![]() ![]() These settings are going to be used when responding to PASV client requests. In your FTPS server, you need to specify a passive IP address and a passive port range. ![]() However, when the packets are encrypted with TLS, as in the case of FTPS, the firewall can't examine the packets and so will have difficulty determining what ports to open and what IP addresses to substitute with. Once they receive the client’s request, they then simply make the necessary substitutions. They also modify the response packet to instruct the client software to connect back at the external IP address, not the FTP server's internal IP address. They then dynamically open that port to the FTP server in anticipation of the request from the client software. Once they have identified the conversation taking place as that of FTP and are able to detect the PASV command, they simply assume that the client is going to be connecting back to the FTP server through another port and IP address. Of course, modern firewalls, NAT routers, reverse proxies, and other routing devices, are smart enough to address this particular situation. In addition, if the port number specified in the response has not been opened on the firewall or routing device, that would also cause the connection to fail. Since the client does not belong to the internal network, it will naturally fail to connect and eventually time-out. What then happens is that, when the client, in turn, attempts to connect, it will attempt to connect to that internal IP address. So now, when the FTPS server responds to the PASV command, its response will specify the FTPS server’s internal IP address and the port number it will be listening on. We’ll be using the term ‘firewall’ but this kind of situation applies to NAT routers, reverse proxies and other routing devices as well. ![]() The client, which is is outside the internal network, is connecting to the FTPS server via the firewall's external IP address. Basically, the FTPS server is in an internal network and has an internal IP address assigned to it. So, let’s say we have an FTPS server sitting behind a firewall. But once you have a firewall or NAT router in between, things can get pretty messy. This shouldn’t be a problem for direct connections.
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