1
0
shorewall_code/Shorewall-docs2/PacketHandling.xml
2005-01-25 23:37:01 +00:00

318 lines
13 KiB
XML
Executable File

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<article>
<!--$Id$-->
<articleinfo>
<title>Packet Handling</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate>2004-07-13</pubdate>
<copyright>
<year>2004</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>
<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 (2.4.2n or 2.6.m) 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>
<title>Packets Entering the Firewall from Outside</title>
<para>Certain processing occurs on packets entering the firewall from the
outside that don&#39;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 Netfilter doesn&#39;t know what to do with the packet
(connection state INVALID) and the protocol is not ICMP then the
packet is silently dropped.</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&#39;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&#39;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&#39;s <emphasis role="bold"><emphasis>interface</emphasis>_in</emphasis>
chain (for example, eth0_in).</para>
</listitem>
<listitem>
<para>If the interface on which the packet entered the firewall has
the <emphasis>norfc1918</emphasis> option specified in
<filename>/etc/shorewall/interfaces</filename>, then the packet is
processed against your rfc1918 file (normally <filename>/usr/share/shorewall/rfc1918</filename>
but that file may be copied to <filename>/etc/shorewall/rfc1918</filename>
and modified). This happens in the <emphasis>filter</emphasis>
table&#39;s <emphasis role="bold">norfc1918</emphasis> chain.</para>
</listitem>
<listitem>
<para>If the interface on which the packet entered the firewall has
the <emphasis>nobogons</emphasis> option specified in
<filename>/etc/shorewall/interfaces</filename>, then the packet is
processed against your bogons file (normally <filename>/usr/share/shorewall/bogons</filename>
but that file may be copied to <filename>/etc/shorewall/bogons</filename>
and modified).</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&#39;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>
<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-&#62;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>If NEWNOTSYN=Yes in /etc/shorewall/shorewall.conf then If the
packet&#39;s protocol is TCP and is not a syn packet then the packet
is processed by the <emphasis role="bold">newnotsyn</emphasis> chain.</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="Documentation.htm#Nested">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>
<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>
<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>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>