glfs/postlfs/security/firewalling.xml

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<sect1 id="fw-firewall" xreflabel="Firewalling">
<sect1info>
<othername>$LastChangedBy$</othername>
<date>$Date$</date>
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<?dbhtml filename="firewall.html"?>
<title>Setting up a network firewall</title>

<para>Before you read this part of the chapter, you should
have already installed iptables as described in the previous section.</para>

<sect2 id="fw-intro" xreflabel="Firewalling Introduction">
<title>Introduction to Firewall Creation</title>

<para>The general purpose of a firewall is to protect a computer or a network
against malicious access.</para> 

<para>In a perfect world, every daemon or service 
on every machine is perfectly configured and immune to flaws such as 
buffer overflows or other problems regarding its 
security. Furthermore, you trust every user accessing your services.
In this world, you do not need to have a firewall.</para>

<para>In the real world however, daemons may be misconfigured
and exploits against essential services are freely available.  You 
may wish to choose which services are accessible by certain machines or 
you may wish to limit which machines or applications are allowed external
access. Alternatively, you may simply not trust some of your 
applications or users. You are probably connected to the Internet.  In this
world, a firewall is essential.</para>

<para>Don't assume however, that having a firewall makes careful configuration
redundant, or that it makes any negligent misconfiguration harmless. It doesn't
prevent anyone from exploiting a service you intentionally offer but haven't
recently updated or patched after an exploit went public.  Despite having a
firewall, you need to keep applications and daemons on your system properly
configured and up to date.  A firewall is not a cure all, but should be an
essential part of your overall security startegy.</para>

</sect2>

<sect2>
<title>Meaning of the word "firewall"</title>

<para>The word firewall can have several different meanings.</para>

<sect3><title><xref linkend="fw-persFw"/></title>

<para>This is a hardware device or software program commercially sold by 
companies such as Symantec which claims that it
secures a home or desktop computer with Internet access. This type of firewall is 
highly relevant for users who do not know how their computers 
might be accessed via the Internet or how to disable that access, 
especially if they are always online and connected via 
broadband links.</para></sect3>

<sect3>
<title><xref linkend="fw-masqRouter"/></title>

<para>This is a system placed between the Internet and an intranet.  To minimize
the risk of compromising the firewall itself, it should generally have only one
role&mdash;that of protecting the intranet.  Although not completely risk free,
the tasks of doing the routing and  IP masquerading (rewriting IP headers of
the packets it routes from clients with private IP addresses onto the Internet
so that they seem to come from the firewall itself) are commonly considered
relatively secure.</para>
</sect3>

<sect3> 
<title><xref linkend="fw-busybox"/></title> 

<para>This is often an old computer you may have retired and nearly forgotten,
performing masquerading or routing functions, but offering non-firewall
services such as a web-cache or mail.  This may be used for home
networks, but is not be considered as secure as a firewall only
machine because the combination of server and router/firewall on one machine
raises the complexity of the setup.</para>
</sect3>

<sect3>
<title>Firewall with a demilitarized zone [not further described here]</title>
<para>This box performs masquerading or routing, but grants public access to 
some branch of your network which, because of public IP's and a physically 
separated structure, is essentially a separate network with direct Internet access.  
The servers on this network are those which must be easily accessible 
from both the Internet and intranet. The firewall protects 
both networks. This type of firewall has a minimum of three network interfaces.</para>
</sect3>

<sect3>
<title>Packetfilter</title>
<para>This type of firewall does routing or masquerading, but does not maintain
a state table of ongoing communication streams. It is fast, but quite limited
in its ability to block inappropriate packets without blocking desired
packets.</para>
</sect3>
</sect2>

<sect2 id="fw-writing" xreflabel="writing the firewalling-setup-scripts">
<title>Now you can start to build your Firewall</title>

<caution><para>This introduction on how to setup a firewall
is not a complete guide to securing systems.  Firewalling is a complex issue
that requires careful configuration.  The scripts quoted here are simply
intended to give examples of how a firewall works. They are not intended to
fit into any particular configuration and may not provide complete protection 
from an attack.</para>

<para>Customization of these scripts for your specific situation will
be necessary for an optimal configuration, but you should make a serious
study of the iptables documentation and creating firewalls in general before 
hacking away.  Have a look at the list of 
<xref linkend="fw-library"/> at the end of this section for 
more details.  There you will find a list of URLs that contain quite 
comprehensive information about building your own firewall.</para>
</caution>

<para>The firewall configuration script installed in the last section differs
from the standard configuration script.  It only has two of the standard
targets: start and status.  The other targets are clear and lock.  For instance when you
run:

<screen><userinput><command>/etc/rc.d/init.d/iptables start</command></userinput></screen>

the firewall will be restarted just as it is upon system startup.  The status target
will present a list of all currently implemented rules.  The clear target turns off all 
firewall rules and the lock target will block all packets in and out of the computer
with the exception of the loopback interface.</para>

<para>The main startup firewall is located in the file 
<filename>/etc/rc.d/rc.iptables</filename>.  The sections below provide three different
approaches that can be used for a system.</para>

<note><para>You should always run your firewall rules from a script.  This ensures
consistency and a record of what was done.  It also allows retention of comments
that are essential for understanding the rules long after they were written.
</para></note>

<sect3 id="fw-persFw" xreflabel="Personal Firewall">
<title>Personal Firewall</title>

<para>A Personal Firewall is designed to let you access all the services
offered on the Internet, but keep your box secure and your data private.</para>

<para>Below is a slightly modified version of Rusty Russell's recommendation
from the 
<ulink url="http://www.netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html">
Linux 2.4 Packet Filtering HOWTO</ulink>.  It is still applicable to the Linux 2.6 kernels.</para>

<screen><userinput><command>cat &gt; /etc/rc.d/rc.iptables &lt;&lt; "EOF"</command>
#!/bin/sh

# Begin $rc_base/rc.iptables

# Insert connection-tracking modules 
# (not needed if built into the kernel)
modprobe ip_tables
modprobe iptable_filter
modprobe ip_conntrack
modprobe ip_conntrack_ftp
modprobe ipt_state
modprobe ipt_LOG

# Enable broadcast echo Protection
echo 1 &gt; /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# Disable Source Routed Packets
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_source_route

# Enable TCP SYN Cookie Protection
echo 1 &gt; /proc/sys/net/ipv4/tcp_syncookies

# Disable ICMP Redirect Acceptance
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_redirects

# Don<6F>t send Redirect Messages
echo 0 &gt; /proc/sys/net/ipv4/conf/all/send_redirects

# Drop Spoofed Packets coming in on an interface, where responses 
# would result in the reply going out a different interface.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/rp_filter

# Log packets with impossible addresses.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/log_martians

# be verbose on dynamic ip-addresses  (not needed in case of static IP)
echo 2 &gt; /proc/sys/net/ipv4/ip_dynaddr

# disable Explicit Congestion Notification 
# too many routers are still ignorant
echo 0 &gt; /proc/sys/net/ipv4/tcp_ecn

# Set a known state
iptables -P INPUT   DROP
iptables -P FORWARD DROP
iptables -P OUTPUT  DROP
 
# These lines are here in case rules are already in place and the
# script is ever rerun on the fly. We want to remove all rules and
# pre-exisiting user defined chains before we implement new rules.
iptables -F
iptables -X
iptables -Z
 
iptables -t nat -F

# Allow local-only connections
iptables -A INPUT  -i lo -j ACCEPT

# Free output on any interface to any ip for any service 
# (equal to -P ACCEPT)
iptables -A OUTPUT -j ACCEPT

# Permit answers on already established connections
# and permit new connections related to established ones 
# (e.g. port mode ftp)
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Log everything else. What's Windows' latest exploitable vulnerability?
iptables -A INPUT -j LOG --log-prefix "FIREWALL:INPUT "

# End $rc_base/rc.iptables
<command>EOF</command></userinput></screen>

<para>This script is quite simple, it drops all traffic coming in into your 
computer that wasn't initiated from your box, but as long as you are simply 
surfing the Internet you are unlikely to exceed its limits.</para> 

<para>If you frequently encounter certain delays at accessing ftp-servers,
take a look at <xref linkend="fw-BB-4"/>.</para>

<para>Even if you have daemons or services running on your system, these
will be inaccessible everywhere but from your computer itself.
If you want to allow access to services on your machine, such as ssh or 
ping, take a look at <xref linkend="fw-busybox"/>.</para> 

</sect3>

<sect3 id="fw-masqRouter" xreflabel="Masquerading Router">
<title>Masquerading Router</title>

<para>A true Firewall has two interfaces, one connected to an intranet, in this
example <emphasis role="strong">eth0</emphasis>, and one connected to the
Internet, here <emphasis role="strong">ppp0</emphasis>.  To provide the
maximum security for the firewall itself, make sure that there
are no unnecessary servers running on it such as <application>X11</application> et
al.  As a general principle, the firewall itself should not access any
untrusted service (Think of a remote server giving answers that makes a daemin on 
your system
crash, or, even worse, that implements a worm via a buffer-overflow).</para>

<screen><userinput><command>cat &gt; /etc/rc.d/rc.iptables &lt;&lt; "EOF"</command>
#!/bin/sh

# Begin $rc_base/rc.iptables

echo
echo "You're using the example configuration for a setup of a firewall"
echo "from Beyond Linux From Scratch."
echo "This example is far from being complete, it is only meant"
echo "to be a reference."
echo "Firewall security is a complex issue, that exceeds the scope"
echo "of the configuration rules below."
echo "You can find additional information"
echo "about firewalls in Chapter 4 of the BLFS book."
echo "http://www.linuxfromscratch.org/blfs"
echo

# Insert iptables modules (not needed if built into the kernel).

modprobe ip_tables
modprobe iptable_filter
modprobe ip_conntrack
modprobe ip_conntrack_ftp
modprobe ipt_state
modprobe iptable_nat
modprobe ip_nat_ftp
modprobe ipt_MASQUERADE
modprobe ipt_LOG
modprobe ipt_REJECT

# Enable broadcast echo Protection
echo 1 &gt; /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# Disable Source Routed Packets
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_source_route

# Enable TCP SYN Cookie Protection
echo 1 &gt; /proc/sys/net/ipv4/tcp_syncookies

# Disable ICMP Redirect Acceptance
echo 0 &gt; /proc/sys/net/ipv4/conf/all/accept_redirects

# Don<6F>t send Redirect Messages
echo 0 &gt; /proc/sys/net/ipv4/conf/all/send_redirects

# Drop Spoofed Packets coming in on an interface where responses
# would result in the reply going out a different interface.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/rp_filter

# Log packets with impossible addresses.
echo 1 &gt; /proc/sys/net/ipv4/conf/all/log_martians

# Be verbose on dynamic ip-addresses  (not needed in case of static IP)
echo 2 &gt; /proc/sys/net/ipv4/ip_dynaddr

# Disable Explicit Congestion Notification 
# Too many routers are still ignorant
echo 0 &gt; /proc/sys/net/ipv4/tcp_ecn

# Set a known state
iptables -P INPUT   DROP
iptables -P FORWARD DROP
iptables -P OUTPUT  DROP
 
# These lines are here in case rules are already in place and the
# script is ever rerun on the fly. We want to remove all rules and
# pre-exisiting user defined chains before we implement new rules.
iptables -F
iptables -X
iptables -Z
 
iptables -t nat -F

# Allow local connections
iptables -A INPUT  -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

# Allow forwarding if the initiated on the intranet
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD  -i ! ppp+ -m state --state NEW      -j ACCEPT

# Do masquerading
# (not needed if intranet is not using private ip-addresses)
iptables -t nat -A POSTROUTING -o ppp+ -j MASQUERADE

# Log everything for debugging 
# (last of all rules, but before policy rules)
iptables -A INPUT   -j LOG --log-prefix "FIREWALL:INPUT  "
iptables -A FORWARD -j LOG --log-prefix "FIREWALL:FORWARD"
iptables -A OUTPUT  -j LOG --log-prefix "FIREWALL:OUTPUT "

# Enable IP Forwarding 
echo 1 &gt; /proc/sys/net/ipv4/ip_forward
<command>EOF</command></userinput></screen>

<para>With this script your intranet should be reasonably secure against
external attacks. No one should be able to setup a new connection to any
internal service and, if it's masqueraded, makes your intranet invisible to the
Internet. Furthermore, your firewall should be relatively safe because there
are no services running that a cracker could attack.</para>

<note><para>If the interface you're connecting to the Internet doesn't connect
via ppp, you will need to change <replaceable>ppp+</replaceable> to the name of
the interface, e.g. <emphasis role="strong">eth1</emphasis>, which you are using.
</para></note>

</sect3>

<sect3 id="fw-busybox" xreflabel="BusyBox">
<title>BusyBox</title>

<para>This scenario isn't too different from the <xref linkend="fw-masqRouter"/>, 
but additionally offers some services to your intranet.
Examples of this can be when you want to administer your firewall from another host 
on your intranet or use it as a proxy or a name server.</para>

<note><para>Outlining a true concept of how to protect a server that offers
services on the Internet goes far beyond the scope of this document. See the references
at the end of this section for more information.</para></note>

<para>Be cautious.  Every service you have enabled makes your
setup more complex and your firewall less secure. You are exposed to the risks of 
misconfigured services or running a service with an exploitable bug.  A 
firewall should generally not run any extra services.  See the introduction to 
the <xref linkend="fw-masqRouter"/> for some more details.</para>

<para>If you want to add services such as internal samba or name servers that do not 
need to access the Internet themselves,  the additional statements are quite
simple and should still be acceptable from a security standpoint.
Just add the following lines
into the script <emphasis>before</emphasis> the logging rules.</para>

<screen>iptables -A INPUT  -i ! ppp+  -j ACCEPT
iptables -A OUTPUT -o ! ppp+  -j ACCEPT</screen>

<para>If daemons, such as squid, have to access the Internet themselves, 
you could open OUTPUT generally and restrict INPUT.</para>

<screen>iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A OUTPUT                                     -j ACCEPT</screen>

<para>However, it is generally not advisable to leave OUTPUT unrestricted. You 
lose any control over trojans who would like to "call home", and a bit of 
redundancy in case you've (mis-)configured a service so that it broadcasts 
its existence to the world.</para>

<para>To accomplish this, you should restrict INPUT and OUTPUT
on all ports except those that it's absolutely necessary to have open.
Which ports you have to open depends on your needs: mostly you will find them 
by looking for failed accesses in your log files.</para>
<itemizedlist spacing="compact" role='iptables'>

<title>Have a look at the following examples:</title>

<listitem><para>Squid is caching the web:</para>
<screen>iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT  -p tcp --sport 80 -m state --state ESTABLISHED \
  -j ACCEPT</screen>
</listitem>

<listitem><para>Your caching name server (e.g., named) does its
lookups via udp:</para>
<screen>iptables -A OUTPUT -p udp --dport 53 -j ACCEPT</screen>
</listitem>

<listitem><para>You want to be able to ping your box to 
ensure it's still alive:</para>

<screen>iptables -A INPUT  -p icmp -m icmp --icmp-type echo-request -j ACCEPT
iptables -A OUTPUT -p icmp -m icmp --icmp-type echo-reply   -j ACCEPT</screen>
</listitem>

<listitem><para><anchor id='fw-BB-4' xreflabel="BusyBox example number 4"/>If you are
frequently accessing ftp servers or enjoy chatting, you might notice certain
delays because some implementations of these daemons have the feature of
querying an identd on your system to obtain usernames.  Although there's really
little harm in this, having an identd running is not recommended because many
security experts feel the service gives out too much additional information.</para>

<para>To avoid these delays you could reject the requests 
with a 'tcp-reset':</para>

<screen>iptables -A INPUT  -p tcp --dport 113 -j REJECT --reject-with tcp-reset</screen>
</listitem>

<listitem><para>To log and drop invalid packets (packets
that came in after netfilter's timeout or some types of network scans):</para>

<screen>iptables -I INPUT -p tcp -m state --state INVALID \
  -j LOG --log-prefix "FIREWALL:INVALID"
iptables -I INPUT -p tcp -m state --state INVALID -j DROP</screen></listitem>

<listitem><para>Anything coming from the outside should not have a
private address, this is a common attack called IP-spoofing:

<screen>iptables -A INPUT -i ppp+ -s 10.0.0.0/8     -j DROP
iptables -A INPUT -i ppp+ -s 172.16.0.0/12  -j DROP
iptables -A INPUT -i ppp+ -s 192.168.0.0/16 -j DROP</screen>

There are other addresses that you may also want to drop: 0.0.0.0/8,
127.0.0.0/8, 224.0.0.0/3 (multicast and experimental), 169.254.0.0/16 (Link 
Local Networks), and  192.0.2.0/24 (IANA defined test network).</para>
</listitem>

<listitem><para>If your firewall is a DHCP client, you need to allow 
those packets:</para>

<screen>iptables -A INPUT  -i ppp0 -p udp -s 0.0.0.0 --sport 67 \
   -d 255.255.255.255 --dport 68 -j ACCEPT</screen>
</listitem>

<listitem><para>To simplify debugging and be fair to anyone who'd like to 
access a service you have disabled, purposely or by mistake, you could REJECT 
those packets that are dropped.</para>

<para>Obviously this must be done directly after logging as the very
last lines before the packets are dropped by policy:</para>

<screen>iptables -A INPUT -j REJECT</screen>
</listitem>
</itemizedlist>

<para>These are only examples to show you some of the capabilities of the
firewall code in Linux. Have a look at the man page of iptables.
There you will find much more information. The port numbers needed for this can be
found in <filename>/etc/services</filename>, in case you didn't find them by
trial and error in your log file.</para>

</sect3>
</sect2>

<sect2 id="fw-finale" xreflabel="Conclusion">
<title>Conclusion</title>

<para>Finally, there is one fact you must not forget: The effort spent
attacking a system corresponds to the value the cracker expects to gain from
it.  If you are responsible for valuable information, you need to spend the
time to protect it properly.</para>

</sect2>

<sect2 id="postlfs-security-fw-extra" xreflabel="Extra Information">
<title>Extra Information</title>

<sect3 id="fw-library" xreflabel="Links for further reading">
<title>Where to start with further reading on firewalls.</title>

<para><blockquote><literallayout>
<ulink url="http://www.netfilter.org/">www.netfilter.org - Homepage of the netfilter/iptables project</ulink>
<ulink url="http://www.netfilter.org/documentation/FAQ/netfilter-faq.html">Netfilter related FAQ</ulink>
<ulink url="http://www.netfilter.org/documentation/index.html#HOWTO">Netfilter related HOWTO's</ulink>
<ulink url="http://en.tldp.org/LDP/nag2/x-087-2-firewall.html">en.tldp.org/LDP/nag2/x-087-2-firewall.html</ulink>
<ulink url="http://en.tldp.org/HOWTO/Security-HOWTO.html">en.tldp.org/HOWTO/Security-HOWTO.html</ulink>
<ulink url="http://en.tldp.org/HOWTO/Firewall-HOWTO.html">en.tldp.org/HOWTO/Firewall-HOWTO.html</ulink>
<ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire.html">www.ibm.com/developerworks/security/library/s-fire.html</ulink>
<ulink url="http://www-106.ibm.com/developerworks/security/library/s-fire2.html">www.ibm.com/developerworks/security/library/s-fire2.html</ulink>
<ulink url="http://www.interhack.net/pubs/fw-faq/">www.interhack.net/pubs/fw-faq/</ulink>
<ulink url="http://www.linuxsecurity.com/docs/">www.linuxsecurity.com/docs/</ulink>
<ulink url="http://www.little-idiot.de/firewall">www.little-idiot.de/firewall (German &amp; outdated, but very comprehensive)</ulink>
<ulink url="http://www.linuxgazette.com/issue65/stumpel.html">www.linuxgazette.com/issue65/stumpel.html</ulink>
<ulink url="http://linux.oreillynet.com/pub/a/linux/2000/03/10/netadmin/ddos.html">linux.oreillynet.com/pub/a/linux/2000/03/10/netadmin/ddos.html</ulink>
<ulink url="http://staff.washington.edu/dittrich/misc/ddos">staff.washington.edu/dittrich/misc/ddos</ulink>
<ulink url="http://www.e-infomax.com/ipmasq">www.e-infomax.com/ipmasq</ulink>
<ulink url="http://www.circlemud.org/~jelson/writings/security/index.htm">www.circlemud.org/~jelson/writings/security/index.htm</ulink>
<ulink url="http://www.securityfocus.com">www.securityfocus.com</ulink>
<ulink url="http://www.cert.org/tech_tips/">www.cert.org - tech_tips</ulink>
<ulink url="http://security.ittoolbox.com/">security.ittoolbox.com</ulink>
<ulink url="http://www.linux-firewall-tools.com/linux/">www.linux-firewall-tools.com/linux/</ulink>
<ulink url="http://logi.cc/linux/athome-firewall.php3">logi.cc/linux/athome-firewall.php3</ulink>
<ulink url="http://www.insecure.org/reading.html">www.insecure.org/reading.html</ulink>
<ulink url="http://www.robertgraham.com/pubs/firewall-seen.html">www.robertgraham.com/pubs/firewall-seen.html</ulink>
</literallayout></blockquote></para>
</sect3>

</sect2>
</sect1>