File: stoddard.html

package info (click to toggle)
lg-issue55 1-5
links: PTS
area: main
in suites: woody
size: 1,600 kB
ctags: 94
sloc: makefile: 36; sh: 7
file content (458 lines) | stat: -rw-r--r-- 21,635 bytes
<!--startcut  ==============================================-->
<!-- *** BEGIN HTML header *** -->
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML><HEAD>
<title>Building a Secure Gateway, part II LG #55</title>
</HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#0000AF"
ALINK="#FF0000">
<!-- *** END HTML header *** -->

<CENTER>
<A HREF="http://www.linuxgazette.com/">
<H1><IMG ALT="LINUX GAZETTE" SRC="../gx/lglogo.jpg" 
	WIDTH="600" HEIGHT="124" border="0"></H1></A> 

<!-- *** BEGIN navbar *** -->
<IMG ALT="" SRC="../gx/navbar/left.jpg" WIDTH="14" HEIGHT="45" BORDER="0" ALIGN="bottom"><A HREF="skjoldebrand.html"><IMG ALT="[ Prev ]" SRC="../gx/navbar/prev.jpg" WIDTH="16" HEIGHT="45" BORDER="0" ALIGN="bottom"></A><A HREF="lg_toc55.html"><IMG ALT="[ Table of Contents ]" SRC="../gx/navbar/toc.jpg" WIDTH="220" HEIGHT="45" BORDER="0" ALIGN="bottom" ></A><A HREF="../lg_frontpage.html"><IMG ALT="[ Front Page ]" SRC="../gx/navbar/frontpage.jpg" WIDTH="137" HEIGHT="45" BORDER="0" ALIGN="bottom"></A><A HREF="http://www.linuxgazette.com/cgi-bin/talkback/all.py?site=LG&article=http://www.linuxgazette.com/issue55/stoddard.html"><IMG ALT="[ Talkback ]" SRC="../gx/navbar/talkback.jpg" WIDTH="121" HEIGHT="45" BORDER="0" ALIGN="bottom"  ></A><A HREF="../lg_faq.html"><IMG ALT="[ FAQ ]" SRC="./../gx/navbar/faq.jpg"WIDTH="62" HEIGHT="45" BORDER="0" ALIGN="bottom"></A><A HREF="tindale.html"><IMG ALT="[ Next ]" SRC="../gx/navbar/next.jpg" WIDTH="15" HEIGHT="45" BORDER="0" ALIGN="bottom"  ></A><IMG ALT="" SRC="../gx/navbar/right.jpg" WIDTH="15" HEIGHT="45" ALIGN="bottom">
<!-- *** END navbar *** -->
<P>
</CENTER>

<!--endcut ============================================================-->

<H4 ALIGN="center">
"Linux Gazette...<I>making Linux just a little more fun!</I>"
</H4>

<P> <HR> <P> 
<!--===================================================================-->

<center>
<H1><font color="maroon">Building a Secure Gateway, part II</font></H1>
<H4>By <a href="mailto:cstod@vvm.com">Chris Stoddard</a></H4>
</center>
<P> <HR> <P>  

<!-- END header -->




<H2>Introduction</H2>
<P> In the 
<A HREF="../issue54/stoddard.html">last article</A>, we installed Linux with
only those packages we absolutly needed.  (If you have not read my previous
article, you should do so now, as it is the base from which this is built on.)
Now comes the detail work, turning your gateway into fortress.  The first thing
to understand is there is no way to be completely secure.  There is just not
enough time to do it all, Corporations employ huge IT departments, whose sole
purpose in life is to secure their networks, and still they get cracked.  Just
accept it and get on with your life.  Our real goal here is to keep honest
people honest, keep the Script Kiddies out and slow the rest down, giving you
opprotunity to discover them.  Ideally, this should be done right after the
clean install, before the system ever gets put on the Internet.  This article
assumes you know something about Linux, how to install it, how to edit
various configuration files, and that you can log in as root.

<P> I also assume you are setting up a firewall system and have
no intention of running DNS, DHCP, web, ftp  or telnet server.
If you intend to run any of these services, of these services,
I recommend setting up seperate machines.  Setup a DMZ on your
network, a system which is secured but allows connections
from system outside your network.  This way if an intruder
does penetrate your server, he will have to start all over
to penetrate your firewall system and you will hopefully
discovered his breakin before he is able to get access to
your internal network.

<H2>System Updates and Security Advisories</H2>
<P> In the world of Computer Security, Knowledge is Power.  Frankly
the Security Experts are always one step behind the Crackers,
most security issues are not discovered by the Experts,
but by the Crackers and are plugged only after they have been
exploited.  You need to keep up to date on new problems, at the
very least you should be updating the packages as they come out. 
Type "rpm -qa &gt; packages.txt", this gives you a list of the
packages and version numbers installed on your system, then go to
Redhat's web site and download the updated packages.  While you
are there you should read the security advisories and implement
any changes they suggest.  If you are really proactive, subscribe
to both the BugTraq and CERT mailing lists.

<H2>Physical Security</H2>
<P>  Since this article is aimed at the home cable modem user, I
will assume physical security is not a problem.  If you have
child or a nosey baby sitter, consider using the BIOS password
protection built into most computers.

<H2>User Accounts and Passwords</H2>
  <P>Besides the root account and the special accounts, which I'll
go into in a moment, there should be only one user account.  The
user and the root accounts should have good passwords.  A good
password is one that is at least 8 characters long, has a mix of
small letters, capital letters and numbers, and is not a
dictionary word.  It is also a good idea to change these passwords
from time to time and do not write the passwords on a sticky note
and put it on the monitor where everyone can see it.  Use different
passwords on each computer on your network, that way, if one system
is cracked an intruder will still not have access to the other
systems on the network.  Again, because password cracking takes
time, you will hopefully discover the cracker before he gets too
far.

<P>  Along this same line, there are several special purpose
accounts, which are installed by default with most Linux
distributions, for our purposes these accounts are useless and
pose a security risk, so we will remove them using the userdel
command.  The syntax for this command is "userdel username",
substituting username with the appropriate account name.  The
accounts we want to remove are;  adm, lp, sync, shutdown, halt,
news, uucp, operator, games, gopher, and ftp.  We also want to
remove the associated groups with groupdel, the syntax is the
same.  Groups to delete are; adm, lp, news, uucp, games, dip,
pppusers, popusers, and slipusers.

<H2>Configuration files</H2>
<P>  This is without a doubt the most important section.  
Poorly-maintained configuration files are the highest risk factor on any
system.  In this section you will be typing many of the same
commands over and over again, this is a good opportunity to write
a shell script to make this easier.  What we want to do, after
we are finished with each file, is to first make sure it's
owned by root; second, that the only account which can read and write to
it is root; and third, that it's unalterable even by root.  This keeps the
files from being accidentally deleted or changed and also
prevents the file from being linked to, which could be a security
risk.  Type "touch secure-it", then type "chmod +x secure-it", now
Open the file in your text editor of choice and put these lines
in: (<A HREF="misc/stoddard/secure-it.sh.txt">text version</A>)

<PRE>
#!/bin/sh

# Change owner to root
chown root.root $1

# Change permissions so only root has access
chmod 600 $1

# Make the file unalterable
chattr +i $1
</PRE>


<P> Now save the file and copy it to /usr/sbin by typing "cp
secure-it /usr/sbin".  Now when we are finished with a file we
can lock it down simply by typing "secure-it filename".

<DL>
<DT><STRONG>/etc/exports</STRONG>
<DD>This file tells the system which system on you network are
allowed to mount NFS drives from this system.  This file should
be empty, if it is not, delete it with "rm /etc/exports" and
create a new empty one with "touch /etc/exports".  Now lock the
file down by typing "secure-it /etc/exports".<P>

<DT><STRONG>/etc/inetd.conf</STRONG>
<DD>This where many of the TCP/IP services start.  Since the only
service we want is ssh--which is not started by inetd--this file
should be empty as well.  So remove it, "rm /etc/inetd.conf",
create an empty one with "touch /etc/inetd.conf".  Lock it down
with "secure-it /etc/inetd.conf".<P>

<DT><STRONG>/etc/hosts.deny</STRONG>
<DD>This tells the system which systems to deny access to your
system's TCP/IP services.  We want to deny everybody not listed
in /etc/hosts.allow everything, so we edit this file so the
following lines are the only thing in it, then we want to lock it
down.

<PRE>
ALL: ALL
</PRE><P>

<DT><STRONG>/etc/hosts.allow</STRONG>
<DD> This file tells the system which other system may access the services
started in inetd.conf, Since inetd.conf is empty, this file should be emty as
well.  Remove it with "rm /etc/hosts.allow", create an empty one with "touch
/etc/hosts.allow", and lock it down with "secure-it /etc/hosts.allow"<P>

<DT><STRONG>/etc/rc.d/rc.local</STRONG>
<DD> The next thing we want to do is keep your system from giving
away too much information about itself through attempted logons
and ICMP packets.  First, delete the /etc/issue and
/etc/issue.net files from the system.  Next open
/etc/rc.d/rc.local in your text editor and remove the following
lines;

<PRE>
echo "" &gt; /etc/issue
echo "$R" &gt;&gt; /etc/issue
echo "Kernel $(uname -r) on $a $SMP$(uname -m)" &gt;&gt; /etc/issue
cp -f /etc/issue /etc/issue.net
echo &gt;&gt; /etc/issue 
</PRE>

Before you save and close the /etc/rc.d/rc.local file, we want 
to keep the system from responding to ICMP requests, such as ping
and traceroute, so we add the following lines right after the
#!/bin/sh line:

<PRE>
echo 1 &gt; /proc/sys/net/ipv4/icmp_echo_ignore_all
echo 1 &gt; /proc/sys/net/ipv4/tcp_syncookies
</PRE>

  This will make your system all but invisible to the outside
world; the Script Kiddies can't crack what they can't find.  The
second line helps protect your system from SYN Denial of Service
Attacks.  Go ahead and save the file and exit.  Please note, this
will also keep you from pinging the system, but should not
interfere with other functions, such as ssh or IP forwarding.
Finally, lock it down.<P>

<DT><STRONG>/etc/hosts.conf</STRONG>
<DD>While we are setting up defenses against outside attacks, we
should add as the last line in /etc/host.conf the following;

<PRE>
nospoof on
</PRE>

This will cause the system to reject any requests coming from a
source outside your network, claiming to be a system on the
inside of your LAN, this type of a attack is called IP Spoofing.
Go ahead and lock it down with secure-it. 

  Other files we don't need to alter, but need to be locked down
are, /etc/services, /etc/passwd, /etc/shadow, /etc/group and
/etc/gshadow.  If you plan to change your passwd or add a user
you will have to run "chattr -i filename" on /etc/passwd,
/etc/shadow, /etc/group and /etc/gshadow or you will get an error
message. <P>

<DT><STRONG>/etc/fstab</STRONG>
<DD>This is where the system gets information about what drives and
partitions should be mounted at bootup and where.  If you
configured your system with one large root partition or you did
not setup a separate partition for /home and /tmp, you can skip
this section and go on to setiing up disk quotas. /home and /tmp
are important areas because they can be written to by users other
than root.  What we want to do is limit what a user can do on
these partitions.  In /home, we do not want the user to be able
to create an SUID program or a device, additionally in /tmp we
don't want any programs to be able to execute.  We do this by
changing the /etc/fstab.  Mine looks like this and likely yours
is similar.

<PRE>
/dev/hda1      /              ext2      defaults        1 1
/dev/hda9      /boot          ext2      defaults        1 2
/dev/cdrom     /mnt/cdrom     iso9660   noauto,owner,ro 0 0
/dev/hda5      /home          ext2      defaults        1 2
/dev/hda6      /tmp           ext2      defaults        1 2
/dev/sda1      /usr           ext2      defaults        1 2
/dev/hda7      /var           ext2      defaults        1 2
/dev/hda8      swap           swap      defaults        0 0
/dev/fd0       /mnt/floppy    msdos     noauto,owner    0 0
none           /proc          proc      defaults        0 0
none           /dev/pts       devpts    gid=5,mode=620  0 0
</PRE>

We want to change the /home and /tmp lines to read as follows:

<PRE>
/dev/hda5      /home          ext2        rw,nosuid,nodev 1 2
/dev/hda6      /tmp           ext2 rw,nosuid,nodev,noexec 1 2
</PRE><P>

<DT><STRONG>Disk Quota</STRONG>
<DD>  If you setup a seperate partition for /home, this step is optional. 
If, instead, you setup your hard drive as one large partition, you
should use the disk quota system, this will limit the amount
of drive space a user can consume and prevent an intruder who has
stolen the account from filling up your hard drive.  The default
line in /etc/fstab will look something like this.

<PRE>
/dev/hda1      /              ext2      defaults        1 1
</PRE>

Change it to this,

<PRE>
/dev/hda1      /              ext2  defaults,usrquota   1 1
</PRE>

Add the following lines to /etc/rc.d/rc.local
  
<PRE>
/sbin/quotacheck -avug
/sbin/quotaon -avug
</PRE>

  now type "touch /quota.user" and then "chmod 700 /quota.user"
and reboot the system.  There may have some error messages about
quota; ignore them.  Once the system is back up, you will need
to set the quota for, what should be the only user account, type
the following command, replacing "username" the the name of your
user account,  type "edquota -u username".  This should bring up
the vi text editor showing something similar to this.

<PRE>
Quotas for user username:
/dev/hda1: blocks in use: 7, limits (soft = 0, hard = 0)
    inodes in use: 6, limits (soft = 0, hard = 0)
</PRE>

By setting a block limit, you are limiting how much drive
space the user can consume in KB, by setting the inodes you
will be limiting the amount of files the user can have.
Soft limits when exceeded will warn the user, hard limits
are absolute.  Unless you have a very good reason to set
them higher, such as you plan on transfering MP3's to this
machine, I suggest setting the limits fairly low, something
like 10 MB of disk space and 100 files.  Edit the lines so
they look like like this, then save the file and exit.

<PRE>
Quotas for user username:
/dev/hda1: blocks in use: 7, limits (soft = 5120, hard = 10240)
    inodes in use: 6, limits (soft = 50, hard = 100)
</PRE>

  This will set a soft limit of 50 files taking up 5 MB and an
absolute limit of 100 files consuming 10 MB of drive space.<P>

<DT><STRONG>/etc/rc.d/init.d/*</STRONG>
<DD>Next we want to make sure all the startup scripts in
/etc/rc.d/init.d have the proper permissions set, so type the
following command, "chmod -R 700 /etc/rc.d/init.d/*".<P>

<DT><STRONG>SUID Programs</STRONG>
<DD>We must find all the SUID files on the system.  These are
programs which take on the identity of root when they are
executed, this is a very large security risk. This makes these
programs targets for buffer overflow attacks and replacement
with Trojans.  To find all the SUID programs on the system type "ls
-alF `find / -perm -4000` &gt; /root/suid.txt".  Now open the
suid.txt file and the output should look something like this.

<PRE>
-rwsr-xr-x 1 root root 35168 Sep 22 23:35 /usr/bin/chage
-rwsr-xr-x 1 root root 36756 Sep 22 23:35 /usr/bin/gpasswd
-r-xr-sr-x 1 root tty 6788 Sep 6 18:17 /usr/bin/wall
-rwsr-xr-x 1 root root 33152 Aug 16 16:35 /usr/bin/at
-rwxr-sr-x 1 root man 34656 Sep 13 20:26 /usr/bin/man
-r-s--x--x 1 root root 22312 Sep 25 11:52 /usr/bin/passwd
-rws--x--x 2 root root 518140 Aug 30 23:12 /usr/bin/suidperl
-rws--x--x 2 root root 518140 Aug 30 23:12 /usr/bin/sperl5.00503
-rwxr-sr-x 1 root slocate 24744 Sep 20 10:29 /usr/bin/slocate
-rws--x--x 1 root root 14024 Sep 9 01:01 /usr/bin/chfn
-rws--x--x 1 root root 13768 Sep 9 01:01 /usr/bin/chsh
-rws--x--x 1 root root 5576 Sep 9 01:01 /usr/bin/newgrp
-rwxr-sr-x 1 root tty 8328 Sep 9 01:01 /usr/bin/write
-rwsr-xr-x 1 root root 21816 Sep 10 16:03 /usr/bin/crontab
-rwsr-xr-x 1 root root 5896 Nov 23 21:59 /usr/sbin/usernetctl
-rwsr-xr-x 1 root bin 16488 Jul 2 10:21 /usr/sbin/traceroute
-rwxr-sr-x 1 root utmp 6096 Sep 13 20:11 /usr/sbin/utempter
-rwsr-xr-x 1 root root 14124 Aug 17 22:31 /bin/su
-rwsr-xr-x 1 root root 53620 Sep 13 20:26 /bin/mount
-rwsr-xr-x 1 root root 26700 Sep 13 20:26 /bin/umount
-rwsr-xr-x 1 root root 18228 Sep 10 16:04 /bin/ping
-rwxr-sr-x 1 root root 3860 Nov 23 21:59 /sbin/netreport
-r-sr-xr-x 1 root root 26309 Oct 11 20:48 /sbin/pwdb_chkpwd
</PRE>

  As you can see right hand side shows the permissions of each
file: anything with an "s" in it has its SUID bit set.  By
disabling SUID bit, only root will be able to run the program. 
What needs to be done now is decide which ones can have the SUID bit
safely turned off--many of these programs require it for normal
operations. Many, however, should be run only by root 
anyway.  How you turn the SUID bit off is with the following
command:"chmod a-s filename".  My suggestions for this step
are; /usr/bin/chage, /usr/bin/gpasswd, /usr/bin/wall,
/usr/bin/chfn, /usr/bin/chsh, /usr/bin/newgrp, /usr/bin/write,
/usr/sbin/usernetctl, /usr/sbin/traceroute, /bin/mount,
/bin/umount, /bin/ping, and /sbin/netreport.

</DL>

<H2>Checking system integrity</H2>
<P>  The last thing you want to do is setup your system to warn you
of any changes to your system.  If any intruders do get in and
plant a Trojan or create a new account, we want the system to be
able to tell what was altered.  There are several good programs
available for this, the easiest to implement that I've found is
fcheck, which can be downloaded from,
<A HREF="http://sites.netscape.net/fcheck/fcheck.html">http://sites.netscape.net/fcheck/fcheck.html</A>.

  Follow the instructions for installing and configuring the
software, it is very straight forward.  Once this is done, you
will want it to run at least once a day and redirect the results to
a file in the root directory.  This can be done through crond,
to setup a cron job, type "crontab -e" this will open vi, now
type the following line:

<PRE>
1 0 * * * /usr/local/fcheck/fcheck -a &gt; /root/fcheck.txt
</PRE>

  Replace the path to check with your own path, save and exit. 
Now at 12:01 every night, fcheck will run and the output will be
placed in /root/fcheck.txt.  If at any time fcheck detects altered
files which you cannot account for, immeaditly remove the package
from the system and reinstall it from the RedHat CD.  Anytime you
make a change to a file, you will need to rerun "fcheck -ca" and
build another baseline.

<H2>Finished</H2>
<P> It is relatively safe to put the system on the internet.  Once
this is done you will want to test your security.  
<A HREF="http://www.grc.com">Gibson Research Corporation</A>
provides a port scanning service.
In a perfect world, all the ports should be in stealth mode,
meaning the ports do not respond to requests at all and will
appear as though there is no system at your IP address.  In a
pinch the ports should be closed, meaning the port responds,
but will not take requests; closed ports are still vulnerable
to some types of attacks.  Open ports are vulnerable ports,
if any of your ports are open go back to the inetd.conf file,
make sure it is empty, check to make sure apache, wu-ftpd or
similar are not installed, also review your ipchains settings
to ensure it is denying packets properly.  It is a good idea to
do this regularly to ensure an intruder has not opened a port
for his personal use.

<P>  Again, as with my last article, I'd like to point out, this is
not the end-all and be-all of Linux security, this is only a
starting point.  I have simplified this down to the very basics,
there are many more things which could be done.  Whether or not
you should seek out these solutions, depends on what you are
protecting.  For a home user this will probably do just fine,
however even a small business with more machines and data to
protect, you should do more research and implement as much
security as is possible.  Better yet, hire a Network Security
Consultant to implement it for you.



<!-- *** BEGIN copyright *** -->
<P> <hr> <!-- P --> 
<H5 ALIGN=center>

Copyright &copy; 2000, Chris Stoddard<BR> 
Published in Issue 55 of <i>Linux Gazette</i>, July 2000</H5>
<!-- *** END copyright *** -->

<!--startcut ==========================================================-->
<HR><P>
<CENTER>
<!-- *** BEGIN navbar *** -->
<IMG ALT="" SRC="../gx/navbar/left.jpg" WIDTH="14" HEIGHT="45" BORDER="0" ALIGN="bottom"><A HREF="skjoldebrand.html"><IMG ALT="[ Prev ]" SRC="../gx/navbar/prev.jpg" WIDTH="16" HEIGHT="45" BORDER="0" ALIGN="bottom"></A><A HREF="lg_toc55.html"><IMG ALT="[ Table of Contents ]" SRC="../gx/navbar/toc.jpg" WIDTH="220" HEIGHT="45" BORDER="0" ALIGN="bottom" ></A><A HREF="../lg_frontpage.html"><IMG ALT="[ Front Page ]" SRC="../gx/navbar/frontpage.jpg" WIDTH="137" HEIGHT="45" BORDER="0" ALIGN="bottom"></A><A HREF="http://www.linuxgazette.com/cgi-bin/talkback/all.py?site=LG&article=http://www.linuxgazette.com/issue55/stoddard.html"><IMG ALT="[ Talkback ]" SRC="../gx/navbar/talkback.jpg" WIDTH="121" HEIGHT="45" BORDER="0" ALIGN="bottom"  ></A><A HREF="../lg_faq.html"><IMG ALT="[ FAQ ]" SRC="./../gx/navbar/faq.jpg"WIDTH="62" HEIGHT="45" BORDER="0" ALIGN="bottom"></A><A HREF="tindale.html"><IMG ALT="[ Next ]" SRC="../gx/navbar/next.jpg" WIDTH="15" HEIGHT="45" BORDER="0" ALIGN="bottom"  ></A><IMG ALT="" SRC="../gx/navbar/right.jpg" WIDTH="15" HEIGHT="45" ALIGN="bottom">
<!-- *** END navbar *** -->
</CENTER>
</BODY></HTML>
<!--endcut ============================================================-->