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Absolute OpenBSD, 2nd Edition by Michael W. Lucas

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FTP and PF

Most modern application protocols run over a single network connection. If you make a web request, your browser opens a connection to the server on port 80, requests information, and receives the answer, all on the same connection. SSH opens a single connection on port 22 and exchanges all information over that port, even if you tunnel a hundred other protocols inside it. Experience and experiments with older protocols taught the wisdom of this approach. FTP is an older protocol, and it provides a wealth of experience on how not to do things.

The original version of FTP (today called active FTP) required the client to connect to the server on port 21. The server would then open a connection back to the client, from port 20 to some ...

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