source: postlfs/security/firewalling/busybox.xml@ 1ea79a1

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

<para>This scenario isn't too different from (<xref linkend="postlfs-security-fw-masqRouter"/>), 
but in this case you want to offer some services to your intranet.
Examples of this can be when you want to admin your box from another host 
on your intranet or use it as a proxy or a name server. Note: 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 <xref linkend="postlfs-security-fw-disclaimer"/>.</para>

<para>Be cautious.  Every service you offer and have enabled makes your
setup more complex and your box less secure. You induce 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 
<xref linkend="postlfs-security-fw-masqRouter"/> for some more details.</para>

<para>If the services you'd like to offer do not need to access the Internet 
themselves, like internal-only samba- or name-servers, it's quite
simple and should still be acceptable from a security standpoint.
Just add the following lines <emphasis>before</emphasis> the logging-rules 
into the script.</para>

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

<para>If your daemons have to access the web themselves, like squid would need
to, 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'd like to "call home", and a bit of redundancy in case 
you've (mis-)configured a service so that it does broadcast its existence to the 
world.</para>

<para>If you prefer to have this protection, you may 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">
<!-- <orderedlist numeration="arabic" spacing="compact"> -->
<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., dnscache) does its
lookups via udp:</para>
<screen>iptables -A OUTPUT -p udp --dport 53                              -j ACCEPT
iptables -A INPUT  -p udp --sport 53 -m state --state ESTABLISHED -j ACCEPT</screen></listitem>

<listitem><para>Alternatively, if 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='postlfs-security-fw-BB-4' xreflabel="example no. 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 box for logging usernames.
Although there's really no harm in this, having an identd running is not 
recommended because some implementations are known to be vulnerable.</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
iptables -A OUTPUT -p tcp --sport 113 -m state --state RELATED -j ACCEPT</screen></listitem>

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

<screen>iptables -I INPUT 1 -p tcp -m state --state INVALID -j LOG --log-prefix \ 
"FIREWALL:INVALID"
iptables -I INPUT 2 -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:</para>

<screen>iptables -t nat -A PREROUTING -i ppp+ -s 10.0.0.0/8     -j DROP
iptables -t nat -A PREROUTING -i ppp+ -s 172.16.0.0/12  -j DROP
iptables -t nat -A PREROUTING -i ppp+ -s 192.168.0.0/16 -j DROP</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 should 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
iptables -A OUTPUT -p icmp --icmp-type 3 -j ACCEPT</screen></listitem>
</itemizedlist>
<!--</orderedlist>-->

<para>These are only examples to show you some of the capabilities of the new 
firewall code in Linux-Kernel 2.4. Have a look at the man page of
iptables.
There you will find more of them. The port-numbers you'll need 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>

<para>If you add any of your offered or accessed services such as the above,
maybe even in FORWARD and for intranet-communication, and delete the
general clauses, you get an old fashioned packet filter.</para>


</sect3>