Re: Passive sftp?

On Wed, Mar 05, 2008 at 09:03:07AM -0800, Bob Rasmussen wrote:
In the FTP protocol, "passive" means that only one connection is
established, originating in the client. All trafic goes over this
connection. (In non-passive FTP, the server opens a second channel back to
the client.)

Actually, passive mode FTP does use a second channel for data transfers,
but it's opened in the opposite direction from the channel used in active
mode. That is, the FTP server selects a "random" TCP port number, tells
it to the client, and then the client initiates the connection to that port.

Active mode works as you said: the client picks a "random" TCP port number,
listens on it, and tells the FTP server to connect to it. This breaks
rather spectacularly when the client is firewalled preventing incoming
connections, or behind a NAT, etc.

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