shorewall_code/docs/Helpers.xml

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  <articleinfo>
    <title>Netfilter Helpers</title>

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

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

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

    <copyright>
      <year>2012</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>Helpers - Introduction</title>

    <para>There are a number of applications that create connections
    dynamically between a client and server. These connections use temporary
    TCP or UDP ports, so static configuration of firewall rules to allow those
    connections would require a very lax firewall configuration. To deal with
    these problem applications, Netfilter supports the concept of a
    <firstterm>helper</firstterm>. Each helper monitors traffic to/from the
    default primary port used by the application and opens the firewall to
    accept temporary connections created by the primary session.</para>

    <para>There are helpers for the following applications; default ports
    monitored by each helper are listed in parentheses:</para>

    <itemizedlist>
      <listitem>
        <para>Amanda (UDP 10080)</para>
      </listitem>

      <listitem>
        <para>FTP (TCP 21)</para>
      </listitem>

      <listitem>
        <para>H323 (UDP 1719, TCP 1720)</para>
      </listitem>

      <listitem>
        <para>IRC (TCP 6667)</para>
      </listitem>

      <listitem>
        <para>Netbios-NS (UDP 137)</para>
      </listitem>

      <listitem>
        <para>PPTP (TCP 1729)</para>
      </listitem>

      <listitem>
        <para>SANE (TCP 6566)</para>
      </listitem>

      <listitem>
        <para>SIP (UDP 5060)</para>
      </listitem>

      <listitem>
        <para>SNMP (UDP 161)</para>
      </listitem>

      <listitem>
        <para>TFTP (UDP 69)</para>
      </listitem>
    </itemizedlist>

    <section>
      <title>Helper Module Loading</title>

      <para>In a modular kernel, each helper is typically packaged as two
      kernel modules. One module handles connection tracking where NAT isn't
      involved and the other module handles NAT. For example, the FTP helper
      consists of these two modules (kernels 2.6.20 and later):</para>

      <itemizedlist>
        <listitem>
          <para>nf_conntrack_ftp</para>
        </listitem>

        <listitem>
          <para>nf_nat_ftp</para>
        </listitem>
      </itemizedlist>

      <para>Note that the naming convention is
      nf_conntrack_<replaceable>application</replaceable> and
      nf_nat_<replaceable>application</replaceable>; more about that
      below.</para>

      <para>The modules are not auto-loaded and must be loaded explicitly
      using the <command>modprob</command> or <command>insmod</command>
      utilities.</para>

      <para>Many of the modules allow parameters to be specified when the
      module is loaded. Among the common parameters is the ports parameter
      that lists one or more ports that the module is to monitor. This allows
      running the application on a non-standard port.</para>
    </section>

    <section>
      <title>Iptables and Helpers</title>

      <para>Iptables supports two ways of interacting with modules:</para>

      <variablelist>
        <varlistentry>
          <term>Helper Match</term>

          <listitem>
            <para>This match (-m helper --helper
            <replaceable>name</replaceable>) allows selection of packets from
            connections monitored or created by the named helper.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>CT Target</term>

          <listitem>
            <para>This target (-j CT --helper <replaceable>name</replaceable>
            ...) , introduced in the 3.4 kernels, allows for explicit
            association of a helper with a connection.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>It is important to note that the name used in iptables is not
      always the same as the name in the kernel module. Names used in iptables
      are shown in the following table:</para>

      <informaltable>
        <tgroup cols="2">
          <tbody>
            <row>
              <entry>Name of kernel module</entry>

              <entry>Name recognized by iptables</entry>
            </row>

            <row>
              <entry>nf_conntrack_amanda</entry>

              <entry>amanda</entry>
            </row>

            <row>
              <entry>nf_conntrack_ftp</entry>

              <entry>ftp</entry>
            </row>

            <row>
              <entry>nf_conntrack_h323</entry>

              <entry><emphasis role="bold">RAS (udp 1719), Q.931 (tcp
              1720)</emphasis></entry>
            </row>

            <row>
              <entry>nf_conntrack_irc</entry>

              <entry>irc</entry>
            </row>

            <row>
              <entry>nf_conntrack_netbios_ns</entry>

              <entry><emphasis role="bold">netbios-ns</emphasis></entry>
            </row>

            <row>
              <entry>nf_conntrack_pptp</entry>

              <entry>pptp</entry>
            </row>

            <row>
              <entry>nf_conntrack_sane</entry>

              <entry>sane</entry>
            </row>

            <row>
              <entry>nf_conntrack_sip</entry>

              <entry>sip</entry>
            </row>

            <row>
              <entry>nf_conntrack_snmp</entry>

              <entry>snmp</entry>
            </row>

            <row>
              <entry>nf_conntrack_tftp</entry>

              <entry>tftp</entry>
            </row>
          </tbody>
        </tgroup>
      </informaltable>

      <para>Netfilter helpers present an opportunity for attackers to attempt
      to breach your firewall by IP address spoofing; See <ulink
      url="https://home.regit.org/netfilter-en/secure-use-of-helpers/">https://home.regit.org/netfilter-en/secure-use-of-helpers/</ulink>
      for a description of the Netfilter facilities available to meet these
      attacks.</para>
    </section>
  </section>

  <section>
    <title>Shorewall Support for Helpers</title>

    <para>Shorewall includes support for helpers is several areas. These areas
    are covered in the sections below.</para>

    <section>
      <title>Module Loading</title>

      <para>Shorewall includes support for loading the helper modules as part
      of its support for loading kernel modules in general. There are several
      options in shorewall.conf (5) that deal with kernel module
      loading:</para>

      <variablelist>
        <varlistentry>
          <term>MODULESDIR</term>

          <listitem>
            <para>This option specifies a comma-separated list of directories
            where Shorewall will look for kernel modules to load.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>MODULE_SUFFIX</term>

          <listitem>
            <para>Lists the possible suffixes for module names.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>LOAD_HELPERS_ONLY</term>

          <listitem>
            <para>Controls whether Shorewall should load only the helpers and
            leave the other modules to the auto-loader. This option
            dramatically reduces the time to process a <command>shorewall
            start</command> or <command>shorewall restart</command>
            command.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>HELPERS</term>

          <listitem>
            <para>This option was added in Shorewall 4.5.7 and lists the
            modules to be enabled for association with connections. This
            option is fully functional only on systems running kernel 3.5 or
            later.</para>

            <para>The module names allowed in this list are <emphasis
            role="bold">amanda</emphasis>, <emphasis
            role="bold">ftp</emphasis>, <emphasis role="bold">h323</emphasis>,
            <emphasis role="bold">irc</emphasis>, <emphasis
            role="bold">netbios-ns</emphasis>, <emphasis
            role="bold">pptp</emphasis>, <emphasis
            role="bold">sane</emphasis>, <emphasis role="bold">sip</emphasis>,
            <emphasis role="bold">snmp</emphasis> and <emphasis
            role="bold">tftp</emphasis>.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>The helper modules to be loaded are listed in the file
      <filename>/usr/share/shorewall/helpers</filename>. If you wish to
      customize that file to load only a subset of the helpers or to specify
      module parameters, then copy the file to <filename>/etc/shorewall/
      </filename>and modify the copy. That way, your changes won't be
      overwritten the next time that Shorewall is updated on your
      system.</para>

      <para>On systems running a a kernel earlier than 3.5, not all of the
      helpers can be totally disabled. The following modules can be disabled
      by using the parameter <emphasis role="bold">ports=0</emphasis> in
      /etc/shorewall/helpers:</para>

      <itemizedlist>
        <listitem>
          <para>ftp</para>
        </listitem>

        <listitem>
          <para>irc</para>
        </listitem>

        <listitem>
          <para>sane</para>
        </listitem>

        <listitem>
          <para>sip</para>
        </listitem>

        <listitem>
          <para>tftp</para>
        </listitem>
      </itemizedlist>
    </section>

    <section>
      <title>Iptables</title>

      <para>The iptables helper match is supported by Shorewall in the form of
      the HELPER column in <ulink
      url="manpages/shorewall-tcrules.html">shorewall-tcrules</ulink>
      (5).</para>

      <para>The CT target is supported directly in <ulink
      url="manpages/shorewall-conntrack.html">shorewall-conntrack</ulink>
      (5).</para>

      <para>In these files, Shorewall supports the same module names as
      iptables; see the table above.</para>
    </section>
  </section>
</article>