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shorewall_code/docs/PacketHandling.xml

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article>
<!--$Id$-->
<articleinfo>
<title>Packet Handling</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2004</year>
<year>2005</year>
<year>2009</year>
<holder>Thomas M. Eastep</holder>
</copyright>
<legalnotice>
<para>Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
License</ulink></quote>.</para>
</legalnotice>
</articleinfo>
<section id="Intro">
<title>Introduction</title>
<para>This article will try to help you understand how packets pass
through a firewall configured by Shorewall. You may find it useful to have
a copy of the <ulink url="NetfilterOverview.html">Netfilter
Overview</ulink> handy to refer to.</para>
<para>The discussion that follows assumes that you are running a current
kernel (2.6.20 or later) with the <ulink url="kernel.htm">recommended
options </ulink>included. Otherwise processing may be somewhat different
from described below depending on the features supported by your
kernel.</para>
<para>Where a packet is covered by steps in more than one of the following
sections, processing occurs in the order in which the sections
appear.</para>
</section>
<section id="Incoming">
<title>Packets Entering the Firewall from Outside</title>
<para>Certain processing occurs on packets entering the firewall from the
outside that don't occur for packets that originate on the firewall
itself.</para>
<itemizedlist>
<listitem>
<para>The TOS field in the packet is conditionally altered based on
the contents of your <filename>/etc/shorewall/tos</filename> file.
This occurs in the <emphasis role="bold">pretos</emphasis> chain of
the <emphasis>mangle</emphasis> table.</para>
</listitem>
<listitem>
<para>Packets are marked based on the contents of your
<filename>/etc/shorewall/tcrules</filename> file and the setting of
MARK_IN_FORWARD_CHAIN in
<filename>/etc/shorewall/shorewall.conf</filename>. This occurs in the
<emphasis role="bold">tcpre</emphasis> chain of the
<emphasis>mangle</emphasis> table.</para>
</listitem>
<listitem>
<para>The destination IP address and/or port number are rewritten
according to DNAT[-] and REDIRECT[-] rules in
<filename>/etc/shorewall/rules</filename>. For new connection
requests, this occurs in a chain in the <emphasis>nat</emphasis> table
called <emphasis role="bold"><emphasis>zone</emphasis>_dnat</emphasis>
where <emphasis>zone</emphasis> is the zone where the request
originated. For packets that are part of an already established
connection, the destination rewriting takes place without any
involvement of a Netfilter rule.</para>
</listitem>
<listitem>
<para>If the destination was not rewritten in the previous step then
it may be rewritten based on entries in /etc/shorewall/nat. For new
connection requests, this occurs in a <emphasis>nat</emphasis> table
chain called <emphasis
role="bold"><emphasis>interface</emphasis>_in</emphasis> where
<emphasis>interface</emphasis> is the interface on which the packet
entered the firewall. For packets that are part of an already
established connection, the destination rewriting takes place without
any involvement of a Netfilter rule.</para>
</listitem>
<listitem>
<para>The packet passes through the accounting rules defined in
<filename>/etc/shorewall/accounting</filename>.</para>
</listitem>
<listitem>
<para>If FASTACCEPT=Yes in <filename>shorewall.conf</filename> and the
packet is part of or related to an existing connection, it is
accepted.</para>
</listitem>
<listitem>
<para>The packet is processed according to your <ulink
url="blacklisting_support.htm">Blacklisting configuration</ulink>
(dynamic blacklist first). If BLACKLISTNEWONLY=Yes in
<filename>/etc/shorewall/shorewall.conf</filename> then only new
connection requests are processed. Processing occurs in the dynamic
and blacklst</para>
</listitem>
<listitem>
<para>If the interface on which the packet entered the firewall has
the <emphasis>nosmurfs</emphasis> option specified in
<filename>/etc/shorewall/interfaces</filename>, then if the packet is
a new connection request is checked for being a smurf in the
<emphasis>filter</emphasis> table's <emphasis
role="bold">smurfs</emphasis> chain.</para>
</listitem>
<listitem>
<para>If:</para>
<itemizedlist>
<listitem>
<para>the packet will be processed by the firewall itself</para>
</listitem>
<listitem>
<para>the interface on which the packet arrived has the
<emphasis>dhcp</emphasis> option in
<filename>/etc/shorewall/interfaces</filename>.</para>
</listitem>
<listitem>
<para>packet's protocol is UDP with destination port 67 or
68.</para>
</listitem>
</itemizedlist>
<para>then the packet is ACCEPTed in the <emphasis>filter</emphasis>
table's <emphasis
role="bold"><emphasis>interface</emphasis>_in</emphasis> chain (for
example, eth0_in). Note that if the interface is its associated zones
only interface, then the <emphasis
role="bold"><emphasis>interface</emphasis>_in</emphasis> chain is
optimized away and its rules are transferred to another chain.</para>
</listitem>
<listitem>
<para>If the interface on which the packet entered the firewall has
the <emphasis>tcpflags</emphasis> option specified in
<filename>/etc/shorewall/interfaces</filename> and the packet's
protocol is TCP then the TCP flags are checked by the <emphasis
role="bold">tcpflags</emphasis> chain (<emphasis>filter</emphasis>
table).</para>
</listitem>
</itemizedlist>
</section>
<section id="All">
<title>All Packets</title>
<para>Regardless of whether the packet originated on the firewall or came
from outside, certain processing steps are common.</para>
<itemizedlist>
<listitem>
<para>Packets are marked based on the contents of your
<filename>/etc/shorewall/tcrules</filename> file and the setting of
MARK_IN_FORWARD_CHAIN in
<filename>/etc/shorewall/shorewall.conf</filename>. This occurs in the
<emphasis role="bold">tcfor</emphasis> chain of the
<emphasis>mangle</emphasis> table.</para>
<para>The remaining processing in this list occurs in the
<emphasis>filter</emphasis> table.</para>
</listitem>
<listitem>
<para>If either the host sending the packet or the host to which the
packet is addressed is not in any defined zone then the all-&gt;all
policy is applied to the packet (including logging). This can occur in
the INPUT, FORWARD or OUTPUT chains.</para>
</listitem>
<listitem>
<para>If the packet is part of an established connection or is part of
a related connection then no further processing takes place in the
filter table
(<emphasis><emphasis>zone1</emphasis>2<emphasis>zone2</emphasis></emphasis>
chain where <emphasis>zone1</emphasis> is the source zone and
<emphasis>zone2</emphasis> is the destination zone).</para>
</listitem>
<listitem>
<para>The packet is processed according to your
<filename>/etc/shorewall/rules</filename> file. This happens in chains
named <emphasis><emphasis
role="bold"><emphasis>zone1</emphasis>2<emphasis>zone2</emphasis></emphasis></emphasis>
chain where <emphasis>zone1</emphasis> is the source zone and
<emphasis>zone2</emphasis> is the destination zone. Note that in the
presence of <ulink url="manpages/shorewall-nested.html">nested or
overlapping zones</ulink> and CONTINUE policies, a packet may go
through more than one of these chains.</para>
</listitem>
<listitem>
<para>Note: If the packet gets to this step, it did not match any
rule.</para>
<para>If the applicable policy has a <ulink url="Actions.html">common
action</ulink> then that action is applied (chain has the same name as
the action).</para>
</listitem>
<listitem>
<para>If the applicable policy has logging specified, the packet is
logged.</para>
</listitem>
<listitem>
<para>The policy is applied (the packet is accepted, dropped or
rejected).</para>
</listitem>
</itemizedlist>
</section>
<section id="Local">
<title>Packets Originating on the Firewall</title>
<para>Packets that originate on the firewall itself undergo additional
processing.</para>
<itemizedlist>
<listitem>
<para>The TOS field in the packet is conditionally altered based on
the contents of your <filename>/etc/shorewall/tos</filename> file.
This occurs in the <emphasis role="bold">outtos</emphasis> chain of
the <emphasis>mangle</emphasis> table.</para>
</listitem>
<listitem>
<para>Packets are marked based on the contents of your
<filename>/etc/shorewall/tcrules</filename> file. This occurs in the
<emphasis role="bold">tcout</emphasis> chain of the
<emphasis>mangle</emphasis> table.</para>
</listitem>
</itemizedlist>
</section>
<section id="Egress">
<title>Packets Leaving the Firewall</title>
<para>Packets being sent to another host undergo additional
processing.</para>
<note>
<para>The source IP address only gets rewritten by the first matching
rule below.</para>
</note>
<itemizedlist>
<listitem>
<para>The source IP address may be rewritten according to DNAT rules
that specify SNAT. If this is a new connection request, then the
rewriting occurs in a <emphasis>nat</emphasis> table chain called
<emphasis role="bold"><emphasis>zone</emphasis>_snat</emphasis> where
<emphasis>zone</emphasis> is the destination zone. For packets that
are part of an already established connection, the destination
rewriting takes place without any involvement of a Netfilter
rule.</para>
</listitem>
<listitem>
<para>If FASTACCEPT=Yes in <filename>shorewall.conf</filename> and the
packet is part of or related to an existing connection, it is
accepted.</para>
</listitem>
<listitem>
<para>The source IP address may be rewritten according to an entry in
the <filename>/etc/shorewall/nat</filename> file. If this is a new
connection request, then the rewriting occurs in a
<emphasis>nat</emphasis> table chain called <emphasis
role="bold"><emphasis>interface</emphasis>_snat</emphasis> where
<emphasis>interface</emphasis> is the interface on which the packet
will be sent. For packets that are part of an already established
connection, the destination rewriting takes place without any
involvement of a Netfilter rule.</para>
</listitem>
<listitem>
<para>The source IP address may be rewritten according to an entry in
the <filename>/etc/shorewall/masq</filename> file. If this is a new
connection request, then the rewriting occurs in a
<emphasis>nat</emphasis> table chain called <emphasis
role="bold"><emphasis>interface</emphasis>_masq</emphasis> where
<emphasis>interface</emphasis> is the interface on which the packet
will be sent. For packets that are part of an already established
connection, the destination rewriting takes place without any
involvement of a Netfilter rule.</para>
</listitem>
</itemizedlist>
</section>
</article>