ch48.htm

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		<TD WIDTH="58" ALIGN="LEFT"><TT>cd</TT> </TD>

		<TD ALIGN="LEFT">Changes directory on the server </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>close</TT> </TD>

		<TD ALIGN="LEFT">Terminates the connection </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>del</TT> </TD>

		<TD ALIGN="LEFT">Deletes a file on the server </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>dir</TT> </TD>

		<TD ALIGN="LEFT">Displays the server directory </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>get</TT> </TD>

		<TD ALIGN="LEFT">Fetches a file from the server </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>hash</TT> </TD>

		<TD ALIGN="LEFT">Displays a pound character for each block transmitted </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>help</TT> </TD>

		<TD ALIGN="LEFT">Displays help </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>lcd</TT> </TD>

		<TD ALIGN="LEFT">Changes directory on the client </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>mget</TT> </TD>

		<TD ALIGN="LEFT">Fetches several files from the server </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>mput</TT> </TD>

		<TD ALIGN="LEFT">Sends several files to the server </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>open</TT> </TD>

		<TD ALIGN="LEFT">Connects to a server </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>put</TT> </TD>

		<TD ALIGN="LEFT">Sends a file to the server </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>pwd</TT> </TD>

		<TD ALIGN="LEFT">Displays the current server directory </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>quote</TT> </TD>

		<TD ALIGN="LEFT">Supplies an FTP command directly </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="58" ALIGN="LEFT"><TT>quit<BR>

			</TT></TD>

		<TD ALIGN="LEFT">Terminates the FTP session<BR>

					</TD>

	</TR>

</TABLE>

For most versions, FTP commands are case sensitive, and using uppercase will display

error messages. Some versions perform a translation for you, so it doesn't matter

which case you use. Since Linux uses lowercase as its primary character set for everything

else, you should probably use lowercase with all versions of FTP, too.

<CENTER>

<H4><A NAME="Heading5<FONT COLOR="#000077">How FTP Uses TCP</FONT></H4>

</CENTER>

<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.</P>

<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 <TT>ftpd</TT>.

The client program is <TT>ftp</TT>.</P>

<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 new-line 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.

This helps in a debugging process. Also, the ASCII commands can be used directly

by a knowledgeable user to communicate with the FTP server component without invoking

the client portion (in other words, communicating with <TT>ftpd</TT> without using

<TT>ftp</TT> on a local machine); this is seldom used except when debugging (or showing

off).</P>

<P>After logging into 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. This is the process followed by FTP when a connection is established:



<DL>

	<DD><B>1.</B> Login--Verify user ID and password<BR>

	<BR>

	<B>2. </B>Define directory--Identify the starting directory<BR>

	<BR>

	<B>3.</B> Define file transfer mode--Define the type of transfer<BR>

	<BR>

	<B>4. </B>Start data transfer--Allow user commands<BR>

	<B><BR>

	5. </B>Stop data transfer--Close the connection

</DL>



<P>These steps are performed in sequence for each connection.</P>

<P>A debugging option is available from the FTP command line by adding <TT>-d</TT>

to the command. This displays the command channel instructions. Instructions from

the client are shown with an arrow as the first character, while instructions from

the server have three digits in front of them. A <TT>PORT</TT> in the command line

indicates the address of the data channel on which the client is waiting for the

server's reply. If no <TT>PORT</TT> is specified, channel 20 (the default value)

is used. Unfortunately, the progress of data transfers cannot be followed in the

debugging mode. A sample session with the debug option enabled is shown here:<FONT

COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">$ ftp -d tpci_hpws4

Connected to tpci_hpws4.

220 tpci_hpws4 FTP server (Version 1.7.109.2 

&#194;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 listing how the mode changed from binary

to ASCII to send the directory listing, and then back to binary (the system default

value).

<CENTER>

<H3><A NAME="Heading6<FONT COLOR="#000077">Configuring FTP</FONT></H3>

</CENTER>

<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 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 with one (especially anonymously). The FTP site name

is of the format<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">ftp.domain_name.domain_type

</FONT></PRE>

<P>where 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 of<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">ftp.tpci.com

</FONT></PRE>

<P>showing that this is the anonymous FTP access for anyone accessing the <TT>tpci.com</TT>

domain. It is usually a bad idea to name your FTP site with a specific machine name,

such as<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">ftp.merlin.tpci.com

</FONT></PRE>

<P>because this 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 it 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 this:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">ftp   IN   CNAME    merlin.tpci.com

</FONT></PRE>

<P>This line points anyone accessing the machine <TT>ftp.tpci.com</TT> to the real

machine <TT>merlin.tpci.com</TT>. If the machine <TT>merlin</TT> has to be taken

out of its FTP server role for any reason, a change in the machine name on this line

will point the <TT>ftp.tpci.com</TT> access to the new server. (A change in the alias

performed over DNS can take a while to become active because the change must be propagated

through all the DNS databases.)

<CENTER>

<H4><A NAME="Heading7<FONT COLOR="#000077">Setting Up ftpd</FONT></H4>

</CENTER>

<P>The FTP daemon, <TT>ftpd</TT>, must be started on the FTP server. The daemon is

usually handled by <TT>inetd</TT> instead of the <TT>rc</TT> startup files, so <TT>ftpd</TT>

is only active when someone needs it. This is the best approach for all but the most

heavily loaded FTP sites. When started using <TT>inetd</TT>, the <TT>inetd</TT> daemon

watches the TCP command port (channel 21) for an arriving data packet requesting

a connection, and then spawns <TT>ftpd</TT>.</P>

<P>Make sure the <TT>ftpd</TT> daemon can be started by <TT>inetd</TT> by checking

the <TT>inetd</TT> configuration file (usually <TT>/etc/inetd.config</TT> or <TT>/etc/inetd.conf</TT>)

for a line that looks like this:<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">ftp   stream   tcp   nowait   root   /usr/etc/ftpd   ftpd -l

</FONT></PRE>

<P>If the line doesn't exist, add it to the file. With most Linux systems, the line

is already in the file, although it may be commented out. Remove the comment symbol

if this is the case. The FTP entry essentially specifies to <TT>inetd</TT> that FTP

is to use TCP, and that it should spawn <TT>ftpd</TT> every time a new connection

is made to the FTP port. In the preceding example, the <TT>ftpd</TT> daemon is started

with the <TT>-l</TT> option, which enables logging. You can ignore this option if

you want. You should replace the pathname <TT>/usr/etc/ftpd</TT> with the location

of your FTP daemon.</P>

<P>There are several <TT>ftpd</TT> daemon options that you can add to the <TT>/etc/inetd.config</TT>

line to control <TT>ftpd</TT>'s behavior. The following list contains the most commonly

used options: 

<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="30" ALIGN="LEFT" VALIGN="TOP"><TT>-d</TT> </TD>

		<TD ALIGN="LEFT" VALIGN="TOP">Adds debugging information to the syslog. </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="30" ALIGN="LEFT" VALIGN="TOP"><TT>-l</TT> </TD>

		<TD ALIGN="LEFT" VALIGN="TOP">Activates a logging of sessions (only failed and successful logins, not debug information).

			If the -l option is specified twice, all commands are logged, too. If specified three

			times, the size of all get and put file transfers are added as well. </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="30" ALIGN="LEFT" VALIGN="TOP"><TT>-t</TT> </TD>

		<TD ALIGN="LEFT" VALIGN="TOP">Sets the timeout period before ftpd terminates after a session is concluded (default

			is 15 minutes). The value is specified in seconds after the -t option. </TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="30" ALIGN="LEFT" VALIGN="TOP"><TT>-T</TT> </TD>

		<TD ALIGN="LEFT" VALIGN="TOP">Sets the maximum timeout period (in seconds) that a client can request. The default

			is two hours. This lets a client alter the normal default timeout for some reason.

					</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD WIDTH="30" ALIGN="LEFT" VALIGN="TOP"><TT>-u</TT> </TD>

		<TD ALIGN="LEFT" VALIGN="TOP">Sets the umask value for files uploaded to the local system. The default umask is

			022. Clients can request a different umask value. </TD>

	</TR>

</TABLE>



<CENTER>

<H4><A NAME="Heading8<FONT COLOR="#000077">FTP Logins</FONT></H4>

</CENTER>

<P>If you are going to set up a user-based FTP service where each person accessing

your system has a valid login name and password, then you must create an account