shorewall_code/docs/Xen.xml

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  <articleinfo>
    <title>Xen and Shorewall</title>

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

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

    <pubdate>2006-03-10</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>Xen Network Environment</title>

    <para><ulink
    url="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/">Xen</ulink> is a
    <firstterm>paravirtualization</firstterm> tool that allows you to run
    multiple virtual machines on one physical machine. It is available on a
    wide number of platforms and is included in recent
    <trademark>SuSE</trademark> distributions.</para>

    <para>Xen refers to the virtual machines as
    <firstterm>Domains</firstterm>. Domains are numbered with the first domain
    being domain 0, the second domain 1, and so on. Domain 0
    (<firstterm>Dom0</firstterm>) is special because that is the domain
    created when to machine is booted. Additional domains (called
    <firstterm>DomU</firstterm>'s) are created using the <command>xm
    create</command> command from within Domain 0. Additional domains can also
    be created automatically at boot time by using the
    <command>xendomains</command> service.</para>

    <para>Xen virtualizes a network interface named <filename
    class="devicefile">eth0</filename><footnote>
        <para>This assumes the default Xen configuration created by
        <command>xend </command>and assumes that the host system has a single
        ethernet interface named <filename
        class="devicefile">eth0</filename>.</para>
      </footnote> in each domain. In Dom0, Xen also creates a bridge
    (<filename class="devicefile">xenbr0</filename>) and a number of virtual
    interfaces as shown in the following diagram.</para>

    <graphic align="center" fileref="images/Xen1.png" />

    <para>I use the term <firstterm>Extended Dom0</firstterm> to distinguish
    the bridge and virtual interfaces from Dom0 itself. That distinction is
    important when we try to apply Shorewall in this environment.</para>

    <para>The bridge has a number of ports:</para>

    <itemizedlist>
      <listitem>
        <para>peth0 — This is the port that connects to the physical network
        interface in your system.</para>
      </listitem>

      <listitem>
        <para>vif0.0 — This is the bridge port that is used by traffic to/from
        Domain 0.</para>
      </listitem>

      <listitem>
        <para>vifX.0 — This is the bridge port that is used by traffic to/from
        Domain X.</para>
      </listitem>
    </itemizedlist>
  </section>

  <section>
    <title>Configuring Shorewall in Dom0</title>

    <para>As I state in the answer to <ulink url="FAQ.htm#faq2">Shorewall FAQ
    2</ulink>, I object to running servers in a local zone because if the
    server becomes compromised then there is no protection between that
    compromised server and the other local systems. Xen allows me to safely
    run Internet-accessible servers in my local zone by creating a firewall in
    (the Extended) Dom0 to isolate the server(s) from the other local systems
    (including Dom0).</para>

    <caution>
      <para>I find Xen Domain 0 to be an arcane environment in which to try to
      use Netfilter (and hence Shorewall). As the number of interfaces and
      bridges increase, complexity increases geometrically. I recommend
      following this guide only if you really need to place a public server in
      your local network. Otherwise, the <ulink url="XenMyWay.html">way that I
      use Xen</ulink> is much more straight-forward.</para>
    </caution>

    <para>Here is an example. In this example, we will assume that the system
    is behind a second firewall that restricts incoming traffic so that we
    only have to worry about protecting the local LAN from the systems running
    in the DomU's.</para>

    <section>
      <title>/etc/shorewall/shorewall.conf</title>

      <para>Because Xen uses normal Linux bridging, you must enable bridge
      support in shorewall.conf</para>

      <blockquote>
        <programlisting>BRIDGING=Yes</programlisting>
      </blockquote>
    </section>

    <section>
      <title>/etc/shorewall/zones</title>

      <para>One thing strange about configuring Shorewall in this environment
      is that Dom0 is defined as two different zones. It is defined as the
      firewall zone and it is also defined as "all systems connected to
      <filename class="devicefile">xenbr0:vif0.0</filename>. In this case, I
      call this second zone <emphasis role="bold">ursa</emphasis> (which is
      the name given to the virtual system running in Dom0); that zone
      corresponds to Dom0 as seen from the outside in the diagram above (see
      more <link linkend="zones">below</link>).</para>

      <blockquote>
        <programlisting>#                                       OPTIONS                 OPTIONS
fw      firewall                        #Domain 0
ursa    ipv4                            #Domain 0 on the bridge
dmz     ipv4                            #Server(s) running in Domains other than 0
net     ipv4                            #The local LAN and beyond
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE</programlisting>
      </blockquote>
    </section>

    <section>
      <title>/etc/shorewall/interfaces</title>

      <para>We must deal with two network interfaces. We must deal with the
      (virtualized) eth0 and we must also deal with the bridge (xenbr0)
      created by Xen.</para>

      <blockquote>
        <programlisting>#ZONE   INTERFACE       BROADCAST       OPTIONS
-       xenbr0          -               dhcp
net     eth0            detect          dhcp
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>
      </blockquote>
    </section>

    <section>
      <title>/etc/shorewall/hosts</title>

      <para>Here we define the zones <emphasis role="bold">ursa</emphasis> and
      <emphasis role="bold">dmz</emphasis> and we extend the definition of the
      zone <emphasis role="bold">net</emphasis>.<blockquote>
          <programlisting>#ZONE   HOST(S)                                 OPTIONS
ursa    xenbr0:vif0.0
dmz     xenbr0:vif+<footnote>
              <para>There is a bug in Shorewall versions prior to 3.0.4 that treats all bridge ports as if they had routeback specified. I recommend that you run a Shorewall verison &gt; 3.0.3 if you run Xen.</para>
            </footnote>
net     xenbr0:peth0
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS LINE -- DO NOT REMOVE</programlisting>
        </blockquote></para>

      <para>Note that the <emphasis role="bold">net</emphasis> zone has two
      different interfaces. From the point of view of Dom0 (which is where
      Shorewall runs), the <emphasis role="bold">net</emphasis> zone comprises
      everything except Dom0. From the point of view of the Extended Domain 0,
      the <emphasis role="bold">net</emphasis> zone is everything connected
      (directly or indirectly) to the <filename
      class="devicefile">peth0</filename> port on the bridge.</para>
    </section>

    <section>
      <title>/etc/shorewall/policy</title>

      <para>The policies shown here effectively isolate Domains 1...N.</para>

      <blockquote>
        <programlisting>#SOURCE         DEST            POLICY          LOG             LIMIT:BURST
#                                               LEVEL
all             fw              ACCEPT
fw              all             ACCEPT
ursa            all             ACCEPT
net             ursa            ACCEPT
net             net             NONE
all             all             REJECT          info
#LAST LINE -- DO NOT REMOVE
</programlisting>
      </blockquote>
    </section>

    <section>
      <title>/etc/shorewall/rules</title>

      <para>These rules determine the traffic allowed into and out of the
      <emphasis role="bold">dmz</emphasis> zone.</para>

      <blockquote>
        <programlisting>#
# "Net' to DMZ
#
ACCEPT          net                dmz                     udp     domain
ACCEPT          net                dmz                     tcp     www,smtp,smtps,domain,ssh,imap,rsync,https,imaps,ftp,10023,pop3,3128
Trcrt/ACCEPT    net                dmz
#
# DMZ to 'Net'
#
ACCEPT          dmz                net:!192.168.0.0/22     udp     domain,ntp
ACCEPT          dmz                net:!192.168.0.0/22     tcp     echo,ftp,ssh,smtp,whois,domain,www,81,https,rsync,cvspserver,2702,2703,8080
ACCEPT          dmz                net:$POPSERVERS         tcp     pop3
Ping/ACCEPT     dmz                net

Ping/ACCEPT     dmz                ursa</programlisting>
      </blockquote>

      <para>Here, 192.168.0.0/22 comprises my local network.</para>

      <para id="zones">From the point of view of Shorewall, the zone diagram
      is as shown in the following diagram.</para>

      <graphic align="center" fileref="images/Xen2.png" />

      <para>Note that the <emphasis role="bold">ursa</emphasis> zone subsumes
      the <emphasis role="bold">fw</emphasis> zone because the <emphasis
      role="bold">ursa</emphasis> zone is defined to be all systems that
      interface to xenbr0's vif0.0 port — it is the rules governing traffic
      to/from the <emphasis role="bold">ursa</emphasis> zone that protect the
      firewall in this configuration.</para>

      <para>More elaborate configurations are possible as described in my
      <ulink url="XenMyWay.html">Xen and the Art of Consolidation</ulink>
      article.</para>
    </section>
  </section>
</article>