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<article id="ECN">
  <!--$Id$-->

  <articleinfo>
    <title>ECN</title>

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

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

    <pubdate>2005-01-17</pubdate>

    <copyright>
      <year>2001</year>

      <year>2002</year>

      <year>2003</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>

  <warning>
    <para>2006-01-17. The ECN Netfilter target in recent 2.6 Linux Kernels is
    broken. Symptoms are that you will be unable to establish a TCP connection
    to hosts defined in the /etc/shorewall/ecn file.</para>
  </warning>

  <section>
    <title>Explicit Congestion Notification (ECN)</title>

    <para>Explicit Congestion Notification (ECN) is described in RFC 3168 and
    is a proposed internet standard. Unfortunately, not all sites support ECN
    and when a TCP connection offering ECN is sent to sites that don't support
    it, the result is often that the connection request is ignored.</para>

    <para>To allow ECN to be used, Shorewall allows you to enable ECN on your
    Linux systems then disable it in your firewall when the destination
    matches a list that you create (the /etc/shorewall/ecn file).</para>

    <para>You enable ECN by</para>

    <programlisting>echo 1 &gt; /proc/sys/net/ipv4/tcp_ecn</programlisting>

    <para>You must arrange for that command to be executed at system boot.
    Most distributions have a method for doing that -- on RedHat, you make an
    entry in /etc/sysctl.conf.</para>

    <programlisting>net.ipv4.tcp_ecn = 1</programlisting>

    <para>Entries in /etc/shorewall/ecn have two columns as follows:</para>

    <variablelist>
      <varlistentry>
        <term>INTERFACE</term>

        <listitem>
          <para>The name of an interface on your system</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>HOST(S)</term>

        <listitem>
          <para>An address (host or subnet) of a system or group of systems
          accessed through the interface in the first column. You may include
          a comma-separated list of such addresses in this column.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <example>
      <title>Your external interface is eth0 and you want to disable ECN for
      tcp connections to 192.0.2.0/24:</title>

      <para><table>
          <title>/etc/shorewall/ecn</title>

          <tgroup cols="2">
            <thead>
              <row>
                <entry align="center">INTERFACE</entry>

                <entry align="center">HOST(S)</entry>
              </row>
            </thead>

            <tbody>
              <row>
                <entry>eth0</entry>

                <entry>192.0.2.0/24</entry>
              </row>
            </tbody>
          </tgroup>
        </table></para>
    </example>
  </section>

  <lot></lot>
</article>