shorewall_code/docs/ScalabilityAndPerformance.xml

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  <articleinfo>
    <title>Scalability and Performance</title>

    <authorgroup>
      <author>
        <firstname>Tom</firstname>

        <surname>Eastep</surname>
      </author>
    </authorgroup>

    <pubdate><?dbtimestamp format="Y/m/d"?></pubdate>

    <copyright>
      <year>2006</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>The performance of the <emphasis role="bold">shorewall
    start</emphasis> and <emphasis role="bold">shorewall restart</emphasis>
    commands is a frequent topic of questions. This article attempts to
    explain the scalability issues involved and to offer some tips for
    reducing the time required to compile a Shorewall configuration and to
    execute the compiled script.</para>
  </section>

  <section>
    <title>Host Groups</title>

    <para>In this article, we will use the term <firstterm>host
    group</firstterm> to refer to a set of IP addresses accessed through a
    particular interface. In a Shorewall configuration, there is one host
    group for:</para>

    <itemizedlist>
      <listitem>
        <para>Each entry in <filename>/etc/shorewall/interfaces</filename>
        that contains the name of a zone in the first column.</para>
      </listitem>

      <listitem>
        <para>Each entry in <filename>/etc/shorewall/hosts</filename>.</para>
      </listitem>
    </itemizedlist>

    <para>As you can see, each host group is associated with a single
    <firstterm>zone</firstterm>.</para>
  </section>

  <section>
    <title>Scaling by Host Groups</title>

    <para>For each host group, it is possible to attempt connections to every
    other host group; and if the host group has the <emphasis
    role="bold">routeback</emphasis> option, then it is possible for
    connections to be attempted from the host group to itself. So if there are
    <emphasis role="bold">H</emphasis> host groups defined in a Shorewall
    configuration, then the number of unique pairs of (<emphasis>source host
    group</emphasis>, <emphasis>destination host group</emphasis>) is
    <emphasis role="bold">H</emphasis>*<emphasis role="bold">H</emphasis> or
    <emphasis role="bold">H</emphasis><superscript>2</superscript>. In other
    words, the number of combinations is the square of the number of host
    groups and increasing the number of groups from <emphasis
    role="bold">H</emphasis> to <emphasis role="bold">H</emphasis>+1 adds
    <emphasis role="bold">H</emphasis> + <emphasis role="bold">H</emphasis> +
    1 = 2<emphasis role="bold">H</emphasis> + 1 additional
    combinations.</para>
  </section>

  <section>
    <title>Scaling by Zones</title>

    <para>A similar scaling issue applies to Shorewall zones. If there are
    <emphasis role="bold">Z</emphasis> zones, then connections may be
    attempted from a given zone <emphasis
    role="bold">Z</emphasis><subscript>n</subscript> to all of the other zones
    (including to <emphasis role="bold">Z</emphasis><subscript>n</subscript>
    itself). Hence, the number of combinations is the square of the number of
    zones or <emphasis
    role="bold">Z</emphasis><superscript>2</superscript>.</para>
  </section>

  <section>
    <title>Scaling within the Shorewall Code</title>

    <para>Shorewall is written entirely in Bourne Shell. While this allows
    Shorewall to run on a wide range of distributions (included embedded
    ones), the shell programming environment is not ideal for writing the
    compiler portion of Shorewall. As a consequency, the code must repeatedly
    perform sequential searches of lists. If a list has <emphasis
    role="bold">N</emphasis> elements (<emphasis role="bold">N</emphasis> &gt;
    1) and a sequential search is made for each of those elements, then the
    number of comparisons is 1 + 2 + 3 + .... + <emphasis
    role="bold">N</emphasis> = <emphasis role="bold">N</emphasis> * (<emphasis
    role="bold">N</emphasis> - 1 ). So again, we see order <emphasis
    role="bold">N</emphasis><superscript>2</superscript> scaling.</para>
  </section>

  <section>
    <title>Improving Performance</title>

    <para>Achieving good performance boils down to two things:</para>

    <itemizedlist>
      <listitem>
        <para>Use a light-weight shell and fast hardware. Especially in the
        compiler, a light-weight shell such as <command>ash</command> or
        <command>dash</command> can provide considerable improvement over
        <command>bash</command>.</para>
      </listitem>

      <listitem>
        <para>With all of the order <emphasis
        role="bold">N</emphasis><superscript>2</superscript> scaling that is
        implicit in the problem being solved, it is vital to keep <emphasis
        role="bold">N</emphasis> small.</para>
      </listitem>
    </itemizedlist>

    <para>So while it is tempting to create lots of zones through entries in
    <filename>/etc/shorewall/hosts</filename>, such configurations
    <emphasis>always</emphasis> perform badly. In these cases, it is much
    better to have more rules than more zones because the performance scales
    linearly with the number of rules whereas it scales geometrically with the
    number of zones.</para>

    <para>Another tip worth noting has to do with the use of shell variables.
    </para>

    <para>Suppose that the following appears in
    <filename>/etc/shorewall/params</filename>: </para>

    <programlisting>HOSTS=&lt;ip1&gt;,&lt;ip2&gt;,&lt;ip3&gt;,...&lt;ipN&gt;</programlisting>

    <para>and suppose that $HOSTS appears in the SOURCE column of <emphasis
    role="bold">M</emphasis> ACCEPT rules. That would generate a total of
    <emphasis role="bold">N</emphasis> * <emphasis role="bold">M</emphasis>
    iptables ACCEPT rules.</para>

    <para>On the other hand, consider the following:</para>

    <blockquote>
      <para><filename>/etc/shorewall/actions</filename>:</para>

      <programlisting>AcceptHosts</programlisting>

      <para><filename>/etc/shorewall/action.AcceptHosts</filename>:</para>

      <programlisting>#TARGET SOURCE          DEST            PROTO   DEST    SOURCE     ORIGINAL     RATE
#                                               PORT    PORT(S)    DEST         LIMIT
ACCEPT  $HOSTS </programlisting>
    </blockquote>

    <para>If the <emphasis role="bold">M</emphasis> ACCEPT rules are now
    replaced with <emphasis role="bold">M</emphasis> AcceptHosts rules, the
    total number of rules will be <emphasis role="bold">N</emphasis> +
    <emphasis role="bold">M</emphasis>.</para>
  </section>
</article>