ch06.htm

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	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Telnet</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">23</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Simple Mail Transfer Protocol (SMTP)</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">25</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Gopher</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">70</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Finger</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">79</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Hypertext Transfer Protocol (HTTP)</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">80</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Network News Transfer Protocol (NNTP)</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">119</TD>
	</TR>
</TABLE>
</P>
<P>I will examine each of the applications described in Table 6.2. All are application-level
protocols or services (that is, they are visible to user and the user can interact
with them at the console).


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>Cross Reference:</B></FONT><B> </B>For a comprehensive list
	of all port assignments, visit <A HREF="ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers"><TT>ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers</TT></A>.
	This document is extremely informative and exhaustive in its treatment of commonly
	assigned port numbers. 
<HR>


</BLOCKQUOTE>

<H3><FONT COLOR="#000077"><B>Telnet</B></FONT></H3>
<P>Telnet is best described in RFC 854, the Telnet protocol specification:

<DL>
	<DD>The purpose of the Telnet protocol is to provide a fairly general, bi-directional,
	eight-bit byte-oriented communications facility. Its primary goal is to allow a standard
	method of interfacing terminal devices and terminal-oriented processes to each other.
</DL>

<P>Telnet not only allows the user to log in to a remote host, it allows that user
to execute commands on that host. Thus, an individual in Los Angeles can Telnet to
a machine in New York and begin running programs on the New York machine just as
though the user were actually in New York.</P>
<P>For those of you who are unfamiliar with Telnet, it operates much like the interface
of a bulletin board system (BBS). Telnet is an excellent application for providing
a terminal-based front end to databases. For example, better than 80 percent of all
university library catalogs can be accessed via Telnet. Figure 6.4 shows an example
of a Telnet library catalog screen.</P>
<P><A NAME="04"></A><A HREF="04.htm"><B>Figure 6.4.</B></A><B><BR>
</B><I>A sample Telnet session.</I></P>
<P>Even though GUI applications have taken the world by storm, Telnet--which is essentially
a text-based application--is still incredibly popular. There are many reasons for
this. First, Telnet allows you to perform a variety of functions (retrieving mail,
for example) at a minimal cost in network resources. Second, implementing secure
Telnet is a pretty simple task. There are several programs to implement this, the
most popular of which is Secure Shell (which I will explore later in this book).</P>
<P>To use Telnet, the user issues whatever command necessary to start his or her
Telnet client, followed the name (or numeric IP address) of the target host. In UNIX,
this is done as follows:</P>
<PRE><FONT COLOR="#0066FF">#telnet internic.net
</FONT></PRE>
<P>This command launches a Telnet session, contacts <TT>internic.net</TT>, and requests
a connection. That connection will either be honored or denied, depending on the
configuration at the target host. In UNIX, the Telnet command has long been a native
one. That is, Telnet has been included with basic UNIX distributions for well over
a decade. However, not all operating systems have a native Telnet client. Table 6.3
shows Telnet clients for various operating systems.
<H4><FONT COLOR="#000077"><B>Table 6.3. Telnet clients for various operating systems.</B></FONT></H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><I>Operating System</I></TD>
		<TD ALIGN="LEFT" VALIGN="TOP"><I>Client</I></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">UNIX</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows 95</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native (command line), ZOC, NetTerm, Zmud, WinTel32, Yawtelnet</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows NT</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native (command line), CRT, and all listed for 95</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows 3.<I>x</I></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Trumptel Telnet, Wintel, Ewan</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Macintosh</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">NCSA Telnet, NiftyTelnet, Comet</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">VAX</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native</TD>
	</TR>
</TABLE>

<H3><FONT COLOR="#000077"><B>File Transfer Protocol</B></FONT></H3>
<P>File Transfer Protocol is the standard method of transferring files from one system
to another. Its purpose is set forth in RFC 0765 as follows:

<DL>
	<DD>The objectives of FTP are 1) to promote sharing of files (computer programs and/or
	data), 2) to encourage indirect or implicit (via programs) use of remote computers,
	3) to shield a user from variations in file storage systems among Hosts, and 4) to
	transfer data reliably and efficiently. FTP, though usable directly by a user at
	a terminal, is designed mainly for use by programs.
</DL>

<P>For over two decades, researchers have investigated a wide variety of file-transfer
methods. The development of FTP has undergone many changes in that time. Its first
definition occurred in April 1971, and the full specification can be read in RFC
114.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>Cross Reference:</B></FONT><B> </B>RFC 114 contains the
	first definition of FTP, but a more practical document might be RFC 959 (<A HREF="http://www.freesoft.org/Connected/RFC/959/index.html"><TT>http://www.freesoft.org/Connected/RFC/959/index.html</TT></A>).
	
<HR>


</BLOCKQUOTE>

<H4><FONT COLOR="#000077"><B>Mechanical Operation of FTP</B></FONT></H4>
<P>File transfers using FTP can be accomplished using any suitable FTP client. Table
6.4 defines some common clients used, by operating system.
<H4><FONT COLOR="#000077"><B>Table 6.4. FTP clients for various operating systems.</B></FONT></H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><I>Operating System</I></TD>
		<TD ALIGN="LEFT" VALIGN="TOP"><I>Client</I></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">UNIX</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native, LLNLXDIR2.0, FTPtool</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows 95</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native, WS_FTP, Netload, Cute-FTP, Leap FTP, SDFTP, FTP Explorer</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows NT</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">See listings for Windows 95</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows 3.<I>x</I></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Win_FTP, WS_FTP, CU-FTP, WSArchie</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Macintosh</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Anarchie, Fetch, Freetp</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">OS/2</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Gibbon FTP, FTP-IT, Lynn's Workplace FTP</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">VAX</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native</TD>
	</TR>
</TABLE>

<H4><FONT COLOR="#000077"><B>How Does FTP Work?</B></FONT></H4>
<P>FTP file transfers occur in a client/server environment. The requesting machine
starts one of the clients named in Table 6.4. This generates a request that is forwarded
to the targeted file server (usually a host on another network). Typically, the request
is sent by inetd to port 21. For a connection to be established, the targeted file
server must be running an FTP server or FTP daemon.</P>
<P><B>FTPD</B> FTPD is the standard FTP server daemon. Its function is simple: to
reply to connect requests received by inetd and to satisfy those requests for file
transfers. This daemon comes standard on most distributions of UNIX (for other operating
systems, see Table 6.5).
<H4><FONT COLOR="#000077"><B>Table 6.5. FTP servers for various operating systems.</B></FONT></H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP"><I>Operating System</I></TD>
		<TD ALIGN="LEFT" VALIGN="TOP"><I>Client</I></TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">UNIX</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Native (FTPD)</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows 95</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">WFTPD, Microsoft FrontPage, WAR FTP Daemon, Vermilion</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows NT</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Serv-U, OmniFSPD, Microsoft Internet Information Server</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Microsoft Windows 3.<I>x</I></TD>
		<TD ALIGN="LEFT" VALIGN="TOP">WinQVT, Serv-U, Beames &amp; Whitside BW Connect, WFTPD FTP Server, WinHTTPD</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">Macintosh</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Netpresenz, FTPD</TD>
	</TR>
	<TR ALIGN="LEFT" rowspan="1">
		<TD ALIGN="LEFT" VALIGN="TOP">OS/2</TD>
		<TD ALIGN="LEFT" VALIGN="TOP">Penguin</TD>
	</TR>
</TABLE>
</P>
<P>FTPD waits for a connection request. When such a request is received, FTPD requests
the user login. The user must either provide his or her valid user login and password
or may log in anonymously.</P>
<P>Once logged in, the user may download files. In certain instances and if security
on the server allows, the user may also upload files.
<H4><FONT COLOR="#000077"><B>Simple Mail Transfer Protocol</B></FONT></H4>
<P>The objective of Simple Mail Transfer protocol is stated concisely in RFC 821:

<DL>
	<DD>The objective of Simple Mail Transfer protocol (SMTP) is to transfer mail reliably
	and efficiently.
</DL>

<P>SMTP is an extremely lightweight and efficient protocol. The user (utilizing any
SMTP- compliant client) sends a request to an SMTP server. A two-way connection is
subsequently established. The client forwards a MAIL instruction, indicating that
it wants to send mail to a recipient somewhere on the Internet. If the SMTP allows
this operation, an affirmative acknowledgment is sent back to the client machine.
At that point, the session begins. The client may then forward the recipient's identity,
his or her IP address, and the message (in text) to be sent.</P>
<P>Despite the simple character of SMTP, mail service has been the source of countless
security holes. (This may be due in part to the number of options involved. Misconfiguration
is a common reason for holes.) I will discuss these security issues later in this
book.</P>
<P>SMTP servers are native in UNIX. Most other networked operating systems now have
some form of SMTP, so I'll refrain from listing them here.


<BLOCKQUOTE>
	<P>
<HR>
<FONT COLOR="#000077"><B>Cross Reference:</B></FONT><B> </B>Further information on
	this protocol is available in RFC 821 (<A HREF="http://sunsite.auc.dk/RFC/rfc/rfc821.html"><TT>http://sunsite.auc.dk/RFC/rfc/rfc821.html</TT></A>).
	
<HR>


</BLOCKQUOTE>

<H4><FONT COLOR="#000077"><B>Gopher</B></FONT></H4>
<P>The Gopher service is a distributed document-retrieval system. It was originally
implemented as the Campus Wide Information System at the University of Minnesota.
It is defined in a March 1993 FYI from the University of Minnesota as follows:

<DL>
	<DD>The Internet Gopher protocol is designed primarily to act as a distributed document-delivery
	system. While documents (and services) reside on many servers, Gopher client software
	presents users with a hierarchy of items and directories much like a file system.