoming connections on FTP port 21, 2 but also a range of ports for the incoming data connections. Notes for Uncommon Local Network Configurations.Use Passive mode session settings to toggle between the active and the passive mode. In the passive mode, the client uses the control connection to send a PASV command to the server and then receives a server IP address and server port number from the server, which the client then uses to open a data connection to the server IP address and server port number received. Using the passive mode is preferable because most of the complex configuration is done only once on the server side, by experienced administrator, rather than individually on a client side, by (possibly) inexperienced users. ADSL modem), unable to accept incoming TCP connections.įor this reason the passive mode was introduced and is mostly used nowadays. built-in Windows firewall) or NAT router (e.g. Nowadays, it is typical that the client is behind a firewall (e.g. In the active mode, the client starts listening on a random port for incoming data connections from the server (the client sends the FTP command PORT to inform the server on which port it is listening).
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