Introduction to ftp server functions for linux operation and maintenance

As one of the oldest protocols, ftp is not widely used yet. It is mainly used for file transfer between the server and the client. The ftp protocol uses a clear text transmission protocol, so it has certain security risks. There is a vsftpd software on the Linux system. Although this software does not have many functions, its focus is on safety. It is recommended that you use this software as an ftp file transfer server. In addition, pure-ftpd is also used more frequently.

ftp function introduction

ftp mainly has the following functions:

• Let the client Files can be transferred and managed between the server and the server. This is the most important function and the core of ftp.

• Different levels of user identities. FTP has three different identities by default: entity user, guest, and anonymous user. The permissions corresponding to these three identities are different. Entity users have the highest permissions, while anonymous users may only have permissions to browse files or may not have any permissions.

• Directory that restricts user activities. This is also the core function of ftp, preventing other directory files on your server from being disgustingly damaged. When you log in using your account and password, you can only operate in the account's home directory and cannot enter other directories.

ftp transmission mode

Compared with ssh, ftp is more complicated. Unlike the ssh service, which only needs to open one port, the ftp service requires two ports. The purposes of these two ports are different. Generally, the server uses port 21 as the command channel, and uses port 20 or other random ports as the data transmission channel. According to the different ways of establishing the data channel, it is divided into two modes:

• Active mode: The server actively initiates a connection to the client. In active mode, port 20 is used by default for data transmission.

• Passive mode: The client initiates a connection to the server. The ports used in this mode are random and can also be customized.

In the next article, I will explain in detail the active mode and passive mode of ftp. We often encounter this problem, why can't we connect to the ftp server? Although the account password and IP are entered normally, we just can't connect? Once we understand the two transmission modes of ftp, this problem can be easily solved.

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