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<refentry>
<refmeta>
<refentrytitle>shorewall-nat</refentrytitle>
<manvolnum>5</manvolnum>
<refmiscinfo>Configuration Files</refmiscinfo>
</refmeta>
<refnamediv>
<refname>nat</refname>
<refpurpose>Shorewall one-to-one NAT file</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>/etc/shorewall/nat</command>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>This file is used to define one-to-one Network Address Translation
(NAT).</para>
<warning>
<para>If all you want to do is simple port forwarding, do NOT use this
file. See <ulink
url="/FAQ.htm#faq1">http://www.shorewall.net/FAQ.htm#faq1</ulink>. Also,
in many cases, Proxy ARP (<ulink
url="/manpages/shorewall-proxyarp.html">shorewall-proxyarp</ulink>(5))
is a better solution that one-to-one NAT.</para>
</warning>
<para>The columns in the file are as follows (where the column name is
followed by a different name in parentheses, the different name is used in
the alternate specification syntax).</para>
<variablelist>
<varlistentry>
<term><emphasis role="bold">EXTERNAL</emphasis> -
{<emphasis>address</emphasis>|[?]COMMENT}</term>
<listitem>
<para>External IP Address - this should NOT be the primary IP
address of the interface named in the next column and must not be a
DNS Name.</para>
<para>If you put COMMENT in this column, the rest of the line will
be attached as a comment to the Netfilter rule(s) generated by the
following entries in the file. The comment will appear delimited by
"/* ... */" in the output of "shorewall show nat"</para>
<para>To stop the comment from being attached to further rules,
simply include COMMENT on a line by itself.</para>
<note>
<para>Beginning with Shorewall 4.5.11, ?COMMENT is a synonym for
COMMENT and is preferred.</para>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">INTERFACE</emphasis> -
<emphasis>interfacelist</emphasis>[<emphasis
role="bold">:</emphasis>[<emphasis>digit</emphasis>]]</term>
<listitem>
<para>Interfaces that have the <emphasis
role="bold">EXTERNAL</emphasis> address. If ADD_IP_ALIASES=Yes in
<ulink
url="/manpages/shorewall.conf.html">shorewall.conf</ulink>(5),
Shorewall will automatically add the EXTERNAL address to this
interface. Also if ADD_IP_ALIASES=Yes, you may follow the interface
name with ":" and a <emphasis>digit</emphasis> to indicate that you
want Shorewall to add the alias with this name (e.g., "eth0:0").
That allows you to see the alias with ifconfig. <emphasis
role="bold">That is the only thing that this name is good for -- you
cannot use it anywhere else in your Shorewall configuration.
</emphasis></para>
<para>Each interface must match an entry in <ulink
url="/manpages/shorewall-interfaces.html">shorewall-interfaces</ulink>(5).
Shorewall allows loose matches to wildcard entries in <ulink
url="/manpages/shorewall-interfaces.html">shorewall-interfaces</ulink>(5).
For example, <filename class="devicefile">ppp0</filename> in this
file will match a <ulink
url="/manpages/shorewall-interfaces.html">shorewall-interfaces</ulink>(5)
entry that defines <filename
class="devicefile">ppp+</filename>.</para>
<para>If you want to override ADD_IP_ALIASES=Yes for a particular
entry, follow the interface name with ":" and no digit (e.g.,
"eth0:").</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">INTERNAL</emphasis> -
<emphasis>address</emphasis></term>
<listitem>
<para>Internal Address (must not be a DNS Name).</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">ALL INTERFACES</emphasis> (allints) -
[<emphasis role="bold">Yes</emphasis>|<emphasis
role="bold">No</emphasis>]</term>
<listitem>
<para>If Yes or yes, NAT will be effective from all hosts. If No or
no (or left empty) then NAT will be effective only through the
interface named in the <emphasis role="bold">INTERFACE</emphasis>
column.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">LOCAL</emphasis> - [<emphasis
role="bold">Yes</emphasis>|<emphasis role="bold">No</emphasis>]</term>
<listitem>
<para>If <emphasis role="bold">Yes</emphasis> or <emphasis
role="bold">yes</emphasis>, NAT will be effective from the firewall
system</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>RESTRICTIONS</title>
<para>DNAT rules always preempt one-to-one NAT rules. This has subtile
consequences when there are sub-zones on an
<replaceable>interface</replaceable>. Consider the following:</para>
<para><filename>/etc/shorewall/zones</filename>:</para>
<programlisting>#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
fw firewall
net ipv4
loc ipv4
smc:net ipv4</programlisting>
<para><filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE OPTIONS
net eth0 dhcp,tcpflags,nosmurfs,routefilter,logmartians,sourceroute=0
loc eth1 tcpflags,nosmurfs,routefilter,logmartians</programlisting>
<para><filename>/etc/shorewall/hosts</filename>:</para>
<programlisting>#ZONE HOST(S) OPTIONS
smc eth0:10.1.10.0/24</programlisting>
<para><filename>/etc/shorewall/nat</filename>:</para>
<programlisting>#EXTERNAL INTERFACE INTERNAL ALL LOCAL
# INTERFACES
10.1.10.100 eth0 172.20.1.100
</programlisting>
<para>Note that the EXTERNAL address is in the <emphasis
role="bold">smc</emphasis> zone.</para>
<para><filename>/etc/shorewall/rules</filename>:</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST SOURCE ORIGINAL RATE USER/ MARK CONNLIMIT TIME HEADERS SWITCH HELPER
# PORT PORT(S) DEST LIMIT GROUP
?SECTION ALL
?SECTION ESTABLISHED
?SECTION RELATED
?SECTION INVALID
?SECTION UNTRACKED
?SECTION NEW
...
DNAT net loc:172.20.1.4 tcp 80</programlisting>
<para>For the one-to-one NAT to work correctly in this configuration, one
of two approaches can be taken:</para>
<orderedlist>
<listitem>
<para>Define a CONTINUE policy with <emphasis
role="bold">smc</emphasis> as the SOURCE zone (preferred):</para>
<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
<emphasis role="bold">smc $FW CONTINUE</emphasis>
loc net ACCEPT
net all DROP info
# THE FOLLOWING POLICY MUST BE LAST
all all REJECT info
</programlisting>
</listitem>
<listitem>
<para>Set IMPLICIT_CONTINUE=Yes in <ulink
url="manpages/shorewall.conf.html">shorewall.conf(5)</ulink>.</para>
</listitem>
</orderedlist>
</refsect1>
<refsect1>
<title>FILES</title>
<para>/etc/shorewall/nat</para>
</refsect1>
<refsect1>
<title>See ALSO</title>
<para><ulink
url="/NAT.htm">http://www.shorewall.net/NAT.htm</ulink></para>
<para><ulink
url="/configuration_file_basics.htm#Pairs">http://www.shorewall.net/configuration_file_basics.htm#Pairs</ulink></para>
<para>shorewall(8), shorewall-accounting(5), shorewall-actions(5),
shorewall-blacklist(5), shorewall-hosts(5), shorewall_interfaces(5),
shorewall-ipsets(5), shorewall-maclist(5), shorewall-masq(5),
shorewall-netmap(5), shorewall-params(5), shorewall-policy(5),
shorewall-providers(5), shorewall-proxyarp(5), shorewall-rtrules(5),
shorewall-routestopped(5), shorewall-rules(5), shorewall.conf(5),
shorewall-secmarks(5), shorewall-tcclasses(5), shorewall-tcdevices(5),
shorewall-mangle(5), shorewall-tos(5), shorewall-tunnels(5),
shorewall-zones(5)</para>
</refsect1>
</refentry>