FTP open control port 21
FTP is divided into two working modes, which are selected by the client
1 Active mode
Control port 21
Data transmission port 20
These two ports are fixed in active mode. The client connects to port 21 and tells the FTP server to adopt the active mode. After the authentication is successful, the client informs the FTP server of its own open data transmission port, and the FTP server uses port 20 to connect to the client. Port for data transmission
2 Passive mode
Control port 21
Data transmission port random
In the passive mode, only port 21 of the FTP server is fixed. The client connects to port 21 and tells the FTP server to adopt the passive mode. After the authentication is successful, the FTP server randomly opens a data transmission port and tells the client that the client connects to the random port that was notified.