Using PASV From an Ubuntu Server to FTP Server

By ServerWatch Staff (Send Email)
Posted May 27, 2010

If you need to transfer files from your Ubuntu server to an FTP server, that requires passive mode (PASV), this blog can help you with firewall blocks. Don't worry though, your system is operating exactly as it is configured to. The explanation just requires a little understanding of FTP and firewalls.

The firewall can be a problem, but with a little understanding of FTP and firewalls the communication can be negotiated.

"The tricky bits concern this second port that is negotiated. This port is not a fixed number, it is a dynamically allocated port above 1023. The port number is encoded in a packet as two numbers that need to be multiplied together to get the port number. The firewalls involved need to be smart enough to recognize the FTP negotiation and extract this data from the data, open that specified port and keep it open during the FTP session."

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