shorewall_code/docs/PacketHandling.xml

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  <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>

      <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 (2.4.2n 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 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 Netfilter doesn'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'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).</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's <emphasis
        role="bold">norfc1918</emphasis> 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>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>