Active FTP

FTP Client a a

21 Command

Figure 2-2 displays a typical FTP mode of operation between a client PC and an FTP server in active mode. The following steps are completed before FTP data can be transferred:

1. The FTP client opens a control channel on TCP port 21 to the FTP server. The source TCP port number on the FTP client is any number randomly generated above 1023.

2. The FTP server receives the request and sends an acknowledgment. FTP commands are exchanged between client and server.

3. When the FTP client requests a directory list or initiates a file transfer, the client sends a command (FTP port command). The FTP server then opens (initiates) a data connection on the FTP data port, TCP port 20.

4. The FTP client generates a new ephemeral (a decimal number above 1023) and sends the information to the server using the port command. The FTP server responds on the data port on the port number requested by the client.

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