tailieunhanh - The Illustrated Network- P56

The Illustrated Network- P56:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 20 File Transfer Protocol 519 The FTP server requires a log-in from the user and in many cases servers will allow a special log-in for anonymous FTP. The user is supposed to use their email address as a password a primitive auditing measure. Anonymous FTP implementations used to allow users to simply press Enter and leave the anonymous password field blank but many FTP implementations now demand at least something at the password prompt. Some do not allow more creative substitutes for an email address and many FTP servers check for things such as the presence of dots and the at sign @ to try to enforce some semblance of honesty. In many cases the FTP server will accept a similar term such as guest or visitor. The point behind anonymous FTP is that users are not required to have a valid user ID or password on the remote system in order to be able to access files in some directories. Of course there are file areas on the FTP server that should only be accessed by authenticated users of the remote system. Private IDs can be combined with anonymous FTP to protect certain areas of the file system while allowing public access to others. Of course this does not stop people from trying to access files they had no business seeing but if the file system permissions are set up correctly or at all FTP is highly secure. However the best way to prevent access to sensitive files is not to put them on an FTP server with public access in the first place. The well-known port of the control connection is TCP port 21. The client runs the FTP client program and uses an ephemeral port to begin the interaction with the server. This connection asks for the user ID and password anonymous or not and is nothing more than a normal remote log-in session using the Telnet application. Once logged in the user is placed in a default file system directory. Navigation outside this directory might be permitted but usually there s a good reason to direct a user to this particular directory and