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252 lines
10 KiB
XML
252 lines
10 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article>
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<!--$Id$-->
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<articleinfo>
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<title>Xen and Shorewall</title>
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<authorgroup>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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</authorgroup>
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<pubdate>2006-03-10</pubdate>
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<copyright>
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<year>2006</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<section>
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<title>Xen Network Environment</title>
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<para><ulink
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url="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/">Xen</ulink> is a
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<firstterm>paravirtualization</firstterm> tool that allows you to run
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multiple virtual machines on one physical machine. It is available on a
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wide number of platforms and is included in recent
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<trademark>SuSE</trademark> distributions.</para>
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<para>Xen refers to the virtual machines as
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<firstterm>Domains</firstterm>. Domains are numbered with the first domain
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being domain 0, the second domain 1, and so on. Domain 0
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(<firstterm>Dom0</firstterm>) is special because that is the domain
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created when to machine is booted. Additional domains (called
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<firstterm>DomU</firstterm>'s) are created using the <command>xm
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create</command> command from within Domain 0. Additional domains can also
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be created automatically at boot time by using the
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<command>xendomains</command> service.</para>
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<para>Xen virtualizes a network interface named <filename
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class="devicefile">eth0</filename><footnote>
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<para>This assumes the default Xen configuration created by
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<command>xend </command>and assumes that the host system has a single
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ethernet interface named <filename
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class="devicefile">eth0</filename>.</para>
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</footnote> in each domain. In Dom0, Xen also creates a bridge
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(<filename class="devicefile">xenbr0</filename>) and a number of virtual
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interfaces as shown in the following diagram.</para>
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<graphic align="center" fileref="images/Xen1.png" />
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<para>I use the term <firstterm>Extended Dom0</firstterm> to distinguish
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the bridge and virtual interfaces from Dom0 itself. That distinction is
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important when we try to apply Shorewall in this environment.</para>
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<para>The bridge has a number of ports:</para>
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<itemizedlist>
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<listitem>
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<para>peth0 — This is the port that connects to the physical network
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interface in your system.</para>
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</listitem>
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<listitem>
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<para>vif0.0 — This is the bridge port that is used by traffic to/from
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Domain 0.</para>
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</listitem>
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<listitem>
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<para>vifX.0 — This is the bridge port that is used by traffic to/from
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Domain X.</para>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>Configuring Shorewall in Dom0</title>
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<para>As I state in the answer to <ulink url="FAQ.htm#faq2">Shorewall FAQ
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2</ulink>, I object to running servers in a local zone because if the
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server becomes compromised then there is no protection between that
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compromised server and the other local systems. Xen allows me to safely
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run Internet-accessible servers in my local zone by creating a firewall in
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(the Extended) Dom0 to isolate the server(s) from the other local systems
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(including Dom0).</para>
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<caution>
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<para>I find Xen Domain 0 to be an arcane environment in which to try to
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use Netfilter (and hence Shorewall). As the number of interfaces and
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bridges increase, complexity increases geometrically. I recommend
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following this guide only if you really need to place a public server in
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your local network. Otherwise, the <ulink url="XenMyWay.html">way that I
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use Xen</ulink> is much more straight-forward.</para>
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</caution>
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<para>Here is an example. In this example, we will assume that the system
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is behind a second firewall that restricts incoming traffic so that we
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only have to worry about protecting the local LAN from the systems running
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in the DomU's.</para>
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<section>
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<title>/etc/shorewall/shorewall.conf</title>
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<para>Because Xen uses normal Linux bridging, you must enable bridge
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support in shorewall.conf</para>
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<blockquote>
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<programlisting>BRIDGING=Yes</programlisting>
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</blockquote>
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</section>
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<section>
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<title>/etc/shorewall/zones</title>
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<para>One thing strange about configuring Shorewall in this environment
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is that Dom0 is defined as two different zones. It is defined as the
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firewall zone and it is also defined as "all systems connected to
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<filename class="devicefile">xenbr0:vif0.0</filename>. In this case, I
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call this second zone <emphasis role="bold">ursa</emphasis> (which is
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the name given to the virtual system running in Dom0); that zone
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corresponds to Dom0 as seen from the outside in the diagram above (see
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more <link linkend="zones">below</link>).</para>
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<blockquote>
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<programlisting># OPTIONS OPTIONS
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fw firewall #Domain 0
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ursa ipv4 #Domain 0 on the bridge
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dmz ipv4 #Server(s) running in Domains other than 0
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net ipv4 #The local LAN and beyond
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#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE</programlisting>
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</blockquote>
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</section>
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<section>
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<title>/etc/shorewall/interfaces</title>
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<para>We must deal with two network interfaces. We must deal with the
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(virtualized) eth0 and we must also deal with the bridge (xenbr0)
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created by Xen.</para>
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<blockquote>
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<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
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- xenbr0 - dhcp
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net eth0 detect dhcp
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#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>
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</blockquote>
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</section>
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<section>
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<title>/etc/shorewall/hosts</title>
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<para>Here we define the zones <emphasis role="bold">ursa</emphasis> and
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<emphasis role="bold">dmz</emphasis> and we extend the definition of the
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zone <emphasis role="bold">net</emphasis>.<blockquote>
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<programlisting>#ZONE HOST(S) OPTIONS
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ursa xenbr0:vif0.0
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dmz xenbr0:vif+<footnote>
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<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 > 3.0.3 if you run Xen.</para>
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</footnote>
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net xenbr0:peth0
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#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS LINE -- DO NOT REMOVE</programlisting>
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</blockquote></para>
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<para>Note that the <emphasis role="bold">net</emphasis> zone has two
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different interfaces. From the point of view of Dom0 (which is where
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Shorewall runs), the <emphasis role="bold">net</emphasis> zone comprises
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everything except Dom0. From the point of view of the Extended Domain 0,
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the <emphasis role="bold">net</emphasis> zone is everything connected
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(directly or indirectly) to the <filename
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class="devicefile">peth0</filename> port on the bridge.</para>
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</section>
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<section>
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<title>/etc/shorewall/policy</title>
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<para>The policies shown here effectively isolate Domains 1...N.</para>
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<blockquote>
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<programlisting>#SOURCE DEST POLICY LOG LIMIT:BURST
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# LEVEL
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all fw ACCEPT
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fw all ACCEPT
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ursa all ACCEPT
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net ursa ACCEPT
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net net NONE
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all all REJECT info
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#LAST LINE -- DO NOT REMOVE
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</programlisting>
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</blockquote>
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</section>
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<section>
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<title>/etc/shorewall/rules</title>
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<para>These rules determine the traffic allowed into and out of the
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<emphasis role="bold">dmz</emphasis> zone.</para>
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<blockquote>
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<programlisting>#
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# "Net' to DMZ
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#
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ACCEPT net dmz udp domain
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ACCEPT net dmz tcp www,smtp,smtps,domain,ssh,imap,rsync,https,imaps,ftp,10023,pop3,3128
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Trcrt/ACCEPT net dmz
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#
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# DMZ to 'Net'
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#
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ACCEPT dmz net:!192.168.0.0/22 udp domain,ntp
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ACCEPT dmz net:!192.168.0.0/22 tcp echo,ftp,ssh,smtp,whois,domain,www,81,https,rsync,cvspserver,2702,2703,8080
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ACCEPT dmz net:$POPSERVERS tcp pop3
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Ping/ACCEPT dmz net
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Ping/ACCEPT dmz ursa</programlisting>
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</blockquote>
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<para>Here, 192.168.0.0/22 comprises my local network.</para>
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<para id="zones">From the point of view of Shorewall, the zone diagram
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is as shown in the following diagram.</para>
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<graphic align="center" fileref="images/Xen2.png" />
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<para>Note that the <emphasis role="bold">ursa</emphasis> zone subsumes
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the <emphasis role="bold">fw</emphasis> zone because the <emphasis
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role="bold">ursa</emphasis> zone is defined to be all systems that
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interface to xenbr0's vif0.0 port — it is the rules governing traffic
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to/from the <emphasis role="bold">ursa</emphasis> zone that protect the
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firewall in this configuration.</para>
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<para>More elaborate configurations are possible as described in my
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<ulink url="XenMyWay.html">Xen and the Art of Consolidation</ulink>
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article.</para>
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</section>
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</section>
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</article> |