Article Title: configuration of FTP services based on various system platforms (RedHatLinux, SUSELinux, CentOS, and SUNSolaris10. Linux is a technology channel of the IT lab in China. Includes desktop applications, Linux system management, kernel research, embedded systems, open source and other basic categories in TCP/IP network applications, file transfer protocol FTP is specially used for file transmission services, it provides services such as file download, WEB site maintenance, file exchange and sharing. It is one of the most important and most widely used Internet services. The FTP server can be built and configured based on a variety of operating system platforms (Windows, RedHat Linux, SUSE Linux, CentOS, and SUN Solaris10. I. FTP service Overview 1. FTP server functions in addition to simple file transmission and management, according to the server software architecture, he can also provide several main functions:
User identities of different levels:
By default, the FTP server is divided into three different identities based on user login: l real account, real user, l guest, guest; l anonymous login, the permissions of the three identities of anonymous vary greatly in the system. For example, a real user can complete system permissions, so more actions can be performed. As for anonymous users, only the user can download resources, anonymous users are not allowed to use too many server resources. In Linux, it also has
Command record and logon file record:
FTP can use the system's syslogd daemon to record data. The recorded data includes commands issued by the user and user data transmission (transmission time, file size, etc). Therefore, you can easily find logon information in/var/log.
Restrict or remove the directory of the user's home
(Change root,
Abbreviation
Chroot)
:
To avoid changing the logon path in Linux. Therefore, the user's work scope is "Limited" under the user's home directory. Such an environment is called change root, or chroot for short. 2. FTP workflow FTP transmission uses a reliable TCP protocol. The TCP packet format performs three handshakes before an online session is established. However, the FTP server is more troublesome because the FTP server uses two online session sessions, namely the control channel and the data stream channel (ftp-data ). Both online sessions must undergo three handshakes. 650) this. width = 650; "height = 376? 420?> A simple online session is shown in. The procedure of an online session is as follows: 1. as shown in the following figure, the client randomly obtains a port greater than 1024 to establish an online session with port 21 on the FTP server. This process requires three handshakes. After an online session is reached, the client can issue commands to the FTP server through this online session, including querying the file name, downloading, uploading, and so on. 2. the FTP server is notified to use the active port and the port number 21 of the connected FTP server is mainly used for issuing commands. However, when data flow is involved, this online session is not used. When the client needs data, it will inform the server of the method used for online sessions. If it is an active online session, the client will first enable a random port, inform the FTP server of the two information through the command channel, and wait for the online session of the FTP server; 3. the FTP server actively communicates with the client online session. After the FTP server understands the client's requirements through the command channel, it will actively forward the port number 20 to the random port online session of the client, of course, this online session will also undergo three handshakes. At this time, the FTP client and the server will establish two online sessions for command issuance and data transmission respectively. The default active online session port number used by the FTP server is port 20. In this way, the "command" and "Data Transmission" channels are successfully established. However, it should be noted that the "data transmission channel" is a channel established only when data transmission is performed. It is not a channel established immediately after you connect to the FTP server.
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