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<TITLE>The Answer Guy 36:
More on: "Remote Login as root"
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LINK="#3366FF" VLINK="#A000A0">
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<H4>"The Linux Gazette...<I>making Linux just a little more fun!</I>"</H4>
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<H1><A NAME="answer">
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<font color="#B03060">The Answer Guy</font>
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<H4>By James T. Dennis,
<a href="mailto:answerguy@ssc.com">answerguy@ssc.com</a><BR>
Starshine Technical Services,
<A HREF="http://www.starshine.org/">http://www.starshine.org/</A>
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<!-- begin 24 -->
<H3 align="left"><img src="../../gx/dennis/qbubble.gif" height="50" width="60"
alt="(?) " border="0">
More on: "Remote Login as root"
</H3>
<p><strong>From Eric Freden on Fri, 04 Dec 1998
</strong></p>
<!-- ::
More on: "Remote Login as root"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:: -->
<P><STRONG><IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
Here is a legitimate use for remote login as root:
</STRONG></P>
<P><STRONG>
My kid plays some svga game on my console and
locks the keyboard (for instance). I want to telnet
in and <TT>/sbin/shutdown -r now</TT>
</STRONG></P>
<P><STRONG>
</STRONG></P>
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" alt="(!)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
Actually you can often recover from this without
a shutdown. But its a trick. Eventually we
might have something like KGI/GGI to provide
more robust SVGAlib support.
</BLOCKQUOTE>
<BLOCKQUOTE>
(The trick is to start X from your telnet/terminal
session. This usually does a complete video
system reset as a side effect. WARNING!:
This might hang the system --- so close any
running text mode apps, save any accessible
documents and issue a few calls to '<tt>sync</tt>' before
trying it).
</BLOCKQUOTE>
<BLOCKQUOTE>
</BLOCKQUOTE>
<P><STRONG><IMG SRC="../../gx/dennis/qbub.gif" ALT="(?)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
For RedHat 5.0 (and all other RedHat versions I've
used) only root can do this. Changing to su
and executing shutdown won't reboot! Perhaps
you could find a workaraound for this scenario?
</STRONG></P>
<P><STRONG>
Eric Freden
</STRONG></P>
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" alt="(!)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
If that's your experience then something is
wrong with your '<tt>su</tt>' command!
</BLOCKQUOTE>
<BLOCKQUOTE>
Did you issue '<tt>su -</tt>' (the dash is pretty important
--- as it forces the '<tt>su</tt>' command to run your
<tt>.login</tt>/<tt>.profile</tt> scripts and initialize root
environment (and shell variables, etc).
</BLOCKQUOTE>
<BLOCKQUOTE>
Another approach is to tweak the permissions
on '<tt>shutdown</tt>' Here's the recommended method:
</BLOCKQUOTE>
<BLOCKQUOTE>
Create a group such as "shutdown"
or use the "wheel" group.
</BLOCKQUOTE>
<BLOCKQUOTE>
Add your regular user account (and mom's?) to that group.
</BLOCKQUOTE>
<BLOCKQUOTE>
Issue a command like:
</BLOCKQUOTE>
<BLOCKQUOTE> <BLOCKQUOTE> <CODE>
chown root.$GROUP $(which shutdown)
</CODE> </BLOCKQUOTE> </BLOCKQUOTE>
<BLOCKQUOTE>
... to set the file group association
appropriately. You could also use
'<tt>chown</tt>' then '<tt>chgrp</tt>' seperately, of course.
</BLOCKQUOTE>
<BLOCKQUOTE>
Make it SUID with a command like:
</BLOCKQUOTE>
<BLOCKQUOTE> <BLOCKQUOTE> <CODE>
chgrp 4550 $(which shutdown)
</CODE> </BLOCKQUOTE> </BLOCKQUOTE>
<BLOCKQUOTE>
N.B. I set the execute bit for owner and group
but <EM>not</EM> for "other/world"
</BLOCKQUOTE>
<BLOCKQUOTE>
This allows people in the associated group to issue
the '<tt>shutdown</tt>' command. That command will run with
root' privileges. The 0 permissions for "other" prevent
"others" from executing this command at all. (Other
users have no valid reason to issue a shutdown command).
</BLOCKQUOTE>
<BLOCKQUOTE>
Setting binaries to be SUID always has implications for
system security. However, it is one of the primary forms
of authority delegation available in Unix/Linux.
</BLOCKQUOTE>
<BLOCKQUOTE>
In this case we minimize the risk by limiting the number
of accounts that can access the command.
</BLOCKQUOTE>
<BLOCKQUOTE>
This technique is generally useful and should be considered
for all Unix/Linux SUID commands.
</BLOCKQUOTE>
<!-- sig -->
<!-- end 24 -->
<!--startcut ======================================================= -->
<P> <hr> <P>
<H5 align="center"><a href="http://www.linuxgazette.com/ssc.copying.html"
>Copyright ©</a> 1999, James T. Dennis
<BR>Published in <I>The Linux Gazette</I> Issue 36 January 1999</H5>
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