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

  <articleinfo>
    <title>Shorewall and Aliased Interfaces</title>

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

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

    <pubdate>2003-11-13</pubdate>

    <copyright>
      <year>2001</year>

      <year>2002</year>

      <year>2003</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>Background</title>

    <para>The traditional net-tools contain a program called
    <emphasis>ifconfig</emphasis> which is used to configure network devices.
    ifconfig introduced the concept of <emphasis>aliased</emphasis> or
    <emphasis>virtual</emphasis> interfaces. These virtual interfaces have
    names of the form <emphasis>interface:integer</emphasis> (e.g., eth0:0)
    and ifconfig treats them more or less like real interfaces.</para>

    <example>
      <title>ifconfig</title>

      <programlisting>[root@gateway root]# ifconfig eth0:0
eth0:0    Link encap:Ethernet  HWaddr 02:00:08:3:FA:55
          inet addr:206.124.146.178  Bcast:206.124.146.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          Interrupt:11 Base address:0x2000
[root@gateway root]# </programlisting>
    </example>

    <para>The ifconfig utility is being gradually phased out in favor of the
    ip utility which is part of the <emphasis>iproute</emphasis> package. The
    ip utility does not use the concept of aliases or virtual interfaces but
    rather treats additional addresses on an interface as objects in their own
    right. The ip utility does provide for interaction with ifconfig in that
    it allows addresses to be <emphasis>labeled</emphasis> where these labels
    take the form of ipconfig virtual interfaces.</para>

    <example>
      <title>ip</title>

      <programlisting>[root@gateway root]# ip addr show dev eth0
2: eth0: &#60;BROADCAST,MULTICAST,UP&#62; mtu 1500 qdisc htb qlen 100
    link/ether 02:00:08:e3:fa:55 brd ff:ff:ff:ff:ff:ff
    inet 206.124.146.176/24 brd 206.124.146.255 scope global eth0
    inet 206.124.146.178/24 brd 206.124.146.255 scope global secondary eth0:0
[root@gateway root]# </programlisting>

      <para><note><para>One <emphasis role="bold">cannot</emphasis> type
      <quote>ip addr show dev eth0:0</quote> because <quote>eth0:0</quote> is
      a label for a particular address rather than a device name.</para><programlisting>[root@gateway root]# ip addr show dev eth0:0
Device &#34;eth0:0&#34; does not exist.
[root@gateway root]#</programlisting></note></para>
    </example>

    <para>The iptables program doesn&#39;t support virtual interfaces in
    either it&#39;s <quote>-i</quote> or <quote>-o</quote> command options; as
    a consequence, Shorewall does not allow them to be used in the
    /etc/shorewall/interfaces file or anywhere else except as described in the
    discussion below.</para>
  </section>

  <section>
    <title>Adding Addresses to Interfaces</title>

    <para>Most distributions have a facility for adding additional addresses
    to interfaces. If you have already used your distribution&#39;s capability
    to add your required addresses, you can skip this section.</para>

    <para>Shorewall provides facilities for automatically adding addresses to
    interfaces as described in the following section. It is also easy to add
    them yourself using the <emphasis role="bold">ip</emphasis> utility. The
    above alias was added using:</para>

    <programlisting>ip addr add 206.124.146.178/24 brd 206.124.146.255 dev eth0 label eth0:0</programlisting>

    <para>You probably want to arrange to add these addresses when the device
    is started rather than placing commands like the above in one of the
    Shorewall extension scripts. For example, on RedHat systems, you can place
    the commands in /sbin/ifup-local:</para>

    <programlisting>#!/bin/sh

case $1 in
    eth0)
        /sbin/ip addr add 206.124.146.177 dev eth0 label eth0:0
        ;;
esac</programlisting>

    <para>RedHat systems also allow adding such aliases from the network
    administration GUI (which only works well if you have a graphical
    environment on your firewall).</para>
  </section>

  <section>
    <title>So how do I handle more than one address on an interface?</title>

    <para>The answer depends on what you are trying to do with the interfaces.
    In the sub-sections that follow, we&#39;ll take a look at common
    scenarios.</para>

    <section>
      <title>Separate Rules</title>

      <para>If you need to make a rule for traffic to/from the firewall itself
      that only applies to a particular IP address, simply qualify the $FW
      zone with the IP address.</para>

      <example>
        <title>allow SSH from net to eth0:0 above</title>

        <para><table><title>/etc/shorewall/rules</title><tgroup cols="7"><thead><row><entry
        align="center">ACTION</entry><entry align="center">SOURCE</entry><entry
        align="center">DESTINATION</entry><entry align="center">PROTOCOL</entry><entry
        align="center">PORT(S)</entry><entry align="center">SOURCE PORT(S)</entry><entry
        align="center">ORIGINAL DESTINATION</entry></row></thead><tbody><row><entry>ACCEPT</entry><entry>net</entry><entry>$FW:206.124.146.178</entry><entry>tcp</entry><entry>22</entry><entry></entry><entry></entry></row></tbody></tgroup></table></para>
      </example>
    </section>

    <section>
      <title>DNAT</title>

      <para>Suppose that I had set up eth0:0 as above and I wanted to port
      forward from that virtual interface to a web server running in my local
      zone at 192.168.1.3. That is accomplised by a single rule in the
      /etc/shorewall/rules file:</para>

      <table>
        <title>/etc/shorewall/rules</title>

        <tgroup cols="7">
          <thead>
            <row>
              <entry align="center">ACTION</entry>

              <entry align="center">SOURCE</entry>

              <entry align="center">DESTINATION</entry>

              <entry align="center">PROTOCOL</entry>

              <entry align="center">PORT(S)</entry>

              <entry align="center">SOURCE PORT(S)</entry>

              <entry align="center">ORIGINAL DESTINATION</entry>
            </row>
          </thead>

          <tbody>
            <row>
              <entry>DNAT</entry>

              <entry>net</entry>

              <entry>loc:192.168.1.3</entry>

              <entry>tcp</entry>

              <entry>80</entry>

              <entry>-</entry>

              <entry>206.124.146.178</entry>
            </row>
          </tbody>
        </tgroup>
      </table>
    </section>

    <section>
      <title>SNAT</title>

      <para>If you wanted to use eth0:0 as the IP address for outbound
      connections from your local zone (eth1), then in /etc/shorewall/masq:</para>

      <table>
        <title>/etc/shorewall/masq</title>

        <tgroup cols="3">
          <thead>
            <row>
              <entry align="center">INTERFACE</entry>

              <entry align="center">SUBNET</entry>

              <entry align="center">ADDRESS</entry>
            </row>
          </thead>

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

              <entry>eth1</entry>

              <entry>206.124.146.178</entry>
            </row>
          </tbody>
        </tgroup>
      </table>

      <para>Shorewall can create the alias (additional address) for you if you
      set ADD_SNAT_ALIASES=Yes in /etc/shorewall/shorewall.conf. Beginning
      with Shorewall 1.3.14, Shorewall can actually create the <quote>label</quote>
      (virtual interface) so that you can see the created address using
      ifconfig. In addition to setting ADD_SNAT_ALIASES=Yes, you specify the
      virtual interface name in the INTERFACE column as follows:</para>

      <table>
        <title>/etc/shorewall/masq</title>

        <tgroup cols="3">
          <thead>
            <row>
              <entry align="center">INTERFACE</entry>

              <entry align="center">SUBNET</entry>

              <entry align="center">ADDRESS</entry>
            </row>
          </thead>

          <tbody>
            <row>
              <entry>eth0:0</entry>

              <entry>eth1</entry>

              <entry>206.124.146.178</entry>
            </row>
          </tbody>
        </tgroup>
      </table>

      <para>Shorewall can also set up SNAT to round-robin over a range of IP
      addresses. Do do that, you specify a range of IP addresses in the
      ADDRESS column. If you specify a label in the INTERFACE column,
      Shorewall will use that label for the first address of the range and
      will increment the label by one for each subsequent label.</para>

      <table>
        <title>/etc/shorewall/masq</title>

        <tgroup cols="3">
          <thead>
            <row>
              <entry align="center">INTERFACE</entry>

              <entry align="center">SUBNET</entry>

              <entry align="center">ADDRESS</entry>
            </row>
          </thead>

          <tbody>
            <row>
              <entry>eth0:0</entry>

              <entry>eth1</entry>

              <entry>206.124.146.178-206.124.146.180</entry>
            </row>
          </tbody>
        </tgroup>
      </table>

      <para>The above would create three IP addresses:</para>

      <programlisting>eth0:0 = 206.124.146.178
eth0:1 = 206.124.146.179
eth0:2 = 206.124.146.180</programlisting>
    </section>

    <section>
      <title>One-to-one NAT</title>

      <para>If you wanted to use one-to-one NAT to link eth0:0 with local
      address 192.168.1.3, you would have the following in /etc/shorewall/nat:<table><title>/etc/shorewall/nat</title><tgroup
      cols="5"><thead><row><entry align="center">EXTERNAL</entry><entry
      align="center">INTERFACE</entry><entry align="center">INTERNAL</entry><entry
      align="center">ALL INTERFACES</entry><entry align="center">LOCAL</entry></row></thead><tbody><row><entry>206.124.146.178</entry><entry>eth0</entry><entry>192.168.1.3</entry><entry>no</entry><entry>no</entry></row></tbody></tgroup></table></para>

      <para>Shorewall can create the alias (additional address) for you if you
      set ADD_IP_ALIASES=Yes in /etc/shorewall/shorewall.conf. Beginning with
      Shorewall 1.3.14, Shorewall can actually create the <quote>label</quote>
      (virtual interface) so that you can see the created address using
      ifconfig. In addition to setting ADD_IP_ALIASES=Yes, you specify the
      virtual interface name in the INTERFACE column as follows:</para>

      <table>
        <title>/etc/shorewall/nat</title>

        <tgroup cols="5">
          <thead>
            <row>
              <entry align="center">EXTERNAL</entry>

              <entry align="center">INTERFACE</entry>

              <entry align="center">INTERNAL</entry>

              <entry align="center">ALL INTERFACES</entry>

              <entry align="center">LOCAL</entry>
            </row>
          </thead>

          <tbody>
            <row>
              <entry>206.124.146.178</entry>

              <entry>eth0:0</entry>

              <entry>192.168.1.3</entry>

              <entry>no</entry>

              <entry>no</entry>
            </row>
          </tbody>
        </tgroup>
      </table>

      <para>In either case, to create rules that pertain only to this NAT
      pair, you simply qualify the local zone with the internal IP address.</para>

      <example>
        <title>You want to allow SSH from the net to 206.124.146.178 a.k.a.
        192.168.1.3.</title>

        <para><table><title>/etc/shorewall/rules</title><tgroup cols="7"><thead><row><entry
        align="center">ACTION</entry><entry align="center">SOURCE</entry><entry
        align="center">DESTINATION</entry><entry align="center">PROTOCOL</entry><entry
        align="center">PORT(S)</entry><entry align="center">SOURCE PORT(S)</entry><entry
        align="center">ORIGINAL DESTINATION</entry></row></thead><tbody><row><entry>ACCEPT</entry><entry>net</entry><entry>loc:192.168.1.3</entry><entry>tcp</entry><entry>22</entry><entry></entry><entry></entry></row></tbody></tgroup></table></para>
      </example>
    </section>

    <section>
      <title>MULTIPLE SUBNETS</title>

      <para>Sometimes multiple IP addresses are used because there are
      multiple subnetworks configured on a LAN segment. This technique does
      not provide for any security between the subnetworks if the users of the
      systems have administrative privileges because in that case, the users
      can simply manipulate their system&#39;s routing table to bypass your
      firewall/router. Nevertheless, there are cases where you simply want to
      consider the LAN segment itself as a zone and allow your firewall/router
      to route between the two subnetworks.</para>

      <example>
        <title>Local interface eth1 interfaces to 192.168.1.0/24 and
        192.168.20.0/24. The primary IP address of eth1 is 192.168.1.254 and
        eth1:0 is 192.168.20.254. You want to simply route all requests
        between the two subnetworks.</title>

        <variablelist>
          <varlistentry>
            <term>If you are running Shorewall 1.4.1 or Later</term>

            <listitem>
              <para>In /etc/shorewall/interfaces:</para>

              <table>
                <title>/etc/shorewall/interfaces</title>

                <tgroup cols="4">
                  <thead>
                    <row>
                      <entry align="center">ZONE</entry>

                      <entry align="center">INTERFACE</entry>

                      <entry align="center">BROADCAST</entry>

                      <entry align="center">OPTIONS</entry>
                    </row>
                  </thead>

                  <tbody>
                    <row>
                      <entry>-</entry>

                      <entry>eth1</entry>

                      <entry>192.168.1.255,192.168.20.255</entry>

                      <entry></entry>
                    </row>
                  </tbody>
                </tgroup>
              </table>

              <para>In /etc/shorewall/hosts:</para>

              <table>
                <title>/etc/shorewall/hosts</title>

                <tgroup cols="3">
                  <thead>
                    <row>
                      <entry align="center">ZONE</entry>

                      <entry align="center">HOSTS</entry>

                      <entry align="center">OPTIONS</entry>
                    </row>
                  </thead>

                  <tbody>
                    <row>
                      <entry>loc</entry>

                      <entry>eth1:192.168.1.0/24</entry>

                      <entry></entry>
                    </row>

                    <row>
                      <entry>loc</entry>

                      <entry>eth1:192.168.20.0/24</entry>

                      <entry></entry>
                    </row>
                  </tbody>
                </tgroup>
              </table>

              <note>
                <para>You do NOT need any entry in /etc/shorewall/policy as
                Shorewall 1.4.1 and later releases default to allowing
                intra-zone traffic.</para>
              </note>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>If you are running Shorewall 1.4.0 or earlier</term>

            <listitem>
              <para>In /etc/shorewall/interfaces:</para>

              <table>
                <title>/etc/shorewall/interfaces</title>

                <tgroup cols="4">
                  <thead>
                    <row>
                      <entry align="center">ZONE</entry>

                      <entry align="center">INTERFACE</entry>

                      <entry align="center">BROADCAST</entry>

                      <entry align="center">OPTIONS</entry>
                    </row>
                  </thead>

                  <tbody>
                    <row>
                      <entry>-</entry>

                      <entry>eth1</entry>

                      <entry>192.168.1.255,192.168.20.255</entry>

                      <entry><xref linkend="multiple_subnets-ex1-n1" /></entry>
                    </row>
                  </tbody>
                </tgroup>
              </table>

              <note id="multiple_subnets-ex1-n1">
                <para>If you are running Shorewall 1.3.10 or earlier then you
                must specify the <emphasis role="bold">multi</emphasis>
                option.</para>
              </note>

              <para>In /etc/shorewall/policy:</para>

              <table>
                <title>/etc/shorewall/policy</title>

                <tgroup cols="5">
                  <thead>
                    <row>
                      <entry align="center">SOURCE</entry>

                      <entry align="center">DESTINATION</entry>

                      <entry align="center">POLICY</entry>

                      <entry align="center">LOG LEVEL</entry>

                      <entry align="center">BURST:LIMIT</entry>
                    </row>
                  </thead>

                  <tbody>
                    <row>
                      <entry>loc</entry>

                      <entry>loc</entry>

                      <entry>ACCEPT</entry>

                      <entry></entry>

                      <entry></entry>
                    </row>
                  </tbody>
                </tgroup>
              </table>
            </listitem>
          </varlistentry>
        </variablelist>
      </example>

      <example>
        <title>Local interface eth1 interfaces to 192.168.1.0/24 and
        192.168.20.0/24. The primary IP address of eth1 is 192.168.1.254 and
        eth1:0 is 192.168.20.254. You want to make these subnetworks into
        separate zones and control the access between them (the users of the
        systems do not have administrative privileges).</title>

        <para>In /etc/shorewall/zones:<table><title>etc/shorewall/zones</title><tgroup
        cols="3"><thead><row><entry align="center">ZONE</entry><entry
        align="center">DISPLAY</entry><entry align="center">DESCRIPTION</entry></row></thead><tbody><row><entry>loc</entry><entry>Local</entry><entry>Local
        Zone 1</entry></row><row><entry>loc2</entry><entry>Local2</entry><entry>Local
        Zone 2</entry></row></tbody></tgroup></table></para>

        <para>In /etc/shorewall/interfaces:<table><title>/etc/shorewall/interfaces</title><tgroup
        cols="4"><thead><row><entry align="center">ZONE</entry><entry
        align="center">INTERFACE</entry><entry align="center">BROADCAST</entry><entry
        align="center">OPTIONS</entry></row></thead><tbody><row><entry>-</entry><entry>eth1</entry><entry>192.168.1.255,192.168.20.255</entry><entry><xref
        linkend="multiple_subnets-ex2-n1" /></entry></row></tbody></tgroup></table><note
        id="multiple_subnets-ex2-n1"><para>If you are running Shorewall 1.3.10
        or earlier then you must specify the <emphasis role="bold">multi</emphasis>
        option.</para></note></para>

        <para>In /etc/shorewall/hosts:<table><title>/etc/shorewall/hosts</title><tgroup
        cols="3"><thead><row><entry align="center">ZONE</entry><entry
        align="center">HOSTS</entry><entry align="center">OPTIONS</entry></row></thead><tbody><row><entry>loc</entry><entry>eth1:192.168.1.0/24</entry><entry></entry></row><row><entry>loc2</entry><entry>eth1:192.168.20.0/24</entry><entry></entry></row></tbody></tgroup></table></para>

        <para>In /etc/shorewall/rules, simply specify ACCEPT rules for the
        traffic that you want to permit.</para>
      </example>
    </section>
  </section>
</article>