lsg43.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

ascii



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Switches to ASCII transfer mode</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



binary



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<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Switches to binary transfer mode</FONT>



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<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



cd



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<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Changes directory on the server</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



close



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<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Terminates the connection</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



del



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<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Deletes a file on the server</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



dir



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Displays the server directory</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



get



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Fetches a file from the server</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



hash



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Displays a pound character for each block transmitted</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



help



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Displays help</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



lcd



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Changes directory on the client</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



mget



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Fetches several files from the server</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



mput



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Sends several files to the server</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



open



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Connects to a server</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



put



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Sends a file to the server</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



pwd



</FONT>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Displays the current server directory</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



quote



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<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Supplies an FTP command directly</FONT>



<TR>



<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



quit



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<TD VALIGN=top  BGCOLOR=#80FFFF ><FONT COLOR=#000080>



Terminates the FTP session</FONT>



</TABLE><P>For most versions, FTP commands are case-sensitive. If you type commands in uppercase, FTP will display error messages. Some versions perform a translation for you, so it doesn't matter which case you use. Because Linux uses lowercase as its primary character set for everything else, you should probably use lowercase with all versions of FTP, too.



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<BR>



<A NAME="E69E245"></A>



<H4 ALIGN=CENTER>



<CENTER>



<FONT SIZE=4 COLOR="#FF0000"><B>How FTP Uses TCP</B></FONT></CENTER></H4>



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<P>The File Transfer Protocol uses two TCP channels: TCP port 20 is used for data, and port 21 is for commands. Both these channels must be enabled on your Linux system for FTP to function. The use of two channels makes FTP different from most other file transfer programs. By using two channels, TCP allows simultaneous transfer of FTP commands and data. FTP works in the foreground and does not use spoolers or queues.



<BR>



<P>FTP uses a server daemon that runs continuously and a separate program that is executed on the client. On Linux systems, the server daemon is called ftpd. The client program is ftp.



<BR>



<P>During the establishment of a connection between a client and server, and whenever a user issues a command to FTP, the two machines transfer a series of commands. These commands are exclusive to FTP, and are known as the internal protocol. FTP's internal protocol commands are four-character ASCII sequences terminated by a newline character, some of which require parameters. One primary advantage of using ASCII characters for commands is that users can observe the command flow and understand it easily, which helps in a debugging process. Also, a knowledgeable user can use the ASCII commands directly to communicate with the FTP server component without invoking the client portion (in other words, communicating with ftpd without using ftp on a local machine). This procedure is seldom done, however, except when debugging (or showing off).



<BR>



<P>After logging in to a remote machine using FTP, you are not actually on the remote machine. You are still logically on the client, so all instructions for file transfers and directory movement must be with respect to your local machine and not the remote one. The process followed by FTP when a connection is established is as follows:



<BR>



<UL>



<LI><B>Log in.</B> This step<B> </B>verifies user ID and password.



<BR>



<BR>



<LI><B>Define directory.</B> This step<B> </B>identifies the starting directory.



<BR>



<BR>



<LI><B>Define file transfer mode.</B> This step<B> </B>defines the type of transfer.



<BR>



<BR>



<LI><B>Start data transfer.</B> User commands are followed.



<BR>



<BR>



<LI><B>Stop data transfer.</B> This step<B> </B>closes the connection.



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<BR>



</UL>



<P>A debugging option is available from the FTP command line by adding -d to the command. This option displays the command channel instructions. Instructions from the client are shown with an arrow as the first character; instructions from the server have three digits in front of them. A PORT in the command line indicates the address of the data channel on which the client is waiting for the server's reply. If no PORT is specified, channel 20 (the default value) is used. Unfortunately, the progress of data transfers cannot be followed in the debugging mode. The following is a sample session with the debug option enabled:



<BR>



<PRE>



<FONT COLOR="#000080">$ ftp -d tpci_hpws4



Connected to tpci_hpws4.



220 tpci_hpws4 FTP server (Version 1.7.109.2 Tue Jul 28 23:32:34 GMT 1992) ready.



Name (tpci_hpws4:tparker):



---&gt; USER tparker



331 Password required for tparker.



Password:



---&gt; PASS qwerty5



230 User tparker logged in.



---&gt; SYST



215 UNIX Type: L8



Remote system type is UNIX.



---&gt; Type I



200 Type set to I.



Using binary mode to transfer files.



ftp&gt; ls



---&gt; PORT 47,80,10,28,4,175



200 PORT command successful.



---&gt; TYPE A



200 Type set to A.



---&gt; LIST



150 Opening ASCII mode data connection for /bin/ls.



total 4



-rw-r----- 1 tparker tpci 2803 Apr 29 10:46 file1



-rw-rw-r-- 1 tparker tpci 1286 Apr 14 10:46 file5_draft



-rwxr----- 2 tparker tpci 15635 Mar 14 23:23 test_comp_1



-rw-r----- 1 tparker tpci 52 Apr 22 12:19 xyzzy



Transfer complete.



---&gt; TYPE I



200 Type set to I.



ftp&gt; &lt;Ctrl-d&gt;



$</FONT></PRE>



<P>You may have noticed in the preceding code how the mode changed from binary to ASCII to send the directory listing, and then back to binary (the system default value).



<BR>



<BR>



<A NAME="E68E229"></A>



<H3 ALIGN=CENTER>



<CENTER>



<FONT SIZE=5 COLOR="#FF0000"><B>Configuring FTP</B></FONT></CENTER></H3>



<BR>



<P>Whether you decide to provide an anonymous FTP site or a user-login FTP system, you need to perform some basic configuration steps to get the FTP daemon active and get the directory system and file permissions properly set to prevent users from destroying files or accessing files they shouldn't. The process can start with choosing an FTP site name. You don't really need a site name, although it can be easier for others to access your machine (especially anonymously) if you have one. The FTP site name is in the following format:



<BR>



<BR>



<PRE>



<FONT COLOR="#000080">ftp.domain_name.domain_type</FONT></PRE>



<P>In this syntax, domain_name is the domain name (or an alias) of the FTP server's domain, and domain_type is the usual DNS extension. For example, you could have an FTP site name like the following example:



<BR>



<BR>



<PRE>



<FONT COLOR="#000080">ftp.tpci.com</FONT></PRE>



<P>This name shows that this is the anonymous FTP access for anyone accessing the tpci.com domain. It is usually a bad idea to name your FTP site with a specific machine name, such as the following:



<BR>



<BR>



<PRE>



<FONT COLOR="#000080">ftp.merlin.tpci.com</FONT></PRE>



<P>This name makes it difficult to move the FTP server to another machine in the future. Instead, use an alias to point to the actual machine on which the FTP server sits. This is not a problem if you are a single machine connected to the Internet through a service provider, for example, but is often necessary with a larger network. The alias is easy to set up if you use DNS. Set the alias in the DNS databases with a line like the following:



<BR>



<BR>



<PRE>



<FONT COLOR="#000080">ftp.tpci.com. IN CNAME merlin.tpci.com.</FONT></PRE>



<P>This line points anyone accessing the machine ftp.tpci.com to the real machine merlin.tpci.com. If the machine merlin has to be taken out of its FTP server role for any reason, a change in the machine name on this line points the ftp.tpci.com access to the new server. (A change in the alias performed over DNS can take a while to become active, as the change must be propagated through all the DNS databases.) The period following the domain name is very important because it prevents expansion of the name to include the domain again (which would result in merlin.tpci.com.tpci.com).



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<A NAME="E69E246"></A>



<H4 ALIGN=CENTER>