FTP Operational Model, Protocol Components and Key Terminology
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Third-Party File Transfer (Proxy FTP)
The FTP standard actually defines a separate model for an alternative way of using the protocol. In this technique, a user on one host performs a file transfer from one server to another. This is done by opening two control connections: one each from the User-PI on the user's machine to the two Server-PI's on the two servers. Then, a Server-DTP is invoked on each server to send data; the User-DTP is not used.
This method, sometimes called third-party file transfer or proxy FTP, is not widely used today. A major reason for this is that it raises security concerns, and has been exploited in the past. Thus, while I felt it was worth mentioning, I will not be discussing it further in my coverage of FTP.
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