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<articleinfo >
<title > Network Mapping</title>
<authorgroup >
<author >
<firstname > Tom</firstname>
<surname > Eastep</surname>
</author>
</authorgroup>
2004-05-29 17:12:25 +02:00
<pubdate > 2004-05-28</pubdate>
2004-04-05 23:11:05 +02:00
<copyright >
<year > 2004</year>
<holder > Thomas M. Eastep</holder>
</copyright>
<legalnotice >
<para > Permission is granted to copy, distribute and/or mify 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 > Why use Network Mapping</title>
<para > Network Mapping is most often used to resolve IP address conflicts.
Suppose that two organizations, A and B, need to be linked and that both
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organizations have allocated the 192.168.1.0/24 subnetwork. There is a
need to connect the two networks so that all systems in A can access the
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192.168.1.0/24 network in B and vice versa without any re-addressing.</para>
</section>
<section >
<title > Solution</title>
<para > Shorewall NETMAP support is designed to supply a solution. The basic
situation is as shown in the following diagram.<graphic
fileref="images/netmap.png" /></para>
<para > While the link between the two firewalls is shown here as a VPN, it
could be any type of interconnection that allows routing of <ulink
url="shorewall_setup_guide.htm#RFC1918">RFC 1918</ulink> traffic.</para>
<para > The systems in the top cloud will access the 192.168.1.0/24 subnet
in the lower cloud using addresses in another unused /24. Similarly, the
systems in the bottom cloud will access the 192.168.1.0/24 subnet in the
upper cloud using a second unused /24.</para>
<para > In order to apply this solution:</para>
<itemizedlist >
<listitem >
<para > You must be running Shorewall 2.0.1 Beta 2 or later.</para>
</listitem>
<listitem >
<para > Your kernel must have NETMAP support. 2.6 Kernels have NETMAP
support without patching while 2.4 kernels must be patched using
Patch-O-Matic from <ulink url= "http://www.netfilter.org" > netfilter.org</ulink> .</para>
</listitem>
<listitem >
<para > NETMAP support must be enabled in your kernel
(CONFIG_IP_NF_TARGET_NETMAP=m or CONFIG_IP_NF_TARGET_NETMAP=y).</para>
</listitem>
<listitem >
<para > Your iptables must have NETMAP support. NETMAP support is
available in iptables 1.2.9 and later.</para>
</listitem>
</itemizedlist>
<para > Network mapping is defined using the <filename > /etc/shorewall/netmap</filename>
file. Columns in this file are:</para>
<variablelist >
<varlistentry >
<term > TYPE</term>
<listitem >
<para > Must be DNAT or SNAT.</para>
<para > If DNAT, traffic entering INTERFACE and addressed to NET1 has
it' s destination address rewritten to the corresponding address
in NET2.</para>
<para > If SNAT, traffic leaving INTERFACE with a source address in
NET1 has it' s source address rewritten to the corresponding
address in NET2.</para>
</listitem>
</varlistentry>
<varlistentry >
<term > NET1</term>
<listitem >
<para > Must be expressed in CIDR format (e.g., 192.168.1.0/24).</para>
</listitem>
</varlistentry>
<varlistentry >
<term > INTERFACE</term>
<listitem >
<para > A firewall interface. This interface must have been defined in
<ulink url= "Documentation.htm#Interfaces" > <filename > /etc/shorewall/interfaces</filename> </ulink> .</para>
</listitem>
</varlistentry>
<varlistentry >
<term > NET2</term>
<listitem >
<para > A second network expressed in CIDR format.</para>
</listitem>
</varlistentry>
</variablelist>
<para > Referring to the figure above, lets suppose that systems in the top
cloud are going to access the 192.168.1.0/24 network in the bottom cloud
using addresses in 10.10.10.0/24 and that systems in the bottom could will
access 192.168.1.0/24 in the top could using addresses in 10.10.11.0.<important > <para > You
must arrange for routing as follows:</para> <itemizedlist > <listitem > <para > Traffic
from the top cloud to 10.10.10.0/24 must be routed to eth0 on firewall 1.</para> </listitem> <listitem > <para > Firewall
1 must route traffic to 10.10.10.0/24 through firewall 2.</para> </listitem> <listitem > <para > Traffic
from the bottom cloud to 10.10.11.0/24 must be routed to eth0 on firewall
2.</para> </listitem> <listitem > <para > Firewall 2 must route traffic to
10.10.11.0/24 through firewall 1.</para> </listitem> </itemizedlist> </important>
The entries in <filename > <filename > /etc/shorewall/netmap</filename> </filename>
in firewall1 would be as follows:</para>
<programlisting > #TYPE NET1 INTERFACE NET2
SNAT 192.168.1.0/24 vpn 10.10.11.0/24 #RULE 1A
DNAT 10.10.11.0/24 vpn 192.168.1.0/24 #RULE 1B</programlisting>
<para > The entry in <filename > /etc/shorewall/netmap</filename> in firewall2
would be:</para>
<programlisting > #TYPE NET1 INTERFACE NET2
DNAT 10.10.10.0/24 vpn 192.168.1.0/24 #RULE 2A
SNAT 192.168.1.0/24 vpn 10.10.10.0/24 #RULE 2B</programlisting>
<example >
<title > 192.168.1.4 in the top cloud connects to 192.168.1.27 in the
bottom cloud</title>
<para > In order to make this connection, the client attempts a connection
to 10.10.10.27. The following table shows how the source and destination
IP addresses are modified as requests are sent and replies are returned.
The RULE column refers to the above <filename > /etc/shorewall/netmap</filename>
entries and gives the rule which transforms the source and destination
IP addresses to those shown on the next line.
<informaltable > <tgroup cols= "5" > <thead > <row > <entry > FROM</entry> <entry > TO</entry> <entry > SOURCE
IP ADDRESS</entry> <entry > DESTINATION IP ADDRESS</entry> <entry > RULE</entry> </row> </thead> <tbody > <row > <entry > 192.168.1.4
in upper cloud</entry> <entry > Firewall 1</entry> <entry > 192.168.1.4</entry> <entry > 10.10.10.27</entry> <entry > 1A</entry> </row> <row > <entry > Firewall
1</entry> <entry > Firewall 2</entry> <entry > 10.10.11.4</entry> <entry > 10.10.10.27</entry> <entry > 2A</entry> </row> <row > <entry > Filrewall
2</entry> <entry > 192.168.1.27 in lower cloud</entry> <entry > 10.10.11.4</entry> <entry > 192.168.1.27</entry> <entry > </entry> </row> <row > <entry > 192.168.1.27
in the lower cloud</entry> <entry > Firewall 2</entry> <entry > 192.168.1.27</entry> <entry > 10.10.11.4</entry> <entry > 2B</entry> </row> <row > <entry > Firewall
2</entry> <entry > Firewall 1</entry> <entry > 10.10.10.27</entry> <entry > 10.10.11.4</entry> <entry > 1B</entry> </row> <row > <entry > Firewall
1</entry> <entry > 192.168.1.4 in upper cloud</entry> <entry > 10.10.10.27</entry> <entry > 192.168.1.4</entry> <entry > </entry> </row> </tbody> </tgroup> </informaltable> </para>
</example>
</section>
<section >
<title > Author' s Notes</title>
<para > This could all be made a bit simpler by eliminating the TYPE field
and have Shorewall generate both the SNAT and DNAT rules from a single
entry. I have chosen to include the TYPE in order to make the
implementation a bit more flexible. If you find cases where you can use an
SNAT or DNAT entry by itself, please let <ulink
url="mailto:webmaster@shorewall.net">me</ulink> know and I' ll add the
example to this page.</para>
<para > In the previous section, the table in the example contains a bit of
a lie. Because of Netfilter' s connection tracking, rules 2B and 1A
aren' t needed to handle the replies. They ARE needed though for hosts
in the bottom cloud to be able to establish connections with the
192.168.1.0/24 network in the top cloud.</para>
</section>
<section >
<title > Can' t I do this with one router? Why do I need two?</title>
<para > The single router would have to be able to route to two different
192.168.1.0/24 networks. In Netfilter parlance, that would mean that the
destination IP address would have to be rewritten after the packet had
been routed; Netfilter doesn' t have that capability.</para>
</section>
</article>
