Question 20 for Active FTP and Passive FTP

main point is using port or pasv cmd.

use pasv cmd port num take from client.

use pasv cmd server will return message with 227.

not use port 20 is for the security .

FTP file transfer uses two sockets.

Active is the IP address with the port number from the server and the PASSIVE request from the client to the server.

A port is automatically generated by the server and sent to the client.

FTP active mode means that FTP will use its own port 20 to actively establish a connection with the client, and an error will occur when the client network does NAT. Think of the NAT port as if the client established a connection to the FTP port 20.

The ports are different, and FTP transmission requires 2 ports. If you are active, it is 21 waiting for connection, 20 is actively initiated, and if you are passive, it is 21 and a random port greater than 1024 is waiting for connection. In active mode, the client's firewall may be blocked, while in passive mode, the server's random ports may be attacked.

If there is no firewall at both ends, it doesn't matter if you are active or passive.

It can be briefly summarized in the following two points:

1. Active FTP:

Command connection: client > port 1024 - > server port 21 data connection: client > port 1024 < - server port 20 port 2, passive ftp:

Command connection: Client > port 1024 - > server port 21 Data connection: client > port 1024 - > server port >1024.

To set up the active mode and passive mode of FTP, the tool used: 8UFTP, the steps are as follows:

2.In the firewall settings, check the passive (pasv) mode (p) to be the passive mode, and remove the check box to be the active mode.

Note: Applicable to 8uftp

Active mode: The FTP client connects from any non-special port (N > 1023) to the command port of the FTP server, port 21. The client then listens on port N+1 (N+1 >= 1024) and sends commands to the FTP server over port N+1 (N+1 >= 1024).

The server will in turn connect to the user's locally specified data port, such as port 20.

Passive mode: In passive FTP, both command connections and data connections are sent by the client, which solves the problem that the inbound connection from the server to the client's data port is filtered out by the firewall. When an FTP connection is opened, the client opens two arbitrary non-privileged local ports (n >; 1024 and n+1).

The first port connects to port 21 of the server, but unlike active FTP, the client does not submit the port command and allows the server to connect to and from its data port, but instead submits the pasv command. The result of this is that the server opens an arbitrary unprivileged port (p >; 1024) and send the port p command to the client. The client then initiates a connection from the local port N+1 to the server's port P to transmit the data.

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