Master the FTP transmission modes for server Establishment

FTP File Transfer Protocol is a commonly used protocol for establishing servers and sites. Therefore, we need to master the basic content of this part. Then establish the server. First, let's figure out what FTP transmission modes are available. FTP (file transfer protocol) plays an important role in many network applications. A very important resource in the Internet is software resources, most of the various software resources are stored on FTP servers. Like most Internet services, FTP is also a client/server system, connect to the FTP server program on the host. The user sends commands to the server program through the client program. The server program executes the commands issued by the user and returns the execution results to the client ｡

The FTP service can be divided into two types based on different service objects: The system FTP server can only be used by legal users on the system; the Anonymous FTP Server (Anonymous FTPServer) allow anyone to log on to the FTP server to obtain files ｡

For FTP data connections, FTP transmission modes include active transmission mode, passive transmission mode, and single-port transmission mode ｡

1. Active Transmission Mode

When the FTP control connection is established and the customer proposes a directory list and transfers files, the client issues The PORT command to negotiate with the server, the FTP server uses a standard Port 20 as the server's data connection port (ftp-data) and establishes a data connection with the customer. Port 20 is only used to connect the source address to the server, and Port 20 does not have a listening process to listen to customer requests ｡

In active transmission mode, FTP data connection and control connection are in the opposite direction, the server initiates a connection for data transmission to the client. The connection port of the client is determined by negotiation between the server and the client ｡

2. Passive Transmission Mode

When the FTP control connection is established and the customer proposes a directory list and transfers files, the client sends the PASV command to make the server in passive transmission mode, the FTP server waits for the customer to contact. the FTP server listens to the customer's requests on other data transmission ports not on Port 20 ｡

In passive transmission mode, the FTP data connection and control connection directions are the same, the client initiates a connection for data transmission to the server. The connection port of the client is the port used to initiate the data connection request. When an FTP client accesses the FTP server outside the firewall, passive transmission mode is required ｡

3. Single Port Mode

In addition to the above two modes, there is also a single port mode. Data connection requests in this mode are initiated by the FTP server, the control connection port of the client is the same as that of the Data Connection port. Because this mode cannot input data or transmit commands continuously for a short time, it is not commonly used ｡

Linux has a lot of available FTP servers, among which the more popular is WU-FTP (Washington University FTP) and VSFTP. Red Hat 8.0 comes with both WU-FTP and VSFTP software. WU-FTP is a famous FTP server software, it is powerful, it can run well in many Unix operating systems. However, VSFTP, as a rising star, is becoming more and more popular. In Red Hat 9.0 release, it only has VSFTP ｡

In VSFTP, VS indicates "Very Secure". From the name, we can see that from the Very beginning, software developers have paid great attention to its security. In addition to inherent security, VSFTP also features high-speed and stable performance. In terms of stability, VSFTP can be connected to more than 4000 concurrent users on a single machine (non-cluster). According to ftp.redhat.com, VSFTP can support a maximum of 15000 concurrent users ｡