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<?xml version="1.0" encoding="iso-8859-1"?>
<!--
The FreeBSD Documentation Project
$FreeBSD$
-->
<chapter id="firewalls">
<chapterinfo>
<authorgroup>
<author>
<firstname>Joseph J.</firstname>
<surname>Barbish</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
<authorgroup>
<author>
<firstname>Brad</firstname>
<surname>Davis</surname>
<contrib>Converted to SGML and updated by </contrib>
</author>
</authorgroup>
</chapterinfo>
<title>Firewalls</title>
<indexterm><primary>firewall</primary></indexterm>
<indexterm>
<primary>security</primary>
<secondary>firewalls</secondary>
</indexterm>
<sect1 id="firewalls-intro">
<title>Introduction</title>
<para>Firewalls make it possible to filter incoming and outgoing
traffic that flows through your system. A firewall can use one
or more sets of <quote>rules</quote> to inspect the network
packets as they come in or go out of your network connections
and either allows the traffic through or blocks it. The rules
of a firewall can inspect one or more characteristics of the
packets, including but not limited to the protocol type, the
source or destination host address, and the source or
destination port.</para>
<para>Firewalls can greatly enhance the security of a host or a
network. They can be used to do one or more of
the following things:</para>
<itemizedlist>
<listitem>
<para>To protect and insulate the applications, services and
machines of your internal network from unwanted traffic
coming in from the public Internet.</para>
</listitem>
<listitem>
<para>To limit or disable access from hosts of the internal
network to services of the public Internet.</para>
</listitem>
<listitem>
<para>To support network address translation
(<acronym>NAT</acronym>), which allows your internal network
to use private <acronym>IP</acronym> addresses and share a
single connection to the public Internet (either with a
single <acronym>IP</acronym> address or by a shared pool of
automatically assigned public addresses).</para>
</listitem>
</itemizedlist>
<para>After reading this chapter, you will know:</para>
<itemizedlist>
<listitem>
<para>How to properly define packet filtering rules.</para>
</listitem>
<listitem>
<para>The differences between the firewalls
built into &os;.</para>
</listitem>
<listitem>
<para>How to use and configure the OpenBSD
<application>PF</application> firewall.</para>
</listitem>
<listitem>
<para>How to use and configure
<application>IPFILTER</application>.</para>
</listitem>
<listitem>
<para>How to use and configure
<application>IPFW</application>.</para>
</listitem>
</itemizedlist>
<para>Before reading this chapter, you should:</para>
<itemizedlist>
<listitem>
<para>Understand basic &os; and Internet concepts.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="firewalls-concepts">
<title>Firewall Concepts</title>
<indexterm>
<primary>firewall</primary>
<secondary>rulesets</secondary>
</indexterm>
<para>There are two basic ways to create firewall rulesets:
<quote>inclusive</quote> or <quote>exclusive</quote>. An
exclusive firewall allows all traffic through except for the
traffic matching the ruleset. An inclusive firewall does the
reverse. It only allows traffic matching the rules through and
blocks everything else.</para>
<para>An inclusive firewall offers much better control of the
outgoing traffic, making it a better choice for systems that
offer services to the public Internet. It also controls the
type of traffic originating from the public Internet that can
gain access to your private network. All traffic that does
not match the rules, is blocked and logged by design. Inclusive
firewalls are generally safer than exclusive firewalls because
they significantly reduce the risk of allowing unwanted traffic
to pass through them.</para>
<note>
<para>Unless noted otherwise, all configuration and example
rulesets in this chapter, create inclusive type
firewalls.</para>
</note>
<para>Security can be tightened further using a <quote>stateful
firewall</quote>. This type of firewall keeps
track of which connections are opened through the firewall and
will only allow traffic through which either matches an existing
connection or opens a new one. The disadvantage of a stateful
firewall is that it can be vulnerable to Denial of Service
(<acronym>DoS</acronym>) attacks if a lot of new connections are
opened very fast. With most firewalls it is possible to use a
combination of stateful and non-stateful behavior to make an
optimal firewall for the site.</para>
</sect1>
<sect1 id="firewalls-apps">
<title>Firewall Packages</title>
<para>&os; has three different firewall packages built
into the base system. They are: <emphasis>IPFILTER</emphasis>
(also known as <acronym>IPF</acronym>),
<emphasis>IPFIREWALL</emphasis> (also known as
<acronym>IPFW</acronym>), and <emphasis>OpenBSD's
PacketFilter</emphasis> (also known as <acronym>PF</acronym>).
&os; also has two built in packages for traffic shaping
(basically controlling bandwidth usage): &man.altq.4; and
&man.dummynet.4;. Dummynet has traditionally been closely
tied with <acronym>IPFW</acronym>, and
<acronym>ALTQ</acronym> with
<acronym>PF</acronym>. Traffic shaping for IPFILTER can
currently be done with IPFILTER for NAT and filtering and
<acronym>IPFW</acronym> with &man.dummynet.4;
<emphasis>or</emphasis> by using <acronym>PF</acronym> with
<acronym>ALTQ</acronym>.
IPFW, and PF all use rules to control the access of packets
to and from your system, although they go about it different
ways and have a different rule syntax.</para>
<para>The reason that &os; has multiple built in firewall packages
is that different people have different requirements and
preferences. No single firewall package is the best.</para>
<para>The author prefers IPFILTER because its stateful rules are
much less complicated to use in a <acronym>NAT</acronym>
environment and it has a built in ftp proxy that simplifies the
rules to allow secure outbound FTP usage.</para>
<para>Since all firewalls are based on inspecting the values of
selected packet control fields, the creator of the firewall
rulesets must have an understanding of how
<acronym>TCP/IP</acronym> works, what the different values in
the packet control fields are and how these values are used in a
normal session conversation. For a good explanation go to:
<ulink
url="http://www.ipprimer.com/overview.cfm"></ulink>.</para>
</sect1>
<sect1 id="firewalls-pf">
<sect1info>
<authorgroup>
<author>
<firstname>John</firstname>
<surname>Ferrell</surname>
<contrib>Revised and updated by </contrib>
<!-- 24 March 2008 -->
</author>
</authorgroup>
</sect1info>
<title>The OpenBSD Packet Filter (PF) and
<acronym>ALTQ</acronym></title>
<indexterm>
<primary>firewall</primary>
<secondary>PF</secondary>
</indexterm>
<para>As of July 2003 the OpenBSD firewall software application
known as <acronym>PF</acronym> was ported to &os; and
made available in the &os; Ports Collection. Released in 2004,
&os;&nbsp;5.3 was the first release that contained
<acronym>PF</acronym> as an integrated part of the base system.
<acronym>PF</acronym> is a complete, full-featured firewall
that has optional support for <acronym>ALTQ</acronym> (Alternate
Queuing). <acronym>ALTQ</acronym> provides Quality of Service
(<acronym>QoS</acronym>) functionality.</para>
<para>The OpenBSD Project does an outstanding job of
maintaining the <ulink
url="http://www.openbsd.org/faq/pf/">PF FAQ</ulink>.
As such, this section of the Handbook will focus on
<acronym>PF</acronym> as it pertains to &os; while providing
some general information regarding usage. For detailed usage
information please refer to the <ulink
url="http://www.openbsd.org/faq/pf/">PF FAQ</ulink>.</para>
<para>More information about <acronym>PF</acronym> for &os;
can be found at <ulink
url="http://pf4freebsd.love2party.net/"></ulink>.</para>
<sect2>
<title>Using the PF Loadable Kernel Modules</title>
<para>To load the PF Kernel Module add the following line to
<filename>/etc/rc.conf</filename>:</para>
<programlisting>pf_enable="YES"</programlisting>
<para>Then run the startup script to load the module:</para>
<screen>&prompt.root; <userinput>service pf start</userinput></screen>
<para>Note that the PF Module will not load if it cannot find
the ruleset config file. The default location is
<filename>/etc/pf.conf</filename>. If the PF ruleset is
located somewhere else, PF can be instructed to look there
by adding a line like the following to
<filename>/etc/rc.conf</filename>:</para>
<programlisting>pf_rules="<replaceable>/path/to/pf.conf</replaceable>"</programlisting>
<para>The sample <filename>pf.conf</filename>
can be found in <filename
class="directory">/usr/share/examples/pf/</filename>.</para>
<para>The <acronym>PF</acronym> module can also be loaded
manually from the command line:</para>
<screen>&prompt.root; <userinput>kldload pf.ko</userinput></screen>
<para>Logging support for PF is provided by the
<literal>pflog.ko</literal> and can be loaded by adding the
following line to <filename>/etc/rc.conf</filename>:</para>
<programlisting>pflog_enable="YES"</programlisting>
<para>Then run the startup script to load the module:</para>
<screen>&prompt.root; <userinput>service pflog start</userinput></screen>
<para>If you need other <acronym>PF</acronym> features you will
need to compile <acronym>PF</acronym> support into the
kernel.</para>
</sect2>
<sect2>
<title>PF Kernel Options</title>
<indexterm>
<primary>kernel options</primary>
<secondary>device pf</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>device pflog</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>device pfsync</secondary>
</indexterm>
<para>While it is not necessary that you compile
<acronym>PF</acronym> support into the &os; kernel, you may
want to do so to take advantage of one of PF's advanced
features that is not included in the loadable module, namely
&man.pfsync.4;, which is a pseudo-device that exposes certain
changes to the state table used by <acronym>PF</acronym>.
It can be paired with &man.carp.4; to create failover
firewalls using <acronym>PF</acronym>. More information on
<acronym>CARP</acronym> can be found in
<xref linkend="carp"/> of the Handbook.</para>
<para>The <acronym>PF</acronym> kernel options can be found in
<filename>/usr/src/sys/conf/NOTES</filename> and are
reproduced below:</para>
<programlisting>device pf
device pflog
device pfsync</programlisting>
<para>The <literal>device pf</literal> option enables support
for the <quote>Packet Filter</quote> firewall
(&man.pf.4;).</para>
<para>The <literal>device pflog</literal> option enables the
optional &man.pflog.4; pseudo network device which can be
used to log traffic to a &man.bpf.4; descriptor. The
&man.pflogd.8; daemon can be used to store the logging
information to disk.</para>
<para>The <literal>device pfsync</literal> option enables the
optional
&man.pfsync.4; pseudo-network device that is used to monitor
<quote>state changes</quote>.</para>
</sect2>
<sect2>
<title>Available <filename>rc.conf</filename> Options</title>
<para>The following &man.rc.conf.5; statements configure
<acronym>PF</acronym> and &man.pflog.4; at boot:</para>
<programlisting>pf_enable="YES" # Enable PF (load module if required)
pf_rules="/etc/pf.conf" # rules definition file for pf
pf_flags="" # additional flags for pfctl startup
pflog_enable="YES" # start pflogd(8)
pflog_logfile="/var/log/pflog" # where pflogd should store the logfile
pflog_flags="" # additional flags for pflogd startup</programlisting>
<para>If you have a LAN behind this firewall and have to forward
packets for the computers on the LAN or want to do NAT, you
will need the following option as well:</para>
<programlisting>gateway_enable="YES" # Enable as LAN gateway</programlisting>
</sect2>
<sect2>
<title>Creating Filtering Rules</title>
<para><acronym>PF</acronym> reads its configuration rules from
&man.pf.conf.5; (<filename>/etc/pf.conf</filename> by
default) and it modifies, drops, or passes packets according
to the rules or definitions specified there. The &os;
installation includes several sample files located in
<filename>/usr/share/examples/pf/</filename>. Please refer
to the <ulink url="http://www.openbsd.org/faq/pf/">PF
FAQ</ulink> for complete coverage of <acronym>PF</acronym>
rulesets.</para>
<warning>
<para>When browsing the <ulink
url="http://www.openbsd.org/faq/pf/">PF FAQ</ulink>,
please keep in mind that different versions of &os; can
contain different versions of PF. Currently,
&os;&nbsp;8.<replaceable>X</replaceable> and prior is
using the same version of <acronym>PF</acronym> as
OpenBSD&nbsp;4.1. &os;&nbsp;9.<replaceable>X</replaceable>
and later is using the same version of <acronym>PF</acronym>
as OpenBSD&nbsp;4.5.</para>
</warning>
<para>The &a.pf; is a good place to ask questions about
configuring and running the <acronym>PF</acronym>
firewall. Do not forget to check the mailing list archives
before asking questions!</para>
</sect2>
<sect2>
<title>Working with PF</title>
<para>Use &man.pfctl.8; to control <acronym>PF</acronym>. Below
are some useful commands (be sure to review the &man.pfctl.8;
man page for all available options):</para>
<informaltable frame="none" pgwide="1">
<tgroup cols="2">
<thead>
<row>
<entry>Command</entry>
<entry>Purpose</entry>
</row>
</thead>
<tbody>
<row>
<entry><command>pfctl
<option>-e</option></command></entry>
<entry>Enable PF</entry>
</row>
<row>
<entry><command>pfctl
<option>-d</option></command></entry>
<entry>Disable PF</entry>
</row>
<row>
<entry><command>pfctl <option>-F</option> all
<option>-f</option> /etc/pf.conf</command></entry>
<entry>Flush all rules (nat, filter, state, table, etc.)
and reload from the file
<filename>/etc/pf.conf</filename></entry>
</row>
<row>
<entry><command>pfctl <option>-s</option> [ rules | nat
state ]</command></entry>
<entry>Report on the filter rules, nat rules, or state
table</entry>
</row>
<row>
<entry><command>pfctl <option>-vnf</option>
/etc/pf.conf</command></entry>
<entry>Check <filename>/etc/pf.conf</filename> for
errors, but do not load ruleset</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect2>
<sect2>
<title>Enabling <acronym>ALTQ</acronym></title>
<para><acronym>ALTQ</acronym> is only available by compiling
support for it into the &os; kernel. <acronym>ALTQ</acronym>
is not supported by all of the available network card drivers.
Please see the &man.altq.4; manual page for a list of drivers
that are supported in your release of &os;.</para>
<para>The following kernel options will enable
<acronym>ALTQ</acronym> and add additional
functionality:</para>
<programlisting>options ALTQ
options ALTQ_CBQ # Class Bases Queuing (CBQ)
options ALTQ_RED # Random Early Detection (RED)
options ALTQ_RIO # RED In/Out
options ALTQ_HFSC # Hierarchical Packet Scheduler (HFSC)
options ALTQ_PRIQ # Priority Queuing (PRIQ)
options ALTQ_NOPCC # Required for SMP build</programlisting>
<para><literal>options ALTQ</literal> enables the
<acronym>ALTQ</acronym> framework.</para>
<para><literal>options ALTQ_CBQ</literal> enables
<emphasis>Class Based Queuing</emphasis>
(<acronym>CBQ</acronym>). <acronym>CBQ</acronym>
allows you to divide a connection's bandwidth into different
classes or queues to prioritize traffic based on filter
rules.</para>
<para><literal>options ALTQ_RED</literal> enables
<emphasis>Random Early Detection</emphasis>
(<acronym>RED</acronym>). <acronym>RED</acronym> is
used to avoid network congestion. <acronym>RED</acronym>
does this by measuring the length of the queue and comparing
it to the minimum and maximum thresholds for the queue. If
the queue is over the maximum all new packets will be dropped.
True to its name, <acronym>RED</acronym> drops packets from
different connections randomly.</para>
<para><literal>options ALTQ_RIO</literal> enables
<emphasis>Random Early Detection In and Out</emphasis>.</para>
<para><literal>options ALTQ_HFSC</literal> enables the
<emphasis>Hierarchical Fair Service Curve Packet
Scheduler</emphasis>. For more information about
<acronym>HFSC</acronym> see: <ulink
url="http://www-2.cs.cmu.edu/~hzhang/HFSC/main.html"></ulink>.</para>
<para><literal>options ALTQ_PRIQ</literal> enables
<emphasis>Priority Queuing</emphasis>
(<acronym>PRIQ</acronym>). <acronym>PRIQ</acronym> will
always pass traffic that is in a higher queue first.</para>
<para><literal>options ALTQ_NOPCC</literal> enables
<acronym>SMP</acronym> support for <acronym>ALTQ</acronym>.
This option is required on <acronym>SMP</acronym>
systems.</para>
</sect2>
</sect1>
<sect1 id="firewalls-ipf">
<title>The IPFILTER (IPF) Firewall</title>
<indexterm>
<primary>firewall</primary>
<secondary>IPFILTER</secondary>
</indexterm>
<para>The author of IPFILTER is Darren Reed. IPFILTER is not
operating system dependent: it is an open source application and
has been ported to &os;, NetBSD, OpenBSD, &sunos;, HP/UX, and
&solaris; operating systems. IPFILTER is actively being
supported and maintained, with updated versions being released
regularly.</para>
<para>IPFILTER is based on a kernel-side firewall and
<acronym>NAT</acronym> mechanism that can be controlled and
monitored by userland interface programs. The firewall rules
can be set or deleted with the &man.ipf.8; utility. The
<acronym>NAT</acronym> rules can be set or deleted with the
&man.ipnat.8; utility. The &man.ipfstat.8; utility can print
run-time statistics for the kernel parts of IPFILTER. The
&man.ipmon.8; program can log IPFILTER actions to the system
log files.</para>
<para>IPF was originally written using a rule processing logic
of <quote>the last matching rule wins</quote> and used only
stateless type of rules. Over time IPF has been enhanced to
include a <quote>quick</quote> option and a stateful
<quote>keep state</quote> option which drastically modernized
the rules processing logic. IPF's official documentation covers
only the legacy rule coding parameters and rule file processing
logic. The modernized functions are only included as additional
options, completely understating their benefits in producing
a far superior and more secure firewall.</para>
<para>The instructions contained in this section are based on
using rules that contain the <quote>quick</quote> option and the
stateful <quote>keep state</quote> option. This is the basic
framework for coding an inclusive firewall ruleset.</para>
<para>For detailed explanation of the legacy rules processing
method see: <ulink
url="http://www.munk.me.uk/ipf/ipf-howto.html"></ulink>
and <ulink
url="http://coombs.anu.edu.au/~avalon/ip-filter.html"></ulink>.</para>
<para>The IPF FAQ is at <ulink
url="http://www.phildev.net/ipf/index.html"></ulink>.</para>
<para>A searchable archive of the open-source IPFilter mailing
list is available at <ulink
url="http://marc.theaimsgroup.com/?l=ipfilter"></ulink>.</para>
<sect2>
<title>Enabling IPF</title>
<indexterm>
<primary>IPFILTER</primary>
<secondary>enabling</secondary>
</indexterm>
<para>IPF is included in the basic &os; install as a separate
run time loadable module. The system will dynamically load
the IPF kernel loadable module when the
<filename>rc.conf</filename> statement
<literal>ipfilter_enable="YES"</literal> is used. The
loadable module was created with logging enabled and the
<literal>default pass all</literal> options. There is no
need to compile IPF into the &os; kernel just to change the
default to <literal>block all</literal>. This can be done
just by adding a <literal>block all</literal> rule at the
end of your ruleset.</para>
</sect2>
<sect2>
<title>Kernel Options</title>
<indexterm>
<primary>kernel options</primary>
<secondary>IPFILTER</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>IPFILTER_LOG</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>IPFILTER_DEFAULT_BLOCK</secondary>
</indexterm>
<indexterm>
<primary>IPFILTER</primary>
<secondary>kernel options</secondary>
</indexterm>
<para>It is not a mandatory requirement to enable IPF by
compiling the following options into the &os; kernel. It is
only presented here as background information. Compiling IPF
into the kernel causes the loadable module to never be
used.</para>
<para>Sample kernel config IPF option statements are in the
<filename>/usr/src/sys/conf/NOTES</filename> kernel source
and are reproduced here:</para>
<programlisting>options IPFILTER
options IPFILTER_LOG
options IPFILTER_DEFAULT_BLOCK</programlisting>
<para><literal>options IPFILTER</literal> enables support for
the <quote>IPFILTER</quote> firewall.</para>
<para><literal>options IPFILTER_LOG</literal> enables the option
to have IPF log traffic by writing to the
<devicename>ipl</devicename> packet logging
pseudo&mdash;device for every rule that has the
<literal>log</literal> keyword.</para>
<para><literal>options IPFILTER_DEFAULT_BLOCK</literal> changes
the default behavior so any packet not matching a firewall
<literal>pass</literal> rule gets blocked.</para>
<para>These settings will take effect only after installing a
kernel that has been built with the above options set.</para>
</sect2>
<sect2>
<title>Available <filename>rc.conf</filename> Options</title>
<para>To activate IPF at boot time, the following statements
need to be added to <filename>/etc/rc.conf</filename>:</para>
<programlisting>ipfilter_enable="YES" # Start ipf firewall
ipfilter_rules="/etc/ipf.rules" # loads rules definition text file
ipmon_enable="YES" # Start IP monitor log
ipmon_flags="-Ds" # D = start as daemon
# s = log to syslog
# v = log tcp window, ack, seq
# n = map IP &amp; port to names</programlisting>
<para>If there is a LAN behind this firewall that uses the
reserved private IP address ranges, the following lines will
have to be added to enable <acronym>NAT</acronym>
functionality:</para>
<programlisting>gateway_enable="YES" # Enable as LAN gateway
ipnat_enable="YES" # Start ipnat function
ipnat_rules="/etc/ipnat.rules" # rules definition file for ipnat</programlisting>
</sect2>
<sect2>
<title>IPF</title>
<indexterm><primary><command>ipf</command></primary></indexterm>
<para>The &man.ipf.8; command is used to load your ruleset file.
Your custom rules would normally be placed in a file, and the
following command could then be used to replace in mass the
currently running firewall rules:</para>
<screen>&prompt.root; <userinput>ipf -Fa -f /etc/ipf.rules</userinput></screen>
<para><option>-Fa</option> means flush all internal rules
tables.</para>
<para><option>-f</option> means this is the file to read for
the rules to load.</para>
<para>This gives you the ability to make changes to your custom
rules file, run the above IPF command, and thus update the
running firewall with a fresh copy of all the rules without
having to reboot the system. This method is very convenient
for testing new rules as the procedure can be executed as many
times as needed.</para>
<para>See the &man.ipf.8; manual page for details on the other
flags available with this command.</para>
<para>The &man.ipf.8; command expects the rules file to be a
standard text file. It will not accept a rules file written
as a script with symbolic substitution.</para>
<para>There is a way to build IPF rules that utilizes the power
of script symbolic substitution. For more information, see
<xref linkend="firewalls-ipf-rules-script"/>.</para>
</sect2>
<sect2>
<title>IPFSTAT</title>
<indexterm><primary><command>ipfstat</command></primary></indexterm>
<indexterm>
<primary>IPFILTER</primary>
<secondary>statistics</secondary>
</indexterm>
<para>The default behavior of &man.ipfstat.8; is to retrieve
and display the totals of the accumulated statistics gathered
as a result of applying the user coded rules against packets
going in and out of the firewall since it was last started,
or since the last time the accumulators were reset to zero
using <command>ipf -Z</command>.</para>
<para>See the &man.ipfstat.8; manual page for details.</para>
<para>The default &man.ipfstat.8; command output will look
something like this:</para>
<screen>input packets: blocked 99286 passed 1255609 nomatch 14686 counted 0
output packets: blocked 4200 passed 1284345 nomatch 14687 counted 0
input packets logged: blocked 99286 passed 0
output packets logged: blocked 0 passed 0
packets logged: input 0 output 0
log failures: input 3898 output 0
fragment state(in): kept 0 lost 0
fragment state(out): kept 0 lost 0
packet state(in): kept 169364 lost 0
packet state(out): kept 431395 lost 0
ICMP replies: 0 <acronym>TCP</acronym> RSTs sent: 0
Result cache hits(in): 1215208 (out): 1098963
IN Pullups succeeded: 2 failed: 0
OUT Pullups succeeded: 0 failed: 0
Fastroute successes: 0 failures: 0
<acronym>TCP</acronym> cksum fails(in): 0 (out): 0
Packet log flags set: (0)</screen>
<para>When supplied with either <option>-i</option> for inbound
or <option>-o</option> for outbound, the command will retrieve
and display the appropriate list of filter rules currently
installed and in use by the kernel.</para>
<para><command>ipfstat -in</command> displays the inbound
internal rules table with rule number.</para>
<para><command>ipfstat -on</command> displays the outbound
internal rules table with the rule number.</para>
<para>The output will look something like this:</para>
<screen>@1 pass out on xl0 from any to any
@2 block out on dc0 from any to any
@3 pass out quick on dc0 proto tcp/udp from any to any keep state</screen>
<para><command>ipfstat -ih</command> displays the inbound
internal rules table, prefixing each rule with a count of how
many times the rule was matched.</para>
<para><command>ipfstat -oh</command> displays the outbound
internal rules table, prefixing each rule with a count of how
many times the rule was matched.</para>
<para>The output will look something like this:</para>
<screen>2451423 pass out on xl0 from any to any
354727 block out on dc0 from any to any
430918 pass out quick on dc0 proto tcp/udp from any to any keep state</screen>
<para>One of the most important functions of
<command>ipfstat</command> is the <option>-t</option>
flag which displays the state table in a way similar to the
way &man.top.1; shows the &os; running process table. When
your firewall is under attack, this function gives you the
ability to identify, drill down to, and see the attacking
packets. The optional sub-flags give the ability to select
the destination or source IP, port, or protocol that you want
to monitor in real time. See the &man.ipfstat.8; manual page
for details.</para>
</sect2>
<sect2>
<title>IPMON</title>
<indexterm><primary><command>ipmon</command></primary></indexterm>
<indexterm>
<primary>IPFILTER</primary>
<secondary>logging</secondary>
</indexterm>
<para>In order for <command>ipmon</command> to work properly,
the kernel option <literal>IPFILTER_LOG</literal> must be
turned on. This command has two different modes that it can
be used in. Native mode is the default mode when the command
is typed on the command line without the <option>-D</option>
flag.</para>
<para>Daemon mode is for when a continuous
system log file is desired, so that logging of past events
may be reviewed. This is how &os; and IPFILTER are configured
to work together. &os; has a built in facility to
automatically rotate system logs. That is why outputting the
log information to &man.syslogd.8; is better than the default
of outputting to a regular file. In the default
<filename>rc.conf</filename>, the
<literal>ipmon_flags</literal> statement uses the
<option>-Ds</option> flags:</para>
<programlisting>ipmon_flags="-Ds" # D = start as daemon
# s = log to syslog
# v = log tcp window, ack, seq
# n = map IP &amp; port to names</programlisting>
<para>The benefits of logging are obvious. It provides the
ability to review, after the fact, information such as which
packets had been dropped, what addresses they came from and
where they were going. These can all provide a significant
edge in tracking down attackers.</para>
<para>Even with the logging facility enabled, IPF will not
generate any rule logging on its own. The firewall
administrator decides what rules in the ruleset he wants to
log and adds the log keyword to those rules. Normally only
deny rules are logged.</para>
<para>It is very customary to include a default deny everything
rule with the log keyword included as your last rule in the
ruleset. This makes it possible to see all the packets that
did not match any of the rules in the ruleset.</para>
</sect2>
<sect2>
<title>IPMON Logging</title>
<para><application>Syslogd</application> uses its own special
method for segregation of log data. It uses special groupings
called <quote>facility</quote> and <quote>level</quote>.
IPMON in <option>-Ds</option> mode uses
<literal>local0</literal> as the <quote>facility</quote>
name by default. The following levels can be used to further
segregate the logged data if desired:</para>
<screen>LOG_INFO - packets logged using the "log" keyword as the action rather than pass or block.
LOG_NOTICE - packets logged which are also passed
LOG_WARNING - packets logged which are also blocked
LOG_ERR - packets which have been logged and which can be considered short</screen>
<!-- XXX: "can be considered short" == "with incomplete header" -->
<para>To setup IPFILTER to log all data to
<filename>/var/log/ipfilter.log</filename>, the file will
need to be created beforehand. The following command will
do that:</para>
<screen>&prompt.root; <userinput>touch /var/log/ipfilter.log</userinput></screen>
<para>The &man.syslogd.8; function is controlled by definition
statements in <filename>/etc/syslog.conf</filename>.
This file offers considerable
flexibility in how <application>syslog</application> will
deal with system messages issued by software applications
like IPF.</para>
<para>Add the following statement to
<filename>/etc/syslog.conf</filename>:</para>
<programlisting>local0.* /var/log/ipfilter.log</programlisting>
<para>The <literal>local0.*</literal>
means to write all the logged messages to the coded
file location.</para>
<para>To activate the changes to <filename>/etc/syslog.conf
</filename> you can reboot or bump the &man.syslogd.8;
daemon into re-reading <filename>/etc/syslog.conf</filename>
by running <command>service syslogd reload</command></para>
<para>Do not forget to change
<filename>/etc/newsyslog.conf</filename> to rotate the new
log created above.</para>
</sect2>
<sect2>
<title>The Format of Logged Messages</title>
<para>Messages generated by <command>ipmon</command> consist
of data fields separated by white space. Fields common to
all messages are:</para>
<orderedlist>
<listitem>
<para>The date of packet receipt.</para>
</listitem>
<listitem>
<para>The time of packet receipt. This is in the form
HH:MM:SS.F, for hours, minutes, seconds, and fractions
of a second (which can be several digits long).</para>
</listitem>
<listitem>
<para>The name of the interface the packet was processed
on, e.g., <devicename>dc0</devicename>.</para>
</listitem>
<listitem>
<para>The group and rule number of the rule, e.g.,
<literal>@0:17</literal>.</para>
</listitem>
</orderedlist>
<para>These can be viewed with
<command>ipfstat -in</command>.</para>
<orderedlist>
<listitem>
<para>The action: p for passed, b for blocked, S for a short
packet, n did not match any rules, L for a log rule.
The order of precedence in showing flags is: S, p, b, n,
L. A capital P or B means that the packet has been logged
due to a global logging setting, not a particular
rule.</para>
</listitem>
<listitem>
<para>The addresses. This is actually three fields: the
source address and port (separated by a comma), the -&gt;
symbol, and the destination address and port, e.g.:
<literal>209.53.17.22,80 -&gt;
198.73.220.17,1722</literal>.</para>
</listitem>
<listitem>
<para><literal>PR</literal> followed by the protocol name
or number, e.g.: <literal>PR tcp</literal>.</para>
</listitem>
<listitem>
<para><literal>len</literal> followed by the header length
and total length of the packet, e.g.:
<literal>len 20 40</literal>.</para>
</listitem>
</orderedlist>
<para>If the packet is a <acronym>TCP</acronym> packet, there
will be an additional field starting with a hyphen followed by
letters corresponding to any flags that were set. See the
&man.ipf.5; manual page for a list of letters and their
flags.</para>
<para>If the packet is an ICMP packet, there will be two fields
at the end, the first always being <quote>ICMP</quote>, and
the next being the ICMP message and sub-message type,
separated by a slash, e.g., ICMP 3/3 for a port unreachable
message.</para>
</sect2>
<sect2 id="firewalls-ipf-rules-script">
<title>Building the Rule Script with Symbolic
Substitution</title>
<para>Some experienced IPF users create a file containing the
rules and code them in a manner compatible with running them
as a script with symbolic substitution. The major benefit
of doing this is that only the value associated with the
symbolic name needs to be changed, and when the script is
run all the rules containing the symbolic name will have the
value substituted in the rules. Being a script, symbolic
substitution can be used to code frequently used values and
substitute them in multiple rules. This can be seen in the
following example.</para>
<para>The script syntax used here is compatible with the
&man.sh.1;, &man.csh.1;, and &man.tcsh.1; shells.</para>
<para>Symbolic substitution fields are prefixed with a dollar
sign: <literal>&dollar;</literal>.</para>
<para>Symbolic fields do not have the &dollar; prefix.</para>
<para>The value to populate the symbolic field must be enclosed
with double quotes (<literal>"</literal>).</para>
<para>Start your rule file with something like this:</para>
<programlisting>############# Start of IPF rules script ########################
oif="dc0" # name of the outbound interface
odns="192.0.2.11" # ISP's DNS server IP address
myip="192.0.2.7" # my static IP address from ISP
ks="keep state"
fks="flags S keep state"
# You can choose between building /etc/ipf.rules file
# from this script or running this script "as is".
#
# Uncomment only one line and comment out another.
#
# 1) This can be used for building /etc/ipf.rules:
#cat &gt; /etc/ipf.rules &lt;&lt; EOF
#
# 2) This can be used to run script "as is":
/sbin/ipf -Fa -f - &lt;&lt; EOF
# Allow out access to my ISP's Domain name server.
pass out quick on &dollar;oif proto tcp from any to &dollar;odns port = 53 &dollar;fks
pass out quick on &dollar;oif proto udp from any to &dollar;odns port = 53 &dollar;ks
# Allow out non-secure standard www function
pass out quick on &dollar;oif proto tcp from &dollar;myip to any port = 80 &dollar;fks
# Allow out secure www function https over TLS SSL
pass out quick on &dollar;oif proto tcp from &dollar;myip to any port = 443 &dollar;fks
EOF
################## End of IPF rules script ########################</programlisting>
<para>That is all there is to it. The rules are not important
in this example; how the symbolic substitution fields are
populated and used are. If the above example was in a file
named <filename>/etc/ipf.rules.script</filename>, these rules
could be reloaded by entering the following command:</para>
<screen>&prompt.root; <userinput>sh /etc/ipf.rules.script</userinput></screen>
<para>There is one problem with using a rules file with embedded
symbolics: IPF does not understand symbolic substitution, and
cannot read such scripts directly.</para>
<para>This script can be used in one of two ways:</para>
<itemizedlist>
<listitem>
<para>Uncomment the line that begins with
<literal>cat</literal>, and comment out the line that
begins with <literal>/sbin/ipf</literal>. Place
<literal>ipfilter_enable="YES"</literal> into
<filename>/etc/rc.conf</filename> as usual, and run script
once after each modification to create or update
<filename>/etc/ipf.rules</filename>.</para>
</listitem>
<listitem>
<para>Disable IPFILTER in system startup scripts by adding
<literal>ipfilter_enable="NO"</literal> (this is default
value) to <filename>/etc/rc.conf</filename>.</para>
<para>Add a script like the following to your
<filename
class="directory">/usr/local/etc/rc.d/</filename>
startup directory. The script should have an obvious
name like <filename>ipf.loadrules.sh</filename>. The
<filename>.sh</filename> extension is mandatory.</para>
<programlisting>#!/bin/sh
sh /etc/ipf.rules.script</programlisting>
<para>The permissions on this script file must be read,
write, execute for owner <username>root</username>.</para>
<screen>&prompt.root; <userinput>chmod 700 /usr/local/etc/rc.d/ipf.loadrules.sh</userinput></screen>
</listitem>
</itemizedlist>
<para>Now, when your system boots, your IPF rules will be
loaded.</para>
</sect2>
<sect2>
<title>IPF Rulesets</title>
<para>A ruleset is a group of IPF rules coded to pass or block
packets based on the values contained in the packet. The
bi-directional exchange of packets between hosts comprises
a session conversation. The firewall ruleset processes both
the packets arriving from the public Internet, as well as the
packets produced by the system as a response to them.
Each <acronym>TCP/IP</acronym> service (i.e.: telnet, www,
mail, etc.) is predefined by its protocol and privileged
(listening) port. Packets destined for a specific service,
originate from the source address using an unprivileged (high
order) port and target the specific service port on the
destination address. All the above parameters (i.e.: ports
and addresses) can be used as selection criteria to create
rules which will pass or block services.</para>
<indexterm>
<primary>IPFILTER</primary>
<secondary>rule processing order</secondary>
</indexterm>
<para>IPF was originally written using a rules processing
logic of <quote>the last matching rule wins</quote> and used
only stateless rules. Over time IPF has been enhanced to
include a <quote>quick</quote> option and a stateful
<quote>keep state</quote> option which drastically modernized
the rule processing logic.</para>
<para>The instructions contained in this section are based on
using rules that contain the <quote>quick</quote> option and
the stateful <quote>keep state</quote> option. This is the
basic framework for coding an inclusive firewall rule
set.</para>
<warning>
<para>When working with the firewall rules, be <emphasis>very
careful</emphasis>. Some configurations <emphasis>will
lock you out</emphasis> of the server. To be on the safe
side, you may wish to consider performing the initial
firewall configuration from the local console rather than
doing it remotely e.g., via
<application>ssh</application>.</para>
</warning>
</sect2>
<sect2>
<title>Rule Syntax</title>
<indexterm>
<primary>IPFILTER</primary>
<secondary>rule syntax</secondary>
</indexterm>
<para>The rule syntax presented here has been simplified to
only address the modern stateful rule context and <quote>first
matching rule wins</quote> logic. For the complete legacy
rule syntax description see the &man.ipf.8; manual
page.</para>
<para>A <literal>#</literal> character is used to mark the
start of a comment and may appear at the end of a rule line
or on its own line. Blank lines are ignored.</para>
<para>Rules contain keywords. These keywords have to be coded
in a specific order from left to right on the line. Keywords
are identified in bold type. Some keywords have sub-options
which may be keywords themselves and also include more
sub-options. Each of the headings in the below syntax has
a bold section header which expands on the content.</para>
<!-- This section is probably wrong. See the OpenBSD flag -->
<!-- What is the "OpenBSD flag"? Reference please -->
<para><replaceable>ACTION IN-OUT OPTIONS SELECTION STATEFUL
PROTO SRC_ADDR,DST_ADDR OBJECT PORT_NUM TCP_FLAG
STATEFUL</replaceable></para>
<para><replaceable>ACTION</replaceable> = block | pass</para>
<para><replaceable>IN-OUT</replaceable> = in | out</para>
<para><replaceable>OPTIONS</replaceable> = log | quick | on
interface-name</para>
<para><replaceable>SELECTION</replaceable> = proto value |
source/destination IP | port = number | flags
flag-value</para>
<para><replaceable>PROTO</replaceable> = tcp/udp | udp | tcp |
icmp</para>
<para><replaceable>SRC_ADD,DST_ADDR</replaceable> = all | from
object to object</para>
<para><replaceable>OBJECT</replaceable> = IP address |
any</para>
<para><replaceable>PORT_NUM</replaceable> = port number</para>
<para><replaceable>TCP_FLAG</replaceable> = S</para>
<para><replaceable>STATEFUL</replaceable> = keep state</para>
<sect3>
<title>ACTION</title>
<para>The action indicates what to do with the packet if it
matches the rest of the filter rule. Each rule
<emphasis>must</emphasis> have an action. The following
actions are recognized:</para>
<para><literal>block</literal> indicates that the packet
should be dropped if the selection parameters match the
packet.</para>
<para><literal>pass</literal> indicates that the packet should
exit the firewall if the selection parameters match the
packet.</para>
</sect3>
<sect3>
<title>IN-OUT</title>
<para>A mandatory requirement is that each filter rule
explicitly state which side of the I/O it is to be used
on. The next keyword must be either <literal>in</literal>
or <literal>out</literal> and one or the other has to be
coded or the rule will not pass syntax checks.</para>
<para><literal>in</literal> means this rule is being applied
against an inbound packet which has just been received on
the interface facing the public Internet.</para>
<para><literal>out</literal> means this rule is being applied
against an outbound packet destined for the interface facing
the public Internet.</para>
</sect3>
<sect3>
<title>OPTIONS</title>
<note>
<para>These options must be used in the order shown
here.</para>
</note>
<para><literal>log</literal> indicates that the packet header
will be written to
<!-- XXX - xref here -->
the <devicename>ipl</devicename> log (as described in the
LOGGING section below) if the selection parameters match the
packet.</para>
<para><literal>quick</literal> indicates that if the selection
parameters match the packet, this rule will be the last
rule checked, allowing a <quote>short-circuit</quote> path
to avoid processing any following rules for this packet.
This option is a mandatory requirement for the modernized
rules processing logic.</para>
<para><literal>on</literal> indicates the interface name to
be incorporated into the selection parameters. Interface
names are as displayed by &man.ifconfig.8;. Using this
option, the rule will only match if the packet is going
through that interface in the specified direction (in/out).
This option is a mandatory requirement for the modernized
rules processing logic.</para>
<para>When a packet is logged, the headers of the packet are
written to the <acronym>IPL</acronym> packet logging
pseudo-device. Immediately following the
<literal>log</literal> keyword, the following qualifiers
may be used (in this order):</para>
<para><literal>body</literal> indicates that the first 128
bytes of the packet contents will be logged after the
headers.</para>
<para><literal>first</literal> If the <literal>log</literal>
keyword is being used in conjunction with a <literal>keep
state</literal> option, it is recommended that this
option is also applied so that only the triggering packet
is logged and not every packet which thereafter matches
the <quote>keep state</quote> information.</para>
</sect3>
<sect3>
<title>SELECTION</title>
<para>The keywords described in this section are used to
describe attributes of the packet to be checked when
determining whether rules match or not. There is a
keyword subject, and it has sub-option keywords, one of
which has to be selected. The following general-purpose
attributes are provided for matching, and must be used in
this order:</para>
</sect3>
<sect3>
<title>PROTO</title>
<para><literal>proto</literal> is the subject keyword and
must be coded along with one of its corresponding keyword
sub-option values. The value allows a specific protocol
to be matched against. This option is a mandatory
requirement for the modernized rules processing
logic.</para>
<para><literal>tcp/udp | udp | tcp | icmp</literal> or any
protocol names found in <filename>/etc/protocols</filename>
are recognized and may be used. The special protocol
keyword <literal>tcp/udp</literal> may be used to match
either a <acronym>TCP</acronym> or a <acronym>UDP</acronym>
packet, and has been added as a convenience to save
duplication of otherwise identical rules.</para>
</sect3>
<sect3>
<title>SRC_ADDR/DST_ADDR</title>
<para>The <literal>all</literal> keyword is essentially a
synonym for <quote>from any to any</quote> with no other
match parameters.</para>
<para><literal>from src to dst</literal>: the
<literal>from</literal> and <literal>to</literal>
keywords are used to match against IP addresses. Rules
must specify <emphasis>both</emphasis> source and
destination parameters. <literal>any</literal> is a special
keyword that matches any IP address. Examples of use:
<literal>from any to any</literal>
or <literal>from 0.0.0.0/0 to any</literal> or
<literal>from any to 0.0.0.0/0</literal> or <literal>from
0.0.0.0 to any</literal> or
<literal>from any to 0.0.0.0</literal>.</para>
<para>There is no way to match ranges of IP addresses which
do not express themselves easily using the dotted numeric
form / mask-length notation. The <filename
role="package">net-mgmt/ipcalc</filename> port may be
used to ease up the calculations. Additional information
is available in the utility's web page: <ulink
url="http://jodies.de/ipcalc"></ulink>.</para>
</sect3>
<sect3>
<title>PORT</title>
<para>If a port match is included, for either or both of
source and destination, then it is only applied to
<acronym>TCP</acronym> and <acronym>UDP</acronym> packets.
When composing port comparisons, either the service name
from <filename>/etc/services</filename> or an integer port
number may be used. When the port appears as part of the
<literal>from</literal> object, it matches the source port
number; when it appears as part of the <literal>to</literal>
object, it matches the destination port number. The use
of the port option with the <literal>to</literal> object is
a mandatory requirement for the modernized rules processing
logic. Example of use: <literal>from any to any port =
80</literal></para>
<!-- XXX: Rewritten, but probably needs more changes -->
<para>Single port comparisons may be done in a number of ways,
using a number of different comparison operators. Port
ranges may also be specified.</para>
<para>port "=" | "!=" | "&lt;" | "&gt;" | "&lt;=" | "&gt;=" |
"eq" | "ne" | "lt" | "gt" | "le" | "ge".</para>
<para>To specify port ranges, port "&lt;&gt;" |
"&gt;&lt;"</para>
<warning>
<para>Following the source and destination matching
parameters, the following two parameters are mandatory
requirements for the modernized rules processing
logic.</para>
</warning>
</sect3>
<sect3>
<title><acronym>TCP</acronym>_FLAG</title>
<para>Flags are only effective for <acronym>TCP</acronym>
filtering. The letters represent one of the possible flags
that can be matched against the <acronym>TCP</acronym>
packet header.</para>
<para>The modernized rules processing logic uses the
<literal>flags S</literal> parameter to identify the tcp
session start request.</para>
</sect3>
<sect3>
<title>STATEFUL</title>
<para><literal>keep state</literal> indicates that on a pass
rule, any packets that match the rules selection parameters
should activate the stateful filtering facility.</para>
<note>
<para>This option is a mandatory requirement for the
modernized rules processing logic.</para>
</note>
</sect3>
</sect2>
<sect2>
<title>Stateful Filtering</title>
<indexterm>
<primary>IPFILTER</primary>
<secondary>stateful filtering</secondary>
</indexterm>
<!-- XXX: duplicated -->
<para>Stateful filtering treats traffic as a bi-directional
exchange of packets comprising a session conversation. When
activated, keep-state dynamically generates internal rules
for each anticipated packet being exchanged during the
bi-directional session conversation. It has sufficient
matching capabilities to determine if the session conversation
between the originating sender and the destination are
following the valid procedure of bi-directional packet
exchange. Any packets that do not properly fit the session
conversation template are automatically rejected as
impostors.</para>
<para>Keep state will also allow <acronym>ICMP</acronym> packets
related to a <acronym>TCP</acronym> or <acronym>UDP</acronym>
session through. So if you get <acronym>ICMP</acronym> type
3 code 4 in response to some web surfing allowed out
by a keep state rule, they will be automatically allowed in.
Any packet that IPF can be certain is part of an active
session, even if it is a different protocol, will be let
in.</para>
<para>What happens is:</para>
<para>Packets destined to go out through the interface connected
to the public Internet are first checked against the dynamic
state table. If the packet matches the next expected packet
comprising an active session conversation, then it exits the
firewall and the state of the session conversation flow is
updated in the dynamic state table. Packets that do not
belong to an already active session, are simply checked
against the outbound ruleset.</para>
<para>Packets coming in from the interface connected to the
public Internet are first checked against the dynamic state
table. If the packet matches the next expected packet
comprising an active session conversation, then it exits the
firewall and the state of the session conversation flow is
updated in the dynamic state table. Packets that do not
belong to an already active session, are simply checked
against the inbound ruleset.</para>
<para>When the conversation completes it is removed from the
dynamic state table.</para>
<para>Stateful filtering allows you to focus on blocking/passing
new sessions. If the new session is passed, all its
subsequent packets will be allowed through automatically and
any impostors automatically rejected. If a new session is
blocked, none of its subsequent packets will be allowed
through. Stateful filtering has technically advanced matching
abilities capable of defending against the flood of different
attack methods currently employed by attackers.</para>
</sect2>
<sect2>
<!-- XXX: This section needs a rewrite -->
<title>Inclusive Ruleset Example</title>
<para>The following ruleset is an example of how to code a
very secure inclusive type of firewall. An inclusive firewall
only allows services matching <literal>pass</literal> rules
through, and blocks all others by default. Firewalls intended
to protect other machines, also called <quote>network
firewalls</quote>, should have at least two interfaces, which
are generally configured to trust one side (the
<acronym>LAN</acronym>) and not the other (the public
Internet). Alternatively, a firewall might be configured to
protect only the system it is running on&mdash;this is called
a <quote>host based firewall</quote>, and is particularly
appropriate for servers on an untrusted network.</para>
<para>All &unix; flavored systems including &os; are designed to
use interface <devicename>lo0</devicename> and IP address
<hostid role="ipaddr">127.0.0.1</hostid> for internal
communication within the operating system. The firewall rules
must contain rules to allow free unmolested movement of these
special internally used packets.</para>
<para>The interface which faces the public Internet is the one
to place the rules that authorize and control access of the
outbound and inbound connections. This can be your user PPP
<devicename>tun0</devicename> interface or your NIC that is
connected to your DSL or cable modem.</para>
<para>In cases where one or more NICs are cabled to private
network segments, those interfaces may require rules to allow
packets originating from those LAN interfaces transit to each
other and/or to the outside (Internet).</para>
<para>The rules should be organized into three major
sections: first trusted interfaces, then the public
interface outbound, and last the public untrusted interface
inbound.</para>
<para>The rules in each of the public interface sections should
have the most frequently matched rules placed before less
commonly matched rules, with the last rule in the section
blocking and logging all packets on that interface and
direction.</para>
<para>The Outbound section in the following ruleset only
contains <literal>pass</literal> rules which contain selection
values that uniquely identify the service that is authorized
for public Internet access. All the rules have the
<literal>quick</literal>, <literal>on</literal>,
<literal>proto</literal>, <literal>port</literal>, and
<literal>keep state</literal> options set. The <literal>proto
tcp</literal> rules have the <literal>flag</literal> option
included to identify the session start request as the
triggering packet to activate the stateful facility.</para>
<para>The Inbound section has all the blocking of undesirable
packets first, for two different reasons. The first is that
malicious packets may be partial matches for legitimate
traffic. These packets have to be discarded rather than
allowed in, based on their partial matches against
<literal>allow</literal> rules. The second reason is that
known and uninteresting rejects may be blocked silently,
rather than being caught and logged by the last rules in the
section. The final rule in each section, blocks and logs all
packets and can be used to create the legal evidence needed
to prosecute the people who are attacking your system.</para>
<para>Another thing that should be taken care of, is to ensure
there is no response returned for any of the undesirable
traffic. Invalid packets should just get dropped and vanish.
This way the attacker has no knowledge if his packets have
reached your system. The less the attackers can learn about
your system, the more time they must invest before actually
doing something bad. Rules that include a <literal>log
first</literal> option, will only log the event the first
time they are triggered. This option is included in the
sample <literal>nmap OS fingerprint</literal> rule. The
<filename role="package">security/nmap</filename> utility is
commonly used by attackers who attempt to identify the
operating system of your server.</para>
<para>Any time there are logged messages on a rule with
the <literal>log first</literal> option,
<command>ipfstat -hio</command> should be executed
to evaluate how many times the rule has actually matched.
Large number of matches usually indicate that the system is
being flooded (i.e.: under attack).</para>
<para>The <filename>/etc/services</filename> file may be used to
lookup unknown port numbers. Alternatively,
visit <ulink
url="http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers"></ulink>
and do a port number lookup to find the purpose of a
particular port number.</para>
<para>Check out this link for port numbers used by Trojans
<ulink
url="http://www.sans.org/security-resources/idfaq/oddports.php"></ulink>.</para>
<para>The following ruleset creates a complete and very secure
<literal>inclusive</literal> type of firewall ruleset that
has been tested on production systems. It can be easily
modified for your own system. Just comment out any
<literal>pass</literal> rules for services that should not
be authorized.</para>
<para>To avoid logging unwanted messages,
just add a <literal>block</literal> rule in the inbound
section.</para>
<para>The <devicename>dc0</devicename> interface name has to
be changed in every rule to the real interface name of the
NIC card that connects your system to the public Internet.
For user PPP it would be <devicename>tun0</devicename>.</para>
<para>Add the following statements to
<filename>/etc/ipf.rules</filename>:</para>
<programlisting>#################################################################
# No restrictions on Inside LAN Interface for private network
# Not needed unless you have LAN
#################################################################
#pass out quick on xl0 all
#pass in quick on xl0 all
#################################################################
# No restrictions on Loopback Interface
#################################################################
pass in quick on lo0 all
pass out quick on lo0 all
#################################################################
# Interface facing Public Internet (Outbound Section)
# Match session start requests originating from behind the
# firewall on the private network
# or from this gateway server destined for the public Internet.
#################################################################
# Allow out access to my ISP's Domain name server.
# xxx must be the IP address of your ISP's DNS.
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
pass out quick on dc0 proto tcp from any to xxx port = 53 flags S keep state
pass out quick on dc0 proto udp from any to xxx port = 53 keep state
# Allow out access to my ISP's DHCP server for cable or DSL networks.
# This rule is not needed for 'user ppp' type connection to the
# public Internet, so you can delete this whole group.
# Use the following rule and check log for IP address.
# Then put IP address in commented out rule &amp; delete first rule
pass out log quick on dc0 proto udp from any to any port = 67 keep state
#pass out quick on dc0 proto udp from any to z.z.z.z port = 67 keep state
# Allow out non-secure standard www function
pass out quick on dc0 proto tcp from any to any port = 80 flags S keep state
# Allow out secure www function https over TLS SSL
pass out quick on dc0 proto tcp from any to any port = 443 flags S keep state
# Allow out send &amp; get email function
pass out quick on dc0 proto tcp from any to any port = 110 flags S keep state
pass out quick on dc0 proto tcp from any to any port = 25 flags S keep state
# Allow out Time
pass out quick on dc0 proto tcp from any to any port = 37 flags S keep state
# Allow out nntp news
pass out quick on dc0 proto tcp from any to any port = 119 flags S keep state
# Allow out gateway &amp; LAN users' non-secure FTP ( both passive &amp; active modes)
# This function uses the IP<acronym>NAT</acronym> built in FTP proxy function coded in
# the nat rules file to make this single rule function correctly.
# If you want to use the pkg_add command to install application packages
# on your gateway system you need this rule.
pass out quick on dc0 proto tcp from any to any port = 21 flags S keep state
# Allow out ssh/sftp/scp (telnet/rlogin/FTP replacements)
# This function is using SSH (secure shell)
pass out quick on dc0 proto tcp from any to any port = 22 flags S keep state
# Allow out insecure Telnet
pass out quick on dc0 proto tcp from any to any port = 23 flags S keep state
# Allow out FreeBSD CVSup
pass out quick on dc0 proto tcp from any to any port = 5999 flags S keep state
# Allow out ping to public Internet
pass out quick on dc0 proto icmp from any to any icmp-type 8 keep state
# Allow out whois from LAN to public Internet
pass out quick on dc0 proto tcp from any to any port = 43 flags S keep state
# Block and log only the first occurrence of everything
# else that's trying to get out.
# This rule implements the default block
block out log first quick on dc0 all
#################################################################
# Interface facing Public Internet (Inbound Section)
# Match packets originating from the public Internet
# destined for this gateway server or the private network.
#################################################################
# Block all inbound traffic from non-routable or reserved address spaces
block in quick on dc0 from 192.168.0.0/16 to any #RFC 1918 private IP
block in quick on dc0 from 172.16.0.0/12 to any #RFC 1918 private IP
block in quick on dc0 from 10.0.0.0/8 to any #RFC 1918 private IP
block in quick on dc0 from 127.0.0.0/8 to any #loopback
block in quick on dc0 from 0.0.0.0/8 to any #loopback
block in quick on dc0 from 169.254.0.0/16 to any #DHCP auto-config
block in quick on dc0 from 192.0.2.0/24 to any #reserved for docs
block in quick on dc0 from 204.152.64.0/23 to any #Sun cluster interconnect
block in quick on dc0 from 224.0.0.0/3 to any #Class D &amp; E multicast
##### Block a bunch of different nasty things. ############
# That I do not want to see in the log
# Block frags
block in quick on dc0 all with frags
# Block short tcp packets
block in quick on dc0 proto tcp all with short
# block source routed packets
block in quick on dc0 all with opt lsrr
block in quick on dc0 all with opt ssrr
# Block nmap OS fingerprint attempts
# Log first occurrence of these so I can get their IP address
block in log first quick on dc0 proto tcp from any to any flags FUP
# Block anything with special options
block in quick on dc0 all with ipopts
# Block public pings
block in quick on dc0 proto icmp all icmp-type 8
# Block ident
block in quick on dc0 proto tcp from any to any port = 113
# Block all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
block in log first quick on dc0 proto tcp/udp from any to any port = 137
block in log first quick on dc0 proto tcp/udp from any to any port = 138
block in log first quick on dc0 proto tcp/udp from any to any port = 139
block in log first quick on dc0 proto tcp/udp from any to any port = 81
# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP's DHCP server as it is the only
# authorized source to send this packet type. Only necessary for
# cable or DSL configurations. This rule is not needed for
# 'user ppp' type connection to the public Internet.
# This is the same IP address you captured and
# used in the outbound section.
pass in quick on dc0 proto udp from z.z.z.z to any port = 68 keep state
# Allow in standard www function because I have apache server
pass in quick on dc0 proto tcp from any to any port = 80 flags S keep state
# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID/PW passed over public Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
#pass in quick on dc0 proto tcp from any to any port = 23 flags S keep state
# Allow in secure FTP, Telnet, and SCP from public Internet
# This function is using SSH (secure shell)
pass in quick on dc0 proto tcp from any to any port = 22 flags S keep state
# Block and log only first occurrence of all remaining traffic
# coming into the firewall. The logging of only the first
# occurrence avoids filling up disk with Denial of Service logs.
# This rule implements the default block.
block in log first quick on dc0 all
################### End of rules file #####################################</programlisting>
</sect2>
<sect2>
<title><acronym>NAT</acronym></title>
<indexterm><primary>NAT</primary></indexterm>
<indexterm>
<primary>IP masquerading</primary>
<see>NAT</see>
</indexterm>
<indexterm>
<primary>network address translation</primary>
<see>NAT</see>
</indexterm>
<para><acronym>NAT</acronym> stands for <emphasis>Network
Address Translation</emphasis>. To those familiar with
&linux;, this concept is called IP Masquerading;
<acronym>NAT</acronym> and IP Masquerading are the same thing.
One of the many things the IPF <acronym>NAT</acronym> function
enables is the ability to have a private Local Area Network
(LAN) behind the firewall sharing a single ISP assigned IP
address on the public Internet.</para>
<para>You may ask why would someone want to do this. ISPs
normally assign a dynamic IP address to their non-commercial
users. Dynamic means that the IP address can be different
each time you dial in and log on to your ISP, or for cable
and DSL modem users, when the modem is power cycled. This
dynamic IP address is used to identify your system to the
public Internet.</para>
<para>Say you have five PCs at home and each one needs
Internet access. You would have to pay your ISP for an
individual Internet account for each PC and have five phone
lines.</para>
<para>With <acronym>NAT</acronym> only a single account is
needed with your ISP. The other four PCs may then be cabled
to a switch and the switch to the NIC in your &os; system
which is going to service your LAN as a gateway.
<acronym>NAT</acronym> will automatically translate the
private LAN IP address for each separate PC on the LAN to
the single public IP address as it exits the firewall bound
for the public Internet. It also does the reverse translation
for returning packets.</para>
<para>There is a special range of IP addresses reserved for
<acronym>NAT</acronym>ed private LANs. According to
RFC 1918, the following IP ranges may be used for private
nets which will never be routed directly to the public
Internet:</para>
<informaltable frame="none" pgwide="1">
<tgroup cols="2">
<colspec colwidth="1*"/>
<colspec colwidth="1*"/>
<colspec colwidth="1*"/>
<tbody>
<row>
<entry>Start IP <hostid
role="ipaddr">10.0.0.0</hostid></entry>
<entry>-</entry>
<entry>Ending IP <hostid
role="ipaddr">10.255.255.255</hostid></entry>
</row>
<row>
<entry>Start IP <hostid
role="ipaddr">172.16.0.0</hostid></entry>
<entry>-</entry>
<entry>Ending IP <hostid
role="ipaddr">172.31.255.255</hostid></entry>
</row>
<row>
<entry>Start IP <hostid
role="ipaddr">192.168.0.0</hostid></entry>
<entry>-</entry>
<entry>Ending IP <hostid
role="ipaddr">192.168.255.255</hostid></entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect2>
<sect2>
<title>IP<acronym>NAT</acronym></title>
<indexterm>
<primary>NAT</primary>
<secondary>and IPFILTER</secondary>
</indexterm>
<indexterm><primary><command>ipnat</command></primary></indexterm>
<para><acronym>NAT</acronym> rules are loaded by using
<command>ipnat</command>. Typically the
<acronym>NAT</acronym> rules are stored in
<filename>/etc/ipnat.rules</filename>. See &man.ipnat.8; for
details.</para>
<para>When changing the <acronym>NAT</acronym> rules after
<acronym>NAT</acronym> has been started, make your changes to
the file containing the NAT rules, then run
<command>ipnat</command> with the <option>-CF</option>
flags to delete the internal in use <acronym>NAT</acronym>
rules and flush the contents of the translation table of all
active entries.</para>
<para>To reload the <acronym>NAT</acronym> rules issue a command
like this:</para>
<screen>&prompt.root; <userinput>ipnat -CF -f
/etc/ipnat.rules</userinput></screen>
<para>To display some statistics about your
<acronym>NAT</acronym>, use this command:</para>
<screen>&prompt.root; <userinput>ipnat -s</userinput></screen>
<para>To list the <acronym>NAT</acronym> table's current
mappings, use this command:</para>
<screen>&prompt.root; <userinput>ipnat -l</userinput></screen>
<para>To turn verbose mode on, and display information relating
to rule processing and active rules/table entries:</para>
<screen>&prompt.root; <userinput>ipnat -v</userinput></screen>
</sect2>
<sect2>
<title>IP<acronym>NAT</acronym> Rules</title>
<para><acronym>NAT</acronym> rules are very flexible and can
accomplish many different things to fit the needs of
commercial and home users.</para>
<para>The rule syntax presented here has been simplified to
what is most commonly used in a non-commercial environment.
For a complete rule syntax description see the &man.ipnat.5;
manual page.</para>
<para>The syntax for a <acronym>NAT</acronym> rule looks
something like this:</para>
<programlisting>map <replaceable>IF</replaceable> <replaceable>LAN_IP_RANGE</replaceable> -&gt; <replaceable>PUBLIC_ADDRESS</replaceable></programlisting>
<para>The keyword <literal>map</literal> starts the rule.</para>
<para>Replace <replaceable>IF</replaceable> with the external
interface.</para>
<para>The <replaceable>LAN_IP_RANGE</replaceable> is what your
internal clients use for IP Addressing, usually this is
something like <hostid
role="ipaddr">192.168.1.0/24</hostid>.</para>
<para>The <replaceable>PUBLIC_ADDRESS</replaceable> can either
be the external IP address or the special keyword
<literal>0/32</literal>, which means to use the IP address
assigned to <replaceable>IF</replaceable>.</para>
</sect2>
<sect2>
<title>How <acronym>NAT</acronym> Works</title>
<para>A packet arrives at the firewall from the LAN with a
public destination. It passes through the outbound filter
rules, <acronym>NAT</acronym> gets its turn at the packet
and applies its rules top down, first matching rule wins.
<acronym>NAT</acronym> tests each of its rules against the
packet's interface name and source IP address. When a
packet's interface name matches a <acronym>NAT</acronym> rule
then the source IP address (i.e.: private LAN IP address) of
the packet is checked to see if it falls within the IP address
range specified to the left of the arrow symbol on the
<acronym>NAT</acronym> rule. On a match the packet has its
source IP address rewritten with the public IP address
obtained by the <literal>0/32</literal> keyword.
<acronym>NAT</acronym> posts an entry in its internal
<acronym>NAT</acronym> table so when the packet returns from
the public Internet it can be mapped back to its original
private IP address and then passed to the filter rules for
processing.</para>
</sect2>
<sect2>
<title>Enabling IP<acronym>NAT</acronym></title>
<para>To enable IP<acronym>NAT</acronym> add these statements to
<filename>/etc/rc.conf</filename>.</para>
<para>To enable your machine to route traffic between
interfaces:</para>
<programlisting>gateway_enable="YES"</programlisting>
<para>To start IP<acronym>NAT</acronym> automatically each
time:</para>
<programlisting>ipnat_enable="YES"</programlisting>
<para>To specify where to load the IP<acronym>NAT</acronym>
rules from:</para>
<programlisting>ipnat_rules="/etc/ipnat.rules"</programlisting>
</sect2>
<sect2>
<title><acronym>NAT</acronym> for a Large LAN</title>
<para>For networks that have large numbers of PC's on the LAN
or networks with more than a single LAN, the process of
funneling all those private IP addresses into a single public
IP address becomes a resource problem that may cause problems
with the same port numbers being used many times across many
<acronym>NAT</acronym>ed LAN PC's, causing collisions. There
are two ways to relieve this resource problem.</para>
<sect3>
<title>Assigning Ports to Use</title>
<!-- What does it mean ? Is there something missing ?-->
<!-- XXXBLAH <- Apparently you can't start a sect
with a <programlisting> tag ?-->
<para>A normal NAT rule would look like:</para>
<programlisting>map dc0 192.168.1.0/24 -&gt; 0/32</programlisting>
<para>In the above rule the packet's source port is unchanged
as the packet passes through IP<acronym>NAT</acronym>. By
adding the <literal>portmap</literal> keyword,
IP<acronym>NAT</acronym> can be directed to only use
source ports in the specified range. For example the
following rule will tell IP<acronym>NAT</acronym> to modify
the source port to be within the range shown:</para>
<programlisting>map dc0 192.168.1.0/24 -&gt; 0/32 portmap tcp/udp 20000:60000</programlisting>
<para>Additionally we can make things even easier by using the
<literal>auto</literal> keyword to tell
IP<acronym>NAT</acronym> to determine by itself which ports
are available to use:</para>
<programlisting>map dc0 192.168.1.0/24 -&gt; 0/32 portmap tcp/udp auto</programlisting>
</sect3>
<sect3>
<title>Using a Pool of Public Addresses</title>
<para>In very large LANs there comes a point where there are
just too many LAN addresses to fit into a single public
address. If a block of public IP addresses is available,
these addresses can be used as a <quote>pool</quote>, and
IP<acronym>NAT</acronym> may pick one of the public IP
addresses as packet-addresses are mapped on their way
out.</para>
<para>For example, instead of mapping all packets through a
single public IP address, as in:</para>
<programlisting>map dc0 192.168.1.0/24 -&gt; 204.134.75.1</programlisting>
<para>A range of public IP addresses can be specified either
with a netmask:</para>
<programlisting>map dc0 192.168.1.0/24 -&gt; 204.134.75.0/255.255.255.0</programlisting>
<para>or using CIDR notation:</para>
<programlisting>map dc0 192.168.1.0/24 -&gt; 204.134.75.0/24</programlisting>
</sect3>
</sect2>
<sect2>
<title>Port Redirection</title>
<para>A very common practice is to have a web server, email
server, database server and DNS server each segregated to a
different PC on the LAN. In this case the traffic from these
servers still have to be <acronym>NAT</acronym>ed, but there
has to be some way to direct the inbound traffic to the
correct LAN PCs. IP<acronym>NAT</acronym> has the redirection
facilities of <acronym>NAT</acronym> to solve this problem.
For example, assuming a web server operating on LAN address
<hostid
role="ipaddr">10.0.10.25</hostid> and using a single public
IP address of <hostid role="ipaddr">20.20.20.5</hostid> the
rule would be coded as follows:</para>
<programlisting>rdr dc0 20.20.20.5/32 port 80 -&gt; 10.0.10.25 port 80</programlisting>
<para>or:</para>
<programlisting>rdr dc0 0.0.0.0/0 port 80 -&gt; 10.0.10.25 port 80</programlisting>
<para>or for a LAN DNS Server on LAN address of <hostid
role="ipaddr">10.0.10.33</hostid> that needs to receive
public DNS requests:</para>
<programlisting>rdr dc0 20.20.20.5/32 port 53 -&gt; 10.0.10.33 port 53 udp</programlisting>
</sect2>
<sect2>
<title>FTP and <acronym>NAT</acronym></title>
<para>FTP is a dinosaur left over from the time before the
Internet as it is known today, when research universities were
leased lined together and FTP was used to share files among
research Scientists. This was a time when data security was
not a consideration. Over the years the FTP protocol became
buried into the backbone of the emerging Internet and its
username and password being sent in clear text was never
changed to address new security concerns. FTP has two
flavors, it can run in active mode or passive mode. The
difference is in how the data channel is acquired. Passive
mode is more secure as the data channel is acquired by the
ordinal ftp session requester. For a real good explanation
of FTP and the different modes see <ulink
url="http://www.slacksite.com/other/ftp.html"></ulink>.</para>
<sect3>
<title>IP<acronym>NAT</acronym> Rules</title>
<para>IP<acronym>NAT</acronym> has a special built in FTP
proxy option which can be specified on the
<acronym>NAT</acronym> map rule. It can monitor all
outbound packet traffic for FTP active or passive start
session requests and dynamically create temporary filter
rules containing only the port number really in use for
the data channel. This eliminates the security risk FTP
normally exposes the firewall to from having large ranges
of high order port numbers open.</para>
<para>This rule will handle all the traffic for the internal
LAN:</para>
<programlisting>map dc0 10.0.10.0/29 -&gt; 0/32 proxy port 21 ftp/tcp</programlisting>
<para>This rule handles the FTP traffic from the
gateway:</para>
<programlisting>map dc0 0.0.0.0/0 -&gt; 0/32 proxy port 21 ftp/tcp</programlisting>
<para>This rule handles all non-FTP traffic from the internal
LAN:</para>
<programlisting>map dc0 10.0.10.0/29 -&gt; 0/32</programlisting>
<para>The FTP map rule goes before our regular map rule. All
packets are tested against the first rule from the top.
Matches on interface name, then private LAN source IP
address, and then is it a FTP packet. If all that matches
then the special FTP proxy creates temp filter rules to let
the FTP session packets pass in and out, in addition to also
<acronym>NAT</acronym>ing the FTP packets. All LAN packets
that are not FTP do not match the first rule and fall
through to the third rule and are tested, matching on
interface and source IP, then are
<acronym>NAT</acronym>ed.</para>
</sect3>
<sect3>
<title>IP<acronym>NAT</acronym> FTP Filter Rules</title>
<para>Only one filter rule is needed for FTP if the
<acronym>NAT</acronym> FTP proxy is used.</para>
<para>Without the FTP Proxy, the following three rules will be
needed:</para>
<programlisting># Allow out LAN PC client FTP to public Internet
# Active and passive modes
pass out quick on rl0 proto tcp from any to any port = 21 flags S keep state
# Allow out passive mode data channel high order port numbers
pass out quick on rl0 proto tcp from any to any port &gt; 1024 flags S keep state
# Active mode let data channel in from FTP server
pass in quick on rl0 proto tcp from any to any port = 20 flags S keep state</programlisting>
</sect3>
</sect2>
</sect1>
<sect1 id="firewalls-ipfw">
<title>IPFW</title>
<indexterm>
<primary>firewall</primary>
<secondary>IPFW</secondary>
</indexterm>
<para>The IPFIREWALL (<acronym>IPFW</acronym>) is a &os; sponsored
firewall software application authored and maintained by &os;
volunteer staff members. It uses the legacy stateless rules
and a legacy rule coding technique to achieve what is referred
to as Simple Stateful logic.</para>
<para>The IPFW sample ruleset (found in
<filename>/etc/rc.firewall</filename> and
<filename>/etc/rc.firewall6</filename>) in the standard &os;
install is rather simple and it is not expected to be used
directly without modifications. The example does not use
stateful filtering, which is beneficial in most setups, so it
will not be used as base for this section.</para>
<para>The IPFW stateless rule syntax is empowered with technically
sophisticated selection capabilities which far surpasses the
knowledge level of the customary firewall installer. IPFW is
targeted at the professional user or the advanced technical
computer hobbyist who have advanced packet selection
requirements. A high degree of detailed knowledge into how
different protocols use and create their unique packet header
information is necessary before the power of the IPFW rules can
be unleashed. Providing that level of explanation is out of the
scope of this section of the Handbook.</para>
<para>IPFW is composed of seven components, the primary component
is the kernel firewall filter rule processor and its integrated
packet accounting facility, the logging facility, the
<literal>divert</literal> rule which triggers the
<acronym>NAT</acronym> facility, and the advanced special
purpose facilities, the dummynet traffic shaper facilities,
the <literal>fwd rule</literal> forward facility, the bridge
facility, and the ipstealth facility. IPFW supports both IPv4
and IPv6.</para>
<sect2 id="firewalls-ipfw-enable">
<title>Enabling IPFW</title>
<indexterm>
<primary>IPFW</primary>
<secondary>enabling</secondary>
</indexterm>
<para>IPFW is included in the basic &os; install as a separate
run time loadable module. The system will dynamically load
the kernel module when the <filename>rc.conf</filename>
statement <literal>firewall_enable="YES"</literal> is used.
There is no need to compile IPFW into the &os; kernel.</para>
<para>After rebooting your system with
<literal>firewall_enable="YES"</literal> in
<filename>rc.conf</filename> the following white highlighted
message is displayed on the screen as part of the boot
process:</para>
<screen>ipfw2 initialized, divert disabled, rule-based forwarding disabled, default to deny, logging disabled</screen>
<para>The loadable module does have logging ability
compiled in. To enable logging and set the verbose logging
limit, there is a knob that can be set in
<filename>/etc/sysctl.conf</filename>. By adding these
statements, logging will be enabled on future reboots:</para>
<programlisting>net.inet.ip.fw.verbose=1
net.inet.ip.fw.verbose_limit=5</programlisting>
</sect2>
<sect2 id="firewalls-ipfw-kernel">
<title>Kernel Options</title>
<indexterm>
<primary>kernel options</primary>
<secondary>IPFIREWALL</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>IPFIREWALL_VERBOSE</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>IPFIREWALL_VERBOSE_LIMIT</secondary>
</indexterm>
<indexterm>
<primary>IPFW</primary>
<secondary>kernel options</secondary>
</indexterm>
<para>It is not a mandatory requirement to enable IPFW by
compiling the following options into the &os; kernel. It is
presented here as background information only.</para>
<programlisting>options IPFIREWALL</programlisting>
<para>This option enables IPFW as part of the kernel</para>
<programlisting>options IPFIREWALL_VERBOSE</programlisting>
<para>Enables logging of packets that pass through IPFW and have
the <literal>log</literal> keyword specified in the
ruleset.</para>
<programlisting>options IPFIREWALL_VERBOSE_LIMIT=5</programlisting>
<para>Limits the number of packets logged through
&man.syslogd.8; on a per entry basis. This option may be
used in hostile environments, when firewall activity logging
is desired. This will close a possible denial of service
attack via syslog flooding.</para>
<indexterm>
<primary>kernel options</primary>
<secondary>IPFIREWALL_DEFAULT_TO_ACCEPT</secondary>
</indexterm>
<programlisting>options IPFIREWALL_DEFAULT_TO_ACCEPT</programlisting>
<para>This option will allow everything to pass through the
firewall by default, which is a good idea when the firewall
is being set up for the first time.</para>
<indexterm>
<primary>kernel options</primary>
<secondary>IPDIVERT</secondary>
</indexterm>
<programlisting>options IPDIVERT</programlisting>
<para>This enables the use of <acronym>NAT</acronym>
functionality.</para>
<note>
<para>The firewall will block all incoming and outgoing
packets if either the
<literal>IPFIREWALL_DEFAULT_TO_ACCEPT</literal> kernel
option or a rule to explicitly allow these connections are
missing.</para>
</note>
</sect2>
<sect2 id="firewalls-ipfw-rc">
<title><filename>/etc/rc.conf</filename> Options</title>
<para>Enable the firewall:</para>
<programlisting>firewall_enable="YES"</programlisting>
<para>To select one of the default firewall types provided by
&os;, select one by reading
<filename>/etc/rc.firewall</filename> and place it in
the following:</para>
<programlisting>firewall_type="open"</programlisting>
<para>Available values for this setting are:</para>
<itemizedlist>
<listitem>
<para><literal>open</literal> &mdash; pass all
traffic.</para>
</listitem>
<listitem>
<para><literal>client</literal> &mdash; will protect only
this machine.</para>
</listitem>
<listitem>
<para><literal>simple</literal> &mdash; protect the whole
network.</para>
</listitem>
<listitem>
<para><literal>closed</literal> &mdash; entirely disables
IP traffic except for the loopback interface.</para>
</listitem>
<listitem>
<para><literal>UNKNOWN</literal> &mdash; disables the
loading of firewall rules.</para>
</listitem>
<listitem>
<para><filename><replaceable>filename</replaceable></filename>
&mdash; absolute path of file containing firewall
rules.</para>
</listitem>
</itemizedlist>
<para>It is possible to use two different ways to load custom
rules for <application>ipfw</application> firewall. One is
by setting <literal>firewall_type</literal> variable to
absolute path of file, which contains <emphasis>firewall
rules</emphasis> without any command-line options for
&man.ipfw.8; itself. The following is a simple example of
a ruleset file that blocksall incoming and outgoing
traffic:</para>
<programlisting>add deny in add deny out</programlisting>
<para>On the other hand, it is possible to set the
<literal>firewall_script</literal> variable to the absolute
path of an executable script that includes
<command>ipfw</command> commands being executed at system
boot time. A valid ruleset script that would be equivalent
to the ruleset file shown above would be the
following:</para>
<programlisting>#!/bin/sh
ipfw -q flush
ipfw add deny in
ipfw add deny out</programlisting>
<note>
<para>If <literal>firewall_type</literal> is set to either
<literal>client</literal> or <literal>simple</literal>, the
default rules found in <filename>/etc/rc.firewall</filename>
should be reviewed to fit to the configuration of the given
machine. Also note that the examples used in this chapter
expect that the <literal>firewall_script</literal> is set to
<filename>/etc/ipfw.rules</filename>.</para>
</note>
<para>Enable logging:</para>
<programlisting>firewall_logging="YES"</programlisting>
<warning>
<para>The only thing that the
<varname>firewall_logging</varname> variable will do is
setting the <varname>net.inet.ip.fw.verbose</varname> sysctl
variable to the value of <literal>1</literal> (see <xref
linkend="firewalls-ipfw-enable"/>). There is no
<filename>rc.conf</filename> variable to set log
limitations, but it can be set via sysctl variable, manually
or from <filename>/etc/sysctl.conf</filename>:</para>
<programlisting>net.inet.ip.fw.verbose_limit=5</programlisting>
</warning>
<para>If your machine is acting as a gateway, i.e., providing
Network Address Translation (NAT) via &man.natd.8;, please
refer to <xref linkend="network-natd"/> for information
regarding the required <filename>/etc/rc.conf</filename>
options.</para>
</sect2>
<sect2 id="firewalls-ipfw-cmd">
<title>The IPFW Command</title>
<indexterm><primary><command>ipfw</command></primary></indexterm>
<para>The <command>ipfw</command> command is the normal vehicle
for making manual single rule additions or deletions to the
active firewall internal rules while it is running. The
problem with using this method is once your system is shutdown
or halted all the rules that were added, changed or deleted
are lost. Writing all your rules in a file and using that
file to load the rules at boot time, or to replace in mass
the currently running firewall rules with changes you made
to the files content, is the recommended method used
here.</para>
<para>The <command>ipfw</command> command is still a very
useful way to display the running firewall rules to the
console screen. The IPFW accounting facility dynamically
creates a counter for each rule that counts each packet that
matches the rule. During the process of testing a rule,
listing the rule with its counter is one of the ways of
determining if the rule is functioning.</para>
<para>To list all the rules in sequence:</para>
<screen>&prompt.root; <userinput>ipfw list</userinput></screen>
<para>To list all the rules with a time stamp of when the last
time the rule was matched:</para>
<screen>&prompt.root; <userinput>ipfw -t list</userinput></screen>
<para>The next example lists accounting information, the packet
count for matched rules along with the rules themselves.
The first column is the rule number, followed by the number
of outgoing matched packets, followed by the number of
incoming matched packets, and then the rule itself.</para>
<screen>&prompt.root; <userinput>ipfw -a list</userinput></screen>
<para>List the dynamic rules in addition to the static
rules:</para>
<screen>&prompt.root; <userinput>ipfw -d list</userinput></screen>
<para>Also show the expired dynamic rules:</para>
<screen>&prompt.root; <userinput>ipfw -d -e list</userinput></screen>
<para>Zero the counters:</para>
<screen>&prompt.root; <userinput>ipfw zero</userinput></screen>
<para>Zero the counters for just the rule with number
<replaceable>NUM</replaceable>:</para>
<screen>&prompt.root; <userinput>ipfw zero <replaceable>NUM</replaceable></userinput></screen>
</sect2>
<sect2 id="firewalls-ipfw-rules">
<title>IPFW Rulesets</title>
<!-- This has already appeared once -->
<para>A ruleset is a group of IPFW rules coded to allow or deny
packets based on the values contained in the packet. The
bi-directional exchange of packets between hosts comprises
a session conversation. The firewall ruleset processes both
the packets arriving from the public Internet, as well as
the packets originating from the system as a response to them.
Each <acronym>TCP/IP</acronym> service (i.e.: telnet, www,
mail, etc.) is predefined by its protocol and privileged
(listening) port. Packets destined for a specific service,
originate from the source address using an unprivileged (high
order) port and target the specific service port on the
destination address. All the above parameters (i.e., ports
and addresses) can be used as selection criteria to create
rules which will pass or block services.</para>
<indexterm>
<primary>IPFW</primary>
<secondary>rule processing order</secondary>
</indexterm>
<!-- Needs rewording to include note below -->
<para>When a packet enters the firewall it is compared against
the first rule in the ruleset and progresses one rule at a
time moving from top to bottom of the set in ascending rule
number sequence order. When the packet matches the selection
parameters of a rule, the rules' action field value is
executed and the search of the ruleset terminates for that
packet. This is referred to as <quote>the first match
wins</quote> search method. If the packet does not match
any of the rules, it gets caught by the mandatory IPFW default
rule, number 65535 which denies all packets and discards them
without any reply back to the originating destination.</para>
<note>
<para>The search continues after <literal>count</literal>,
<literal>skipto</literal> and <literal>tee</literal>
rules.</para>
</note>
<para>The instructions contained here are based on using rules
that contain the stateful <literal>keep state</literal>,
<literal>limit</literal>, <literal>in</literal>,
<literal>out</literal> and <literal>via</literal> options.
This is the basic framework for coding an inclusive type
firewall ruleset.</para>
<warning>
<para>Be careful when working with firewall rules, as it is
easy to end up locking yourself out.</para>
</warning>
<sect3 id="firewalls-ipfw-rules-syntax">
<title>Rule Syntax</title>
<indexterm>
<primary>IPFW</primary>
<secondary>rule syntax</secondary>
</indexterm>
<para>The rule syntax presented here has been simplified to
what is necessary to create a standard inclusive type
firewall ruleset. For a complete rule syntax description
see the &man.ipfw.8; manual page.</para>
<para>Rules contain keywords: these keywords have to be coded
in a specific order from left to right on the line.
Keywords are identified in bold type. Some keywords have
sub-options which may be keywords them selves and also
include more sub-options.</para>
<para><literal>#</literal> is used to mark the start of a
comment and may appear at the end of a rule line or on its
own lines. Blank lines are ignored.</para>
<para><replaceable>CMD RULE_NUMBER ACTION LOGGING SELECTION
STATEFUL</replaceable></para>
<sect4>
<title>CMD</title>
<para>Each new rule has to be prefixed with
<parameter>add</parameter> to add the
rule to the internal table.</para>
</sect4>
<sect4>
<title>RULE_NUMBER</title>
<para>Each rule is associated with a rule_number in the
range 1..65535.</para>
</sect4>
<sect4>
<title>ACTION</title>
<para>A rule can be associated with one of the following
actions, which will be executed when the packet matches
the selection criterion of the rule.</para>
<para><parameter>allow | accept | pass |
permit</parameter></para>
<para>These all mean the same thing which is to allow
packets that match the rule to exit the firewall rule
processing. The search terminates at this rule.</para>
<para><parameter>check-state</parameter></para>
<para>Checks the packet against the dynamic rules table.
If a match is found, execute the action associated with
the rule which generated this dynamic rule, otherwise
move to the next rule. The check-state rule does not
have selection criterion. If no check-state rule is
present in the ruleset, the dynamic rules table is checked
at the first keep-state or limit rule.</para>
<para><parameter>deny | drop</parameter></para>
<para>Both words mean the same thing which is to discard
packets that match this rule. The search
terminates.</para>
</sect4>
<sect4>
<title>Logging</title>
<para><parameter>log</parameter> or
<parameter>logamount</parameter></para>
<para>When a packet matches a rule with the
<literal>log</literal> keyword, a message will be logged
to &man.syslogd.8; with a facility name of SECURITY.
The logging only occurs if the number of packets logged
so far for that particular rule does not exceed the
<literal>logamount</literal> parameter. If no
<literal>logamount</literal> is specified, the limit is
taken from the sysctl variable
<literal>net.inet.ip.fw.verbose_limit</literal>. In both
cases, a value of zero removes the logging limit. Once
the limit is reached, logging can be re-enabled by
clearing the logging counter or the packet counter for
that rule, use <command>ipfw reset log</command>.</para>
<note>
<para>Logging is done after
all other packet matching conditions have been
successfully verified, and before performing the final
action (accept, deny) on the packet. It is up to you
to decide which rules you want to enable logging
on.</para>
</note>
</sect4>
<sect4>
<title>Selection</title>
<para>The keywords described in this section are used to
describe attributes of the packet to be checked when
determining whether rules match the packet or not.
The following general-purpose attributes are provided for
matching, and must be used in this order:</para>
<para><parameter>udp | tcp | icmp</parameter></para>
<para>Any other protocol names found in
<filename>/etc/protocols</filename> are also recognized
and may be used. The value specified is the protocol to
be matched against. This is a mandatory
requirement.</para>
<para><parameter>from src to dst</parameter></para>
<para>The <literal>from</literal> and <literal>to</literal>
keywords are used to match against IP addresses. Rules
must specify <emphasis>both</emphasis> source and
destination parameters. <literal>any</literal> is a
special keyword that matches any IP address.
<literal>me</literal> is a special keyword that matches
any IP address configured on an interface in your &os;
system to represent the PC the firewall is running on
(i.e.: this box) as in <literal>from me to
any</literal> or <literal>from any to me</literal> or
<literal>from 0.0.0.0/0 to any</literal> or
<literal>from any to 0.0.0.0/0</literal> or
<literal>from 0.0.0.0 to any</literal> or <literal>from
any to 0.0.0.0</literal> or
<literal>from me to 0.0.0.0</literal>. IP addresses are
specified as a dotted IP address numeric form/mask-length
(CIDR notation), or as single dotted IP address numeric
form. This is a mandatory requirement. The <filename
role="package">net-mgmt/ipcalc</filename> port may be
used to ease up the calculations. Additional
information is available in the utility's web page: <ulink
url="http://jodies.de/ipcalc"></ulink>.</para>
<para><parameter>port number</parameter></para>
<para>For protocols which support port numbers (such as
<acronym>TCP</acronym> and <acronym>UDP</acronym>), it
is mandatory to code the port number of the service that
will be matched. Service names (from
<filename>/etc/services</filename>) may be used instead
of numeric port values.</para>
<para><parameter>in | out</parameter></para>
<para>Matches incoming or outgoing packets, respectively.
The <literal>in</literal> and <literal>out</literal>
are keywords and it is mandatory that
one or the other is coded as part of your rule matching
criterion.</para>
<para><parameter>via IF</parameter></para>
<para>Matches packets going through the interface specified
by exact name. The <literal>via</literal> keyword causes
the interface to always be checked as part of the match
process.</para>
<para><parameter>setup</parameter></para>
<para>This is a mandatory keyword that identifies the
session start request for <acronym>TCP</acronym>
packets.</para>
<para><parameter>keep-state</parameter></para>
<para>This is a mandatory keyword. Upon a match, the
firewall will create a dynamic rule, whose default
behavior is to match bidirectional traffic between source
and destination IP/port using the same protocol.</para>
<para><parameter>limit {src-addr | src-port | dst-addr |
dst-port}</parameter></para>
<para>The firewall will only allow
<replaceable>N</replaceable> connections with the same
set of parameters as specified in the rule. One or more
of source and destination addresses and ports can be
specified. The <literal>limit</literal> and
<literal>keep-state</literal> can not be used on the
same rule. The <literal>limit</literal> option provides
the same stateful function as
<literal>keep-state</literal>, plus its own
functions.</para>
</sect4>
</sect3>
<sect3>
<title>Stateful Rule Option</title>
<indexterm>
<primary>IPFW</primary>
<secondary>stateful filtering</secondary>
</indexterm>
<!-- XXX: duplicated -->
<para>Stateful filtering treats traffic as a bi-directional
exchange of packets comprising a session conversation. It
has the matching capabilities to determine if the session
conversation between the originating sender and the
destination are following the valid procedure of
bi-directional packet exchange. Any packets that do not
properly fit the session conversation template are
automatically rejected as impostors.</para>
<para>The <literal>check-state</literal> option is used to
identify where in the IPFW rules set the packet is to be
tested against the dynamic rules facility. On a match the
packet exits the firewall to continue on its way and a new
rule is dynamically created for the next anticipated packet
being exchanged during this bi-directional session
conversation. On a no match the packet advances to the
next rule in the ruleset for testing.</para>
<para>The dynamic rules facility is vulnerable to resource
depletion from a SYN-flood attack which would open a huge
number of dynamic rules. To counter this attack, &os;
added another new option named <literal>limit</literal>.
This option is used to limit the number of simultaneous
session conversations by checking the rules source or
destinations fields as directed by the
<literal>limit</literal> option and using the packet's IP
address found there, in a search of the open dynamic rules
counting the number of times this rule and IP address
combination occurred, if this count is greater that the
value specified on the <literal>limit</literal> option, the
packet is discarded.</para>
</sect3>
<sect3>
<title>Logging Firewall Messages</title>
<indexterm>
<primary>IPFW</primary>
<secondary>logging</secondary>
</indexterm>
<para>The benefits of logging are obvious: it provides the
ability to review after the fact the rules you activated
logging on which provides information like, what packets
had been dropped, what addresses they came from and where
they were going, giving you a significant edge in tracking
down attackers.</para>
<para>Even with the logging facility enabled, IPFW will not
generate any rule logging on its own. The firewall
administrator decides what rules in the ruleset will be
logged, and adds the <literal>log</literal> verb to those
rules. Normally only deny rules are logged, like the deny
rule for incoming <acronym>ICMP</acronym> pings. It is
very customary to duplicate the <quote>ipfw default deny
everything</quote> rule with the <literal>log</literal>
verb included as your last rule in the ruleset. This
way it is possible to see all the packets that did not
match any of the rules in the ruleset.</para>
<para>Logging is a two edged sword, if you are not careful,
you can lose yourself in the over abundance of log data
and fill your disk up with growing log files. DoS attacks
that fill up disk drives is one of the oldest attacks
around. These log messages are not only written to
<application>syslogd</application>, but also are
displayed on the root console screen and soon become very
annoying.</para>
<para>The <literal>IPFIREWALL_VERBOSE_LIMIT=5</literal>
kernel option limits the number of consecutive messages
sent to the system logger &man.syslogd.8;, concerning the
packet matching of a given rule. When this option is
enabled in the kernel, the number of consecutive messages
concerning a particular rule is capped at the number
specified. There is nothing to be gained from 200 log
messages saying the same identical thing. For instance,
five consecutive messages concerning a particular rule
would be logged to <application>syslogd</application>, the
remainder identical consecutive messages would be counted
and posted to <application>syslogd</application> with a
phrase like the following:</para>
<programlisting>last message repeated 45 times</programlisting>
<para>All logged packets messages are written by default to
<filename>/var/log/security</filename>, which is
defined in <filename>/etc/syslog.conf</filename>.</para>
</sect3>
<sect3 id="firewalls-ipfw-rules-script">
<title>Building a Rule Script</title>
<para>Most experienced IPFW users create a file containing
the rules and code them in a manner compatible with running
them as a script. The major benefit of doing this is the
firewall rules can be refreshed in mass without the need
of rebooting the system to activate them. This method is
very convenient in testing new rules as the procedure can
be executed as many times as needed. Being a script,
symbolic substitution can be used to code frequent used
values and substitute them in multiple rules. This is
shown in the following example.</para>
<para>The script syntax used here is compatible with the
&man.sh.1;, &man.csh.1;, &man.tcsh.1; shells. Symbolic
substitution fields are prefixed with a dollar sign
&dollar;. Symbolic fields do not have the &dollar; prefix.
The value to populate the symbolic field must be enclosed
in "double quotes".</para>
<para>Start your rules file like this:</para>
<programlisting>############### start of example ipfw rules script #############
#
ipfw -q -f flush # Delete all rules
# Set defaults
oif="tun0" # out interface
odns="192.0.2.11" # ISP's DNS server IP address
cmd="ipfw -q add " # build rule prefix
ks="keep-state" # just too lazy to key this each time
&dollar;cmd 00500 check-state
&dollar;cmd 00502 deny all from any to any frag
&dollar;cmd 00501 deny tcp from any to any established
&dollar;cmd 00600 allow tcp from any to any 80 out via &dollar;oif setup &dollar;ks
&dollar;cmd 00610 allow tcp from any to &dollar;odns 53 out via &dollar;oif setup &dollar;ks
&dollar;cmd 00611 allow udp from any to &dollar;odns 53 out via &dollar;oif &dollar;ks
################### End of example ipfw rules script ############</programlisting>
<para>That is all there is to it. The rules are not important
in this example, how the symbolic substitution field are
populated and used are.</para>
<para>If the above example was in
<filename>/etc/ipfw.rules</filename>, the rules could
be reloaded by entering the following on the command
line.</para>
<screen>&prompt.root; <userinput>sh /etc/ipfw.rules</userinput></screen>
<para>The <filename>/etc/ipfw.rules</filename> file could be
located anywhere you want and the file could be named any
thing you would like.</para>
<para>The same thing could also be accomplished by running
these commands by hand:</para>
<screen>&prompt.root; <userinput>ipfw -q -f flush</userinput>
&prompt.root; <userinput>ipfw -q add check-state</userinput>
&prompt.root; <userinput>ipfw -q add deny all from any to any frag</userinput>
&prompt.root; <userinput>ipfw -q add deny tcp from any to any established</userinput>
&prompt.root; <userinput>ipfw -q add allow tcp from any to any 80 out via tun0 setup keep-state</userinput>
&prompt.root; <userinput>ipfw -q add allow tcp from any to 192.0.2.11 53 out via tun0 setup keep-state</userinput>
&prompt.root; <userinput>ipfw -q add 00611 allow udp from any to 192.0.2.11 53 out via tun0 keep-state</userinput></screen>
</sect3>
<sect3>
<title>Stateful Ruleset</title>
<para>The following non-<acronym>NAT</acronym>ed ruleset is an
example of how to code a very secure 'inclusive' type of
firewall. An inclusive firewall only allows services
matching pass rules through and blocks all other by
default. Firewalls designed to protect entire network
segments, have at minimum two interfaces which must
have rules to allow the firewall to function.</para>
<para>All &unix; flavored operating systems, &os; included,
are designed to use interface <devicename>lo0</devicename>
and IP address <hostid role="ipaddr">127.0.0.1</hostid>
for internal communication with in the operating system.
The firewall rules must contain rules to allow free
unmolested movement of these special internally used
packets.</para>
<para>The interface which faces the public Internet is the
one to place the rules that authorize and control access
of the outbound and inbound connections. This can be your
user <acronym>PPP</acronym> <devicename>tun0</devicename>
interface or your NIC that is connected to your DSL or cable
modem.</para>
<para>In cases where one or more than one NICs are connected
to a private LAN behind the firewall, those interfaces must
have rules coded to allow free unmolested movement of
packets originating from those LAN interfaces.</para>
<para>The rules should be first organized into three major
sections, all the free unmolested interfaces, public
interface outbound, and the public interface inbound.</para>
<para>The order of the rules in each of the public interface
sections should be in order of the most used rules being
placed before less often used rules with the last rule in
the section blocking and logging all packets on that
interface and direction.</para>
<para>The Outbound section in the following ruleset only
contains <literal>allow</literal> rules which contain
selection values that uniquely identify the service that
is authorized for public Internet access. All the rules
have the <literal>proto</literal>, <literal>port</literal>,
<literal>in/out</literal>, <literal>via</literal> and
<literal>keep state</literal> option coded. The
<literal>proto tcp</literal> rules have the
<literal>setup</literal> option included to identify the
start session request as the trigger packet to be posted
to the keep state stateful table.</para>
<para>The Inbound section has all the blocking of undesirable
packets first, for two different reasons. The first is
that malicious packets may be partial matches for legitimate
traffic. These packets have to be discarded rather than
allowed in, based on their partial matches against
<literal>allow</literal> rules. The second reason is that
known and uninteresting rejects may be blocked silently,
rather than being caught and logged by the last rules in
the section. The final rule in each section, blocks and
logs all packets and can be used to create the legal
evidence needed to prosecute the people who are attacking
your system.</para>
<para>Another thing that should be taken care of, is to
insure there is no response returned for any of the
undesirable stuff. Invalid packets should just get dropped
and vanish. This way the attacker has no knowledge if his
packets have reached your system. The less the attackers
can learn about your system, the more secure it is. Packets
with unrecognized port numbers may be looked up in
<filename>/etc/services/</filename> or go to <ulink
url="http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers"></ulink>
and do a port number lookup to find the purpose of the
particular port number is. Check out this link for port
numbers used by Trojans: <ulink
url="http://www.sans.org/security-resources/idfaq/oddports.php"></ulink>.</para>
</sect3>
<sect3>
<title>An Example Inclusive Ruleset</title>
<para>The following non-<acronym>NAT</acronym>ed ruleset is
a complete inclusive type ruleset. It is safe to use this
ruleset on your own systems. Just comment out any
<literal>pass</literal> rules for services that are not
required. To avoid logging undesired messages, add a
<literal>deny</literal> rule in the inbound section. The
<devicename>dc0</devicename> interface will have to be
changed in every rule, with the actual name of the
interface (NIC) that connects your system to the public
Internet. For user <acronym>PPP</acronym>, this would be
<devicename>tun0</devicename>.</para>
<para>There is a noticeable pattern in the usage of these
rules.</para>
<itemizedlist>
<listitem>
<para>All statements that are a request to start a session
to the public Internet use
<literal>keep-state</literal>.</para>
</listitem>
<listitem>
<para>All the authorized services that originate from
the public Internet have the <literal>limit</literal>
option to stop flooding.</para>
</listitem>
<listitem>
<para>All rules use <literal>in</literal> or
<literal>out</literal> to clarify direction.</para>
</listitem>
<listitem>
<para>All rules use <literal>via</literal>
<replaceable>interface-name</replaceable> to specify
the interface the packet is traveling over.</para>
</listitem>
</itemizedlist>
<para>The following rules go into
<filename>/etc/ipfw.rules</filename>.</para>
<programlisting>################ Start of IPFW rules file ###############################
# Flush out the list before we begin.
ipfw -q -f flush
# Set rules command prefix
cmd="ipfw -q add"
pif="dc0" # public interface name of NIC
# facing the public Internet
#################################################################
# No restrictions on Inside LAN Interface for private network
# Not needed unless you have LAN.
# Change xl0 to your LAN NIC interface name
#################################################################
#&dollar;cmd 00005 allow all from any to any via xl0
#################################################################
# No restrictions on Loopback Interface
#################################################################
&dollar;cmd 00010 allow all from any to any via lo0
#################################################################
# Allow the packet through if it has previous been added to the
# the "dynamic" rules table by a allow keep-state statement.
#################################################################
&dollar;cmd 00015 check-state
#################################################################
# Interface facing Public Internet (Outbound Section)
# Interrogate session start requests originating from behind the
# firewall on the private network or from this gateway server
# destined for the public Internet.
#################################################################
# Allow out access to my ISP's Domain name server.
# x.x.x.x must be the IP address of your ISP.s DNS
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
&dollar;cmd 00110 allow tcp from any to x.x.x.x 53 out via &dollar;pif setup keep-state
&dollar;cmd 00111 allow udp from any to x.x.x.x 53 out via &dollar;pif keep-state
# Allow out access to my ISP's DHCP server for cable/DSL configurations.
# This rule is not needed for .user ppp. connection to the public Internet.
# so you can delete this whole group.
# Use the following rule and check log for IP address.
# Then put IP address in commented out rule &amp; delete first rule
&dollar;cmd 00120 allow log udp from any to any 67 out via &dollar;pif keep-state
#&dollar;cmd 00120 allow udp from any to x.x.x.x 67 out via &dollar;pif keep-state
# Allow out non-secure standard www function
&dollar;cmd 00200 allow tcp from any to any 80 out via &dollar;pif setup keep-state
# Allow out secure www function https over TLS SSL
&dollar;cmd 00220 allow tcp from any to any 443 out via &dollar;pif setup keep-state
# Allow out send &amp; get email function
&dollar;cmd 00230 allow tcp from any to any 25 out via &dollar;pif setup keep-state
&dollar;cmd 00231 allow tcp from any to any 110 out via &dollar;pif setup keep-state
# Allow out FBSD (make install &amp; CVSUP) functions
# Basically give user root "GOD" privileges.
&dollar;cmd 00240 allow tcp from me to any out via &dollar;pif setup keep-state uid root
# Allow out ping
&dollar;cmd 00250 allow icmp from any to any out via &dollar;pif keep-state
# Allow out Time
&dollar;cmd 00260 allow tcp from any to any 37 out via &dollar;pif setup keep-state
# Allow out nntp news (i.e., news groups)
&dollar;cmd 00270 allow tcp from any to any 119 out via &dollar;pif setup keep-state
# Allow out secure FTP, Telnet, and SCP
# This function is using SSH (secure shell)
&dollar;cmd 00280 allow tcp from any to any 22 out via &dollar;pif setup keep-state
# Allow out whois
&dollar;cmd 00290 allow tcp from any to any 43 out via &dollar;pif setup keep-state
# deny and log everything else that.s trying to get out.
# This rule enforces the block all by default logic.
&dollar;cmd 00299 deny log all from any to any out via &dollar;pif
#################################################################
# Interface facing Public Internet (Inbound Section)
# Check packets originating from the public Internet
# destined for this gateway server or the private network.
#################################################################
# Deny all inbound traffic from non-routable reserved address spaces
&dollar;cmd 00300 deny all from 192.168.0.0/16 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 00301 deny all from 172.16.0.0/12 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 00302 deny all from 10.0.0.0/8 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 00303 deny all from 127.0.0.0/8 to any in via &dollar;pif #loopback
&dollar;cmd 00304 deny all from 0.0.0.0/8 to any in via &dollar;pif #loopback
&dollar;cmd 00305 deny all from 169.254.0.0/16 to any in via &dollar;pif #DHCP auto-config
&dollar;cmd 00306 deny all from 192.0.2.0/24 to any in via &dollar;pif #reserved for docs
&dollar;cmd 00307 deny all from 204.152.64.0/23 to any in via &dollar;pif #Sun cluster interconnect
&dollar;cmd 00308 deny all from 224.0.0.0/3 to any in via &dollar;pif #Class D &amp; E multicast
# Deny public pings
&dollar;cmd 00310 deny icmp from any to any in via &dollar;pif
# Deny ident
&dollar;cmd 00315 deny tcp from any to any 113 in via &dollar;pif
# Deny all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
&dollar;cmd 00320 deny tcp from any to any 137 in via &dollar;pif
&dollar;cmd 00321 deny tcp from any to any 138 in via &dollar;pif
&dollar;cmd 00322 deny tcp from any to any 139 in via &dollar;pif
&dollar;cmd 00323 deny tcp from any to any 81 in via &dollar;pif
# Deny any late arriving packets
&dollar;cmd 00330 deny all from any to any frag in via &dollar;pif
# Deny ACK packets that did not match the dynamic rule table
&dollar;cmd 00332 deny tcp from any to any established in via &dollar;pif
# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP.s DHCP server as it.s the only
# authorized source to send this packet type.
# Only necessary for cable or DSL configurations.
# This rule is not needed for .user ppp. type connection to
# the public Internet. This is the same IP address you captured
# and used in the outbound section.
#&dollar;cmd 00360 allow udp from any to x.x.x.x 67 in via &dollar;pif keep-state
# Allow in standard www function because I have apache server
&dollar;cmd 00400 allow tcp from any to me 80 in via &dollar;pif setup limit src-addr 2
# Allow in secure FTP, Telnet, and SCP from public Internet
&dollar;cmd 00410 allow tcp from any to me 22 in via &dollar;pif setup limit src-addr 2
# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID &amp; PW are passed over public
# Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
&dollar;cmd 00420 allow tcp from any to me 23 in via &dollar;pif setup limit src-addr 2
# Reject &amp; Log all incoming connections from the outside
&dollar;cmd 00499 deny log all from any to any in via &dollar;pif
# Everything else is denied by default
# deny and log all packets that fell through to see what they are
&dollar;cmd 00999 deny log all from any to any
################ End of IPFW rules file ###############################</programlisting>
</sect3>
<sect3>
<title>An Example <acronym>NAT</acronym> and Stateful
Ruleset</title>
<indexterm>
<primary>NAT</primary>
<secondary>and IPFW</secondary>
</indexterm>
<para>There are some additional configuration statements that
need to be enabled to activate the <acronym>NAT</acronym>
function of IPFW. The kernel source needs
<literal>option IPDIVERT</literal> statement added to the
other IPFIREWALL statements compiled into a custom
kernel.</para>
<para>In addition to the normal IPFW options in
<filename>/etc/rc.conf</filename>, the following are
needed.</para>
<programlisting>natd_enable="YES" # Enable <acronym>NAT</acronym>D function
natd_interface="rl0" # interface name of public Internet NIC
natd_flags="-dynamic -m" # -m = preserve port numbers if possible</programlisting>
<para>Utilizing stateful rules with
<literal>divert natd</literal> rule (Network Address
Translation) greatly complicates the ruleset coding
logic. The positioning of the
<literal>check-state</literal>, and
<literal>divert natd</literal> rules in the ruleset becomes
very critical. This is no longer a simple fall-through
logic flow. A new action type is used, called
<literal>skipto</literal>. To use the
<literal>skipto</literal> command it is mandatory that
each rule is numbered, so the
<literal>skipto</literal> rule number knows exactly where
it is jumping to.</para>
<para>The following is an uncommented example of one coding
method, selected here to explain the sequence of the packet
flow through the rulesets.</para>
<para>The processing flow starts with the first rule from the
top of the rule file and progress one rule at a time deeper
into the file until the end is reached or the packet being
tested to the selection criteria matches and the packet is
released out of the firewall. It is important to take
notice of the location of rule numbers 100 101, 450, 500,
and 510. These rules control the translation of the
outbound and inbound packets so their entries in the
keep-state dynamic table always register the private LAN
IP address. Next notice that all the allow and deny rules
specify the direction the packet is going (i.e.: outbound
or inbound) and the interface. Also notice that the start
outbound session requests, all
<literal>skipto rule 500</literal> for the network address
translation.</para>
<para>Say a LAN user uses their web browser to get a web
page. Web pages are transmitted over port 80. So the
packet enters the firewall. It does not match rule 100
because it is headed out rather than in. It passes rule
101 because this is the first packet, so it has not been
posted to the keep-state dynamic table yet. The packet
finally comes to rule 125 a matches. It is outbound through
the NIC facing the public Internet. The packet still has
its source IP address as a private LAN IP address. On
the match to this rule, two actions take place. The
<literal>keep-state</literal> option will post this rule
into the keep-state dynamic rules table and the specified
action is executed. The action is part of the info
posted to the dynamic table. In this case it is
<literal>skipto rule 500</literal>. Rule 500
<acronym>NAT</acronym>s the packet IP address and out it
goes. Remember this, this is very important. This packet
makes its way to the destination, where a response
packet is generated and sent back. This new packet
enters the top of the ruleset. This time it does match
rule 100 and has it destination IP address mapped back to
its corresponding LAN IP address. It then is processed by
the <literal>check-state</literal> rule, it is found in
the table as an existing session conversation and released
to the LAN. It goes to the LAN PC that sent it and a new
packet is sent requesting another segment of the data from
the remote server. This time it gets checked by the
<literal>check-state</literal> rule and its outbound
entry is found, the associated action,
<literal>skipto 500</literal>, is executed. The packet
jumps to rule 500 gets <acronym>NAT</acronym>ed and released
on its way out.</para>
<para>On the inbound side, everything coming in that is part
of an existing session conversation is being automatically
handled by the <literal>check-state</literal> rule and the
properly placed <literal>divert natd</literal> rules. All
we have to address is denying all the bad packets and only
allowing in the authorized services. Say there is an
apache server running on the firewall box and we want people
on the public Internet to be able to access the local web
site. The new inbound start request packet matches rule
100 and its IP address is mapped to LAN IP for the firewall
box. The packet is then matched against all the nasty
things that need to be checked for and finally matches
against rule 425. On a match two things occur. The packet
rule is posted to the keep-state dynamic table but this
time any new session requests originating from that source
IP address is limited to 2. This defends against DoS
attacks of service running on the specified port number.
The action is <literal>allow</literal> so the packet is
released to the LAN. The packet generated as a response,
is recognized by the <literal>check-state</literal> as
belonging to anexisting session conversation. It is then
sent to rule 500 for <acronym>NAT</acronym>ing and released
to the outbound interface.</para>
<para>Example Ruleset #1:</para>
<programlisting>#!/bin/sh
cmd="ipfw -q add"
skip="skipto 500"
pif=rl0
ks="keep-state"
good_tcpo="22,25,37,43,53,80,443,110,119"
ipfw -q -f flush
&dollar;cmd 002 allow all from any to any via xl0 # exclude LAN traffic
&dollar;cmd 003 allow all from any to any via lo0 # exclude loopback traffic
&dollar;cmd 100 divert natd ip from any to any in via &dollar;pif
&dollar;cmd 101 check-state
# Authorized outbound packets
&dollar;cmd 120 &dollar;skip udp from any to xx.168.240.2 53 out via &dollar;pif &dollar;ks
&dollar;cmd 121 &dollar;skip udp from any to xx.168.240.5 53 out via &dollar;pif &dollar;ks
&dollar;cmd 125 &dollar;skip tcp from any to any &dollar;good_tcpo out via &dollar;pif setup &dollar;ks
&dollar;cmd 130 &dollar;skip icmp from any to any out via &dollar;pif &dollar;ks
&dollar;cmd 135 &dollar;skip udp from any to any 123 out via &dollar;pif &dollar;ks
# Deny all inbound traffic from non-routable reserved address spaces
&dollar;cmd 300 deny all from 192.168.0.0/16 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 301 deny all from 172.16.0.0/12 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 302 deny all from 10.0.0.0/8 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 303 deny all from 127.0.0.0/8 to any in via &dollar;pif #loopback
&dollar;cmd 304 deny all from 0.0.0.0/8 to any in via &dollar;pif #loopback
&dollar;cmd 305 deny all from 169.254.0.0/16 to any in via &dollar;pif #DHCP auto-config
&dollar;cmd 306 deny all from 192.0.2.0/24 to any in via &dollar;pif #reserved for docs
&dollar;cmd 307 deny all from 204.152.64.0/23 to any in via &dollar;pif #Sun cluster
&dollar;cmd 308 deny all from 224.0.0.0/3 to any in via &dollar;pif #Class D &amp; E multicast
# Authorized inbound packets
&dollar;cmd 400 allow udp from xx.70.207.54 to any 68 in &dollar;ks
&dollar;cmd 420 allow tcp from any to me 80 in via &dollar;pif setup limit src-addr 1
&dollar;cmd 450 deny log ip from any to any
# This is skipto location for outbound stateful rules
&dollar;cmd 500 divert natd ip from any to any out via &dollar;pif
&dollar;cmd 510 allow ip from any to any
######################## end of rules ##################</programlisting>
<para>The following is pretty much the same as above, but uses
a self documenting coding style full of description comments
to help the inexperienced IPFW rule writer to better
understand what the rules are doing.</para>
<para>Example Ruleset #2:</para>
<programlisting>#!/bin/sh
################ Start of IPFW rules file ###############################
# Flush out the list before we begin.
ipfw -q -f flush
# Set rules command prefix
cmd="ipfw -q add"
skip="skipto 800"
pif="rl0" # public interface name of NIC
# facing the public Internet
#################################################################
# No restrictions on Inside LAN Interface for private network
# Change xl0 to your LAN NIC interface name
#################################################################
&dollar;cmd 005 allow all from any to any via xl0
#################################################################
# No restrictions on Loopback Interface
#################################################################
&dollar;cmd 010 allow all from any to any via lo0
#################################################################
# check if packet is inbound and nat address if it is
#################################################################
&dollar;cmd 014 divert natd ip from any to any in via &dollar;pif
#################################################################
# Allow the packet through if it has previous been added to the
# the "dynamic" rules table by a allow keep-state statement.
#################################################################
&dollar;cmd 015 check-state
#################################################################
# Interface facing Public Internet (Outbound Section)
# Check session start requests originating from behind the
# firewall on the private network or from this gateway server
# destined for the public Internet.
#################################################################
# Allow out access to my ISP's Domain name server.
# x.x.x.x must be the IP address of your ISP's DNS
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
&dollar;cmd 020 &dollar;skip tcp from any to x.x.x.x 53 out via &dollar;pif setup keep-state
# Allow out access to my ISP's DHCP server for cable/DSL configurations.
&dollar;cmd 030 &dollar;skip udp from any to x.x.x.x 67 out via &dollar;pif keep-state
# Allow out non-secure standard www function
&dollar;cmd 040 &dollar;skip tcp from any to any 80 out via &dollar;pif setup keep-state
# Allow out secure www function https over TLS SSL
&dollar;cmd 050 &dollar;skip tcp from any to any 443 out via &dollar;pif setup keep-state
# Allow out send &amp; get email function
&dollar;cmd 060 &dollar;skip tcp from any to any 25 out via &dollar;pif setup keep-state
&dollar;cmd 061 &dollar;skip tcp from any to any 110 out via &dollar;pif setup keep-state
# Allow out FreeBSD (make install &amp; CVSUP) functions
# Basically give user root "GOD" privileges.
&dollar;cmd 070 &dollar;skip tcp from me to any out via &dollar;pif setup keep-state uid root
# Allow out ping
&dollar;cmd 080 &dollar;skip icmp from any to any out via &dollar;pif keep-state
# Allow out Time
&dollar;cmd 090 &dollar;skip tcp from any to any 37 out via &dollar;pif setup keep-state
# Allow out nntp news (i.e., news groups)
&dollar;cmd 100 &dollar;skip tcp from any to any 119 out via &dollar;pif setup keep-state
# Allow out secure FTP, Telnet, and SCP
# This function is using SSH (secure shell)
&dollar;cmd 110 &dollar;skip tcp from any to any 22 out via &dollar;pif setup keep-state
# Allow out whois
&dollar;cmd 120 &dollar;skip tcp from any to any 43 out via &dollar;pif setup keep-state
# Allow ntp time server
&dollar;cmd 130 &dollar;skip udp from any to any 123 out via &dollar;pif keep-state
#################################################################
# Interface facing Public Internet (Inbound Section)
# Check packets originating from the public Internet
# destined for this gateway server or the private network.
#################################################################
# Deny all inbound traffic from non-routable reserved address spaces
&dollar;cmd 300 deny all from 192.168.0.0/16 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 301 deny all from 172.16.0.0/12 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 302 deny all from 10.0.0.0/8 to any in via &dollar;pif #RFC 1918 private IP
&dollar;cmd 303 deny all from 127.0.0.0/8 to any in via &dollar;pif #loopback
&dollar;cmd 304 deny all from 0.0.0.0/8 to any in via &dollar;pif #loopback
&dollar;cmd 305 deny all from 169.254.0.0/16 to any in via &dollar;pif #DHCP auto-config
&dollar;cmd 306 deny all from 192.0.2.0/24 to any in via &dollar;pif #reserved for docs
&dollar;cmd 307 deny all from 204.152.64.0/23 to any in via &dollar;pif #Sun cluster
&dollar;cmd 308 deny all from 224.0.0.0/3 to any in via &dollar;pif #Class D &amp; E multicast
# Deny ident
&dollar;cmd 315 deny tcp from any to any 113 in via &dollar;pif
# Deny all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
&dollar;cmd 320 deny tcp from any to any 137 in via &dollar;pif
&dollar;cmd 321 deny tcp from any to any 138 in via &dollar;pif
&dollar;cmd 322 deny tcp from any to any 139 in via &dollar;pif
&dollar;cmd 323 deny tcp from any to any 81 in via &dollar;pif
# Deny any late arriving packets
&dollar;cmd 330 deny all from any to any frag in via &dollar;pif
# Deny ACK packets that did not match the dynamic rule table
&dollar;cmd 332 deny tcp from any to any established in via &dollar;pif
# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP's DHCP server as it is the only
# authorized source to send this packet type.
# Only necessary for cable or DSL configurations.
# This rule is not needed for 'user ppp' type connection to
# the public Internet. This is the same IP address you captured
# and used in the outbound section.
&dollar;cmd 360 allow udp from x.x.x.x to any 68 in via &dollar;pif keep-state
# Allow in standard www function because I have Apache server
&dollar;cmd 370 allow tcp from any to me 80 in via &dollar;pif setup limit src-addr 2
# Allow in secure FTP, Telnet, and SCP from public Internet
&dollar;cmd 380 allow tcp from any to me 22 in via &dollar;pif setup limit src-addr 2
# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID &amp; PW are passed over public
# Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
&dollar;cmd 390 allow tcp from any to me 23 in via &dollar;pif setup limit src-addr 2
# Reject &amp; Log all unauthorized incoming connections from the public Internet
&dollar;cmd 400 deny log all from any to any in via &dollar;pif
# Reject &amp; Log all unauthorized out going connections to the public Internet
&dollar;cmd 450 deny log all from any to any out via &dollar;pif
# This is skipto location for outbound stateful rules
&dollar;cmd 800 divert natd ip from any to any out via &dollar;pif
&dollar;cmd 801 allow ip from any to any
# Everything else is denied by default
# deny and log all packets that fell through to see what they are
&dollar;cmd 999 deny log all from any to any
################ End of IPFW rules file ###############################</programlisting>
</sect3>
</sect2>
</sect1>
</chapter>
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