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<TITLE>The Answer Guy 50: Closing Ports, Disabling Unwanted Services</TITLE>
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<H4>By James T. Dennis,
<a href="mailto:linux-questions-only@ssc.com">linux-questions-only@ssc.com</a><BR>
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<H3 align="left"><img src="../../gx/dennis/qbubble.gif"
height="50" width="60" alt="(?) " border="0"
>Closing Ports, Disabling Unwanted Services</H3>
<p><strong>From Troy Miller on Tue, 18 Jan 2000
</strong></p>
<!-- ::
Closing Ports, Disabling Unwanted Services
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:: -->
<P><STRONG>
I'm new to Linux and I'm trying to secure my PC. I have a DSL line and I
found out I have 4 open ports (FTP, Finger, Telnet and SMTP). I'm trying to
find a resource on how to close these ports. I do not plan to use them at
this time. I don't want to compromise my system.
</STRONG></P>
<P><STRONG>
Thanks,
<br>Troy
</STRONG></P>
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
Bravo! You are doing the right thing.
</BLOCKQUOTE>
<BLOCKQUOTE>
Three of the services you mention are usually launched
dynamically by inetd. So you can disable them by
editing your <TT>/etc/inetd.conf</TT> file, find these services
by name, and commenting them out of the file (inserting
a "<tt>#</tt>" (hash/pound sign) at the beginning of those lines).
After that you can send a HUP signal to inetd to force it
to re-initialize itself. You can use the following command
to do that:
</BLOCKQUOTE>
<BLOCKQUOTE><BLOCKQUOTE><CODE>
kill -HUP $(cat <TT>/var/run/inetd.pid</TT>)
</CODE></BLOCKQUOTE></BLOCKQUOTE>
<BLOCKQUOTE>
Of course, in this case you aren't running ANY other
inetd services so you can even just kill you inetd process
using a command like:
</BLOCKQUOTE>
<BLOCKQUOTE><BlockQuote><Code>
/etc/rc.d/init.d/inetd stop
</Code></BlockQuote></BLOCKQUOTE>
<BLOCKQUOTE>
... or something like that. (The exact path to your
<TT>/etc/.../init.d</TT> scripts might differ depending on
your distribution).
</BLOCKQUOTE>
<BLOCKQUOTE>
You can also find an remove the <tt>rcX.d</tt> scripts that
start inetd during boot if you like. Your system
might have a '<tt>chkconfig</tt>' command to manage those
SysV init symlinks for you, or you can use the
technique that I've described here before
(create <TT>/etc/.../rc*.d/disabled</TT> directories, and
move the S??... symlinks into those as appropriate).
</BLOCKQUOTE>
<BLOCKQUOTE>
Search the back issues for "sysv init" to hear the
long-winded discussion of how all of that works.
</BLOCKQUOTE>
<BLOCKQUOTE>
That leaves the SMTP service. That is probably being
provided by sendmail, which is probably being loaded
"standalone" (not dynamically through inetd). So you
also want to find the <TT>/etc/.../rcX.d/S*</TT> script that's
starting sendmail and disable that.
</BLOCKQUOTE>
<BLOCKQUOTE>
After you've done that you'll have a machine that
doesn't provide any network services to anyone.
</BLOCKQUOTE>
<BLOCKQUOTE>
You can double check that using a command like:
</BLOCKQUOTE>
<BLOCKQUOTE><BLOCKQUOTE><CODE>
netstat --inet -an
</CODE></BLOCKQUOTE></BLOCKQUOTE>
<BLOCKQUOTE>
... and looking for anything that's in the LISTEN
state. You can also check it using a port scanner
like nmap from another system. (I'm guessing that
you've already been playing with one of those,
to get the information on which you've based this
question).
</BLOCKQUOTE>
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<H5 align="center"><a href="http://www.linuxgazette.com/copying.html"
>Copyright ©</a> 2000, James T. Dennis
<BR>Published in <I>The Linux Gazette</I> Issue 50 February 2000</H5>
<H6 ALIGN="center">HTML transformation by
<A HREF="mailto:star@starshine.org">Heather Stern</a> of
Starshine Technical Services,
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