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| Subject: | Re: Passive sftp? |
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| Date: | Wed, 5 Mar 2008 13:44:40 -0500 |
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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