When the NAT happens on a client side, what the FTP server cannot know, the IP address it provides is wrong too (from a client’s perspective). Notes for Uncommon Local Network Configurations Route port range for data connections on Microsoft Azure firewall/NAT.Configure port range for data connections and external IP address on Microsoft IIS FTP Server.Server sent passive reply with unroutable address. 3 You can tell that this happened from a session log: By default WinSCP detects, when an unroutable IP address is provided, and uses a server (control connection) address instead. It is common, that the FTP server is not configured properly and provides its internal IP address, that cannot be used from a client network. 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. The server administrator should setup the server as described below. With the passive mode, most of the configuration burden is on the server side. 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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